摘要:

Copper ion binding to N-phenylphthalamic acid studied by 13Cnuclear magnetic resonance and electron paramagnetic resonance: model interaction of polyamic acid with copper Toshifumi Hiraoki,"" Noriyuki Kinjo,tb Kunio Miyazaki,b Osamu Miurab and Akihiro Tsutsumi*" aDepartment of Applied Physics, Hokkaido University, Sapporo 060, Japan Hitachi Research Laboratory, Hitachi Ltd., Hitachi 319-12, Japan The interaction of copper with N-phenylphthalamic acid (PPA) cured at various temperatures was investigated by 13C NMR and EPR spectroscopies, as the model system of polyamic acid on copper. EPR spectra prove that copper is dissolved into the PPA- N-methylpyrrolidone solution, producing paramagnetic Cu2 ions. The 13C NMR resonances of the phthalic group of PPA are + selectively broadened due to the paramagnetic interaction between the 13C nuclei and Cu2+ ions, showing binding of Cu2+ to the carboxylate group. Cu2+ ion has no effect on the amide and phenyl groups.Cu2+ ion exchanges rapidly between the carboxylate groups at an exchange rate > lo3s-' at 23 "C. PPA is imidized to N-phenylphthalimide (PPI)at 150 "C, accompanied by the dissociation of Cu2+. Paramagnetic effects from the Cu2+ ions is not exerted on PPI. The results obtained are compared with the interfacial interaction between polyamic acid and copper. Polyimides are one of the most important classes of high- performance polymers.' Owing to their excellent electrical, thermal and high-temperature mechanical properties, polyim- ides have found many applications, in particular for advanced microelectronics.The polyimide-metal interface characteristics, such as chemical, adhesion and electrical properties are very important for microelectronic devices. A number of extensive studies of polyimide-metal interface interactions have been performed.'q2 Understanding the nature of the interacting processes is essential to basic device reliability. There are basically two types of interface between polyimide and metal: metal-on-polymer and polymer-on-metal. The metal-on-polymer type of interface, normally formed by vapour deposition of metal films onto polyimide, has been charac- terized extensively by surface spectros~opies.~-~ The polymer- on-metal type of interface is created by coating a polyimide precursor onto a metal followed by curing to form the polyimide. Among the metals investigated, copper has been found to exhibit an important differen~e.~~' Copper is commonly used as the metal in microelectronic applications because of its high conductivity and low cost.Two reactions have been reported to occur at the interface between pyromellitic dianhydride- oxydianiline (PMDA-0DA)-derived polyamic acid and copper during ~uring.~-'~ At first, the polyamic acid reacts directly with copper or copper oxide and the copper-polyamic acid complex is proposed to form. Secondly, this complex decom- poses to the polyimide during subsequent curing. Both reac- tions are controlled by the curing temperature and the oxygen level supplied to the interface.The thermal imidization in the presence of oxygen was found to produce a cuprous oxide (Cu20), which degrades the polyimide ~atalytically.~-'~ The reverse, copper-on-polyimide, interface, however, does not produce CU,O.~ The nature of the interactions at the interface still remains to be understood concerning the binding site and exchange rate of copper. In the copper-polyamic acid complex the copper ion is believed to bind the carboxylate group of the polyamic We have shown that N-phenylphthalamic acid (PPA; Fig. l), as a model compound of the main compo- -f Present address: Ibaraki Research Laboratory, Hitachi Chemical Co., Ltd., Higashi 4-13-1, Hitachi 317, Japan. nent of polyamic acid, interacts with copper to form the PPA-copper ion complex in N-methylpyrrolidone (NMP), and that the copper ion binds to the carboxylate group of PPA.12 The present study clarifies the interaction of PPA with copper and the curing of the PPAXopper complex, as a model system of polyamic acid on copper, by means of I3C NMR and EPR spectroscopies. The interfacial interaction of polyamic acid with copper will be discussed, and compared with the present study.Experimental N-phenylphthalamic acid (PPA) was synthesized from phthalic anhydride and aniline in N-methylpyrrolidone (NMP).I2 PPA solutions in NMP (10%) were treated at 50, 100 and 150°C with and without copper particles for 3 h each, and then poured into water. The resultant precipitates were fil- tered off, washed several times with water, and dried in U~CUO at room temperature.The copper content of each sample was obtained using a Hitachi 2-8000 atomic absorption spectrometer. 13C NMR spectra in solution were obtained on a Bruker AM-500 spectrometer operating at 125.76 MHz at 23 "C un- 38 PPA 02 II PPI Fig. 1 Structures of N-phenylphthalamic acid (PPA) and N-phenyl- phthalimide (PPI) J. Muter. Chem., 1996, 6(5),727-731 727 less otherwise stated The sample concentrations used were 0 1-0 05 mol dm-3 solutions in deuteriated dimethyl sulfoxide [(CD3)ZSO, MSD Isotopes] Chemical shifts are expressed relative to the resonance of (CD,),SO at 6 39 5 Two-dimen-sional 'H 2QF-COSY,15 l6 C-H C0SYl7 and HMBCI8 experi- ments were performed to obtain resonance assignments EPR spectra in the solid state were obtained at 9 25 GHz and a microwave power of 1mW using a JEOL FES-1XG spec- trometer at room temperature The 100 kHz field modulation width was 1 mT Results and Discussion Paramagnetic copper ion The copper concentrations of PPA cured at 50,100 and 150 "C in the presence of copper were estimated to be 3 6, 3 0 and 0 16%, respectively Fig 2 shows the EPR spectra of PPA cured at three different temperatures with copper The spectra show that the cunng of PPA in the presence of copper produces paramagnetic Cu2+ ions Analogous results have been obtained from visible spectra in NMP l2 Paramagnetic effect on 13CNMR spectra of PPA Resonance assignments of the 13C NMR spectra of PPA without copper were made with the two-dimensional NMR spectroscopies described in the Experimental section The chemical shifts of PPA obtained are summarized in Table 1 The paramagnetic interaction between the 13C nucleus and the Cu2+ ion is clearly observable in NMR spectra Resonances around the metal-ion binding site will be broadened because J I I I I300 400 BlmT Fig.2 EPR spectra of copper with PPA in the solid state at 23°C Cunng temperature and recording gain of each spectrum (a) 50°C and 6 3, (b) 100"Cand 5, (c) 150"C and 5 Table 1 I3C NMR chemical shifts (6) for PPA and PPI at 296 K PPA PPI carbon atom 6 carbon atom 6 C' 138 92 C' 131 54 CZ 129 97 c3 129 54 CZ 123 72 c4 129 38 c3 134 72 c5 131 72 C6 127 79 c7 16740 c4 167 03 C8 167 48 c9 139 58 c5 131 91 C'O 119 53 C6 127 42 C" 128 63 c7 128 86 CIZ 123 32 C8 128 08 of the hyperfine interaction between a nucleus and an unpaired electron with a large magnetic moment l9 24 Since the Cu+ ion contains no unpaired electron, its complex is always diamagnetic Therefore, for Cu +,paramagnetic broadening is not observed in the NMR spectrum and the EPR spectrum is silent The solution of PPA containing copper was added to the PPA solution without copper, both of which were cured at 50 "C Fig 3 and 4 show the copper concentration dependence of the aromatic and carbonyl regions, respectively, of the 13C NMR spectra of PPA The resonance of the carboxylate car- bon Cs of PPA is at first selectively broadened and its intensity decreases remarkably with the addition of copper Simultaneously, the C1 and C2 resonances are considerably broadened This observation illustrates that the paramagnetic Cu2+ ion interacts with PPA and is located close to the carboxylate group of PPA The addition of increasing concen- trations of copper ions results in the broadening of the C3, C6 and C4 resonances and the disappearance of the C1, C2 and Cs resonances Copper ion exhibited hardly any influence on the carbons of the phenyl group, C5 of the phthalic group and the amide group at the concentration used This is confirmed by 'H NMR spectra, which show that the amide proton resonance is not affected by the presence of Cu2+ ion (data not shown) These results show that Cu2+ ion is bound to the carboxylate group and forms a complex in solution The paramagnetic line broadening effect of copper ions on the carbon resonances of PPA is of the following order Cs >C2> C1 >C3> C6 > C4 No paramagnetic shift was observed for resonances of PPA within the copper concen- tration used The paramagnetic contribution of the line width, Avp, is I (4I I I I I 1 140 I35 130 125 120 6 Fig.3 Aromatic region of 13C NMR spectra of PPA Cu PPA molar ratio (a) 0, (b) 17 x 10 3, (c) 3 4 x 10 3, (d) 5 1 x 10 3, (e) 8 4 x 10 3, (f) 16 x lo-' Resonances in (a) are labelled with the PPA carbon number as shown in Fig 1 728 J Mater Chem, 1996,6(5), 727-731 A-8 ,7I I I I68 1676 Fig.4 Carbonyl region of I3C NMR spectra of PPA us. Cu: PPA molar ratio: (a) 0, (b) 1.7 x (c) 3.4 x Resonances in (a) are labelled with the PPA carbon number as shown in Fig. 1. I I I I TPC Fig. 5 Temperature dependence of Av, for C4 (0)and C6 (0)reson-ances of PPA. Cu: PPA molar ratio, 1.6 x lop2. defined as shown in eqn. ( 1):24 AvP = AVobs -AVO (1) where Avobsand Avo are the observed linewidth in the presence and absence of copper ion, respectively. Fig. 5 shows the temperature dependence of Avp for the C4 and C6 resonances. Both Avp values decrease with increasing temperature, showing the fast chemical exchange between free and bound Cu2+ to PPA on the NMR timescale in the temperature range investi- gated.This observation implies that copper ion does not remain bound to a particular carboxylate group, but migrates rapidly between different carboxylate groups. The simultaneous imidization of PPA was not detected in the spectrum during the experiment at 80 "C. In the case of no paramagnetic shift, Avp is expressed as follows [eqn. (2)]:24 nAvp = fq/( GM + zM) (2) where f is the Cu : ligand molar ratio, q the number of ligands attached to the copper ion, GMthe spin-spin relaxation time of the nucleus of the molecule bound to copper ion, and T~ the mean lifetime of a nucleus in the bound state. As the present system is undergoing rapid exchange, GM>> z~. Eqn. (2) is thus simplified to: GM= fq/nAvp >> TM (3) We can estimate the upper limit value of T~ from eqn.(3) iff and q are known. Although no data is available for the binding constant of Cu2+ ion to PPA, all Cu2+ ions are assumed to bind to PPA. zM is roughly evaluated to be << 1 ms for f = 1.6 x in solution at 23 "C,assuming q = 1-4. It is known that the scalar interaction between a paramag- netic centre and a remote nucleus is the major contributor to GM,rather than the dipole-dipole interaction, in Cu2+ ion Paramagnetic broadening of the signals of the C4 and C6 carbons, as shown in Fig. 3, indicates that the unpaired electron of Cu2+ is transmitted to both carbons through the chemical bonds. Curing effect on PPA Fig.6 shows the effect of curing on PPA in the absence of copper.As resonances of imidized PPA, namely N-phenyl- phthalimide (PPI), are not detected in the spectrum shown in Fig. 6(a), PPA is judged not to be imidized at 50°C. With increasing curing temperature, new resonances of PPI as well as resonances of PPA are observed simultaneously in Fig. 6(b) and only the resonances of PPI appear in Fig. 6(c). The chemical shifts of PPI are summarized in Table 1. These results indicate that curing at 150°C induces PPA to imidize com- pletely to PPI. The spectrum in Fig. 6(b) is composed of resonances of both PPA and PPI. As expected, the C7 and C8 resonances of PPA and the C4 resonance of PPI are simultaneously observed in the carbonyl region. The degree of imidization of PPA can be determined from a comparison of the relative integrated inten- sities for the corresponding signals between PPI and PPA. The amount of PPI is estimated to be approximately 17% at a curing temperature of 100 "C.Fig. 7 shows the 13C NMR spectra of PPA cured in the presence of copper. Note that the signal-to-noise ratio of the spectrum of PPA cured at 50°C is quite poor, as shown in Fig. 7(a). Very broad and sharp signals are observed simul- taneously. The latter can be assigned to signals of PPI, from comparison with chemical shifts and the spectrum shown in Fig. 6(c). The former can be assigned to the CIO, C'l and C12 resonances of the phenyl group of PPA in the same way, 34 I 6 12 lo II 91 5 2 (a 1 A (I I I I I I 140 I35 130 6 12s 120 Fig.6 13C NMR spectra of PPA in the absence of copper at different curing temperatures: (a) 50°C, (b) 100°C, (c) 150°C. The numbered resonances in (a) and (c) correspond to the PPA and PPI carbon numbers shown shown in Fig. 1. J. Mater. Chem., 1996, 6(5), 727-731 729 I 1;D 135 1io 1Is 1io 6 Fig. 7 13C NMR spectra of PPA in the presence of copper at different cunng temperatures (a) 50 "C, (b) 100 "C, (c) 150 "C Arrows see text indicated by the arrows in Fig 7(a) and (b) However, no other signals belonging to PPA can be observed PPA cured at 50 "C contains copper at a Cu PPA molar ratio of 0 14 There is a large excess of paramagnetic Cu2+ ions in this system, though we could not determine quantitatively the amount of paramag- netic species present in the sample Therefore, all 13C NMR signals of PPA are broadened out and there is no selectivity of broadening of the signals for PPA This bulk paramagnetic effect is observed in the slightly broadened solvent signal as well 23 The spectrum of PPA cured at 100"C is similar to that at 50°C The appearance of PPI in Fig 7(a) implies that the presence of copper may stimulate the imidization of PPA to PPI, as resonances of PPI cannot be observed for PPA cured in the absence of copper at 50 "C It is difficult to estimate the amount of PPI from the spectrum, because signal intensities of PPA cannot be obtained owing to marked line broadening The relative amount of PPI in the sample cured at 100"C is clearly more than that at 50 "C, by comparison of the signal intensities of PPI and PPA The spectrum shown in Fig 7(c) is character- istic of PPI and shows the absence of broadened signals, in spite of the presence of paramagnetic copper ions These results imply that paramagnetic copper ions do not interact with PPI All chemical shifts of PPI shown in Fig 7 are different by 0 28 to 0 02 ppm from the equivalent signals in the absence of Cu2+ ions Since PPI does not interact with Cu2+ ions, this is probably due to the bulk paramagnetic effect of the excess of copper ions in the system23 Comparison of the interactions of PPA-Cu and polyamic acid-Cu The 13C NMR and EPR spectroscopic results presented in this work are compared with the polyamic acid-Cu interaction during imidization of polyamic acid In the PPA-Cu sys-tem, metallic copper or copper oxide is dissolved into the PPA-NMP solution, producing paramagnetic Cu2+ ions Simultaneously, Cu2+ ions form paramagnetic complexes with the carboxylate groups of PPA and exchange between the carboxylate groups at a rate >lo3s-' at room temperature in solution When the PPA-copper complex in solution is heated to 150"C, PPA is imidized to PPI, accompanied by the dis- 730 J Muter Chem, 1996, 6(5),727-731 sociation of the copper ions As the copper ions are reported to interact weakly with the solvent NMP,12 the resultant copper ions are dispersed into the solvent and washed out by water during the purification of samples Therefore, the copper content of PPI is very low, as described previously A similar interaction scheme can be applied to the case of polyimide formation from polyamic acid solution on copper, since it is widely believed that a sequence of reactions occurs when the polyamic acid precursor is coated and cured on a copper surface l425 Copper is dissolved in the polyamic acid-NMP solution7 25 and forms the paramagnetic Cu2+ ion complex with the carboxylate group of the polyamic acid This is confirmed by various surface spectros~opies~ lo I4 as well as our preliminary 13C NMR and EPR measurements26 Since the fast exchange behaviour of resonances is observed in the 13C NMR spectra of the polyamic acid-Cu2+ ion complex,26 the Cu2+ ions can migrate into the polyamic acid from the interface through chemical exchange between carboxylate groups It is furthermore demonstrated that the solvent pro- vides the mobility for the copper ions25 As this process accelerates with increasing temperature, the copper ions can easily percolate deeply into the polyamic acid solution from the interface Therefore, cuprous oxide is found in the polyimide far from the interface l2 When the solution is heated, polyamic acid is dehydrated to become polyimide, eliminating copper ions With simultaneous evaporation of the solvent NMP and water, the eliminated copper ions become cuprous oxide in the polyimide in the presence of oxygen7 Although the precise mechanism of this process is not yet clear, the degradation of the polyimide was found to be reduced significantly by the complete or almost complete exclusion of oxygen14 It was confirmed that cuprous oxide rather than metallic copper has a catalytic effect on polyimide degradation 7-14 Conclusion We have examined the interaction of PPA and PPI with copper by NMR and EPR spectroscopies Paramagnetic Cu2+ ion is produced and exchanges rapidly between the carboxylate groups of PPA in solution As these processes have also been observed in the polyamic acid-copper system by our prelimi- nary I3C NMR measurements, Cu2+ ion could easily percolate into the polyamic acid solution from the interface, and after curing copper particles or Cu20 would be found in the polyimide far from the interface PPA is imidized to PPI with curing at 150 "C This reaction is simultaneously accompanied by the dissociation of Cu2+ ion A paramagnetic effect is not directly exerted on PPI The authors are grateful to S Kawahara for his assistance in EPR measurements References 1 Polyimides Synthesis, Characterization and Applications, ed K L Mittal, Plenum Press, New York, 1984, vol 1and 2 2 L B Rothman, J Electrochem SOC,1980,127,2216 3 N J Chou, D W Dong, J Kim and A C Liu, J Electrochem SOC,1984,131,2335 4 N J ChouandC H Tang,J Vac Sci Technol, A, 1984,2,751 5 J W Bartha, F L P 0 Hahn and P S Ho, J Vac Sci Technol, A, 1985,3,1390 6 P S Ho, P 0 Hahn, J W Bartha, G W Rubloff, F K Legoues and B D Silverman, J Van Sci Technol, A, 1985,3,739 7 Y-H Kim, J Kim, G F Walker, C Feger and S P Kowalczyk, J Adhes Sci Technol, 1988,2,95 8 H G Linde, J Appl Polym Sci ,1990,40,2049 9 M C Burrell, P J Codella, J A Fontana and J J Chera, J Vac Sci Technol, A, 1989,7,1778 10 K Kelley, Y Ishino and H Ishida, Thin Solid Films,1987,154,271 11 Y-H.Kim, G. F. Walker, J. Kim and J. Park, J. Adhes. Sci. 20 N. Higuchi, T. Hiraoki and K. Hikichi, Macromolecules, 1980, Technol., 1987, 1, 331. 13, 81. 12 0. Miura, H. Watanabe, K. Miyazaki and S. Numata, Trans. 21 T. Hiraoki, M. Kaneko and K. Hikichi, Polym. J., 1979,11, 397. 13 ZEZCE Japan, 1988, J71-C, 1516. 0. Miura, H. Watanabe, K. Miyazaki and S. Numata, Trans. 22 23 T. Hiraoki, A. Tsutsumi and K. Hikichi, Polym. J., 1979, 11, 591. NMR of Paramagnetic Molecules, ed. G. N. LaMar, IEZCE Japan, 1988, J71-C, 1510. J. W. D. Horrocks and R. H. Holms, Academic Press, New York, 14 D-Y. Shih, J. Paraszczak, N. Klymko, R. Flitsch, S. Nunes, J. Lewis, C. Yang, J. Cataldo, R. McGouey, W. Graham, R. Serino 24 1973. R. A. Dwek, in Nuclear Magnetic Resonance in Biochemistry, 15 and E. Gallingan, J. Vac. Sci. Technol., A, 1989,7, 1402. U. Piantini, 0. W. Serrensen and R. R. Ernst, J. Am. Chem. Soc., 25 Clarendon Press, Oxford, 1973. S. Kowalczyk, Y-H. Kim, G. Walker and J. Kim, Appl. Phys. Lett., 1982,104,6800. 1988,52, 375. 16 A. J. Shaka and R. Freeman, J. Magn. Reson., 1983,51,169. 26 T. Hiraoki, N. Kinjo, 0. Miura, K. Miyazaki and A. Tsutsumi, 17 18 A. Bax and G. Morris, J. Magn. Reson., 1981, 42, 501. A. Bax and M. F. Summers, J. Am. Chem. Soc., 1986,108,2093. manuscript in preparation. 19 N. Higuchi, T. Hiraoki and K. Hikichi, Polym. J., 1979, 11, 139. Paper 5/05092H; Received 1st August, 1995 J. Muter. Chem., 1996,6(5), 727-731 731

ISSN:0959-9428    

DOI:10.1039/JM9960600727

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Mesogenic properties of novel enamino ketone ligands and their copper (11) complexes Jadwiga Szydlowska, Wiesl-aw Pyiuk, Adam Krbwczynski and Ildar Bikchantaev? Laboratory of Dielectrics and Magnetics, Department of Chemistry, University of Warsaw, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland Several novel homologous series of ligands incorporating an enamino ketone quasi-ring and their copper complexes are synthesized and studied by thermal microscopy, DSC and EPR methods. The ligands { 1-[4'-( 4"-hexyloxyphenyloazo)phenyl]-3-alkylaminoprop-2-en-1-ones}and their complexes form enantiotropic nematic and smectic C phases with the nematic phase of some of the complexes broader than 80 "C.The EPR spectra of the paramagnetic complexes show that the chelate centre is planar in dilute solution as well as in the magnetically concentrated smectic C phase.Exchanging the hexyloxy moiety for a hexylamino or N-hexyl-N-methylamino group gives a series of ligands and complexes with lower melting and clearing temperatures. Further molecular modifications including a reversal of the substitution pattern on the enamino ketone ring are also presented. Organometallic mesogens have been intensively studied during the last decade. Studies of paramagnetic mesogens provide a logical basis for the design and synthesis of ferromagnetic liquid crystals, which is important because of their potential applications. Also of interest are the optical properties of these complexes. As a rule, they exhibit vivid colours,' pronounced birefringence,2 dichroism3 and unusually strong optical nonlin- ear effect^.^ Their electrical properties, among them one-dimensional conductivity,' are also a promising area.An investigation of the properties of these compounds give an opportunity for the application of several new techniques in the field of liquid crystal research. For example, Mossbauer spectroscopy (a method limited, however, to only some metals) and EXAFS, which provides information about the environ- ment of the metal centre.6 EPR spectros~opy~-~ and mag- netochemical methods are vital for studies of paramagnetic mesogens, which are often easily obtained by complexation of transition metal ions with appropriate ligands. In general, liquid-crystalline phases are formed by disc-like or rod-like molecules.For geometrical reasons, it is easy to obtain discotic compounds, which are well known for several classes of 1igands.l' In contrast, calamitic paramagnetic com- plexes are restricted, in practice, to two classes of ligands,lO.'l P-diketonates and salicylaldimines. The latter seem to be more valuable materials, especially in view of their lower melting points and wider mesophase range.12 Recently, compounds incorporating quasi-ring enamino ketones, stabilized by an intermolecular hydrogen bond, were reported to be calamitic liquid-crystalline materials.13 They also appeared to be promising ligands, which enabled the synthesis of low melting, thermally stable paramagnetic nema- togens.l4.l5 In this work we present results of our studies of some novel mesomorphic enamino ketone ligands as well as their copper complexes, both designed as distinctly elongated molecules.The complexes are paramagnetic and are charac- terized by relatively good thermal stability with moderate melting temperatures. Molecular structure In order to obtain liquid crystalline molecules and to ensure a high length: breadth ratio for both ligands and complexes we synthesized compounds in which the enamino ketone group was substituted by a terminal alkyl chain in the 3-position and two hexyloxyazobenzene groups in the 1-position. The 7 Also: Kazan Physical-Technical Institut, Sibirsky Trakt 10/7,420029 Kazan, Russia. resulting ligands and complexes of the main series 1 and 2, respectively were studied in detail.For comparison, some compounds of a comparative series 3 and 4, having the substituents in reversed positions, were also examined. Besides the 'reversed' azo-derivatives some 'reversed' azoxy-com-pounds, series 5 and 6, were synthesized in order to extend the mesophase temperature range. Further important modifi-cations of the parent series 1,2 were obtained by replacing the oxygen atom of the alkoxy chain by an -NH-group, series 7 and 8. Modifications in which the -NH-group was exchanged by an -N(CH3)-moiety were also synthesized and studied, series 9 and 10. series 2 Y = H13C80 8 Y = H13CeNH 10 Y = H13C*N(CH3) series3 x = N=N 6 X=N(O)N series 4 x = N=N 6 X=N(O)N J.Mater. Chem., 1996, 6(5), 733-738 733 The abbreviations, n-n, used to denote the structures in the text denotes the series to which the compound belongs, n, and its homologue number, n Experimental Synthesis All the ligands and their copper(I1) complexes were prepared in the same manner from the appropriate acetyl derivatives of azobenzene and aliphatic amines (series 1, 2 and 7-10) or aliphatic methyl ketones and derivatives of p-aminoazo- or p- aminoazoxy-benzenes (senes 3-6) The starting materials are well known compounds, and their synthesis is routine Typical synthetic procedures are described below Preparation of 1-[ 4-(4-hexyloxyphenylazo) phenyll-3-pro- pylaminoprop-2-en-1-one (1-3). 4-( 4-Hexyloxypheny1azo)- a~etophenone'~'(324 g, 10 mmol) and ethyl formate (1 2 g) were added to molecular sodium (023 g, 10 mmol) in Et20 (20 cm3) and stirred vigorously for 12 h The resulting suspen- sion of formyl ketone sodium salt was neutralized with dilute aq HC1 and the formyl ketone diethyl ether solution was separated The solution, after the addition of 1-aminopropane (06Og, ca 10mmol) in MeOH (20cm3), was kept at room temperature for 2 h Next, the diethyl ether was evaporated and the remaining solution cooled and the resulting precipitate filtered off and recrystallized from methanol Red-orange crystals of the final product 1-3 were obtained (ca 70%) (Found C, 732, H, 79, N, 105 Calc for C24H31N302 C, 7324, H, 796, N, 10 68%) The NMR spectrum is consistent with the molecular structure, GH(CDC13) 0 80-1 95 [m, 16 H, OCH2(CH,),CH3, NCH,CH2CH3], 325 (m, 3 H, NCH,), 4 04 (t, J 6 8Hz, 3 H, OCH,), 5 74 (d, J 73,l H, H2), 6 85-7 20 (m, H', H3", H'"), 7 80-8 10 (m, 6 H, H2', H3', H", H6', H2", H6"), 10 40-10 55 (m, 1 H, NH) Preparation of bis{ 1-[ 4-(4-hexyloxyphenylazo)phenyl]-3-propylaminoprop-2-en-1-onato}copper(11) (2-3).A solution of Cu,(AcO), .2H20 (0 3 g) in MeOH (10 cm3) was added to a boiling solution of 1-3 (065 g, 2 mmol) in MeOH (20cm3) After 5 min at reflux, the solution was allowed to cool, and the precipitated complex (ca 90YO)filtered off and recrystallized from hexane (Found C, 68 2, H, 70, N, 9 9 Calc for C48H60N604CU c, 67 93%, H, 714%, N, 9 goo/,) Other ligands and complexes were obtained in a similar way The only difference was that ligands having terminal alkyl chains longer than decyl were recrystallized from ethanol Measurements The identification of mesophase was based on microscopic observations of the textures and miscibility tests with reference compounds For ortho- and cono-scopy, a Zeiss Jenapol-U polarizing microscope, equipped with a Mettler FP82HT hot stage was used A typical schlieren texture was observed in both nematic (N) and smectic C (S,) phases In addition to the schlieren texture the Sc phase revealed also, a focal-conic fan or broken fan texture In the smectic A (S,) phase homeotropic or fan textures were obtained depending on the glass surface preparation The smectic F (S,) phase was recognized from its focal-conic fan texture decorated with L-shaped patches In the smectic G (S,) phase a charactenstic mosaic texture was observed l6 Calorimetric measurements were performed using a DSC7 Perkin-Elmer set-up Routine runs were performed in a dry nitrogen atmosphere at a scanning rate of 5 K min-' In the case of thermally unstable complexes the scanning rate was increased to 20 "Cmin-' EPR spectra at different tempera- tures were taken in the X-band on a Radiopan spectrometer equipped with a flowed-nitrogen-atmosphere heater 734 J Muter Chem , 1996, 6(5), 733-738 Results and Discussion The phase-transition temperatures and phase-transition en-thalpy changes for the compounds of series 1-10 are collected in Tables 1-4 Main series Phase properties.Most of the compounds studied exhibited simple polymorphism For the homologues of the series 1 and 2 (Fig 1)only the nematic and smectic C phases are observed In agreement with their molecular elongation, both ligands and complexes reveal a wide enantiotropic uniaxial nematic phase with an almost 90 "C temperature range For complexes 2-9 and 2-10 the nematic phase is broader than in previously examined enamino ketone derivatives l4 The phase diagrams of the homologous series 1 and 2 are similar to those detected for other series of 1-phenyl-3-alkylaminoprop-2-en-1-one derivatives 14b For the ligands 1 the melting points monoton- ically decrease with increasing terminal alkyl chains length while for complexes 2 a shallow minimum of the melting temperatures is observed For both series 1 and 2, the long terminal alkyl chains depresses the clearing temperatures, destabilizing the nematic phase This effect is particularly strong for the complexes and suggests that the alkyl chain substituted on the nitrogen atom is slightly nonplanar with respect to the enamino ketone ring The isotropisation entropy, which in terms of the Landau- de Gennes model reflects the magnitude of the critical contri- bution into the free energy of a system, shows a pronounced odd-even effect for the ligands 1 (Table 1) In the homologous series of organometallic compounds 2, the clearing entropy varies in a more complex way The initial, distinct entropy decrease is followed by an entropy rise with increasing terminal chain The shape of the thermal DSC isotropisation peaks for both ligands and complexes 1 and 2 is typical, showing the presence of the specific heat anomalies I I(a) tLL Is0 150 -'"1 0 0L 250 150 5 15 n Fig.1 Phase diagram for (a) series 1ligands and (b) senes 2 complexes Table 1 Phase transition temperatures ("C) and enthalpy changes in parentheses (kJ mol-') for ligands 1 and their copper(11) complexes 2 comp. Cry SC N Is0 1-3 135.8 (17.9) - 0 182.1 (2.76) 1-4 1-5 141.9 (18.7) 137.2 (17.8) -- 0 165.2 (2.13) 166.9 (2.73) 1-6 1-7 132.4 (17.4) 127.5 (15.4) -- 0 157.3 (2.22) 157.4 (2.76) 1-8 1-9 1-10 1-11 122.7 (14.6) 117.9 (12.6) 112.5 (9.00) 107.0 (8.19) --121.6 (3.63) 126.0 (3.3) 0 0 151.4 (2.35) 151.8 (2.86) 148.8 (2.79) 148.3 (3.2) 1-12 102.0 (7.43 ) 128.0 (3.59) 145.0 (3.17) 1-13 97.6 (5.98) 131.7 (3.78) 145.2 (3.45) 1-15 1-18 91.2 (3.53) 97.1 (4.59) 133.3 (4.26) 131.9 (11.8)" 0 140.7 (3.48) 134.8 (0.72) 2-3 195.3 (60.0) - 256 (dec.) 2-4 2-5 160.0 (44.7) 141.8 (69.2) -- 0 235 (4.3) 221.5 (3.8) 2-6 2-7 125.2 (59.4) 136.4 (50.6) -- 0 213.6 (3.5) 214.9 (4.2) 2-8 2-9 121.5 (43.2) 116.6 (43.2) -- 0 206.6 (2.9) 202.7 (2.90) 2-10 108.0 (44.3) - 0 194.1 (2.57) 2-1 1 2-12 2-13 115.8 (53.8) 116.2 (50.3) 122.7 (56.8) --- 0 0 189 (0.97) 185.9 (1.85) 182.5 (1.25) 2-15 125.5 (63.4) 122.6 (3.61) 0 172.2 (2.90) 2-18 118.5 (75.4) 130.2 (5.76) 0 159.2 (1.74) "Subsequent peaks not resolved.In both series 1 and 2 for the long homologues, besides the nematic phase the smectic C phase is observed as well. In the ligands (series 1) it appears for the terminal alkyl chain n= 10 and in the complexes (series 2) the smectic C is stabilized by the longer terminal alkyl substituents, (n3 15). Compounds of series 1 and 2 showed strong dichroism (yellow-orange) in the nematic and smectic phases. This points to high optical anisotropy and/or a relatively high degree of molecular orientation in the mesophases. EPR studies. For rod-like mesogenic copper complexes, the organization of their mesophases has not yet been well eluci- dated. For complexes of the enamino ketones crystal molecular structures are not even known.However, from a comparison of the optical properties of some of the complexes with their parent ligands it was suggested that the ligand axes are not colinear. Distortion of the chelate core from planarity and pronounced intermolecular interactions have been suggested as responsible fa~t0rs.l~' To verify these assumptions further information needs to be obtained. In this work the EPR method was used to study the complex 2-18. Its spectra in dilute solution as well as in the condensed crystalline, smectic C, nematic and isotropic phases were analysed. The parameters of the spectra in toluene solution (go= 2.114, A,= 1.4 mT) are close to those observed for copper complexes with a nitrogen-oxygen environment for copper ion and a planar trans configuration of the chelate core.I7 The spectrum of the solution in the glassy state is not informative because it is not sufficiently resolved. The EPR spectra for all condensed phases consisted of exchange narrowed lines and their evolution upon heating from crystal to the isotropic phase is shown in Fig.2. The solid phase spectrum indicates the rhombicity of the magnetic parameters: g, = 2.168, g, = 2.048, gx=2.101. The isotropic phase spectrum appears to be incompletely averaged due to insufficiently fast rotational motion of the molecules. For the mesophases, the spectrum of the Sc phase is axially symmetrical and computer simulation of the line shape (Fig. 3) provides parameters: g, = 2.056, A,,= 35 Oe; g, =gx= 2.1 1, AI = 155 Oe (A = EPR linewidthj.The high field y-line is related to N I 300 320 BlmT Fig. 2 EPR spectra obtained for the polycrystalline, liquid crystalline and isotropic phases of the complex 2-18 I measured _cL culated 3QO 320 BlmT Fig. 3 Comparison of the experimental and simulated spectra for the Sc phase of the complex 2-18 the long molecular axis and the second line corresponds to the mean value between g-factors of the two short molecular axe~.'',~' Such a spectrum suggests an usual phase structure for the S, and S, with the long axes being parallel to each other and the short axes randomly distributed. Assuming axial symmetry of the molecular magnetic parameters, the g-tensor for the individual species can be calculated from the above presented smectic phase g data.20 For 2-18, gll'=2.164, gl'= 2.056 and the mean g-factor go'=2.092 were found.The last value points toward a planar trans configuration of the chelate core for the copper complex 2-18 not only in dilute solutions but also in magnetically concentrated mesophases. In the nematic phase only one line was observed, coinciding with the position of the y-line for the Sc phase. This suggests a strong orientation of the long molecular axes along the external magnetic field, indicating that the anisotropy of the diamagnetic susceptibility exceeds the anisotropy of the para- magnetic susceptibility. This confirms previously reported results,21,22 that high diamagnetic anisotropy reflects the pres- ence of a large number aromatic rings in the organometallic complexes examined.It seems, that in the case of the enamino ketone copper complexes the four phenyl rings contained in the core are sufficient to orient the molecular long axes parallel to the magnetic field.23 Comparative series In order to promote other mesophases and allow discussion of the influence of substituents and bridging groups on mesog- enity, some modifications were introduced into the structure of compounds 1 and 2. As the first modification, compounds 3 and 4 having an enamino ketone ring with reversed substitu- ents to the parent series 1 and 2 were synthesized. The resulting 'reversed core' compounds appear to have markedly higher J.Mater. Chem., 1996, 6(5j, 733-738 735 Table 2 Phase transition temperatures ("C) and enthalpy changes in parentheses (kJ mol ') for the reversed ligands 3 (azo) and 5 (azoxy) and their copper(I1) complexes, 4 and 6 comp Cry 3-3 161 9 (9 9) 3-5 162 5 (9 9) 3-1 1 136" 4-3 167" 4-5 143 3 (59 1) 4-1 1 156 1 (566) 5-11 922 (172) 6-1 1 141 5 (53 6) "From microscopy melting points It is also seen (Table2) that the phenyl substituent, if attached to the amino group, favours more ordered phases than if attached to the ketone group In both series of ligands and complexes an additional, not observed in the series 1 and 2, orthogonal smectic A phase appears In series 3, a narrow smectic A phase is stabilized by the short terminal alkyl chains In contrast, for complexes 4, the smectic A phase is stabilized for the longer homologues For the ligands 4, for the higher homologues below the smectic C phase, some tilted hexatic and crystalline smectic phases (most probably smectic F and smectic G) are also observed In addition to the differences in the mesophase sequence, the thermal stability of the mesophases and their chain length dependence is different for the alkylamino- (series 1 and 2) and the arylamino- (series 3 and 4) derivatives l5 In contrast to series 2, in series 4 the long terminal alkyl chains do not destroy the nematic phase This difference can be attributed to changes in simple geometrical factors Whether it is also connected to differences in electron density within the enamino ketone ring, cannot be decided based on our limited data As a second modification, the azo group in some compounds of the reversed series 3, 4 was replaced by an azoxy group resulting in series 5 and 6 Such a change leads t0 an increase in the clearing temperatures, destabilization of the more ordered liquid crystalline phases and depression of the melting points More flexible moieties, such as -CH,O- were also tried as bridging groups However in this case, a significant destabilisation of mesophase thermal stability was observed As a result, only narrow enantiotropic nematic phases exist for the complexes (eg [Cu{H5C,0-CC,H, -CH20-C6H, -CO(CH)2N-C9H,9>2] has a melting point at 127"C and an isotropisation temperature of 124 oC14a) As a third modification, the hexyloxy group in the parent SA N Is0 - 1948 (04) 197 4 (2 2) 2052 (1 7) 197 3 (4 0) 1873 (114) - --- 0 1998(19) 171 (dec) - - 180 8 (2 2) 145 5" 2046 (10 1) 0 156" - 170 4 (2 9) 137" - 0 177 8 (2 6) series 1 and 2 was exchanged for a hexylamino group Not all of the resulting alkylamino- ligands of the homologous series 7 are liquid crystals (Table 3) The ligands (7) with an inter- mediate length of the terminal alkyl chain do not show mesfgenic properties, whereas for the shorter homologues the monotropic nematic phase was observed and for the longer ones only the smectic C phase appears In the complexes of the series 8 (Fig 4) the hexylamino group suppresses the clearing temperatures profoundly, and this is accompanied by a relatively smaller depression of the melting points As a result a very narrow enantiotropic nematic phase is detected However for some homologues (eg n =3), the nematic phase, if supercooled, is observed even at 80°C, about 60°C below the clearing temperature The tendency of the long substituents SC II II 1 5 15 n Fig.4 Phase diagram for the series 8 complexes I) for ligands 7 and their copper(I1) complexes 8Table 3 Phase transition temperatures ("C) and enthalpy changes in parentheses (in kJ mol comp Cry sc 7-2 7-3 7-4 7-5 7-6 7-8 7-10 7-12 7-18 8-2 8-3 8-4 8-5 8-6 8-8 8-10 8-12 8-18 1044 (247) 122 5 (17 2) 989 (32) 104 3 (5 7) 1000 (11 9) 974(177) 109 7 (15 9) 122 6 (30 6) 0907 (422) 200" (dec ) 160" (dec) 130" 104 1 (37 2) 102 (37 0) 101 1 (39 4) 112 1 (45 5) 99 4( 45 5) SA N -103 (8 1) -0 -0 -0 -0 -0 90 6 (0 3) 0 108 l(65) 1048 (5 3) 101 1 (64) Is0 106 (0 4) -97 2 (0 4) 102 5 (0 6) 90" 228" (dec ) 145" 171 5" (dec) 149 8 (0 6) 125 3 (07) 1226 (1 1) ~~~~~ ~ ~ ~ "From microscopy 736 J Mater Chem , 1996, 6(5),733-738 to destabilize the liquid crystalline phases is similar to series 2.The thermal stability of the compounds of series 8 is low and they are easily decomposed above 150°C. It is worthwhile mentioning a phenomenon that is observed in the nematic phase for some homologues of the series 8.The conoscopic observation of a homeotropically aligned sample reveals a uniaxial nematic phase. However, when the sample is rapidly heated or cooled, the branches, visible in the conos- copic image (Fig. 5)separate, which is evidence of the biaxiality of the phase. The optical axes of the sample are located randomly and the angle between them depends on the rate of temperature change. In the orthoscopic observation the initially uniform dark texture becomes slightly lightened when the sample is biaxial. A similar effect can be obtained by subtle stress on a homeotropically aligned sample. The observed induced biaxiality can be plausibly explained as created by a faint horizontal flow of matter which tilts the initially vertically oriented molecules.The flow results from the stress or the temperature gradient. Since almost all the compounds presented above have melting points close to 100°C or higher, to lower the melting temperatures compounds of series 9 and 10 were synthesized. For this purpose, in line with previous reports,24 molecules with fork-like tails were designed. The hexylamino group of the series 7 and 8 was exchanged for the N-hexyl-N-methylam- ino group. As expected, the resulting ligand series 9 and complex series 10 (Fig. 6) have distinctly lower melting points. However, the clearing temperatures were also suppressed. Unlike the amino series 7 all the compounds with N-methyl- ated ligands showed mesogenic properties, although in some cases only the monotropic nematic phase was detected (Table4).In contrast to the series 1, 2, 4, 8 and 10, in the ligand series 9 the nematic phase is stabilized by increasing the length of the terminal alkyl chains, This reflects the comparatively smaller influence of the branching CH, group Fig. 5 Conoscopic images of complex 8-3 for the homeotropically aligned (a) relaxed and (b)rapidly heated sample ( 10 "C min-') I I I I' (a) , 75 -Is0 50 -vi= 125 100 75 5 115 n Fig. 6 Phase diagram for (a)series 9 ligands and (b)series 10 complexes Table 4 Phase transition temperatures ("C) and enthalpy changes in parentheses (kJ mol-l) for ligands 9 and their copper(11) complexes 10 comp. Cry SA N Is0 9-6 84.2 (41.4) -0 46.4 (0.5) 9-10 49.0 (13.3) -0 58.9 (1.4) 9-11 70.5 (36.3) -0 65.9 (2.1) 9-12 64.8 (34.7) -0 65.6 (2.0) 9-13 57.0 (33.3) -0 68.4 (2.0) 9-14 46.4 (37.9) -0 67.7 (1.6) 9-15 48.6 (38.4) -0 68.7 (3.9) 9-18 62.6 (52.0) 55.3 (0.8) 68.1 (2.7) 10-6 110.8 (38.1) -131.9 (1.5) 10-10 77.5 (53.8) -0 96.4 (1.1) 10-1 1 86.7 (68.7) -0 101.9 (1.20) 10-12 81.7 (50.4) -0 95.6 (1.2) 10-13 73.5 (55.5) -0 94.1 (1.2) 10-14 72.7 (52.0) -0 88.4 (0.97) 10-15 89.8 (71.1) -0 87.6 (1.5) 10-18 81.6 (72.5) -0 82.6 (2.1) on the distortion of the molecular linearity for the longer species than for the shorter ones. For the complexes (10) only the nematic phase is observed and the mesophase stability is similar to the parent series 2.Similarly to the main series 1 and 2, all compounds of the comparative series (3-10) showed dichroic properties (yellow- orange colour) in all liquid crystalline phases. Conclusions We have synthesized several homologous series of elongated calamitic copper(I1) complexes and their parent ligands series is connected with subtle differences of molecular structure and electrical charge distribution. Although the phase polymorphism depends on factors not entirely elucidated some general rules can be found. For the J. Muter. Chem., 1996, 6(5),733-738 737 ligands and their copper complexes, elongation of the alkyl chain N-substituted on the enamino ketone ring destabilizes mesophases This effect can be explained by the non-planar environment of the N-atom resulting in the non-colinear alignment of the alkyl chain with respect to the molecular core l4 In the series with reversed positions on the enamino ketone ring, where the alkyl chain is attached to the carbonyl group the influence of the terminal chain is less pronounced In this case planarity of the mesogenic core is induced by n-electron interactions between the enamino ketone and the phenyl rings l5 For the ligands and complexes, substitution on the alkyl- amino (hexylamino) group as the terminal chain significantly depresses both isotropisation temperatures and melting points The melting temperatures decrease especially strongly if a branched N-hexyl-N-methylamino chain is applied The influ- ence of this group is similar to that of the -CH(CH3)- moiety The branched fork-like tail containing the -N(CH3)- group seems to be promising for further syntheses of low- melting liquid crystals These studies can throw light on the molecular organization of liquid-crystalline phases in organometallic compounds Previous X-ray measurements showed that the layer thickness in the smectic A phase formed by the enamino ketone Cu" complexes is significantly smaller than the molecular length (d/L-0 7-0 8) 14' This result was explained either by the non- planar structure of the chelate core or by interaction between the Cu" complexes, which form molecular clusters The present EPR studies of the Cu" complexes confirm the planarity of the enamino ketone chelate core This indicates that the mesophases of the complexes might be composed of small clusters rather than individual molecules Among possible stabilizing factors interactions of Cu 0, Cu N, Cu Cu type should be considered Copper-oxygen interactions are well known for crystals of some salicylaldymine copper com- plexes l1 25 26 Similar interactions for nickel complexes have also been reported 25 Copper-copper interactions were pre- viously assumed to be present in isotropic melts to explain EPR signals of polymeric salicylaldimine complexes 27 Significant interdigitation of alkyl chains of molecules from neighbouring layers (up to three carbon atoms) or a weak nematic orientational order parameter (N 0 6) in the smectic layer could also be responsible for a d/L value< 1 However, high entropy loss associated with the interdigitation or con- siderably constrained in-plane translational motion of mol- ecules with strongly disordered molecular axes make both mechanisms less plausible 28 This work is a part of a research project sponsored by KBN grant 2P303 02407 Financial assistance for the EPR research part and scholarship for I Bikchantaev granted from the 12- 5O1/VII/BW-11301/33/95 is also acknowledged References 1 A Krowczynski, W Pyzuk and E Gorecka, Polish J Chem ,1994, 68,281 2 D W Bruce, D A Dunmurr, P M Maitlis, M M Manterfield and R Orr, J Muter Chem, 1991,1,255 3 D W Bruce, D A Dunmurr, S E Hunt, P M Maitlis and R Orr, J Muter Chem, 1991,1,857 4 C Cipparone, C Versace, D Duca, D Pucci, M Ghedini and C Umeton, Mol Cryst Liq Cryst, 1992,212,217 5 H Schultz, H Lehnann, M Rein and M Hanack, Struct Bonding (Berlin), 1991,74,43 6 G Albertini, A Guido, G Mancini, S Stizzaand and R Bartolino, Europhys Lett, 1990,12,629 7 Yu G Galyametdinov, 0 N Kadkin and I V Ovtchinnikov, Izu Akad Nauk USSR, Ser Khim , 1992,12,402 8 (a)I V Ovtchinnikov, I G Bikchantaev, Yu G Galyametdinov and R M Galimov, in Proc 24th Ampere Congress, Poznan 1988, p 567, (b) I V Ovtchinnikov, Yu G Galyametdinov and I G Bikchantaev, Izu Akad Nauk USSR, Ser Fiz, 1989,53,1870 9 P J Alonso, M Marcos, J I Martinez, V M Orera, M L Santjuan and J L Serrano, Liq Cryst, 1993,13,585 10 A M Giroud-Godquin and P M Maitlis, Angew Chem Int Ed Engl ,1991,30,375 11 D W Bruce, in Inorganic Materials, ed D W Bruce and D O'Hara, Wiley, England, 1992 12 (a)M Marcos, P Romero and J L Serrano, J Chem Soc, Chem Commun, 1989,1641,(b)J L Serrano, P Romero, M Marcos and P J Alonso, J Chem Soc, Chem Commun, 1990,859 13 W Pyzuk, A Krowczynski and E Gorecka, Liq Cryst, 1991, 10,593 14 (a)W Pyzuk, E Gorecka and A Krowczynski, Liq Cryst, 1992, 11, 797, (b) W Pyzuk, A Krowczynski, E Gorecka and J Przedmojski, Liq Cryst, 1993,14, 773 15 W Pyzuk, A Krowczynski and E Gorecka, Proc SPIE, 1992, 1845,277, Mol Cryst Liq Cryst ,1994,249, 17 16 G W Gray and J W G Goodby, in Smectic Liquid Crystal- Textures and Structures, Leonard Hill, Glasgow and London, 1984 17 H Yokoi, Bull Chem SOC Jpn , 1974,47,3037 18 R M Galimov, I G Bikchantaev and I V Ovtchinnikov, Zh Strukt Khim ,1989,30,65 19 R M Galimov, I G Bikchantaev, I V Ovtchinnikov and V N Konstantinov, Zh Strukt Khim ,1989,30,59 20 I G Bikchantaev, Yu G Galyametdinov and I V Ovtchinnikov, Zh Strukt Khim, 1987,28,61 21 J Barbera, A M Levelut, M Marcos, P Romero and J L Serrano, Liq Cryst ,1991,10,119 22 B Borchers and W Haase, Mol Cryst Liq Cryst, 1991,209,319 23 I G Bikchantaev, Yu G Galyametdinov, A Prosvinn, K Gnesar, E A Soto-Bustamante and W Haase, Liq Cryst, 1995,18,231 24 W Weissflog, G Pelzl, I Letko and S Diele, Mol Cryst Liq Cryst ,1995,260,157 25 B 0 West, in New Pathways in Inorganic Chemistry, ed E A V Ebsworth, A G Maddock and A G Sharpe, Cambridge University Press, 1968 26 H Tamura, K Ogawa, A Takeuchi and S Yamada, Chem Lett, 1977,889, Bull Chem Soc Jpn , 1979,52,5322 27 M Marcos, L Oriol, J L Serrano, P J Alonso and J A Puertolas, Macromolecules, 1990,23,587 28 R Holyst, Phys Rev A, 1991,44,3692 738 J Muter Chem, 1996, 6(5),733-738

ISSN:0959-9428    

DOI:10.1039/JM9960600733

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Structural variation of liquid crystalline trioxadecalins Volkmar Vill,” Hanns-Walter Tunger and Markus von Minden Institute of Organic Chemistry, University of Hamburg, 0-20146 Hamburg, Germany Synthesis and mesogenic properties of new liquid crystals bearing a chiral trioxadecalin system are described. Boron-containing three-ring systems with a lateral methoxy group show cholesteric, TGBA and smectic A phases. Molecules containing four or five rings show mostly smectic C* phases. The insertion of a triple bond leads to ferroelectric smectic C* phases, but compounds with a flexible spacer between the rings show only monotropic smectic A phases. Lateral fluorination of the aromatic rings leads, depending on the position of the fluorine, either to stabilised smectic phases with lower transition temperatures or to cholesteric phases with complete suppression of all smectic phases.New technical applications for chiral liquid crystals and the discovery of new chiral mesophases have increased the interest in easily accessible mesogens with chiral centres.’ Most of the investigated and commercially used liquid crystals possess one chiral centre in the flexible side chain. However, we base our syntheses on carbohydrates, which allows the introduction of a chiral ring system into the molecular core, so that the chirality is located in the part of the molecule that determines the general mesogenic properties. Compounds with a chiral trioxadecalin ring system are obtained in a short synthesis from tri-0-acetyl-D-gluca1.2 They show an interesting and sometimes unusual mesogenic behav- iour, e.g.cholesteric helix inversion^^.^ and re-entrant TGBA phases.’ In mixed systems, the re-entrant TGBA phase is stabilized,6 and for some compounds a sign inversion of the spontaneous polarization in induced smectic C* phases is obser~ed.~ Here we present detailed structural modifications of com- pounds having a trioxadecalin ring system to study the scope of their chiral and mesogenic properties. We lengthened the mesogenic group by increasing the number of rings and fitted rigid alkynyl and flexible alkyl spacers between the rings. In addition, the effect of terminal methoxy groups instead of longer alkyl chains is examined. Lateral fluorine atoms should modify the global molecular shape and introduce dipole-dipole interactions. We synthesized compounds with lateral fluoro substituents at the aromatic rings in the molecular core and studied the influence of the position of the fluorine on the mesogenic behaviour.Experimenta1 The synthesis of the adducts is shown in Scheme 1. A three- step synthesis starting from tri-0-acetyl-D-glucal 1 (or a five- step synthesis starting from cheap glucose) leads in an overall yield of 28% to the enantiomerically pure diol4 (with a fluoro substituent the yield decreases to 6%). The diol can easily be combined with different boronic acids and aldehyde dimethyl acetals giving a broad variety of products (Scheme 2)., The synthesis of the alkynyl aldehyde 13 which is used for the synthesis of compounds 7 is outlined in Scheme 3.899 General reaction conditions Conditions A.A flask with diol 4 (0.076 mmol), 4-alkoxybenzaldehyde dimethyl acetal ( 1.2 equiv., 0.092 mmol) and toluene-p-sulfonic acid (monohydrate) (5.0 mg) in N,N-dimethylformamide (5 ml) was fitted to a rotary evaporator. The mixture was allowed to react for 1 h at reduced pressure (29-33 mbar) in a 60 “C water-bath, the methanol formed, distilled off during the reaction. Then the solvent was evapor- ated in uucuo (10 mbar) at 75 “C water-bath temperature. The solid residue was washed with saturated aqueous sodium hydrogen carbonate, filtered, washed with water and cold ethanol and then recrystallized from ethanol.Conditions B. Diol 4 (0.076 mmol) and 4-alkoxyphenyl- boronic acid (1.2 equiv., 0.092 mmol) were dissolved in toluene (5 ml). The water produced in the reaction was coevaporated three times with toluene (5 ml). The remaining crystalline solid was recrystallized from ethanol. Conditions C. The unsaturated compound (0.025 mmol) was dissolved in ethyl acetate (15 ml) and ethanol (5 ml) and stirred after the addition of a catalytic amount of palladium on charcoal (10%) under a hydrogen atmosphere for 1 h. The catalyst was filtered off, the solvent evaporated and the residue recrystallized from ethanol. Conditions D. These conditions were similar to conditions A, but using diol 4 (0.126 mmol) and the dialdehyde tetramethyl acetal (0.9 equiv., 0.057 mmol).Materials Amberlyst IR 120 ion-exchange resin (protonated form), dichloromethane (99”/), N,N-dimethylformamide (%WOO), etha- nol, ethyl acetate (WYO), light petroleum (50-70 “C), mag- nesium sulfate (99”/), methanol, palladium (10% on charcoal), sodium carbonate (98%), sodium hydrogen carbonate (99”/0), sodium methoxide, stannic(1v) chloride, toluene (99”/), tolu- ene-p-sulphonic acid (98%, monohydrate) were used as received. In all cases dry solvents were used. Dichloromethane was refluxed over phosphorus pentoxide and stored over molecular sieves 4 A after distillation. N,N-Dimethylform-amide was filtered over silica gel and freshly distilled before use. Techniques Thin-layer chromatography (TLC) was performed on silica gel (Merck GF,,,), and detection was effected by spraying with a solution of ethanol-sulfuric acid (9: l), followed by heating, and UV-absorbance.Column chromatography was performed on silica gel 60 (230-400 mesh, Merck). Optical rotations were recorded using a Perkin-Elmer 241 polarimeter. The NMR spectra (‘H: 400 MHz, I3C: 100.6 MHz) were recorded on a Bruker AMX-400 spectrometer with tetramethylsilane (TMS) as an internal standard (m, =centred multiplet). An Olympus BH optical polarizing microscope equipped with a Mettler FP 82 hot stage and a Mettler FP 80 central processor was used J. Muter. Chem., 1996, 6(5), 739-745 739 phenyi alkyl ether SnCI4 * Acr%Oh AcO CHzCIz -0OC"H2"+1 1 2 H2 PdIC ethanol ethyl acetate separation of anomers 4 X=H F 3 Scheme 1 Synthetic route to chiral building block 4 conditions A X=H F OC"H2" + 1 OCfi2n + 1 6 H2 PdJC 7(conditions C) I 8\conditions AfI Scheme 2 Structural variation of the trioxadecalines triphenylp hosphine carbon tetrabromide H1 7C800cH0 dichloromethane * 11 12 1 butyII ith ium N N-dimethyl-formamide trimethyl orthoformate 12 HCI 14 13 Scheme 3 Synthetic route to propynal 13 740 J Muter Chem , 1996, 6(5),739-745 to identify thermal transitions and characterize anisotropic textures. For further verification of the textures a contact preparation with p-butyl-p’-methoxyazoxybenzene (K 16 N 76 I) was carried out.The NMR data of a homologous series of compounds differ only in the integral of the signal at 6 1.3 (number of protons, -CH2 -).Thus, the experimental and NMR data of only one member of every series are given below. The yields and optical rotations of all compounds are listed in Table 1. Synthesis of 1-(2’,3-dideoxy-~-~-eryythro-hexopyranosyl)-2-fluoro-4-hexyloxybenzene4(n=6,x =F) To a mixture of tri-O-acetyl-D-glucal (3.0 g, 11 mmol) and 3- fluorohexyloxybenzene (3.3 g, 16.9 mmol) in dry dichloro- methane (50 ml) were added 2 drops of stannic tetrachloride. After stirring the reaction mixture for 2 h at room temperature solid sodium carbonate (2.0 g) was added. The reaction mixture was stirred for a further 15 min and then filtered. The solvent was removed in uucuo. The product was dissolved in ethanol Table 1 Yields and optical rotations of the synthesised compounds yield opt.rotation (CHC1,) comp. conditions mg YO [a];’ c/g per 100ml 5a 5b 5c 5d A A A A 19 22 25 20 42 53 62 25 +24.1 +27.0 +22.3 +25.0 1.o 0.1 0.5 0.1 5e 5f ” 5g6a 6b A A A B B 23 28 35 29 16 59 50 66 56 30 +26.2 +20.1 +20.8 +28.5 +30.0 1.o 0.5 0.5 1.o 0.1 6c 6d B B 18 11 32 18 +21.0 +22.0 0.1 0.1 6e B 14 22 +26.0 0.1 6f 6h 6i 6k 61 6m“ 6n 60“ 6P 6q“6r 6g 6.j B B B B B B B B B B B B B 13 17 11 25 18 25 10 18 29 33 24 35 32 32 30 27 63 46 65 26 39 60 73 50 79 65 +26.0 +23.0 +20.0 + 18.4 + 19.0 + 18.2 +22.0 + 19.0 +19.4 + 17.6 +17.6 +20.0 + 15.8 0.1 0.1 0.1 0.5 0.1 1.o 0.1 0.1 0.5, 1.o 0.2 0.1 0.5 6s” 6t 6u“ 6v 6w” 6x 7a 7b 7c 7d 7e 8a 8b 9a 9b 9c 9d 1Oa 10b 1oc 1Od 1Oe 10f 1% B B B B B B A A A A A C C A A A A D D D D D D D 29 28 12 25 25 13 24 12 14 12 27 10 15 26 32 10 40 20 16 14 12 22 24 43 57 49 21 50 52 29 44 24 28 24 57 80 74 43 60 15 66 28 23 20 18 33 36 34 +21.4 +26.8 +22.0 +25.0 +24.0 +26.0 +24.0 +20.0 + 19.0 +23.0 +23.0 +20.0 +20.0 +24.3 +22.0 +24.0 +27.0 +23.2 +22.0 +27.6 +28.0 +25.5 +27.5 +18.3 0.5 0.5 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 1.o 1.o 0.1 0.5 0.5 0.5 0.5 0.1 1.o 1.o 1.o “Ref.5.(40ml) and ethyl acetate (120ml) and, after addition of palladium on charcoal (lo%, 10 mg), hydrogenated under a hydrogen atmosphere. After stirring for 4 h at room tempera- ture, the solution was filtered, the solvent evaporated and the residue purified uiu column chromatography (eluent: light petroleum (bp 50-70 “C)-ethyl acetate, 20: 1) to give 3 which was stirred at room temperature for 4 h with sodium methoxide ( 10.0 mg) in methanol (50 ml).The solution was neutralized with acidic ion exchange resin (Amberlite IR 120, H+-form), filtered and evaporated to give the title compound (0.22 g, 6%), syrup, [a];’+ 19.0 (c=0.5, CHC1,); d~(cDC1,) 4.67 (dd, 1 H, H-1’), 1.77 (mc, 4 H, H-2’,,, H-3’,,, P-CH,), 1.96 (mc, 1 H, H-2’eq), 2.17 (mc, 1 H, H-3’eq), 3.35-3.95 (m, 6 H, H-4, H-5’, H-6’,, H-6’b7 a-CH,), 6.56 (dd, 1H, H-3), 7.34 (dd, 1H, H- 5), 6.68 (dd, 1 H, H-6), 1.44 (mc, 2 H, 7-CH,), 1.27 (br s, 4 H, -CH2-)7 0.88 (t7 3 H7 CH3); 3Jl!,2!ax 11.0, 3Jlr,2!eq 2-09 3J5,6 8.6, 4J3,6 2.4, ,J3,F 12.2, 4J5,~8.6 HZ; GC(CDC1,) 83.2 (c-l’), 32.9, 33.9 (C-2’, C-3’), 74.5 (C-4), 68.3 (C-5’), 64.5 (C-6’), 160.4 (C-2), 101.9 (C-3), 110.4 (C-5), 127.6 (C-6), 68.8 (a-CH,), 31.5, 29.2, 26.0, 22.7 (-CH2-), 14.1 (CH,); 1J2,F165, 2J3,F19 Hz.Synthesis of (lS,3R,6R,SR)-S-(4‘-hexyloxyphenyl)-3-(4”-methoxyphenyl)-2,4,7-trioxabicyclo[4.4.01decane 5b Reaction conditions A using 4-methoxybenzaldehyde dimethyl acetal. d~(cDC1,) 3.65 (ddd, 1 H, H-I), 5.56 (s, 1 H, H-3), 4.29 (dd, 1 H, H-5,), 3.77 (dd, 1 H, H-5b), 3.58 (ddd, 1 H, H-6), 4.47 (dd, 1 H, H-8), 1.85 (mc, 2 H, H-9,,, H-loax), 2.02 (mc, 1 H, H-94, 2.20 (mc, 1 H, H-lOeq), 7.26 (d, 2 H, H-2’, H-6’), 7.43 (d, 2 H, H-2”, H-6”), 6.88 (d, 4 H, H-3’, H-5’, H-3”, H-5”), 3.80 (s, 3 H, OMe), 3.94 (t, 2 H, a-CH,), 1.76 (mc, 2 H, P-CH,), 1.43 (mc, 2 H, y-CH,), 1.32 (br s, 4 H, -CH,-)), 0.90 (t, 6 H, CH3); ,Jl,6 9-57 ,Jl,lOax l0.87 3J5a,5b 10.27 ,J5a,6 10.2, ,J5b,6 5.47 ,J8,9ax 10.2, ,Jg,geq 2.4, ’JAryl 8.5 HZ; Gc(CDCl3) 74.1 (C-I), 101.8 (C-3), 69.6 (C-5), 78.3 (C-6), 79.7 (C-8), 31.6 (C-9), 33.1 (C-lo), 137.0 (C-1’), 130.1 (C-1”), 127.5, 127.2 (C-2’, C-6’, C- 2”, C-6”), 113.7, 114.5 (C-3’, C-5’, C-3”, C-5”), 158.8 (C-4’), 160.0 (C-4”), 55.3 (OMe), 68.1 (a-CH,), 29.3, 29.2, 25.7, 22.6 (-CH2-), 14.0 (CH,).Synthesis of (lS,3R,6R,SR)-S-(2-fluoro-4-hexyloxyphenyl)-3-(4-dodecyloxyphenyl)-2,4,7-trioxabicyclo[4.4.01decane 5g Reaction conditions A using 4-dodecyloxybenzaldehyde dimethyl acetal. 6,(CDC13) 3.64 (ddd, 1 H, H-1), 5.54 (s, 1 H, H-3), 4.28 (dd, 1 H, H-5,), 3.75 (dd, 1 H, H-5b), 3.58 (ddd, 1 H, H-6), 4.76 (dd, 1 H, H-8), 1.85 (mc, 2 H, H-9,,, H-loax), 2.03 (mc, 1 H, H-9,,), 2.19 (mc, 1 H, H-loeq), 6.56 (dd, 1 H, H-3’), 6.67 (dd, 1 H, H-5‘), 7.31 (dd, 1 H, H-6’), 7.41 (d, 2 H, H-2”, H-6”), 6.87 (d, 2 H, H-3”, H-S’), 3.93 (mc, 4 H, a-CH,), 1.75 (q,4 H, p-CH,), 1.42 (mc, 4 H, y-CH,), 1.30 (br s, 24 H, -CH,-), 0.88 (t7 6 H7 CH3); ,J1,6 8.97 3J1,10ax 10.8, 3J5a,5b 10.2, ,J5a,6 10.2, 3J5b,6 5*4, 3J8,9ax 3J8,9eq 2-073JAryl,, 8.67 3J5r,6r 8-67 4J31,5t 2.5, 3J3,,F12.1, 4J6,,F 8.6 Hz; Gc(CDC13) 73.5 (C-1), 101.8 (C-3), 69.5 (C-5), 78.2 (C-6), 74.2 (C-8), 31.9 (C-9), 32.2 (C-lo), 160.4 (C- 2’), 101.9 (C-3’), 110.4 (C-5’), 127.6 (C-6), 130.0 (C-l”), 127.4 (C-2”, C-6”), 114.4 (C-3”, C-5”), 159.7 (C-4”), 68.1 (N-CH,), 31.9, 29.7, 29.6, 29.4, 29.3, 29.2, 26.0, 22.7 (-CH2-), 14.1 (CH3); ‘J2t.F 162, ,J3,,F 19 Hz.Synthesis of (lS,6R,SR)-S-(4‘-methoxyphenyl)-3-( 4-hexyloxyphenyl)-2,4,7-trioxa-3-borabicyclo[4.4.01decane 6b Reaction conditions B using 4-hexyloxyphenylboronic acid. GH(CDC13) 3.87 (ddd, 1 H, H-1), 4.24 (dd, 1 H, H-5,), 3.95 (dd, 1 H, H-5,), 3.62 (ddd, 1 H, H-6), 4.48 (dd, 1 H, H-8), 1.70-1.91 (m, 4 H, H-9,,, H-lo,,, P-CH,), 2.03 (mc, 1 H, H- geq), 2.37 (mc, 1 H, H-lOeq), 7.26 (d, 2 H, H-2’, H-6’), 7.75 (d, 2 H, H-2”, H-6”), 6.87 (d, 4 H, H-3’, H-5’, H-3”, H-5”), 3.82 (s, J. Muter. Chem., 1996, 6(5),739-745 741 3 H, OMe), 3 94 (t, 2 H, a-CH,), 145 (mc, 2 H, y-CH,), 1 27 Synthesis of (1&3R,6R,SR)-8-( 4‘-hexyloxyphenyl)-3-[ 2-( 4- (brs,4H, -CH,-),090(t,6H,CH3),3J1691,3J110axoctyloxyphenyl ethyl]-2,4,7-trioxabicyclo [4.4.01 decane 8a 109, 5b 10 2, ,J5a 6 10 2, ,J5b 6 54, 3J8 9ax 10 9, 9eq 2 0, ,JAryl 8 0 Hz, Gc(CDC1,) 71 6 (C-1), 64 9 (C-5), 76 0 (C-6), 79 8 (C- 8), 31 2 (C-9), 32 9 (C-lo), 133 4 (C-l’), 127 3 (C-2, C-6’), 114 5 (C-3’, C-5’), 158 5 (C-4), 135 8 (C-2”, C-6”), 113 2 (C-3”, C-5”), 162 5 (C-4”), 55 1 (OMe), 68 1 (a-CH,), 31 2, 29 2, 25 7, 22 6 (-CH2-), 14 0 (CH3) Synthesis of (lS,6R,SR)-8-( 4’-octyloxyphenyl)-3-( 3”-fluoro-4”- hexyloxyphenyl)-2,4,7-trioxa-3-borabicyclo[4.4.01 decane 6n Reaction conditions B using 3-fluoro-4-hexoxyphenylboronic acid d~(cDC13)3 87 (ddd, 1 H, H-1), 4 23 (dd, 1 H, H-5,), 3 94 (dd, 1 H, H-5b), 3 61 (ddd, 1 H, H-6), 4 48 (dd, 1 H, H-8), 1 70-1 91 (m, 6 H, H-9,,, H-lo,,, 8-CH,), 203 (mc, 1 H, H- geq), 2 36 (mc, 1 H, H-lOeq), 7 26 (d, 2 H, H-2’, H-67, 6 87 (d, 2 H, H-3’, H-5’), 748 (dd, 1 H, H-2”), 692 (dd, 1 H, H-5”), 7 50 (dd, 1 H, H-6”), 3 98 (mc, 4 H, a-CH,), 145 (mc, 4 H, y-CH,), 131 (br s, 12 H, -CH,-), 089 (t, 6 H, CH,), ,J16 9 1, ”l lOax lo 9, ,J5a 5b lo 2, ,J5a 6 lo 2, ,55b 6 5 4, 3J8 9ax 10 9, ,J8 9eq O, ,JAryl 8, ,J5,, 6~ 2, 4J2t1 611 17, 3J2,/ F 12 2, 4J51 F 8 2 Hz, Gc(CDCl3) 71 6 (C-l), 64 9 (C-5), 760 (C-6), 79 8 (C-8), 31 1 (C-9), 32 8 (C-lo), 133 3 (C-1’), 127 2 (C-2’, C-6’), 114 5 (C-3’, C-5’), 158 9 (C-4’), 135 7 (C-2”, C-6”), 113 8 (C-3”, C-5”), 160 5 (C-4”), 68 1, 69 2 (a-CH,), 31 8, 31 6, 29 4, 29 3, 29 2, 29 1, 26 0, 25 6, 22 7, 22 6 (-CH2-), 14 1, 14 0 (CH,), ‘J3,, F 169, 2J2,, F 16 Hz Synthesis of (lS,6R,SR)-&(2’-fluoro-4’-hexyloxyphenyl)-3-(4-octyloxyphenyl)-2,4,7-trioxa-3-borabicyclo[4.4.01 decane 6t Reaction conditions B using 4-octyloxyphenylboronic acid &(CDCl,) 3 88 (ddd, 1 H, H-1), 4 24 (dd, 1 H, H-5,), 3 94 (dd, 1 H, H-5,), 3 62 (ddd, 1 H, H-6), 478 (dd, 1 H, H-8), 171-1 87 (m, 6 H, H-9,,, H-lO,,, P-CH,), 205 (mc, 1 H, H- geq), 2 35 (mc, 1 H, H-loeq), 6 58 (dd, 1 H, H-3’), 6 68 (dd, 1 H, H-5’), 7 33 (dd, 1 H, H-6’), 7 73 (d, 2 H, H-2”, H-6”), 6 87 (d, 2 H, H-3”, H-5”), 3 95 (mc, 4 H, a-CH,), 144 (mc, 4 H, y-CH,), 1 39 (br s, 12 H, -CH, -), 0 90 (t, 6 H, CH,), ,J16 9 1, ,J1 10 9, ,J5a 5b 10 2, ,J5a 6 10 2, ,J5b 6 5 4, 3J8 9ax 10 9, 3J8 9eq 2 0, 4J31,JAryl,, 8 7, ,J5’ 6, 8 2, 5, 2 6, ,J3( F 12 2, 4J6tF 8 5 Hz, Gc(CDCI3) 71 5 (C-1), 64 8 (C-5), 76 1 (C-6), 73 6 (C-8), 31 1 (C-9), 32 0 (C-lo), 120 4 (C-1’), 160 8 (C-2’), 101 9 (C-3’), 110 6 (C-5’), 135 7 (C-2”, C-6”), 113 8 (C-3”, C-5”), 162 5 (C-4”), 68 1, 67 8 (a-CH,), 31 8, 31 5, 29 4, 29 3, 29 2, 29 1, 26 0, 25 6, 22 7, 22 6 (-CH2-), 14 1,14 0 (CH,), ‘J2, F 169,,J3/ F 25, 3J6, F 6 HZ Synthesis of (lS,3R,6R,SR)-8-( 4‘-hexyloxyphenyl)-3-( 4”- octyloxyphen ylethyny1)-2,4,7- trioxabicyclo [4.4.01 decane 7a Reaction conditions A using l-(4-octyloxyphenyl)prop-l-ynal dimethyl acetal The product was dissolved in dichloromethane, stirred with charcoal and filtered through a pad of silica gel before recrystallisation h~(CDc13)3 69 (ddd, 1 H, H-l), 5 51 (s, 1 H, H-3), 424 (dd, 1 H, H-5,), 3 69 (dd, 1 H, H-5b), 3 56 (ddd, 1 H, H-6), 4 45 (dd, 1 H, H-8), 185 (mc, 2 H, H-9,,, H-loax), 201 (mc, 1 H, H-9,,), 2 20 (mc, 1 H, H-loeq), 7 23 (d, 2 H, H-2’, H-6’), 6 86 (d, 2 H, H-3’, H-5’), 7 42 (d, 2 H, H-2”, H-6”), 6 81 (d, 2 H, H- 3”, H-5”), 3 94 (mc, 4 H, a-CH,), 175 (mc, 4 H, P-CH,), 143 (mc, 4 H, y-CH,), 126 (brs, 12 H, -CH,-), 089 (t, 6 H, CH3), 3J1 6 5, 3Jl 1Oax lo 8, 3J5a 5b lo 5, ,J5a 6 lo 0, ,J5b 6 4, ,J89ax 10 2, 3J89eq 2 4, ,JArYl8 5 Hz, Gc(CDCl,) 73 6 (C-1), 92 5 (C-3), 69 6 (C-5), 78 6 (C-6), 79 7 (C-8), 31 9 (C-9), 33 0 (C-lo), 133 0 (C-1’), 113 0 (C-l”), 127 2 (C-2’, C-6’), 133 6 (C-2”, C-6”), 114 4, 114 5 (C-3’, C-5’, C-3”, C-5”), 158 8 (C-4’), 159 9 (C-4’7, 82 0, 85 5 (alkynyl-C), 68 1 (a-CH,), 31 8, 29 7, 29 6, 29 4, 29 3, 29 2, 26 0, 22 7 (-CH2-), 14 1 (CH,) 742 J Muter Chem , 1996, 6(5), 739-745 Reaction conditions C using 7a GH(CDC1,) 3 41 (mc, 2 H, H-1, H-6), 4 59 (t, 1 H, H-3), 4 14 (dd, 1 H, H-5,), 3 54 (dd, 1 H, H-5b), 442 (dd, 1 H, H-8), 1 76 (mc, 4 H, H-gax, H-lOax, P-CH,), 199 (mc, 3 H, H-9,,, C3-CH2-), 2 12 (mc, 2 H, H-loeq), 270 (t, 2 H, Aryl-CH,-), 7 23 (d, 4 H, H-2’, H-6’), 6 85 (d, 4 H, H-3’, H-5’), 7 10 (d, 2 H, H-2”, H-6”), 6 82 (d, 2 H, H-3 ’, H-5”), 3 93 (t, 4 H, a-CH,), 120-1 70 (m, 40 H, -CH2-)), 088 (t, 6 H, CH3), 3Jl 6 9 5, ,Jl lOax lo 8, 3J3 CH, 1, 3JAryl CH,,CH, 8 1, ,J5a 5b 9 77 3J5a 6 9 7, ,Jsb 6 3 8, 9ax 100, 9eq 1 7, ’J~ryl 8 5 HZ, Gc(CDC1,) 74 5 (C-1), 102 0 (C-3), 69 5 (C-5), 78 0 (C-6), 79 5 (C-8), 31 9 (C-9), 33 1 (C-lo), 133 5 (C-1’), 131 0 (C-1”), 127 2 (C-2’, C-6’), 129 3 (C-2”, C-6”), 114 5 (C-3’, C-5’, C-3”, C-5”), 158 5 (C-4’, C-4”), 68 1 (a-CH,), 31 8, 37 0 (ethyl-C), 29 7, 29 6, 29 3, 29 2, 26 0, 22 7 (-CH, -), 14 1 (CH,) Synthesis of (lS,3R,6R,SR)-8-( 4‘-hexyloxyphenyl)-3-[ 4-(4”‘-dodecyloxybenzoyloxy)phenyl]-2,4,7-trioxabicyclo [4.4.01 decane 9c Reaction conditions A using 4-( 4-dodecyloxybenzoyl) benzal- dehyde dimethyl acetal h~(CDc13)3 65 (ddd, 1 H, H-l), 5 62 (s, 1 H, H-3), 4 32 (dd, 1 H, H-5,), 3 78 (dd, 1 H, H-5b), 3 55 (ddd, 1 H, H-6), 445 (dd, 1 H, H-8), 170-1 90 (m, 6 H, H-9,,, H-lo,,, P-CH,), 203 (mc, 1 H, H-9,,), 2 20 (mc, 1 H, H-lOeq), 7 29 (d, 2 H, H-2’, H- 6’), 6 85 (d, 2 H, H-3’, H-5’), 7 57 (d, 2 H, H-2”, H-6’), 7 21 (d, 2 H, H-3”, H-5”), 8 13 (d, 2 H, H-2”’, H-6’”), 696 (d, 2 H, H- 3”’, H-S”), 3 93, 4 03 (t, 4 H, a-CH,), 1 40 (mc, 4 H, y-CH,), 130 (br S, 20 H, -CH2-), 090 (t, 6 H, CH3), 3J16 9 5, 3J1loax 10 8, ,J5a 5b 10 2, 3Jsa 6 10 2, ,J5b 6 5 4, 3Js 9ax 10 2, 9eq 2 4, 3J~ryl 8 5 Hz, Gc(CDCl3) 74 1 (C-I), 101 2 (C-3), 68 2 (C-5), 78 3 (C-6), 79 7 (C-8), 31 9 (C-9y, 33 1 (C-lo), 133 5 (C-l’), 121 0 (C-1’), 127 4, 127 2 (C-2’, C-6’, C-2”, C-6”), 132 3 (C-2”, C-,”’), 114 3, 1144 (C-3’, C-5’, C-3”’, C-5”’), 121 6 (C-3”, C-5”), 158 5 (C-4’), 152 5 (C-4”), 164 5 (C-4”’), 68 1 (a-CH,), 31 6, 297, 296, 294, 293, 292, 260, 257, 227, 226 (-CH2-), 14 1, 140 (CH,) Synthesis of (lS,3R,6R,SR)-8-( 2’-fluoro-4‘-hexyloxyphenyl)-3-[4-(4”’-hexoxybenzoyloxy)phenyl]-2,4,7-trioxabicyclo[4.4.01 decane 9d Reaction conditions A using 4-( 4-dodecyloxybenzoyl) benzal- dehyde dimethyl acetal GH(CDC1,) 3 65 (ddd, 1 H, H-1), 5 62 (s, 1 H, H-3), 4 31 (dd, 1 H, H-5,), 3 79 (dd, 1 H, H-5,), 3 55 (ddd, 1 H, H-6), 4 78 (dd, 1 H, H-8), 171-1 95 (m, 6 H, H-9,,, H-lo,,, P-CH,), 205 (mc, 1 H, H-9,,), 2 21 (mc, 1 H, H-loeq), 6 58 (dd, 1 H, H-3’), 6 68 (dd, 1 H, H-5’), 7 33 (dd, 1 H, H-6’), 7 57 (d, 2 H, H-2”, H-6”), 7 21 (d, 2 H, H-3“, H-5”), 8 13 (d, 2 H, H-2”’, H-6’”), 6 96 (d, 2 H, H-3”’, H-5”’), 3 93, 4 03 (t, 4 H, a-CH,), 1 40 (mc, 4 H, y-CH,), 130 (brs, 8 H, -CH,-), 090 (t, 6 H, CH,), 3Jl 6 5, 3J1 lOax lo 8, 3J5a 5b lo 3, 3J5a 6 lo 3, 3J5b 6 4 3, 3J8 9ax 110, 9eq 2 1, ’JA,~ 8 5, ,J5! 6, 8 5, 4J31 5, 2 4, ,J31 F 12 1, 4J6, F 8 5 Hz, Gc(CDCI3) 73 6 (C-1), 101 9 (C-3), 68 3 (C-5), 78 3 (C- 6), 74 1 (C-8), 31 5 (C-9), 31 2 (C-lo), 135 2 (C-l’), 1604 (C- 2’), 101 9 (C-3’), 110 4 (C-5’), 127 6 (C-6’), 120 2 (C-1”), 127 4 (C-2”, C-6”), 121 6 (C-3”, C-5”), 151 6 (C-4”), 132 3 (C-2”’, C- 6”’)) 114 3 (C-,”’, C-5”’), 68 3 (a-CH,), 29 2, 29 0, 25 7, 22 6 (-CH2-), 14 0 (CH,), ‘J2, F 147, ,J3, F 25 HZ Synthesis of (l’S,3’R,6’R,SR)-1,4-bis[8’-(4”-hexyloxypheny1)-2’,4’,7’-trioxabicyclo[4.4.01 decan-3-yll benzene lob Reaction conditions D using terephthalaldehyde tetramethyl acetal d~(cDC13) 3 65 (ddd, 2 H, H-l’), 5 50 (s, 2 H, H-3’), 4 30 (dd, 2 H, H-5’,), 3 75 (dd, 2 H, H-5’b), 3 55 (ddd, 2 H, H-6’), 4.47 (dd, 2 H, H-8’), 1.85 (mc, 4 H, H-9’,,, H-lO’ax), 2.02 (mc, 2 H, H-9’eq), 2.20 (mc, 2 H, H-lO’eq), 7.25 (d, 4 H, H-2”, H-6”), 6.85 (d, 4 H, H-3”, H-5”), 7.52 (s, 4 H, H-Aryl), 3.94 (t, 4 H, a- CH,), 1.76 (mc, 4 H, P-CH,), 1.40 (mc, 4 H, y-CH2), 1.32 (br s, 8 H, -CH,-), 0.90 (t, 6 H, CH,); 3Jl,,6, 10.8,9.5, 3J1,,10,ax 3J5ar,5b, 10.2, 3J5a,,6, 10.2, 3J5b1,6t 5-49 3J8/,9/ax 10-2, 3J8,,9req 2-47 3JAryl8.5 Hz; GC(CDCl3) 74.1 (C-1’), 101.5 (C-3’), 69.5 (C-57, 78.3 (C-6’), 79.7 (C-S’), 31.9 (C-9’), 33.1 (C-lo), 133.5 (C-1”), 127.2 (C-2”, C-6”), 114.5 (C-3”, C-5”), 158.8 (C-4”), 138.5 (C-1, C-4), 126.1 (C-2, C-3, C-5, C-6), 68.1 (a-CH,), 29.7, 29.6, 29.4, 29.2, 26.0, 22.7 (-CH2-), 14.1 (CH,).Synthesis of (l’S,3’R,6‘R,8’R)-1,4-bis[8’-(2”-fluoro-4”-hexyloxy-phenyl)-2’,4’,7’-trioxabicyclo[4.4.01 decanyl] benzene 10h Reaction conditions D using terephthalaldehyde tetramethyl acetal. GH(CDCl3) 3.64 (ddd, 2 H, H-1’), 5.60 (s, 2 H, H-3’), 4.28 (dd, 2 H, H-5’,), 3.75 (dd, 2 H, H-5’b), 3.58 (ddd, 2 H, H-6’), 4.76 (dd, 2 H, H-8’), 1.87 (mc, 4 H, H-9’,,, H-lO’ax), 2.02 (mc, 2 H, H-9’eq), 2.20 (mc,2 H, H-lO’eq), 6.56 (dd, 2 H, H-3”), 6.67 (dd, 2 H, H-5”), 7.31 (dd, 2 H, H-6”), 7.53 (s, 4 H, H-Aryl), 3.94 (t, 4 H, a-CH,), 1.76 (mc, 4 H, P-CH,), 1.42 (mc, 4 H, y-CH,), 1.32 (br s, 8 H, -CH,-), 0.90 (t, 6 H, CH,); 3Jl,,6, 9.5, 3Jl!,10,ax 3J5a,,5br 3J5a,,6, 3J5bt,6r 5-49 3J8,,9,ax 10.27 3J8,,9teq 2.4, 3JAryl 8.5, 3J51,,611 8.6, 4J3u,5u 2-57 3J311,F 12.1, 4J6rr,~ 8.6 Hz; Gc(CDCl3) 74.1 (C-1’), 101.2 (C-3’), 69.5 (C-5’), 78.7 (C-6’), 79.7 (C-S’), 31.9 (C-9’), 32.2 (C-lo’), 120.5 (C-1”), 160.4 (C-2”), 102.1 (C-3”), 110.5 (C-5”), 127.6 (C-6”), 138.5 (C-1, C- 4), 126.1 (C-2, C-3, C-5, C-6), 68.1 (a-CH,), 29.7, 29.6, 26.0, 22.7 (-CH2-), 14.1 (CH3); ‘J2,,,F 162 Hz.Results and discussion All new compounds show liquid crystalline behaviour of less ordered mesophases such as cholesteric (Ch), smectic A (SA), smectic C* (S,*), blue (BP) and twist grain boundary (TGBA) phases. The mesogenic properties are shown in Table 2. The cholesteric phase exhibits in most cases a fan texture, and is miscible with the nematic compound p-butyl-p’-methoxy- azoxybenzene. The smectic A phase shows a fan texture or is homeotropic.The smectic C* phase is observable as a fan texture with strongly developed pitch lines or as a schlieren texture. The TGBA phase appears with its typical filament tex- ture from the homeotropic smectic A phase. The blue phase with a pitch in the UV is determined by the characteristic paramorphic cholesteric texture that develops continuously in the whole sample from the isotropic state via a blue phase. In a contact preparation with p-butyl-p’-methoxyazoxybenzene, the pitch of the blue phase changes to visible light, and it is observable by its platelet texture. The compounds 5a-5e show a cholesteric mesophase, the preferred mesophase for methoxy-substituted trioxadecalines like compounds 5 and 6. In compounds 6, a tetrahedral carbon atom is replaced by a planar boron atom and, hence, the binding angle of the left alkoxyphenyl unit is different from compounds 5.This structural change causes a broader choles- teric mesophase from 10“C for 5 to 60 “C for 6 and the forming of a blue phase with the pitch in the UV or, in contact preparation, in the visible light. When the methoxy group is placed alongside the boron atom, what is realized for 6f-61, the mesogenic behaviour differs from that for 6a-6e. The cholesteric phase still dominates, but for an alkyl chain length greater than eight a TGBA and smectic A phase are observed Table 2 Transition temperatures of non-fluorinated compounds comp. m n transition temperaturesrc 5a 1 1 K 195.0 Ch 206.0 I 5b 1 6 K 142.9 Ch 155.3 I 5c 1 8 K 134.7 Ch 151.7 I 5d 1 10 K 133.0 Ch 146.9 I 5e 1 12 K 130.6 Ch 140.5 I 6a 1 1 K 158.0 Ch 211.1 (BP) I 6b 6 1 K 92.5 Ch 176.5 (BP) I 6c 8 1 K 90.1 Ch 170.0 (BP) I 6d 6e 10 12 1 1 K 93.5 K 96.6 Ch 163.3 (BP) Ch 151.5 (BP) I I 6f 6g6h 6i 6j6k 61 1 1 1 1 1 1 1 6 8 9 10 11 12 14 K 100.9 K 95.0 K 92.3 K 94.7 K 90.5 K 93.2 K 92.8 (SA 90.0) SA 93.5 SA 101.3 SA 112.0 Ch 177.9 (BP) Ch 165.5 (BP) (TGBA 78.O)Ch 159.5 (BP) (TGBA 90.5)Ch 156.8 (BP) TGBA 94.3 Ch 152.5 (BP) TGBA 103.0 Ch 148.8 (BP) TGBA 113.5 Ch 141.2 (BP) I I I I I I I .7a 8 6 K 163.3 Sc* 165.0 SA 166.6 Ch 173.7 7b 8 8 K 158.6 Sc* 166.9 SA 168.0 I 7c 8 10 K 148.4 Sc* 167.2 I 7d 8 12 K 142.9 Sc* 165.5 I 7e 8 14 K 131.2 Sc* 157.2 I 8a 8b 8 8 6 14 K 124.5 K 120.3 (SA 120.5) (SA 120.3) 9a 6 6 K 126.3 Ch 282.0 I 9b 6 12 K 108.5 Sc* 175.0 SA 206.6 9c 12 6 K 109.6 Sc* 174.8 SA 210.0 I 10a 1 1 K 220.0 Ch 300.0 dec.10b 6 6 K 193.4 Sc* 233.0 Ch 275.0 dec. 1oc 8 8 K 179.3 Sc* 209.0 SA 254.5 Ch 260.0 dec. 1Od 1Oe 10 12 10 12 K 173.2 Sp 211.5 K 163.3 Sc* 234.5 SA 250.0 SA 245.0 dec. dec. 10f 14 14 K 156.5 Sc* 255.0 dec. J. Mater. Chem., 1996, 6(5), 739-745 743 (6h-61) On heating, the TGBA filament texture develops from the homeotropic smectic A phase For chain lengths greater than ten, the smectic A and TGBA phases are enantiotropic, for shorter chains monotropic An analogy may be drawn with compounds such as 11,lo here SA phases are exhibited if the short alkyl chain is attached to the benzoate core We reported earlier that compounds of the general structure 5, eg 5f (Scheme 4) and boron-containing compounds of the general structure 6, eg 6u (Scheme 4) with longer alkoxy chains on both sides show smectic A and monotropic ferroelec- tric phases2 The smectic C* phase is shown only by boron- containing systems with a planar centre in the trioxadecalin system The changed binding angle results in a different molecular shape and, in this case, favours the forming of a smectic C* phase This effect can as well be achieved for non- boron containing compounds by lengthening a wing group with a rigid spacer relative to the molecular core TGBA and smectic C* phases have been found by Goodby et a1 for the phenyl propiolates" So we tried to fit a triple bond as a rigid spacer into our systems creating a structural analogy to these compounds In the compounds 7a-e, the alkynyl group is fitted between the aromatic ring and the trioxadecalin ring which lengthens the lateral group relative to the molecular core All these materials show a smectic C* phase, which for chains longer than eight carbon atoms is the only existing mesophase Shorter chain lengths give a smectic A phase as well as a cholesteric phase The hydrogenation of the rigid alkynyl bridge to a flexible alkyl spacer shows that the linearity of the spacer is essential for the exhibition of the smectic C* phase The hydrogenation causes the complete suppression of the smectic C* phase, leaving only a monotropic smectic A phase for the compounds 8 Even the cholesteric phase of 7a is suppressed in 8a The mesogenic behaviour is similar to the mesogenic properties of the two-ring systems such as 5h (Scheme 4) These compounds only show a monotropic SA phase A comparison of 8a with 5h shows that the right side of the molecule and the molecular core are identical They differ in the left wing group, for 5h an alkyl chain, and for 8a an alkyl chain with a phenyl ring located in the chain The phenyl ring is flexibly relative to the rigid molecular core, so it behaves more like a part of a flexible chain than like a rigid structural unit Thus, it shows like the two-ring compounds a monotropic smectic A phase Another possibility for introducing a lengthened wing group is the introduction of a further aromatic ring which is accomplished for compounds 9 by using a 44 4-alkoxybenzoyl- oxy) benzaldehyde for synthesis The mesogenic behaviour strongly depends on the terminal chain lengths The four-ring systems show for longer alkyl chains a smectic C* schlieren texture and a smectic A phase, but the symmetrically hexyloxy- substituted compound 9a exhibits only a cholesteric mesophase A different class of molecules is synthesized by the dimeris- ation of 4 using terephthalaldehyde as a reagent The resulting structure contains two trioxadecalin units, and the molecule is still chiral, no meso form is obtained Compound 10a with terminal methoxy groups forms a cholesteric phase at high temperatures, but for longer alkyl chains a smectic C* phase is observed, accompanied by a smectic A phase Only 10f with the longest terminal chain lengths shows exclusively the smectic C* phase All compounds possess high melting and clearing temperatures, and they are liable to decompose at high temperatures Next we wanted to study the influence of the introduction and position of a lateral fluoro atom on the mesogenic behaviour The effect of lateral fluoro substitution on the aromatic rings is examined with two different systems The fluorine atom is either located next to the decalin ring system or next to the terminal side chain The general structure of the compounds synthesized is shown in Scheme2 The influence of the position of the fluoro atom is studied for compounds 6 with either X1or X2=F In the case of X1=F, the fluorine is positioned in the aromatic 2-position next to the trioxadecalin ring system, protruding into the molecular core In the case of X2=F, the fluorine is positioned in the aromatic 3-position next to the terminal alkoxy chain, directed to the area of the terminal chains The small change from 2-F to a 3-F substitution causes a large difference in the mesogenic behaviour In Table 3 the transition temperatures are compared with the temperatures of the non-fluorosubstituted compounds Looking at the 3-fluoro compounds with the fluorine next 10 the terminal chain one can see that all compounds show the same phase sequence as the non-fluoro-substituted compounds, but at lower tem- peratures The monotropic smectic C* phase shown by 6m-6r is stabilized and is easier to observe In contrast, the 2-fluorinated compounds 6t, 6v, 6x (in which the fluorine pro- 5h m = 7, n = 6 K 75 (SA59)I H2rn+ crno~ 0 ~ 0 C , H 2 ,+ 1la rn = 2, n = 12, K 93 N 87 I 1 1 b m = 12, n = 2, K 76 5 (SA76) N 91 I Scheme 4 Structures with related mesogenic effects 744 J Muter Chem , 1996, 6(5), 739-745 Table 3 Transition temperatures of fluorinated compounds, compared with the corresponding unfluorinated compounds comp.m n XI 5f " 12 6 H 5g 12 6 F 6m" 6 8 H 6n 6 8 H 60" 6 10 H 6P 6 10 H 6q"6r 6 6 12 12 H H 6s" 8 6 H 6t 8 6 F 6u" 12 6 H 6v 12 6 F 6w" 6 6 H 6x 6 6 F 9a 6 6 H 9b 6 6 F 10b 6 6 H 10h 6 6 F "Ref.2. trudes into the molecular core) show only a cholesteric meso- phase and all smectic phases are completely suppressed. This effect is observed for all compounds having the fluorine in 2- position, even in 6x with two fluorine atoms in both positions (X1=X2=F) the effect of the fluoro substituent in the 2-position is larger. The position of the lateral fluorine causes a steric disturbance of the molecular shape. In the 2-position it changes the form of the rotational cylinder of the central ring system. The shape of the molecular core does not allow the aggregation of the ring segments to form a smectic phase anymore.Protruding out from the ring segment, the fluorine changes the molecular structure as well, but now it is located near the terminal chains that do not need as much space as the central ring systems, so the protruding fluorine atom does not cause a disturbance that is as large as the disturbance caused in 2-position. It only lowers the transition temperatures, and diminishes the risk of recrystallisation while cooling down to the smectic C* phase,12 so observation of this monotropic fan textured phase with pitch lines is more easily achieved. This 'inside-outside-effect' of the lateral substitution can be confirmed using the database LiqCryst.I3 It is possible to compare all mesogenic compounds known in the present literature with a substitution pattern similar to that described above.The result of this comparison is that the observation of lowering of the transition temperatures in one case and suppression of smectic phases in the other case for the different fluoro substitution positions is a general effect of molecules with this substitution pattern and not a special effect of the compounds described.12 x2 transition temperatures/"C H K 138.0 SA 155.0 I H K 93.6 Ch 106.0 I H F H F H F K K K K K K 83.0 66.7 77.0 67.7 82.0 69.0 (Sc* 60.0) (Sp 52.4) (Sc* 57.0) (Sp 39.3) (Sp 44.0) (Sp 35.9) SA 182.0 SA 163.7 S, 177.0 SA 158.7 S, 155.3 SA 168.0 I I I I I I H K 88.0 (Sp 73.0) SA 179.0 I H K 71.5 Ch 132.5 I H H K K 89.0 71.6 (Sp 59.0) S, 165.0 Ch 118.6 I I H K 84.0 SA 180.0 T F K 65.3 Ch 118.0 I H K 126.3 Ch 282.0 I H K 97.0 Ch 250.0 I H K 193.4 Sc* 233.0 Ch 275.0 dec.F K 143.9 Ch 280.0 dec. We thank the Deutsche Forschungsgemeinschuft for financial support. References 1 J. W. Goodby, J. Muter. Chem., 1991, 1, 307. -7 V. Vill and H.-W. Tunger, Liebigs Ann., 1995, 1055. 3 V. Vill, H.-W. Tunger, H. Stegemeyer and K. Diekmann, Tetrahedron: Asymmetry, 1994, 12,2443. 4 V. Vill, H.-W. Tunger, K. Hensen, H. Stegemeyer and K. Diekmann, Liq. Cryst., in the press. 5 V. Vill and H.-W. Tunger, J. Chem. SOC., Chem. Commun., 1995, 1047. 6 V. Vill, H.-W. Tunger and D. Peters, Liq. Cryst., in the press. 7 H. Stegemeyer,A. Sprick, M. A. Osipov, V. Vill and H.-W. Tunger, Phys. Rev. E, 1995,51,5721. 8 E. J. Corey and P. L. Fuchs, Tetrahedron Lett., 1972,36,3769. 9 E. R. H. Jones, L. Skattebol and M. C. Whiting, J. Chem. SOC., 1958, 1054. 10 T. T. Blair, M. E. Neubert, M. Tsai and C.-C. Tsai, J. Phys. Chem. Ref. Data, 1991, 20, 189; J. Malthete, J. Billard, J. Canceill, J. Gabard and J. Jacques, J. Phys. (Paris), 1976,37, Suppl. C3, 1. 11 J. W. Goodby, I. Nishiyama, A. J. Slaney, C. J. Booth and K. J. Toyne, Liq. Cryst., 1993, 14, 37. 12 V. Vill, unpublished results. 13 V. Vill, LiqCryst-Liquid Crystal Database, Fujitsu Kyushu Systems (FQS) Ltd, Fukuoka, Japan, 1995, LCI Publisher, Hamburg 1995. Paper 5/05032D; Received 28th July 1995 J. Muter. Chem., 1996, 6(5),739-745 745

ISSN:0959-9428    

DOI:10.1039/JM9960600739

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis and mesogenic properties of 3,6-disubstituted cyclohex-2-en-1-ones Roger Brettle, David A. Dunmur, Louise D. Farrand".? and Charles M. Marson Department of Chemistry, University of Shefield, Shefield, UK S3 7HF A series of 3,6-substituted cyclohex-2-en- 1-ones have been prepared by an efficient convergent Robinson-type annulation route. The cyclohex-2-en-1-one ring is the basis of a novel mesogenic core and when substituted at the 6-position provides a chiral centre, which is adjacent to a strong transverse dipole moment. This system has the advantage that both of the two main features required for new electro-optical applications (a dipole for switching and a chiral centre to reduce the symmetry) are located in the molecular core. The synthesis and mesogenic properties of these novel enones are reported.Extensive research in the liquid-crystal field has been directed towards attempting to correlate the relationship between mol- ecular structure and mesogenic properties.' A systematic way of studying this relationship is to introduce small changes into the chosen mesogenic structure and to observe the effects of different substituents. In the design of liquid crystals for new electro-optical applications this aim has been partially achieved by many researchers who have studied the effect of chirality in optically active systems.2 In particular, new electro-optical applications based on the ferroelectric (Sc*) and electroclinic effects (S,*) have been found to require a high degree of chirality coupled with a large transverse dipole rn~ment.~ Chirality is usually introduced into the molecule by means of a chiral centre incorporated into an aliphatic chain.A closer examination of ferroelectricity in liquid crystals shows that the magnitude of spontaneous polarization depends upon the tilt angle, the size of the dipole at the chiral centre, and the amount of freedom that the chiral centre has to rotate about the long axis of the molecule. In many cases, the effectiveness of the chiral centre in promoting macroscopic chiral properties in liquid crystals has been shown to increase as the chiral centre is brought closer to the rigid molecular The aim of this work was to prepare a range of mesogenic compounds for new electro-optical applications based on chiral smectic liquid crystals, which if enantiomerically enriched will exhibit a large spontaneous polarization.Knowledge of the factors which influence a high spontaneous polarization value suggested a target structure that would combine a chiral centre as part of the rigid core with a transverse dipole in order to reduce the rotation of the chiral centre about its long axis (which if not rigid may reduce the value of spontaneous polarization). In order to produce chiral liquid crystals for a variety of applications, it is possible to use either mesogenic compounds, or non-mesogenic chiral compounds in a suitable liquid-crystal host. It is more desirable to use a chiral smectic material than a non-mesomorphic compound as a dopant in displays because incompatibility between dopant and host material is likely to reduce the effectiveness of the chiral dopants.These considerations suggested a study of the cyclo- hex-2-en-1-one ring system, which, when substituted at the 6-position, provides a chiral centre that is adjacent to a strong transverse dipole moment. In the design of molecules for ferroelectric liquid crystals based on tilted chiral smectics, it is desirable that the electric dipole has a fixed orientation with respect to the molecular core, and that the symmetry breaking chiral element is also rigidly attached to the core. The structures reported here have the advantage that both of the two main t Present address: Merck Ltd., West Quay Road, Poole, UK BHlS 1HX.features required for ferroelectric purposes (a dipole for switch- ing and a chiral centre to reduce the symmetry) are present in the molecular core. 0 I In order to make the system anisotropic, the core can be made longer by substitution of an aromatic ring placed at the 3-position (I). Such substitution also renders the core more polarizable. The aromatic ring may then be further substituted at the 4-position to elongate the molecule (R =alkyl or alkoxy), and the cyclohex-2-en-1-one ring can be substituted at the 6-position to provide an extended calamitic structure with a centrally located chiral centre (R' =alkyl). The 3-arylcyclohex- 2-en-1-one system contains a novel rigid core, and the aim of this work was to determine whether this new system did in fact exhibit mesogenic behaviour when appropriately substi- tuted.The approach was to prepare several racemic modifi- cations of 3-arylcyclohex-2-en-1-onesby a convergent route. This allowed the possibility of preparing as many analogous compounds as possible with minimum structural alterations of the precursors. One route which could permit such con- vergency was a Robinson-type ann~lation.~ The synthetic route chosen would have to provide a variety of analogues in order to allow optimization of the molecular structure to give desired properties, preferably smectic A or smectic C phases, a low transition temperature from the crystal to the smectic phase, and stability over a wide temperature range.The ultimate aim of this work was to prepare optically active liquid crystals, but effort was initially focused on the preparation of racemic compounds in order to establish the mesophase behaviour. The preparation of pure enantiomers of related compounds is described elsewhere.8 Synthesis The arylcyclohex-2-en-1-oneswere prepared as shown by the convergent route in Scheme 1. The phenyl ethers (1, 2) were prepared by alkylation of phenol with the appropriate alkyl bromide according to the literature proced~re.~~~~ Subsequent Friedel-Crafts acylation of the aromatic compounds using 3- chloropropionyl chloride ( 1 equiv) and aluminium trichloride (1.1 equiv) in pentane (20°C, 1h) afforded the P-chloroke- tones (3-7).The short chain-length P-keto esters (R' =C,, C,) (8-10) were obtained by alkylation of the acetoacetate with an alkyl J. Muter. Chern., 1996, 6(5),747-751 747 I 13-23 Scheme 1 Preparation of arylcyclohex-2-en-1-ones halide using sodium methoxide as a nucleophilic base In the reactions with longer chain alkyl halides, R1 =cg, Clo (11, 12), the use of a nucleophilic base served only to cleave ethyl acetoacetate vzu attack at the site of the electrophilic carbonyl group Therefore, the non-nucleophilic base sodium hydride was chosen to replace sodium ethoxide No reaction was observed at 0 or 20"C, however, under reflux in tetrahydro- furan, an extremely clean reaction was observed and both alkylations proceeded in high yields The desired 3-arylcyclohex-2-en- 1-ones ( 13-23) were obtained in a one-pot procedure from the appropriate chloro ketones (3-7) and the keto esters (8-12) The chloro ketone was treated with potassium tert-butoxide in ethanol to generate the enone (11) zn sztu To this mixture was added a mixture of the keto ester (1 equiv) and sodium ethoxide (1 equiv) in ethanol The mixture was stirred under reflux for 3 h, concen- trated and extracted with CH,Cl,, to give an oil which was dissolved in ethyl acetate-light petroleum (1 4) On cooling, white crystals of the desired 3-arylcyclohex-2-en- 1-one ( 13-23) precipitated To our knowledge, this is the first time that 3- arylcyclohex-2-en-1-oneshave been prepared by this [3C +3C] type condensation This convergent condensation has the advantage that the unwanted activating alkoxycarbonyl group is cleaved during the one-pot procedure, presumably by a retro-Claisen process However, in the preparation of 14, minor quantities of the keto ester (14b) were isolated The reaction of ethyl 2-undecylacetoacetate (11) with 1-(4- decylphenyl)-3-chloropropan-l-one(7) was not successful, probably because of the presence of the bulky terminal alkyl chains In general, the alkylphenyl derivatives were isolated in slightly higher yield, and proved easier to isolate than the alkoxy compounds This may be a consequence of the mes- omerism involving the 4'-alkoxy substituent further dimin- ishing the already low electrophilicity of the carbonyl group of the vinyl ketones (11) Instrumentation and procedures Moisture-sensitive reactions were conducted in oven-dried glassware assembled under a positive pressure of argon Solvents were dried according to literature methods Thin-layer chromatography (TLC) was used to monitor the extent of the reaction and was performed on type 5244 Merck 0 2 mm 748 J Muter Chem, 1996, 6(5),747-751 aluminium-backed silica plates The plates were visualized using UV light or iodine vapour Column chromatography was performed with silica gel (Sorbsil 60) as the stationary phase Organic solutions were dried over anhydrous mag- nesium sulfate Evaporation refers to removal of the solvent under reduced pressure 'H NMR spectra were recorded using a Bruker AC250 (250 MHz) with the residual proton signal of chloroform 6= 7 25 as the internal standard, 13C NMR spectra were obtained on a Bruker AC250 instrument operating at 62 9 MHz 13C shifts were measured in ppm relative to the central peak of deuteriochloroform at 6 =77 0 Mass spectra were obtained on a Kratos MS-25 instrument Microanalytical data were obtained on a Perkin-Elmer 2400 CHN analyser The phase assignments13 and transition temperatures were determined by thermal polarized light microscopy using a Zeiss Universal microscope equipped with crossed polarizers and a Linkam hot-stage with an integrated temperature con- troller Heating and cooling rates were usually 5 or 10 "C min Differential scanning calorimetry (DSC) was performed using a Perkin-Elmer DSC-7 at scan rates of 10 min-' Sample masses of between 1 and 2 mg were typically used Onset temperatures were taken as transition tempera- tures, and they and the associated thermodynamic parameters were usually taken from the second heating cycle Experimental Butyloxybenzene (1) Prepared as described in the literature' and was obtained as a clear, colourless liquid (74%), bp 41 "C, 0 4 mmHg (lit ,9 bp 82-83 5 "C, 10 mmHg) Hexyloxybenzene (2) Prepared according to the literature procedure lo A clear, colourless liquid was isolated (84%), b p 80 "C, 0 4 mmHg (lit ,lo bp 118-124 "C, 12 mmHg), Cl2HI80 requires C, 80 90, H, 10 11%, Found C, 81 03, H, 10 24% l-[(4-HexyZoxy)phenyl]-3-chloropropun-l-one(3) A solution of hexyloxybenzene ( 15 0 g, 84 mmol) in pentane (30 cm3) was added to an ice cooled slurry of aluminium chloride (12 7 g, 95 mmol) in pentane (50 cm3) 3-Chloropropionyl chloride (8 0 ml, 84 mmol) was dripped into the slurry After stirring at room temperature for 1 h, water (100cm3) was added to quench the reaction The pentane layer was removed and washed with water (50 cm3) The first aqueous layer was washed with an equal amount of pentane The organic extracts were dried and evaporated off to give a brown oil which was dissolved in ethyl acetate Addition of light petroleum to the solution precipitated 3 as greenish-brown crystals (19 3 g, 85%), mp 43-44 "C, (lit ,14 m p 31-34 "C) Cl,H210,Cl requires C, 67 03, H, 7 87, Cl, 13 19%, Found C, 66 73, H, 7 83, C1, 1308% 6, 793 (2H, d, J=8), 691 (2H, d, J=8), 401 (2 H, t, J=6), 3 91 (2 H, t, J=6), 3 40 (2 H, t, J=6), 170 (2 H, quintet, J =6), 1 52-1 25 (6 H, m), 0 90 (3 H, t, J =6) 6, 195 2 (s), 1662 (s), 130 3 (d), 129 3 (s), 114 3 (d), 68 3 (t), 41 0 (t), 39 0 (t), 31 5 (t), 29 0 (t), 25 6 (6), 22 5 (t), 14 0 (9) m/z (+CI) 269 (M+, 76), 233 (42), 205 (52), 121 (100%) Similarly prepared were 1-[(4-Buty/oxy)phenyl]-3-chloropropan-l-one (4) White prisms (85%), m p 54-55 "C, (lit ,14 m p 54 "C) 1-(4-Hexy/phenyl)-3-chloropropun-l-one(5) White prisms (67%), m p 40-41 "C C19H290C1 requires C, 71 27, H, 8 37, Cl, 1402Y0, Found C, 71 30, H, 8 28, C1, 13 86% 1-(4-Heptylphenyl)-3-chloropropun-1-one (6) White prisms (67%), m p 36-37 "C C16H,,0CI requires C, 72 03, H, 8 69, C1, 13 29%, Found C, 72 08, H, 8 44, C1, 13 13% 1-(4-Decylphenyl)-3-chloropropun-l-one (7).Large white needles (97%), m.p. 59-60 "C. C,,H,,OCl requires C, 73.88; H, 9.46; C1, 11.48%; Found C, 73.88; H, 9.37; C1, 11.47%. The following were prepared according to literature procedures. Ethyl 2-pentylacetoacetate (S)." 76%, b.p. 72-74 "C, 0.4 mmHg, (lit.,I6 b.p. 121 "C, 10 mmHg). Ethyl 2-allylacetoacetate (9).1757%. The 'H and I3C NMR data were in accordance with those reported in the literature." Methyl 2-propylacetoacetate ( 10). Isolated as a clear, colour- less oil (65%), b.p. 101 "C, 0.7 mmHg. 6,: 3.75 (3 H, s), 3.44 (1 H, t, J=8), 2.23 (3 H, s), 1.90-1.79 (2 H, m), 1.38-1.24 (2 H, m), 0.93 (3 H, t, J=7).6,: 203.0 (s), 170.2 (s), 59.2 (d), 52.1 (q), 29.9 (t), 28.3 (q), 20.5 (t), 14.3 (9). Ethyl 2-undecylacetoacetate (11). A solution of ethyl aceto- acetate (3.90 g, 30 mmol) in THF (40 cm3) was added to a stirred suspension of sodium hydride (80% in oil) (0.83 g, 30.0 mmol) in THF (50 cm3) at 0 "C. After 1h, a solution of 1-iodoundecane (8.47 g, 30.0 mmol) in THF (20 cm3) was added and the solution was stirred at room temperature for 2 h. The solution was then stirred overnight under reflux and a white precipitate appeared. The mixture was allowed to cool, the solvent removed, and the residue taken up into dichloro- methane (50 cm3), washed with water (2 x 50 cm3) and dried. Evaporating off the solvent afforded a viscous oil, which was subjected to flash column chromatography using dichloro- methane: hexane (2 : 1) as eluant (R,0.51) to give a colourless oil which crystallized on standing.Recrystallization from light petroleum afforded 11 as colourless crystals (7.01 g, 82%), m.p. 31 "C, (lit.,', b.p. 145-15OoC, 1.0mmHg). 6,: 4.18 (2 H, q, J= 7), 3.37 (1 H, t, J=7), 2.40 (3 H, t, J=7), 2.09 (3 H, s), 1.57 (2 H, quintet, J=7), 1.32-1.17 (16 H, m), 0.9 (3 H, t, J=7). 6,: 204.2 (s), 167.0 (s), 61.2 (t), 60.0 (d), 43.8 (t), 31.9 (t), 29.8 (q), 29.4 (t), 29.4 (t), 29.3 (t), 29.2 (t), 28.2 (t), 27.4 (t), 23.9 (t), 22.7 (t), 14.1 (2) (9). ~,,,/cm-~ (Nujol mull) 2920 (C-H), 1720 (C=O), 1430 (C-H), 1380 (C-H). Similarly prepared was: Ethyl 2-hexylacetoacetate (12)." 77%, b.p.100-102 "C, 0.07 mmHg, (lit.," b.p. 127-129 "C, 10 mmHg). 3-(4-Butyloxyphenyl)-6-propylcyclohex-2-en-1-one ( 13). 1-(4-Butyloxyphenyl)-3-chloropropan-l-one(2.25 g, 9.4 mmol) was treated with potassium tert-butoxide (1.05 g, 9.4 mmol) in super-dry ethanol. In a separate flask, methyl 2-propylaceto- acetate (1.48 g, 9.4 mmol) was added to a solution of sodium (0.23 g, 10 mmol) previously dissolved in ethanol (30 cm'). This solution was then added to the first ethanolic solution. The reaction was heated under reflux for 3 h, and then cooled and concentrated to give a brown oil which was dissolved in dichloromethane (100 cm3) and washed with brine (2 x 50 cm3). The organic layer was removed and dried, then concentrated to give a yellow oil.The oil was dissolved in ethyl acetate; light petroleum was then added and the solution was left to stand in a freezer for 24 h. The white precipitate was collected by vacuum filtration. Recrystallization from ethyl acetate and light petroleum gave 13 as a white crystalline solid (1.27 g, 46%). C1,H2,02 requires C, 79.68; H, 9.15%; Found C, 79.82; H, 9.10%. 6,: 7.50 (2H, d, J=8), 6.90 (2H, d, J= 8), 6.35 (1 H, m), 3.97 (2H, t, J=6), 2.85-2.65 (2H, m), 2.32 (1 H, m), 2.23 (1 H, m), 1.87 (2 H, m), 1.83 (2 H, quintet), 1.52 (2 H, sextet), 1.40 (3 H, m), 1.00-0.92 (6 H, m). 6,: 202.1 (s), 160.8 (s), 157.7 (s), 130.4 (s), 127.5 (d), 123.2 (d), 114.6 (d), 67.8 (t), 45.3 (d), 31.5 (t), 31.2 (t), 27.7 (t), 26.8 (t), 20.2 (t), 19.2 (t), 14.2 (q), 13.8 (9).m/z (+CI) 287 (M', 70), 244 (100%). Similarly prepared were: 3-(4-Butyloxyphenyl)-6-pentylcyclohex-2-en-l-one ( 14). White prisms (41%). C21H30O2 requires C, 80.21; H, 9.62%; Found C, 79.91; H, 9.78%. 3-(4-Butyloxyphenyl)-6-pentyl-(6-ethoxycarbonyl)cyclohex-2-en-1-one (14b). Yellow crystals (12%), m.p. 61 "C. C24H3404 requires C, 74.58; H, 8.87%; Found C, 74.61; H, 8.72%. 6,: 196.4 (s), 171.8 (s), 161.0 (s), 157.5 (s), 129.8 (s), 127.6 (d), 114.6 (d), 67.8 (t), 61.1 (t), 56.1 (s), 33.7 (t), 32.2 (t), 31.2 (t), 29.8 (t), 25.2 (t), 24.2 (t), 22.4 (t), 19.2 (t), 14.1 (q), 14.0 (q), 13.8 (4). 3-(4-Hexyloxyphenyl)-6-allylcyclohex-2-en-1-one (15).Prisms (45%). C21H2802 requires C, 80.73; H, 9.03%; Found C, 80.75; H, 9.00%.3-(4-Hexyloxyphenyl)-6-propylcyclohex-2-en-1 -one ( 16).Prisms (39%). C21H3OO2 requires C, 80.21; H, 9.62%; Found C, 80.06; H, 9.51%. 3-(4-Hexyloxyphenyl)-6-pentylcyclohex-2-en-1-one ( 17). Prisms (46%). C&&2 requires C, 80.66; H, 10.01%; Found C, 80.58; H, 10.31%. 3-(4-Hexylphenyl)-6-propylcyclohex-2-en-1 -one ( 18). Fine prisms (43%). HRMS, C21H3oO requires 298.2295; M+ found 298.2297. 3-(4-Hexylphenyl)-6-pentylcyclohex-2-en-l-one(19). Prisms (47%). C24H360 requires C, 84.65; H, 10.66%; Found C, 84.74; H, 10.80%. 3-(4-Heptylphenyl)-6-pentylcyclohex-2-en-1 -one (20).Microprisms (47%). C24H360 requires C, 84.65; H, 10.66%; Found C, 84.74; H, 10.80%. 3-(4-Heptylphenyl)-6-hexylcyclohex-2-en-1 -one (21).Microprisms (45%).C2,H3,0 requires C, 84.69; H, 10.80%; Found C, 84.73; H, 10.93%. 3-(4-Decylphenyl)-6-propylcyclohex-2-en-1-one (22).Microprisms (46%). HRMS, C2sH380 requires 354.2919; M+ found 354.2923. 3-(4-Decylphenyl)-6-pentylcyclohex-2-en-l-one(23). Prisms (45%). C27H420 requires C, 84.75; H, 11.06%; Found C, 84.51; H, 11.06%. Liquid-crystal Properties The transition temperatures of the mesogenic cyclohex-2-en- 1-ones are listed in Table 1. Phase types were identified by the optical texture^.'^ and phase transitions and temperatures were confirmed by DSC. All of the homologues synthesized exhibited smectic A phases, which can be attributed to the presence of a localized dipole in the core of the molecule. All of the smectic A phases were observed as the common focal-conic fan texture.This texture was always seen to separate from the isotropic, on cooling in the form of battonets, which themselves consist of growing focal-conic domains. A texture characteristic of all compounds exemplified by compound 21 is shown in Plate 1. DSC analyses show that the values obtained for the enthalpy change from the smectic A to the isotropic liquid are quite large, typically 5-8 kJ mol-I, which are within the expected range of values for a smectic A to isotropic transition. All of the alkoxyphenyl cyclohex-2-en- 1-ones exhibited rela- tively low K+SA (40-60 "C) transition temperatures. The smectic A phases were maintained over a wide temperature range, typically 20-60 "C. The relatively short compound (13) exhibited a crystal to crystal transition before exhibiting a J.Mater. Chem., 1996, 6(5), 747-751 749 Table 1 Phase types and transition temperatures of cyclohex-2-en-l-ones (R and R' as in Scheme 1) transition temperature enone R R' (optical microscopy)/"C 13 C3H7 K(60.1)Kz( 68.2)S~( 97.1 )I 14 C5H1 1 15 C3H5 K(48.9)SA( 63.2)I 16 C3H7 K(45.O)S,(96.2)1 17 C5H11 18 C3H7 19 C5H11 20 GHl, 21 C6H13 22 C3H7 I(36.0)s~(32.O)K monotropic 23 C5H11 K(48.3)SA(63.1)I Plate 1 Focal-conic fan texture of the smectic A phase of compound 21 formed at 60°C on cooling from the isotropic phase Fig. 1 Stackbar chart to illustrate the phase types and transition temperatures of the cyclohex-2-en- l-ones smectic A phase.The liquid-crystal properties of this compound may verge on the limits of anisotropic requirements for liquid- crystal behaviour since the known compound 3-(4-ethoxy-phenyl)-6-methylcyclohex-2-en-l-one is not mesogenic.20 No mesogenic properties were observed for compound 14b, the isolated cyclohex-2-en- l-one keto ester, which has a melting point of 62°C. The alkyl chain in 15 contains a double bond. A comparison with compound 16, which does not possess a 750 J. Muter. Chem., 1996, 6(5), 747-751 T onset/"C AH/kJ mol-' AS/J K-' mol-I 53.6 7.6 23.2 64.6 10.3 31.0 89.5 7.5 20.7 42.8 20.3 54.3 63.1 7.9 20.8 47.5 17.6 64.9 59.5 5.2 15.6 46.4 14.6 45.7 89.9 6.9 19.0 51.1 15.7 48.4 102.2 8.1 21.6 27.1 13.8 46.3 33.2 6.3 20.5 21.2 10.3 34.8 59.3 6.2 18.7 21.5 11.0 37.4 60.0 6.9 20.7 43.6 33.3 105.2 57.5 5.6 17.1 34.7 37 9 123.2 43.8 38.9 122.8 59.4 7.3 21.9 double bond in the terminal chain, shows that the main effect of the alkene is to lower the clearing point considerably.It is known that liquid crystals with alkenyl side chains are chemi- cally and photochemically stable as long as the double bond is not conjugated.21 All of the alkylphenyl cyclohex-2-en- l-ones prepared were stable and showed no decomposition during heating and cooling cycles. In general, the alkylphenyl derivatives exhibited both lower melting and clearing transition temperatures than the alkoxy homologues.Compounds 18, 19 and 20 exhibited extremely low temperature crystal to smectic A transitions. The smectic A phases were observed over a 10-30°C range. The optimum alkyl chain lengths were hexyl with pentyl, and heptyl with pentyl. Enone (22),which has a large difference in alkyl chain lengths within the molecule, possesses a monotropic smectic A phase. When R' is increased to C5Hll, as in enone (23), a smectic A phase is observed on heating and cooling. These results indicate that it is likely that appropriate modifi- cation of the groups attached to the arylcyclohex-2-en-1 -one core will result in the formation of liquid-crystal phases of other symmetries. Optically active mesogens based on alkyl- phenyl cyclohex-2-en- l-ones have been prepared using highly efficient enzymic resolution of acetoxy-derivatives, and the synthesis and mesophase properties of these have been described previously.* Direct comparison of the mesophase properties of the racemic compounds reported in this paper with the optically active compounds previously reported is not possible since the terminal groups are different. However, both series of mesogens exhibit smectic A phases, the optically active carbonyloxy-compounds having higher melting points and shorter mesophase ranges, possibly due to their increased dipole moments.Conclusions A series of new mesogenic compounds based on the novel core structure of phenylcyclohex-2-en-1 -one has been prepared.The core structure was chosen to combine a lateral dipole moment in the core with a chiral centre, although the compounds reported in this paper were all prepared as racemates. All compounds of this series showed smectic A phases below 100"C: those having alkoxy-chain substituents generally had higher transition temperatures and wider smectic A ranges. Increasing the alkyl chain length beyond seven carbon atoms diminished the liquid crystallinity of the materials. The dis- covery of a new basic core material should allow the prep- aration of many more liquid crystals having a variety of properties. The fact that these mesogens have relatively low melting point temperatures is also of significance in the devel- opment of new materials for applications.The cyclohex-2-en- 1-one core introduces two new structural features, which can be useful in particular display devices, since a transverse dipole moment rigidly linked to a chiral centre is necessary for the appearance of ferroelectricity in chiral smectic C phases and for electroclinism in chiral smectic A phases. Both these phenomena can be used as a basis for new liquid-crystal devices. The present route to 6-alkyl-3-arylcyclohex-2-en-l-ones complements the synthesis of chiral 6-carbonyloxy-3- arylcyclohex-2-en- 1-ones as previously reported.* We thank Hitachi Research Laboratory, Hitachi Ltd., Japan, for financial support of this work. References 1 J. W. Goodby, J. Mater. Chem., 1991,1,307. 2 J. W. Goodby, I.Nishiyama, A. J. Slaney, C. J. Booth and K. J. Toyne, Liq. Cryst., 1993, 14, 37. 3 H. Stegemeyer, R. Meister, H-J. Altenbach and D. Szewczyk, Liq. Cryst., 1993, 14, 1007. 4 J. W. Goodby, J. S. Pate1 and E. Chin, J. Phys. Chem., 1987, 91, 5151. 5 S. Takehara, M. Osawa, K. Nakamura, T. Kusumoto, K-I. Sato, A. Nakayama and T. Hiyama, Ferroelectrics, 1994,148,195. 6 K. Itoh, M. Takeda, M. Namekawa, S. Nayuki and Y. Murayama, T. Yamashi and T. Kitazume, Ferroelectrics, 1993,148,85. 7 W. S. Rapson and R. Robinson, J. Chem. SOC., 1935,1285. 8 R. Brettle, D. A. Dunmur, L. D. Farrand and C. M. Marson, J. Chem. SOC. Chem. Commun., 1994,2041. 9 E. Profft, Chem. Abstr., 1959,53,2222. 10 M. Protiva, M. Radsner, E. Aolerova, V. Seidlova and Z. J. Vejoelek, Chem. Abstr., 1965,62, 524c. 11 F. J. Marshall and W. N. Cannon, J. Org. Chem., 1956,21,245. 12 R. Brettle, D. A. Dunmur, L. D. Farrand, N. J. Hindley and C. M. Marson, Chem. Lett., 1993, 1663. 13 D. Demus and L. Richter, Textures in Liquid Crystals, Verlag Chemie, Weinheim, 1980. 14 N. J. Hindley, PhD Thesis, University of Sheffield, 1991. 15 A. I. Vogel, A Textbook of Practical Organic Chemistry, Longman, Harlow, 5th edn., p. 620. 16 A. J. Birch and R. Robinson, J. Chem. SOC., 1942,494. 17 D. Gravel and M. Labelle, Can. J. Chem., 1985,63,1875. 18 Beilstein, 3rd suppl., vol. 3, p. 1269. 19 Beilstein, 1st suppl., vol. 3, p. 252. 20 V. S. Bezborodov and D. A. Trohimets, Zh. Org. Khim., 1991, 27, 1958. 21 M. Schadt, R. Buchecker and A. Villiger, Liq. Cryst., 1990,7, 519. Paper 5/06286A; Received 22nd September 1995 J. Mater. Chem., 1996, 6(5),747-751 751

ISSN:0959-9428    

DOI:10.1039/JM9960600747

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Stability of the antiferroelectric phase in dimeric liquid crystals having two chiral centres with CF3or CH, groups; evaluation of conformational and electric interactions Yoshi-ichi Suzuki," Tadaaki Isozaki? Shigeharu Hashimoto," Tetsuo Kusumoto,b Tamejiro Hiyama,' Yoichi Takanishi,d Hideo Takezoe*d and Atsuo Fukudad aCentral Research and Development Laboratory, Showa Shell Sekiyu K.K., 123-1 Shimokawairi, Atsugi, Kanagawa 243-02, Japan bSagami Chemical Research Center, 4-4-1 Nishiohnuma, Sagamihara, Kanagawa 229, Japan 'Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226, Japan dDepartmentof Organic and Polymeric Materials, Tokyo Institute of Technology, 0-okayama, Meguro-ku, Tokyo 152, Japan The synthesis of a novel series of dimeric liquid crystals with two chiral centres with CF, or CH, groups connected by an alkylene spacer is described.The two mesogenic units in the dimer tend to align in anticlinic zigzag ordering or in parallel ordering depending on whether the alkylene spacer has an odd or even number of -CH2 -groups, n, stabilizing the antiferroelectric SCA* phase or the ferroelectric Sc* phase, respectively. This odd-even rule in the conformational effect is no longer valid when the length of the alkylene spacer is increased to over n =10: the SCA*phase emerges instead of the Sc* phase for n =12, as already reported for the dimer with CF, groups. Furthermore, we found that the dimers with n =10 also show a SCA*phase, if the dimers with CH, or CF, groups are sandwiched between thick cells. In thin cells, on the other hand, the Sc* phase emerges instead of the SCA*phase in the dimers with CH, (n=10 and 12) and CF3 (n= lo), although the SCA*phase is stable even in thin cells in the dimer with CF, (n=12).Thus, the cell thickness dependence is pronounced in the dimer with CH, groups, indicating that the dimer with CF, groups shows more stable antiferroelectricity, the same as in the monomers.These facts suggest the importance of an electric interaction between mesogens for the emergence of the SCA*phase. A new chiral smectic liquid crystal phase exhibiting an anticlinic ordering of the molecular orientation in successive layers has been discovered. This phase, called the antiferro- electric liquid crystal (AFLC) phase, typically appears in 4-( 1-methylheptyloxycarbony1)phenyl4'-octyloxybiphenyl-4-carboxylate (MHPOBC)' and 44 l-trifluoromethyl-heptyloxycarbony1)phenyl 4'-octyloxybiphenyl-4-carboxylate (TFMHPOBC).2 AFLCs have been extensively studied, since they exhibit unique characteristics for device application: dc threshold and double hysteresis., On the other hand, extensive studies have also been carried out to clarify the origin of the antiferroelectric It was reported that the antiferroelec- tric SCA* phase is stabilized by a molecular pairing uiu a dipole-dipole interaction between two molecules.8 Recently, the importance of a dipole component parallel to the smectic layer was also pointed out.g With a view to the construction of new types of display devices,"." many AFLC compounds have been developed and the correlation between the molecular structure and the appear- ance of the antiferroelectric SCA*phase has been investigated. As a consequence, some empirical rules for the design of AFLC compounds have been rep~rted.".'~ According to these rules, a core structure composed of three phenyl rings more reliably induces the antiferroelectric phase than compounds with two phenyl rings.Most of the AFLC compounds have two ester groups, one in the mesogenic unit and the other in the linking group, which align in the same direction and in the same sense. This structure effectively induces polarization and conjugation along the molecular long axis in antiferroelectric molecule^.'^ In practice, however, it is not yet clear how to design new AFLC compounds which show the stable SCA* phase.We have already presented the synthesis of dimeric liquid crystal com- pounds 1 consisting of two chiral centres with two CF, groups in an alkylene spacer which connects the two mesogenic units, a dimer model of TFMHPOBC.159'6 These dimers exhibited a highly stabilized SCA*phase dependent on the odd-even number of the alkylene spacer. In this article, we report the synthesis of dimeric liquid crystals 1 and 2 having CF, or CH, groups at the chiral centres and discuss the importance of the electric interaction in addition to the conformational effect on the appearance of the SCA*phase.Experimental Synthesis Two series of dimeric liquid crystal compounds were synthe- sized as shown in Schemes 1 and 2. The chiral diol 3 was prepared by the treatment of the Grignard reagent obtained from 1,5-dibromopentane with (S)-(trifluoromethy1)oxirane (75% ee). Esterification of 3 with 4-benzyloxybenzoic acid followed by debenzylation of the resulting 4-benzyloxybenzoate 4 afforded bisphenol 5 as a diastereoisomeric mixture (S,S :S,R =3 :1). Resolution of the diastereoisomeric mixture of 5 by HPLC (Daicel, Chiralpak AD, hexane-propan-2-01, 7 : 1) afforded (S,S)-5 which was condensed with 4'-octyloxybi- phenyl-4-carboxylic acid to give rise to l(7).A similar trans- formation with 1,8-dibromooctane and (S)-propylene oxide (> 96% ee), followed by esterification and debenzylation, afforded bisphenol8.Bisphenol8 was also condensed with the corresponding acid to give rise to 2(,0,. In a similar manner 1(8)-1(12),2(*)and 2(11)-2(12) were obtained. The structures of the target products as well as intermediates were confirmed by IR, 'H NMR (J values are given in Hz) and mass spectral data, specific rotation ([@IDvalues are given in units of lo-' deg cm2 g-') and elemental analysis. J. Muter. Chem., 1996, 6(5), 753-760 753 iii / 3 Scheme 1 Reagents 1, Mg, 11, CuI, (S)-(trifluoromethyl)oxirane, 111, PhCH,OC,jH,COOH, PPE, IV, H2, 10% Pd-C, V, C8H,,0C6H,C6H4COOH, DCC, DMAP Scheme 2 Reagents I, Mg, 11, CuI, (S)-propylene oxide, 111, PhCHzOC,H,COOH, DCC, DMAP, IV, H2, 10% Pd-C, V, C8H170C6H4C6H4COOH,DCC, DMAP l,l,1,11,11,11-Hexafluoroundecane-2,10-dio1(3).In the pres- ence of a catalytic amount of copper(1) iodide (500 mg, 2 6 mmol), (S)-3,3,3-Trifluoro-1,2-epoxypropane(75% ee, 6 6 g, 59 mmol) was added slowly to the Grignard reagent prepared by the treatment of 1,5-dibromopentane [7 g, 30 mmol in tetrahydrofuran (THF) 90 ml] and magnesium (1 5 g, 62 mmol) The reaction mixture was stirred at room tempera- ture for 3 h, quenched with 2 M hydrochlonc acid, and then extracted with diethyl ether After removal of the solvent by evaporation, the crude product was purified by column chrom- atography (hexane-thy1 acetate, 5 1) followed by distillation (1 10 "C/O 1Torr) to give the title compound 3 (2 6 g, 29%) as a colourless oil, dB(CDC13) 1 30-1 48 (m, 8 H), 151-1 74 (m, 6 H), 2 06 (d, J 6 2, 2 H), 3 92 (m, 2 H), v(neat)/cm-l 3400, 2940, 2870, 1280, 1170, 1140, 700, m/z (re1 intensity) 297 (M' + 1, 30%), 139 (46), 90 (28), 73 (24), 70 (22), 69 (28), 68 (28), 67 (32), 57 (22), 55 (loo), 43 (41), 42 (23), 41 (80), 39 (28), 31 (25), 27 (32) l,l,l,ll,ll,ll-Hexafluoroundecane-2,lO-diylbis( 4-benzyloxy- benzoate) (4) Ethyl polyphosphate (solution win chloroform, 3 ml) was added to a mixture of 1,1,1,11,11,1l-hexafluoro-undecane-2,lO-diol [0 28 g, 0 94 mmol, (S,S) (S,R)=3 13 and 4-benzyloxybenzoic acid (048 g, 2 1mmol) suspended in dichloromethane (2 ml) The reaction mixture was stirred at room temperature for 17 h, quenched with sat aq sodium hydrogen carbonate and extracted with diethyl ether After removal of the solvent, the crude product was purified by column chromatography (hexane+thyl acetate, 9 1) to afford the title compound 4 [0 49 g, 72%, (S,S) (S,R)=3 11 as a colourless oil, dH(CDC1,) 124-1 46 (m, 10 H), 1 82 (br q, J 7 3, 4 H), 5 13 (s, 4 H), 5 50 (sextet, J 6 8, 2 H), 7 01 (d, J 9 0, 4 H), 7 3-7 42 (m, 10 H), 8 02 (d, J 9 0, 4 H), v(neat)/cm-l 2950, 2880, 1730, 1600, 1510, 1260, 1170, 1100, 1010, 850, 770, 740, 700, m/z (re1 intensity) 718 (M++2, trace), 716 (M', trace), 302 (34%), 211 (51), 121 (21), 91 (100) l,l,l,ll,ll,ll-Hexafluoroundecane-2,lO-diyl bis( 4-hydroxy- benzoate) (5).Debenzylation of 4 was carried out by stirring 6 [0 4 g, 0 56 mmol, (S,S) (S,R)=3 11 with 10% Pd-C (002 g) in ethyl acetate (6 ml) for 3 h Filtration of the catalyst, concentration, followed by column chromatography (hexane- ethyl acetate, 3 1) gave the title compound 5 [030 g, 99%, (S,S) (S,R)=3 11 Pure (S,S)-5 was obtained by HPLC separa- tion (Daicel, Chiralpak AD, hexane-propan-2-01, 7 1) as a COlOUrleSS Oil, [O!]D20 -58 8 (C =105, CHCl,), dH(CDC13) 120-1 40 (m, 10 H), 1 82 (br q, J 7 3, 4 H), 5 49 (sextet, J 6 7, 2H), 551 (s, 2H), 688 (d, J 88, 4H), 799 (d, J 88, 4H), v(neat)/cm-' 3400, 2940, 1700, 1610, 1510, 1440, 1260, 1170, 1100, 850, 770, 700, m/z (re1 intensity) 536 (M+, trace), 138 (15%), 121 (100) (S,S)-l,l,l,ll,ll,1l-Hexafluoroundecane-2,l0-diyl bis(4-[ 4-octyloxybiphenyl-4-yl)carbonyloxy] benzoate) [l,,,].Dicyclo-hexylcarbodiimide (1 15 mg, 0 56 mmol) was added to a solu- tion of 5 (0 1 g, 0 19 mmol), 4'-octyloxybiphenyl-4-carboxylic acid (125 mg, 0 38 mmol), dimethylaminopyridine (DMAP) (30 mg, 0 24 mmol) in dichloromethane (6 ml), and the result- ing solution was stirred at room temperature for 7 h before filtration of the precipitated material Concentration zn vucuo followed by column chromatography (hexane-dichlorometh- ane, 2 1 ) and recrystallization (hexane-dichloromethane, 10 I), gave the title compound (172 mg, 8O%), [O!]DZo -39 8 (c= 105, CHCI,), &(CDCl,) O 90 (t, J 7 0, 6 H), 124-1 54 (m, 30 H), 177-1 90 (m, 8 H), 401 (t, J 6 6, 4 H), 5 55 (sextet, J 6 6, 2 H), 7 00 (d, J 8 8,4 H), 7 36 (d, J 8 8, 4 H), 7 59 (d, J 8 8, 4 H), 7 70 (d, J 8 6, 4 H), 8 17 (d, J 8 8, 4 H), 8 23 (d, J 8 6, 4H), v(KBr)/cm-' 2930, 1740, 1600, 1260, 1180, 1160,830,770 (Found c, 69 89, H, 6 49 Calc for C67H74F6010 C, 69 78, H, 6 47%) Other homologues 1,81-1(121were prepared in a similar manner (S,S)-l,l,l,12,12,12-Hexa~uorododecane-2,1l-dzylbzs{ 4-[( 4'- octyloxybzphenyl-4-yl)carbonyloxy]benzoate} [l,,,] [O!]DZ0 -390 (c=lOl, CHCl,), dH(CDC13) 090 (t, J 70, 6 H), 1 24-1 54 (m, 32 H), 1 77-1 92 (m, 8 H), 4 02 (t, J 6 6, 4 H), 5 55 (sextet, J 6 6, 2 H), 7 01 (d, J 8 8, 4 H), 7 36 (d, J 8 8, 4 H), 7 59 (d, J 8 8, 4H), 7 70 (d, J 8 6, 4H), 8 17 (d, J 8 8,4H), 823 (d, J 86, 4H), v(KBr)/crn-l 2930, 1735, 1600, 1260, 1180, 1160, 1070, 830, 770 (Found C, 70 08, H, 6 52 Calc for C68H76F6010 C, 69 97, H, 6 56%) (S,S)-1,1,1,13,13,13-Hexa~uorotrzdecane-2,12-dzylbzs(4-[( 4'- octyloxybzphenyl-4-yl)carbonyloxy]benzoate) [l,,,] [a]D20 -33 8 (c=I 01, CHCI3), d~(cDCl3) 090 (t, J 70, 6 H), 1 23-1 53 (m, 34 H), 177-1 92 (m, 8 H), 401 (t, J 6 6, 4 H), 5 55 (sextet, J 6 6, 2 H), 7 01 (d, J 8 8,4 H), 7 36 (d, J 8 8, 4 H), 759 (d, J 88,4H), 770(d, J 86,4H), 8 17(d, J 88,4H), 823 (d, J 8 6, 4H), v(KBr)/cm-' 2920, 2850, 1730, 1600, 1260, 754 J Muter Chem, 1996, 6(5),753-760 1175, 1160, 1110, 1060, 830, 765 (Found: C, 70.15; H, 6.80.Calc. for C,j9H78F,Olo: C, 70.15; H, 6.66%). (S,S)-1,1,1,14,14,14-Hexa~uorotetradecane-2,13-diyl bis(4-[( 4'-octyloxybiphenyl-4-yl)carbonyloxy]benzoate} Cl(lOI* [a]~~'-40.5 (c= 1.01, CHC1,); d~(cDC1,) 0.90 (t, J 7.0, 6 H), 1.21-1.54 (m, 36 H), 1.77-1.92 (m, 8 H), 4.02 (t, J 6.6, 4 H), 5.55 (sextet, J 6.7, 2 H), 7.01 (d, J 8.8,4 H), 7.36 (d, J 8.8,4 H), 7.59 (d, J 8.8,4 H), 7.70 (d, J 8.5, 4 H), 8.17 (d, J 8.8,4 H), 8.23 (d, J 8.5, 4H); v(KBr)/cm-' 2930, 2850, 1735, 1600, 1260, 1180, 1160, 1100, 1070, 1010, 830, 765 (Found: C, 70.20; H, 6.81. Cak.for C~OH~OF~O~O: c, 70.34; H, 6.75%). (S,S)-1,1,1,15,15,15-Hexu~uoropentudecane-2,14-d~yl bis(4-[( 4'-oxtyloxybiphenyl-4-yl)carbonyloxylbenzoate} [l,,l,]. [a]D2' -38.3 (C= 1.01, CHC1,); &(CDCl,) 0.90 (t, J 7.0, 6 H), 1.20-1.53 (m, 38 H), 1.77-1.92 (m, 8 H), 4.01 (t, J 6.6, 4 H), 5.55 (sextet, J 6.6, 2 H), 7.01 (d, J 8.8, 4 H), 7.36 (d, J 8.8, 4 H), 7.60 (d, J 8.8, 4 H), 7.70 (d, J 8.6, 4 H), 8.17 (d, J 8.8, 4 H), 8.23 (d, J 8.6, 4H); v(KBr)/cm-' 2930, 2850, 1735, 1600, 1260, 1160, 11 15, 1165, 830, 765 (Found: C, 70.46; H; 6.92.Calc. for C,,Hg2F6Olo: C, 70.52; H, 6.83%). (S,S)-1,1,1,16,16,16-~exu~uorohexadecane-2,15-diyl bis(4-[( 4'-octyloxybiphenyl-4-yl)carbonyloxylbenzoate} [1(12J. [a]~~'-39.5 (c= 1.05, CHCl,); GH(CDCI3) 0.90 (t, J 7.0, 6 H), 1.20-1.53 (m, 40 H), 1.78-1.93 (m, 8 H), 4.02 (t, J 6.6, 4 H), 5.56 (sextet, J 6.6, 2 H), 7.01 (d, J 8.8, 4 H), 7.36 (d, J 8.8, 4 H), 7.60 (d, J 8.8, 4H), 7.71 (d, J 8.6, 4 H), 8.17 (d, J 8.8, 4 H), 8.23 (d, J 8.6, 4 H); v(KBr)/cm-' 2930, 2860, 1735, 1600, 1260, 1180, 1160, 1070, 830, 770 (Found: C, 7.082; H, 7.08.Calc. for C~~H,,F~O~O:c, 70.69; H, 6.92%). (S,S)-Tetradecane-2,13-diol (6). To the Grignard reagent prepared by the treatment of 1,8-dibromooctane (1.7 g, 6.2 mmol) in THF (8 ml) and magnesium (0.43 g, 18 mmol), (S)-propylene oxide (>96% ee, 0.85 g, 15 mmol) was added slowly in the presence of a catalytic amount of copper(1) iodide (0.130 g, 0.68 mmol). The reaction mixture was stirred at 0°C for 10 h and then quenched with 2 M hydrochloric acid. The mixture was extracted with diethyl ether. The solvent was removed by evaporation and the crude product was recrys- tallized from hexane-ethyl acetate (7: 1) to give the title compound 6 (0.59 g, 41%); mp 61 "c; [a],''+ 10.0 (c= 1.04, CHCI,); bH(CDC13) 1.19 (d, J 6.2, 6 H), 1.25-1.50 (m, 22 H), 3.75-3.84 (m, 2 H); v(KBr)/cm-' 3350, 2920, 2850, 1470, 1370, 1130, 1040, 1015, 945, 720cm-'; m/z (rel. intensity) 231 (Mf + 1, trace), 83 (20%), 69 (23), 55 (29), 45 (loo), 43 (22), 41 (22).(S,S)-Tetradecane-2,13-diyl bis(4-benzylox ybenzoate) ( 7). DCC (0.80g, 3.9mmol) was added to a solution of (S,S)-tetradecane-2,13-diol (0.40 g, 1.5 mmol), 4-benzyloxybenzoic acid (0.8 g, 3.5 mmol) and DMAP (80mg, 0.65 mmol) in dichloromethane (20 ml), and the resulting solution was stirred at room temperature for 24 h. The precipitated material was filtered, and the solvent was removed under reduced pressure. The residue was purified by column chromatography (hexane-ethyl acetate, 9 : 1) to give the title compound 7 (380 mg, 39%) as a colourless oil; [alD2' +32.9 (c =1.03, CHCI,); dH(C~cl,) 1.20-1.45 (m, 16H), 1.31 (d, J 6.3, 6H), 1.52-1.62 (m, 2 H), 1.64-1.74 (m, 2 H), 5.07-5.15 (m, 2 H), 5.12 (s, 4 H), 6.99 (d, J 9.0,4 H), 7.30-7.45 (m, 10 H), 7.99 (d, J 9.0,4 H); v(neat)/cm-' 2930, 2860, 1710, 1600, 1510, 1450, 1380, 1280, 1250, 1165, 1100, 1020, 1000, 845, 770, 700; m/z (rel.intensity) 651 (M' + 1, trace), 149 (27%), 91 (loo), 83 (20), 55 (36), 43 (24), 41 (38), 28 (21). (S,S)-Tetradecane-2,13-diyl bis( Qhydroxybenzoate) (8). A mixture of 7 (0.38 g, 0.58 mmol) and 10% Pd-C (0.02g) in ethyl acetate (6 ml) was stirred under an atmospheric pressure of hydrogen at room temperature for 5 h. Filtration of the catalyst, concentration in uucuo, followed by purification by column chromatography (hexane-thy1 acetate, 3 :1), afforded the title compound 8 (0.23 g, 84%) as a colourless oil; [C!lD20+22.2 (c=0.99, CHCl,); dH(CDC13) 1.16-1.41 (m, 16 H), 1.32 (d, J 6.3, 6 H), 1.53-1.63 (m, 2 H), 1.65-1.75 (m, 2 H), 5.09-5.19 (m, 2 H), 6.1-7.1 (br, 2 H), 6.88 (d, J 8.9, 4 H), 7.94 (d, J 8.9,4 H); v(neat)/cm-' 3350,2920,2850, 1680, 1605, 1590, 1510, 1440, 1355, 1310, 1280, 1230, 1160, 1110, 850, 770, 700, 620; m/z (rel.intensity) 470 (M', 2%), 139 (loo), 121 (67). (S,S)-Te tradecane-2,13-diyl bis{4-[( 4-oct ylox ybiphen yl-4-yl )-carbonyloxy] benzoate} [2(1J.DCC (245 mg, 1.2 mmol) was added to a solution of 8 (230mg, 0.49 mmol), 4'-octyloxybiphenyl-4-carboxylicacid (0.35 g, 1.1 mmol) and DMAP (10 mg, 0.08 mmol) in dichloromethane (10 ml), and the resulting solution was stirred at room temperature for 24 h.The precipitated material was filtered, and the filtrate was concentrated. The residue was purified by column chroma- tography (dichloromethane). The product was further purified by recrystallization from hexane-dichloromethane (5 : 1) to give analytically pure title compound 2(,,, (0.45 g, 85%); [a]D2'+22.9 (c= 1.02, CHCl,); GH(CDC1,) 0.90 (t, J 6.9, 6 H), 1.24-1.52 (m, 36 H), 1.34 (d, J 6.3, 6 H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4 H), 4.02 (t, J 6.6, 4H), 5.16 (br sextet, J 6.3, 2H), 7.01 (d, J 8.7, 4H), 7.31 (d, J 8.7, 4 H), 7.59 (d, J 8.7, 4 H), 7.70 (d, J 8.5, 4 H), 8.13 (d, J 8.7, 4 H), 8.23 (d, J 8.5, 4 H); v(KBr)/cm-' 2920, 2850, 1730, 1720, 1600, 1500, 1270, 1200, 1160, 1115, 1070, 1010, 830, 770, 690 (Found: c, 77.10; H, 8.14.Calc. for C70H86010: c, 77.32; H, 7.97%). Compounds 2(8), 2(,,,and 2(12Jwere prepared in a similar manner. (S,S)-Dodecane-2,ll-diyl bis(4-[( 4'-octyloxybiphenyl-4-yl)-carbonyloxylbenzoate) ~2(~,].[C!]DZo +23.2 (c=1.02, CHC1,); dH(CDC1,) 0.90 (t, J 7.0, 6 H), 1.24-1.52 (m, 32 H), 1.34 (d, J 6.2, 6H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4 H), 4.02 (t, J 6.6, 4 H), 5.16 (br sextet, J 6.3, 2H), 7.00 (d, J 8.8, 4H), 7.31 (d, J 8.8, 4H), 7.59 (d, J 8.8, 4 H), 7.70 (d, J 8.7, 4H), 8.13 (d, J 8.8, 4 H), 8.23 (d, J 8.7, 4H); v(KBr)/cm-l 2930, 2850, 1730, 1720, 1600, 1265, 1190, 1160, 1110, 1070, 1010, 830, 765 (Found: C, 76.86; H, 8.01.Calc. for C68H82010: C, 77.10; H, 7.80%). (S,S)-Pentadecane-2,14-diylbis(4-[( 4-octyloxybiphenyl-4-yl)-carbonyloxy]benzoate} [2(,,,]. [aID2'+21.8 (c =1.05, CHC1,); dB(CDC13) 0.90 (t, J 7.0, 6 H), 1.24-1.52 (m, 38 H), 1.34 (d, J 6.2, 6H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4 H), 4.02 (t, J 6.6, 4 H), 5.16 (br sextet, J 6.2, 2H), 7.01 (d, J 8.8, 4H), 7.31 (d, J 8.8, 4H), 7.59 (d, J 8.8, 4H), 7.70 (d, J 8.6, 4H), 8.13 (d, J 8.8, 4H), 8.23 (d, J 8.6, 4H); v(KBr)/cm-l 2900, 2850, 1730, 1710, 1600, 1260, 1190, 1160, 1110, 1070, 1010, 830, 760 (Found: C, 77.28; H, 8.20. CdC. for C71H88010: c, 77.42; H, 8.05Yo). (S,S)-Hexadecune-2,15-diylbis(4-[( 4-octyloxybiphenyl-4-yl)-carbonyloxy]benzoate) [2(12J.[a]D2' +22.9 (c = 1.01, CHCI,); GH(CDC1,) 0.90 (t, J 7.0, 6 H), 1.24-1.52 (m, 40 H), 1.34 (d, J 6.2, 6 H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4H), 4.02 (t, J 6.6, 4H), 5.16 (br sextet, J 6.2, 2H), 7.01 (d, J 8.8, 4H), 7.31 (d, J 8.7, 4H), 7.59 (d, J 8.8, 4H), 7.70 (d, J 8.5, 4H), 8.13 (d, J 8.7, 4H), 8.23 (d, J 8.5, 4H); v(KBr)/cm-' 2900, 2850, 1730, 1715, 1600, 1500, 1465, 1265, 1200, 1160, 1110, 1070, 1010, 830, 760, 690 (Found: C, 77.42; H, 8.30. Calc. for C7,H9oOlo: C, 77.53; H, 8.13%). Measurements The phase and transition temperatures of the compounds were determined by polarizing optical microscopy using an Olympus J. Muter. Chem., 1996, 6(5),753-760 755 BH2 instrument equipped with a temperature controller (Mettler FP82) and by differential scanning calorimetry (DSC) using a Rigaku DSC-8240D instrument with a TAS-200 data analysis system.Homogeneously aligned cells of ca. 2 pm thickness were prepared by rubbing polyimide thin films coated on the substrate glass plates with indium tin oxide (ITO). The spontaneous polarization was measured by the triangu- lar wave voltage method (5 Hz). The electrooptic properties were measured by applying a triangular wave voltage of 0.01 Hz frequency. The light transmittance through the sample cell was detected by a photomultiplier tube. The apparent tilt angle was determined by the extinction direction between crossed Nicols as a function of electric field.Results Phase sequences and transition temperatures The phase sequences and the transition temperatures of l(,,) and Z(,)were determined by DSC measurement, thermal polar- izing optical microscopy and their electrooptic behaviour. As shown in Fig. 1, the dimers l(,,)enantiotropically exhibit the SCA*phase or the Sc* phase depending on whether n Fig. 1 Phase sequences and transition temperatures of dimeric liquid crystals l(")and 2,"). The appearance of Sc* in parentheses indicates that it appears after field application and/or in thin cells (see text). (nd9) is odd or even, respectively. For an explanation of use of Sc* in parentheses, see below (microscopic observation section). This odd-even behaviour may be attributed to a conformational effect of the alkylene spacer -(CH,), -, as we In our previous paper,16 the chiral smectic phase in l(lo)was assigned to S,*.In the present study, we noticed that the SCA*phase emerges instead of the S,* phase, when the cell thickness is thick enough, as will be shown later. For 2,,,, the phase sequences are the same as those of l(,)for n d 11, and are also shown in Fig. 1. In Fig. 2, the molecular alignment based on the confor- mational effect of the alkylene spacer is schematically illus- trated. For odd values of n [e.g. l,,,], the two mesogenic groups are expected to align in a zigzag manner (a),whereas for even values of n [e.g. l,,,], they are arranged in a parallel manner (b).This odd-even rule is no longer valid when n is large as in l(lO),1(12), Z(10)and 2(12).We should note that l(,,)and Z(,,)with odd n values always show the Iso-SCA* phase transition. This is an unusual phase sequence in normal monomeric compound systems. The direct transition from Is0 to SCA*is interpreted by the suppression of the SA phase due to the bent structure shown in Fig.2(u). In fact the same phase sequence was observed for main chain polymers, in which mesogens are connected by a polymethylene chain with odd numbers of CH2 gro~ps.'~-'~ Microscopic observation In order to identify the phases in l(,)and 2,,,, texture obser- vation was made. The micrographs of the homogeneously aligned cells of 2(,) are shown in Plate 1; (a)a 2 pm cell of Z18, at 160"C, (b)a 3.5 pm cell of 2(10,at 155 "C and (c) a 9 pm cell of 2(,,, at 145 "C.In Plate l(a), two domains with different colours are observed and are attributed to twisted states in the Sc* phase. Hence we can conclude that 2(8)shows Sc* phase at 160°C. In Plate l(b) and (c), a typical herringbone texture was observed and the extinction direction under the crossed polarizers is parallel to the layer normal, suggesting the presence of the antiferroelectric SCA*phase at 155°C in 2(,,, and at 145 "C in Z(',). However, the situation is not so simple. The SCA*phase irreversibly changes to the Sc* phase on application of a field. After the application of a triangular voltage wave (0.1 Hz, Fig. 2 (a)Molecular orientational structures of the antiferroelectric SCA*phase of 1(7)and (b)the ferroelectric Sc* phase of l,,, 756 J.Muter. Chem., 1996, 6(5),753-760 Plate 1 Optical micrographs of (a)a 2 pm cell of 2(*, at 160"C, (b)a 3.5 pm cell of 2(,,, at 155 "Cand (c) a 9 pm cell of 2(,,, at 145 "C +5 V) to the 3.5 pm cell of 2(,,,, the texture changes from Plate 2(u) to (b).In Plate 2(b), ferroelectric domains appear, although antiferroelectric domains still remain. Thus, the SCA*phase is so unstable that the application of an electric field easily converts the phase to Sc*. The phase also depends on the cell thickness. Plate 3 shows the textures of 2(,,, and 2(12,.In thin cells they show the S,* phase, while they show the SCA*phase in thick cells. We confirmed that 1(,,, also shows the Sc* phase in a thin cell as already reportedi6 and that 4,,,shows the SCA*phase even in thin cells such as 2.5 pm, as shown in Plate 4.In this respect, l,,, has relatively strong antiferroelectricity compared with 2(n). In Fig. 1, shown above, Sc* in parentheses means that it appears after field application and/or in thin cells. Elec troop tic response Dimers with an odd value of n (n=7, 9 and 11) did not respond electrooptically, indicating the presence of the stable SCA*phase. In contrast, an electrooptic response was obtained in dimers with an even value of n (a= 8, 10 and 12). As already shown in Fig. 2(u) in our previous paper,16 a 2.5 pm thick cell of 1(12)shows antiferroelectric switching, while a 2.5 pm thick cell of l,,,, shows ferroelectric switching.In a thick cell, however, l(,,)shows antiferroelectric behaviour as shown in Fig. 3. In this figure, which displays the antiferroelec- Plate 2 Optical micrographs of the 3.5 pm cell of 2(,,, at 155"C (a) before and (b) after the application of a triangular voltage wave (0.1 Hz, +5 V) Plate 3 Optical micrographs of the thin and thick cells of (a)2,,,! at 140 "C(thin) and at 155 "C (thick) and (b)2(12)at 145 "C(thin and thick) tric transmittance change on application of a triangular voltage wave, one of the crossed polarizers was set parallel to the smectic layer. Fig. 4 shows the dependence on thickness of the electrooptic response in 2(,,, for (a) a 2 pm thick cell at 120"C, (6) a 3.5 pm thick cell at 155 "C and (c) a 7 pm thick cell at 150 "C.In the measurement showing ferroelectric switching [Fig. 4(u)],one of the polarizers was adjusted so as to obtain a dark state for one of the ferroelectric states. The thin [Fig. 4(a)] and thick J. Muter. Chem., 1996, 6(5),753-760 757 Plate 4 Optical micrographs of the 2.5 pm cells of (a) l(lo)at 130°C and (b)1(,,, at 120°C I 1I I I -20 -10 0 10 20 applied voltageN Fig. 3 Electrooptic responses in l(lo) [Fig. 4(c)] cells exhibit ferroelectric and antiferroelectric behav- iour, respectively. In the medium thick (3.5 pm) cell, ferroelec- tric behaviour overlaps with antiferroelectric behaviour because of the coexistence of the S,* and SCA*domains, as shown in Plate 2(6). The phase of 2(12) is also dependent on the cell thickness, while 1(12)always shows the SCA*phase even in thin cells.16 Fig.5 shows ferroelectric [Fig. 5(u)] and antiferroelectric [Fig. 5(b)] switching behaviour in thin (2.5 pm) and thick (9 pm) cells, respectively. Spontaneous polarization and tilt angle Fig. 6 shows the temperature dependence of the spontaneous polarization P, in five dimers. The temperature dependence and the absolute values are almost the same. The absolute values are much smaller than those in the monomers (MHPOBC and TFMHPOBC).2 This may be because the 758 J. Muter. Chem., 1996, 6(5),753-760 I 1 1 I -20 -10 0 10 20 I.'.'I....I ....-C E' $-a'b I ....1 ,,..I ..., -10 -5 0 5 10 -10 -5 0 5 10 applied voltageN Fig. 4 Thickness dependence of the electrooptic responses in 2(10,; (a) 2 pm, 120"C, (b)3.5 pm, 155 "C and (c) 7 pm, 150°C intramolecular directional relationship between the dipole moments of the carbonyl and CH, or CF, groups is fixed by the molecular conformation.The tilt angles of these dimers were also measured. The temperature dependences of the tilt angle in five dimers are shown in Fig. 7. The tilt angle of l,,,is larger than that in 2(") by about 5". Discussion Switching behaviour It is possible to consider a switching model of the mesogenic unit of a dimer and a monomer in the antiferroelectric SCA* phase, as illustrated in Fig. 8(u) and (b), respectively. In the dimer, the mesogenic units meet strong friction, since the two mesogenic units are interlinked and can only rotate around the end of the alkylene chain along a large cone.Thus, the antiferroelectric phase is highly stabilized and a higher energy is required for the switching of one of the mesogenic units, in other words for the conformational change of the dimer molecules, than for the switching in the monomer assembly, which can switch without changing the centre of mass of the molecules. This schematic model is consistent with the exper- imental results in that electrooptic switching cannot be I 1 I I I I D r r le I B-31::Y 8 -4 -2 0 2 4 c .... .... ....,.." .g I(b) -10 -5 0 5 10 applied voltageN Fig. 5 Thickness dependence of the electrooptic responses in 2(12,; (a) 2.5 pm, 150 "C and (b) 9 pm, 130 "C 60 50 .- OR .y 4052 30 .. 4" 20 lo t2 oli . I I I I 0 10 20 30 40 T-T, /"C 301 1I I I Lt 8200, 0 0 10 20 30 40 T,-TI"C Fig. 7 Temperature dependence of tilt angle in five dimers; (0)l(lo), (0)1(12)>(A)2@,>(m) 2(10,,(0)412, Fig. 8 Switching models in dimer (a)and monomer (b) systems Fig. 9 Formation of the anticlinic structure in l(lo,,1(,,,, 2(,,, and 2(12, Origin of stabilizationof AFLC For the inherent stabilization of the antiferroelectric ordering, the pairing of transverse dipole moments in the neighbouring layers is believed to be the major interaction in the formation of the antiferroelectric SCA* phase.* As mentioned above, antiferroelectric SCA*or ferroelectric Sc* appear depending on whether there is an odd or even number of the alkylene spacer -(CH,),-, respectively.Hence, at least in the dimer systems, a conformational effect is responsible for the formation of the antiferroelectric SCA* phase. In l(,,),1(12),2(,,, and 2(,,,, however, the dimeric mesogenic units tend to form the anticlinic structure, as shown in Fig. 9(b), resulting in the emergence of the SCA*phase instead of the Sc* phase. We can interpret this phenomenon as follows. Some electric interaction such as a dipole-dipole interaction between mesogenic units overcomes the conformational effect due to the increased flexibility in the longer alkylene chain. Thus, the antiferroelectric SCA*phase appears, even if the value of n of -(CH,), -is even.Conclusion A series of novel dimeric liquid crystals l(,)and 2(,) were synthesized. These dimers were found to show the antiferroelec- tric SCA* or the ferroelectric Sc* phases depending on whether they have an odd or even number of n in the alkylene spacer -(CH,), -chain, respectively. This odd-even rule becomes invalid when n increases: 1(,,), 1(,,), 2(,,, and 2(,,, exhibit an SCA*phase, though n is even. This fact indicates that an electric interaction is important for the stabilization of the SCA*phase. Thus, there exist two major forces responsible for the stabiliz- ation of the antiferroelectric SCA* phase, conformational and electric interactions. The emergence of the SCA* phase is governed by the balance between these two interactions.A complicated phase behaviour which depends on field appli- cation and cell thickness is also reported. The details will be reported in a separate paper. This work was partly supported by a Grant-in-Aid for Scientific Research (Specially Promoted Research No. 06102005) from the Ministry of Education, Science, Sports and Culture. J. Muter. Chem., 1996, 6(5), 753-760 759 References 1 K Furukawa, K Terashima, M Ichihashi, S Saitoh, K Miyazawa and T Inukai, Ferroelectrzcs, 1988,85, 63 2 Y Suzuki, T Hagwara, I Kawamura, N Okamura, T Kitazume, M Kakimoto, Y Imai, Y Ouchi, H Takezoe and A Fukuda, Liq Cryst, 1989,6,167 3 A D L Chandani, T Hagiwara, Y Suzuki, Y Ouchi, H Takezoe and A Fukuda, Jpn J Appl Phys ,1989,28, L1261 4 A D L Chandani, E Gorecka, Y Ouchi, H Takezoe and A Fukuda Jpn J Appl Phys, 1989,28, L1265 5 H Takezoe, K Hiraoka, T Isozaki, K Miyachi, H Aoki and A Fukuda, in Modern Topics in Liquid Crystals-From Neutron Scattering to Ferroelectriczty, ed A Buka, World Scientific Publishing, Singapore, 1994, p 290 6 T Isozaki, T Fujikawa, H Takezoe and A Fukuda, Phys Rev B, 1994,48,13439 7 A Fukuda, Y Takanishi, T Isozaki, K Ishikawa and H Takezoe, J Mater Chem ,1994,4,997 8 Y Takanishi, K Hiraoka, V K Agrawal, H Takezoe, A Fukuda and M Matsushita, Jpn J Appl Phys ,1991,30,2023 9 K Miyachi, J Matsushima, Y Takanishi, K Ishikawa, H Takezoe and A Fukuda, Phys Rev E, 1995,52, R2153 10 Y Yamada, N Yamamoto, K Mon, K Nakamura, T Hagwara, Y Suzuki, I Kawamura, H Onhara and Y Ishibashi, Jpn J Appl Phys, 1990,29,1757 11 Y Yamamoto, Y Yamada, N Koshobu, K Mori, K Nakamura, H Onhara, Y Ishibashi, Y Suzuki and I Kawamura, Jpn J Appl Phys, 1992,31,3186 12 Y Suzuki, 0 Nonaka, Y Koide, N Okabe, T Hagiwara, I Kawamura, N Yamamoto, Y Yamada and T Kitazume, Ferroelectrzcs, 1993, 147, 109 13 I Nishiyama and J W Goodby, J Mater Chem , 1992,2,1015 14 I Nishiyama, E Chin and J W Goodby, J Muter Chem, 1993, 3,161 15 Y Suzuki, T Isozaki, T Kusumoto and T Hiyama, Chem Lett, 1995,719 16 T Kusumoto, T Isozaki, Y Suzuki, Y Takanishi, H Takezoe, A Fukuda and T Hiyama, Jpn J Appl Phys ,1995,34, L830 17 J Watanabe, H Komura and T Niion, Lzq Cryst, 1993,13,455 18 Y Nakata and J Watanabe, J Mater Chem ,1994,4,1699 19 J Watanabe and M Hayashi, Macromolecules, 1988,21,278 Paper 5/07080E, Received 26th October 1995 760 J Mater Chem, 1996,6(5), 753-760

ISSN:0959-9428    

DOI:10.1039/JM9960600753

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Oriented cadmium oxide thin solid films Metodija 2. Najdoski," Ivan S. Grozdanov and Biljana Minceva-Sukarova Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Sts Cyril and Methodius University, POB 162, Arhimedova 5, 91000 Skopje, Republic of Macedonia A chemical bath deposition technique has been developed for the preparation of Cd(02)o.88(OH)o.24 thin films on glass and quartz substrates from alkaline media at room temperature. These films were then annealed at 473 K in order to obtain cadmium oxide films. Two baths were used for film preparation. X-Ray diffraction analysis clearly showed that the CdO films obtained from a bath without KBr have a preferred crystalline orientation in the (200) direction while those obtained from a bath containing KBr had only slightly preferred crystalline orientation in the (1 11) direction.The optical bandgap, E,, was evaluated from the VIS absorption spectra. It was found that E, for the films oriented in the (200) direction is 2.63 eV, while it is 2.57 eV for those with the slight (1 11) orientation. The different values of the cross-over terminal points of absorption and transmittance spectra confirm the difference between the films with different crystalline orientation. Thin solid films of cadmium oxide have been prepared by different methods, e.g. ion-beam sputtering,' reac-tive sputtering,' activated reactive evaporation6 and chemical Interest in the preparation of these films is due to their wide applications such as photovoltaic ~ells,'~'~ heat-reflecting coatings, large-area transparent conductors, displays, gas sensors for ambient air, C2HSOH, as well as gas sensors in conjunction with Sn02 films for CO detection, and other electrical devices.One of the most common and low-cost methods for the fabrication of CdO thin films from aqueous solutions is the electrodeless chemical deposition method. By this method, Cd(OH), thin films are obtained initially, which after annealing are converted into CdO thin CdO crystallizes with the sodium chloride str~cture.~ It is an n-type semiconductor with high conductivity which is due to either the presence of anion vacancies or doping effects. As expected, many of its physical properties were found to be dependent on the method and conditions of preparation.In this paper, we present an electrodeless chemical bath deposition method for the fabrication of oriented cadmium oxide thin solid films, along with some basic properties of the as-deposited and annealed flms. Experimenta1 Cd(02)o~88(OH)o~24films were prepared from two baths, A and B, as described below. The overall chemical reaction can be written as shown in eqn. (1). [Cd(NH,)41(OH)2+0.88H202~/cd(02)0.88(OH)0.24 + 1.78H20+4NH, (1) Besides the tetraamino complex of cadmium, bromine com- plexes of cadmium were also formed in the bath containing KBr (bath B; see below). The depositions were carried out in alkaline media (pH z10) at room temperature. Thin films were deposited on planar glass (75 mm x 25 mm x 1 mm) and quartz (40 mm x 10 mm x 1 mm) substrates.Since the temperature required does not exceed 298 K, a glass or plastic laboratory beaker can be used and stirring is necessary for good quality thin films. Preparation of Cd(02)0.88(OH)0.24thin films from bath A Chemical deposition bath A contained a mixture of two solutions. Solution 1 consisted of Cd(N03)2 (17 cm3; c= 1 mol dm-3), NH, (23 cm3; 20%) and deionized water (52 cm3); solution 2 contained H202 (2 cm3; 25%0), deionized water (2 cm3) and NH, (1 cm3; 20%). The substrates (which had been cleaned previously in chro- mic acid) were inserted into the bath. Then, 3 cm3 of deionized water were added in small portions. This volume corresponds to 293 K solution temperature; less water is added for higher temperatures (e.g.1cm3 for 297 K). In cu. 3-10 min, a white precipitate of Cd(02)0.88(OH)0.24 began to fill the bath. The coated substrates were then taken out, rinsed with deionized water and dried in air before annealing. Preparation of Cd(02)0.88(OH)0.24thin films from bath B In order to fabricate very thin films (< 100 nm), we used bath B which was a mixture of the following two solutions. Solution 1 consisted of Cd(N03)2 (6cm3; c=l mol drn-,), NH, (8.5 an3;20%), KBr (5 cm3; c= 1 mol drn-,) and deionized water (52 cm3); Solution 2 contained H202 (2 cm3; 25%0), deionized water (6 cm3) and NH, (1 cm3; 20%). Having combined the two solutions at 293 K, 1 cm3 of deionized water was also added.For higher solution tempera- tures, there is no need to add water. The deposition procedure was the same as that described for bath A. Preparation of CdO thin films Cadmium oxide films were prepared by annealing the Cd(02)0.88(OH)0.24 thin films at 468 K for 30min. Cd(02)o.88(OH)0.24thin films were transformed into CdO according to the following pyrolytic reaction [eqn. (2)]. Films characterization The thickness of the films was determined by the gravimetric method. The sheet resistance of the films was measured between two silver-pasted electrodes, 1 cm in length and 1 cm apart. The deposited films as well as the bulk precipitates were studied by X-ray diffraction, using a JEOL Model JDX diffractometer and nickel-filtered Cu-Kcr radiation.Optical studies were carried out on a (Hewlett Packard) HP 8452 A UV-VIS spectrophotometer. J. Muter. Chem., 1996, 6(5),761-764 761 r 'I"3.05 O! 1I I 1 50 loo 150 200 250 300 t Imin Fig. 1 Thickness of CdO films us. deposition time for bath A 200 I 1 0 20 40 60 80 100 t Imin Fig. 2 Thickness of CdO films us. deposition time for bath B Plate 1 Micrograph of CdO film of about 700 nm thickness (magnifi- cation ca. 200) Results and Discussion The thickness of CdO films at 293 K as a function of the deposition time is shown in Fig. 1 for films obtained from bath A and in Fig. 2 for those from bath B. A terminal thickness of cu. 800 nm was achieved in ca. 275 min from bath A and 170nm thickness for films prepared from bath B.Thicker films up to 500nm were obtained by re-inserting the initially deposited Cd(02)0.88(OH)0,24 films; films thicker than 500 nm could not be obtained from bath B. The sheet resistance varied between 20 and 2 kQ0 -'and were not reproducible. Thicker films had higher sheet resist- ances, a phenomenon reported previously.' We believe that the reason for this unusual behaviour is fracturing of the films, shown in Plate 1. Namely, Cd(02)0.88(OH)0.24 decomposes violently, according to reaction (2), therefore conversion to CdO is difficult to control. The fracturing is probably caused by the liberation of formed gases. X-Ray investigations X-Ray diffractograms of the bulk precipitate from the chemical deposition are shown in Fig.3. During the preparation of the precipitate for X-ray analysis, CO, from the air was chemi- 762 J. Muter. Chem., 1996, 6(5), 761-764 5 15 25 35 45 55 65 75 05 2Bldegrees Fig. 3 XRD pattern of (a) carefully prepared (with minimal exposure to CO,) precipitate; (b) ordinary prepared precipitate with CdCO, peaks present (indicated by *). d Values are shown above the peaks. sorbed. As a result of this, CdCO, reflections were detected in the precipitate, as shown in Fig. 3(b).When precautions were taken (i.e. the precipitate was washed with ethanol, filtered, dried on filtered paper, and at 373 K for 15 min) hardly any CdC03 peaks were detected, as shown in Fig. 3(u).Comparison of these peaks with the standards'' confirmed that the material is Cd (02 )0.88(OH)o.24. XRD patterns of Cd(02)0.8g(OH)0.24 (bulk and films) are shown in Fig. 4.,while the corresponding bulk and films XRD patterns of CdO are shown in Fig. 5. The hkl indices were identified according to available literature XRD analy- sis clearly indicates a highly preferred crystalline orientation in the (200) direction for films prepared from bath A (without KBr), while a slightly preferred crystalline orientation in the (111) direction was noticed for films prepared from bath B. It has been reported13 that the presence of H,O, in the solution influences the crystalline orientation of PbS films in the (200) direction. At the moment, we have no appropriate explanation for the differently preferred orientation of our films obtained from the two baths (A and B). In order to estimate the dptimal annealing time, the phase transformation was studied by XRD analysis of a single film (from bath A; thickness ca.500 nm). Fig. 6 shows the results, Fig. 4 XRD patterns of Cd(02)0.88(OH)0.24: (a) bulk precipitate; (b) films obtained from bath A; (c) films obtained from bath B r I 5 15 25 35 45 552Bldegrees Fig.5 XRD patterns of CdO: (a) bulk precipitate; (b) films obtained from bath A; (c) films obtained from bath B 15 25 35 45 55 2Bldegrees Fig. 6 Gradual chemical conversion detected with XRD: (a) film of Cd(02), ss(OH)O24; (b)-(d) intermediate phases; (e) film of CdO. As deposited (a);annealing times: 2-3 min (b); 4-6 min (c);6-10 min (d); and 16-20 min (e).which indicate that the chemical conversion was completed in less than 30min. Optical investigations In order to study the optical changes during the chemical conversion, a very thin film (ca. 30 nm) of Cd(02)o.8,(OH)o.24 on quartz was deposited from bath B. Successive annealing and spectra recording were performed on this single film. The results are shown in Fig. 7. The notable shifts in the absorption edges correspond to the observed changes from colourless for Cd(02)~.,8(OH)~24 to bright yellow for the CdO films. Owing to the very low thickness of the film, the applied annealing time was shorter than 15 min. Using optical transmission data, in the region between 434 and 660 nm, the room-temperature optical bandgaps were determined from plots of us.E. Extrapolation of the linear plots gave bandgap energies of 2.63 eV for films obtained from bath A and 2.57 eV for those 2b 460 600 Alnm Fig. 7 UV-VIS spectra of the films: (a)film of Cd(O,), ss(OH), 24; (b), (c)mixtures of Cd(02), 88(OH)024 and CdO; (d)film of CdO Table 1 A comparison of CdO films prepared using different deposition methods thickness/ optical deposition method ref. nm bandgaplev ion-beam sputtering 1 500 2.4-2.42 spray pyrolysis chemical bath deposition 1 9 500 - 2.36 2.2 chemical deposition chemical bath deposition 7 this work 320 78, 115 2.6 2.57, 2.63 from bath B. Table 1 gives a comparison of our results with those reported elsewhere.Conclusion Thin solid films of CdO were prepared by chemical bath deposition with thicknesses from ca. 100 to 800 nm (bath A) and from 40 to 170 nm (bath B). XRD analysis showed a highly preferred crystalline orientation in the (200) direction for films prepared from bath A (where no KBr was added), while a slightly preferred crystalline orientation in the (1 11) direction was found for films prepared from bath B. In both cases, the crystalline orientation of the CdO films was the same as the crystalline orientation of the previously deposited films of Cd(O~)o~88(0H)o~,4. During the annealing process, a chemical conversion took place and the liberation of formed gases caused fracturing of the films. As a result, most of the annealed films were not conductive.This is one of the disadvan- tages of our method at the moment. Further efforts will be aimed at finding ways to prevent film fracturing during annealing. References 1 T. L. Chu and S. S. Chu, J. Electron. Muter., 1990,19, 1003. 2 W. Shixing and C. Michael, Chem. Muter., 1993,5, 577. 3 C. Saravani, R. P. Sredhara and R. P. Jayarama, Muter. Lett., 1992, 12,406. 4 I. Moriguchi, I. Tanaka, Y. Teraoka and S. Kagawa, Nippon Kaguku Kuishi, 1991, 10, 1392. 5 C. H. Champness, Y. F. Go and Z. A. Shukri, in Proc. 4th Int. Symp. Uses Selenium and Tellurium, ed. S. C. Carapella, Jr. Selenium-Tellurium Dev. Assoc., Darien, CT, 1989, p. 600. 6 C. Saravani, R. K. T. Ramakrishna and R. Jayarama, Muter. Lett., 1993,15, 356. 7 M. Ristov, Gj. Sinadinovski, I. Grozdanov and M. Mitreski, Ann. Phys., 1990,40, 53. 8 R. L. Call, N. K. Sesan and J. R. Whyte, Sol. Energy Muter., 1980, 2, 375. J. Muter. Chem., 1996,6(5), 761-764 763 9 M Ocampo, A M Fernandez and P J Sebastian, Semicond Sci Technol, 1993,8,750 12 SOC,1959,81,3830 C W Hoffman, R C Ropp and R W Mooney, J Am Chem 10 11 C H Champness and C H Chan, Sol Energy Muter Sol Cell, 1993,30,65 Inorganic Index to the Powder Difraction File, 1972, Joint Committee on Powder Diffraction Standards 13 I C Torriani, M Tomyiama, S Bilac, G B Rego, J I Cisneros and Z P Arguello Thin Solid Films,1981,77, 347 Paper 5/06059A, Received 13th September, 1995 764 J. Mater. Chem., 1996, 6(5),761-764

ISSN:0959-9428    

DOI:10.1039/JM9960600761

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Morphology control of thin LiCoO, films fabricated using the electrostatic spray deposition (ESD) technique Chunhua Chen," Erik M. Kelder, Paul J. J. M. van der Put and Joop Schoonman Laboratory for Applied Inorganic Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Electrostatic spray deposition (ESD) is a technique recently developed for the fabrication of inorganic thin films. Several process steps involved in an ESD process are overviewed. A variety of surface morphologies of LiCoO, thin layers fabricated by this technique are presented, indicating correlations between the morphologies and deposition conditions. Electrostatic atomization of liquids has been investigated for many years. It has been applied in crop spraying' and for painting.However, recently it has been used for the preparation of particles of metal oxides by pyrolysing solution spray droplet^.^,^ Compared with other spray techniques, e.g. ultra-sonic or mechanical atomization, it has the advantage that nonosized droplets may be produced under proper conditions. More recently, it was used for preparing thin films of metal oxides, such as LiMn204, Y,O,-stabilized ZrOz, BaCeO,, L~COO,,~-~and a CdS-polymer composite film.7 This tech- nique may be termed electrostatic spray deposition (ESD), and it has also been called electrostatic spray pyrolysis (ESP) in our previous reports. In addition to the very simple set-up, which is a common advantage of spray film-fabrication tech- niques over techniques using vacuum systems, e.g.physical vapour deposition, the high deposition efficiency attained using this technique appears to be another attractive feature. This is mainly due to a well defined trajectory of spray droplets directed towards the substrate by the electric field. In this respect, it is similar to the so-called corona spray te~hnique."~ However, in the ESD process, the charged aerosol is generated more directly and usually consists of monodispersed primary particles, while the corona spray techniques produces its spray by other means, e.g. ultrasonically combined with electrical discharge. Like other spray deposition techniques, the electrostatic spray deposition technique usually atomizes a precursor solu- tion into an aerosol, which is then directed to a heated substrate to form a thin layer.The history of applying the ESD process to deposit thin films is short, so that only a few reports exist concerning the deposition mechanism. Here we present the results of LiCoO, thin layers prepared by ESD of ethanol solutions containing lithium and cobalt precursors. The focus will be on the control of the morphology of the layers by controlling the deposition parameters. Furthermore, a number of different morphologies produced by this technique will help us to find a unified deposition mechanism model. Processes involved in ESD There are several physical and chemical processes involved in the ESD of layers, occurring either sequentially or simul- taneously.Possible sequential steps are (see also Fig. 1):spray formation; droplet transport, evaporation, disruption; preferen- tial landing of droplets; discharge, droplet spreading, penetra- tion of droplet solution, drying; surface diffusion, reaction. All of these processes can influence the morphology of the deposited layer. They are described below separately. Spray production In the ESD technique a capillary-plate configuration is usually adopted.'O The precursor solution is placed in a container which is connected to a metal capillary tube. When a voltage is applied to the capillary, an electrostatic field is immediately set up across the capillary and the grounded plate. This field also penetrates the liquid surface and acts on the ions in the solution. In the case of a positive potential at the capillary, positive ions move to the surface of the solution at a rate which depends upon the electrical relaxation time constant 5, which in turn depends upon the electrical conductivity K and the absolute permittivity of the solutions E (=ereO, where E, is the relative permittivity of the solution and e0 is the absolute permittivity of free space), as shown in eqn.(1): For ethanol solutions, the absolute permittivity is approxi- mately 2 x lo-'' F m-I." The resistivity of an ethanol solution containing a salt has typical values between lo-' and 1 S m-', therefore its electrical relaxation time is 2 x 10-'-2 x lO-''s, which means the surface charge can develop fully in <<1 ps, while this surface charge completely shields the bulk of the solution so that there is no free charge inside the solution.The surface charge density 0 is given by cr = eOE, where E is the electric field strength. The surface charge causes an outward electrostatic pressure on the solution, which is opposite to the Fig. 1 Processes involved in ESD. 1, spray formation; 2. droplet transport, evaporation, disruption; 3, preferential landing; 4, discharge, droplet spreading, penetration, drying; 5, surface diffusion, reaction. J. Muter. Chem., 1996,6(5), 765-771 765 inward directed pressure from the surface tension. This leads to surface instabilities, which are normally called Rayleigh- Taylor instabilities.12 Taylor has shown that when the electric field is strong enough, the electrostatically stressed liquid surface can be distorted into a stable conical shape (Taylor cone).13 The cone surface is equipotential but the net electric field Esurfthat exists at the surface of a Taylor cone is given by eqn.(2): where y is the surface tension of the liquid with respect to the surrounding gas, a the semi-vertex angle of the cone, and r the radius. An ideal Taylor cone has a semi-vertex angle of 49.3”. At the very apex of a Taylor cone the liquid surface is unstable since according to eqn. (2) the electric field would tend to be infinite with r close to zero. In order to compensate for this ‘infinite’ electric field, the Taylor cone will emit charged droplets immediately after the application of an electric potential.In the so-called cone-jet model4 (as shown in Fig. 1)these primary charged particles are usually monodispersed. Having con-sidered capillary equilibrium, liquid continuity and monument and charge continuity at the jet, Ganan-Calvo deduced the following relation for polar liquids [eqn. (3)]:15 (3) where d and Q are the diameter of the droplets emitted at the jet (the primary droplet size) and the feed rate or flow rate of the solution, respectively. As a result, the primary droplet size depends on the flow rate, the conductivity and the permittivity of the solution. Aerosol transport An electrostatically produced charged droplet of mass m will be attracted towards a grounded substrate by a Coulombic force qEsp, where q and ESPare the droplet charge and the electric field strength in the travelling space, respectively.Simultaneously, a gravitational force, mg (where m is the mass of the droplet and g is the gravitational acceleration constant), and a viscous drag force 37cdyvC (where q is the dynamic viscosity of air, v is the drop velocity and C is a correction coefficient) also act on the droplet. The trajectory as well as the flight time taken from the nozzle to the substrate for this droplet will be determined mainly by these forces. The gravi- tational force may be neglected in the case of electrostatic spraying because droplets produced in this way are very small. Assuming a homogeneous electric field in the travelling space (i.e.a constant ESP)and a short nozzle-to-substrate distance L so that the drag force and the solvent evaporation can be neglected, the flight time t can be calculated according to eqn. (4): t %%J2 (4)( On the other hand, if L is long enough, the equilibrium between the Coulombic force and the drag force determines the terminal velocity of the droplet, i.e.: and In addition, there are space-charge forces arising from the 766 J. Muter. Chem., 1996, 6(5), 765-771 repulsive interaction between charged droplets. Moreover, the real situation is further complicated by: (i) the non-uniform temperature profile and the resulting thermophoresis force; and (ii) the evaporation of the solvent and the resulting possible droplet disruption (see below).These factors also change the flying speed and time. Solvent evaporation and droplet disruption Alcohol solutions have been used frequently in the ESD process. Solvent evaporation during the flight of a solution droplet is inevitable, especially under heating conditions. The evaporation rate for small volatile drops can be calculated by using eqn. (7):16 2/2+ d dt d + 5.33(A2/d)+ 3.42; where d is the droplet diameter, D, the diffusion coefficient of the vapour of the solvent in air, M the molecular mass of the solvent, R the gas constant, p the density of the solution, P, the partial vapour pressure of the solvent away from the droplet, Pd the partial vapour pressure at the droplet surface, T the ambient temperature, Td the droplet temperature which is normally <T due to cooling by evaporation, and 2 the mean free path of air.For a charged droplet, it is not clear whether the evaporation time is influenced by the charge. Even if it is negligible, the calculation of the evaporation time is still difficult because the system is not isothermal, and, thus, T and & change from place to place. Evaporation of the solvent results in shrinkage of the droplet, keeping the total charge the sarne.I7 A charged droplet may be disrupted into a few smaller droplets, after reaching a maximum attainable charge density, qR, for a liquid droplet with radius a. This is the so-called Rayleigh limit,” which can be expressed as shown in eqn. (8): The disruption of a droplet (‘mother droplet’) usually occurs with the ejection of a few highly charged, very tiny drops (‘daughter droplets’).Therefore, for solutions with rather volatile alcohols as solvents and/or a long nozzle-to-substrate distance and/or a high deposition temperature, the effect of droplet disruption should be taken into account. In that case, there is no longer a monodispersed particle size distribution. In contrast, for solutions with a relatively non-volatile solvent and/or a short nozzle-to-substrate distance and/or a low deposition temperature, the monosized distribution may remain during droplet flight. Preferential landing of droplets on the substrate In the strong electrostatic field, induced charges exist on the surface of the grounded substrate, with a sign opposite to that of the droplets or the nozzle.The charge distribution generally is not uniform, but depends on the position relative to the nozzle and, in particular, on the local curvature of the surface. The charges concentrate more at the places where the curvature is greater. Therefore, the electric field there is stronger than at other places. When a charged droplet approaches the surface, it will be attracted more towards these more curved areas; this is referred to as ‘preferential landing’. This action will cause agglomeration of the particles, especially when the incoming droplets are small (see below). Also, this means that the roughness of the substrate surface may influence the mor-phology. An increase in the surface roughness will lead to more particle agglomeration. Discharge, spreading and penetration of solution droplets on the surface As soon as a charged droplet contacts with the surface of the substrate or the earlier formed layer, it starts to discharge by transferring the charge to the grounded substrate either immediately or through the layer to the substrate.This process is very fast according to eqn. (1) when the electronic conduc- tivity of the substrate (usually a metal in ESD) and the deposited layer is relatively high. In this case, the discharge process is not expected to determine the morphology of the layer. However, in the cases of using insulating substrates or depositing insulating layers, the discharge may proceed slowly and: hence, it influences the morphology.Nevertheless, when wet droplets reach the substrate surface (see below), the discharge process could also be completed through electrical conduction in the concentrated solution on the surface. If the evaporation of all of the solvent has not been completed when a droplet reaches the surface of the heated substrate. the solution wets the surface of the substrate or the earlier deposited layer. This is usually true when using a high boiling point solvent or depositing at low temperatures. The type and dynamics of spreading depend strongly upon the so-called spreading coefficients [eqn. (9)]:19920 S ='isv -'is1 -Ylv (9) where ySy, ysl and 7," denote the interfacial tensions between the substrate and ambient gas, between the substrate and the drop liquid, and between the drop liquid and ambient gas, respectively.If S <0 only partial wetting occurs with equilib- rium reached at a finite contact area. If S 2 0 the drop spreads until it completely covers the surface. The value of S is intimately related to the spreading rate. For S =0, r9cct where r is the radius of the contact circle and t is the time, so the rate of evolution of the drop slows rapidly. If S>O, r4cct. Therefore, the choice of substrate will affect the spreading rate of the liquid droplet, and may finally affect the morphology of the layer. The spreading rate is also influenced by the viscosity of the liquid.Qualitatively, the spreading rate decreases with increasing viscosity. However, even if S >0 in a ESD process, the spreading may not be complete when the simultaneous drying process proceeds rapidly. This is why many lamellar particles are usually formed in an ESD layer. When cracks or pinholes are formed in the earlier deposited layer, the subsequently arriving solution droplets may pen- etrate into these defects by capillary action. Therefore, the earlier formed defects are 'repaired in this way, and a crack- free layer is easily obtained. This appears to be another advantage of the ESD technique. Decomposition, reaction and surface diffusion of the solute(s) The decomposition and reaction (either partial or complete) of the solute(s) may have occurred before the droplets reach the substrate, which is expected if the surrounding tempera- ture is high enough and dried droplets have been formed.Rearrangement of these dry particles on the substrate surface by surface diffusion is not expected at moderate deposition temperatures <5OO"C used in this ESD experiment. In this case, a grain-like structure is expected to be formed instead of a very dense morphology. On the other hand, at relatively low temperatures the spreading of solution droplets on the surface and the following process, which is actually a wet chemical process of an alcohol solution of metal salt precursors, determine the layer mor- phology. For the spreading process, the viscosity change of the solution droplets is important.For the wet chemical process, there are many factors which influence the morphology. Specifically, among them are the solution chemistry including the solvation state, for instance whether there is complexation of the metal ions by the alcohol, evaporation and/or reaction with ambient gas of the solvent on the heated substrate, nucleation and precipitation of the solutes, and dissociation and chemical reaction of the solutes. This may be the only way to form a relatively dense morphology except at an extremely high surrounding temperature at which the whole droplets will be vaporized before reaching the substrate surface. Other morphologies, like a unique porous structure found in our experiment (see below), can also be formed by using different solutes and/or different solvents.Furthermore, unlike a normal wet chemical process such as a sol-gel process, this method proceeds via the continuous repetition of many small steps. Therefore a crack-free layer is more easily formed due to the aforementioned defect-repairing mechanism. In general, the final morphology of the layer depends upon the relative rate of spreading, precipitation, decomposition and reaction. For example, if the spreading is slow but the precipi- tation and the decomposition are fast, the morphology will be granular. The ideal conditions for the formation of a dense layer include at least: (i) the particles arriving at the substrate must still be wet (solutes); (ii) the solubilities of the solutes in alcohol must be sufficiently large; and (iii) the spreading of the solution droplets must be rapid.Experimental Ethanol (100%) solutions of Li(CH3COO)* 2H,O and Co(N03), * 6H,O were prepared separately. The solutions were mixed in a molar ratio of Li: Co = 1:1; these solutions were used as the precursors. The Li (or Co) concentrations were 0.003 to 0.05 mol dm-3. To investigate the solvent effect, mixtures of alcohols, ethanol (C,H,OH) and butyl carbitol (C4H90CzH40CzH40H),were also used. A horizontal ESD set-up with capillary-plate configuration was used in most cases (Fig. 2). Circular stainless-steel (and sometimes platinum or aluminium) disks (1.4 cm in diameter) were chosen as the substrate and acted as the 'plate'.A heating element was used to control the substrate temperature. A positive high voltage up to +15 kV was applied to the nozzle (a hollow needle or 'capillary') through which the precursor solution was forced to flow by the pressure difference between the top level of the precursor solution and the solution at the needle orifice, and from which a positively charged spray was generated. The flow rate was controlled using valve G. The nozzle-to-substrate distance was 6 cm. Another set-up with a vertical configuration was also used to obtain a special morphology. To investigate the effect of the substrate, an unpolished alumina square plate partly covered with some aluminium foil was used as the substrate, in order to produce the same conditions, necessary for a good comparison.Besides this, a thin (0.1 mm thick) smooth yttria-stabilized zirconia [YSZ or cubic (Y01,5)o,16(Zr02)o,s4] was also used as a substrate under similar deposition conditions. substrate layer spray heating nozzle ....~ -ground Fig. 2 Horizontal ESD set-up J. Muter. Chem., 1996, 6(5), 765-771 767 Fig.3 Four types of layer morphology obtained by ESD. I, dense layer; 11, dense layer with incorporated particles; 111, porous top layer with dense bottom layer; IV, fractal-like porous layer. Results and Discussion We reported previously that there exists an almost proportional relationship between layer mass and deposition time for ESD derived layers.6 Furthermore, the growth rate in terms of the layer mass per unit time is found to be independent of the deposition temperature.Therefore, over-spray is apparently not a problem in ESD. The overall composition of the layer should be the same as that of the precursor solutions. Elemental analysis by atomic absorption spectroscopy (AAS) for an LiCoO, layer deposited at 340 "C confirmed this conclusion. This is in fact one of the advantages of the ESD technique in comparison with other spray and non-spray deposition techniques. In addition to a unique porous microstructure, four types of layer morphologies were observed in this study; they are schematically shown in Fig. 3. Type I is a relatively dense layer; type I1 is a relatively dense layer with some particles incorporated; type 111 consists of a relatively dense bottom layer containing some lamellar particles, and agglomerates of lamellar particles on top of this dense layer, forming a porous and sometimes fractal-like structure, while type IV is a very porous structure made of fractal-like agglomerates of tiny particles. These four types of morphologies are formed under certain deposition parameters, as will be described below.Effect of deposition time The effect of deposition time on layer morphology deposited at 340 "C is shown in Fig. 4. It can be seen that the layer deposited within 1 h (ca. 1.5 pm thick) is relatively dense, belonging to type I1 [Fig. 4(a)]. With increasing deposition time and thus increasing layer thickness the morphologies of the layers are shifted to type 111 [Fig.4(b)]. With a deposition time of 6 h the top section of the layer is very porous [Fig. 4(c)]. This morphology development can be explained by consider- ing the competitive effect between the rates of evaporation, decomposition and spreading. Probably most of the spray droplets arriving at the substrate are still wet. Initially these drops spread on the metal substrate surface at high speed, because the surface tension of a metal [ysv in eqn. (9)] is usually much greater than that of a metal oxide. Therefore, the solution droplets can spread rapidly. Also, the solubilities of Li(CH,COO) -2H20 and Co(NO,), * 6H20 in ethanol (100 g at 12.5 "C and 21.5 g at 25 "C, respectively) are suffic- iently high.Combining the two factors, a continuous layer is formed. In the meantime, evaporation of ethanol and decompo- sition of the acetates Li(CH,COO) and Co(NO,), take place, producing a relatively dense morphology. It has been proved by X-ray diffraction that LiCoO, is formed at this deposition 768 J. Mater. Chem., 1996, 6(5),765-771 Fig. 4 Surface morphologies of layers deposited at 340 "C for different deposition times: (a)1; (b)3; (c) 6 h. Precursor solution, 0.04 mol dm-3 Li(CH,COO) -2H,O + Co(N03),* 6H20 ethanol solution; substrate, stainless steel; applied voltage, 11 kV. temperature, i.e. 340 oC.6 Therefore, the reaction between the lithium intermediates such as Li,O and cobalt intermediates such as COO may also contribute to the formation of this dense morphology. The submicron-sized particles incorporated in the layer could be from the disruption of larger 'mother droplets' owing to the evaporation of ethanol during the droplet flight and the reaching of the Rayleigh limit.When these disrupted particles arrive at the surface they are likely to be dry. They can be incorporated into the continuous layer. With the increase of deposition time and layer thickness, the spreading of the solution droplets will occur on the surface of the LiCoO, layer, which usually has a smaller surface tension than a metal (stainless-steel here). In other words, the wettability of the ethanol solution on the LiCoO, surface is less than that on a metal surface. Therefore, discrete particles may be formed on the surface owing to the slow spreading.Some extent of spreading of the solution leads to the lamellar particles. These discrete particles also increase the surface roughness, which enhances the possibility of preferential landing and agglomeration. In addition, no crust or hollow particles have been observed when ethanol is used as the solvent. This suggests that the evaporation of ethanol and precipitation of the solutes proceed homogeneously. Effect of deposition temperature Fig. 5 shows the differences in morphology of layers deposited at different temperatures. As already shown in Fig. 4(c) the layer deposited at 340 "C for the same deposition time is quite porous and consists of discrete agglomerates of lamellar par- ticles, belonging to the type 111 morphology.At lower depos- ition temperatures [Fig. 5(a) and (b)]the morphologies of the layers are less porous and belong to type 11, but consist of many large particles (4-20 pm), most of which are also lamellar, 'buried' in an amorphous matrix. The continuous 'semi-trans- parent' matrix appears to be formed from the spreading of large droplets. This provides further evidence that the particles landing on the substrate surface are still wet drops. At these Fig. 5 Surface morphologies of layers deposited at different tempera- tures for 6 h: (a) 230; (b)280; (c)400; (d) 500°C. Precursor solution, 0.04 rnol dm-, Li(CH,C00) -2H,O + Co(NO,), -6H,O ethanol solution; substrate, stainless steel in (a),(b)and (c),Pt in (d); applied voltage, 11 kV.relatively low temperatures the incoming droplets are larger and heavier than those at a higher deposition temperature, owing to less solvent evaporation. Therefore, their movement direction cannot be changed considerably by the attraction of induced charges at the substrate surface to form agglomerates. There are hardly any agglomerates in the layer at the deposition temperature of 230 "C [Fig. 5(a)],while minor agglomeration occurs at the deposition temperature of 280°C [Fig. 5(b)].In addition, slower precipitation steps at low temperatures also favour the formation of the relatively dense morphology. With increasing deposition temperature, the agglomeration extent increases by increasing the effect of the preferential landing.The morphology of the layer deposited at 400°C belongs to the type 111. The agglomeration is substantial but the agglomer- ates still constitute small lamellar particles [Fig. 5(c)], implying that even at this temperature the incoming droplets are not completely dry. It seems that the incoming droplets are com- pletely dried at 500 "C because lamellar particles are no longer observed and drying traces are absent [Fig. 5(d)]. Actually, the morphology of the layer is fractal-like, belonging to type IV. Therefore, with increasing deposition temperature the mor-phology of the deposited layer changes from type I1 to type IV, i.e. from relatively dense to highly porous. Effect of precursor solution concentration Fig.6 shows two layers, both with type I11 morphology, prepared with two concentrations of precursor solution. The deposition times are different but the layer thicknesses are similar, i.e. ca. 0.8 pm, which is about the thickness of a layer deposited over 1 h from a 0.04 mol dmP3 precursor solution [Fig. 4(a)]. Therefore, the influence of the concentration on the morphology is not remarkable as long as the layer thicknesses are similar. However, more scattered agglomer- ates and lamellar particles are present, in the layer obtained with a 0.0038 mol dm-3 solution [Fig. 6(a)] than that with a 0.01 mol dm-3 solution [Fig. 6(b)], and with a 0.04 mol dm-3 solution [Fig. 4(a)]. This is due to the fact that a longer Fig. 6 Surface morphologies of layers deposited at 350 "C for 2 h with solutions of different concentrations: (a) 0.0038; (b)0.010 rnol dm-, Precursor solution, Li(CH,COO) -2H,O + Co(NO,), -6H,O ethanol solution; substrate, stainless steel; applied voltage, 11 kV deposition time will result in an increased roughness, as shown in Fig.4. Interestingly, there is no large variation in particle sizes when precursor solutions with different concentrations are used. According to eqn. (3) a lower concentration, and hence a smaller conductivity, will result in a larger primary particle size. However, the solid particle size after drying should also increase with the concentration of the precur- sor solution. The combination of these two opposing factors may lead to comparable final particle sizes for different concentrations.Effect of electric field strength Fig. 7 shows two layers deposited by applying 8 and 15 kV, respectively, to the nozzle. Their morphologies both belong to type 111. However, it can be seen that the extent of particle agglomeration increases with decreasing electric field. Therefore, the layer from a stronger electric field [Fig. 7(b)] looks denser than that from a weaker electric field [Fig. 7(a)]. Also, the particle size using a weak electric field is smaller. This can be attributed to a shorter flight time of droplets under the stronger field according to either eqn. (4) or eqn. (6), and, hence, this results in less solvent evaporation and larger incoming droplets at the substrate surface.Another reason might be a stronger preferential landing effect existing in a stronger electric field. Effect of substrate Three layers deposited on different types of substrate are shown in Fig. 8. For the two layers simultaneously deposited on aluminium and alumina, their morphologies are quite different. The layer on aluminium [Fig. 8(u)] is rather dense whereas that on alumina [Fig. 8(b)]is not. The latter consists Fig. 7 Surface morphologies of layers deposited at 350 "C for 4 h with different applied high voltages: (a) 8; (b) 15 kV. Precursor solution, 0.04 mol dmP3 Li(CH3C00)* 2H,O + Co(NO,), 6H,O ethanol solution; substrate, stainless steel. J. Muter. Chem., 1996, 6(5),765-771 769 Fig. 8 Surface morphologies of layers deposited at 350 "C for 2 h on different substrates: (a) A1 foil; (b) Al,O, plate (1 mm thick); (c) YSZ disk (0.1 mm thick).Deposition on the first two substrates was conducted simultaneously. Precursor solution, 0.04 mol dm-, Li(CH,COO) * 2H,O + Co(NO,), * 6H,O ethanol solution; applied voltage, 11 kV. of more agglomerates than that on the aluminium substrate. Note that originally present in the alumina substrate are some cracks or cavities, which enhance the preferential landing effect and accordingly lead to the formation of more agglomerates. In addition, a large difference in the dielectric property between aluminium and alumina may also cause the electric field to be stronger near aluminium than near alumina.This might also contribute to the different morphologies obtained on these substrates. However, under similar deposition conditions, a rather dense layer can also be formed on a thin (0.1mm thick) and smooth YSZ substrate [Fig. 8(c)]. This suggests that the presence of cracks and cavities in the alumina substrate used in Fig. 8(bj is the main reason for the formation of agglomer- ates. The effect of the thickness of the ceramic substrate on the layer morphology is unclear and is worth further study. Effect of solvent As discussed above, the morphology of a deposited layer is largely determined by the droplet size and some physical properties, such as boiling point and spreading behaviour on the substrate, of incoming droplets, and in particular the solubilities of the precursor solutes.By changing the solvent composition, the layer morphology may also be modified. Fig. 9(u) and (b) show that the morphology changes from type IV to type 111, when a mixture of 67 vol% ethanol + 33 vol% butyl carbitol is used as the solvent instead of 100% ethanol. The boiling point of butyl carbitol is cu. 230 "C, while that of ethanol is only 78 "C. Therefore, at 450 "C the droplets arriving at the substrate are probably dried particles when pure ethanol is used as solvent, but they are still wet in the instance of mixtures with a higher boiling point. When using 50 vol% ethanol + 50 vol% butyl carbitol as the solvent mix- ture at 250°C the layer [Fig. 9(c)] is relatively dense, and hardly any particles can be discerned. It belongs to the type I morphology.Compared with the layers formed using pure ethanol solution [Fig. 5(a) or (b)] the morphology is denser. 770 J. Muter. Chem., 1996, 6(5j, 765-771 Fig. 9 Surface morphologies of layers deposited using precursor solu- tions with different solvent compositions: (a) 100 vol% ethanol solution (0.04mol dm-,), at 450°C for 2 h; (b) 67 vol% ethanol+ 33 vol0/o butyl carbitol solution (0.04 mol dm-,), at 450 "C for 2 h; (c) 50% ethanol + 50 vol% butyl carbitol solution (0.02 mol dm-3), at 250 "C for 4 h; (d) 15 vol% ethanol + 85 vol% butyl carbitol solution (0.005mol dm-,), at 230 "C for 2 h. (u)-(c): Li(CH,C00) -2H20 + Co(NO,), -6H,O precursor, using the hori- zontal set-up; (d) Li(CH,COO) -2H20+ Co(CH,COO), -4H,O pre- cursor, using the vertical set-up.This is due to slower evaporation of solvent during both droplets travelling in air and spreading on the substrate surface, and accordingly, the slower precipitation step. Therefore, by using high boiling point solvents the morphology of a layer becomes denser. Fig. 9(dj shows the unique morphology of a layer deposited at 230°C using a vertical set-up with a solution containing 15 vol% ethanol + 85 vol% butyl carbitol as solvent. Note that cobalt acetate instead of nitrate was used in this case. The layer is a highly porous and three-dimensional interconnected structure with a narrow pore-size distribution. The pore size is ca. 8 pm. It appears to be a stable structure as the network remains unchanged after annealing at 450 "C.The formation mechanism for this unique reticulate structure is not yet clear. However, the precipitation step must play a crucial role because it is found that such a structure cannot be obtained by using cobalt nitrate. According to our experiment, the solubility of cobalt nitrate in the solvent is much larger than that of cobalt acetate. Therefore, this morphology is probably formed during the spreading of wet droplets. In addition, there could be chelation between cobalt acetate and butyl carbitol, as the colour of the precursor solution is dark green, rather than pink which is the colour of cobalt acetate and the precursor solution using cobalt nitrate. The possible chelating effect might cause an increase in the viscosity of the solution during the spreading step.This may also contribute to the formation of the structure. Owing to the unique structure and its potential application, it is worth further study. Conclusions The electrostatic spray deposition (ESD) technique opens the opportunity to control the morphology of a layer. The mor- phology of the layer deposited by ESD is determined by the spray droplet size (especially the size of the incoming droplets), the deposition temperature, the spreading rate of solution droplets on the substrate and, if at low deposition temperatures or using a high boiling point solvent, the solution chemistry including the precipitation process and the pyrolysis or reaction of the solutes.The main factor which determines the layer morphol- ogy is the substrate temperature. The higher the substrate temperature, the more porous the layer. The layer deposited at elevated temperatures with ethanol as solvent is very porous and has a fractal-like morphology. At a moderate temperature and at an early stage the deposited layer on a metal substrate is relatively dense, but it becomes porous with increasing thickness. The concentration of the precursor solution has a minor effect on the layer morphology. Basically, it is easier to obtain a denser layer using a higher concentration compared to using a lower concentration. The electric field strength can influence the flying time of the charged particles.Above the onset voltage, the higher the applied voltage, the denser the layer. For a ceramic substrate, its surface roughness (or smooth- ness) can affect the morphology of a deposited layer. Cracks or cavities present in the substrate will lead to the formation of more agglomerates. At the same deposition temperature, a layer formed using a precursor solution with a high boiling point solvent is generally denser than one obtained using a low boiling point solvent. A unique porous structure, however, can be obtained with a high boiling point solvent. The Foundation for Chemical Research in the Netherlands (SON) under the Netherlands Organization for Scientific Research (NWO) is acknowledged for financial support. References 1 R.A. Coffee, Outlook Agr., 1981,10,350. 2 E. B. Slamovich and F. F. Lange, Muter. Res. Soc. Symp. Proc., 1988,121,257. 3 P. H. W. Vercoulen, D. M. A. Camelot, J. C. M. Marijnissen, S. Pratsinis and B. Scarlett, in Synthesis and measurement qf ultrajine particles, ed. J. C. M. Marijnissen and S. Pratsinis, Delft University Press, Delft, 1993, p. 71. 4 A. A. van Zomeron, E. M. Keller, J. C. M. Marijnissen and J. Schoonman, J. Aerosol Sci., 1994,25, 1229. 5 E. M. Kelder, 0.C. J. Nijs and J. Schoonman, Solid State lonics, 1994,68, 5. 6 C. H. Chen, A. A. J. Buysman, E. M. Kelder and J. Schoonman, Solid State Ionics, 1995,80, 1. 7 0.V. Salata, P. J. Dobson, P. J. Hull and J. L. Hutchison, Thin Solid Films, 1994,251, 1. 8 W. Siefert, Thin Solid Films, 1984, 120,267. 9 W. Siefert, Thin Solid Films, 1984, 120,275. 10 J. M. Grace and J. C. M. Marijnissen, J. Aerosol Sci., 1994, 25, 1005. 11 R. C. Weast, Handbook of Chemistry and Physics, 56th edn., CRC Press, Boca Raton, FL, 1975, E-56. 12 A. G. Bailey, Electrostatic spraying of liquids, John Wiley & Sons, New York, 1988. 13 G. 1. Taylor, Proc. R. Soc. London, A, 1964,280,383. 14 M. Cloupeau and B. Prunet-Foch, J. Electrost., 1990,25, 165. 15 A. M. Ganan-Calvo, J. Aerosol Sci.. 1994,25, suppl. 1, S309. 16 W. C. Hinds, Aerosol technology, John Wiley and Sons, New York, 1982. 17 M. A. Abbas and J. Latham, J. Fluid Mech., 1967,30,663. 18 L. Rayleigh, Philos. Mag., 1882,5, 184. 19 J. T. Davies and E. K. Rideal, Interfacial Phenomena, Academic, New York, 1961. 20 D. Beaglehole, in Fluid Interfacial Phenomena, ed. C. A. Croxton, John Wiley and Sons, New York, 1986, p. 523. Paper 5/05211D; Received 4th August, 1995 J. Mater. Chem., 1996, 6(5),765-771 771

ISSN:0959-9428    

DOI:10.1039/JM9960600765

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

_____~ ~ Synthesis, properties and performances of electrodeposited bismuth telluride films Pierre Magri, Clotilde Boulanger and Jean-Marie Lecuire Laboratoire d'Electrochimie des Matkriaux, URA CNRS 158, Uniuersitk de Metz, Ile du Saulcy, 57045 Metz Cedex, France Bismuth telluride alloy films of uniform thickness have been successfully prepared by electrodeposition from a solution containing Bi3+ and HTe02+ ions in 1 mol dm-3 nitric acid (pH=O) on stainless steel. The electrodeposited films are monophasic and exhibit a polycrystalline structure (R3rn).The film composition is dependent on the electrolyte composition and the current density. The electrical properties of the electrodeposited samples have been determined. The obtained films are n-type semiconductors with high carrier concentration.Thermoelectricity is the phenomenon which results from the direct conversion of heat into electrical energy (or vice versa). Since 1950, much effort has been devoted to the use of this phenomenon for applications in static coolers. The relative efficiency of a thermoelectrical material is measured in terms of the figure of merit, Z, which is defined by: 2=a2/p/l where a is the Seebeck coefficient, p the electrical resistivity and 2. the thermal conductivity. In order to improve values of 2,the material should be a good electrical conductor, a poor thermal conductor and should have a large thermoelectric power. Mildly degenerate semiconductors have the best combi- nations of these intrinsic properties.' Bismuth telluride (Bi,Te,) and its derivative compounds are considered to be the best materials for use in thermoelectric refrigeration at room temperature.',2 These materials are generally synthesised by directional crystallisation powder metallurgy process7,* or evaporation However, these tech- niques do not readily lend themselves to the production of large-area thermoelements. Electrochemical deposition may provide an alternative process to these classical methods.Furthermore, electrodeposition techniques have been success- fully employed for preparation of chalcogenide semiconductors, e.g. CdS, CdSe, CdTe, InSe, PbTe.13,14 With regard to bismuth chalcogenides, thin films of the sulfide (Bi2S3) have been prepared by anodisation of bismuth metal in polysulfide solu- tions'' and by direct electrodeposition from non-aqueous media.16 We have previously shown that an electrodeposition process leads to the formation of textured bismuth telluride films."7'* The process is based on the electroreduction of telluride ions, in the presence of bismuth salts, according to the reaction: 3TeIV+2 Bi"' +18 e --+Bi2Te3 Takahashi et al." have also obtained Bi-Te alloy films in potentiostatic conditions. The present work concerns in particular the definition of the optimum conditions for a galvanostatic process of bismuth telluride films, and the electrical characterisation of the syn- thesised compounds.Analytical Study under Voltamperometric Conditions Experimental details The reaction analysis was realised by voltamperometric tech- niques on a rotating platinum disk electrode using a classical three-electrode device in a thermostatted cell (25 "C) and under an inert atmosphere (argon HP).All the potentials were measured and expressed by reference to the aqueous KCl saturated calomel electrode (SCE). The auxiliary electrode was a platinum disk. The linear sweep voltammograms were obtained using a Tacussel PJT 24.1 potentiostat, IMT.l interface, Voltamaster 2 software and an IBM-compatible machine. Electrolytes The electrolytes were prepared in solution with deionized water. To ensure the stability and the solubility of bismuth(m) solutions, the selected solvent was 1 mol dmP3 aqueous HNO,. The Bi"' solutions were obtained by dissolution of Bi(N03),.5H20 (analytical grade).The concentrations were determined by EDTA volumetry.20 The TetV solutions were prepared from the reaction of nitric acid on high-purity elemental tellurium. Under these acidic conditions, tellurium was in the form of HTe02+ (telluryl ion). The concentrations were controlled by an oxido-reduction titration based on TeIV oxidation by a titrated CrV' solution and on a back titration of this reagent (in excess) against an iron@) solution.20 The Bi3 + and HTe02+ electrolyte mixtures were obtained from the above solutions, and the Bi3+ :HTe02+ ratios were varied to determined values. Results The aim of the analytical study was to investigate the behaviour of Bi3+, HTe02+ and mixtures of them during cyclic voltammetry.Bi3+ solution. The voltamperogram of the Bi3 + solution is shown in Fig. 1. During a cathodic exploration, the obtained curve shows a reduction wave at -150 mV due to the reduction of Bi3+ to bismuth metal. This wave presents a diffusion-limited current which increases with increasing Bi"' concen-tration. A shift towards more negative potentials causes solvent reduction from -350mV. The X-ray identification of the electrodeposited product after this cathodic scan confirms the formation of bismuth metal. The deposit is oxidisable by a reverse sweep. Indeed, an anodic scan gives evidence of a peak (E=-50 mV) which corresponds to bismuth anodic stripping. The anodic :cathodic charge ratio is 0.82.The cathodic charge excess is due to hydrogen evolution which was not observed if the cathodic scan was stopped at -250 mV. HTe02+ solution. The voltamperogram of the HTe02+ solution is shown in Fig. 2. In contrast with the bismuth behaviour, the tellurium system appears to be a slower system. J. Mater. Chem., 1996, 6(5), 773-779 773 6001 i 1 L 450 -a 30°-3.. \.r--150 0 -1SOl " " " " " ' " ' -500 0 500 loo0 EImV (vs SCE) Fig. 1 Voltamperogram in 1mol dm HNO, of a BI3+ solution, [BI3+] =3 5 x 10 mol dm Working electrode, Pt, surface area, 3 14 mm', rotation rate, 600 rpm, potential sweep rate, 60 mV min 6001 I 1 450 -300 -Te' +TP + 4e a5 150-J1+ I -600 -400 -200 0 200 400 600 800 lo00 EImV (vsSCE) Fig.2 Voltamperogram in 1rnol dm HNO, of an HTe02+ solution, [HTeOJ =3 5 x 10 mol dm Working electrode, Pt, surface area, 3 14 mm', rotation rate, 600 rpm, potential sweep rate, 60 mV min In this solution, the solvent reduction occurs at ca -350 mV The reduction process due to the deposition of elemental tellunum occurs at a potential similar to that of bismuth but the oxidation peak is observed at ca +600mV The syn- thesised film was removed from the electrode, ground and analysed by X-ray diffraction The obtained pattern shows that the product is crystalline tellurium In this case, the anodic ca-thodic charge ratio is 092 The cathodic charge excess is due to hydrogen evolution which was not observed if the cathodic scan was stopped at -250 mV The deposition potentials of each element are very similar, which suggests that codeposition should be possible and should lead to alloy formation after an appropriate thermal treatment Mixture solutions.We carried out similar experiments in solutions containing Bi3+ and Te4+ in different ratios The investigations led to proton reduction (-350 mV) and, for the reverse scans, to the oxidation of electrodeposited material In order to obtain Bi2Te3, the Bi3+ HTe02+ ratio was firstly fixed at 2 3 The voltammetric curve for such a solution (Fig 3) evidences only one signal in the form of a reduction wave with deposit formation = -50 mV), and one anodic peak (E= +400 mV) which is well defined The voltamperog- ram (Fig 4) for a solution corresponding to Bi3+ HTe02+ = 4 3 presents two successive reduction waves that are not well defined We can also see two anodic stripping peaks (El=+ 180 mV, E2= +400 mV) If the cathodic exploration is stopped at -200mV, the first peak disappears, and the predominant signal situated at +400mV remains For a solution containing an excess of tellurium (Bi3+ HTeO,' = 1 2), the current-potential curve (Fig 5) shows a cathodic wave A reversal of the potential scanning reveals not only the same signal (E = +400 mV) but also a shoulder which could 774 J Muter Chem , 1996,6(5), 773-779 -500 -250 0 250 4(Jo 500 EIrnV (vs SCE) Fig.3 Voltamperogram in 1 mol dm HNO, of a mixture containing [HTeO2+]=525x10 3,[Bi3f]=35~10 ,rnoldm ,,[Bi] [Te]= 2 3 Working electrode, Pt, surface area, 3 14mrn2, rotation rate 600 rpm, potential sweep rate, 60 mV min 500 250 0 a -250 s, -500 - -750 - -1000 -.c * I * * - * * ' , ' * , ' * - ' ' 050 025 0 0 25 050 0 75 EImV (vs SCE) Fig. 4 Voltamperogram in 1 mol dm HNO, of a mixture containing [HTeO2+]=525x10 ,, [Bi3+]=7x10 ,moldm [Bi] [Te]= 4 3 Working electrode, Pt, surface area, 3 14 mmz rotation rate, 600 rpm, potential sweep rate 60 mV min 46 04 -02 0 02 04 EImV (vsSCE) Fig. 5 Voltamperogram in 1 mol dm HNO, of a mixture containing [HTeO2+]=7x10 ,, [Bi3+]=35x10 ,rnoldm [Bi] [Te]= 2 4 Working electrode, Pt, surface area, 3 14mm2 rotation rate, 600 rpm, potential sweep rate, 60 mV min be attributable to the tellurium behaviour, as indicated previously These first results indicate that there is no codeposition of bismuth metal and elemental tellurium for an electrolyte composition corresponding to a Bi Te ratio of 2 3 because the curve does not present the specific anodic signals of each element The presence of one signal during reduction and oxidation is expected to be the signature of the electrochemical process according to the following reaction 3 HTe02++2 Bi3++18 e-+9 H++Bi2Te3+6 H20 In order to verify this hypothesis, we studied the anodic oxidation behaviour of Bi2Te, obtained by directional crystal- lisation The powdered sample synthesised in the S Scherrer Laboratory' (Laboratoire de Physique du Solide, INPL, Nancy) was attached to the section of a glassy carbon rod by means of a colloidal graphite slurry according to a previously described procedure.2' We investigated the anodic oxidation of the latter compound in 1mol dm-3 HNO, electrolyte.A comparison of this anodic voltammogram (Fig. 6) with that obtained with a deposit obtained under stoichiometric con- ditions (Fig. 3) shows a similarity in feature and in potential range. Furthermore, electrolysis at a fixed potential on the diffusion current limit was performed in a stoichiometric electrolyte on a platinum electrode to obtain a sufficient bulk. The elec- trodeposited product was ground and analysed by X-ray diffraction. The diffraction pattern shows that the product had good crystallinity and all diffraction lines could be indexed to the hexagonal rhombohedra1 structure of Bi2Te3 (space group R3m)(Fig.7). The lattice parameters of the hexagonal structure were determined from the observed reticular distance, using a least-squares method (Table 1). The obtained values were in Table 1 Lattice parameters of the Bi,Te, hexagonal structure source ah/A Ch/A ref. 22 4.3835( 5) 30.487(1) ref. 23 4.3850(2) 30.487(2) electrodeposited films 4.384(2) 30.11 (3) 0 250 rw 500 750 EImV (ws. SCE) Fig. 6 Anodic oxidation in 1 mol dm-3 HNO, of a directional crystallised Bi, 06Te2 94. Working electrode, Pt; surface area, 3.14 mm2; rotation rate, 600 rpm; potential sweep rate, 60 mV min-'. '9 N \Y good agreement with those observed by Francombe2, and Brebrick,, with, however, a much smaller c parameter, indica- tive of a composition other than Bi2Te3.Galvanostatic Synthesis The galvanostatic process is commonly used in the electroplat- ing industry. Therefore, we concentrated our efforts on a technique using a constant current to define the best parameters for galvanostatic cathodic deposition. Deposition conditions Electrodeposition was carried out at a constant temperature of 25 "C. The working electrodes (stainless-steel disk) were polished with carborundum paper and with diamond paste (1 pm size). An area of 2 cm2 was exposed for deposition. The electrolyte composition was imposed by the solubility of HTeO,' in nitric acid solvent (1 mol dm-3 HNO,) at pH =0.Under these acidic conditions, the HTe02 maximal concen- + tration is 5 x rnol dm-3 because insoluble TeO, precipi- tates at higher concentrations according to Pourbai~.~~ Therefore, the Bi3+ content of solution is adjusted to 3.33 x mol dm-3. Controls Samples were prepared after the electrodeposition by thorough rinsing in three steps [O.l mol dm-3 nitric acid solution (pH= l), deionized water and ethanol] followed by drying in air. The composition of products removed from the support was analysed using two techniques: electron probe microanalysis (CAMECA SX50) calibrated with tellurium (purity 99.9%) and bismuth (purity 99.9%) standards, or a volumetric method.25 The bismuth and tellurium microanalyses were performed in ten different sections of the samples.The stoichi- ometry corresponds to the average of these ten values and is calculated with a total atom number assigned as 5. Analyses were reproducible within & 1YO.The stoichiometry volumetric determination was elaborated in our laboratory according to previously reported procedure^.^^^^^ Analyses were reproduc- ible to 0.5-1.5Y0. h 0 \ h m0 .. . . . . . . . 0 m om NWcU4*4* 4 ti1.-E u)C B 1.-c i 1I i 11 I I 1 J I I 10.0 5.04.0 3.0 2.5 2.0 1.5 1.2 1.0 dhkl =Fig. 7 Reflecting XRD pattern obtained on a ground electrodeposited compound under potentiostatic conditions: Edeposlt -150mV J. Muter. Chem., 1996,6(5), 773-779 775 The phase identification and the estmation of lattice param- eters were carned out by X-ray diffraction using a curve detector (INEL, Co-Kcx radiation), and the morphology was studied using a scanning electron microscope (HITACHI model S 2500 LB) We realised the physical characterisation, after removing the films from their supports, through measurements of the electri- cal resistivity and the Hall effect in a direction perpendicular to the cleavage planesz5 [using the device from the S Scherrer Laboratory5 (Laboratoire de Physique du Solide, INPL, Nancy)] The Hall effect and resistivity measurements were made on rectangular samples over the temperature range 100-300K Current contacts were made on the underside of the sample by soldering gold spring wires (5 pm diameter) with a bismuth tin eutectic to minimise interfacial and thermoelectric effects Measurements of the Hall effect were conducted with the magnetic field parallel to the cleavage planes and the electric current and Hall voltage perpendicular to the cleavage planes, using a Van der Pauw technique 27 z8 To achieve an accuracy of 2% for resistivity values and 3% for Hall effect, twenty measurements were made for each sample Chemical composition, structure and morphology The optimal value of the applied current density was deter- mined using the Hull's cell method 29 The obtained films were regular, metallic, and pearl-grey for current densities varying from 03 to 12AdmP2, corresponding to a growth rate of 6-25 pm h-l At higher values, the material was black and did not adhere to the support electrode At lower values, the growth rate of films was very slow After electrodeposition, the current density dependence of the stoichiometry of the Bi-Te alloys was studied for films removed from the support Table 2 shows the obtained stoichi- ometry values From these expenmental values, two main features are observed First, the electrodeposited alloys always present an excess of tellurium in comparison with the Bi,Te3 composition In the Bi2Te3 structure, two major defaults are observed An excess of bismuth induces a substitution of Te for Bi accepting one electron For an excess of tellunum, there is a substitution of Bi for Te, with the loss of one electron3' In our case, where the tellurium atomic percentage is >6O%, Te,, defaults are evident, which induces an n-type conductivity Secondly, the tellunum atomic percentage decreases and tends towards the theoretical value in BizTe3 with increasing current density The optimal obtained value is 644 atom% tellurium, corresponding to a Bi, 78Te3 22 compound In order to reach the ideal value of Bi Te (=2 3), we can imagine two possibil- ities either an increase of the current density, or enrichment of the Bi3+ concentration in the solution The first possibility was eliminated because values higher than 1 2 A dm-2 are not compatible with a good matenal, as discussed above Also, we added Bi3+ to the electrolytes Two solutions were studied with Bi Te=3 3 and 4 3 Fig 8 indicates the atomic percent- age of tellunum evolution as a function of the current density for the different electrolyte compositions It can be seen that Table 2 Current density dependence of stoichiometry' current density/A dm Te (YO) Bi,Te, <O 17 69 8 B1151Te3 49 0 17-0 25 67 6 B1l 6ZTe3 38 0 25-0 35 66 0 B1l 70Te3 30 0 35-0 47 65 0 B1l 7STel 25 0 47-0 61 64 8 B1l 76Te3 24 0 6140 70 64 5 B1l 77STe3 225 0 70+0 79 64 6 B1l 77Te3 23 0 7940 90 64 5 B1l 77STe3 225 090-,105 64 4 B117*Te322 "Solution with Bi Te =2 3, Hull's cell IA=0 35 A, t = 30 min 776 J Muter Chem , 1996,6(5), 773-779 mooo0 25 050 075 1 00B"Te' current density/A dm-* Fig.8 Evolution of Te atomic percentage with current density and Bi Te ratio (Rso,)in electrolyte the tellurium percentage depends on the current density and that a high Bi Te furthers an alloy formation containing 62 4 atom% tellurium, 1 e Bi, 88Te3 12 For a current density below 12Adm and for three electrolyte compositions, the faradaic efficiencies were meas- ured for the real alloy stoichiometry from the mass increase of the electrode after deposition, and by comparing the exper- imental increase to that expected from Faraday's law Efficiencies were calculated for the synthesis of alloys using the theoretical mass and the real electron number used in the electrochemical process reaction, taking into account the alloy stoichiometry The results are collected in Table 3 Note that the efficiencies are found to be close to 100% These results reveal that the assumed deposition mechanism is indeed operat- ive and that the applied current density does not induce hydrogen formation The cathodically deposited materials were analysed by reflecting X-ray diffraction The patterns show a single phase, whatever the applied current density All the diffraction lines could be completely indexed on the basis of the hexagonal cell or an equivalent rhombohedra1 cell cor_responding to the structure of Bi2Te3 or its solid solution (R3m) The diffraction peaks remain sharp for all alloys with no detectable shoulders However, the intensity ratios of the peaks are not in good agreement with those obtained by X-ray diffraction on a ground product This fact indicates an orientational effect in the film growth, which we have studied in detail l7 The analysis of the film stoichiometries show an excess of tellurium This fact could suggest tellurium metal deposition, but there are no diffraction peaks corresponding to tellurium metal Furthermore, the possibility of the electroformation of an amorphous tellurium should be ruled out Indeed, galvanostatic deposition under the same conditions (current density, support) from an electrolyte containing only Te" ions led to the formation of crystallised tellurium films The experimental reticular distances are slightly different A least-squares fit was performed using all lines with O(hkl)> 20 O that could be indexed unambiguously The procedure yielded lattice parameters us tellurium percentage in the material (Table 4) The parameters vary smoothly, suggesting the mate- rial is a single phase and corresponds to a solid solution Moreover, the ah lattice parameter of electrodeposited films is Table 3 Electrodeposition efficiencies faradaic efficiency (YO) current density/A dm Bi Te=2 3 Bi Te=3 3 Bi Te=4 3 <O 17 92 49 95 24 95 37 0 17-+0 25 93 16 95 36 95 85 0 2540 35 93 22 96 14 96 90 0 35-0 47 94 24 96 70 97 30 0 47-0 61 95 20 97 34 98 50 0 70-0 79 95 46 97 56 98 80 0 904 05 95 38 98 30 99 20 Table 4 Hexagonal lattice parameters of electrodeposited Bi-Te alloys Te (%) 60" 4 3835( 5) 30.487( 1) 63 6 4 399( 2) 30.13( 3) 67 8 4.403 (6) 29.92( 4) 68 04 4.409(4) 29 97( 3) 68 5 4.41 1 (2) 29.90( 4) 'BiZTe3 single crystal, ref.22. Fig. 9 SE micrograph of a Bi,Te, film electrodeposited onto stainless steel from an HNO, electrolyte solution containing Bi :Te =4 :3 larger than that of a single crystal obtained by solid-state reaction, and the ah lattice parameter seems to increase with increasing tellurium content, while the ch parameterdecrease^.^^.^^ The reason for such a difference is certainly due to the substitution in bismuth planes by tellurium in our tellurium-enriched compounds. This leads to a more compact structure because the tellurium atom is smaller than the bismuth atom. The SEM image of the surface of the electrodeposited compound (Fig.9) shows a polycrystalline assembly of regular needles of length 1.5 pm for the face which is in contact with the electrolyte. This appearance persists up to a thickness of 0.1 mm. For the synthesis of thicker films, dendrites grow and prevent the formation of a regular film. After removing the film, the face which is in contact with the support electrode exhibits a uniform surface with metallic lustre. Transport properties The measurements were made to determine the influence of the thickness and the composition of samples at room tempera- ture, and over a temperature range of 100-300 K. Influence of the thickness. Three films series were prepared with a current density of 0.9 A dm-2 and for deposition times of 2, 3 and 4h.In these cases, the thicknesses of the films were 38, 56 and 74 pm respectively. Experimental values of electrical resistivity p, Hall coefficient R,, carrier concentration C, and Hall mobility pH were determined. The values obtained for the Bi, ,,Te3 23 stoichiometry series (Table 5) were representative of all series. First, note that the Hall coefficient is negative in all cases. In accordance with theory, a negative Hall coefficient induces a n-type semiconductor. Therefore, our electrodepos- ited compounds are n-type and these results confirm the excess of tellurium found by the stoichiometry measurements. Secondly, the values of resistivity and the Hall coefficient can be considered as constant, with no significant effect of thickness.This point implies that the electrodeposited alloy is homo-geneous and retains the same composition during its formation. Moreover, the film resistivity (cu. 11 pi2 m) is clearly lower than that of the single crystal (cu. 75 pl2 m).' Note that a low value is favourable for increasing the figure of merit 2 as regards its literal expression. If we compare the carrier concentrations, the film shows a higher concentration (C,=57 x lo1' ~m-~)than that of the single crystal (C, =0.7 x 10'' cm-'). However, these high values are certainly due to the significant presence of grain boundaries in the films. Influence of the composition. Films with the same thickness were investigated in the wide composition range 63.6< Te <70%.They were prepared under galvanostatic conditions and Table 6 gives the results. The variation of the resistivity value is slight compared with our predictions. The RH values and the Hall mobility increase when the tellurobismuthite stoichiometry is approached. However, the obtained RHvalues, corresponding to a donor density of ca. 2.0 x lo2' e- cmP3, do not correspond to those observed for a single crystal, and the Hall mobility values are larger than those observed for a single crystal. The significant differences are certainly due to the polycrystalline state of the films. Evolution vs. temperature. The resistivity variations for the Bi, ,,Te, 21 sample were studied between 100 and 300 K. The results are plotted in Fig.10. The electrical resistivity increases with increasing temperature and reaches a maximum value at 270K, in agreement with the results of Smirnov et uL3' and Satterthwaite and Ure.32 The RH coefficient [Fig. 1 l(u)] appears to be temperature independent as expected for a semiconductor with high carrier concentration. The carrier concentration C, [Fig. 11 (b)]decreases slightly when the tem- perature increases, as for a single crystal. The values are larger than those of the single crystal; this is caused by the high grain-to-grain connectivity and polycrystalline state. A decrease in the Hall mobility occurs with increasing tempera- ture [Fig. 11(c)]. An analysis of a theoretical mobility model (pH=pOT-X)was carried out where pH is the Hall mobility and T is the temperature.The x exponent was found to be Table 5 Influence of film thickness on electrical properties for Bi, 77Te, 23 carrier Hall mobility/ thickness/pm temperature/# resistivity/pZ2 m cm3 C-' concentration/lo-l9 cm-3 cm2 V-' s-l 38 301.3 11.92 -11.77 -53.10 -9.88 56 304.2 10.87 -10.84 -57.67 -9.97 74 303 8 10.27 -10.38 -60.43 -10.12 J. Muter. Chem., 1996, 6(5),773-779 777 Table 6 Influence of stoichiometry on electrical properties carrier Hall mobility/ Te (YO) Bi,Te, resistivity/jd2 m R,/10 3cm3C ~oncentration/l0'~cm cm2V 's 70 0 B1l SOTe, 50 1206 -4 20 -146 3 -3 54 69 2 B1l 54Te3 46 9 43 -6 37 -98 47 -6 75 65 4 B1l 73Te3 27 1168 -8 72 -71 76 -8 76 647 B1l 77Te3 23 11 09 -11 02 -57 06 -9 99 642 B1179Te3 21 12 33 -15 25 -40 99 -12 37 63 6 B1l82Te3 18 12 74 -19 26 -32 45 -15 12 equal to 0 5 This value does not correspond to any theoretical 11 1 model of diffusion carrier mechanism Note that the high defect concentration and the polycrystalline state induce a large grain boundary quantity and therefore mask the carrier diffusion E lo: xx mechanisms X n g 9-2-28-a w -> x x x x X X Conclusion This study shows that an electrochemical process has success-fully synthesised bismuth-tellunum alloys which possess ther- 7~"""""""'"''"'' 100 150 200 250 300 TfK Fig.10 Temperature dependence of resistivity for an electrodeposited B1179Te3 21 film 0 0 0 0 0 mo 0 -0 00 SO 50 100 150 200 250 300 350 -5 0.L -7 5 1-0'Y)-1001 o00 00>-125-0 cu W moelectric properties The analytical study of the reduction of Bi3+,HTeOzf and their mixtures has established the direct electroformation of non-stoichiometric bismuth tellunde compounds with an excess of tellurium The galvanostatic process may be a simple and economic method for the large-scalefabrication of thermo-electric convertors The electrodeposited samples are polycrystalline and their stoichiometries vary according to the solution composition and the applied current density Physico-chemicaland electrical characterisations showed that the film composition remains constant during the synthesis The electrodeposited compounds show n-type semiconducting behaviour with a significant car-rier concentration References 1 A Ioffe, Semiconductors Thermoelements and Thermoelectricity Cooling, Infosearch, London, 1957 2 B Yim and F Rosi, Solid State Electronics, 1972,15, 1121 3 Harmon, J Phys Chem ,1957,2,181 4 F Rosi, B Abeles and R Jensen, J Phys Chem Solids, 1959, 10,191 5 J P Fleunal, These de I'INPL, Nancy, 1988 6 T Caillat, M Carle, P Pierrat, H Scherrer and S Scherrer, J Phys Chem Solids, 1992,53,1121 7 T Ohta, T Uesugi, T Tokiai, N Nosaka and T Kajikawa, Proc 8th Intl Conf Thermoelectric Energy Conversion, ed S Scherrer and H Scherrer, INPL, Nancy, 1988, p 7 8 R Griot, G Brun and J C Tadenac, Proc 8th Intl Conf Thermoelectric Energy Conversion,ed S Scherrer and H Scherrer, INPL, Nancy, 1988, p 27 9 C Machet, P Lebon and A Septier, le vide-les couches minces, 1982,211,125 10 J Przyluski and K Borkowski, Proc 6th Intl Conf Thermoelectrics,Arlington, TX, 1986, p 100 11 Y Shing, Y Chang, A Mirshafii, L Hayashi, S Roberts, J Josefourier and N Tran, J Vac Sci Technol, 1983,171,903 12 E Charles, E Groubert and A Boyer, J Mater Sci Lett, 1988, 7,579 13 C Lokhande and S Pawar, Phys Status Solidi A, 1989,111,17 \6-150; 14 C De Mattei and R Feigelson, Electrochemistry of SemiconductorsI: and Electronics Process and Devices, ed J McHardy and=t-17 51 F Ludwig, Noyes, New Jersey, 1992, p 1 15 B Miller and A Heller, Nature (London),1976,262, 680-20 0* 50 100 150 200 250 300 350 16 A Baranski, W Fawcett and C Gilbert, J Electrochem Soc , 1983, TIK 130,2423 Fig.11 Temperature dependence of the Hall constant RH (a), carrier concentration C, (b) and Hall mobility pH (c) for an electrodeposited B1179Te3 21 film 17 18 H Chaouni, P Magri, J Bessieres, C Boulanger and J J Heizmann, Proc Icotom 10, Mater Forum, 1994, 157-162, 1371 P Magri, C Boulanger and J M Lecuire, Proc 13th Intl Conf 778 J Mater Chem , 1996,6(5), 773-779 Thermoelectrics,ed. B. Mathiprakasam and P. Heenan, AIP Press, New York, 1995,p. 277. 19 M. Takahashi, Y. Katou, K. Nagata and S. Furuta, Thin Solid Films, 1994,240, 70. 20 G. Charlot, Chimie Analytique Quantitatiue, Masson, Paris, 1974, p. 355. 21 C. Boulanger and J. M. Lecuire, Electrochim. Acta, 1987,32,345. 22 M. H. Francombe, Br. J. Appl. Phys., 1958,9,415. 23 R. F. Brebrick, J. Appl. Crystallogr., 1968,1, 248. 24 M. Pourbaix, Atlas d’Equilibres Electrochimiques, Gauthier-Villars, Paris, 1963,p. 568. 25 P. Magri, These de 1’UniversitC de Metz, 1995. 26 P. Magri, C. Boulanger and J. M. Lecuire, Analusis, to be published. 27 L. J. Van Der Pauw, Philips Res. Rep., 1958,13, 1. 28 L. J. Van Der Pauw, Philips Tech. Rev., 1958,20,220. 29 L. Lacourcelle, Protection contre la corrosion, Presses Universitaires de France, Pans, 1976,p. 120. 30 J. P. Fleurial, L. Gaillard, R. Triboulet, H. Scherrer and S. Scherrer, J. Phys. Chem. Solids, 1988,49, 1237. 31 I. Smirnov, E. Shadrichev and V. Kusatov, Sou. Phys. Solid State, 1970,11,2681. 32 C. Satterthwaite and R. Ure, Phys. Rev., 1957, 108, 1164. Paper 5/06632H; Received 9th October, 1995 J. Muter. Chem., 1996, 6(5), 773-779 779

ISSN:0959-9428    

DOI:10.1039/JM9960600773

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Oxidation of alkaline-earth-metal sulfide powders and thin films Janos Madarasz,?Tuula Leskela, Janne Rautanen and Lauri Niinisto" Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-021 50 Espoo, Finland The oxidation of CaS and SrS powders and thin films was studied in situ up to 1300 "C by thermoanalytical techniques (TG, DTA and high-temperature XRD) while FTIR and powder XRD were used to analyse ex situ the solid reaction intermediates and products. CaS powder starts to oxidize to CaSO, around 500 "C but the oxidation is not complete because of a competing reaction, i.e. the decomposition of CaSO, to CaO, which is significant above 1000"C. SrS0, is more stable and therefore SrS completely oxidizes to SrSO, before the decomposition starts.FTIR and XRD failed to detect CaSO, or SrS0, at any stage of the heating process. Particle size appears to have a marked effect on the decomposition of powders, while moisture plays only a minor role. Thin films appear to be more stable towards oxidation but they react around 1000 "C with the silicon substrate. Alkaline-earth-metal sulfides, especially the isostructural CaS and SrS, are important matrix materials for electroluminescent (EL) thin films used in flat-panel full-colour displays. When these sulfide layers are doped with rare-earth-metal ions, e.g. trivalent Eu, Tb or Ce, they emit in red, green and green-blue, re~pectively.'-~ Although the emissive layer in an electrolumi- nescent structure is well protected from the atmosphere, there is a risk that at least its surface may become oxidized or degraded during deposition or annealing.Decreasing the extent of oxidation and degradation would enhance the luminescence efficiency and stability of these devices. While systematic studies on oxidation and degradation of alkaline-earth-metal sulfide thin films are lacking, there are several reports on the effects of oxygen contamination. Okamoto and Hanaoka6 reported that the luminance of elec- tron-beam (EB)-deposited SrS :Ce3+ thin films was reduced owing to the presence of a significant amount of oxygen in form of SrC0, in the precursor pellet. Abe et aL7 have systematically examined the effects of oxygen partial pressure in the EB evaporation atmosphere and found out that an increase in the oxygen pressure led to a higher oxygen content in the CaS :Eu2+ films and simultaneously decreased the electroluminescent efficiency owing to the oxidation of some Eu2+ ions to Eu3+.A reductive atmosphere appears generally advantageous for the Eu2+ and Ce3+ dopants. Thus, for instance, H, gas applied during EB evaporation could improve up to 2.3 times the luminance of a SrS:Ce,Eu,K thin film.' Heat treatment of an SrS :Pr,Ce EB precursor pellet in a H,S atmosphere resulted in films showing luminances five times greater than was found for films without the heat treatment.g On the other hand, there are reports that oxygen in EL layers appears in some cases not to be detrimental. Park et a!." observed (by XPS) oxygen-related luminescence centres in a CaS :Bi3+ phosphor.Furthermore, they found that CaS :Eu and SrS :Eu phosphors can be stabilized against atmospheric moisture and carbon dioxide by annealing above 700 "C in air for several hours." According to their XPS study such a treatment results in the formation of CaSO, and a spurious 'strontium oxysulfide (SrSO)'. Based on the XPS spectra of the SrS :Pr,Ce powder, several sulfur species including elemen- tal S and SO3,-are suggested to appear on the surface owing to the oxidizing effect of moist air.g These suggestions do not seem to be well founded because, for instance, the chemical shifts of the S species show SO4,-formation.', Poelman et al.I3 correlated the additional peaks in the emission spectra of an t Permanent address: Institute of General and Analytical Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary.SrS, -$ex :Ce layer to its SrSO, contamination. When oxygen is present the rare-earth-metal dopant ions in alkaline-earth- metal or zinc sulfide matrices may form (LnO),,"+ polycation clusters as shown by extended X-ray absorption fine structure (EXAFS) studies.14 The thermal oxidation processes of CaS are frequently studied above 500°C in order to investigate the complex desulfurization processes of gases obtained from coal gasifi- cation or from combustion using lime or limestone as absorb- ent~.'~,'~Some of the model components in such a system were recently studied by thermogravimetry (TG), differential thermal analysis (DTA) and X-ray powder diffraction (XRD).17 Chemical species formed in reactions of SO, with CaO and CaCO, have been monitored by Fourier-transform IR pho- toacoustic spectroscopy (FTIR-PAS)." The primary product found in the desulfurization process of combustion gases was CaSO, which, however, is thermodynamically unstable in comparison to CaSO,.As part of our ongoing research into the deposition and characterization of SrS and CaS thin films for EL appli- ~ations,'~-~'the present investigation was undertaken using a combination of thermal and diffraction methods as well as by employing FTIR spectroscopy. The main emphasis was on SrS, but CaS was studied also and the results compared with literature data.Experimenta1 Preparation of the samples SrS and CaS powders. SrS and CaS powders were prepared by two slightly different methods. In the first22 (Method A) we used two different strontium carbonates [from Schering A.-G. and Baker, containing 130 and 60 pg (g SrC03)-' barium, respectively] as starting materials for SrS while the starting material for CaS was calcium carbonate (from Merck). The carbonates (3-4 g) were weighed in A1,0, boats which were placed inside a ceramic tube in the tube furnace (Carbolite Furnaces CTF 1200). The temperature of the furnace was raised to 1000 "C and a stream of H2S (purity 99.999%) diluted with Ar-H, (5%) gas (flow rate 3-5ml s-') was passed through the furnace for 3 h. During cooling only the Ar-H, mixture was passed through the furnace.,, The other SrS and CaS samples (B) were prepared in CNRS, Talence, France.These samples were synthesized from the corresponding sulfates by reduction in an Ar-H, (10%) stream at 1000 "Cfor 15 h. After grinding the products were annealed at 1100°C under H2S for 1 h. J. Mater. Chem., 1996, 6(5), 781-787 781 SrS and CaS thin films. Sr(thd),, Ca(thd), (thd= 2,2,6,6,- tetramethylheptane-3,5-dione)and H,S were used as precursors for the deposition of SrS and CaS thin films by atomic layer epitaxy (ALE) ''23 The hot-wall, flow-type F-120 reactor, manufactured by Microchemistry Ltd (Espoo, Finland), was employed in the depositions Phosphorus-doped Si( 100) wafers were used as substrates The substrate and source temperatures were 360, 225 "C for SrS and 350, 197 "C for CaS, respectively The thicknesses of the SrS and CaS films were 620 and 630 nm, respectively, as evaluated from reflectance spectra measured using a Hitachi U 2000 spectrophotometer, and the program of Ylilammi and Ranta-aho 24 Thermal analysis TG measurements were carried out in a Seiko Instruments TG-DTA 320 analyser, equipped with a SSC/5200 disk station The heating rates were 2 and 10°C min-' A120, and Pt crucibles were employed and Al,O, was used as a reference material The experiments were carried out in flowing air using flow rates of 80 or 220 ml min-' The sample size was typically 20-30 mg Some experiments were also performed in moist air produced by passing air at a rate of 80ml min-' through a gas washing bottle filled with water IR, XRD and SEM measurements SrS and CaS powders and the products after heating in air were characterized by IR spectroscopy and X-ray diffraction IR spectra of the samples were recorded in the region 4000-400 cm -'with a Nicolet Magna-FTIR 750 spectrometer using the KBr disk technique X-Ray diffractograms were recorded with a PhilipsoMPD 1880 diffractometer using Cu-Ka radiation (A= 154060 A) An Anton Paar HTK 10 high-temperature goniometer attachment was used for zyt sztu X-ray diffraction (XRD) measurements during heating of CaS and SrS powders and thin films in air The powder samples were glued onto Pt wire and measured using a linear position-sensitive detector (PSD) while the thin films were measured with a normal proportional counter equipped with a graphite monochromator Cu-Ka was used as the X-ray source in both cases For powder samples the Pt(ll1) reflection was used as an internal standard Scanning electron microscopy (SEM) was used to check the particle size and its distribution in the CaS samples prepared by the two methods Results and Discussion Thermal analysis of powder samples Oxidation of SrS powders in air.The thermal behaviour of SrS powders in air is basically straightforward and proceeds as expected the oxidation to SrS0, begins slowly around 700"C, the reaction rate is increased at 800°C and the mass at 1300°C corresponds to SrSO, (Fig 1) Oxidation to SrSO, does not appear to take place under these conditions According to FTIR measurements the formation of SrSO, was clearly observed in the samples heated to 600 "C but the first indications of oxidation to sulfate were observed at 450 "C because in this case IR spectroscopy is a much more sensitive method than TG SrS synthesized from SrSO, (method B) was oxidized almost completely to sulfate by 1300°C when a heating rate of 10°C min-' was used The flow rate of air influenced the oxidation slightly (97 8% with 80 ml min-' and 99 7% with 220 ml min-') In the case of method A samples only 70% of the theoretical yield of SrS0, was achieved even at 1300°C The remaining SrS was unchanged according to XRD analysis (Fig 2) The conversion could be increased to 90% using the slower heating rate (2°C min-l) and smaller sample size (5 mg), but 100% conversion to sulfate could not 782 J Mater Chem, 1996, 6(5),781-787 160 I G 110 100 ex0 80 70 -tendo Fig.1 Simultaneously recorded TG and DTA curves for SrS (sample B) in flowing air (80 ml min-I), recorded with a heating rate of 10 "C min Initial mass 22 55 mg be reached The main reason for the different behaviour between the two SrS samples dunng heating in air is most probably the different particle size, which will be discussed in more detail below After SrSO, has been formed there is an additional change as the DTA curve reveals an endothermic transition at 1150°C without any change in the TG curve This reversible transition can be assigned to the orthorhombic- cubic phase transition of SrSO, 25 The oxidation in moist air does not deviate much from the oxidation in dry air, except that at temperatures above 1000"C the oxidation takes place somewhat quicker in moist air Therefore, in contrast to concern expressed in the literat~re,~ SrS phosphors appear not to be significantly moisture-sensitive Oxidation of CaS powders in air.The oxidation of CaS samples begins around 500 "C (Fig 3, 4) The oxidation rate in sample B is initially very slow, but it increases above 750 "C and is very fast up to 1050°C (Fig 3) According to the IR measurements the formation of CaSO, was clearly observed in the samples heated to 600°C but, also in this case, the first indications of oxidation to sulfate were observed at 450°C Again, no formation of sulfite could be seen (Fig 5) The theoretical yield of CaS0, (188 7% of the original mass) was not achieved by either sample, however The factors affecting the amount of CaSO, formed include the heating rate, sample amount and particle size When the heating rate was slow (2°C min-') the mass gain was greater (77%) compared to the faster heating rate (10 "C min-') which resulted in a mass gain of 62% According to thermodynamic data26 27 SrSO, is more stable at higher temperatures than CaSO, Theoretically it is not possible to completely oxidize CaS to CaSO, because the decomposition of CaSO, to CaO begins while the oxidation of CaS is still occuring The decomposition reaction cannot be seen from the TG curve of sample B (Fig 3) because the two competing reactions balance each other In the A sample a sudden mass loss between 1060 and 1115 "C followed by a rapid mass increase can be seen (Fig 4) According to XRD analysis the amount of CaO is small until 950"C, but when the sample is heated to 1085°C the product contains a considerable amount of CaO as well as CaSO, and the unreacted CaS (Fig 6) At 1200°C CaS could no longer be observed The main reason for the different thermal behaviour of the two CaS samples is the particle size, which in the A sample is >5 pm while in the B sample it is t2 pm as measured by SEM When CaS with a larger particle size (sample A) was ground in an agate mortar the sudden mass loss at 1085°C almost disappeared A higher conversion to sulfate was also 20.00 30.00 40.00 50.00 60.00 2e/degrees Fig.2 XRD patterns of SrS (sample A) at room temperature (a) and after heating up to 600 (b),950 (c), 1300 "C (d)at 10 "C min- rate in flowing air (80 ml min -') :e% i r140 130s!;120 g 110 100 90 80 endo J70' * ' ' ' 1 ' ' ' ' ' 0 200 LOO 600 800 1000 1200 1400 TI"C Fig.3 Simultaneous TG and DTA curves for CaS (sample B) in air (80ml min-'), recorded with a heating rate of 10°C min-'. Initial mass 22.04mg. 11 0 -105k? v) Y 1002 95 90 85 80 0 200 400 600 800 1000 1200 1400 TPC Fig.4 Simultaneous TG and DTA curves for CaS (sample A) in air (80ml min-'), recorded with a heating rate of 10°C min-'.Initial mass 18.56 mg. achieved by grinding the sample. An obvious explanation is that if the particle size is larger the diffusion of oxygen into the particles is slower and the CaSO, formed at the outer surface of CaS particles begins to decompose before the inner part has oxidized. It may also be possible that the CaSO, acts 21 0 220200 k677 I 592190 610 170 120 -110 -100 7 (b) -90 80 -70 -60 -50 -(a) 1800 1600 lL00 1200 1000 800 600 Fig.5 FTIR spectra of CaS (sample A) recorded in the range 1800-400 cm-' after heating the sample up to (a) 350, (b)600, (c) 950 and (d) 1300"C as a barrier for the oxygen diffusion and slows the oxidation.Grinding the powder provides a larger surface for oxidation and shortens the diffusion length needed to complete oxidation. However, even with small particle sizes a 100% conversion to sulfate cannot be achieved because of the instability of CaSO, at high temperatures. The DTA curve of CaS also reveals an endothermic trans- ition at 1220°C. This reversible transition is due to the orthorhombic-cubic phase transition of CaSO, .28 Our results can be compared with those presented recentlyI7 where it was found that the oxidation of CaS began, under the experimental conditions used, at 500"C and the maximum J. Mater. Chem., 1996, 6(5),781-787 783 C8S (220 o',,,,,,,,, .,,,,,,,, ,,",",, ,,.,,,,,,10 d0 40 $0 I 0 2@/degrees Fig.6 XRD pattern of CaS (sample A) cooled from 1085 "C to room temperature The unassigned reflections belong to CaSO, mass was reached around 950°C The total mass increase was only 7%, which can be explained by a small gas flow rate (15 ml min-l) and a large sample size (42 mg) The oxidation was a two-step process and a possible explanation, according to the author, was that CaS oxidizes first to CaSO, which then oxidizes to CaSO, The existence of CaSO, was not proven by any analysis, however According to the ex sztu XRD analysis the different phases present at 1100°C were CaSO,, CaO and CaS The oxidation in moist air is very similar to the oxidation in dry air initially, but at temperatures above 850°C the oxidation rate is higher in moist air The total mass gain is also ca 10% higher in moist air Although the differences in the observed oxidation behaviour for CaS in dry and moist air are small, they are clearly observable below 1000 "C whereas a similar effect was not seen for SrS This may be connected to the higher oxidation tendency of CaS compared with SrS High-temperature XRD of powders and thin films Mainly because of the small amount of sample, thin films cannot normally be studied using the conventional thermo- analytical techniques such as TG and DTA 29 High-tempera-ture X-ray diffraction (HTXRD) is one of the few methods available for zn sztu monitoring of the thermally induced changes occurring in thin films3' One of the problems in HTXRD studies is sample alignment, which may lead to erroneous peak positions Also, the linear PSD detector causes errors to the peak positions near the edges of the measuring windows A further source of error is the sample temperature, which may significantly deviate from the programmed one if the conduction of heat from the sample through the substrate to the thermocouple is not good32 In this work the errors in the peak positions were minimized by using the Pt(ll1) reflection and the silicon substrate reflections as internal standards for the powder and thin film samples, respectively The temperature gradients between the sample and the Pt heating element were minimized by waiting 10min at each measuring temperature before performing the XRD measure- ment Also the heating rate between each temperature was relatively slow A fourth, more serious problem in the HTXRD study of thin films is caused by the reaction of the substrates with the 784 J Muter Chem, 1996, 6(5),781-787 thin film For silicon substrate this takes place at higher temperatures and may lead to a number of silicon-containing crystalline phases which are sometimes hard to identify Fig 7 displays the XRD pattern of an SrS powder recorded zn sztu at temperatures from 30 to lOOO"C, and Fig 8 displays the XRD pattern of CaS powder recorded zn sztu at tempera- tures from 30 to 1100 "C According to the HTXRD measure- ment, the SrS powder began to oxidize at 750 "C The oxidation product was SrSO, but the SrS phase was still present after heating the powder to 1000 "C 33 The CaS powder also started to oxidize at 750°C and the product was initially CaSO, but at 1000°C the formation of CaO started because of the decomposition of CaSO, 34 The CaS peaks disappeared between 1000 and 1100°C The intensity of the CaSO, peaks decreased after 1000 "C while the intensities of the reflections due to CaO increased Fig 9 shows the zn sztu HTXRD results for an SrS thin film sample from room temperature to 1000°C It appears that the (111) oriented SrS thin films are thermally very stable because the intensity of the SrS reflections begins to decrease only above 800 "C At 1000 "C a considerable amount of SrS is still left together with SrSO, and the silicon-containing phases SrSiO, and Sr,SiO, (Fig 9) 35 Prolonged heating (15 h) at 1000°C causes the SrS phase to disappear, however Fig 10 shows the zn sztu HTXRD results for a CaS thin film sample heated from room temperature up to 1000°C The pattern at room temperature displays a typical mixed orien- tation with (lll), (200), (220) and (222) as the most dominant reflections l9 The first signs of thermal reactions are visible at 600°C where, for instance, the (111) and (200) peaks of CaS have a diminished intensity as compared to a sample recorded at room temperature At 800°C CaS is still present but part of it has been converted to CaSO, At the same time CaSO, has partly decomposed to CaO The most prominent peaks at 1000"C are due to silicon-containing phases, however, showing that oxygen diffuses into the substrate and the SiO, formed reacts with CaO Indeed, the presence of CaSiO, can be concluded from the XRD pattern (Fig 10) when comparing it to the JCPDS data 36 A difference to the SrS thin film pattern at the same temperature is the absence of the 2 1 phase (Ca2Si0,), instead the (111) and (222) reflections of SiO, (cri~tobahte)~~are relatively strong Although the SiO, detected 50 62 00 22.00 26.00 30.00 34.00 2Wdegrees Fig.7 SrS powder (sample B) measured in situ by high-temperature XRD at (a) 30, (b)500, (c) 750 and (d) 1OOO"C in air using linear PSD. The measuring time for each spectrum was 500 s. Intensities are in root scale. -00N Y 1 .60 3 L.60 I I I 40 (f 1 0.ool I I I I I I I e00 23.00 27.00 31 .OO 35.00 26,/degrees Fig.8 CaS powder (sample B) measured in situ by HTXRD at (a) 30, (b)500, (c) 750, (d) 900, (e) 1000 and (f) 1100"C in air using linear PSD. The measuring time for each spectrum was 500 s. Intensities are in root scale. is a high-temperature phase (>1200"C), its presence at 1000"C be seen in the TG curves. The sample characteristics, uiz. its can be explained by the effect of calcium ions which lower the synthesis and especially the particle size, have a significant transition temperature. 37 The peak at 28= 17.0" could not be effect on the reaction temperatures and mechanism. This is indexed with certainty; it probably belongs to a transient phase especially obvious for CaS where the formation of CaSO, and because it almost disappeared when the sample was kept at its decomposition to CaO are competing reactions.1000°C for 1.5 h. The alkaline-earth-metal sulfide thin films, in particular the (111) oriented SrS, appear to be remarkably resistant towards Conclusions oxidation which is a promising sign in view of their processing and for possible application in EL devices. The reaction with The oxidation of both CaS and SrS powders in air leads to the substrate observed by high-temperature XRD at 1000"C the formation of the sulfate phase without the sulfite intermedi- is not a problem in real applications where the deposition and ate. The first signs of oxidation are observed in the IR spectra annealing temperatures used are much lower, for instance in at 450 "C, but only at higher temperatures can a mass increase the atomic layer epitaxy (ALE)technique around 400-500 0C.38 J.Muter. Chem., 1996, 6(5),781-787 785 40 10 00 20 00 24 00 28 00 32 00 36 00 40 00 2Bldegrees Fig. 9 SrS thin film sample measured zn situ by HTXRD at (a) 28, (b) 600, (c) 800 and (d) 1000 "C using normal detector The data collection time per spectrum was 1 25 h and the spectrum was collected in the range 20 =20-60" a =SrSO,, b =Sr2Si04,c =SrSiO, and d =unidentified peak Si(200) is a forbidden reflection of Si Intensities are in root scale '1 nn zoo e75 cc C ccc00I e 00I I I I I I I I 15 00 25 .OO 3s 00 45 00 55 00 2Bldegrees Fig. 10 CaS thin film sample measured zn situ by HTXRD at (a) 28, (b) 600, (c) 800 and (d) 1000°C using normal detector The data collection time per spectrum was 1 25 h and the spectrum was collected in the range 20 =20-60" a =CaO, b=CaSO,, c =CaSiO, and d =unidentified peak (see text) Intensities are in root scale Professor C Foaussier (CNRS, Talence) is gratefully acknowl- 2 M Leskela and L Niinisto, Mater Chem Phys 1992,31,7 edged for providing the CaS and SrS powder samples The 4 M Leskela, M Makela, E Nykanen and M Tammenmaa, Chemtronics 1988,3, 113 authors are also indebted to Ms E Nykanen for growing the 4 M Leskela, L Niinisto, E Nykanen, P Soininen and M Tiitta, SrS thin film sample and to Dr M Ritala for the SEM J Less-Common Met 1989,153,219 measurements An Eotvos scholarship from the State of 5 M Leppanen, M Leskela, L Niinisto, E Nykanen, P Soininen Hungary and partial support from the National Scientific and M Tiitta, SID91 Digest 1991,282 Research Foundation (OTKA, Budapest, Grant No F014518) 6 K Okamoto and K Hanaoka, Jpn J Appl Phys 1988,27, L1923 to Janos Madarasz are gratefully acknowledged 7 Y Abe, K Onisawa, Y A Ono and M Hanazono, Jpn J Appl Phys 1990,29,1495 8 Q Z Gao, J Mita, T Tsuruoka, M Kobayashi and K Kawamura, J Crystal Growth 1992,117,983 References 9 K Onisawa, Y Abe, K Tamura, T Nakayama, M Hanazono and Y A Ono, J Electrochem SOC 1991,138,599 1 S Tanaka, H Deguchi, Y Mikami, M Shiiki and H Kobayashi, 10 H L Park, H K Kim and C H Chung, Solid State Commun SID86 Digest 1986,28 1988,66,867 786 J Muter Chern, 1996, 6(5), 781-787 M Han, S-J Oh, J H Park and H L Park, J Appl Phys, 1993, 73,4546 24 25 M Ylilammi and T Ranta-aho, Thin Solid Films, 1993,232, 56 F D Rossini, D D Wagman, W H Evans, S Levine and I Jaffe, T A Carlson, Photoelectron and Auger Spectroscopy, Plenum Circ Bur Stand, 1952, No 500,785 Press, New York and London, 1975, pp 200,205,356 D Poelman, R Vercaemst, R L Van Meirhaeghe, W H Lafrere and F Cardon, Jpn J Appl Phys Part 1, 1993,32,3477 26 27 I Barin, Thermochemical Data of Pure Substances, VCH Verlag, Weinheim, 1989 M Fredriksson and E Rosen, Chem Scr , 1980,16,34 15 16 17 18 19 20 21 22 Y Charreire, 0 Tolonen-Kivimaki, E Nykanen, M Leskela, L Niinisto, D Bonnin and 0 Heckmann, Thin Solid Films, 1994, 247, 151, and references therein K Schwerdtfeger and I Barin, Erdol Kohle, Erdgas, Petrochem, 1993,46,103 P F B Hansen, K Dam-Johansen and K Oestergaard, Chem Eng Sci, 1993,48,1325 K Wieczorek-Ciurowa, J Therm Anal, 1992,38,523 M A Martin, J W Childers and R A Palmer, Appl Spectrosc, 1987,41,120 J Rautanen, M Leskela, L Niinisto, E Nykanen, P Soininen and M Utriainen, Appl Surf Sci, 1994,82/83, 553 P Soininen, L Niinisto, E Nykanen and M Leskela, Appl Surf Sci , 1994,75,99 S Lehto, P Soininen, L Niinisto, J Likonen and R Lappalainen, Analyst, 1994,119, 1725 J W Brightwell, B Ray and C N Buckley, J Crystal Growth, 1982,59,210 28 29 30 31 32 33 34 35 36 37 38 0 W Florke, Naturwissenschaften, 1952,39,478 M Leskela, T Leskela and L Niinisto, J Therm Anal, 1993, 40,1077 D D L Chung, P W DeHaven, H Arnold and D Ghosh, X-Ray Diflractzon at Elevated Temperatures-A Method for In Situ Process Analysis, VCH, New York, 1993 Shinichi Ohya and Yasuo Yoshioka, Adv X-Ray Anal 1990, 33,397 N E Brown, S M Swapp, C L Bennett and A Navrotsky, J Appl Crystallogr ,1993,26,77 JCPDS cards 8-489,5-593 JCPDS cards 8-464,37-1496,4-777 JCPDS cards 34-99,39-1256 JCPDS cards 3 1-300,27-605 F Liebau, Structural Chemistry of Silicates, Structure, Bonding and Classrjication, Springer-Verlag, Berlin, 1985 L Niinisto and M Leskela, Appl Surf Scz , 1994,82/83,454 23 M Tammenmaa, H Antson, M Asplund, L Hiltunen, M Leskela, L Niinisto and E Ristolainen, J Crystal Growth, 1987,84, 151 Paper 5/06493G, Recieved 2nd October, 1995 J.Muter Chem, 1996, 6(5),781-787 787

ISSN:0959-9428    

DOI:10.1039/JM9960600781

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

LiSb(edta) (H,O): a convenient precursor to LiSbS, and LiSb03 Bertrand Marrot, Chantal Brouca-Cabarrecq and Alain Mosset" CEMESf CNRS, 29 rue Marvig, 31055 Toulouse Cedex, France The synthesis and crystal structure of the edta complex LiSb(edta)( HzO) are reported (H,edta =ethylenediaminetetraacetic acid). The compound crystallizes in the monoclinic system. Sb(edta) entities are connected by cyclic [Li(COO)], dimers resulting in parallel layers stacked in the [1lo] direction. Pyrolysis of this complex under sulfur vapour between 700 and 850 "C leads to the mixed sulfide 0-LiSbS,. Pyrolysis in air above 660 "Callows the preparation of the mixed oxide LiSb03. Much research has been carried out in attempts to find new fast ion conducting materials for applications in solid-state batteries or other ionic devices.These compounds are mainly oxides, but sulfides also have been investigated extensively and a number of compounds have been shown to possess interesting electrical properties. For instance, AgIn,S8 shows high ionic conduction, while AgSbSz and Cu3BiS3 exhibit mixed electronic-ionic conduction. Li3,Sb6 -,s, presents elec- tronic behaviour for x=+ and mainly ionic behaviour for x=4.1-4 Improvements in conductivity of several orders of magni- tude can be obtained through substitutions as shown e.g. for Li,GeO,. This material is a moderately good conductor (lo-, K1cm-l at 300°C) but high conductivity (10-1!X1cm-' at 300°C) is found in Li3,,Zn,.,,Ge04.5 We intend to apply this idea to the chemistry of the group 15 metals antimony and bismuth, and to study not only mixed oxides but also mixed sulfides.Indeed, investigation of the phase diagrams of the systems M2S-M',S3 (M=Li, Na, Cu, Ag; M' =Sb, Bi) shows numerous, extremely stable sulfides, some of which occur as minerals (e.g. pyrargyrite, emplectite, pavonite). These systems are interesting because the trivalent nature of the group 15 element and the stereoactivity of the electron lone pair could lead to a great diversity of structural types and, perhaps, to frameworks optimized for ionic conduction. One of the goals in modern solid-state chemistry is to find new synthesis routes at low temperatures through so-called soft chemistry. Different methods can be used to prepare sulfides with complicated stoichiometries.The coprecipitation method has problems of reproducibility and stoichiometry control owing to the different solubilities of cation salts. The all-alkoxide sol-gel method is very promising, and has been used successfully for the preparation of CaLa,S, .6 However, the alkoxide precursors are unstable towards moisture, expens- ive and sometimes difficult to prepare in the case of heterobime- tallic compounds. Our approach to the problem is to use edta coordination complexes as starting products. The ligand itself and several sodium salts are commercially available and are rather cheap. A large number of metallic and heterobimetallic complexes are known and are easy to prepare in aqueous media.The great majority are extremely stable under ambient conditions and can be stored. Further, the sulfurization process using sulfur vapour is more efficient on organometallic compounds because of the formation of carbon disulfide, a very powerful sulfid- ing agent. In this paper, we report the initial results of this study. A new edta complex, LiSb(edta)(H,O), has been prepared and its structure has been solved. Results of pyrolyses under air and sulfur vapour are reported. They prove the feasability of the synthesis method in the case of the bimetallic, i.e. non-substituted, sulfides and oxides. Experimental Crystals of LiSb(edta)(H,O) were prepared by the following procedure. Antimony oxide, Sb203 (20 mmol), and H,edta (40mmol) were dissolved in 150ml of water and heated at reflux for 1 h.Lithium acetate (40 mmol) was then added and refluxing was continued for a further 2 h. The pH of the resulting solution was 4.5. After cooling, the solution was evaporated slowly at room temperature. After 30 days, suitable colourless single crystals were obtained. The chemical formula was established by elemental analysis (see Table 1).Diffracted intensities were measured on a CAD- 4 Enraf-Nonius automatic diffractometer. The space group was determined from the examination of systematic reflection conditions. The crystal structure was solved using the Patterson method and difference Fourier syntheses. All hydrogen atoms were located. Refinement calculations were performed using SHELX76 with a weighting scheme w =k/[02(F)+k'F2].Crystal data, measurement and refinement parameters are given in Table 2. Scattering factors for neutral atoms and f,f' were taken from the International Tables for X-Ray Crystallography.8 X-Ray powder diffraction measurements were recorded on a Seifert diffractometer using Cu-Kar. radiation. Diffraction patterns for single phases were indexed by comparison to the JCPDS library and using the LAZY PULVERIX program.' Pyrolyses were conducted in a Thermolyne tube furnace. The apparatus used for sulfide preparation is schematically drawn in Fig. 1. To avoid the presence of oxygen and moisture, the furnace was flushed with dry nitrogen for 14 h. Samples, placed in a mullite boat, were heated to 200°C in a dry nitrogen flow and then, at a fixed temperature for 2 h, in a flow of nitrogen and sulfur vapour (partial pressure = 10 Torr).During cooling, the sulfiding atmosphere was stopped when the sample temperature reached 200°C but the nitrogen flow was maintained. Table 1 Elemental analysis of the edta complex element experimental (Yo) Sb 26.42( 9) Li 1.56(3) C 27.88( 6) H 3.29(5) N 6.63(6) 0" 34.22 a Obtained by difference to 100%. calculated for LiSb(edta)(H,O) (%) 27.99 1.59 27.62 3.24 6.44 33.1 1 J. Muter. Chem., 1996, 6(5), 789-793 789 Table 2 Crystal data, intensity measurements and refinement para- Table 3 Final least-squares atomic parameters with estimated standard meters for LiSb(edta)(H,O) deviations for SbLi(edta)(H,O) formula mass 434.9 ~~ ~space group min z 4 0.24512( 5) 0.24332(2) 0.06845(4) 1.46(1)44 7.348( 1) 0.5107( 15) 0.0471 (6) 0.6459( 11) 2.27(2) blA 16.833(6) 0.3285(6) 0.2227(2) -0.125 2.21(4) CIA 10.822(2) 0.4484( 6) 0.1350( 3 J -0.2502(4) 2.62(5) Bl" 92.84(2) 0.18 16( 6) 0.1200( 3) 0.1565( 5) 2.47(5)VIA' 1336.9(6) 0.2995( 6) 0.0207( 3) 0.2734( 5) 2.81(5) diffractometer Enraf-Nonius CAD4 0.3356(6) 0.2697( 2) 0.2521 (4) 2.10(6) radiation, monochr. Mo-Ka, graphite 0.4590( 7) 0.3582(3) 0.3814(5) 2.78(4) T/K 293 0.1355(6) 0.3896( 3) 0.0242 (4) 2.13( 5) crystal size/mm3 0.25 x 0.125 x 0.05 0.2087( 6) 0.4937(2) -0.091 3( 5) 2.28(5) picm -21.2 0.1427( 7) 0.9870(4) 0.8980(4) 3.73(8) D,/g cm-3 2.16 0.5049( 7) 0.1668( 3) 0.0808(5) 1.39(8) F(000) 856 0.4772( 7) 0.3356( 3) 0.0463( 5) 1.46( 9) scan mode w scan 0.6711(9) 0.2191 (4) 0.0928 (7) 1.8( 1) data collection limits O< 04 30" 0.6486(8) 0.2929( 4) 0.0157(6) 1.8(1) -ll<h<ll; O<k<24, 0.5051( 10) 0.1 172(4) -0.0342( 7) 1.9(2) 0<1<16 0.4240( 8) 0.1609(4) -0.1470( 6) 1.7(2) number of reflections total 4147, unique 3892, 0.5012( 10) 0.1 129(4) 0.1903(7) 1.8( 1) with I z 3a(I) 2051 0.31 16( 9) 0.0805( 4) 0.2085(6) 2.1(1) no.of variables, R, R, 251, 0.033, 0.032 0.5018(9) 0.3778 (4) 0.168 1 (7) 1.7(2) pmaxin final AF synthesisle k3 1.61 0.4298( 8) 0.3336( 4) 0.2767(6) 1.7(2) 0.4308( 9) 0.3936(4) -0.053 l(6) 1.7(2) 0.2426(9) 0.4296( 3) -0.0391 (5) 1.6(2) Results and Discussion Description of the crystal structure of LiSb(edta) (H,O) Final positions and equivalent isotropic thermal parameters are given in Table 3, and selected bond angles and distances appear in Table 4.As shown in Fig. 2, the ethylenediaminetetraacetate ligand is coordinated to five metallic centres, one antimony atom and four lithium atoms. Among the eight oxygen and two nitrogen atoms of the ligand, only one oxygen atom [0(6)] is not involved in the coordination scheme. The six-coordination around Sb results in the formation of five chelated rings: four glycinate rings fused equatorially, G( 1) [Sb-N(2)-C(9)- C(10)-0(7)] and G(2) [Sb-N(l)-C(5)-C(6)-0(3)], or axially, R(l) [Sb-O(l)-C(4)-C(3)-N(l)] and R(2) [Sb-O(5)-C(8)-C(7)-N(2)], to the ethylenediamine E ring [Sb-N( 1)-C( l)-C(2)-N(2)].The resulting confor- mation is E,G/R according to the notation suggested by Porai- Koshits and co-workers,l0''' a conformation frequently ob- served in six-coordinate edta complexes. The five-membered rings deviate greatly from planarity, with dihedral angles (NCCN or NCCO) of 53.8, 21.8, 20.2, 16.8 and 8.6" for E, G(l),G(2), R( 1) and R(2), respectively. The three dicoordinate carboxylate groups show a classical dissymmetry* in the C-0 distances with a mean 'shyt' distance of 1.23( 1) A and a mean Fig. 2 ORTEP view of the edta coordination to antimony and lith- 'long' distance of 1.28(2) A. The only no?-coordinated oxygen ium atoms atom gives a C-0 distance of 1.215A. According to the + H20 NaOH Fig.1 Diagram of the sulfiding apparatus. 1, Sulfur mullite boat; 2, sample mullite boat. 790 J. Muter. Chem., 1996, 6(5),789-793 Table 4 Interatomic distances (A)and angles(") Sb-N( 1) Sb-O(1) Sb-O(5) Li-0(2) Li-0(7) 2.3007( 3) 2.2385( 4) 2.1119( 3) 1.9277(4) 1.9574( 3) N(l)-C( 1) N(2)--C(2) C(l)-C(2) C(4)-0(2) C(6)-0(4) C(8)-0(6)C( 10)-O( 8) 1.5053( 2) 1.501 1 (2) 1.5012(4) 1.2204( 2) 1.23 19( 3) 1.21 53( 2) 1.2390( 3) N( 1)-Sb-O( 1) N( l)-Sb-0(7) N(2)-Sb-O( 3) O(l)-Sb-0(3) 0(3)-Sb-0(5) 0(2)- L1- 0(4) O(4)- L1- O(7) 72.71( 1) 141.35( 1) 142.70( 1) 108.20( 1) 82.15( 1) 11 1.67( 1) 105.56( 1) N( 1 )-Sb-O(3) N( 1)-Sb-N(2) N(2)-Sb-O( 5) O(l)-Sb-0(5) O(3)- Sb- O(7) O(2)-Li-O( 7) 0(4)-Li-0(8) C( 1)-N( 1)-C(3) C(2)-N(2)-C( 7) N( 1)-C( 1)-C(2) C( 3)-C(4)-0( 1) N( l)-C(5)-C(6) O(3)-C( 6)-0(4) C( 7)- C( 8)-O(6) C(9)-C( 10)-0(7) 11 1.28( 1) 110.79( 1) 11 1.63 ( 1) 116.10( 1) 112.00(1) 127.25( 1) 120.36(2) 115.84(2) C( 1)-N( 1)-C(5) C( 2)-N( 2)- C( 9) C( l)-C(2)-N(2) C( 3)-C(4)-0( 2) C( 5)-C( 6)-O( 3) N(2)-C( 7)-C( 8) O(5)-C(8)-O(6) C( 9)- C( 10)- O(8) standard notation for coordinated carboxylates, the classifi- cation is a-2-a for C(4) and C(6), 1-a for C(8) and a-3-sa for C( 10).12 The six-coordination of the antimony atom corresponds to an extremely irregular polyhedron.Two facts explain this characteristic: the stereoactivity of the Sb"' electron lone p?ir and the wide range of Sb-0 distances (from 2.112 to 2.628 A). The longest Sb-0 distance is observed for the p2-0(7) atom.The lithium atom shows a distorted tetrahedral surrounding; the greatest deviation appears for 0(2)-Li-0(7) (96.15'). The coordination of the edta ligand to five metal atoms results in an intricate three-dimensional network. The main characteristic is the formation of lithium cyclic dimers, localized on centres of symmetry, through the coordination of the C( ly) carboxylate group. The lithium-lithium distance is 3.53 A. This type of association of M' metal atoms (M=Li, Na, Ag) is not uncommon in the crystal structures of edta com-plexes. It has already been observed, for example, in Na,In(edta)(SO,)(H,O), (Na-Na= 3.43 A), LiNi(Hedta) Fig. 3 ORTEP view of a layer 2.3282( 4) Sb-Li 4.033( 1) 2.3402( 7) Li-Li 3.530( 1) 2.6281 (8) 1.9723( 3) 1.9126(2) 1.4996( 3) 1.4937( 3) 1.5001(2) 1.48OO( 3) 1.5214( 2) 1.283 1 (3) 1.5185(2) 1.2726( 2) 1.5089( 2) 1.2987(3) 1.5240( 2) 1.2625 (2) 70.22( 1) 76.67(2) 76.01( 1) 145.63( 1) 147.13( 1) 96.15( 2) 115.59(2) N( 1)-Sb-0(5) N(2)- Sb- O(1) N(2)-Sb-O(7) O(l)-Sb-0(7) 0(5)-Sb-0(7) 0(2)-Li-0(8) O(7)- Li-O( 8) 80.87( 1) 76.79( 1) 64.87( 2) 94.05( 1) 93.07( 1) 108.66( 1) 117.60( 1) 109.44( 1) 108.75(1) 110.18( 1) 119.50( 1) 115.30( 1) 114.76(2) 122.70( 1) 117.66( 1) C(3)-N( 1)-C(5) C( 7)- N(2)- C (9) N( l)-C(3)-C(4) O(l)-C(4)-0(2) C(5)-C( 6)- O(4) C(7)-C(8)-0( 5) N(2)-C(9)-C( 10) O(7)-C( 10)-O(8) 108.68(2) 109.86( 2) 112.32( 2) 124.37( 1) 117.43( 1) 116.94( 1) 11 1.54( 1) 126.50( 1) 1 (a) () f 24ii h i+! Y 2? 5 15 20 25.._.-id (b) I Fig. 4 Experimental (a) and calculated (b) X-ray patterns for LiSb(edta)(H,O). All the experimental peaks were indexed but this indexation is only shown for the 35 strongest peaks on the experimen- tal pattern. (H20) (Li- Li =3.74 A), Na,Mo,O,(edta)( JNa-Na = 3.74 A) and AgCu(Hedta)(H,O), (Ag-Ag=2.86 The three-dimensional structure can be described in terms of parallel layers. Fig. 3 shows such a layer. For the sake of clarity, only the atoms involved in the connections between metal atoms have been included. Fused rings built from four lithium dimers and four antimony atoms can be seen. In these rings, alternating long Sb-Li distances [through the C(6) carboxylate group] and short Sb- Li distances C4.03 A through the p2-o(7) atom] are found.Alternatively, the struc- ture of the layer can be described as a pseudo-hexagonal J. Muter. Chem., 1996, 6(5), 789-793 791 arrangement of lithium dimers These layers are stacked in the [ 1lo] direction and connected through Sb' atoms X-Ray powder data Large quantities of the edta complex can be prepared con-veniently using the above procedure with rapid evaporation at 25 "C The as-prepared microcrystalline powder is identical to the single crystals, as shown by comparison of the exper- imental diffraction pattern with the pattern calculated for the crystal structure (Fig 4) Preparation of LiSbSz The hexagonal form of the bimetallic sulfide, P-LiSbS2, has been obtained at 800°C by pyrolysis of the edta precursor in a sulfiding atmosphere This phase can be obtained free from impurities between 700 and 850°C The sulfide is slightly hygroscopic but it can be stabilized by subsequent drying at 3 1 1 ? 5 10 15 20 25 -60I 5 10 15 20 25 II -Fig.5 X-Ray patterns for LiSbS, (a)experimental, (b)calculated from the crystal structure (c)JCPDS file no 40-1330 for P-LiSbS," 792 J Muter Chem , 1996, 6(5),789-793 100 10 I0 00 a0 a 3DOID a01 32 43s 14 4 a I 1IZ IZt 02 I21 I2 0 I 0 2 Bldegrees Fig. 6 Guinier-Lenne pattern of the precursor pyrolysis in air (a)incre-asing temperature, (b)plateau at 1000 "C, (c) decreasing temperature 120"C for 20 min The structural characterization was made by powder X-ray diffraction A comparison between the exper- imental pattern [Fig 5(u)] and the pattern calculated from the crystal structure17 [Fig 5(b)] shows an unambiguous indexation, which does not, however, correspond to the JCPDS file (no 40-1330)18 given for P-LiSbS, [Fig 5(c)] In this original work," the bimetallic sulfide was prepared by reaction, in a silica tube, of a mixture of Sb2S3 and Li2S and slow cooling from the melt Thus, the phase registered as the p phase in the JCPDS file does not correspond to the crystal structure and, most probably, was spoiled by oxygen owing to reaction between L1,S and the tube Indeed, in some of our expenmental runs, the obtained sulfide corresponds to the JCPDS file but contains up to 64 mass% of oxygen This result seems to originate from the sulfur vapour pressure being too low during the sulfiding procedure Preparation of LiSb03 The mixed oxide can also be prepared from the edta precursor by pyrolysis in air above 660°C Fig 6 shows the thermal behaviour as registered on a Guinier-Lenne camera The precursor is heated slowly to 1000 "C (0 17 "C min-l), kept at this temperature for 10 h and then cooled to room temperature Above 3OO0C, the precursor is destroyed totally and the product is amorphous At 660"C, the mixed oxide LiSbO, crystallizes This is in good agreement with the TG analysis which shows a gradual loss of water below 300 "C, a decompo- sition of the organic part leading to Sb20, + Li2C03 at 410 "C (experimental loss 49 1%, calc loss 49 5%) and finally the formation of the mixed oxide above 550°C (experimental loss 59 6%, calc loss 59 4%) Conclusions This study demonstrates that edta coordination complexes can be convenient precursors for mixed oxides and sulfides In a first stage, non-substituted materials were prepared The same procedure will be used to prepare these compounds with appropriate substitutions in the antimony site in order to control and improve the ionic conductivity This can be achieved easily through coprecipitation from the aqueous solution by adding a solvent like acetone l9 References 1 J Flahaut, L Domange, M Guittard, M Ourmitchi and J Kamsu, Bull SOC Chim Fr , 1961,2382 2 V Valiukenas, R Jasinskaite, A Orliukas and A Sakalas, Liet Fiz Rinkinys, 1980,20,49 3 N F Lugakov, E A Movchanskii and I N Pokrovskii, Fiz Tverd Tela Poluprovodn , 1974,23 4 J Olivier-Fourcade, L Izghouti, E Philippot and M Maurin, Rev Chim Miner, 1983,20,186 5 A. R.West, Basic Solid State Chemistry, Wiley, New York, 2nd 13 V. M. Agre, N. P. Kozlova, V. K. Trunov and S. D. Ershova, Zh. 6 edn., 1991, p. 324. H. W. Li, H. H. Min, L. H. Wen and Y. L. Whai, Muter. Res. Bull., 14 Strukt. Khim., 1981,22, 138. T. N. Polynova, T. V. Filippova and M. A. Porai-Koshits, Koord. 199 1,26, 649. Khim., 1986, 12, 273. 7 G. M. Sheldrick, SHELX76, Program for Crystal Determination, University of Cambridge, 1976. Structure 15 J. J. Park, M. D. Glick and J. L. Hoard, J. Am. Chem. SOC., 1969, 91, 301. 8 A. J. C. Wilson, International Tables for X-Ray Crystallography, Kluwer Academic, London, 1992, vol. C, Table 4.2. 16 C. Brouca-Cabarrecq, B. Marrot and A. Mosset, Acta Crystallogr., Sect. C, 1996,52. 9 K. Yvon, W. Jeitschko and E. Parthe, J. Appl. Crystallogr., 1977, 10, 73. 17 J. Olivier-Fourcade, M. Maurin and E. Philippot, Rev. Chim. Miner., 1983,20, 196. 10 M. A. Porai-Koshits, A. I. Pozhidaev and T. N. Polynova, Zh. 18 J. Olivier-Fourcade, L. Izghouti, E. Philippot and M. Maurin, Rev. Strukt. Khim., 1974, 15, 1117. Chim. Miner., 1983,20, 186. 11 M. A. Porai-Koshits, Ya. M. Nesterova, T. N. Polynova and D. T. Gacia Banus, Koord. Khim., 1975,1,682. 19 A. Mosset, J. Galy, E. Coronado, M. Drillon and D. Beltran, J. Am. Chem. SOC., 1984,106,2864. 12 M. A. Porai-Koshits, Zh. Strukt. Khim., 1980,21, 146. Paper 5/07097J; Received 27th October, 1995 J. Muter. Chem., 1996, 6(5), 789-793 793

ISSN:0959-9428    

DOI:10.1039/JM9960600789

出版商:RSC    

年代:1996

数据来源: RSC