摘要:

J. MATER. CHEM., 1994, 4(8), 1201-1204 Electrochromic Behaviour and X-Ray Structure Analysis of a Pechmann Dye, (€)=5,5’=Diphenyl-3,3’-bifuranylidene-2,2’=dione Jack Silver,*” Mustafa T. Ahmet,” Keith Bowden,a John R. Miller,” Shahdah Rahmat,” Christopher A. Reynolds,” Alan Bashall: Mary McPartlin*’ and Jill Trottee a Department of Chemistry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, UK C04 3SQ School of Applied Chemistry, University of North London, Holloway Road, London, UK N7 8DB Studies on the title compound have established that, although it could be oxidised with a concomitant change in its visible spectrum, this process is not reversible. This is discussed in the light of the crystal structure which IS also reported in this paper.The title compound crystallises in the monoclinic space group P2,/n, a =27.033(6) A, b =4.988(2) A, c =5.372(2)A, p =94.59(2)”. The molecules have close intermolecular contacts along stacks packing in the c direction. The packing is compared to that found in two other Pechmann dyes. Electrochromic behaviour has been observed in four different types of chemical material: inorganic, organic, organometallic and chlathrate compounds.’ Most electrochromics offer only a monochromatic colour change, though the lanthanide bisph- thalocyanines offer polychromic changes.2 A monochromatic electrochromic material which produces a good red colour, even for a limited number of colour changes (cycles), has yet to be reported in the literature.Red dyes are not rare in the literature, but, in order to be sublimed to form useful films, they must be relatively thermally stable. One such group of materials is a family of dyes known as ‘Pechmann dyes’. The 1.2 h 07 -.-C 0.9 2 (dv 5 0.62s:Llparent compound is (E)-5,5’-diphenyl-3,3’-bifuranylidene-2,2’-(ddione (1).3 We report here its electrochromic properties and its crystal structure. Results and Discussion Fig. l(u) shows the electronic absorption spectrum of the parent Pechmann dye (1) in chloroform solution. This solution is pink-orange and fluorescent. Subliming the dye onto an optically transparent electrode [indium-doped tin oxide glass (ITO)] produced an orange-red non-fluorescent film with the electronic absorption spectrum presented in Fig.2(a). This spectrum is substantially different from that in Fig. l(a) and indicates either that the conformation of the molecule changes between the solid state and solution, or that there are signifi- cant intermolecular interactions in the solid state. The IR spectrum of the film is identical to the Nujol mull spectrum previously reported3 and confirms that no structural change is induced by the sublimation process. Oxidation of the Pechmann dye film on the IT0 at +1.0 V changed the visible spectrum, giving a loss of intensity across the entire visible absorption band between 390 and 620nm, and a slight increase between 750 and 850 nm. The shape of the band between 400 and 550nm also changes.Reduction of the oxidised form back towards the neutral form suggested only a slight recovery of the original material [Fig. 2(b)]. IR spectra of the sublimed films are presented in Fig. 3 for 1 0.3 0 0.40 400 600 800 400 600 800 Wnm Fig. 1 (a) Electronic absorption spectrum of 1 in chloroform solution. (b)Electronic absorption spectrum of 1 dissolved in polystyrene film. the neutral and oxidised states. On oxidation there are several changes in the IR spectrum, particularly in the catbonyl- stretching frequency region. The absorption area of the band at 1745 cm-’ is rcduced (by about one half) and a new band appears at 1781 cm-’. Several explanations are possible: (i) only a fraction of the molecules are oxidised giving rise to a spectrum which is a superposition of the original spectrum and the oxidiseti form, the latter having one band at 1781 cm-l and possibly a second band near 1745 cm-l; (ii) oxidation (complete) is accompanied by a structural change involving loss of the centre of symmetry giving rise to two IR-allowed C-0 stretching frequencies; (iii) oxidation occurs in only one half of the molecule and the oxidised molecule is asymmetrical, with two non-equivalent environments for the ca rbonyl groups; (iv) intermolecular interactions change on oxidation, a distinct possibility because of the close intermolecular approach of the C-0 bonds (see the crystal sti-ucture reported herein).The third possibility fits in with previous findings that it is possible for a functional group on one ring to react indepen- dently of the ~ther.~,~ There is also a change in the IR spectrum near 1100 cm- ’.The nature of the vibration cannot be assigned here but the general change is similar to the carbonyl-stretching region, J. MATER. CHEM., 1994, VOL. 4 1.88 1.32 0.7E h cu).- C3 4 v Q)0 0.2c 500 700 1 IB investigated using the AM l5 semi-empirical method, as implemented within the MOPAC 6.0 program,6 for both the neutral molecule and the radical cation. We note that the latter is not directly relevant as the radical cation would be short-lived and would be an intermediate to a more stable neutral molecule of unknown structure. The calculations predict both modelled molecules to be planar, as confirmed by second-derivative calculations which show that in both cases the Hessian is positive definite.This result suggests that the asymmetry observed between the two carbonyl groups is a result of inter- rather than intra-molecular interactions. The highest occupied molecular orbital, HOMO, of the neutral molecule is spread over the entire molecule (see Fig. 4). Moreover, the change in Coul~on~.~charge distributions between the neutral molecule and the radical cation is in accord with the electron being removed from a highly delocal- ised orbital. As stated above, the solid-state electronic absorption spec- trum [Fig. 2(u)]was clearly different from the solution spec- trum [Fig. l(u)].The origins of this difference are important. To try to resolve the reasons for this, the dye was dissolved in a chloroform-polystyrene solution and the solvent was allowed to evaporate.The resulting pink fluorescent film gave the visible absorption which is presented as Fig. l(b). Clearly, this spectrum is closer to the solution spectrum [Fig. l(u)] than to the spectrum of the sublimed solid film [Fig. 2(u)]. This finding supports the earlier premise that the molecules of the Pechmann dye interact in the solid state. Indeed, such an interaction is found in the crystal structure reported below. 0.20 1The films did not undergo reduction at -1.0 V either in 500 700 Vnm Fig. 2 A, Electronic absorption spectra of a thin sublimed film of 1: (a) as sublimed, (b)oxidised.B, Electronic absorption spectrum of the sublimed film of Fig. 2A (b) reduced back to neutral [little change is seen, compare Fig. 2A (b)]. 59.56r h Y VI 1800 1760 1720 iVcm-’ Fig.3 IR spectra of 1 thin sublimed film from Fig. 2A (-) (as sublimed), (---) and after oxidation in the 1850-1660 cm-’ range i.e. loss of intensity of the original spectrum accompanied by the appearance of a new band. From the differences in the electronic absorption spectra [Fig. 2(a), (b)] we feel that explanation (i) can be ruled out. However, we are not able to choose between explanations (ii), (iii) and (iv) on the basis of experimental data alone. For this reason computer-modelling programs were used to study molecular structure of 1 and its radical cation.The geometry, molecular orbitals and charge distribution were the neutral or in any state once oxidised. Therefore, the electrochromic properties of this particular material are not likely to be of practical use. X-Ray Structural Data The molecular structure of 1 is illustrated in Fig. 5 showing it to be almost planar overall. For comparison, the bond lengths in the diphenyl compound 1 are listed in Table l(u), together with those of the two polymorphs of the correspond- ing dimesityl compound, red 2a and black 26;’ the interbond angles for 1 are in Table l(b). Th? central double lactone unit in 1 is planar to within 0.02 A an! the bridging bond length, C(1)-C(l’), of 1.361(11) A is similar to those in 2a and 2b (mean 1.365 A).The phenyl substituent in 1 is almost coplanar with the H H H H H’ H -H H H Fig.4 Semiempirical optimized geometry showing a schematic rep- resentation of the HOMO (a n: orbital). The contributions to the orbital with positive phase are shown in grey; those with negative phase are shown in black. The radii of the circles are roughly proportional to the coefficient of the orbitals in the LCAO expansion. J. MATER. CHEM., 1994, VOL. 4 Fig. 5 Almost planar molecular structure of 1 Table 1 Comparison of bond lengths (A)and angles (degrees) found in the three knowp Pechmann structures (u) Bond lengths/A band 1, reda 2b, red" 2b, blackb 1.361(11-) 1.364 1.365 1.492(8) 1.476 1.480 1.415(8) 1.359(8) 1.427 1.340 1.422 1.343 1.403 (7) 1.393( 7) 1.396 1.382 1.400 1.380 1.191(8) 1.189 1.187 1.433(8) 1.468 1.459 1.406( 8) 1.402 1.407 1.361(8) 1.392( 9) 1.402 1.385 1.383 1.382 1.390(9) 1.386 1.389 1.368(8) 1.384 1.376 1.407(8) 1.408 1.415 (b)Inter-bond angles/degrees for 1 C(6)-C( 1)-C(2) 106.1(5) 0(3)-C(2)-C(l) 132.1(6) 0(4)-C(2)-C( 1) 106.5(5) 0(4)-C(2)-0(3) 121.3(5) C(5)-O(4)- C( 2) 107.8( 4) C( 6)- C( 5)-O(4) 11 1.4( 5) C(7)-C(5)-0(4) 116.4(5) C( 7)-C( 5)-C( 6) 132.3( 5) C(5)-C(6)-C( 1) 108.2(5) C(8)-C(7)-C(5) 121.5(5) C( 12)-c(7)-c(5) 120.3(5) C(12)-C(7)-C(8) 118.2(5) C(9)-C( 8)-C(7) 120.1 (5) C( lO)-C(9)-C(8) 121.3(6) C( 11)-C( 10)-c(9) 118.7(6) C( 12)-C( 11)-C( 10) 120.8(6) C( 11)-C( 12)-c(7) 120.9( 6) lactone, giving a dihedral angle between the two rings of 1.2", in marked contrast to the mesityl substituents in 2a and 2b, which are, respectively, at angles of 56" and 65" to the lactone rings.This observation is reflected in the shorter bond length between !he lactone and phenyl rings in 1, S(5)-C(7) of 1.433(8)A [Table l(a)], cornpaTed with 1.468 A for the equiv- alent length in 2a and 1.459 A in 2b. These differences are consistent with much greater conjugation between lactone and substituent in the almost planar molecule of 1; in 2a and 2b coplanarity is prevented by the two methyl groups at the 2,6-positions of the dimesityl units [i.e. equivalent to C(6) and C( 12) of the diphenyl compound 1, shown in Fig.51. Part of the solid-state structure of 1 is illustrated in Fig. 6(a) and (b) which shows the 'stepped' arrangement of three adjacent molecules along the c axis viewed (a)onto the plane of the molecule and (b) at 75" to (a). The molecules in 1 mainly interact via one carbonyl group at x,y, z of the central molecule (A) and one at -x,-y, -1-z of an adjacent molecule (B), with a second set of identical interactions between the second carbonyl (at -x, -y, -z) of the first molecule (A) and one carbonyl (at x, y, 1+z) of the molecule on the other side (C); both sets correspond to the short contact distances \ \ \ A \ \ \ \ B Fig.6 Arrangement of the molecules along the c axis of the crystal viewed: (a) onto the plane of the molecule, (b)at 75" to the plane of the molecule C(2)..-0(3) 3.19, 0(3)..-0(3) 3.11, and H(6)...0(3) 2.35 A (Table 2).In contrast, the blue-black mesityl analogiie (2b) has no interactions involving the carbonyl groups: the only short contacts being four identical distances of 3.31 A bctween carbon atoms of one molecule and those on either side; the red form (2a) shows no interactions at less than the van der Waals distances and the material may be thought of as composed of essentially non-interacting molecules. The struc- ture of the diphenyl red material (1) is therefore interesting as its solid-state interactions (at less than the relevant 1 an der Waals distances) all involve the oxygen atom of the carbonyl group, 0(3), which may well be involved in the changes encountered in the IR spectrum on oxidation of the thin sublimed films.We note that the blue-black phase of the dimesityl deriva- tive did not have good conducting properties. From our electrochromic investigation, which showed that sublimed films of 1 were slow to oxidise and would not cycle to neutral, we would not expect 1 to be a very good conductor either. Table 2 Selected intermolecular interactions (A)with first atom at x, Y, z equivalent position of second atom 0(3)...0(3) 3.11 -x, -y, -1-z C(2)*..0(3) C( 5)--0( 3) H( 12)-0(3) 3.19 3.30 2.35 -x, -y, -1-z x,1 +y, z x, 1+y, l+z J. MATER. CHEM., 1994, VOL. 4 Conclusions The Pechmann dye sublimed readily and formed good-quality even films. However, it proved to have limited electrochromic potential (one shot use). Experimental (E)-5,5'-Diphenyl-3,3'-bifuranylidene-2,2'-dione(1) was pre-pared as previously de~cribed.~,~ Crystal Data for 1 C20H1204,0M=316.31, monoclinic, spate group P2,/n, a= 27.033(Q) A, b=4.988(2) A,~=5.372(2) A, p=94.59(2)", U= 722.04 A3, F(000)=328, ,u(Mo-Ka)=0.60 cm-l, Z=2, D,= 1.455 g cmP3.A crystal of size 0.38 mm x 0.14 mm x 0.06 mm was used in the data collection. Data were collected in the 8 range 3-23' using a 8-28 scan mode with a scan width of 0.90'. Equivalent reflections were merged to give 684 unique data with I/g(I)>2.0. Structure solution and refinement were carried out using SHELX 76.'' The structure was solved using direct methods. All the hydrogen atoms were inc!uded in the refinement at calculated positions (C-H 1.08 A), assuming idealised sp2 hybridisation, except that bonded to C(6) which was located from a difference-Fourier synthesis and inserted without refinement at that position.All the non-hydrogen atoms of the five-membered ring were assigned anisotropic thermal parameters in the final cycles of the full-matrix refinement (which converged at R =0.0749 and R' =0.0686 with weights of w = l/02Foassigned to the individual reflections). In a final difference-Fourier synthesis there were no residual maxima greater than 0.5 of an electron. The final atomic coordinates are given in the supplementary data.? ~~ t Observed and calculated structure factors fractional atomic coordi- nates and thermal parameters for 1 (1 1 pp.) are deposited with the Cambridge Crystallographic Data Centre.Details available from the Editorial Office. IR Spectroscopy The IR spectra were collected on material sublimed as thin films onto ITO-covered glass. A Perkin-Elmer 1700 Fourier transform infrared spectrometer was used to collect the spec- trum by reflection from the film. Electrochromic Studies The films were oxidised by applying + 1.0 V to the IT0 in an electric cell using 10% KCl-water as the electrolyte and a silver wire as the counter electrode. They were subjected to voltages of +1 both in the neutral and oxidised states. UV-VIS Spectroscopy Spectra were recorded on thin films [deposited under vacuum (lo-' Torr) on IT0 glass using an Edwards 306a vacuum sublimation unit] on a Perkin-Elmer Lambda 5 spectrometer.References 1 J. Silver, New Scientist, 1989, 30th September, p. 48. 2 C. S. Frampton, M. J. O'Connor, J. Peteraon and J. Silver, Displays Techn. Appl., 1988,9, 174, and references therein. 3 E. Klingsberg, Chem. Rev., 1954,54,59. 4 K. Bowden, R. Etemadi and R. J. Ranson, J. Chem. Soc., Perkin Trans. 2,1991,743. 5 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. SOC.,1985,107,3902. 6 J. J. P. Stewart, Mopac 6.0, Frank J. Seiler Research Laboratory, US Air Force Academy, Colorado Springs, CO 80840. 7 C. A. Coulson and H. C. Longuet-Higgins, Proc. R. Soc. London, Ser. A, 1947,191,39. 8 B. H. Chiurguin and C. A. Coulson, Proc. R. Soc. London, Ser. A, 1950,201,196. 9 M. J. Begley, L. Crombie, G. L. Griffiths, R. C. F. Jones and M. Rahman, J. Chem. SOC.,Chern. Commun., 1981,823. 10 G. M. Sheldrick, SHELX 76 Program for Crystal Structure Determination, University of Cambridge. Paper 4/00602J; Received 31st January, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401201

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1205-1213 Monolayer Behaviour and Langmuir-Blodgett Film Properties of some Amphiphilic Phthalocyanines: Factors influencing Molecular Organisation within the Film Assembly Michael J. Cook,*aJim McMurdo,a David A. Miles," Richard H. Poynter,a John M. Simmons,a Simon D. Haslam,tb Robert M. Richardsonb and Kevin Welford" a School of Chemical Sciences, University of East Anglia, Nomvich, UK NR4 7TJ School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK BS8 ITS DRA, St. Andrews Road, Malvern, Worcs, UK WR743PS The monolayer and LB film-forming properties of 20 structurally related amphiphilic octa-substituted phthalocyanine derivatives have been assessed. The molecular packing within examples of the films has been probed by visible-region spectroscopy and low-angle X-ray diffraction methods.Among derivatives where the aliphatic substituents are attached by ether linkages, there is a variation in the behaviour according to the length of the chains and whether or not the chains are branched. Films are not highly ordered but may contain domains of ordered structure, giving rise to ri red-shifted absorption band in the visible spectrum. Analogues where the chains are attached by carbon-carbon bonds show superior monolayer behaviour and are excellent materials for deposition as LB films. Furthermore, there is good evidence from the spectra that change in the length of the alkyl chains provides a means of controlling the type of molecular packing within the films. Hughes' and Alexander2 investigated monolayer behaviour of phthalocyanine (Pc) derivatives in the 1930s, but it was Roberts and co-workers' research3 in the early 1980s which marked the start of the extensive interest in both Pc monolayer and LB films evident over the last ten year^.^,^ Most of the attention during this period has centred on substituted derivatives, those bearing three or four substitu- ent groups having been particularly well studied. Not all give good monolayer or deposition behaviour, but those that do often give rise to LB films with a promising degree of molecular order. The most common type of order involves the molecules stacking more or less cofacially in aligned columns whose axes are parallel to the substrate surface. This type of packing leads to a characteristic blue shift of the visible absorption band relative to that observed in the solution phase.6 Our own interests have centred on the use of octa-substituted compounds.Unlike the tetra-substituted com-pounds, which are normally formed as a mixture of isomers and used as such, the octa-substituted derivatives can be synthesized isomerically pure, a feature which should encour- age molecular ordering. In earlier work we noted the contrasting behaviour of the octa-alkoxy series 1 and the octa-alkyl series 2. The former give LB films with some degree of order depending upon chain length.7 The latter form rigid compressed aggregates at the air/water interface which cannot be deposited onto substrates8 Series 3, analogues of 2 but containing both hydrophilic and hydro- phobic groups, behaved very differently.The introduction of amphiphilic character encourages ordering at the water surfaceg and the monolayers can be deposited as highly ordered LB The molecular packing is quite different from that in the films of the tri-and tetra-substituted compounds; the visible spectrum shows both a red- and blue-shifted band characteristic of Davydov splitting, implying the presence of translationally non-equivalent mol- ecules within the unit cell.lo,ll The present paper reports an investigation of a range of t-Present address: DRA, St. Andrews Road, Malvern, Worcs, WR14 3PS. RC M la R = -(CH2),CH3; M = H,H b R = --(CH2),CH*(CH&; M = H,H c R = --(CH2)2CH.(CH3),; M = Ni R' 2 R=R'=alkyl; M=H,HorCu 3 R = alkyl; R' = -(CH2),C02H; M = H ,H or Cu new amphiphilic octa-substituted compounds, 4-14, closely related in terms of their structures.They were chosen to appraise how ring substituents, in particular, chain-lengt h and functionality, can influence both monolayer and LB film-forming properties.12 The structural packing within examples of the films has been probed by visible spectroscopq and low-angle X-ray diffraction methods. J. MATER. CHEM., 1994, VOL. 4 CI CI OH HO OH 4a R = -(CH,),CH,; M = H,H b R = -(CH2)4CH3; M = CU c R = -(CH2)4CH3; M = Ni n= 1 5a R = -(CH2),CH*(CH3),; M = H,H b R = -(CH2)2CHm(CH3)2; M = CU c R = -(CH,),CH.(CH,),; M = Ni n= 1 6 R = -(CH2)2CH*(CH3)2; M = CU n=2 7e R = -(CH2)2CH*(CH3)2; M = H,H b R =-(CH2)2CH*(CH3)2; M = CU n=3 9 R = -(CH,)&H3; M = H,H n= 1 Experimental Materials Metal-free Alkoxy Amphiphiles The metal-free amphiphilic compounds of series 4-9 were obtained in yields of 2-5% by allowing the appropriately substituted 3,6-bis-alkoxyphthalonitrileand 3,6-di( hydroxy- a1koxy)phthalonitrile to react in a 9 : 1 ratio.These precursors were prepared by reaction of the appropriate bis-cyano- hydroquinone with an alkylating agent using standard conditions, cf. ref. 13. The required amphiphilic phthalocyan- ine product was separated chromatographically from the octaalkoxyphthalocyanine by-product of type 1 [in partic- ular, 1,4,8,11,15,18,22,25-octapentyloxyphthalocyanine,la, mp 109"C (lit.,I3 116-117.5 "C) and 1,4,8,11,15,18,22,25-octa-iso-pentyloxyphthalocyanine, lb, see Table 11.The general procedure, an adaptation of the synthesis of compounds of series 1 described earlier,13 is exemplified by the following. 1,4-Di (3-hydroxypropyloxy)-S, 1 1,15,18,22,25-hexaisopentyl-oxyphthalocyanine, (5a):Using a typical procedure, a mixture of 3,6-di(isopenty1oxy)phthalonitrile (2.16 g, 9 mmol) and 3,6-di(3-hydroxypropyloxy)phthalonitrile (0.28 g, 1mmol) in dry pentan-1-01 (10 ml) was heated to reflux and lithium metal (0.1 g) added in small pieces. Reflux was continued for 45 min and then the mixture allowed to cool to room temperature (rt) when glacial acetic acid (10 ml) was added and stirring continued for 1 h.The solvents were removed under reduced pressure and the residue taken up in chloroform (50 ml) and washed with water (50 ml), saturated brine (50 ml), dried with (MgS04), filtered and the chloroform removed under reduced pressure to give a dark green viscous oil. This was chromato- graphed over silica gel Merck grade 7734 using as eluent a mixture of 40-60 light petroleum-THF 9: 1 to give a green solid which was recrystallised from THF-methanol to give 1,4,8,11,15,18,22,25-octaisopentyloxyphthalocyanine( 1b) (612 mg, 28%) as green needles mp 201 "C. (Found: C, 71.8; HO 10 R = --(CH,),CH,; M = H,H 11 R = -(CH,)&H,; M = H,H 12 R = -(CHp)$H3; M = H,H 13a R = --(CH,)&H$ b R=--(CH,)&H,; c R = -(CH2)&H3; M = H,H M=CU M = Ni 14a R = -(CH,),CH,; M = H,H b R = --(CH,)&H,; M = CU H, 8.2; N, 9.3YO.C,,H,,N,O, requires C, 7 1.5; H, 8.3; N, 9.3YO). 6H (60 MHz, CDC1,): 0.24 (s, 2H), 1.05 (d, 48H), 2.2 (m, 24H), 5.0 (t, 16H), 7.6 (s, 8H).Increasing the eluent polarity (40-60 light petroleum-THF 4 : 1)gave a second green fraction which was chromatographed a second time over silica gel (eluent cyclohexane-THF 3 :2). (In later preparations it was shown that chromatographic separations could be improved by modifying the eluent through the addition of triethylamine at 1YO.)Recrystal-lisation from THF-MeOH afforded dark green crystals of 1,4-di( 3-hydroxypropyloxy)-8,11,15,18,22,35-hexaisopenty1-oxyphthalocyanine [63 mg, 6% based on the 3,6-di(3-hydroxypropyloxy)phthalonitrile] mp 196-197 "C.(Found: C, 69.5; H, 7.6; N, 9.5%. C68H90N@10 requires C, 69.2; H, 7.7; N, 9.5%). 6, (400 MHz, CDCl,): 0.24 (s, 2H), 1.02 (d, 12H), 1.06 (d, 12H), 1.08 (d, 12H), 1.60 (m, 6H), 2.00 (m, 4H), 2.15 (m, 8H), 2.27 (m, 4H), 3.93 (m, 4H), 4.27 (t, 2H), 4.87 (m, 12H), 5.04 (t, 4H), 7.58 (d, 2H), 7.6 (s, 2H), 7.63 (d, 2H), 7.66 (s, 2H). Metal-free Alkyl Amphiphiles Compounds 10, 12 and 14a were available from a recent study of mesogenic phthalocyanines. l4 The homologues 11 and 13a were prepared similarly in yields of 8 and 6%, respectively. Metallated Phthalocyanines Metal-free phthalocyanines were converted into their metallated derivatives by reaction with the corresponding metal acetate, e.g.:1,4,8,11,15,18-Hexadecyl-22,25-di(4-hydroxy-butyl)phthalocyaninatocopper(n), ( 14b): In a typical experiment, a mixture of 1,4,8,11,15,18-hexadecyl-22,25-di-(4-hydroxybuty1)phthalocyanine(91 mg, 61 pmol) and cop- per@) acetate (400 mg, 2 mmol) in dry pentan-1-01 (20 ml) was heated under reflux for 30 min.The mixture was allowed to cool to rt and the pentan-1-01 removed under reduced J. MATER. CHEM., 1994, VOL. 4 Table 1 Characterisation of novel compounds compound C H N Q-band no. M substituent mp/"C" formula found (reqd.) LaJmmb lb 20 1 71.8 8.2 9.3 762 (l.52)c (71.85 8.3 9.3) 740 (1.31) lc 220 68.9 7.7 8.8 734 ( 1.73)d (68.6 7.7 8.9)4a 6 x OC,H1, 101-103 69.1 7.9 9.4 762 (1.76) 2 x O(CH2hOH (69.2 7.7 9.5) 738 (1.49) 4b 6 x OC5H11 149-150 65.8 7.3 8.9 740 (1.87) 2 x O(CH,),OH (65.8 7.15 9.0)4c 6 x OC5Hll 209-210 65.7 7.4 9.45 733 (1.20) 2 x O(CHz),OH (66.1 7.2 9.1)5a 6 (CH2)2CH(CH3 12 196-197 69.5 7.6 9.5 763 (1.60)2 x O(CH,),OH (69.2 7.7 9.5) 740 ( 1.36)5b 6 0(CH2)2CH(CH3)2 225-226 65.4 7.0 8.9 740 (2.10) 2 x O(CH,),OH (65.8 7.15 9.0)0(CH2)2CH(CH3)2 269-271 65.9 6.9 9.0 733 (1.74) 2 x O(CH2),0H (66.1 7.2 9.1)0(CH2)2CH(CH3)2 230-232 66.5 7.05 8.5 739 (2.111) 2 x O(CH2),0H (66.25 7.3 8.8)7a 6 0(CH2)2CH(CH3)2 65-67 70.4 8.2 8.6 762 (1.33) 2 x O(CH2)50H (70.0 8.0 9.0) 740 (1.59) 7b 6 x O(CH2)2CH(CH3)2 87-88 66.3 7.4 8.3 739 2 x O(CH2),0H (66.7 7.5 8.6)0(CH2)2CH(CH3)2 104 59.3 6.3 7.8 757 (1.78) 6 x C1; 2 x O(CH2),0H (58.9 6.1 8.1) 731 (1.56) 0(CH2)2CH(CH3)2 132-133 55.8 5.9 6.9 739 (1.38) 6 x C1; 2 x O(CH2),0H (56.1 5.75 7.3)9 6 x OC7H15 66-67 71.1 8.5 8.1 762 (1.43) 2 x O(CH2)30H (71.3 8.5 8.3) 738 (1.25) C7H15 K-Dl18 78.9 9.7 8.6 700 ( 1.25)" 2 x (CH,),OH D+I152 (78.9 9.5 9.0) 735 (1.72) 13a 6 CgHl9 K+D98 79.6 10.25 7.5 695 (1.32)" 2 x (CH2),0H D-+I122 (79.8 10.0 7.9) 730 (1.47) C9H19 K+D154 76.05 9.6 7.4 710 (2.17)' 2 x (CH2),0H D-I170 (76.4 9.55 7.6) 13c 6 CgHl9 K+D95 76.5 9.5 7.4 705 (1.01 )" 2 x (CH2),0H D-I123 (76.65 9.6 7.6) 14b 6 x C10H21 151-1 52 76.8 10.0 6.9 710 (1.98)" 2 x (CH,),OH I-D151 (76.8 9.9 7.2) D-K136 "Melting points and transition temperatures for liquid-crystalline materials.K, crystal state; D, discotic mesophase; I, isotropic liquid.bMeasured as solutions in toluene unless indicated otherwise, as for: 'in cyclohexane, din dichloromethane. 'in l,l,l-trichloroethane. pressure. The residue was chromatographed over silica gel available to the material at a rate of 50 cm2 min-' until (Merck grade 7734;eluent 40-60 light petroleum-THF 2:1) a compressed state was achieved at surface pressurcs of and recrystallised from THF-methanol to give as green crys- ca. 25-40 mN m-l depending upon the sample. The stability tals 1,4,8,11,15,18-hexadecyl-22,25-di(4-hydroxybuty1)phthal-of the compressed monolayer as a function of time was ocyaninatocopper(II), (84mg, 88%). (Found: C, 76.8;H, 10.0; assessed by monitoring the percentage decrease in area over N, 6.9%.C100H154CuN802requires: C,76.8;H, 9.9;N,7.2%). a particular timespan. Characterisation data for all novel compounds are presented LB films for X-ray diffraction and optical spectroscopic in Table 1. Mps were measured using a Linkam hot-stage studies were prepared by depositing compressed monolayers attached to an Olympus polarising microscope. Mesophase (typically at surface pressures of 30-35 mN m-') onto silicon behaviour was observed for compounds 10-14. and glass slides cleaned as described earlier." These were rendered hydrophobic by silanising the slides in a 2-4% solution of dichlorodimethylsilane in l,l,l-trichloroet hane Monolayer Behaviour and LB Film Deposition for 30 min. Experiments were performed on either a Joyce-Loebl con-Vertical dipping through the molecular monolayer was stant-perimeter trough or a NIMA 622 dual-compartment undertaken with the surface pressure maintained constant trough.Water purification procedures, equipment and pro- during the dipping process. Dipping speeds were 10 mm niin-' cedure for preparing multilayer LB films have been discussed for compound la and 8 or 15mm min-' for all others. A previou~ly.',~ drying time was frequently incorporated into the dipping Material was transferred to the water surface as solutions sequence depending upon the material being deposited, see in l,l,l-trichloroethane at known concentrations of ca. Discussion. 0.5 mg ml-' with the trough barriers fully open. Monolayer 'Horizontal lifting' of monolayers of 10, 12 and 14a was behaviour was examined once the solvent had been deemed attempted onto glass slides.The slides were arranged horizon- to have evaporated (30 min). The barriers were then slowly tally and lowered onto the the monolayer at ca. 1 mm rnin-l closed to 5-15 mN m-' and then reopened. Monolayer behav- and then raised, the process then repeated to obtain the iour was then plotted as a PA isotherm by reducing the area required number of layers. Optical Spectroscopy Visible spectra were measured using either a Hitachi U2000 or U3000 spectrophotometer. The substrate was aligned such that it was normal to the sample beam. Polarised spectra were measured using Polaroid sheet polarisers placed between the incident beam and the sample holder and recorded with the incident light polarised both parallel and perpendicular to the dipping direction.X-Ray Diffraction Analysis of the films was performed using a high-resolution X-ray reflectometer which has been describcd elsewhere." Collimated Cu-Ka X-rays were used (A=1.54 A). The sample LB film was placed on a horizontal table and the angle of incidence and angle of reflection were scanned simultaneously so that they remained equal. The reflected intensity was recorded as a function of the scattering vector Q =4nsinO/A, where 6' is the angle (in radians) between the incident (or reflected) beam and the substrate. Results and Discussion Materials Of the compounds 4-14 all bar 10, 12 and 14a are novel, the synthesis of the last three having been described re~ent1y.l~ The remainder were prepared by analogous routes, gave satisfactory elemental analyses (Table 1) and, in the case of the metal-free and nickel derivatives, 'H NMR signals fully consistent with their structures.Proton signals in the aromatic region proved particularly diagnostic, compounds of series 4, 5, 7 and 9 showing two singlets and an AB pattern while an accidental equivalence of protons of 11 and 13 reduces the signals to two apparent singlets in the ratio of 3 : 1, cf. ref. 14. A detailed 'H and 13C NMR analysis of compounds 5a and 9 will be presented elsewhere.16 Previously we demonstrated that 10, 12 and 14a exhibit discotic columnar liquid-crystalline beha~iour.'~ Of the new materials, 11, 13a,b,c and 14b also give rise to a liquid-crystal phase and transition temperatures are summarised in Table 1.Monolayer Behaviour Material at the air/water interface was compressed and decom- pressed to give the conventional n-A isotherms. Examples obtained for the first compression/relaxation cycle are shown in Fig. 1. Area per molecule, Ao, data (Table 2) were obtained by extrapolating the region of the isotherm which corresponds to the condensed phase to zero pressure. Table2 also gives the results of monolayer stability tests, which are presented as the percentage decrease in surface area of films compressed to a specified pressure over a specified time. Octaalkoxyphthalocyanines, la, 1b, lc: Compound la was examined in our earlier work,7 but has been re-measured alongside the new derivatives lb and lc for comparative purposes. Data for la confirmed the finding that this com- pound gives isotherms which show substantial hysteresis during relaxation [Fig.1(a)]. Inspection of a spate-filling model of the compound suggtsts a width of ca. 22A and a packing thickness of ca. 4.5-5 A. Thus the measured A, value, 70 A, is inconsistent with monolayer behaviour. Inspection of the TC-Aisotherm indicates that there is some instability in the condensed phase at surface pressures >15 mN m-l and there is a significant decrease in surface area over time. Compound lb gives a similar type of isotherm. Again there is hysteresis but the condensed phase appears to be stable to ca.20 mN m-l, there is a much smaller decrease in area with J. MATER. CHEM., 1994, VOL. 4 201 n 10, , , , 0 30-z 20-E--.2 - 20- L2 Q 10-. 10- 0 1 0 \ 0 0 100 200 0 100 200 area per molecule/A* Fig. 1 Examples of n-A isotherms showing the initial compression/ relaxation cycle for (a) 1,4,8,11,15,18,22,25-octapentyloxyphthalo-cyanine (la), (b) 1,4-di(3-hydroxypropyloxy)-8.11,15,18,22,25-hexa-isopentyloxyphthalocyanine (5a), (c) 1,4,8,1 1,15,18-hexaheptyloxy- 22,25-di(3-hydroxypropyloxy)phthalocyanine(9), (d) 1,4,8,11,15,18-hexadecyl-22,25-di(4-hydroxybutyl)phthalocyaninatocopper (11) ( 14b), (e) 1,4-di(4-hydroxybuty1)-8,11,15.18,22,25-hexa~~ctylphthalocyanine (12) and (f) the n-A isotherm for 1,4-di(4-hydroxybutyl)-8,11,15,18,22,25-hexaoctylphthalocyanine(12) obtained for the second compression/relaxation cycle time, and the value for A, is 113 A.The latter is more consistent with a monolayer. Thus the behaviour of the branched-chain isomer at the air/water interface is substan- tially superior to that of the straight-chain compound. The nickel analogue lc behaves similarly but exhibits a small enhancement in monolayer stability. Alkoxy amphiphiles, 4-9: Compounds of series 4-7 are structurally related to la and lb differing insofar as two of the pentyloxy or isopentyloxy groups have been replaced by two hydroxylated alkoxy groups. The introduction of amphiphilic character provides a marked improvement in monolayer behaviour which is broadly shared by each member of the series.Relative to la and lb, compounds of series 4-7 show much less hysteresis in the n-A isotherm and the compressed states are more stable, retaining their integrity at pressures of ca. 30-35 mN m-l. The n-A isotherm for com- pound 5a is fairly typical and is shown in Fig. l(b). During compression of the film, the surface pressure starts to rise at a higher surface area per molecule than for la and lb. The slope is not steep but becomes steeper at cu. 20 mN m-l and continues to increase as such, at least until a surface pressure of 35mN m-l is reached. The less steep region may corre- spond to the liquid-expanded region or a phase in which the molecules are reorganising from a flat-on to a more perpen- dicular arrangement.Earlier workg on a longer-chained hom- ologue of la indicated that the molecules are lying flat on the water surface at low surface pressures, an orientation presum- J. MATER. CHEM., 1994, VOL. 4 Table 2 Monolayer and deposition behaviour stability %dec. (surface deposition transfer dipping compound A,-,/A2 a pressure/mN m-’)’ tYPe ratio rate/mm inin-’ la lb IC 4a 4b 4c 5a 5b 5c 70 113 134 120 131 123 135 155 140 lO(25) 10( 15)* 6( 15)* < l(30) <l(30) <1(30) 1(35)<l(35) < l(35) Z Y Y Y-z Y-z Y+Z Y Y Y 0, 0.6 0.9, 1 0, 0.8 0, 0.9 0, 1 0.8, 1 0.7, 1 0.9, 1 C 10 10 10 15 15 15 15 15 15 6 7a 7b 8a 8b 129 125 146 115 140 5(25) 1(35) 1(30) < l(35) < l(30) Y Y Y Y d 0.8, 1 1, 1 0.9, 1 0.9, 1 15 15 15 15 9 > 150 26( 15)* e 10 124 5-10( 30)* Y 0.9, 0.9 8 11 12 13a 121 124 124 <2( 30)* < 1(30)* <1 (30)* Y Y Y 1, 1 1, 1 0.9, 1 8 8 8 13b 13c 14a 14b 137 117 137 125 <2(30)* < 1(30)* <1 (30)* <1 (30)* Y Y Y Y 1, 1 1, 1 1, 1 1, 1 8 8 8 8 “Value for the average area per molecule in the compressed state, extrapolated to zero surface pressure.’Stability of monolayer measured as the percentage decrease in surface area at the surface pressure indicated in parentheses () over 20 min or, in the case of measurements noted by *, over 1 h. ‘The monolayer collapsed during deposition, rendering measurement impossible. dDeposition not attempted. ‘No deposition observed.ably encouraged by weak interaction between the oxygen atoms in the ether links and the surface water molecules. As the surface pressure is increased the molecules are compressed up towards the vertical. Similar behaviour is postulated for the series 4-7, though the values for A. are sometimes larger than expected for molecules fully perpendicular to the water surface. Where comparisons are possible it appears that monolayer behaviour is marginally improved for the metallated species and that straight-chain pentyloxy compounds, 4, give some- what less stable monolayers than their isomers of series 5. The length of the hydrophilic chain, which is varied through series 5-7, appears not to be a factor in controlling monolayer properties in a significant way.The introduction of six chlorine substituents onto the ring system, as in 8, also has little apparent effect on the monolayer properties, indicating that the factors which dominate the behaviour are the isopentyloxy groups and the presence of the hydroxy group at the end of the other two chains. The longer-chain homologue of 4a, the heptyloxy analogue 9, behaves less well. The n-A isotherm, Fig. l(c), is of the same general form as those of the other ‘alkoxy amphiphiles’ but the surface pressure starts to rise at a higher surface area per molecule. The inflection occurs at ca. 15 mN m-’ and there is rather more hysteresis on decompression from 25 mN m-l. Above 25 mN m-l the film collapses. It is unlikely that the molecules reach the fully perpendicular orientation because the value for A, extrapolated from the isotherm in Fig.1(c)is too large to be consistent with this. The compressed film is much less stable over a period of time than the films of 4a and 4b, suggesting that the length of the alkoxy chain is rather critical in controlling monolayer behaviour. This is in accord with our earlier work on series 1 which showed that the octapentyloxy derivatives were better behaved than those with either shorter or longer chains.’ Alkyl amphiphiles, 10-14: Replacement of the ether linkages by methylene groups has a very marked effect on the appear- ance of the n-A isotherm. These compounds, with the excep- tion of 12, behave very similarly.They show well defined transitions from the two-dimensional gas phase to the condensed phase, the latter characterised by a near-vertical region in the isotherm, e.g. Fig. l(d), with no evidence tor the intervening state apparent for the alkoxy amphiphiles. Values for A, indicate that the molecules are oriented with their planes more or less perpendicular to the surface. The isotherms are very similar in form to those recorded earlier for series 3. An earlier X-ray reflectivity study of a monolayer of the latter indicated’ that the molecules are essentially perpendicular even in the uncompressed film and the new compounds may behave similarly. Compound 12 gave rise to a slightly different isotherm which showed a transitional state prior to the formation of the fully condensed phase, Fig.l(e). This inter- mediate state is not observed on the second compression/ decompression cycle, Fig. l(f),which gives an isotherm com- parable to that of the other compounds on the first cycle, Fig. l(d). Comparisons of 4a and 10, and of 9 and 12, are particularly interesting. These pairs of compounds are ‘isosteric’, having the same number of linking atoms in the chains, arid the superior monolayer behaviour of the two alkyl amphiphiles illustrates how replacement of the ether links by methylene groups markedly affects the monolayer properties. A property which distinguishes the alkyl amphiphiles from the (ilkoxy compounds is that they give rise to a liquid-crystal phase in the bulk material.The reason for this difference between the two series is unclear at the present time but may be the key to the differences in the monolayer behaviour. Thus the alkyl amphiphiles are examples of amphotropic material^'^ in that they may self-assemble through both their capacity to form a mesophase and through their amphiphilic character. ,4s the surface area is reduced the molecules may well mimic the behaviour observed in the liquid-crystal state wherein the aromatic cores align in columns. The n-A isotherms, which show minimal hysteresis, suggest that such columns break up as readily as they are formed when the surface pressure decreases. Compound 10 in the compressed state showed a small decrease in surface area over time, which was unexpected in view of its otherwise good monolayer behaviour.To investi- gate this further, the monolayers of each of the alkyl amphi- philes were compressed to the point where they collapsed. The surface pressure required to cause collapse of the films was similar in each case at ca. 50 mN m-l. This suggests that the monolayer of 10 is not intrinsically less stable, and we propose that the reduction in surface area reflects a gradual reorganisation of the molecules in the monolayer to give a more closely packed monolayer. It may be significant that the deposited films of 10 contain a different type of molecular assembly to the others in this series, vide infra. Deposition Behaviour Octaalkoxy phthalocyanines, la, lc: The present experiments on the deposition of the simple octapentyloxy compound la gave rise to 2-type deposition in contrast with Y-type observed in our earlier series of experiment^.^ We have no explanation for this inconsistent behaviour.A characteristic of the films was the manner in which they were wetted during the emersion process. To alleviate this, a drainage time of up to 20 min was incorporated into the cycle after each dip. Films were deposited with an irregular transfer ratio and were of variable quality and often visibly non-uniform. The behaviour of the branched-chain derivative, lc, was strikingly different. Y-type transfer was observed under conditions where the straight- chain compounds deposited 2-type and there was apparently less wetting after emersion.Transfer ratios were satisfactory (0.8-1.0) and the films appeared uniform on visual inspection. Alkoxy amphiphiles, compounds 4a, 4b, 4c, deposited Y- type over the first dipping cycle with subsequent 2-type deposition on subsequent cycles. An attempt to deposit 4b onto hydrophilic rather than hydrophobic substrates gave 2- type deposition throughout. In common with the symmetri- cally substituted compounds, la and lc, the films were found to have been wetted after the upstroke. Transfer ratios ranged from 0.7-1.0, which suggested an even coverage of the sub- strate, but visual inspection showed that the films were usually non-uniform. The longer-chain analogue 9 failed to deposit. Unlike the straight-chain compounds, the isopentyloxy substituted amphiphiles, 5-8, deposited in a regular Y-type manner, giving even green films.Transfer ratio data indicated consistent coverage of the substrate throughout the deposition experiment. The presence of three, four or five methylene units in the hydrophilic tails of these compounds, 5, 6 and 7, respectively, made no apparent difference to the film-forming properties. The chlorinated derivatives, 8, behaved in a similar fashion to the unchlorinated compounds. The uniformity of transfer of a representative example, 5b, was investigated by optical spectroscopy, plotting absorbance against number of layers and, in a separate study, bY capacitance methods. Both techniques showed that the build- up of the film is uniform.There was limited wetting of these films during dipping, suggesting that the branched chains are more hydrophobic than the straight chains. Alkyl amphiphiles, 10-14: Each compound was deposited by the vertical dipping method and 10, 12 and 14a were also transferred to a substrate by the horizontal lifting technique. The former method of transfer gave very even films by Y-type deposition with constant transfer ratios close to unity. There was little apparent wetting of the films after emersion. However, it was found to be advantageous to allow the films to drain for 1h after the first cycle. Subsequent dipping was carried out without incorporating a drying time between cycles. Plots of absorbance against number of dips were linear at least for films of up to 20 dips.J. MATER. CHEM., 1994, VOL. 4 Films deposited by the vertical-lifting method at 30 mN m-' gave a transfer ratio greater than unity, viz. 10, 1.90; 12, 1.80; 14a, 1.42. While X-type films are expected by this technique, there are precedents for the fabrication of Y-type structures, molecules apparently turning over and being drawn in underneath as the substrate is lifted. For the present compounds, lower values for the transfer ratios were obtained when the surface pressure was 25 mN m-'. Characterisation of LB Films Visual inspection revealed that the most even films were obtained from the metallated isopentyloxy amphiphiles and the alkyl amphiphiles. Where comparisons were possible it appeared that films obtained of the latter by the horizontal- lifting method approached the quality of those obtained by the vertical-dipping procedure.Alkoxy amphiphiles: Compounds 5b and 7b deposited to give particularly even films and were evaluated by X-ray reflectivity. Neither film gave any Bragg peaks, suggesting a lack of layer ordering. However, the diffraction from the film of 7b does exhibit thickness fringFs which transform to a coherent film thickness of c?. 280A. This corresponds to an amount of material ca. 22 A thick deposited during each Y- type dipping cycle. The electronic spectra of phthalocyanines show a character- istic absorption in the visible region referred to as the Q-band. Non-aggregated metal@) Pcs in the solution or gas phase show a single main absorption assigned to the doubly degener- ate transition alu-eg, see Fig.2. For metal-free Pcs the lower symmetry of the system lifts the degeneracy and the Q-band is split into two components. In the solid state the spectra are rendered more complex through exciton coupling which broadens peaks and leads to shifts in the band positions which are dependent upon molecular pa~king.~ The visible absorption spectra were recorded of films of 4b, 4c,5b, 5c and 7b and each gave a remarkably similar spectrum; data are collected in Table 3 and the spectrum of the LB film of 5b is shown as Fig. 2(a). In each case the major component of the Q-band is broad and red shifted relative to the toluene solution spectrum.There is a second, lower-intensity absorp- tion to higher energy. The spectra of films of 4b and 4c show no dichroism. However, films of 5b,5c, and 7b give rise to dichroism such that the absorption intensity is lower when the electric field vector is polarised perpendicular to the dipping direction, El than parallel to it, Ell. The dichroic Table3 Visible spectral data for LB films of some alkoxy and alkyl amphiphiles compound i,,,/nm (El : El,)" lc 4b 4c 5b 5c 7b 10 11 12 13a 13b 13c 14a 14b 752 (1.0) 758 (1.0) 752 (1.0) 758 (0.75) 753 (0.96) 750 (0.77) 740 (1.7) 768 (0.92) 767 (0.93) 770 (0.71) 769 (0.64) 743 (0.74) 771 (0.75) 767 (0.78) 637 (1.36) 634 (1.23) 638 (1.32) 635 (1.07) 638 (1.02) 639 (1.12) 632 (1.0) "Main absorption band(s) in the visible region.Data in parentheses () show the dichroic ratio, (El:El,),the ratio of the absorbances recorded with the electric field vector, E, perpendicular and parallel to the dipping direction. J. MATER. CHEM., 1994, VOL. 4 1211 0.500T (a 1.000 @) I A I Q,0 0.500 0.500 -e (d) v)a a 0.400 0.400 -0.300 0.300-0.200 0.200-0.100 0.100-0.00~ o.ooo+ wavelengthlnm Fig. 2 Visible spectra of LB films (glass slides) of (a) 1,4-di( 3-hydroxypropyloxy)-8,11,15,18,22,25-hexaisopentyloxyphthalocyaninatoco~~per(11) (5b), (b) 1,4,8,11,15,18-hexahexyl-22,25-di(4-hydroxybuty1)phthalocyanine(lo), (c) 1,4-di(4-hydroxybutyl)-8,11,15,18,22,25-hexanonylphthalo-cyaninatocopper(rr) (13b) and (d) 1,4-di(4-hydroxybuty1)-8,11,15,18,22,25-hexanonylphthalocyaninatonickel(11)(13c). Spectra are recorded with the electric field vector polarized: A, perpendicular to the dipping direction, El; and B, parallel to the dipping direction, Ell.The third line, C, in each plot corresponds to the solution phase spectrum and is shown for comparison. ratio is essentially constant over the visible region with El: Ell =0.75, 0.94 and 0.77 for 5b, 5c and 7b, respectively. In general, the observation of dichroism points to an overall anisotropic arrangement of the molecules in the film. The greater absorption observed for films of 5b, 5c and 7b when E is parallel to the dipping direction indicates that the rings are preferentially aligned such that the mean angle of the planes of the rings relative to the dipping direction is <45 "C.The absence of well defined layer spacing deduced from the X-ray reflection study, even in the films which do show dichroism, points to a lack of long-range order through the film, i.e. perpendicular to the substrate. This does not, however, preclude short-range in-plane molecular order and it is this which may give rise to the red-shifted absorption. Using Kasha's 'dimer' model," based on a pair of interacting parallel transition dipoles, the molecular exciton theory predicts a red-shifted absorption if the transition dipoles are offset by a specific amount. Extrapolated in terms of a columnar arrange- ment of phthalocyanines, a red-shifted absorption is predicted for a column in which the planes of the molecules form an angle, 8, to the stack axis which is <54.7 "C.The absence of dichroism observed in the films of the other alkoxy amphiphiles points to a lack of anisotropic alignment, though note that an arrangement of the molecules with the planes of the rings parallel to the substrate surface will not produce dichroism either. We suggest that, as a series, the alkoxy derivatives deposit to form films with varying degrees of short-range order. The domains of order may contain molecules arranged with their planes offset from their nearest neighbours; where dichroism is observed, the domains are overall anisotropically ordered. Alkyl amphiphiles: X-Ray reflectivity studies were per-formed on 30-layer films (15 vertical dips) of 10, 12, 14a and 14b which were taken as representative examples.These gave results different from those of the alkoxy amphiphiles. At higher angles they show two Bragg peaks, indicative of molecular layers parallel to the substrate, e.g. Fig. 3. Tht: layer spacing, d, was calculated by applying Bragg's law io the second-order peaks (i.e. nA=2d sin 0, with n =2), Table 4. Earlier experience suggests that the position of these peaks (unlike the position of the 001 peaks) is not significantly distorted by X-ray refractive index effects in the film, arid the 8L6 I I , I ,A 0 0.1 0.2 0.3 0.4 0.5 QIA Fig. 3 X-Ray reflections recorded for a 30-layer film of 1.4-di(4-hydroxybutyl)-8,11,15,18,22,25-hexaoctylphthalocyanine (12) on glass Table 4 Summary of XRR data measured for LB films (15 dips) of compounds 10, 12, 14a and 14b deposited on silicon (unless indicated otherwise) d-spacing frqm thickness calculate4 compound 002 peak/A (from fringes)/A thickness/A" ~~ 10 35 540-620 525 lob 35 540-620 525 12 40 540-620 600 14U 41 640-710 615 14b 42 630/658' 630 a Calculated from the proposed number of layers and observed bilayer spacing.bOn glass substrate, not silicon. 'Value obtained from fit (see text ). layer spacings quoted are considered to have an accuracy of +0.5 A. From molecular models we estimate the distance, w, from the OH groups to the end of fully extended alkyl chain: on the opposite side of the molecules to be 23, 24 and 25 A for 10, 12 and 14a,b, respectively.For each compound the measured value for d is greater than w, consistent with a bilayer assembly deposited by Y-type deposition. However, for each film d <2w, and considerably so in the case of 10, which implies that the molecules are to some extent tilted and/or there is interdigitation of the chains of molecules in adjacent layers. In each case the second-order peak was far stronger than the first-order peak, indicative of a bilayer structure with low electron density in the middle of the bilayer. The most intense Bragg peaks were observed for the copper-containing com-pound, 14b. This is expected due to the increased scattering contrast provided by the ion.For the metal-free derivatives, the intensities of the peaks decrease as the chains are length-ened. This may be associated with a genuine increase in disorder (layers intermix and become less well defined) or a loss in contrast in the X-ray scattering profile induced by greater interdigitation of substituent chains. All of the films exhibit Keissig fringes implying that they are reasonably uniform. The fringes are not well defined, except for the film of 14b, but fringe intensity does seem to increase as the substituent chain length is increased, suggesting that more uniform films are formed. The thicknesses of the films esti-mated from the positions of the fringes using the formula AQ =27+ are given in Table 4.AQ is the separation of adjacent fringes in the reflectivity profile and t is the thickness of the film. The calculated thicknesses do not, in general, correlate well with the thicknesses calculated from the number of dips (15 'bilayers') and the observed bilayer spacings. This may be due to the poor definition of the fringes or interference between the secondary maxima associated with the 001 peak 3 0 0 0 0 0.05 0.10 0.15 0.20 0.25 QIA Fig. 4 Fitted X-ray reflectivity profile of the LB Film of 14b J. MATER. CHEM., 1994, VOL. 4 and the fringes. This would produce a shift in fringe position. Each film exhibits a number of fringes between the critical angle and the 001 peak that supports the development of 15 bilayer films.An attempt was made to model the reasonably well defined fringes yielded by the film of 14b by calculating and fitting the reflectivity using an optical matrix formalism. This would be expected to give a more accurate determination of the film thickness. The resultant fit and the associated parameters are given in Table 5. The data were only fitted up to but excluding the first-order Bragg peak using a model of a single film of uniform electron density. The principal variable parameters in the fitting were the mean electron density of the film, the film thickness and the roughnesses of the air/film and film/substrate interfaces. The other parameters (scaling factor, background, beam divergence and electron density of air and the substrate) were kept constant at physically re$sonable values.The value of 658 & 1 A implies a film thickness of 15.7 bilayers. The large drop in reflected intensity after the 001 peak is usually indicative of the formation of a non-integral number of repeat units due to interference between the 001 peak and the Keissig fringes. It therefore appears that, con-sidering the X-ray reflectivity data and the fitted parameters, an 'extra' layer has been deposited onto the film. Interfacial roughnesses of films of phthalocyanines have not been reported before, but the roughnesses are very high when compared to those found by Mu~grove'~[or LB filmsooffatty acids deposited onto silicon (ualf.= 1.9 A, uf,,=4.4 A). The value of the mean electron density is sensible when compared to those of othet closely packed aromatic systems (for example, Pbe.,zene =0.281 A comparison of the data yielded by films of 10 on glass and silicon gives interesting results.The 'bilayer' spacings are the same, but the first-order Bragg peak is stronger in the case of the film on silicon. This implies a slightly different packing of the molecules to generate a bilayer with greater electron density at its centre. The visible absorption spectra were recorded of a film of each compound (Table 3). Spectra of LB films of 10 are somewhat similar to those of the alkoxy-substituted materials but show a higher degree of structure and a much sharper main band which replaces the split Q-band of the solution phase, Fig.2(b). The interpretation of the spectral bandshape is not straightforward and will not be pursued at the present time. The dichroism, with E, :E,,x 1.3 over the whole of the visible region, is indicative of a preferential alignment of the planes of the rings towards the normal to the dipping direction rather than along the dipping direction. In contrast, LB films of 11 through to series 14 give spectra with prominent red-and blue-shifted absorptions relative to the solution phase spectrum [Table 3 and Fig. 2(c) and (d)]. With one exception, the spectrum of the nickel derivative, 13c, where the separation of the main absorptions is smaller, Fig. 2(d), the spectra are closely similar in terms of band position to those obtained earlier for LB films of series 3.All show a similar dichroism such that E,:EI,> 1 for the blue-shifted band and E,:EII<l for the red-shifted band. As discussed in our earlier work, the splitting of the absorption Table 5 Parameters used to fit fringes of the LB film of 14b parameter value ~ layer thickness/A 658k 1 electron density/e k3 0.30f0.01 roughness of filmlsubstrate interface, u,,,/A 15.3f0.01 roughness of air/film interface, u,,,/A 11.7f0.01 J. MATER. CHEM., 1994, VOL. 4 band and the dichroism is indicative of Davydov splitting arising from the presence of translationally non-equivalent molecules within the 'unit cell', as would be found for a 'herring-bone' arrangement of the molecules within columns. On the basis of the simplified exciton model for an oblique arrangement of transition dipoles described by Kasha," the observed polarisation would point to the column axes aligned preferentially perpendicular to the dipping direction.Precedents for this type of column alignment where molecules are fully cofacial have been interpreted in terms of deposition of preformed columnar structures within the monolayer at the air/water interface.21 Conclusion The n-A isotherms and deposition characteristics observed for the alkoxy derivatives show that monolayer behaviour and film deposition are highly sensitive to the type of alkoxy group attached to the ring, and whether or not the compounds have been rendered amphiphilic through the modification of two of the side chains with terminal hydroxy groups.Metallated derivatives showed consistently better behaviour than the metal-free derivatives, with the latter giving LB films which appeared patchy on visual inspection. Introduction of amphiphilic character and incorporation of isopentyloxy groups in place of straight-chain pentyloxy substituents proved beneficial. Indeed, the best monolayer and deposition properties were exhibited by compounds 5b, 5c, 6, 7b and 8b. The poorest behaviour was shown by the amphiphilic com- pound 9, which suggests that the longer heptyloxy chains are disruptive to surface ordering. We believe these results are significant in the light of our recent X-ray crystallographic study of three compounds of series 1.22Compound lb shows an ordered structure in which the molecules stack in offset columns, similar to the proposed type of assembly within the ordered domains in the LB films.However, the nickel analogue of the straight-chain octapentyloxy derivative and octahep- tyloxy phthalocyanine both recrystallise to form solids which do not give a significant diffraction pattern, i.e. there is no long-range order. Thus it appears that the more compact isopentyloxy groups help to confer order, whereas the straight- chain alkoxy groups disfavour order. Replacement of the alkoxy functionalities by alkyl groups, as in the alkyl amphiphiles 10-14, gives rise to a further improvement in monolayer and deposition behaviour. LB films of the alkyl amphiphiles are more highly ordered, as judged by the X-ray diffraction data which provide evidence of bilayer spacing and dichroism within the visible spectra.The spectra also show that the molecular packing within the film is dependent upon the length of the hydrophobic chains, with that for the shortest-chain derivative, 10, differing from that for the others. The contrast in behaviour within the pairs of isosteric compounds from the alkyl amphiphile and alkoxy amphiphile series, 10 and 4a, 12 and 9, is remarkable. It reveals that the lack of order associated with the straight- chain alkoxy groups, referred to above, is not merely a function of their length. Presumably the presence of the ether linkages introduces local interactions which are sufficient to disrupt ordered molecular packing which, in phthalocyanines, is normally associated with interactions between the aromatic cores of adjacent molecules within columnar stacks.This effect may provide the basis for an explanation as to why the alkyl amphiphiles exhibit columnar mesophase behaviour whereas the alkoxy amphiphiles do not, core: core interactions within the former overwhelming the disruptive effects of the mobile alkyl side chains. The present results illustrate the potential for controlling molecular packing within phthalocyanine LB films through modification of the substituents on the ring and the assemblies described here clearly differ from the fully cofacial packing commonly observed for the tri- and tetra-substituted deriva- tives referred to earlier.The ability to control the molecular packing in phthalocyanine LB films is potentially important for the application of these formulations in devices. Dif'ferent types of molecular packing may show different interactions with gases, of potential value in sensing devices and, :is has been demonstrated, will have different electronic absorption signatures which may prove to be of value in research into laser addressed data storage systems. The authors thank SERC for a research grant to support J.McM., CASE studentships with DRA for D.A.M, K.H.P. and S.D.H. and an SERC-IT studentship for J.M.S. References 1 A. Hughes, Proc. R. Soc. London, Ser. A, 1936,155,710. 2 A. E. Alexander, J. Chem. Soc., 1937,1813.3 S. Baker, M. C. Petty, G. G. Roberts and M. V. Twigg, Thibi Solid Films, 1983,99, 53; S. Baker, G. G. Roberts and M. C. Petty, IEE Proc. Solid State Electron. Devices, 1983, 130, 260; J. Batey, M. C. Petty, G. G. Roberts and D. R. Wight, Electronic: Lett., 1984,20,489. 4 A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-assembly, Academic Press, San Diego, 1991. 5 M. J. Cook, in Spectroscopy of New) Materials, ed. R. J. H. Clark and R. E. Hester, Wiley, Chichester, 1993, pp.87-150. 6 e.g. M. Yoneyama, M. Sugi, M. Saito, K. Ikegami, S.-I. Kuroda and S. Iizima, Jpn. J. Appl. Phys., 1986, 25, 961; K. Ogawa, H. Yonehara, T. Shoji, S-I. Kinoshita, E. Maekawa, H. Nahahara and K. Fukuda, Thin Solid Films, 1989, 178, 439; K.Clgawa, S-I. Kinoshita, H. Yonehara, H. Nakahara and K. Fiikuda, J. Chem. SOC., Chem. Commun., 1989,477; M. Fujiki, H. Tatlei and S. Imamura, Jpn. J. Appl. Phys., 1987,26, 1224; M. Fujiki, H Tabei and T. Kurihara, Langmuir, 1988,4,1123. 7 M. J. Cook, A. J. Dunn, M. F. Daniel, R. C. 0. Hart, R. M. Richardson and S. J. Roser, Thin Solid Films, 1988,159,395. 8 N. B. McKeown, M. J. Cook, A. J. Thomson, K. J. Harrison, M. F. Daniel, R. M. Richardson and S. J. Roser, Thin Solid Films, 1988,159,469. 9 N. Dent, M. J. Grundy, R. M. Richardson, S. J. Roser, N. B. McKeown and M. J. Cook, J. Chim. Phys., 1988,85,1003. 10 M. J. Cook, M. F. Daniel, K. J. Harrison, N. B. McKeovn and A. J. Thomson, J. Chem. SOC., Chem. Commun., 1987,1148. 11 M. J. Cook, N. B. McKeown, J. M. Simmons, A. J. Thomson, M. F. Daniel, K. J. Harrison, R. M. Richardson and S. J. Roser, J. Muter. Chem., 1991, 1, 121. 12 For preliminary accounts of aspects of this study see: (a) M. A. Chesters, M. J. Cook, S. L. Gallivan, J. M. Simmons and D. A. Slater, Thin Solid Films, 1992, 210/211 538; (b)S. Mukopadhyay, A. K. Ray, M. J. Cook, J. M. Simmons and C. A. Hogarth, J. Muter. Sci., Muter. Electron., 1992,3, 139. 13 M. J. Cook, A. J. Dunn, S. D. Howe, A. J. Thomson and K. J. Harrison, J. Chem. SOC., Perkin Trans. 1, 1988,2453. 14 I. Chambrier, M. J. Cook, S. J. Cracknell and J. Mchlurdo, J. Muter. Chem., 1993,3, 841. 15 R. M. Richardson and S. J. Roser, Liquid Crystals, 1987,2, 797. 16 K. Bergesen, L. Haugland, M. J. Cook and J. McMurdo, in preparation. 17 H. Ringsdorf, B. Schlarb and J. Venzmer, Angew. Chem., lnt. Ed. Engl., 1988,113,27; A. Laschewsky, Angew. Chem., Int. Ed Engl., 1989,28,1574. 18 M. Kasha, in Spectroscopy of the Excited State, NATO Ad\. Stud. Ser. B, Physics, No 12, ed. B. D. Bartolo, Plenum Press, New York, 1976, p. 337. 19 R. J. Musgrove, Ph.D. Thesis, University of Bristol, 1991. 20 A. Zarbahksh, personal communication. 21 H. Itoh, T. Koyama, K. Hanabusa, E. Masuda, H. Shiri and T. Hayakawa, J. Chem. SOC., Dalton Trans., 1989,1543. 22 M. J. Cook, J. McMurdo and A. K. Powell, J. Chem. SOC., Chem. Commun., 1993,903. Paper 4/01269K; Received 2nd March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401205

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(8), 1215-1218 Cyclic Voltammetry of Zeolite-supported Manganese Porphyrins Laurent Gaillon, Fethi Bedioui* and Jacques Devynck Laboratoire d'Electrochimie et de Chimie Analytique (URA no 216 du CNRS), Ecole Nationale Superieure de Chimie de Paris, I I rue Pierre et Marie Curie, 75231 Paris Cedex 05, France The electrochemical behaviour of adsorbed 5,10,15,20-tetra(4-N-methyIpyridinium)porphyrinatomanganese(111)complex on ZSM-5 and EMC-2 zeolites and VPI-5 molecular sieve has been investigated by cyclic voltammetry. The results show that the redox potential of the Mn"'/Mn" process of the fixed porphyrins can be measured by this technique by using a pressed powder composite electrode. These potentials are positively shifted, by 50 to 380 rnV according to the zeolite matrices, relative to the unlinked complex. Several recent publications on the fixation of metal complexes on synthetic zeolites have pointed out the advantages of this type of new molecular material in the field of photochemistry and (e1ectro)catalysis.Thus, the new trends in the encapsul- ation of metal-Schiff base, metal-phthalocyanine,'-'' metal-polypyridinediyl and metal-phenanthroline complexes12-'* have prompted us to explore their electrochemical behav- iour.11,18-22 However, in spite of their potential use in many fields, relatively few examples of zeolite fixation of metallopor- phyrins by ion-exchange process haye been rep~rted.~~-~' In fact, owing to their large size (12-14 A; larger than the opening diameter of the zeolite networks) and their relatively low stability, porphyrins cannot be fixed within the porous struc- tures of zeolites (either by ion exchange or by in situ synthesis).They are simply supported on the external surface of the microporous minerals. Relating to this field, we reported in 1985 the first example of Y-type zeolite-adsorbed multicharged M(TMPyP) p~rphyrins~~ [M =Mn"', Co"' or Fe"'; TMPyP = 5,10,15,20-tetra(4-N-methy1pyridinium)porphyrinatomangan-ese(111)1 and their use in electro~atalysis.~~ Since the construction of organized molecular microstruc- tures on electrode surfaces has become of considerable interest in the fields of photo- and electro-chemistry, we describe in this paper the electrochemical characterization, by cyclic voltammetry of the multicharged manganese porphyrin, Mn(TMPyP) (Fig.1) supported on ZSM-5 and EMC-2 zeo- lites and VPI-5 molecular sieve. This provides new examples of zeolite-supported porphyrins and extends the use of such molecular materials to different types of zeolites from the most commonly used Y-type faujasite. Experimental Zeolite ZSM-5 is a member of the silicate family in which the porous network is made up of 10-rings (with a free opening of Fig. 1 Structure of the manganese porphyrin, Mn(TMPyP) 5.3 A x 5.6 A and 5.1 A x 5.5 A)interconnected in zigzag chan- nelsq2' EMC-2 is a member of the pure hexagonal phase of the faujasite family (structure type EMT) in which the porous netyork is omade up 12-rings with pore sizes of 7.6 A and 7.6 Ax 5.7 A.29 VPI-5 molecular sieve is a member of the aluminophosphate family in which the porous network as made up of 18-membered rings with a pore size of ca.13 A.30 All these materials were prepared and provided by the Labordoire de Reactivitk de Surface de YUniversite Paris VI (U.R.A. no 1106 du CNRS) according to the cited literat~re.~*-~' M~(TMPYP)~'was supported on EMC-2 and ZSM-5 zeolites using an ion-exchange reaction by stirring 0.8 g of the zeolites (sodium form) in 6 ml of H20+ 1mmol 1-' Mn(TMPyP) solution for 1 h. The zeolite samples wert: then carefully washed with methanol, dichloromethane and aceto- nitrile and air-dried at 80°C for 24 h. The fixation of Mn(TMPyP) on VPI-5 molecular sieve was achieved by simple adsorption of the complex on the surface of the neutral mineral according to the experimental procedure used for the zeolites.UV-VIS spectrophometry (performed in a Nujol mull) provided evidence for the fixation of porphyrin on the solid minerals, since all the Soret bands observed for the supported complexes appeared around 475 nm, which is characteristic of Mn"' p~rphyrins.~~ Elemental analysis (Mn) of the solids showed that they contained ca. 0.5 wt.% of bound manganese complex. All electrochemical studies were performed using a three- electrode potentiostatic system [Princeton Applied Research assembly including a potentiostatic galvanostat (model 173) with an interface (model 276) monitored with an IBM-PC computer]. The working electrodes were prepared by car efully mixing 30 mg of the solid manganese porphyrin supported on the mineral matrix with 30 mg of graphite powder and pressing the mixture onto a platinum or gold grid (total area, 2cm2) according to the literature method.23 The electrochemical behaviour of such electrodes was analysed by cyclic voltamme- try in dimethyl sulfoxide (DMSO), acetonitrile (CH3CN) and H20 with different supporting electrolytes. The electrolytic solutions were routinely deoxygenated by bubling argon through them for 30min prior experiments and by main- taining an argon atmosphere above the solutions during the experiments.The potentials are reported with reference to an aqueous saturated calomel electrode (SCE) located in a separ- ate compartment containing the supporting electrolyte.Results and Discussion Fig. 2 shows the cyclic voltammogram of Mn(TMPyPr fixed on ZSM-5 zeolite in acetonitrile-0.1 mol 1-' tetrabutylam-monium tetrafluoroborate (TBABF,)-0.1 mol 1-li thiurn tetrafluoroborate (LiBF,) solution. The well defined pair of J. MATER. CHE:M., 1994, VOL. 4 -EN vs. SCE -0.2 0 0.2 EN vs. SCE Fig. 2 Cyclic voltammetry (c= 10 mV s-') of Mn(TMPyP) adsorbed on ZSM-5 in acetonitrile-0.1 mol 1-'(TBABF,-LiBF,) solution Fig. 3 Cyclic voltammetry (u = 10 mV s-') of Mn( TMPyP) adsorbed on EMC-2 in acetonitrile-0.1 mol l--' LiBF, solution peaks observed in this voltammogram can be attributed to the well known Mn"'/Mn" redox of the sup- Nevertheless, note that in DMSO-0.1 mol 1 TBABF, solu- ported porphyrin.Table 1 shows the redox potential noted tion, addition of LiBF, leads to the appearance of a new Ee,="(Epa+Ep,)/2 (Epaand Epc are the anodic and cathodic couple of peaks situated at Eeq= -0.075 V (see Fig. 5). The peak potentials, respectively) for this redox process determined coexistence of both pairs of peaks in DMSO-0.1 mol 1-l for the dissolved Mn(TMPyP) porphyrin (entry 1) and com- (TBABF,-LiBF,) may be a reflection of there being two pared with those obtained for the complex supported on different redox sites located on the zeolite surface. They may (entry 3), on Y-type also be related to two different types of interaction between silica34 (entry 2) on m~ntmorillonite~~ zeolite23 (entry 4) and on ZSM-5 zeolite (entry 5).These the complex and the mineral support (adsorbed or ion-results show that the electrochemical behaviour of the manga- exchanged species). The different observation of the two redox nese porphyrin in acetonitrile solution is not significantly sites could be a consequence of the electrochemical visibility affected by its irreversible fixation on the zeolite. The observed of the different couples. This was observed only in the case of potential shifts measured for the Mn"'/Mn" redox process of the EMC-2 zeolite and in DMSO solution. However, similar the ZSM-5 supported complex in different electrolytic media splitting of the redox peaks in H,O-0.02 rnol 1-l KC10, was show an effective influence of both the solvent and the also observed, but the cyclic voltammetric data were too ill electrolyte cation on the electrochemical reaction.This may defined to be considered further. be explained by a solvation effect of the supported porphyrin By taking into account the quantities of charge passed combined with charge compensation by the electrolyte cation during the Mn"'/Mn" reduction or oxidation peaks during upon the electrochemical reduction of the Mn"' centres. the cyclic voltammetric measurements (after substraction of Fig. 3 and 4 show the cyclic voltammograms of the background currents, as indicated on the voltammograms Mn(TMPyP) porphyrin adsorbed on EMC-2 type zeolite by the dashed lines) we can estimate the amount of electro- in acetonitrile-0.1 mol 1-' LiBF,, and DMSO-0.1 mol 1-' chemically active supported species.In the case of ZSM-5 and TBABF, solution. These data show that the electrochemical EMC-2 zeolite-supported porphyrin; the total loading of the behaviour of the supported porphyrin remains well defined solid minerals is 3.5 x mol of porphyrin per g of zeolite. without any significant changes (see Table 1, entry 6). As the outer surface of the zeolites considered is c'u. 30-40 m2 Table 1 Redox potential values E,, =(Epa+Ep,)/2 of the Mn"'/Mn" process for the Mn( TMPyP) porphyrin dissolved in solution and supported on different mineral matrices system solvent €,,IF' cs. SCE ref. 1 dissolved Mn( TMPyP) DMSO-0.1 moll-' CH,CN-0.1 mol I-' TBABF, TBABF, -0.080 0.100 34 34 H,O-0.02 mol 1-' KClO, -0.250 this work 2 Mn(TMPyP) adsorbed on DMSO-0.1 moll-' TBABF, -0.110 34 silica CH,CN-0.1 mol 1-' (TBABF,-LiBF,) 0.010 34 3 Mn( TMPyP) intercalated into montmorillonite DMSO-0.1 moll-' TBABF, CH,CN-0.1 mol 1-l TBABF, -0.100 0.070 33 33 4 5 Mn(TMPyP) adsorbed on Y Mn(TMPyP) adsorbed on faujasite CH,CN-0.1 mol I-' Et,NC10, H20-0.02 mol I-' KClO, CH,CN-0.1 moll-' (TBABF,-LiBF,) 0.030 0.100 0.140 23 23 this work ZSM-5 DMSO-0.1 mol 1-' (TBABF,-LiBF,) 0.125 this work H,O-0.02 moll-' KClO, 0.00 this work 6 Mn(TMPyP) adsorbed on CH,CN-0.1 mol 1-' LiBF, 0.00 this work ECM-1 DMSO-0.1 mol 1-1 TBABF, 0.160 this work DMSO-0.1 moll-' LiBF, -0.075 this work 7 Mn(TMPyP) adsorbed on CH,CN-0.1 moll-' TBABF, 0.050 this work VPI-5 DMSO-0.1 moll-' TBABF, 0.175 this work H20-0.02 mol 1-' KClO, 0.120 this work J.MATER. CHEM., 1994, VOL. 4 --0.4 0 0.4 EN vs. SCE Fig. 4 Cyclic voltammetry (v= 10 mV s-') of Mn(TMPyP) adsorbed on EMC-2 in DMSO-0.1 mol 1-' TBABF, solution 100 -1 00 -0.4 0 0.4 EN vs.SCE Fig. 5 Cyclic voltammetry (u = 10 mV s-') of Mn(TMPyP) adsorbed on EMC-2 in DMSO-0.1 mol 1-1 (TBABF,-LiBF,) solution g-', the manganese porphyrins (cross-section 1.8 nm2) are present as a monolayer on the external surface of the zeolite crystals. The calculated quantities of charge measured in DMSO, CH,CN and H,O solutions (from the voltammetric peaks after the deduction of the background capacitive cur- rent) are small and suggest that 2% of the supported complex is electroactive.It is obvious from this result that the number of electrochemically accessible sites is small. However, we have observed that the measured electroactivity remains con- stant during several experiments with no significant loss. This provides helpful evidence for the stability of the supported complexes. Table 1 reports the results obtained with the supported complex and especially those provided by VPI-5 adsorbed Mn(TMPyP). These data are the first reported results of the electrochemical characterization in organic and aqueous sol- vents of such chemically modified zeolites and a molecular sieve. Conclusion One main result is clear from Table 1: the surface adsorption of the porphyrin to EMC-2, ZSM-5 and VPI-5 lcads a significant positive shift of the potential value of the Mn"'/Mn" redox process in DMSO and H,O solution (up to 380mV).This indicates that the fixation of the porphyrin to the mineral by surface adsorption makes the metal centre more easily reducible in DMSO and H,O solutions relative to the urilinked complex. The origin of this phenomenon may arise from a strong interaction between the central manganese cation of the porphyrin and the negatively charged zeolite surface. Further studies comparing the electroanalysis of these sup- ported materials with their catalytic efficiency (for oxidation reactions where the Mn"'/Mn" redox process is in1 olved) should provide more evidence of the benefit of zeolite adsorp- tion of the manganese porphyrin, similar to our previously reported studies on the clay-intercalated Mn porphyri ns and their electrocatalytic behavi~ur.~~,~~ We thank Dr. J-M.Manoli and Dr. C. Potvin from the Laboratoire de Rkactivite de Surface de I'Universite Paris VI (U.R.A. no 1106 du CNRS) for providing us with the samples of ZSM-5, EMC-2 and VPI-5. References 1 G. A. Ozin and C. Gil, Chem. Rev., 1989,89,1979. 2 P. C. H. Mitchell, Chem. Ind., 1991, 308, and references tht rein. 3 N. Herron, J. Coord. Chem., 1988,19,25. 4 K. J. Balkus Jr., A. A. Welch and B. E. Gnade, J. Inclusion l'henom. Mol. Recog. Chem., 1991,10, 141. 5 M. Nakamura, T. Tatsumi and H.Tominaga, Bull. Chrm. SOC. Jpn., 1990,63, 3334. 6 C. Bowers and P. K. Dutta, J. Catal., 1990, 122. 271. 7 N. Herron, Chemtech, 1989,542. 8 K. J. Balkus Jr., A. A. Welch and B. E. Gnade, Zeolites. 1990, 10, 722. 9 S. Kowalak, R. C. Weiss and K. J. Balkus Jr., J. Chem. SOC, Chem. Commun., 1991, 57. 10 A. G. Gabrielov, K. J. Balkus Jr., S. L. Bell, F. Bedioui and J. Devynck, Microporous Muter., 1994,2, 119. 11 K. J. Balkus Jr., A. G. Gabrielov, S. L. Bell, F. Bedioui, I,. Roue and J. Devynck, Inorg. Chem., 1994,33,67. 12 W. De Wilde, G. Peeters and J. H. Lunsford, J. Phys. Chew., 1980, 84,2306. 13 W. H. Quayle and J. H. Lunsford, Inorg. Chem., 1982,21,97. 14 K. Mizuno and J. H. Lunsford, Inorg. Chem., 1984,23,3510. 15 K. Mizuno, S.Tmamura and J. H. Lunsford, Inorg. Chent., 1984, 23, 3510. 16 P. K. Dutta and J. A. Incavo, J. Phys. Chem., 1987,91,4443. 17 K. Maruszewski, D. P. Strommen, K. Handrich and J. R. Kincaid, Inorg. Chem., 1991,30,4579. 18 K. Mesfar, B. Carre, F. Bedioui and J. Devynck, J. Muter Chem., 1993,3, 873. 19 F. Bedioui, E. De Boysson, J. Devynck and K. J. Balkus Jr., J. Electround. Chem., 1991,315, 313. 20 F. Bedioui, E. De Boysson, J. Devynck and K. J. Ba!kus Jr., J. Chem. SOC.,Furaduy Trans., 1991,87,3831. 21 L. Gaillon, N. Sajot, F. Bedioui, J. Devynck and K. J. Balkus Jr., J. Electround. Chem., 1993,345, 157. 22 F. Bedioui, L. Roue, E. Briot, J. Devynck, S. L. Bell and K. J. Balkus Jr., J. Electround. Chem., in the press. 23 B. De Vismes, F.Bedioui, J. Devynck and C. Bied-Charreton, J. Electround. Chem., 1985, 187, 197. 24 B. De Vismes, F. Bedioui, J. Devynck, M. Perree-Fauvet and C. Bied-Charreton, Nouv. J. Chim., 1986, 10, 81. 25 L. Persaud, A. J. Bard, A. Campion, M. A. Fox, T. E. hlallouk, S. E. Weber and J. M. White, J. Am. Chem. Soc., 1987,109,7309. 26 Z. Li, C. M. Wang, L. Persaud and T. E. Mallouk, J. Phy;,. Chem., 1988,92,2592. 1218 J. MATER. CHEM., 1994, VOL. 4 27 A. J. Bard and T. E. Mallouk, in Molecular Design of Electrode 32 L. Barloy, J. P. Lallier, P. Battioni, D. Mansuy, Y. Piffard and Surfaces, ed. R. W. Murray, Wiley, New York, 1992, p. 271 and references therein. 33 M. Tournoux, New J. Chem., 1992,16,71, and references therein. L. Gaillon, F. Bedioui, J. Devynck, P. Battioni, L. Barloy and 28 29 J. V. Smith, Chem. Rev., 1988,88, 149, and references therein. F. Dougnier, J. Patarin, J. L. Guth and D. Anglerot, Zeolites, 1992, 12, 160. 34 D. Mansuy, J. Electroanal. Chem., 1991,303,383. L. Gaillon, F. Bedioui, J. Devynck and P. Battioni, J. Electroanal. Chem., 1993,347,435. 30 31 M. E. Davis, C. Saldarriaga, C. Montes, J. Garees and C. Crowder, Nature (London), 1988,331,698. A. Harriman and G.Porter, J. Chem. SOC., Faraday Trans. 2, 1979, 75, 1532. 35 L. Gaillon, F. Bedioui, J. Devynck and P. Battioni, J. Mol. Catal., 1993,78, L23. Paper 4/01820F; Received 28th March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401215

出版商:RSC    

年代:1994

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J. MATER. CHEM., 1994, 4(8), 1219-1226 Magnetic Properties and Crystal Structure of the p-Fluorophenyl Nitronyl Nitroxidet Radical Crystal: Ferromagnetic Intermolecular Interactions'leading to a Three-dimensional Network of Ground Triplet Dimeric Molecules Yuko Hosokoshi, Masafumi Tamura, Minoru Kinoshita,* Hiroshi Sawa, Reizo Kato, Youko Fujiwara* and Yutaka Ueda Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106,Japan The crystal of p-fluorophenyl nitronyl nitroxide [2-(4-fluorophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-l H-imidazol-1 -oxyl 3-oxide, abbreviated as p-FPNN] radical has been found to possess a dimeric structure. Each dimer is related to adjacent ones by four-fold screw symmetry to form a three-dimensional network.The temperature dependence of the paramegnetic susceptibility is explained by the formation of a triplet state within the dimer and additional interdimer ferromagnetic interactions. The intra- and inter-dimer exchange interactions are estimated to be J/k, =5.0 K and J'/kB= 0.02 K, respectively. The molecular arrangement in the dimer suggests that the intermolecular interactions between the NO group and the phenyl ring are favourable to ferromagnetic coupling. Slight structural changes below ca. 100 K have been studied by use of electron paramagnetic resonance and powder X-ray diffraction measurements. In 1991 it was found that P-p-nitrophenyl nitronyl nitroxide (p-p-NPNN) undergoes a ferromagnetic transition at 0.6 K.'T~ This is the first example of a purely organic radical ferromag- net consisting only of light elements such as C, H, N and 0.Following this, a second organic ferromagnet with T,=1.48 K has been rep~rted.~ These findings have initiated investigations into the magnetic properties of molecular materials. However, the detailed mechanism of the ferromagnetic intermolecular interactions in these materials is still open to research. Understanding the origin of the ferromagnetic interactions would help to improve the rational design of molecular magnets in terms of molecular and crystal structures, which are not fully available yet. In order to elucidate the physics behind the magnetism of molecular materials, knowledge of the relation between the molecular packing and the intermol- ecular magnetic interactions is of primary importance.Some other organic radical crystals, such as galvinoxyl [2,6-di-tert- butyl-4-( 3,5-di-tert-butyl-4-oxocyclohexa-2,5-di-enylidenemethyl) phenoxyl] ,4 TANOL suberate { 4,4'-[1,8-dioxoctane- 1,8-diyl) bis (oxy)] bis( 2,2,6,6-tetramethylpiperidin-1-yloxyl)},5 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-yloxyl (MOTMP)6 and Y-P-NPNN,~ are known to possess dominant ferromagnetic intermolecular interactions as sug- gested by the positive Weiss constants. However, the last three compounds have been found to undergo antiferromagnetic transitions owing to antiferromagnetic secondary interactions. As typically demonstrated by these facts, the molecular pack- ing that leads to three-dimensional (3D) ferromagnetic inter- actions in the crystal is crucial to bulk ferromagnetism.Hence, the control of the crystal structures is one of the most important factors in designing molecular magnets. The series of phenyl a-nitronyl nitroxides afford satisfactory crystallinity and stability. Many derivatives of this series can be readily obtained. These characteristics are suitable for the purpose of surveying the interrelations between the molecular form, crystal structure and the magnetism in organic radical crystals. By classifying the molecular packing of these com- pounds in the crystals, useful information can be found for the control of molecular magnetism. ?The use of the term nitroxide is discouraged by IUPAC; the preferred term is aminoxyl.$ Present address: Department of Chemistry, Faculty of Science, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171, Japan. In this paper, an example of a 3D network of ferromagnetic intermolecular interaction is presented for p-fluorophenyl nitronyl nitroxide (p-FPNN). The crystal structure of this compound is first described, with emphasis on its dimeric character and its three-dimensionality. From the analysis of the static magnetic results, it can be seen that both intra- and inter-dimer interactions are ferromagnetic and that the system is thus ferromagnetic in three dimensions. The molecular packing is compared with that of other related compounds. Intermolecular interactions between the NO group and the phenyl ring are suggested as favourable for ferromagnetic coupling.It is found from electron paramagnetic resonance (EPR) measurements that structural distortion occurs below ca. 100 K. The change in crystal symmetry and lattice param- eters were analysed by means of powder X-ray diffraction techniques. Experimental Sample Preparation p-FPNN was prepared basically by following the route given in the literature (Scheme l).7,8 Slight modifications to the firstg and the second procedures'O in Scheme 1afforded higher yields (see Appendix). Dark-blue single crystals of p-FPNN were obtained by slow evaporation of a concentrated solution. The crystals from the solutions of various organic solvents, hexane, diethyl ether, benzene etc., were examined by means of the X-ray Weissenberg photograph.No polymorphism was found for room temperature crystal growth. Crystals obtained from a hexane solution were used for all the experiments. X-Ray Crystallographic Analysis X-Ray intensity data at 298 K were collected on an MAC Science automated four-circle diffractometer with graphite monochromatized Cu-Ka radiation by 8-20 scans up to 20 =130". Unit-cell parameters were determined from the least-squares refinement for 20 reflections within 56" <28 < 60". The independent 1782 reflections with (F,I >4a((F, ) were used for structural analysis. The structure was solved by the direct method and refined by the full-matrix least-squares procedure with an anisotropic approximation for non- J.MATER. CHEM., 1994, VOL. 4 Scheme 1 hydrogen atoms. An analytical absorption correction was performed." All the procedures were carried out with the 'Crystan' program package of MAC Science. Static Magnetic Measurements The magnetization below 10 K for the field up to 55 kOe and the susceptibility from 1.8 to 300K were measured using a Quantum Design MPMS SQUID magnetometer. The field was applied parallel (HIlc) or perpendicular (Hlc) to the c-axis. The susceptibility below 10 K was measured under a small applied field (500 Oe) to avoid saturation effects as far as possible. The diamagnetic contribution was estimated by fitting the data above 200 K to the Curie law. EPR Measurements EPR spectra of single crystals were measured by using a JEOL JES-FElXG EPR spectrometer (X band) with rotation of the crystal around the crystallographic axes.The sample was placed at the centre of a cylindrical cavity in the TEoll mode. The microwave magnetic field, H,, was always in the vertical direction and the static field, H, was in the horizontal direction. The sample was rotated around the vertical axis. An Air Products LTR-3-110 helium flow type cryostat was used for low-temperature measurements of down to 2.2K. The spectra were recorded at various temperatures on the heating process. The crystallographic axes were checked by means of the X-ray Weissenberg photograph every time before and after the low-temperature experiments. Powder X-Ray Diffraction Measurements Powder X-ray diffraction data were collected using an MAC Science MXP18 system with a rotating anode generator and a monochromator of single crystalline graphite for Cu-Ka radiation. A CIT closed cycle helium gas refrigerator model 22C was used for low-temperature measurements of down to 10 K.Powdered silicon was added as an internal standard for diffraction angles. Results and Discussion Crystal Structure p-FPNN crystallizes in the tetragonal system (space group 141/a). The crystallographic data and the final positional parameters are summarized in Tables 1 and 2. The unit cell contains 16 crystallographically equivalent molecules. Fig. 1 shows an ORTEP12 drawing of the molecule. The O(l)-N(3)-C(5)-N(4)-0(2) moiety is planar within 0.014 A.Table 1 Crystallographic data formula formula weight crystal size/mm crystal system spaoce group vik3 a/+ CIA z DJg cm -radiation scan mode total reflections measured unique reflections reflections used (IFol>4o(lF,I)) residuals: R; R," goodness of fit: S C,,H,,N,O,F251.30 0.25 x 0.40 x 0.30 tetragonal 14,la 5343 [21 21.931 [3] 11.110 [2] 16 1.25 Clu-Kx(i= 1.54178 A 8-28 2566 2214 1782 0.061; 0.086 2.32 "The function minimized was sum [W(IF~~~-(F~(~)~],in which w = [(olFo +0.00061F012] -1)' Theodeviationof C(12) and C(13) from this plane is about 0.1 A. The dihedral angle between the best planes of the ONCNO moiety and the phenyl ring is 36.0".According to EPRI3 and nuclear magnetic resonance (NMR)14 measurements, most of the spin densities of 2-nitronyl nitroxide radicals are concentrated on the ONCNO moiety. Therefore, in the following, attention is directed mainly toward the structural features related to the ONCNO moieties. Fig. 2 and 3 display the molecular packing in the crystal viewed along the c and b axes, respectively. Short inter- molecular atomic distances are selected in Table 3, in which interatomic distances concerning the methyl groups are omitted. As can be seen in Fig. 2, there is a noticeable dimeric structure. Short intermolecular contacts linking the NO group and the phenyl ring are found between the two molecules in the dimer. The relevant intermolecular atomic distancfs are 0(2)..-C(lO), 3.117[4] A and 0(2)...H(10), 2.23[4] A, the latter being shorter than the sum of the van der Waals radii.The intermolecular contacts are doubled as a result of the inversion symmetry at the centre of each dimer. A dimer is surrounded by four nearest-neighbours. Between the neighbouring dimers, ,a relatively short intermolecular atomic distance, 3.538[3] A, is found, which is between the terminal oxygen, O(l), of molecule i (x,y, z) and the bridge- head carbon atom of the ONCNO moiety, C(5), of molecule iii (1/4+7, -1/4+x, -1/4+z). As shown in Fig. 3, each partner of the dimers arrayed along a four-fold screw axis can interact with one another by the above relation. J. MATER. CHEM., 1994, VOL. 4 1221 Table 2 Positional parameters and equivalent isotropic thermal parameters with standard deviations in square brackets atom x Y Z 0(1) 0.25328 [8] 0.0985 [l] -0.0046 [2] 7.42 [7] O(2) 0.46249 [9] 0.1094 [11 0.0131 [3] 9.15 [9] N(3) 0.30725 [9] 0.1177 [l] 0.0171 [2] 5.17 [6] N(4) 0.4059 [l] 0.1224 [l] 0.0277 [a] 5.80 [6] C(5) 0.3593 [1] 0.0898 [1] -0.0139 [2] 4.69 [6] C(6) 0.3640 [l] 0.0325 [l] -0.0803 [2] 4.61 [6] C(7) 0.3219 [1] 0.0180 [l] -0.1696 [2] 5.16 [7] C(8) 0.3251 [2] -0.0368 [l] -0.2303 [31 6.01 [8] C(9) 0.3698 [l] -0.0766 [l] -0.1987 [3] 6.10 [8] C(10) 0.4123 [1] -0.0646 [l] -0.1123 [3] 6.45 [9] C(11) 0.4098 [l] -0.0094 [l] -0.0527 [3] 5.62 [8] C(12) 0.3177 [1] 0.1729 [l] 0.0936 [31 5.79 [S] ~(13) 0.3876 [ij 0.1796 [i] 0.0879 [31 6.57 [9] C(14) 0.2942 [2] 0.1561 [2] 0.2179 [4] 9.0 [l] ~-X a C(15) 0.2822 [2] 0.2256 [2] 0.0390 [61 9.3 c21 C(16) 0.4100 [3] 0.2306 [2] 0.0011 [7] 12.6 [2] Fig.2 Crystal structure of p-FPNN viewed along the c axis, showing C(17) 0.4200 [3] 0.1857 [5] 0.2056 [6] 14.0 [3] the relation between the dimers. Symmetry operations: i (.x, y, z),F( 18) 0.3723 [11 -0.13187 [9] -0.2558 [2] 8.97 [7] ii( 1+X,F,Z), iii (1/4+J, -1/4 +x,-1/4+z). The closest intradimer H(7) 0.290 [l] 0.047 [2] -0.185 [3] spacing it indicated by the dotted line together with the interiitomic H(8) 0.296 [2] -0.048 [2] -0.293 [3] distance/A.H(10) 0.444 [2] -0.094 [2] -0.088 [3] H(11) 0.437 [2] 0.002 [2] 0.011 [3]H(14A) 0.305 [2] 0.188 [2] 0.267 [4]H(14B) 0.250 [2] 0.151 [2] 0.210 [4]H(14C) 0.314 [2] 0.114 [2] 0.252 [4]H(15A) 0.292 [2] 0.267 [2] 0.090 [4]H(15B) 0.240 [2] 0.210 [2] 0.063 [4] H(15C) 0.303 [2] 0.231 [2] -0.047 [5] H(16A) 0.407 [3] 0.258 [3] 0.078 [6] H(16B) 0.382 [3] 0.223 [3] -0.052 [7] H(16C) 0.459 [3] 0.229 [3] -0.015 [6] H(17A) 0.405 [3] 0.212 [3] 0.251 [7] H(17B) 0.405 [3] 0.144 [3] 0.247 [6] H(17C) 0.459 [3] 0.191 [3] 0.184 [16] 0 Fig.3 Crystal structure of p-FPNN projected onto the bc plane. The closest intra- and inter-dimer spacing are indicated by the dotted an9 broken lines, respectively, together with the interatomic distarrces/A. The symmetry operation notation is the same as that in Fig. 2 Table 3 Short intermolecular atomic distances with standard devi- Fig.1 ORTEP drawing of the p-FPNN molecule showing the atom- ations in square brackets‘ numbering scheme. For simplicity, the hydrogen atoms of methyl ib ... groups are not shown. 11 distance/A 1 111 distmce/W ~~~ ~ ~ O(2) C(10) 3.117 [4] 0(1) C( 5) 3.538 [3] Fig. 4 is a schematic view of the unit cell, where the open O(2) C(11) 3.583 [4] O(1) C(11) 3.576 [4] C(7) C( 6) 3.587 [3] circles and the ‘bonds’ represent the dimers and the interdimer C(7) C( 7) 3.606 [2] interactions, respectively. It can be seen that dimers form a C(7) C(11) 3.706 [4] 3D network by the four-fold coordination of the interdimer C(l) C( 6) 3.718 [3] interaction. “Atomic distances concerning the methyl groups are omitted. bi (x, y, z);ii (l+X, -6 3;iii (1/4+Y, -1/4+x, -1/4+z).Static Magnetic Measurements The molar diamagnetic susceptibility was estimated to be The temperature dependence of the paramagnetic suscepti- -1.70 x emu mol-’ (= -2.14 x m3 mol-l) for Hllc bility is shown in Fig. 5. The Curie-Weiss law reproduces the and -1.50x lop4emu mol-’ for Hlc. observations above 25 K with the Curie constant, C,= The paramagnetic susceptibility (x,) for Hlc was slightly 0.375 emu K mol-’ and the Weiss constant, 0,=2.5 K. The (about 0.3%) larger than that for HJlc above 10 K. The Curie constant agrees with the existence of 1 mol of S= 1/2 anisotropy gradually increased below 10 K with decreasing species. The positive Weiss constant indicates the ferromag- temperature. For Hlc, xp was larger than that for Hllc by netic interactions between these spins. Below ca.lOK, the only about 1% even at the lowest temperature. Since this value of the susceptibilities are close to the prediction by the anisotropy plays no substantial role in the following dis- Curie law with C =0.5 emu K mol-’, suggesting the formation cussion, we refer only to the Hl\c data in this paper. of 1/2 mol of S =1 (triplet) species. A 0 a Fig. 4 Schematic display of the 3D network of the dimers. The circles and ‘bonds’ stand for the dimers and the interdimer interactions, respectively. The hatched circles and the filled circles are correspond- ing to the dimers depicted in Fig. 2 and 3. ’,I. s = 112 Curie-Weiss law e =2.5 K I-E E’ %* 1o4 1 10 100 TIK Fig.5 Paramagnetic susceptibility (x,) plotted against temperature The formation of triplet species at low temperature is also supported by the magnetization data. The magnetization curves below 10K are shown in Fig. 6, together with the theoretical curves for non-interacting S =1 and 1/2 species calculated on the basis of the expression, WHIT)=(N4/2S)gP,SBs(x) (1) where B,(x)is the Brillouin function and x=gSpBH/kBT.It is evident that the curves at 1.8 and 4.0 K are explained by eqn. (1)with S=l. Taking account of the dimeric structure of this material, we surmise that the two S= 1/2 spins form a triplet state within 6000F T 4000 E J. MATER. CHEM., 1994, VOL. 4 each dimer below ca. 4 K. Namely, the intradimer intermolecu- lar interactions are considered to be ferromagnetic. The plot of xpTversus T (Fig.7) is a convenient tool to find magnetic interactions; xpT is proportional to the Curie constant or to the square of the so-called effective moment. Starting from the value for non-interacting S =1/2 spins, 0.375 emu K mol-’, xpTincreases as T decreases. Below ca. 5 K, the increase in xPT is suppressed, suggesting that the triplet formation completes around this temperature. However, xpT continues to increase even at the lowest tem- peratures, and there is a large excess over the value for the S= 1 dimers (0.5 emu K mol-l) below about 3 K. It follows that there exist additional ferromagnetic interactions between the dimers from these observations. In view of the molecular arrangement in the crystal, a model can now be introduced to describe the whole tempera- ture dependence of the susceptibility. The mean-field treatment of the weak interdimer interactions yields xp=7T-8‘i3+exp(-2J/k,T) -1 where J denotes the intradimer interactions and C =0.5 emu K mol-’.The factor, 3/[3 +exp(-2J/kBT)], describes the temperature dependence of the number of the triplet species. The mean-field parameter, 8’, is related to the interdimer exchange coupling, J’, by 8’=2zJ’S(S+1)/3kB (3) where S =1 and z is the number of the nearest-neighbour dimers. As each dimer is surrounded by four dimers in the crystal, it is reasonable to assume z =4. In Fig. 7, the fit of this model to the experimental data is shown by the solid curve.The best fit is obtained, when J/kB= 5.0 K and 8’=0.1 K are used. The interdimer inter- action is weak enough to justify the application of this model to the present case; the validity of the mean-field approxi- mation holds when T>>8’and J/kB>>@. By putting 8’=O.l K into eqn. (3), the interdimer interaction, J’/kB,is estimated to be about 0.02 K. Only this assignment of the interactions to the intra- and inter-dimer ones is consistent with the molecular arrangement at 298 K. Let us now briefly summarize the magnetic state of the system as a function of temperature. As all the intermolecu- lar magnetic couplings are thermally disturbed above ca. 100 K, the system behaves as an assembly of independent S= 1/2 spins.The triplet species are formed within the dimers to raise xpTon cooling from 100 K, as a result of the intradimer interactions (J/kB=5.0K). At ca. 4 K, almost all the dimers are triplets. Further cooling makes the triplets interact ferro- 0.55~ 0 0 10 20 30 HT -l/koeK-l Fig. 7 Temperature dependence of xpT.The solid curve representsFig. 6 Magnetization curves. Solid curves are theoretical ones based the values calculated on the basis of eqn. (2) with J/k, =5.0 K and on the Brillouin function. 0, 1.8 K; 0,4 K; 0,10 K. 8‘=0.1 K. J. MATER. CHEM., 1994, VOL. 4 magnetically with each other, which affords an extra increase in x,T. It should be noted that the interdimer interactions form a 3D network as can be seen from the crystal structure (Fig.4). This would allow the system to exhibit ferromagnetism at very low temperatures. The mean-field parameter, 8’=0.1 K, is the upper limit of the transition temperature.? Magneto-structural Correlation Let us consider the correlation between the magnetic inter- actions and the molecular arrangements. The present data show that the dimer is responsible for the ferromagnetic exchange coupling, J/k, =5.0 K. This interaction is charac- terized by the close spacing between the terminal oxygen of the nitroxide group in the one molecule and the phenyl-side hydrogens and carbons in the other. Hereafter, we write this type of interaction as the NO..-Ar interaction, where Ar stands for an aryl group. We have also shown that the interdimer interaction is responsible for the additional ferromagnetic exchange coup- ling, J’/k, =0.02 K.The close spacing of the terminal oxygen of the nitroxide group in the one molecule and the bridgehead central carbon in the other is characteristic of this interaction. This conformation is expected to be stable from the viewpoint of electrostatic energy, which is potentially useful in designing the molecular form preferable to ferromagnetic interaction. Similar molecular arrangements have also been found in the crystals of p-~hlorophenyl,’~ p-iodophenyl16p-br~mophenyl,’~ and ~yrimidinyl’~ nitronyl nitroxides. However, the sign and amplitude of the exchange inter- actions related to this arrangement is very sensitive to the detailed structure.For example, the p-chloro derivative shows an antiferromagnetic interaction, whereas the p-bromo and p- iodo derivatives exhibit ferromagnetic couplings. This suggests that the observed net interactions are determined by the competition between the interactions of different signs; the interaction between the terminal oxygen and the central carbon and that between the oxygen and other atoms in the phenyl ring may compensate each other. Therefore, more improvement in the design of the molecular structure is needed in order to control this type of interaction efficiently. The NO-..Ar interactions are also found in the 8-,18,19y-,’ and of p-NPNN. All of them are known to exhibit ferromagnetic interactions.A 8-p-NPNN crystal consists of layers of dimers with the dimer structure similar to that of p- FPNN. A 7-p-NPNN crystal is composed of molecular chains along the [Oll] direction and the molecular contacts along the chain are also similar to those in the p-FPNN dimer. All these NO.. .Ar interactions are accompanied by the inversion symmetry at the centre of the dimer. The NO.--Ar interaction is also observed in a P-p-NPNN crystal, but there is no inversion symmetry in the P-phase ~rysta1.l~ These three phases of p-NPNN have another common structural feature, i.e. the intermolecular interactions between the nitroxide (NO) and nitro (NO,) groups. The ferromagnetic couplings in the p-hydroxyphenyl nitronyl nitroxide,,’ is also related to the interaction between the NO group and the para substituent (OH group).Therefore, the role of each molecular packing in affording the ferromagnetic couplings in the three phases cannot be uniquely concluded. However, the definite corre- spondence between the dimeric packing and the triplet forma- tion in p-FPNN enables us to conclude that the NO--.Ar interactions should bring ferromagnetic couplings. This t No magnetic phase transition was detected by heat capacity measurements down to 0.4K. The authors acknowledge Dr Y. Nakazawa and Prof. M. Ishikawa for the heat capacity measurements. conclusion ensures the three-dimensionality of P-p-NPNN, which is required for its bulk ferromagnetism,1,2,18 without the NO.-.Ar interactions P-p-NPNN is a two-dimensional ~ystern.’~ The close intermolecular spacing between the NO group and the aromatic ring has also been found in p-PYNN2, (p-pyridyl nitronyl nitroxide), which is reported to evhibit ferromagnetic intermolecular interactions.We thus point out that the NO..-Ar interaction is one of the key factors for the ferromagnetic interactions in a series of aryl nitronyl nitroxide derivatives. The magnitude of the ferromagnetic coupling due to the NO.-.Ar interaction ranges from J/k,=0.27 K in p- PYNN2, to J/k,=5.0 K in the present case; it is sensitive to subtle change in the geometry in the crystal. A sizeable enhancement of this type of ferromagnetic interaction has been achieved by use of cation radical salts based on pyridin- ium derivatives of nitronyl nitr~xide.,~ These finding would be useful in molecular designing for the search of an organic ferromagnet and should be chvcked by further investigations.EPR Measurements The principal values of the EPR g factor at 298 K were gaa(=gbb)=2.0080 and g,, =2.0034. A Lorentzian shaped spectrum was observed at 298 K. The peak-to-peak line width, AH,,, was about 4G, being almost independent of the direction of the static field. The temperature dependence of the EPR spectra was exam- ined down to 2.2 K. Observed were only Am= 1 transirions. The directions of the three principal axes were retaincd at any temperature. Fig. 8 shows an example of the temperature dependence of the g factor. The inequivalence between the HIJa’and H Ilb’ results clearly indicates that the tetragonal symmetry is no longer retained below about 100 K.As discussed in the next section, evidence for the lowering of the crystal symmetry is given by the powder X-ray diffraction measurements. The low-temperature shifts of the g factor are different from sample to sample. The line shape always becomes asymmetric below 10 K. Most puzzling are the H(lc results; the g shift goes upwards or downwards even for the same specimen on different runs. These features are due probably to twinning in the crystal at low temperatures. Nevertheless, it is certain for any crystals examined that the departure of the g factor for Hlla’ from that of Hllb’ starts at ca. 100K. Fig. 9 shows the temperature dependence of AH,, obtained from the spectra for the g values in Fig.8. All the data have similar temperature dependence, so that AHpp is not con-sidered to be seriously influenced by the structural change. Below 100 K, AHpp gradually increased with decreasing T down to ca. 20 K, where AH,, becomes about twice as large as that at room temperature, followed by a rapid increase below 10K. The temperature dependence of AHpp is thus characterized by the two anomalies around 100 and 10 K. By comparison with the susceptibility results, the first anomaly can be related to the beginning of the triplet formation Blithin each dimer, while the second anomaly possibly reflect$ the growth of short-range magnetic order as a result of interdimer interactions.Powder X-Ray Diffraction Measurements As was indicated by the EPR measurements, the tetragonal symmetry is lost at low temperature. This structural change was investigated by assessing the temperature dependence of the powder X-ray diffraction patterns. The diffraction patterns at 298 and at 10K are shown in J. MATER. CHEM., 1994, VOL. 4 2.020"'-4b 2ob 30 2.0001d 4 0 50 100 150 300 0 50 100 150 300 t 9 2.00017;:oO po; , o1000,, o1500,:: ,200I 0 50 12.0034 0 50 100 150 )*O 2.020 ' ' ' ' ' 'I ' '1 " ' 1 ' .t30!72ok 2*010-0. 0 0. 00 0 0 . . 2.0080 ~,0@1008800 000 0 0 0 2.0034 0 50 100 150 300 TIK Fig. 8 Temperature dependence of the principal values of the EPR g factor within the ab, bc and ca planes of the same specimen; Hlla' (@), Hllb' (A)and Hllc (0).The a' and b' axes are the principal axes within the plane perpendicular to the c axis at low temperatures; the minimum and maximum of the g factor were observed for Hila' and Hljb', respectively.Fig. l0.t Splitting of peaks was clearly observed at 10 K, indicating the breaking of the tetragonal symmetry. However, no marked extra peaks were detected; they are possibly too weak to be observed. No peak contradicts the extinction rule for body-centred (I-) lattice. And the splitting does not seem to affect the intensity of each peak. These suggest that the change in the structure is not drastic. The cell parameters were refined by means of the peak- profile fitting24 of the whole diffraction pattern using the data within 7"<20<35". In order to index all the observed peaks including the split ones, /i'# 90" is required.Among the subgroups of 14,/a, only the triclinic ones satisfy this require- ment. Therefore, it is appropriate to assume the triclinic lattice for the low-temperature structure. The final parameters obtained from the data at 10 K are reported in Table4(a), together with those at 298 K. The solid lines in Fig. 10 represent the fitting results. Fig. 11 shows the single peak profiles of the 211 diffraction at 298 K and that at 10K, which shows the splitting into the 211 and 121 diffractions. The structural change is not abrupt but gradual, as we had anticipated from the EPR results. In Fig.12 the full widths of t All the diffraction patterns for the sample kept for 2 h after puri- fication are indexed on the basis of the room-temperature crystal structure. However, standing of samples for a few days often gives rise to impurity peaks, whose positions are not reproducible. OOd0 50 100 150 300 T/K Fig.9 EPR linewidth (AHpp) as a function of temperature. The notation of the axes is the same as that in Fig. 8. half maxima (fwhm) versus temperature are plotted for some diffraction peaks. The broadening of these peaks below about 100 K is obvious. Some of them shows splitting at 20 K. The comparison of the cell parameters at 298 and 10 K is given in Table 4(b). In addition to the smallness of the changes in the cell parameters, it is also recognized that almost all the peaks obey the extinction rules for the I-lattice.From this result, we surmise that the characteristic dimeric structure of p-FPNN established at 298 K basically holds at low tempera- tures. We conclude that the essence of the correspondence between the structure and the magnetic interactions in p-FPNN is not influenced by the structural change. Conclusions The crystal structure and the magnetic properties of p-FPNN have been investigated. The molecules are arranged in a dimeric fashion in order to afford a 3D network. Two types of ferromagnetic intermolecular interactions have been found from the static magnetic measurements. The intradimer inter- actions are responsible for the formation of the triplet species at low temperatures.In addition, the interdimer interactions bring about the 3D ferromagnetic network below 4 K. From the temperature dependence of the EPR signals and the powder X-ray diffraction patterns we have found a slight structure deformation below 100 K. The assignment of the couplings to the molecular packing has lightened a magneto-structural correlation. The inter- actions between the NO group and the phenyl ring found J. MATER. CHEM., 1994, VOL. 4 15000 10000 5000 u)c. t 035! 10 15 20 15 20 2Bldegrees Fig. 10 Powder X-ray diffraction patterns at (a) 298 K and (b) 10 K for the same specimen. Solid curves represent the fitting results. Ranges of 7.56" d 28 d 7.64" at 298 K and 7.58" d 28 d 7.68" at 10 K were excluded from the final refinement, because of impurity peaks appeared at 7.58" and 28= 7.62', respectively, see the footnote to the text.Table 4 (a) Cell parameters determined from powder X-ray diffraction data with standard deviations in square brackets. (b) Temperature variation of cell parameters between 298 K and 10 K (4 cell parameter 298 K 10 K 44 21.927 [2] 21.795 [3] hi+ -21.888 [3] CIA 11.1113 [7] 10.860 [11 xldegrees -90.10 [l] Pldegrees -89.24 [l] ?;/degrees __ 89.86 [2] X Ax" IAxl/x(198 K) -0.132 A 6.0 x 10-3 -0.039 4 1.8 x 10-3 -0.251 A 22.6 x 10-3 0.10" 1.1 x 10-3 -0.76" 8.4x 10-3 -0.14" 1.6 x 10-3 ~ "Ax=x(~OK)-x(298 K). within the dimer are suggested to be a key factor for bringing ferromagnetic couplings.The architecture of p-FPNN crystal would give insight into construction of a new type of 3D network of ferromagnetic intermolecular interactions. The authors thank a referee for his profound and constructive comments, which helped us to clarify the symmetry of the low-temperature crystal structure. This work was supported by the Grant-in-Aid for Scientific Research on Priority Area 'Molecular Magnetism' (Area No. 228/04242103) from the 2Wdegrees Fig. 11 Peak profiles of the 21 1 diffractions at (a) 298 K and (h)10 K 0 0 50 100 300 TIK Fig. 12 Temperature dependence of the full widths of half maxima (fwhm) of peaks 101 (O), 21 1 (0)and 222 (A).For split peaks, the values of the sum of the actual fwhm and the difference in the diffraction angles for the hkl and kh 1 are used.Ministry of Education, Science and Culture, Japaii. The authors are grateful to Dr. D. Shiomi and Mr. K. Kozawa for valuable discussions. Appendix Preparation of Compounds 2 and 3 Compound 2 can be prepared by a one-pot operation \x ithout isolating the oily intermediate 2-iodo-2-nitropropane. To a solution of 80 g of sodium hydroxide (2.0 mol) in water-ethanol (900 cm3+250 cm3), 180 cm3 of 2-nitropropane (1) (2.0mol), 200 g of iodine (0.79 mol) and 1OOg of sodium iodide (0.67mol) were added. The mixture was refluxed for 1 h and then cooled to 0°C to yield a white crybtalline 1226 J. MATER. CHEM., 1994, VOL. 4 precipitate of compound 2.The precipitate was filtered off, washed with water until the washings became colourless and then dried. The typical yield was 85%. It is possible to extract compound 3 directly from the aqueous slurry resulting from the zinc reduction of 2 with a 5 6 T. Sugano and M. Kinoshita, J. Chem. Phys., 1986,57,453; Chem. Phys. Lett., 1987, 141, 540. G. Chouteau and C1. Veyret-Jeandey, J. Phjs. (Paris), 1981, 42, 1441. M. Kamachi, H. Sugimoto, A. Kajiwara, A. Harada, Y. Morishima, W. Mori, N. Ohmae, M. Nakano, M. Sorai, satisfactory yield, although a route through the hydro-chloride salt of 2 has been de~cribed.~ In a solution of 54g of ammonium chloride (1.0 mol) in water-methanol (180 cm3+450 cm3), 90 g of 2 (0.51mol) was suspended. The mixture was kept below 20 "C with vigorous stirring during slow addition of 200 g of zinc powder (3.1 mol) during ca.7 8 T. Kobayashi and K. Amaya, Mol. Cryst. Liq. Cryst., 1993, 232, 53; T. Kobayashi, M. Takiguchi, K. Amaya, H. Sugimoto, A. Kajiwara, A. Harada and M. Kamachi, J. Phys. SOC.Jpn., 1993, 62, 3239. E. F. Ullman, J. H. Osiecki, D. G. B. Boocock and R. Darcy, J. Am. Chem. SOC., 1972,94,7049. J. Goldman, T. E. Petersen and K. Torssell. Tetrahedron, 1973, 3 h. After the temperature ceased to increase, the stirring was continued at room temperature for an additional 3 h. The mixture was then filtered. The separated zinc compound was washed with methanol (3 x 100 cm3). The washings were combined with the filtrate and concentrated under reduced pressure, until the methanol was entirely removed.Further removal of water by evaporation is usually accompanied by 9 10 11 12 13 14 29, 3833. L. W. Seigle and H. B. Hass, J. Org. Chem., 1940, 5, 100. M. Lamchen and T. W. Mittag. J. Chem. SOC.C, 1966,2300. C. Katayama, Acta. Crystallogr., Sect. A, 1986,42, 19. C. K. Johnson, ORTEPII, Report ORNL5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976, vol. 22, p. 833. J. A. D'anna and J. H. Wharton, J. Chem. Phjs., 1970,53,4074. J. W. Neely, G. F. Hatch and R. W. Krilick, J. Am. Chem. Soc., appreciable sublimation of 3. Instead, the residual slurry was moderately dried by adding anhydrous sodium carbonate; the loss of 3 due to sublimation was thus avoided as far as possible. From this, 3 was extracted with dichloromethane using a Soxhlet apparatus for 2 days.The extract was concen- trated to yield crude 3 as a pale-yellow creamy material. 15 16 17 1974,96,652. M. Tamura, D. Shiomi, Y. Hosokoshi, N. Iuasawa, K. Nozawa, M. Kinoshita, H. Sawa and R. Kato, Mol. Crjst. Liq. Cryst., 1993, 232,45. Y. Hosokoshi, M. Tamura, H. Sawa, R. Kato and M Kinoshita, in preparation. F. L. de Panthou, D. Luneau, J. Laugier and P. Rey, J. Am. Chem. Triturating this in cold diethyl ether left sufficiently pure 3 as a white solid, with the impurity, amines, washed off. The typical yield was 40%. 18 19 SOC.,1993, 115,9095. M. Kinoshita, Mol. Cryst. Liq. Cryst., 1993,232, 1. K. Awaga, T. Inabe, Umpei. Nagashima and Y. Maruyama, J. Chem. SOC., Chem. Commun., 1989,1617; 1990,520. 20 P. Turek, K. Nozawa, D. Shiomi, K. Awaga, T. Inabe, . References Y. Maruyama and M. Kinoshita, Chem. Phys. Lett., 1991, 180, 327. 1 M. Tamura, Y. Nakazawa, D. Shiomi, K. Nozawa, Y. Hosokoshi, 21 E. Hernandez, M. Mas, E. Molins, C. Rovira and J. Veciana, M. Ishikawa, M. Takahashi and M. Kinoshita, Chem. Phys. Lett., 1991,186,401. 22 Angew. Chem., Int. Ed. Engl., 1993,32,882. K. Awaga, T. Inabe and Y. Maruyama, Chem. Phys. Lett,, 1992, 2 Y. Nakazawa, M. Tamura, N. Shirakawa, D. Shiomi, 190, 349. M. Takahashi, M. Kinoshita and M. Ishikawa, Phys. Rev. B: Condens. Matter., 1992,46, 8906. 23 K. Awaga, T. Inabe, Y. Maruyama, T. Nakamura and M. Matsumoto, Chem. Phys. Lett., 1992,195,31. 3 R. Chiarelli, M. A. Novak, A. Rassat and J. L. Tholence, Nature 24 H. Toraya, J. Appl. Crystallogr.. 1986, 19,440. (London), 1993,363,147. 4 K. Mukai, Bull. Chem. SOC. Jpn., 1969, 42, 40; K. Awaga, Paper 3/06820J; Received 15th Nouember, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401219

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1227-1232 Synthesis and Optical Spectroscopy of Linear Long-chain Di-terminal Alkynes and their Pt-a-Acetylide Polymeric Complexes Muhammad S. Khan," Ashok K. Kakkar," Nicholas J. Long," Jack Lewis,*" Paul Raithby," Paul Nguyen,b Todd B. Marder,*b Felix Wittmann" and Richard H. Friend*" " University Chemical Laboratory, Lensfield Road, Cambridge, UK CB2 I EW Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada Cavendish Laboratory, Madingley Road, Cambridge, UK CB3 OHE A variety of straight-chain alkynes with extended n-conjugation through benzene, anthracene and thiophene linker units in the backbone, H-C=C-R'-C=C-R-C=C-R'-C=C-H (R=p-C6H4, 9,10-C,4H8, 2,5-C4H,S; R'=p-CsH4, p-C6H4-C6H4-~) has been synthesized.The alkynyl chromophores with an anthracene spacer unit are highly emissive in solution with luminescence quantum yields of up to 0.5. The platinum o-acetylide polymeric complexes of the above ligands show strong absorptions associated with metal-to-alkynyl ligand charge transfer (MLCT) transitions. It is clear that the n-conjugation is maintained through the metal centres and the optical gap for the polymer, fPt(PBu",),-C~C-p-C,H4-c~c-9,1 O-c,4H,-c~c-p-c,H4-c~~~~is lower than for the complexes fPt(PB",),-C= :C-R'-C-c-R-c~C-R'-C=c], (R =P-C6H4, 2,5-C4H,S; R' =p-C,H4; p-C6H4-C6H4-p). Organometallic polymers containing transition-metal centres connected by conjugated alkynyl ligands are of significant current interest due to their rigid-rod molecular structure and n-electron conjugation along the polymer chain.These proper- ties lead to liquid-crystalline behaviourl and third-order non- linear optical proper tie^.^-^ Other materials properties, such as one-dimensional conductivity may be possible if the optical gaps can be lowered sufficiently. Our interest in this area5p11 has been to develop new routes to the polyyne polymers, and to optimize the n-conjugation between the building blocks which constitute the polymer backbone while maintaining the desirable stability of the polymers. Recent reports concerning the bonding in metal alkynyl,12 b~tadiynyl'~and bis(alkynyl)14 complexes, based on a combi- nation of photoelectron spectroscopic studies and molecular orbital calculations, support previous calculational find-ing~'~.'~which indicate that there is considerable mixing of the filled metal d-orbitals with the filled 7c-system of the alkynyl moiety.The energy mismatch between the metal d-levels and the alkynyl n*-levels, at least in cases in which there are no strong n-acceptor substitutents in conjugation with the alkynyl moiety, leads to a lack of significant n-backbonding. However, the perturbation of the filled metal d-levels by the n-donor character of the alkynyl groups leads to low-lying metal-to-ligand charge transfer (MLCT) absorp- tions. Recently reported extended Huckel band calculation^^^ on metal polyyne polymers indicate that in fL,M-C=C-R- C=C3, systems, the highest occupied crystal orbital (HOCO, which is roughly analogous to the HOMO in a molecular system) is delocalized along the chain through the ML, groups for n =2 (square planar) and II =4 (octahedral) geometries.It was also sugge~tedl~.~~ that extending the n-conjugation length in the alkynyl linker groups would lower the optical gap in such metal polyyne systems predominantly by lowering the energy of the LUMO/LUCO (lowest unoccupied molecular/ crystal orbital). With these ideas in mind, we set out to prepare a new set of Pt polyyne polymers containing extended alternating aryl and alkynyl n-systems. We also hoped to prepare novel fluorescent and soluble rigid-rod polymers employing bis( pheny1ethynyl)anthracene-based n-linkers in the main chain. These are extended analogues of a diethynyl- anthracene-based platinum polyyne, the absorption and emis- sion properties of which were reported briefly by Sonogashira et ~21.'~We report herein the synthesis of rigid-rod alkynyl arene chromophores ( 1-5) which possess extended n-conju- gation, and their platinum polymeric complexes (6-13 I.An examination of the linear optical properties (e.g. quan- tum yields) of the organic chromophores containing anthra- cene linker units (2-3)indicates that they are highly emissive with quantum yields of up to 0.5. Such chromophores may be useful as laser dyes, scintillation agents and electroluniinesc- ence florescers.20 As will be seen, the optical gaps are lower for the organometallic polymers than for their organic counterparts, indicating that the n-conjugation is maintained through the metallic centres.Substitution of the benzene ring with an anthracene unit in the conjugation backbone (2-3) leads to a lower bandgap in the resulting organonietallic polymers which indicates that anthracene is more effective in n-electron delocalisation along the backbone. Experimental General Solvents were predried and distilled from appropriate drying agents. 1,4-Diethynylben~ene,~l Pt (PBu",)~C~~ and Pt(A~Bu"~)~cl, via literature procrhdures. were p~epared~~.~~ NMR spectra were recorded on Bruker AC-200 or AM-400 spectrometers. 31PNMR spectra were referenced to external trimethylphosphite, and the 'H and 13C NMR spectra were referenced to solvent resonances.The IR spectra were recorded on a Perkin-Elmer 1710 fourier-transform spectrometer. The molecular weights were determined by GPC.24 Optical a bsorp-tion and luminescence quantum yield measurements were carried out using dilute solutions of the compounds in dichloromethane. For quantum yield measurements, an exci- tation wavelength of 364nm obtained from an Ar-ion laser was used and 1,1,4,4-tetraphenylbutadienewas employed as a standard (quantum yield between and 0.84-0.8626,27 in cyclohexane). The quantum yields were calculated by integration of the emission in the spectral region from 3.2 to 1.7 eV. The measurements are accurate to within 10% of the figures quoted. The measurements (except for the standard) were taken in dichloromethane, and the solution wds not nitrogen bubbled.Synthesis Extended diterminal alkynes were prepared by following a general procedure outlined below for 1. However, lor the J. MATER. CHEM., 1994, VOL. 4 1 preparation of the anthracenyl derivatives, 2 and 3, the limited solubility of 9,lO-diiodoanthracene in diisopropylamine re-quired that this solution be gently warmed in the addition funnel during dropwise addition to the appropriate alkyne solution. 1,4-Bis(p-diethynylpheny1)benzene 1 A solution of 1,4-diiodobenzene (1.11g, 3.36 mmol) in 40 ml of diisopropylamine was added dropwise to a suspension of 1,4-diethynylbenzene (1.72 g, 13.66 mmol), PdC12( PPh,), (0.047 g, 0.067 mmol), and CuI (0.013 g, 0.068 mmol) in 50 ml of diisopropylamine under nitrogen over a period of 20h.Diisopropylamine was then removed in vacuo, and the crude product was triturated with hexane (4 x 10 ml) to remove the excess of 1,4-diethynylbenzene. The product was purified further by dissolving it in hot toluene and filtering it through a 1 cm pad of silica gel (70-230 mesh). Compound 1 was obtained as a yellow solid in three crops by cooling the hot toluene solution. The yield was 0.65 g (59%). 'H NMR (200 MHz, CDCl,) 6: 3.17 (s, 2 H, -CrC-H), 7.46 (s, 8 H, terminal -C6H4-), 7.49 (s, 4 H, central -C6H4-). 13C NMR (50.4 MHz, CDCI,) 6: 79.1, 83.2, 90.8, 90.9, 122.2, 123.0, 123.5, 131.5, 131.6, 132.1. IR (KBr) v/cm-': 3276 (-C=C-H), 2213, 2101 (-C=C-). Calcd. for C&II4: C, 95.68; H, 4.32%.Found: C, 95.63; H, 4.27%. 9,10-Bis(p-diethynylphenyl)anthracene 2 Dark orange solid (75% yield). 'H NMR (200 MHz, CDCl,) 6: 3.22 (S, 2 H, pC=C-H), 7.54-7.58 (m, 4 H, -C6H4-), 7.62-7.66 (m, 4 H, anthracenyl), 7.70-7.73 (m, 4 H, -C6H4-), 2 3 4 6-13 8.63-8.67 (m, 4 H, anthracenyl). IR (KBr) v/cm-': 3269 (-C-C-H), 2194, 2103 (-C=C-). Calcd. for C,,H,,: C, 95.75; H, 4.25%. Found: C, 95.77; H, 4.15%. 9,10-Bis(p-diethynylbiphenyl)anthracene3 Bright orange solid (40% yield). 'H NMR (200 MHz, CDCI,) 6,: 3.15 (s, 2 H, -C=C-H), 7.55-7.69 (m, 16 H, overlapped, anthracenyl and -C6H, -Hs), 7.82-7.86 (m, 4 H, -C6H4-), 8.68-8.73 (m, 4H, anthracenyl). IR (KBr) v/cm-': 3294 (-c-c-H), 2146, 2105 (-c=C-). Calcd. for C4,H,,: C, 95.47; H, 4.53%.Found: C, 95.58; H, 4.70%. 2,5-Bis(p-diethynylpheny1)thiophene4 Yellow solid (58% yield). 'H NMR (200MHz, CD,Cl,) 6,: 3.17 (s, 2 H, -C-C-H), 7.16 (s, 2 H, thiophenyl), 7.46 (s, 8 H, -C6H4-). IR (KBr) v/cm-': 3281 (-C-C-H), 2197, 2105 (-C=C-). Calcd. for C,,H,,S: C, 86.72; H, 3.64%. Found: C, 86.57; H, 3.79%. 2,5-Bis(p-diethynylbipheny1)thiophene5 Yellow solid (87% yield). 'H NMR (200 MHz, CDCI,) 6,: 3.13 (s, 2 H, -C=C-H), 7.17 (s, 2 H, thiophenyl-H), 7.56 (s, 8H, -C6H4-), 7.58 (s, 8H, -C6H4-). IR (KBr) v/cm-': 3280 (-C-C-H), 2192, 2105 (-C=C-). Calcd. for C36H20S: C, 89.23; H, 4.16%. Found: C, 89.44; H, 4.39%. The polymeric complexes (6-8) were prepared by the general procedure outlined below for 6. J. MATER.CHEM., 1994, VOL. 4 f Pt (PBu",),-C-C-p-C6H4-C-C-p-C,H4-c~c-p-c6H4-C=Cj, 6 1 (0.032 g, 0.1 mmol) was dissolved in 100 ml of refluxing diethylamine by stirring over a period of 1 h. To the resulting solution, Pt(PBu",),Cl, (0.067 g, 0.1 mmol), followed by CuI (2 mg) were added. The reaction mixture was refluxed under nitrogen for 40 h. Diethylamine was then removed in uucuo, the residue was dissolved in dichloromethane and the solu- tion was passed through an alumina column. After removal of dichloromethane in uucuo, the product was dissolved in toluene, and to the resulting solution, methanol was added. Compound 6 was obtained as a yellow solid in 57% yield. 31PNMR (162 MHz, CDC1,) 6 137.95. IR (CH,Cl,) v/cm-l: 2097. Calcd. for C~OH~~P,P~: c, 64.99; H, 7.20%.Found: C, 64.88; H, 7.18%. M, =58 482 (TI,= 63). 1229 fpt(PBUn~)2-C-C-p-C~H~-C6H~-~-c~c-9,lO-C~~H~-C~c-p-c6H4-c6H4-p-cE cjn10 Orange solid (51% yield). 31P NMR (162 MHz, CDCl,) 6 134.47. IR (CH,Cl,) v/cm-': 2097. Calcd. for C70H-8P2 =: C, 71.47; H, 6.68%. Found: C, 71.58; H, 6.64% M,=28748 (n, =24). The polymeric complexes (11-13) were prepared by the general procedure outlined below for 11. Complexes 12 and 13 were found to be insoluble in common organic solvents and were purified by washing the crude products repeatedly with boiling dichloromethane. fPt(PBUn,),-C-C-p-C6H4-C=C-2,5-C,H2S-C-C-p-C6H4-c=cj,11 Compound 4 (0.013 g, 0.1 mmol) was dissolved in 25 ml of refluxing diethylamine by stirring it for 1 h.To the resulting c=c+,7 Light yellow solid (55% yield). IR (CH,Cl,) v/cm-'): 2099. Calcd. for Cs0H66A~2Pt: c,59.34; H, 6.57%. Found: c, 59.22; H, 6.51%. M,=55 589 (n,=55). The polymeric complexes (8-10) were prepared by the general procedure outlined below for 8. fpt(PBUn,),-C~C-P-C6H4-C-c-9,10-C1,H,-C~C-p-C6H4-C-Cj, 8 Compound 2 (0.042 g, 0.1 mmol) was dissolved in 25 ml of refluxing piperidine by stirring it over a period of 1h. To the resulting solution, Pt( PBun3),Cl, (0.067 g, 0.1 mmol), followed by CuI (2 mg) and 5 pl of PBu", were added. The reaction mixture was refluxed under nitrogen for 48 h. Piperidine was then removed in vucuo, the residue was dissolved in dichloro- methane, and the solution was passed through an alumina column.After removal of dichloromethane in uucuo, the product was washed with methanol. Compound 8 was obtained as an orange solid in 45% yield. 31PNMR (162 MHz, CDC1,) 6 134.52. IR (CH,Cl,) v/cm-': 2099. Calcd. for C5,H7,P,Pt: C, 68.01; H, 6.88%. Found: C, 67.87; H, 6.92%. M, =29 882 (n, =29). fPt(AsBun3)2-C-C-~-C6H4-C-C-9,1O-C14H8-C=C-p-c6H4-cE Cj,, 9 Orange solid (53% yield). IR (CH,Cl,) v/cm-l: 2096. Calcd. for C&7,AS2Pt: C, 62.63; H, 6.34%. Found: c, 62.74; H, 6.37%. M, =32 499 (n, =29). ~P~(ASBU",),-C-~C-~-C~H~-C-~C-~-C~H~-C~C-P-C~H~-solution, Pt(PBu",),Cl, (0.067 g, 0.1 mmol), followed by CuI (2 mg) were added. The reaction mixture was refluxed under nitrogen for 24 h. Diethylamine was then removed in uacuo, the residue was dissolved in dichloromethane and the solution was passed through an alumina column.After remcval of dichloromethane in uucuo, the product was washed with methanol. Compound 11 was obtained as a dark brown solid in 55% yield. 31P NMR (162 MHz, CDC1,) 6 134.92. IR (CH,Cl,) v/cm-l: 2087. Calcd. for C,,H,,P,SPt: c, 52.66; H, 7.73%. Found: C, 52.78; H, 7.81%. M,=22545 (n,=31). fPt(PBUn3)2-C-C-p-C6H4-C,H,-C-C-2,5-C,H,S-C-C-p-C&-C= c], 12 Orange solid (59% yield). Insoluble polymer. IR ( Nujol) v/cm-l: 2098. Calcd. for C,,H,,P,SPt: C, 68.71; H, 691%. Found: C, 68.71; H, 6.94%. fPt(AsBu"3)yC-C-p-C6H4-C6H4-C-C-2,5-c4H2s-c~ C-PC6H4-CfC3,, 13 Orange solid (60% yield). Insoluble polymer. IR (Yujol) v/cm-': 2096. Calcd.for C,,H,,As,SPt: C, 61.58; H, 6.20%. Found: C, 61.94; H, 6.38%. Results and Discussion Synthesis and Characterization The alkynyl ligands were prepared by a Pd"/CuI-cat alysed cross-coupling rea~tion,~-,~ aryl halides with terminal of alkynes (Scheme 1). The coupling of diiodobenzene/anthra- I-R-I + H-W-H 1 Scheme 1 1230 EBu"~ I CI-Pt-CI + H* R'-R-fl+HI EBu"~ (E = P, AS) 8 E=P:R= 10 E=P;R= ;R= 12 E=P;R= ;R'= Scheme 2 cene/thiophene with H-C=C-R'-C?C-H (R' =p-C6H4, p-C,H,-C,H,-p) proceeds smoothly at room temperature in the presence of a 2 mol% PdCl,(PPh,), and CuI in diisopro-pylamine to give the alkynes 1-5 in 58-87% yields. For purposes of economy, the excess di-terminal alkynes (e.g. 174-diethynylbenzene)can be recovered by washing the crude product repeatedly with hexane, followed by eluting this filtrate through a column of silica gel.In addition to the desired extended di-terminal alkyne products, some oligomeric species were also formed in the reaction mixtures. These insoluble by-products, along with the catalysts and amine salts, were removed by dissolution of the product mixture in hot toluene and passage through a short column of silica gel. The polymeric complexes (Scheme 2) were prepared by adaptation of the dehydrohalogenation route developed originally by Hagihara.' A typical polymerisation reaction involved the reaction of equimolar quantities of the appro-priate platinum dihalide complex and the corresponding J.MATER. CHEM., 1994, VOL. 4 r ,. h , -. a -.-C ..__-' '.'. 2.0 3.0 4.0 5.0 energyleV Fig. 1 Optical absorption (---) and photoluminescence (-) spectra for the alkyne ligands (1,2,3) r 1.o 2.0 3.0 4.0 5.0 energyleV Fig. 2 Optical absorption (---) and photoluminescence (-) spectra for the platinum o-acetylide polymers (64) I-a. c.-. C-3-@:a-v c-.-0.-_2-0-a- a, 1.0 2.0 3.0 4.0 5.0 6.0 7.0 energyleV Fig. 3 Optical absorption spectra for the ethynyl thiophene ligand and the corresponding platinum polymer (4, 11) substituted alkyne in refluxing diethylamine or piperidine in the presence of a small amount of CuI. For the synthesis of anthracenyl derivatives (8-lo), addition of a small amount of PBu", during the reaction was necessary for optimized yields.Except for compounds 12 and 13, all of the polymeric complexes were soluble in dichloromethane, tetrahydrofuran and toluene and, in fact, they were found to be much more soluble than the free ligands. All new organic and organo-metallic compounds were characterised using analytical and spectroscopic techniques, and the details are given in the J. MATER. CHEM., 1994, VOL. 4 Table 1 Optical gaps for the alkynyl ligands (1-4) and for the platinum a-acetylide polymeric complexes (6, 8, 11) compound optical gap EgIeV 1 3.42 2 2.58 3 2.55 4 3.30 6 3.11 8 2.48 11 2.70 Table2 Relative quantum yields for the organics (1-3) and the corresponding Pt-o-acetylide polymers (6, 8, 10) alkyne quantum yield polymer quantum yield 1 0.57 6 0.043 2 0.31 8 0.034 3 0.49 10 0.091 Experimental section.The IR spectra of the organo-metallic polymers displayed a single (vCrC) absorption at ca. 2096 cm-' (for 11, 2087 cm-') which indicates a trans-configuration of the ligands around Pt"L, moieties in these square-planar complexes. The weight-average molecular weights (M,) for the polymeric complexes (6-11) were obtained by gel permeation chromatography (GPC).24 Optical Spectra: Optical Gap Measurements The optical absorption and photoluminescence spectra of the organic ligands and polymers are presented in Fig. 1-3 with the corresponding spectral data in Table 1.The complexes show strong absorptions, which are assigned to MLCT trans- itions. It is noteworthy that the strongest minimum-energy peaks (2.75-3.25 eV) are lower in energy for the organo- metallic polymeric complexes than for the alkyne ligands (3.45-3.74eV). This is in agreement with our previous result^^,^.^ and shows that the n-conjugation is maintained through the platinum metal centres. The optical gap for the organometallic polymer containing an anthracene bridge (8) in the backbone (2.48 eV) is lower than for the corresponding polymers containing a benzene (6, 3.11 eV) or a thiophene (11, 2.70 eV) bridge. This indicates that the anthracene bridge containing three fused aromatic rings is more effective in the delocalisation of n-electrons along the backbone.The optical gap for the organometallic polymer with a thiophene bridge (11) is lower than that containing a benzene bridge (6). Similar behaviour has also been observed in other species containing thiophene as a linker unit.38 Photophysical Studies Excitation of the organic and organometallic polymers in dichloromethane solutions at 364 nm at room temperature results in strong emissions. The relative quantum yields are listed in Table 2. The ligands show an approximate mirror symmetrical (lowest-energy) absorption and photoluminescence peaks as expected in the Frank-Condon picture.39 For the ligands 1 and 3, however, there are additional higher-energy photo- luminescence peaks. These peaks (ca. 3-3.2 eV) have cor-responding features in the absorption spectrum.If these photoluminescence peaks arise from one molecule then this constitutes a violation of Kasha's Although excep- tions to Kasha's rule are rare, there are well known examples such as azulene which luminesces from the second excited S2 singlet.40 Weak photoluminescence from higher excited singlet states in anthracene have also been reported.41 In the materials we consider here, it is conceivable that there is more than one luminescence centre. There is a correlation between absorp- tion (and emission) around 2.5 eV and the presence of the CEC-A-C-C group. The red shift in this main absorbance (photoluminescence) band at 2.5 eV compared to anthracene is ca. 0.9 eV.42 We note that the energy spacings observed in the sidebands of 2.5 eV in photoluminescence and absorption in 9,10-bis(pdiethynylbipheny1)anthracene is 0.17 eV and matches those observed in anthracence.We have previously investigated simpler versions of related platinum oligo-yne polymer^.^,^^ Our early interest wa.; domi- nated by the question of how much conjugation thcre was through the metal site. The interaction between the conjugated electronic system and the metal is also expected to lead to interesting effects by virtue of the mixing of singlet and triplet manifolds by the influence of the metal. For solid thin films of fPt( PBun3),-C-C-p-C6H,-C-C~, we found long-lived phosphorescence at helium temperatures which was quenched at room temperature. We attributed this phosphorescence to a triplet-singlet transition.Triplet-singlet transitions have a known susceptibility to the presence of heavy atoms .md we found a strong decrease of the triplet phosphorescence lifetime in the isostructural palladium polymer, fPd( PBu",),-C=C- p-C6H,-C=C+,. We found that quenching was especially strong in aerated solutions at room temperature.,, The polymers also show mirror-image relationships between the lowest-energy absorption and photoluminescence. It is noteworthy that the quantum yields for the polymers are ca. one-tenth of those for the free ligands. In the Pt polymer, 10, below the strong absorption one observes a strong absorp- tion at 3.26 eV. We also see much weaker absorption at 2.6 eV as shoulders on the lower-energy tail of the 3.26 eV transition.Similarly, we find mirror-symmetrically matched photc dumin- escence peaks at and below 2.5 eV observed as shoulders on the lower-energy tail of the 3 eV photoluminescence peak. This is similar to what we observed for a related platinum polymer, fpt(PBUn3)2-C~C-p-C6H4-C-c~,.43We think, therefore, that the lower quantum yields seen in our measure- ments in the platinum polymers are a consequence of their formation of long-lived transitions with increased susceptibil- ity to quenching. Conclusions Alkynyl chromophores with extended n-conjugation through benzene, anthracene and thiophene linker units show very interesting linear optical properties. Relative luminescence quantum yields were obtained from the emission spectra of these highly emissive compounds, and potential exists for their applications in the materials industry.The optical spectra of the corresponding platinum polymeric complexes indicate that the n-conjugation along the backbone extending through the Pt" metal centres in these a-acetylide polymeric complexes is maintained. We thank SERC and the Kobe Steel Europe Ltd. (M.S.K.), Darwin College, Cambridge (N.J.L.) and NSERC of Canada (A.K.K.; T.B.M., grant support; P.N., post-graduate fellow- ship), the NSERC/Royal Society Bilateral Exchange Program (T.B.M.), and the British Council (Ottawa, Canada) (P.N.) for financial support. We also thank Dr. I. Hinton at CIBA- GEIGY Plastics, UK for molecular weight determinations. References 1 N.Hagihara, K. Sonogashira and S. Takahashi, Ado. Poivrn. Sci., 1981,41,149; S. Takahashi, H. Morimoto, E. Murata, S. Kataoka, 1232 J. MATER. CHEM., 1994, VOL. 4 2 K. Sonogashira and N. Hagihara, J. Polym. Sci., Polym. Chem. Ed., 1982,20, 565, and references therein. W. J. Blau, H. J. Byrne, D. J. Cardin and A. P. Davey, J. Muter. Chem., 1991,1,245. 14 15 16 J. N. Louwen, R. Hengelmolen, D. M. Grove, A. Oskam and R. L. DeKock, Organornet., 1984,4908. N. M. Kostic and R. F. Fenske, Organomet., 1982,1,974. D. Zargarian, P. Chow, N. J. Taylor, and T. B. Marder, J. Chem. 3 A. P. Davey, D. J. Cardin, H. J. Byrne and W. J. Blau, in Organic Molecules for Nonlinear Optics and Photonics, ed.J. Messier, F. Kajzar, P. Prasad and D. Ulrich, Kluwer, Dordrecht, 1991, p. 391. 17 18 SOC., Chem. Commun., 1989,540. G. Frapper and M. Kertesz, Inorg. Chem., 1993.32, 732. M. S. Khan, A. K. Kakkar, S. L. Ingham, P. R. Raithby, J. Lewis, B. Spencer, F. Wittmann and R. H. Friend, J. Organomet. Chem., 4 C. C. Frazier, S. Guha, W. P. Chen, M. P. Cockerham, 1994, in the press. P. L. Porter, E. A. Chauchard and C. H. Lee, Polymer, 1987, 28, 19 K. Sonogashira, K. Asami and N. Takeuchi, J. Muter. Sci. Lett., 553; C. C. Frazier, E. A. Chauchard, M. P. Cockerham and 1985,4, 737. P. L. Porter, Muter. Res. SOC. Symp. Proc., 1988,109,323;S. Guha, 20 S. K. Gill, Aldrichim. Acta, 1983, 16(3), 59. C. C. Frazier, K. Kang and S. E. Finberg, Opt. Lett., 1989,14,952; 21 R.Nast and H. Grouhi, J. Organomet. Chem., 1979,182, 197. C. C. Frazier, S. Guha and W. Chen, Pat. C.T. Int. Appl. WO 89 22 G. B. Kauffmann and L. A. Teter, Inorg. Synth.. Vll, 1963,248. 01, 182, Feb. 1989, U.S. Appl. 81, 785, Aug. 1987 (Chem. Abs., 23 Gmelins Handbuch der Anorganischem Chemir, 8th ed., vol. 68 1989,111,lO 544613). Platinum (D). 5 S. J. Davies, B. F. G. Johnson, M. S. Khan and J. Lewis, J. Chem. 24 For GPC procedural details see: S. Takahashi, M. Kariya, Soc., Chem. Commun., 1991,187. T. Yatake, K. Sonogashira and N. Hagihara. Macromolecules, 6 H. B. Fyfe, M. Mlekuz, D. Zargarian, N. J. Taylor and 1978,11,1060. T. B. Marder, J. Chem. SOC., Chem. Commun., 1991, 188; H. B. Fyfe, M. Mlekuz, G. Stringer, N.J. Taylor and T. B. Marder, 25 1. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 1965. in Inorganic and Organometallic Polymers with Special Properties, ed. R. M. Laine, NATO AS1 Ser. E, Kluwer, Dordrecht, 1992, 26 M. J. Adams, J. G. Highfield and G. F. Kirkbright, Anal. Chem., 1980,52,1260. 11 12 vol. 206, pp. 331-344; T. B. Marder, G. Lesley, Z. Yuan, H. B. Fyfe, P. Chow, G. Stringer, I. R. Jobe, N. J. Taylor, I. D. Williams and S. K. Kurtz, in Materials for Nonlinear Optics: Chemical Perspectives, ed. S. R. Marder, S. John and G. D. Stucky, ACS Symp. Ser., 1991, vol.455, pp.605-615; H. B. Fyfe, M. Mlekuz, D. Zargarian and T. B. Marder, in Organic Materials for Nonlinear Optics 11, ed. R. A. Hann and D.Bloor, Spec. Publ. No. 91, Royal Society of Chemistry, Cambridge, 1991, pp. 204-209; G. Lesley, Z. Yuan, G. Stringer, I. R. Jobe, N. J. Taylor, L. Koch, K. Scott, T. B. Marder, I. D. Williams and S. K. Kurtz, in Organic Materials for Nonlinear Optics 11, ed. R. A. Hann and D. Bloor, Spec. Publ. No. 91, Royal Society of Chemistry, Cambridge, 1991, p. 197. B. F. G. Johnson, A. K. Kakkar, M. S. Khan, J. Lewis, A. E. Dray, F. Wittmann and R. H. Friend, J. Muter. Chem., 1991,1,485. B. F. G. Johnson, A. K. Kakkar, M. S. Khan and J. Lewis, J. Organomet. Chem., 1991,409, C12. M. S. Khan, S. J. Davies, A. K. Kakkar, D. Schwartz, B. Lin, B. F. G. Johnson and J. Lewis, J. Organomet. Chem., 1992,424,87. J. Lewis, M. S. Khan, A. K. Kakkar, B. F. G. Johnson, T. B. Marder, H.B. Fyfe, F. Wittmann, R. H. Friend and A. E. Dray, J. Organomet. Chem., 1992,425,165. M. S. Khan, N. A. Pasha, A. K. Kakkar, P. R. Raithby, J. Lewis, K. Fuhrmann and R. H. Friend, J. Muter. Chem., 1992,2,759. D. L. Lichtenberger, S. K. Renshaw and R. M. Bullock, J. Am. Chem. SOC., 1993,115,3276. 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 A. T. R. Williams, S. A. Winfield and J. N. Miller, Analyst (London), 1983,108,1067. L. Cassar, J. Organornet. Chern., 1975,93,253. H. A. Dieck and F. R. Heck, J. Organomet. Chem., 1975,93,259. K. Sonogashira, Y. Toda and N. Hagihara, Tetrahedron Lett., 1975,4467. L. D. Ciana and A. J. Haim, Heterocycl. Chem.. 1984,21,607. T. X. Neenan and G. M. Whitesides, J. Org. Chrm., 1988,53,2489. A. E. Steigman, E. Graham, K. J. Percy, L. R. Khundkar, L. T. Cheng and J. W. Perry, J. Am. Chem. SOC.. 1991,113,7658. K. J. d’Alarcoa and N. J. Leonard, J. Org. Chem., 1985,50,2462. A. N. Tischler and T. J. Lanza, Tetrahedron Lett., 1986,27, 1653. B. M. Trost, C. Chan and G. Ruhter, J. Am. Chem. SOC., 1987, 109,3486. P. J. Hanhela and D. B. Paul, Aust. J. Chem., 1981,34, 1701. A. K-Y. Jen, V. P. Rao, K. Y. Wong and K. J. Drost, J. Chem. SOC., Chem. Commun., 1993,90. C. A. Parker, Photoluminescence in Solutions, Elsevier, Amsterdam, 1968. J. N. Miller, Standards in Fluorescence Spectrometry, Chapman and Hall, London, 1981. R. Katoh and M. Kotani, Chem. Phys. Lett., 1993,201,141. H. F. Wittmann, Ph.D. Thesis, University of Cambridge, 1993. H. F. Wittmann, K. Fuhrmann, R. H. Friend, M. S. Khan and J. Lewis, Synth. Met., 1993,55-57, 56. 13 D. L. Lichtenberger, S. K. Renshaw, A. Wong and C. D. Tagge, Organornet., 1993, 12, 3522. Paper 4/01624F; Received 18th March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401227

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1233-1237 Soluble Nickel Bis(dithio1ene) Oligomers for Third-order Non-linear Optical Studies Callum A. S. Hill," Adam Charlton," Allan E. Underhill," Steven N. Oliver,b Steven Kershaw,b Robert J. Manningb and B. J. Ainslieb a Department of Chemistry, University of Wales, Bangor, Gwynedd, UK LL572UW British Telecom Laboratories, Martlesham Heath, Ipswich, UK IP5 7RE The synthesis and characterisation of two new nickel bis(dithio1ene) oligomers is described. The third-order non-linear optical index of refraction (n2)and real-to-imaginary ratio has been determined using the z-scan technique for one ditholene system as a guest in a poly(methylmethacry1ate)(PMMA) host. A real-to-imaginary ratio (n2/p)of 4.3 was determined at a concentration of 5 x 1O2' molecules cm -3.The metal bis(dithio1enes) constitute a large class of com- pounds which exhibit many interesting optical, magnetic and electrical properties.lT2 For example, the metal dmit complexes (where H,dmit = 1,3-dithiol-2-thione-4,5-dithiolate)have been shown to exhibit superconducting beha~iour.~ Recently, atten- tion has been given to the potential of metal bis(dithio1enes) for all optical device We have for some time been investigating the resonant-enhanced third-order nonlinearities of these compounds both in solution5 and in the solid state.7 Solid-state measurements have been performed with dithiolene guests in a PMMA host at up to 20% loadings with results in agreement with solution studies.Unfortunately, higher loadings of the metal complexes resulted in severely degraded figures of merit due to intermolecular interactions.' Tn order to overcome these problems of polymer incompatibility, we have been engaged upon a programme to synthesize soluble and processable metal bis(dithio1ene) polymers. Early attempts at producing metal bis(ditho1ene) polymers often produced intractable, insoluble and poorly characterised material^.^'^ Recently, Fang and Reynolds have attempted to synthesize polymeric systems with enhanced solubility." This approach was only partially successful in that soluble dianionic forms were produced, which upon oxidation gener- ally gave intractable black powders of variable purity, as evidenced by elemental analyses.A much improved method involving copolymerisation of monomeric dithiolene units in polycarbonate and polyurethane chains has been reported recently." This approach has yielded dithiolene-containing polymers with up to 20% (w/w) of the metal complex. These polymers are reported to be soluble and of sufficiently high molecular weight to form films. The work described here is a modification of the above methods to synthesize processable metal bis(dithio1ene) poly- mers, formed from the metal complex in order to maximise the concentration of the third-order non-linear optically active species in the material. We report the synthesis and character- isation of two new dithiolene oligomeric systems of the type shown in Fig. 1, where R=H (structure la) and R=C4H9 r Fig.1 Structures of dithiolene oligomers prepared (la, R =H; lb R=C4H9) (structure lb). In materials of this type the linkage between the dithiolene moieties is provided via alkyl bridges with the phenyl substituents present as pendant groups. The advantage of systems of this type is the ability to attach solubilising groups to the phenyl substituents, thus producing 01 igomers with improved solubility or polymer compatibility. Thin films of material lb codispersed with poly(methy1me- thacrylate) have been formed by spin coating at concentrations of the oligomer in the host polymer of ca. 50%, without crystallisation of the guest species. Third-order non-linear optical studies on these films have been performed using the z-scan technique.Such studies have shown that itt these high guest loadings a very good real/imaginary ratio is observed, in marked contrast with the behavi our of monomeric species at these concentrations. Experimental Third-order Non-linear Optical Measurements Preliminary third-order measurements were performed on thin films of compound lb in PMMA, using the z-scan technique. Samples of the ditholene and PMM4 were co-dissolved in chlorobenzene and spun onto silica substrates using a photoresist spinner, giving layers of ca. 1 pm per spin. Absorption spectra of the films were measured using a Perkin- Elmer 119 Spectrophotometer balanced for reflectilm loss. Measurements of the non-linear refractive index and real-to- imaginary ratios of n2 were performed using tht z-scan technique at a wavelength of 1064nm using 100 ps pulses at a 10 Hz repetition rate.Full experimental details have been reported el~ewhere.~ Materials Characterisation and Cyclic Voltammetry Near-infrared (NIR) spectra were recorded on a Beckman DK2A or a Perkin-Elmer A9 Spectrophotometer in ciicholor- methane. Infrared (IR) spectra were recorded as thin films formed by slow evaporation of dichloromethane solutions dropped onto NaCl plates, in a Perkin-Elmer 1600 FT-IR spectrometer. Elemental analyses were performed on a Carlo Erba 1106 elemental analyser, using trifluoroacetanilide as reference standard. Proton NMR were recorded on CDCl, solutions using a Bruker AC-250 250 MHz spectrometer with TMS as internal standard, all spectra are reported in ppm on the 6 scale.Mass spectra were run on a Finnigan hlat 1020 mass spectrometer, using the solid probe facility, and in the EI mode. Cyclic voltammetry was performed using an EG&G Princeton Applied Research Model 264A Polarographic Analyser, with software data processing. Experiments were conducted using dry, distilled dichloromethane as solvent with 0.1 mol dm-3 of [NBu:][BF,] as supporting electrolyte. A platinum button working electrode and platinum spade auxil- liary electrode were used in conjunction with either silver wire or an Ag/AgCl electrode as reference. A ferrocene standard was run before and after cyclic voltammetry experiments and all El, values are quoted relative to the ferrocene/ferrocenium couple El, value.Synthesis The synthetic procedure adopted was a modification of that used by Mueller-Westerhoff et a1 for the synthesis of liquid-crystalline metal bis(dithiolenes).12 Our method (Fig. 2) involved the Friedel-Craft acylation of a bifunctional acid chloride with either benzene or n-butylbenzene to produce the diketone (2a). Bromination of this compound yielded the CI-C, 0 0 R 2a R 2b l(U0 0 0 R R 2c 2d r H. Bu Fig. 2 Synthetic route used in preparation of metal bis(dithio1ene) oligomers. Reagents and conditions: (i) RC6H5/CH,C12, 0 "C; (ii) Br2/CH2C12, rt; (iii) EtOCS, Kf/CH3COCH3, rt; (iv) conc. H,SO,, 0 OC; (v) MeO-Na+/MeOH, then TBA, Br and NiC12.6H20 and iodine/CH3COCH3 .J. MATER. CHEM., 1994, VOL. 4 corresponding bis(a-bromoketone) 2b. Reaction of 2b with potassium o-ethyl xanthate in acetone afforded the xanthate ester 2c. Cyclisation of this product was effected by stirring 2c in concentrated H,SO, at 0°C to yield 2d. It was found that the use of 48% HBr/H,O :HOAc (1: 1) gave a product of lower purity. The metal complexes were prepared from the tetrasodium salt (2e).The sodium salt was produced by gently heating a suspension of the ligand in a solution of sodium methoxide or ethoxide (excess) in dry methanol or ethanol, under argon. In an important modification of the published procedure, an equimolar quantity (based on the ligand) of tetrabutylammonium bromide was added to the yellow solu- tion of the sodium salt. To this solution was added, extremely slowly,0.5 equivalents of NiC12.6H20 dissolved in dry alcohol.In this way the dianionic nickel complex was isolated as the tetrabutylammonium salt. This species was then dissolved in acetone, filtered and oxidised to the neutral form by the addition of a solution of iodine in acetone. The neutral complex precipitated and was collected. The solid was then dissolved in dichloromethane and precipitated with methanol. This procedure was repeated to yield the purified product. 1,PDiphenylnonane-1,Qdione (2a, R =H) To a mixture of benzene (17.8 g, 0.23 mol) and anhydrous aluminium chloride powder (29.5 g, 0.22 mol) in 500 ml of dry dichloromethane was added a solution of azelaoyl chloride (24.9 g, 0.11 mol) in 50 ml dry dichloromethane dropwise over a period of 2 h.The reaction mixture had been previously cooled in a salt-ice bath and moisture was excluded through- out. The solution was stirred overnight, poured into a mixture of 1kg of cracked ice and 100ml of concentrated HCl, and stirred for 1h. The product was extracted with diethyl ether (3 x 100 ml), dried over anhydrous magnesium sulfate and the solvent taken off on a rotary evaporator. The light brown oily residue was dissolved in 600 ml of boiling methanol and then placed in a freezer set at -25 "C overnight. The product was obtained as a white powder (19.65 g, 0.064 mol, 58.2%) which was collected by vacuum filtration and dried in a vacuum oven at 30°C.'H NMR 6: 1.4 (m, 6 H), 1.8 (m, 4 H), 3.0 (t, 4 H), 7.4 (t, 4 H), 7.5 (d, 2 H), 7.9 (d, 4H). IR v,,,/cm-': 2931, 1681 (C=O str), 1448, 1266, 739. Analysis: C, 81.26; H, 8.30%; C21H24O2 requires: C, 81.78; H, 7.84. Mpt 49- 51 "C. m/z (El) 308 (M+,5%). 2,8-Dibromo-1,9-diphenylnonane-1,9-dione(2b, R =H ) To the diketone (2a) (18.5 g, 0.06 mol) dissolved in 500 ml of dichloromethane was added bromine (19.1 g, 0.24 mol) dis- solved in 50 ml dichloromethane, dropwise with stirring. Moisture was excluded from the flask throughout the addition procedure. After addition of a few drops of the bromine solution the reaction mixture was gently warmed until the bromine colouration disappeared. Addition was then con-tinued dropwise.After the completion of the reaction the solvent was removed on a rotary evaporator (care HBr fumes!) to yield a brown oil. This was dissolved in 500 ml of boiling methanol, the flask was then placed in a freezer set at -25 "C overnight. The product was obtained as a white powder (17.3 g, 0.037 mol, 61.8%). 'H NMR 6: 1.4 (m, 6H), 2.1 (m, 4H), 5.1 (t, 2 H), 7.4 (t, 4H), 7.5 (t, 2 H), 8.0 (d, 4 H). IR v,,,/cm-l: 2933, 1684 (C=O str). Analysis: C, 53.85; H, 5.13%: C,,H,,Br,O, requires: C, 54.10, H, 4.76%. Mp 77-80°C. m/z (EI) 465 (M', 0.5%),385 (M+-Br, 8%), 305 (Mf-2Br, 22%). J. MATER. CHEM., 1994, VOL. 4 1,9-Dipheny1-2,8-bis[Ethoxy (Thiocarbonyl )thio] nonane- 1,Pdione (2c, R =H) To a solution of the a-bromoketone (2b) (4.6g, 0.01 mol) in acetone at room temperature was added freshly recrystallised potassium o-ethyl xanthate (5.0 g, 0.03 mol), with stirring.A white precipitate of potassium bromide appeared immediately. The mixture was stirred for 30 min then the KBr was filtered off to leave a light yellow solution. The acetone was taken off on rotary evaporator to reveal a light yellow oil (5.2 g, 9.5 x mol, 94.8%). 'H NMR 6: 1.3 (m, 12H, 1.8 (m, 2H), 2.0 (m, 2H), 4.6 (q, 4 H), 5.4 (t, 2 H), 7.4 (t, 4 H), 7.6 (t, 2 H), 8.0 (d, 4H). IR vmax/cm-': 2933,1682 (C=O str), 1224(C-0), 1050 (C-0). Analysis: C, 60.45; H, 5.97%; C27H3204S4requires: C, 59.09; H, 5.88%. 1,5-Di [5-(Cphenyl)-2-oxo-l,3-dithiolyl]pentane (2d, R =H) To 30 ml of concentrated H2S04cooled to 0 "C was added the xanthate ester (2b) (4.6 g, 8.4 x lop3mol) with vigorous stirring.The mixture was stirred for 1 h at 0°C then poured over ice (250 8). The product was extracted with 3 x 200 ml portions of dichloromethane, dried over anhydrous mag- nesium sulfate, filtered and the solvent reduced to ca. 50ml using a rotary evaporator. To the pink solution was added 50 ml of methanol and the flask placed in a freezer overnight. The product was collected by filtration as an off-white micro- crystalline solid (2.40 g, 5.3 x lop3mol, 62%). 'H NMR 6: 1.3 (m, 2H), 1.5 (q, 4H), 2.5 (t, 4H), 7.3 (m, 4 H), 7.4 (m, 6 H). IR v,,,/cm-l: 2858, 1645 (C=O str). Analysis: C, 60.13; H, 4.51%; C23H2002S4 requires: C, 60.49; H, 4.42.m/z (EI) 456 (M', 50%). Preparation of Nickel Complex (Compound la) To sodium (0.2 g, 8.6 x mol) dissolved in 200ml dry degassed methanol under an argon blanket was added the ligand 2d (0.57 g, 1.2 x loW3mol). The mixture was gently heated to 50 "C with stirring for 1 h to form a yellow solution of the tetrasodium salt. To this solution (at ambient tempera- ture) was added a solution of tetrabutylammonium bromide (0.4 g, 1.2x mol) dissolved in 50 ml dry degassed methanol. To this solution was then added a solution of NiC1,.6H20 (0.15 g, 6.3 x mol) dissolved in 100 ml of dry degassed methanol dropwise, over a period of 2 h with vigorous stirring. The yellow solution darkened immediately upon addition of the nickel salt, and a red-brown precipitate gradually formed. After completion of the addition step, the reaction mixture was left stirring for 1h then opened to the air and the nickel salt collected by vacuum filtration. The yield of the crude dianionic salt was 0.44 g.This material was dissolved in acetone (0.26 g of insoluble material remaining) and to the red solution was added 0.023 g (9.0 x lop4mol) of iodine dissolved in 10ml acetone. A blue precipitate was obtained, which was collected by vacuum filtration. The solid was dissolved in dichloromethane to give a blue-black solu-tion and reprecipitated with methanol to yield a dark blue powder (0.07 g, 12.7% based on the ligand). Analysis: C, 54.30; H, 4.64%; C21H,oS4Ni requires: C, 54.91; H, 4.40%. MS (FAB), weak peak cluster at m/z 919.NIR (CH,Cl,), A,,, (E,,,): 795 nm (1.3 x lo4 1373 dm3 mol-' cm-l); IR vmax/cm-': 2360. 1,9-Di(Cbutylphenyl)nonane-1,9-dione (2a, R =C4H9) To n-butyl benzene (29.8 g, 0.22 mol) dissolved in 500 ml of dry dichloromethane in a 11 round-bottomed flask equipped with a magnetic stirrer follower and cooled in a salt-ice bath was added anhydrous aluminium chloride (29.6 g, 0.22 mol). To the resulting yellow solution was added a solution of azelaoyl chloride (25 g, 0.11mol) dissolved in 50 ml of dry dichloromethane dropwise; moisture was excluded through- out. After 12 h the brown solution was poured onto a mixture of 1kg ice and 100ml of concentrated HCl with vigorous stirring. The yellow organic layer was extracted with 3 x 300 ml washings of diethyl ether and the solution dried over anhydrous magnesium sulfate.The solvent was removed on a rotary evaporator to yield a yellow oil, which upon recrystallisation from methanol yielded lustrous white platelets (34.9 g, 0.082 mol, 75% based on azelaoyl chloride). 'H NMR 6: 0.9 (t, 6 H), 1.3 (m, 10 H), 1.6 (m, 4 H), 1.7 (m, 4H), 2.6 (t, 4 H), 2.9 (t, 4 H), 2.9 (t, 4 H), 7.2 (d, 4 H), 7.9 (d, 4 H). Analysis: C, 82.95; H, 10.24%, C29H4002 requires: C, 82.81; H, 9.59%. m/z (E/I): 420 (M', 30%); IR v,,,/cm-' 2930, 1681 (C=O), 1606, 737. l,9-Di (Cbutylphenyl)-2,8-dibromononane-1,9-dione (2b, R =C4H9) To the diketone (2a) (6.61 g, 0.016 mol) dissolved in 250 ml of dichloromethane was added a solution of bromine (5.1 g, 0.032 mol) dissolved in 50 ml of dichlormethane dropwise.Moisture was excluded from the reaction throughout. After complete addition of the bromine solution, the contenls were left stirring for several hours to yield a light yellow oil which was recrystallised from methanol to yield a white powder (5.8 g, 0.01 mol, 62.7%). 'H NMR 6: 0.9 (t, 6H), 1.4 (m, lOH), 1.6 (m, 4H), 2.2 (m, 4 H), 2.7 (t, 4H), 5.1 (t, 2 H), 7.3 (d, 4H), 7.9 (d. 4 H). Analysis: C, 59.42; H, 6.97%; C,&38Br20, requires: C. 60.22; H, 6.62%. m/z (EI): 578 (M+,weak), 497,499 (M' -BF, 1%), 417 (M++ -2Br, 2%); IR v,,,/cm-l: 2929, 1682 (<:=O), 1605. 1,9-Di (Cbutylphenyl)-2,8-bis [ethoxy (thiocarbonyl )thio ]-nonane-l,9-dione (2c, R =C4H9) To a solution of the a-bromoketone (2b) (4.78 g, 8.3 x lop3mol) dissolved in 250 ml of HPLC-grade acetone was added freshly recrystallised (acetone-ther) potassium o-ethylxanthate (3.46 g, 2.16 x mol), with stirring af room temperature. The solution turned pale yellow and a white precipitate formed immediately; after 30 min the precipitate was removed by vacuum filtration. The acetone was removed by suction to yield a yellow solid, this was stirred with diethyl ether and filtered off.The yellow solution was taken down to dryness on a rotary evaporator to yield a light yellow oil (4.08 g, 6.2 x lop3mol, 74.4%). 'H NMR 6: 0.9 (t, 6 H), 1.4 (m, 16 H), 1.6 (m, 4 H), 1.9 (m, 2 H), 2.1 (m, 2H), 2.7 (t, 4 H), 4.6 (q, 4 H), 5.4 (t, 2 H), 7.3 (d, 4 H), 7.9 (d, 4 H).Analysis: C, 63.13; H, 6.71%; C35H,804S4 requires: C, 63.59; H, 7.32%. IR v,,,/cm-': 2931, 1679 (C=O), 1226, 1050. l,5-Di [5-[ 4-( 4-butyl )phenyl]-2-oxo-l,3-dithiolylpentane (2d, R =C4H9) To 30 ml of vigorously stirred concentrated sulfuric acid in a 500ml round bottom flask in an ice bath was added the xanthate (2c) (3.4 g, 5.1 x mol) as a yellow oil. As the oil was added a colour change from yellow to orange and then back to yellow was observed. After 30 min stirring the reaction mixture was poured onto ice and the product was extracted with 3 x 300 ml washings of dichloromethane and dried over anhydrous magnesium sulfate. The solvent was removed on J. MATER. CHEM., 1994, VOL. 4 a rotary evaporator to yield a pink oil which was dissolved in a minimal volume of diethyl ether and the flask placed in a freezer set at -30°C overnight.A crop of off-white waxy crystals was obtained which was filtered off, washed with methanol and dried in a vacuum oven overnight (0.69 g, 1.2 x mol, 23.5%). 'H NMR 6: 0.9 (t, 6 H), 1.4 (m, 16H), 2.5 (t, 4H), 2.6 (t, 4 H), 7.2 (s, 8 H). Analysis: C, 65.10; H, 6.54%; C31H36S402 requires: C, 65.45; H, 6.38%. m/z (EI): 568 (M', 15%); IR v,,,/cm-': 2929, 1674 (C=O), 1119. Preparation of Nickel Complex (Compound lb) To sodium (0.68 g, 3.0 x lo-' mol) dissolved in 250 ml dry degassed methanol under an argon blanket was added of the ligand (2d) (0.38 g, 6.7 x lop4mol). The mixture was stirred with gentle heating until all of the ligand had dissolved to form a yellow solution, heating was continued for a further 30 min to ensure completion of the ring-opening reaction.After cooling the mixture to room temperature 0.86 g (2.7 x mol) of tetrabutylammonium bromide dissolved in 50ml of dry degassed methanol was added. This was followed by the addition of a solution of 0.19 g NiC1,-6H20 (8.0 x mol) dissolved in 50 ml of dry degassed methanol over a period of 90 min with vigorous stirring. After a further 90min the contents of the flask were filtered to yield a dark brown solid. This was dissolved in acetone and filtered to give a dark red solution to which was added a solution of 0.08 g (3.2 x lop3mol) of iodine dissolved in 10 ml of acetone. A green precipitate formed which was collected by vacuum filtration (0.14 g, 72% based on the ligand).Purification was by repeated reprecipitation from chloroform-methanol. Analysis: C, 60.81; H, 6.47%; C29H36S4Ni requires: C, 60.94; H, 6.35%. MS (FAB): weak cluster at 1139 correspond- ing to dimer. NIR (CH,Cl,), Lax(E,~,): 810nm (1.55 x lo4dm3 mol-' cm-'); IR v,a,/cm-l: 2363, 1375. Results and Discussion The electronic spectra of these materials show strong NIR absorption bands typical of neutral nickel bis(dithio1ene) c~mplexes.',~~.~~The shape of this band was found to be concentration-dependent, with more concentrated solutions showing band broadening with a concomitant decrease in the absorption coefficient. More concentrated solutions show two NIR bands (Fig.3). 'H NMR studies of solutions of these complexes also exhibit broadened absorptions, suggesting aggregation is taking place. No such effects have been observed with monomeric analogues of these complexes. The presence r of the electron-denoting p-butyl substituent on the phenyl ring of compound lb, induced a red shift of the NIR band by 15 nm compared with the unsubstituted species (Table 1). The presence of bulky alkyl groups adjacent to the phenyl ring prevents coplanarity of the benzene ring with the dithiolene core, severely restricting conjugation. Thus the effect of elec- tron-donating species on the phenyl ring exerts only a minor influence on the position of the NIR band. This can be compared with compounds 4 (R =H, R'=C4H9) and 6 (R= H, R'=H), in Table 1.In these compounds the phenyl ring is coplanar with the dithiolene core and p-alkyl substitution causes a bathochromic shift of 55 nm. The electrochemical behaviour of the two oligormeric sys- tems la and lb show two quasi-reversible redox processes typical of dithiolene systems (Table 1, Fig. 4, system la) exhibited two redox processes at E12= -1.37 and -0.62 V relative to ferrocene, with peak separations of 116.5 and 79.3 mV. System lb gave El, values of -1.40 and -0.64 V with peak separations of 90.1 and 74.1 mV. These processes have been assigned to dianion monoanion and mono-anion + neutral redox processes, respecti~ely,'~ i.e. [ML,I2-=$[ML2I1-$ [ML,]'. It can be seen that butyl substitution on the phenyl ring has little effect upon the electrochemical behaviour of complexes with a 1,2-aryl/alkyl substitution pattern.(This is again due to the steric nature of the alkyl group effectively limiting conjugation between the phenyl ring and the dithiolene core.) Referring to Table 1, it can be seen that where the alkyl ring is coplanar to the dithiolene core (systems 5 and 6), butyl substitution on the phenyl ring has little effect upon the El, redox potentials. This behaviour can be rationalised in terms of a simple two- level model developed by Mueller-Westerhoff et The frontier orbitals of the dithiolene system are shifted in energy by the influence of electron-donating substituents on the dithiolene core. The HOMO level (2B1, in DZh)is considered to be destabilised by a greater extent that the LUMO (3B,,) level.The NIR band which is due to a 2B,,+3B2, transition Table 1 Comparison of electrochemical and NIR behaviour E E [ML,I2-+[MLJ-+[ML2I0 R R' E (relative to ferrocene)/V Am,,/nm la H -(CH2)5--1.37 -0.62 795 lb C4H9 -(CH2)5--1.40 -0.64 810 2 C4H9 CH, -1.37 -0.61 805 3 H C6H5 -1.27 -0.47 865 4 C4H9 H -1.30 -0.45 865 5 H C4H9 -1.42 -0.56 800 6H H 810 I, I I 600 800 1000 1200 J Wnm 600 800 1000 WnmFig. 3 Concentration dependence of NIR band structure. (a) 7.5 x lop5rnol dm-, (b)2.5 x mol dm-, (compound la). Fig. 4 Difference in solid state and solution spectra for compound lb J. MATER. CHEM., 1994, VOL. 4 exhibits a bathochromic shift due to the presence of electron- donating substitutents since the energy gap between the HOMO and LUMO levels decreases.However, the redox couples investigated involve population and depopulation of the LUMO level which is less sensitive to ligand variation on the dithiolene core. The presence of weak FAB signals indicating dimeric species, coupled with the analytical results strongly suggests that the dichloromethane soluble fraction is composed of dimers of the dithiolene units. It is suggested that the pentyl linking groups afford sufficient flexibility to allow rings of the dithiolene dimers to form. The insoluble fraction will no doubt have other length oligomers present, and studies are currently underway to determine the exact composition of the samples depending upon the reaction conditions.The preparation of polymers of this type via the method of metal complexation polymersiation presents a problem when the nature of the termination step is considered. Unless groups such as alkyl halides are deliberately introduced during the metal addition process, the termination step can only be provided by the metal centre and therefore oligomers composed of different sized rings will be formed. In an attempt to promote solubility of these systems reactions have been performed under rela- tively dilute conditions in an attempt to limit chain growth. A much more detailed study of the preparation step is currently underway. Preliminary third-order non-linear optical measurements have been performed on compound lb using the z-scan technique.Unfortunately, thin films of the oligomer sample alone could not be prepared directly, but it was possible to prepare a guest-host film containing 48% w/w of the oligomer in PMMA in excess of the loading achieved with monomeric materials. The spectrum of system lb in PMMA is presented in Fig. 4 compared with the same material dissolved in dichloromethane. It can be seen that the oligomeric sample codispersed in PMMA exhibits a A,,, at 825 nm, a red shift of 15 nm compared with the material dissolved in dichloro- methane. This red shift is associated with a general broadening on the long-wavelength side of the band and a tail extending out to 1220nm. A red shift of absorption bands between solution and solid-state spectra is a well known phenomenon, although the origin of the long-wavelength tail is not known.A comparison of z-scan results for samples lb and 5 is given in Table 2. Both samples were measured as thin films of nickel dithiolene codispersed in PMMA at the concen- trations quoted, experimental details have been given else- where.7 Both samples exhibited comparable values for the non-linear refractive index (n2) at similar concentrations. However, evaluation of the materials using the Stegeman figure of merit [W=An,,,/(a,A)] shows that sample lb exhibits a lower W value due to the increased linear absorption coefficient associated with the long-wavelength tail mentioned previously. The real/imaginary ratio of f3) measured for the two samples (nz/p)was also comparable, being of the order of 3.The ratio of the real to imaginary non-linear molecular hyperpolarisability forms the basis for a device-based figure Table 2 Non-linear optical data concentration/molecules cmP3 a/cm-' n2/cm2 kW-' w B/n2 lb 5 5 x lozo 2 x 1O2O 240 167 -4.6 -8.3 x lop9 10-9 0.5 1.1 4.3 3.1 5 3.2 x lo2' 784 -1.0 x low8 0.3 1.4 of merit.17 Depending upon the optical device in question, the real/imaginary x(3)ratio should exceed a value of ca. 2rc for an ideal case. Referring to Table 2, note that whilst acceptable ratios of n2 and are observed at concentrations of 2.0 x lo1* molecules cmP3 for both the samples, at higher concentrations of the monomeric species a severe degradation in this ratio occurs.18 The reason for this is not known, but may be due to molecular aggregation occurring at these higher loadings.The oligomeric species (lb) does not show this rapid fall-off in nz/P at higher concentrations, which is a most encouraging result. A study of the variation of the n2/P FOM of the oligomer us. concentration in the host species is required in order to quantify this effect. Further studies are underway to determine the highest concentration of oligomer that may be incorporated into the PMMA matrix without reduction of this figure of merit. Conclusions By modifying a nickel bis(dithio1ene) to improve miscibility with a polymeric matrix, it has been found that an improve- ment in the real to imaginary (n2/P) figure of merit arid the W figure of merit has been achieved.This occurs at concen- trations where the similar monomeric species exhibits a severe degradation of these parameters. Studies are currently underway to determine the maximum concentration that the oligomers can be incorporated into the polymer matrices. References 1 J. A. McCleverty, Prog. Inorg. Chem., 1968, 10,49. 2 U. T. Mueller-Westerhoff, Comp. Coord. Chem., 1986,2, 595. 3 P. Cassoux, L. Valade, H. Kobayashi, A. Kobayashi, R. A Clark and A. E. Underhill, Coord. Chem. Rev., 1991,110,115. 4 A. E. Underhill, C. A. S. Hill, C. S. Winter, S. N. Oliver and J. D. Rush, Mol. Cryst. Liq. Cryst., 1993,217,7. 5 C. S. Winter, S.N. Oliver, R. J. Manning, J. D. Rush, C. A. S. Hill and A. E. Underhill, J. Muter. Chem., 1992,2,443. 6 C. A. S. Hill, A. E. Underhill, C. S. Winter, S. N. Olivx and J. D. Rush, Organic Materials for Nonlinear Optics II,RSC Special Publication No. 91, Royal Society of Chemistry, Cambridge, 1991, pp. 217-222. 7 S. N. Oliver, C. S. Winter, R. J. Manning, C. Hill and A. E. Underhill, SPIE Vol 1775 Nonlinear Optical Properties of Organic Materials V,ed. D. J. Williams, SPIE, 1992, pp. 110-120. 8 J. R. Anderson, V. V. Patel and E. M. Engler, Tetrahedrojr Lett., 1978,3,239. 9 T. Vogt, C. Faulmann, R. Soules, P. Leconte, A. Riosset, P. Castan, P. Cassoux and J. Galy, J. Am. Chem. Soc. 1988, 110,1833. 10 F. Wang and J. R. Reynolds, Macromolecules, 1990,23,3219. 11 F. Wang, Y-J. Qiu and J. R. Reynolds, Macromolecules 1991, 24, 4567. 12 U. T. Mueller-Westerhoff,A. Nazzal, R. J. Cox and A. M. (iiroud, Mol. Cryst. Liq.Cryst. (Lett.), 1980,249. 13 G. N. Schrauzer and V. P. Mayweg, J. Am. Chem. SOC.. 1965, 87, 3585. 14 Z. S. Herman, R. F. Kirchner, G. M. Loew, U. T. Mueller-Westerhoff, A. Nazzal and M. C. Zerner, Inorg. Chem., 1982, 21, 46. 15 G. A. Bowmaker, P. D. W. Boyd and G. K. Campbell. Inorg. Chem., 1983,22,1208. 16 U. T. Mueller-Westerhoff,B. Vance and D. I. Yoon, Tetrahedron, 1991,47,909. 17 V. Mizrahi, K. W. DeLong and G. I. Stegemann, Opt. Lett, 1990, 14, 1140. 18 C. S. Winter, R. J. Manning, S. N. Oliver and C. A. S. Hill, Opt. Commun., 1992,90, 139. Paper 4/02659D; Received 5th May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401233

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(8), 1239-1244 In situ Study of a Strontium P-Diketonate Precursor for Thinlfilm Growth by Atomic Layer Epitaxy Jaan Aarik,*" Aleks Aidla," Andres Jaek," Markku Leskela*b and Lauri Niinisto*" a Laboratory of Electroluminescence and Semiconductors, Tartu University, EE-2400 Tartu, Estonia b Department of Chemistry, University of Helsinki, FIN-0001 4 Helsinki, Finland Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02 150 Espoo, Finland Precursor properties of Sr(thd), (thd =2,2,6,6-tetramethyl heptane-3,5-dione) have been investigated using in situ mass monitoring of thin films during atomic layer epitaxy growth cycles. H20 and H2S were used as the other source materials. The Sr(thd), source temperature, T,, significantly affects the growth rate.At T, =240-270 "C, a decrease of growth rate and in some cases even etching of the grown film takes place. These phenomena can be explained by the decomposition of Sr(thd),, which also explains the dependence of the growth rate on the reactor temperature, T,, observed when T,<240 "C and 240 <T,/"C <400. The growth experiments were complemented and the conclusions supported by separately studying (by mass spectrometry thermogravimetry, and differential thermal analysis) the thermal stability and fractionation of the precursor. The interest in strontium compounds has increased recently owing to the fact that SrS doped with rare-earths ions is a possible matrix/activator candidate for full-colour, thin-film electroluminescent (TFEL) displays.' In the development of materials for full-colour TFEL devices the blue phosphor has been the most difficult to achieve.For this purpose, SrS :Ce3+ has extensively been studied with considerable success.2 For instance, a full-colour EL device recently demonstrated was based on SrS : Ce and ZnS :Mn emitting layer^.^ SrO, on the other hand, is a constituent of high-T, superconducting oxides4y5 and of dielectrics for high-capacitance-density thin-film capacitors.6 For these applications a controllable depos- ition of SrS and SrO thin films is of great importance. In general, the alkaline-earth metals form very few volatile complexes which can be applied in thin-film deposition from the gas phase. The P-diketonates, especially the Sr(thd), complex (thd =2,2,6,6-tetramethyl heptane-3,5-dione) are among the few sufficiently stable volatile complexes of stron- tium.The P-diketonate polyether complexes form another group of volatile alkaline-earth-metal compounds recently reported in the Sr( thd), has been successfully applied in the SrS deposition by the atomic layer epitaxy (ALE) method.' ALE is a novel deposition technique in which the reactants are pulsed separately onto the substrate where exchange reactions take place and a monolayer of the desired product (or fraction thereof) is formed;" for instance, in the case of SrS the overall reaction is Sr(thd),(g) +H,S(g)+SrS(s) +2 Hthd(g). The actual reaction mechanism is more complicated owing to the thermal instability of the precursor Sr( thd),.11,12 Indeed, it has been shown that M(thd), as well as M(thd), M,(thd), (M =Ca, Sr) and other dissociation fragments can be found in the gas phase when thd chelates of alkaline-earth metals are ~olatilized.'~*'~Oligomeric and mixed (0x0 or aqua) ligand complexes can easily form with Ba p-dike ton ate^.'^,^^ In order to obtain a better understanding of the reaction mechanism and thus improved control of the deposition process and the resulting thin film properties, in situ studies are highly desirable.It was recently shown that monitoring the film mass during an ALE cycle is a powerful method which yields valuable information concerning adsorption and exchange reaction^.'^ In this study we have employed this method for in situ investigation of the ALE growth of SrO and SrS, where Sr(thd), and H,O or H2S were used as source materials. Ex situ thermogravimetry (TG), differential thermal analysis (DTA) and mass spectrometry (MS) studies of the precursor were also carried out. Parallel to the present study on Sr(thd),, we have also investigated the precursor properties of Ca(thd),18 and Ba(thd),." Experimental Experiments were carried out in a hot-wall flow-type ALE reactor (Fig.1). Fused quartz was used for all components of the reactor which were in contact with active gases at high temperatures. Argon was used as the carrier gas. Its pressure measured at the reactor outlet was kept at 150-200 Pa while the flow rate was 4m s-l inside the reactor tube.The commutation of the active gases was performed uia electro-magnetic valves operated by a personal computer. A gas manifold enabled us to switch the reactant flows on and off within 0.5-1.0s.The reactor temperature, TR,as well as the temperature of the effusion cell used as the Sr(thd), source, T,, could be controlled with an accuracy of 0.3 K. During the experiments TR and Ts were varied in the ranges 200-420°C and 200-340 "C, respectively. The Sr(thd), source chemical was synthesized as described earlier,12 and its purity and volatility were routinely checked for each batch by X-ray diffraction (XRD), differential scan- ning calorimetry (DSC)and TG/DTA. H20 or H2S were used as the other source materials. More detailed investigations into the volatility and gas- phase fractionation were carried out by TG/DTA measure- reactant gas inlet reactor mass sensor qocouple thermocouple solid reactant evaporation cell Fig.1 Schematic diagram of the flow-type hot-wall ALE equipment used ments in vucuo and by MS. For the thermoanalytical studies a Seiko TG/DTA 320 instrument of the series SSC 5200 was used and operated at a pressure of cu. 700Pa. The typical heating rate/sample weight combination was 10"C min-' and 5 mg. The UHV high-resolution mass spectra were recorded in a JEOL MS DX-303/DA5000 instrument. The solid samples were introduced into the mass spectrometer through a sample stage equipped with programmable heating.The mass of the deposited films in the ALE reactor was determined by a quartz crystal mass sensor. The sensor had Ag electrodes and its resonance frequency was 30 MHz. The films were grown directly on the Ag electrodes or on a buffer layer of aluminium or tantalum oxide. The increments of oscillation period, AT, proportional to the mass changes were plotted with a time resolution of 1s. The proportionality constant between AT and the corresponding mass change was (1.12+0.04)x 106gcm-2 s-'. Before starting the measurements a layer of SrS or SrO was grown onto the mass sensor using 20-50 ALE cycles. An ALE cycle included the injection of the Sr thd complex and H20 or H2S into the reactor, the reactants being separated by an inert- gas pulse.The first purge time following the injection of the thd complex was chosen to be sufficiently long (25 s) in order to avoid intermixing of the source materials in the gas phase; the second purge, following H,O or H,S injection, was set even longer (50-200 s). This time was needed to obtain the necessary data for the correction of experimental curves in respect of the temperature drift. Indeed, as the temperature coefficient of the mass sensor was as high as 530fs K-' at 250 "C, temperature increments as low as 0.01 K had a notice- able effect on AT. Therefore the reaction heat, valving of the gas flows as well as the instability of the reactor heater could affect the mass sensor signal. The temperature drift caused by the last factor was a slow one and was taken into account using the extrapolation of the time dependences of AT recorded during the second purge time.To minimize the role of the other two factors, appropriate cycle times were used. As the time constant of the mass sensor response to the temperature changes was ca. 10s the mass increments AT',, corresponding to the complete ALE cycle were recorded no sooner than 50-100 s after the end of the cycle. During the AzfO/Azl ratio measurements, where AT^ is the mass sensor signal increment caused by the adsorption of 80 1 I I Sr(thd), on H20off 1 J. MATER. CHEM., 1994,VOL. 4 Sr-containing precursor, we used short (1---2 s) exposures to the Sr(thd), source to reduce the concurrent temperature efTec t s.Results The oscillation period (AT) of the quartz crystal mass sensor as a function of time at two source temperatures is shown in Fig. 2, which also shows the different sequence of the pulse and purge times (tl-t4). The figure shows that an increase of T, from 235 to 265 "C has a significant effect on the curves. At 235°C the behaviour of the oscillation period resembles that observed for AlC13/H20, Ca( thd),/H,O and Ca(thd),/H,S precursor sy~terns.'~-'~ Similarly, the increase of the mass sensor signal during the time interval tl can be interpreted as the adsorption of metal-containing precursor, while the decrease of AT during the time interval t3 is mostly caused by the replacement of the heavy ligands by lighter oxygen atoms or OH groups.The specific features of the present case are that: (i) during exposure to the Sr(thd), source, the mass sensor signal does not reach a constant level but slowly continues to increase with the exposure time and (ii) the value of AT changes during the purge times. Plotting AT as a function of time under conditions when there was no Sr(thd), present as source chemical enabled us to determine that the increase of AT during time interval t, is caused by the decrease of the sensor temperature following switching-off the H,O flow. The origin of the incomplete saturation of AT, observed during the Sr-containing precursor pulse, is evident from Fig. 3 which shows that the saturation of the mass sensor signal increment AT'^ corresponding to a complete ALE cycle is also incomplete.As the temperature effects were neglected during AZ'~measurements, we concluded that this incomplete saturation as well as the decrease of AT during the time interval t, were caused by continuous CVD- type reactions which could be due to the decomposition of the source material or irreversible exchange reactions between adsorbate and H20 traces in the carrier gas. However, refer- ence measurements with the AlCl, precursor show that the role of the H20 background is negligible. The adsorption of A1C13 was self-limiting and no change of AT was observed after closing the AlCl, valve in spite of the fact that AlCl, is more reactive towards H20 than Sr( thd),. Consequently, the 40 I H,O on 0 -40 \ ri H200ff -80 Sr(thd), offP -80 1 JI I 1 1 I I I I I 0 100 200 300 100 200 0L tls Fig.2 Increment of mass sensor oscillation period A7 at TR=250 "C as a function of time at two different source temperatures (a) 235 "C and (b)265 "C J. MATER. CHEM., 1994, VOL. 4 0 a 16 24 pulse duration/s Fig.3 Increment of mass sensor oscillation period for a complete ALE cycle Af0 as function of thd complex pulse duration at TR= 280 "C and T,=290 "C. AT'^ obtained by extrapolation of the exper- imental data is the increment of mass sensor signal caused by the pure ALE growth during one ALE cycle. thermal decomposition of the precursor seems to be the only reason for the steady-state oscillation period increase. Fig.3 also demonstrates that the rate of this process, (dz/dt),,,, (slope of the broken line in Fig. 3), is more than an order of magnitude lower than the value of the derivative d(Az',)/dt recorded at low tl values. The latter determines the lowest deposition rate which can be limited by the arrival of precursor molecules. Therefore the thermal decomposition rate, which at all reactor temperatures was significantly lower, could not be transport-limited. Plotting (dz/dt)fi,,l us. reciprocal reactor temperature (Fig. 4), we find that the activation energy of the decomposition is 39 kJ mol-'. In order to avoid the uncer- tainties caused by this unintentional process, the values of Az0 presented in Fig. 5 and 6 were obtained via linear extrapol- ation of the respective experimental data to t, =0 as shown in Fig. 3.In this way quantities corresponding to the 'pure' ALE were obtained. The most unexpected phenomenon observed in this study is that AT, as a function of cycle time, changes its shape at some combinations of TR and Ts [Fig. 2(b)]. Moreover, in some cases when Ts increases, etching occurs instead of growth [Fig. (b)].The etching of the as-grown film took place at TR=250-265 "C, while at other TR values a decrease of the growth rate in the Ts region 240-250°C was observed [Fig. S(u)]. Relevant information concerning this anomalous behaviour was obtained from the dependence of the derivative 40 30 20 10 0 I I I I-1 0 1 2 1 240 260 280 source T/"C 1.0-m 0.6 -r9 -0.4 C-c h-+-.F -s0.2 0.1 I I 1 I I I I 1 1.4 1.6 1.8 2.0 lo3 T~IK Fig. 4 Rate of the steady-state oscillation period increase (dt/dt)f,,,l as function of l/TR (dz/dt)initi,l on the source temperature. Here (dz/dt)i,iti,l is the slope of the mass sensor signal us. time curve at the beginning of the Sr(thd), pulse. At this moment the surface coverage of Sr(thd), is small compared with the saturation level and the adsorption rate, which is proportional to dz/dt, is also pro- portional to the partial pressure of active species and their sticking coefficient. Fig. 5(b) shows that (dz/dt)initi,l has a sharp maximum at those source temperatures where a decrease of growth rate occurs, provided that the data were recorded during the increase of T,.This maximum indicates an anomal- ous increase in the vapour pressure and/or the activity and is most plausibly connected with the decomposition of Sr( thd), in the effusion cell. This supposition is consistent with the fact that no maxima or minima were recorded during the decrease of source temperature. It is interesting to note that at source temperatures below 240"C, where no anomalies were observed, Az0 as a function of the reactor temperature has a minimum which is also located at 240-250 "C [Fig. 6(u)].A steep increase at lower TR is mostly due to the unsaturated physisorption of Sr precursor. The increase in AT^ at TR3250 "C is very similar to the increase in the growth rate observed earlier', and will be discussed below.Note that Azo is nearly constant at Ts>270 "C [Fig. 6(b)]and its value is close to that obtained at Ts<240 "C and TR=380-400 "C. Fig. 6(b) also demon- strates that there is no significant difference in AT^ when H20 is replaced by H,S. Evidence indicating the decomposition of the Srl thd), precursor enables us to expect that the Sr complex which is chemisorbed on the solid surface in the ALE process is not source T/"C Fig. 5 Increments of mass sensor oscillation period for a complete ALE cycle AT^ (a) and derivative of oscillation period (dz/dt)initial (h) as a function of thd complex source temperature. (a)@, TR=320 "C; 0,TR=250 OC; (b) TR=320 "C. J. MATER.CHEM., 1994, VOL. 4 I I 1 40 20 PU 200 280 360 440 200 280 360 440 reactor TIoC Fig. 6 Increment of mass sensor oscillation period for a complete ALE cycle Ar0 as a function of reactor temperature for (a) T, =230 "C and (b) Ts=280 "C. 0,Sr(thd),/H,S; 0,Sr(thd),/H,O. 100 200 300 400 500 TI'C Fig.7 TG and DTA curves showing the volatilization and partial decomposition of Sr(thd),. The sample weight is 16.8 mg and the heating rate is loomin-' at a pressure of 700 Pa. Sr(thd),. In order to obtain data concerning this adsorbate we measured the ratio AZ'~/AZ~,where Az~is the change of the mass sensor signal increment caused by the adsorption of the Sr-containing precursor. This ratio is equal to the corre- sponding mass ratio Amo/Am, and, provided that the product of the complete ALE cycle is known, makes it possible to estimate the metal-to-ligand ratio in the chemisorbed com-plex.17 At TR,T, 2270 "C we obtained Amo/Am, =0.44 f0.06 and 0.46 0.05 for H2S and H20, respectively.At TR =200 "C and Ts=235 "C, Am,/Aml =0.25 0.05 when H,S was used as source material. With H,O the values of Amo/Aml range from 0.3 to 0.6 at TR,Tsd 240 "C. Such a large dispersion is probably due to incomplete exchange reactions between adsorbed thd complexes of strontium and H20 at these low temperatures. The results described above can be compared with those obtained in ex situ TG and MS studies. Fig. 7 shows typical thermoanalytical curves for the Sr( thd), precursor. According to the TG and DTA curves the volatilization at 220-320°C is rapid and smooth, but a residue (ca.10%) and the peaks in the DTA curve indicate decomposition and probably also melting. The small weight loss just above lOO"C, seen in the TG and especially in the DTA curve, is caused by the release of moisture and low-molecular-weight decomposition prod- ucts and its amount varies from batch to batch. The weight residue depends somewhat on the experimental conditions (heating rate, sample size etc.) but is typically 5-10%. That low-molecular-weight fragments are expelled from the precursor is corroborated by the mass chromatograms recorded at source temperatures of 100, 200 and 300°C (Fig. 8). Typical mass spectra corresponding to these plateaus are presented in Fig.8. The dominant peaks at m/z 54, 127, 271 and 725 correspond to C3H20+, C3H202C(CH3)3f, I 1 I I I I I I I 20 40 60 80 100 120 140 300oc scan no. l04III39 57 95 111 scan no. Fig. 8 Mass chromatogram of Sr(thd), showing the relative intensities of the main peaks as function of temperature (see the heating profile) Sr(thd) and Sr,(thd),, respectively. The peak at mjz 391, most notably present during the fast heating from 100 to 200"C, is probably a ligand-recombination product as it was found also in the spectra of Ca(thd), and Ba(thd),.,' The occurrence of Sr(thd) and Sr,(thd), at higher temperatures is in agreement with the behaviour of the corresponding calcium precur- Note, however, that the volatilization temperatures in various experiments (TG, ALE, MS) are not directly comparable because of the pressure differences.Discussion Our experimental results demonstrate that Sr( thd), as a source material for ALE growth displays noticeable peculiari- ties in the temperature range 240-270°C. As the behaviour of the Sr(thd), complex is not reversible it can be concluded that Sr( thd), partly decomposes at these temperatures. This conclusion is in agreement with the data published by Schwarberg et ~1.'~reporting that Sr( thd), melts at 248-268 "C with decomposition and also with the present thermoanalyt- ical and MS studies, which show that the gas phase already contains a significant amount of decomposition products (Fig. 9). Using the assumption of decomposition, a satisfactory explanation to all our results can be given.The explanation is based on the fact that the free ligand or its frag-ments volatilize at these temperatures more easily than an Sr-containing complex. One can also compare this case with J. MATER. CHEM., 1994, VOL. 4 'O01 127 271 I I 'II 100 200 300 400 500 600 700 800 mlz Fig. 9 Mass spectra of Sr(thd), obtained at (a) lOO"C, (b) during heating from 100 to 200 "C and (c)at 200 "C Ce(thd),, for which it has been shown that mass spectra of the vapour phase contain a number of peaks corresponding to dissociation fragments which do not contain metal atoms.21 Interacting with the surface of Sr-containing films, these thd fragments form an adsorbate that restricts the adsorption of the strontium complex.Furthermore, the as-formed adsorbate can be desorbed, resulting in etching.22 However, as the etching was not observed at reactor temperatures exceeding 265 "C, the process should be limited by the surface reactions rather than by the desorption rate. The experimental results enable us to conclude that the reaction between the free ligand and solid surface is exother- mic. Indeed, the time dependence of AT characterizing etching [Fig. 2(b)]shows that the sensor temperature decreases when the Sr source is switched off. This behaviour can only be caused by a preceeding increase of temperature because there was no such decrease in the cases when the growth was regular [Fig.2(u)] and when there was no Sr(thd), in the crucible. Consequently, the reaction rate between the solid film and the free ligand should decrease with increasing reactor tem- perature. This supposition is in good agreement with our experimental data where etching of the grown film took place in a narrow range of reactor temperatures above 250 "C while only a decrease of growth rate was observed at higher TR. The dependence of growth rate on the reactor temperature observed at TR 3240 "C and T,<240 "C [Fig. 6(u)] can also be explained by the decomposition of Sr(thd),. In this case, Sr(thd), decomposes on the substrate or in the reactor tube before reaching the substrate. All the dissociation fragments interact with the solid surface simultaneously and their contri- bution depends only on the reactor temperature.As the reaction between the solid film and the free ligand is exother- mic the effect of the latter should be reduced at higher temperatures. Thus, the growth rate should have a minimum at the lowest reactor temperatures where Sr( thd), decomposes. One can see from Fig. 5(u) that this conclusion is in good agreement with experimental data. Unfortunately the existing experimental data do not enable us to ascertain which kind of ligand-dissociation fragments are responsible for reducing the growth rate and etching. Correspondingly, it cannot be concluded whether or not these reactive intermediates can be neutralized, e.g.by incorporating additional components into the precursor.However, by pre- heating the precursor at Ts>270"C the effect of these dis- sociation products on the growth rate can be decreased. Assuming that at T,3270 "C Sr(thd), is, to a considerable degree, decomposed and that the free ligands are completely removed from the source material, the blocking of substrate surface by free ligands should be absent at these source temperatures. Consequently, the growth rate cannot depend on the reactor temperature in this case but is constant over a 1243 wide temperature range. As one can see from Fig. 5(h),our experimental data also confirm this conclusion. Additional information about less volatile decomposition products could be obtained from the chemical analysis of the residues which remain in the crucible and on the walls of the inlet tube.However, in our case the results of this analysis are not sufficiently reliable because we could not perform the analysis without exposing these reactive residues to ail-. As shown in our previous study,17 surface reactions can be determined using the ratio of the film mass increments corre- sponding to different steps of the ALE cycle. Unfortunately, the mass increments caused by the adsorption of the thd complex contain some experimental uncertainty which enables us to deduce only an approximate scheme of surface reactions. Assuming that thd ligands coordinated to Sr do not decom- pose during chemisorption, and excluding etching, the surface reactions for the first step of the ALE cycle can be expressed as Sr(thd),( g)+Sr(thd),(ads) +(x-y) thd(g) In some cases Sr( thd), can replace chemisorbed hydrogen: (x-y)H(ads)+Sr(thd),(g)+Sr(thd),(ads) +(x-y)H(thd)(g) (2) However, the values of Am, corresponding to these two cases differ by less than 1% and this difference cannot be used as basis for making any decision concerning the type of reaction.The reactions which take place during the second step of the ALE cycle can be expressed as Sr( thd),(ads) +H,S(g) SrSH, -,(ads) +yH (thd) ( g) (3) provided that the bonds between Sr and solid surface are stable. Similar equations can be written when H2S is replaced by H20. However, in the latter case the analysis is more compli- cated because Sr(OH), can be formed instead of SrO.Moreover, we still do not have an authentic corroboration of exchange reactions between thd complex of strontium and H20 being complete. Therefore the values of y can only be estimated for reactions between Sr(thd), and H,S. This was done using the experimental values of Amo/Am,. We estab- lished that at 270 <(TR,TS)/"C<400 the y value calculated according to eqn. (3) is 1.03f0.24. Hence at temperatures above 270°C Sr(thd), has lost a thd ligand before and/or during adsorption. Our MS results show that in the gas phase y is mainly 1.5. Thus the thd complex decomposes further on the substrate surface. This conclusion is also confirxned by the data presented in Fig. 3 and 4, which show that the deposition does not stop while the substrate is exposed to the Sr source and the role of this unsaturated adsorption increases with the increasing substrate temperature.For comparison only, we also calculated the value of y for reactions where H20 takes part instead of H2S. The values range from 0.6 to 1.0 at 270<(TR,Ts)/"C<400 and from 0.3 to 1.7 at 200 <(TR, Ts)/"C <240. We also estimated the mass of ligand(s) coordinated to one Sr atom adsorbed during the first step of the ALE cycle at TR=200 "C and Ts=235 "C. The m/z of this ligand calculated from Amo/Am, is 390 f100. This value is the mass of d single thd ligand multiplied by a factor of 2.1f0.6 and, within the limits of experimental error, coincides with the mass of a ligand coordinated to an Sr atom in the correspondrng MS measurements.Thus one can conclude that during adsorption at such a low temperature Sr-containing thd complexes do not lose any ligands. It is obvious that the size of a thd ligand is greater than that of an Sr site. Accordingly thd ligands included in the adsorbed particles limit the growth rate. In this connection 1244 J. MATER. CHEM., 1994, VOL. 4 the increase of the number of ligands per Sr atom with decreasing temperature explains the corresponding decrease of growth rate very well. 3 Workshop, ed. V. P. Singh and J. S. McClure, Cinco Puntos Press, 1992. M. Leppanen, G. Harkonen, A. Pakkala. E. Soininen and R. Tornqvist, Eurodisplay 93, Conference Proceedings, Conclusions This study demonstrates that Sr(thd), behaves in a compli- cated way as a source material for ALE growth.At source temperatures below 240°C the growth is rather stable. However, the growth rate depends on the reactor temperature. The molar mass of the main Sr-containing precursor adsorbed 4 5 Strasbourg, 1993, p. 229. T. Kimura, H. Nakao, H. Yamawaki, M. Ihara and M. Ozeki, ZEEE Trans. Magn., 1991,27,1211. J. M. Zhang, B. W. Wessels, D. S. Richeson, T. J. Marks, D. C. DeGroot and C. R. Kannevurf, J. Appl. Phys., 1991, 60, 2143. T. Sukuma, S. Yamamichi, S. Matsubara, H. Yamaguchi and Y. Miyasaka, Appl. Phys. Lett., 1990,57, 2431. J. A. T. Norman and C. P. Pez, J. Chem. Soc., Chem. Commun., appears to correspond closely to Sr(thd),. At the same time mass spectra indicate that some decomposition of thd ligands can take place during evaporation of the source material.At source temperatures ranging from 240 to 270°C the Sr source fractionates further. The vapour pressure of free ligands exceeds that of Sr-containing complexes. The adsorption of free ligands or their fragments dominates, resulting in very 10 1991,971. K. Timmer, K. I. M. A. Spee, A. Mackor. H. A. Meinema, A. L. Spek and P. van der Sluis, Znorg. Chim. Actu, 1991,190, 109. M. Tammenmaa, M. Asplund, H. Antson, L. Hiltunen, M. Leskela, L. Niinisto and E. Ristolainen, J. Crystal Growth, 1987,84, 151. M. Leskela and L. Niinisto, in Atomic Layer Epitaxy, ed. T. Suntola and M. Simpson, Blackie, Glasgow. 1990, p. 1. low or even negative values of growth rates (etching). At source temperatures above 270 "C, those dissociation fragments that do not contain Sr have mostly left the source material and the growth becomes stable again.Moreover, the main strontium complex is dimeric, giving rise to an increased 11 12 13 M. Leskela, Acta Polytech. Scand., Chem. Trrhnol. Metall. Ser., 1990, 195,67. M. Leskela, L. Niinisto, E. Nykanen, P. Soininen and M. Titta, Muter. Res. SOC.Symp. Ser., 1991,222, 315. J. E. Schwarberg, R. E. Sievers and R. W. Moshier, Anal. Chem., 1972,42,1828. growth rate, independent of the reactor temperature. 14 M. Leskela, L. Niinisto, E. Nykanen, P. Soininen and M. Tiitta, Thermochim. Acta, 1991,75,91. The authors thank A-A. Kiisler, K. Kukli and J. Laine-Ylijoki for experimental assistance. The cooperation of the Laboratory of Organic Chemistry at the Helsinki University of Technology (Prof.M. Lounasmaa and Mr. P. Sarkio) in obtaining the mass spectra is gratefully acknowledged. This 15 16 17 18 S. B. Turnipseed, R. M. Barkley and R. E. Sievers, Inorg. Chem., 1991,30,1164. J. G.Hubert-Pfalzgraf, Appl. Organornet. Chem., 1992,6,627. J. Aarik, A. Aidla, A. Jaek, A-A. Kiisler and A-A. Tammik, Acta Polytech. Scand., Chem. Technol. Metall. Ser., 1990, 195,201. J. Aarik, A. Aidla, A. Jaek, M. Leskela and L. Niinisto, Appl. Surf. work was partly supported by HUMAL Electronics Ltd. (Tartu, Estonia) and by the Technology Development Centre (TEKES in Helsinki, Finland; grant 4173/91). 19 20 Sci., 1994,75, 33. J. Aarik, A. Aidla, A. Jaek, M. Leskela and L. Niinisto, to be published. M. Lounasmaa, P. Sarkio, M. Leskela and L. Niinisto, to be published. References 21 M. Leskela, R. Sillanpaa, L. Niinisto and M. Tiitta, Acta Chem. Scand., 1991,45, 1006. 1 2 M. Leskela and L. Niinisto, Muter. Chem. Phys., 1991,31,7. See e.g. Proceedings of Electroluminescence Workshops, Acta 22 F. Rousseau, A. Jain, T. T. Kodas, M. Hampden-Smith, J. D. Farr and R. Muenchausen, J. Muter. Chem., 1992,2,893. Polytechn. Scand., Ser. Appl. Phys., Ph 179, ed. M. Leskela and E. Nykanen, 1990 and Electroluminescence, Proc. 6th Znt. Paper 3/07129D; Received 2nd December, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940401239

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( S), 1245-1247 Investigations into the Growth of AlN by MOCVD using Trimethylsilylazide as Nitrogen Source John Auld," David J. Houltont Anthony C. Jones,*" Simon A. Rushworth" and Gary W. Critchlov@ a Epichem Limited, Power Road, Bromborough, Wirt-a/, Merseyside, UK L62 3QF Institute of Surface Science and Technology, University of Loughborough, Loughborough, Leicestershire, UK LE113TU Thin films of AIN have been deposited by atmospheric-pressure MOCVD using trimethylaluminium (Me,AI) and trimethyl- silylazide (Me,SiN,) as precursors. The films were deposited at 300 or 450"C and had growth rates of up to 3 pm h-' Thin films of aluminium nitride (AlN) have a number of important applications, such as passive barrier layers and substrates in VLSI and ULSI silicon technology, transparent high-temperature windows, and as optical enhancement layers in magneto-optic multilayer structures.In addition, the mixed ternary alloy Al,Ga,-,N has a large potential application in optoelectronic devices operating in the UV-blue spectral region. The development of these various applications is critically dependent on the capability to deposit thin films of AlN at low to moderate substrate temperatures. Metalorganic chemical vapour deposition (MOCVD) is an attractive technique for the controlled deposition of AlN thin films, having the advantages of large area growth capability, excellent conformal step coverage and precise control of layer thickness. The deposition of AlN by MOCVD has tradition- ally been carried out using mixtures of trimethylaluminium (Me,A1) and ammonia (NH,).',, However, the high thermal stability of NH, necessitates the use of high substrate tempera- tures (typically >9OO0C).This leads to the problem of nitrogen loss from the A1N film which is only partially alleviated by the use of excessively high V/IIT ratios (e.g. >2000: 1). In an effort to achieve A1N growth at lower temperatures, a variety of 'single-source' precursors which already contain an (Al-N) bond have been investigated. These include CA1(NR2)312, CHAI(NR,),I, (R =Me,Et),3 (Me2A1NH2)3,4 ( Et2AlN3),,5 and (Me,A1NR2), (R =Pr'),6 from which A1N has been deposited successfully at low to moderate substrate temperatures (400-800 "C). However, these 'single-source' pre- cursors generally have very low vapour pressures (<1 Torr at room temperature), which necessitates the heating of source and reactor inlet lines and the use of vacuum CVD equipment.It is therefore desirable to develop alternative precursors which may be more conveniently utilized in MOCVD, and hydrazine (N2H4) has been used in combination with Me,Al to deposit AlN at 220°C.7 However, N,H4 is highly toxic (TLV,,, =0.01 ppm) and unstable, which is likely to restrict its large-scale application in MOCVD seriously. In contrast, the primary alkylamines tevt-butylamine (Bu'NH,) and iso- propylamine (PriNH2) are of low toxicity and are stable, which permits their manufacture and use on an industrial scale.In addition, Bu'NH, and PtNH, have convenient vapour pressures of 340 Torr (25 "C) and 476 Torr (20 "C), respectively. Both Bu'NH, and Pr'NH, have recently been used in combination with Me,A1 to deposit A1N films by atmospheric-pressure MOCVD in the temperature range 400-600°C.' Although the mechanism of AlN growth from Me,Al-RNH, mixtures has not been established, it was proposed' that directly bonded species such as (Me,AlNHR), are formed in situ in the gas phase prior to layer growth. The recent report' of AIN growth from the single source molecules (Me,AINHR), (R =But, Pr') strongly supports this proposal. The successful growth of AlN by low-pressure C\'D using the trimeric 'single-source' molecule diethylaluminium azide, (Et2A1N3),,5 has encouraged us to investigate methods of forming such species in situ in the vapour phase using volatile Group I11 and Group V components.This approach, which is an extension of our earlier work,* aims to combine the advantages of convenient source temperatures and high growth rates associated with the use of high vapour pressure reagents, with the low growth temperatures associated with single-source precursors. In this paper, we report the successful growth oj A1N by atmospheric-pressure MOCVD using Me,Al in Combination with trimethylsilylazide (Me3SiN3). Experimental General Techniques The reagents used were electronic-grade Me,A1 ( Epichem Limited, vapour pressure =9.7 Torr at 20 "C) and Ue,SiN, (Aldrich; bp =94 "C at 760 Torr).The Me,SiN3 was dcoxygen- ated prior to use by repeated freeze-pump-thaw cycles in a nitrogen atmosphere. Proton nuclear magnetic resonance (lH NMR) data were obtained on a Bruker WM 250 spectrometer operating at 250 MHz, and mass spectral data were obtained using a VG Pegasus, quadrupole mass spectrometer operating at an ionization energy of 70 eV. Auger electron spectroscopy (AES) was carried out on a Varian scanning Auger spectrometer. The atomic c omposi-tions qupted are from the bulk of the film (depth from surface >2000 A) and were obtained by combining AES with sequen- tial Ar +-ion bombardment until comparable compositions were obtained for consecutive data points. Film thicknesses were estimated by the time taken to sputter through the layer using Ar +-ion bombardment.Scanning electron microscopy (SEM) was performed on a JEOL JS 35 CM scanning electron microscope. AlN Film Growth The A1N films were deposited at atmospheric pressure in a simple cold-wall horizontal quartz reactor (Elecrro Gas Systems Ltd.) using radiant substrate heating. The siLbstrates used were Si( 11 1) single-crystal wafers and these were cleaned (20% nitric aciddeionized water), degreased with acetone and dried before use. The Me,Al and Me,SiN, sources were operated at room temperature and were mixed in a 'T-piece' at the reactor inlet. A full summary of growth conditions is given in Table 1. J. MATER. CHEM., 1994, VOL. 4 Table 1 Growth conditions used to deposit A1N films from Me,Al-Me,SiN, mixtures' run no.H2 carrier gas flow H, carrier gas flow Me,SiN, :Me,Al' growth growth through Me,A1 (sccm) through Me,SiN, (sccm) temp./"C rate/pm h -lb 252 35 20.0 1.3 300 253 35 15.6 0.99 450 255 35 81.5 4.99 450 "Cell pressure 760 Torr; Me,Al and Me,SiN, sources at 20 "C; substrates Si( 111) single crystal wafers. bFilm thickness estimated from AES sputter time. 'Based on an estimated Me,SiN, vapour pressure of ca. 4.3 Torr at 20 "C. Table 2 AES analysis of A1N films grown on Si( 11 1) using Me3A1 and Me,SiN, film no. composition (atom Yo) A1 :N ratio A1 N C 0 Si 252 41.6 38.8 11.0 8.6 0.0 1.07 253 44.9 40.9 11.4 2.9 0.0 1.10 255 45.4 43.9 9.8 1.1 0.0 1.04 Table3 AES depth profile through a typical A1N film (255) grown using Me,Al-Me,SiN, mixtures sputter time composition (atom %) /min A1 N C 0 ~ ~~ ~ ~ ~~ ~~~ 1 36.9 0.0 0.0 63.1 21 33.6 8.3 5.1 53.0 31 42.6 36.8 8.4 12.3 60 44.8 43.5 9.0 2.7 68 45.4 43.7 9.8 1.1 Results and Discussion A1N films were successfully deposited using Me,Al and Me3SiN, at substrate temperatures of 300 and 450°C.The atomic composition of the films was determined by AES and the data are summarized in Table 2. These data show that the films have an A1 :N ratio close to unity, although relatively large levels of oxygen and carbon are present. Significantly, silicon was not detected in the films.The high strength of the aluminium-oxygen bond makes it difficult to exclude oxygen from A1N films deposited in relatively unsophisticated MOCVD equipment of the type used in this study. Comparable levels of oxygen contamination (4-5 atom%) were detected by AES in AlN films grown using (Me2A1NH2),, and this was attributed to trace oxygen present in the MOCVD reactor. Support for this proposal is provided by an AES profile through a typical film, see Table 3, which shows that the oxygen content decreases greatly with increas- ing depth, suggesting that oxidation has occured after film deposition and not during film growth. The carbon contamination in the films is likely to be intrinsic to the use of a methyl-based A1 precursor, as pre- viously observed in the deposition of A1 metal films" and AlGaAs layers'' by MOCVD.Comparable levels of carbon (2.7-17 atom%) have also been detected by AES in AlN films grown using Me,A1-RNH, mixtures (R =But, Pr').' The AlN films were extremely hard and scratch resistant and demonstrated specular surface morphology. SEM data for a 1 pm thick film grown at 450°C, see Fig. 1, indicate that the film is amorphous with an average grain size of <0.1 pm. The mechanism of AIN growth from mixtures of Me,A1 and Me,SiN, has not been fully established. However, the very low growth temperatures (300-450°C) and low V/IIT ratios (approx. 1:1 to 5: 1) used successfully in the present study strongly suggest that a 'directly bonded' (Al- N) species is the active precursor to A1N deposition.It is significant that Si is not detected by AES in the films and this indicates that the (Me,Si) fragment is efficiently removed from the growth zone during film growth. Information concerning the possible growth mechanism has been obtained by the ex situ addition of Me,SiN, (9.2 g, 0.08 mol) to Me,Al (5.9 g, 0.08 mol) at room temperature. This resulted in a volatile liquid product which was isolated by distillation in uucuo, and which had a mass spectrum identical with an authentic sample of tetramethylsilane (Me,Si) [rnlz; 88 (Me,%+), 74 (Me,SiH+), 73 (Me,Si+), 45 (MeSiH,+)] and which showed no evidence of any residual Me,SiN, [rnlz; 115 (Me,SiN,+), 100 (Me2SiN3+) 73 (Me,%+), 45 (MeSiH,+), 28 (N2+)].The lower volatility liquid residue had an identical 'H NMR spectrum to that reported" for Me,AlN, (6 Al-CH, =0.53) which is signifi- cantly different from the 'H NMR spectrum of Me,Al Fig. 1 Scanning electron micrograph of an AlN film (no. 255) grown at 450 "C on Si( 111) (magnification x 10000) J. MATER. CHEM., 1994, VOL. 4 (6 A1-CH, =0.3). A similar displacement of an alkyl group from an organometallic A1 compound has been reported', in the room-temperature reaction between Me,AlI and Me,SiN,, which leads to methylaluminiumiodo azide, MeAlIN, . These data strongly suggest that Me3A1 and Me,SiN, will react similarly in the gas phase, either at room temperature, or in the hot boundary layer, to form Me,AlN, and the volatile, relatively stable species tetramethylsilane (Me&).Tetramethylsilane is unlikely to be significantly pyrolysed at the low growth temperatures used in the present study and thus Si will be transported effectively away from the growth zone. Subsequent pyrolysis of Me,AlN, leads to the deposition of AlN, although in the absence of any added gettering agent, the presence of methyl radicals strongly bound to an A1 atom on or near the growth surface, followed by their subsequent decomposition, will lead to the significant carbon contami- nation observed in the A1N films. Further support for this proposed mechanism is provided by the observation that pyrolysis of the Me,SiN, precursor alone failed to deposit a film at substrate temperatures below 550 "C, indicating that a more active species which already contains a direct (Al-N) bond is the precursor to A1N deposition.At 600 "C a film containing both nitrogen (49.9 atom%) and silicon (22.5 atom%) was deposited, which indicates that Me,SiN, is unlikely to be suitable for AlN growth at more elevated temperatures. It was thought that mixtures of trimethylgallium (Me,Ga) and Me3SiN, might prove suitable for the deposition of GaN by MOCVD. However, AES data indicate that high levels of silicon (18-35 atomyo), in addition to oxygen and carbon, are present in films deposited at 450°C. This suggests that a different growth mechanism is in operation, and that the (Me,Si) group is no longer eliminated efficiently from the growth surface.Further studies are in progress aimed at elucidating these various growth mechanisms. Conclusions AlN films have been deposited by atmospheric-pressure MOCVD using Me,Al in combination with Me,SiN,. High growth rates of 3 pm h-' were obtained at low substrate temperatures (300-450 "C).The films contained relatively high levels of carbon and oxygen impurities, although silicon was notably absent from the films. A growth mechanism involving the formation of intermediate gas-phase species such as Me,A1N3 has been proposed and is supported by tdie ex situ addition of Me,SiN, to Me,Al. This work was supported by the Department of Trade and Industry under the LINK/ATP initiative and the Teaching Company Scheme. David Houlton is a Teaching Company Associate (University of Keele).References 1 M. Morita, S. Isogai, N. Shimizu, K. Tsubouchi and N. Mikoshiba, Jpn. J. Appl. Phys., 1981, 19, L173. 2 M. Morita, M. Useugi, S. Isogai, K. Tsubouchi and N. Mikoshiba, Jpn. J. Appl. Phys., 1981, 20, 17. 3 Y. Takahashi, K. Yamashita, S. Motojima and K. Singiyama, Surf. Sci., 1979,86, 238. 4 L. V. Interrante, W. Lee, M. McConnell, N. Lewis and E. Hall, J. Electrochem Soc., 1989, 136,472. 5 K. L. Ho, K. F. Jensen, J. W. Hwang, W. L. Gladfelter and J. F. Evans, J. Crystal Growth, 1991,107,376. 6 D. C. Bradley, D. M. Frigo and E. A. D. White, Euro. Rat. Appl., 1989 EPO 331 448. 7 M. Mizuta, S. Fujieda, T. Jitsukawa, Y. Matsumoto, Proc. Znt. Symp. GaAs and Related Compounds, Las Vegas, Nevida, 1986, IOPP, Bristol, 1987. 8 A. C. Jones, J. Auld, S. A. Rushworth, E. W. Williams, P. W. Haycock, C. C. Tang and G. W. Critchlow, Adu. Muter., 1994,6,229. 9 M. M. Sung, H. D. Jung, J. K. Lee, S. H. Kim, J. T. Park and Y. Kim, Bull. Korean Chem. Soc., 1994,15,79. 10 D. R. Biswas, C. Ghosh and R. L. Layman, J. Electroc hem. Soc., 1983,130,234. 11 T. F. Kuech, E. Veuhoff, T. S. Kuan, V. Deline and R. Potemski, J. Crystal Growth, 1986,77,257. 12 J. Muller, 2.Nuturforsch., 1979,34, 531. 13 N. Wiberg, W. C. Joo and H. Henke, Inorg. Nucl. Cllem. Lett., 1967,3,267. Paper 4/02530J; Received 28th Aid, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401245

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(8), 1249-1253 Growth of ZnO by MOCVD using Alkylzinc Alkoxides as Single-source Precursors John Auld," David J. Houlton," Anthony C. Jones,*" Simon A. Rushworth," M. Azad Malik,b Paul O'Brien*b and Gary W. Critchlow" " Epichem Limited, Power Road, Bromborough, Wirral, Merseyside, UK L62 3QF Department of Chemistry, Queen Mary & Westfield College, University of London, Mile End Road, London UK El 4NS " Institute of Surface Science and Technology, University of Loughborough, Loughborough, Leicestershire, UK LE77 3TU Thin films of ZnO have been grown by low-pressure MOCVD using methylzinc isopropoxide, MeZn(OPi), and methylzinc tert-butoxide, MeZn(OBu*), in the absence of an added oxygen source. The films were grown on to glass substrates in the temperature range 250-400 "C with growth rates of between 0.2 and 4.4 pm h-'.Thin films of zinc oxide (ZnO) have a number of important applications in heterojunction solar cells, surface acoustic wave devices, optical waveguides, varistors and gas sensors. ZnO films have been grown by a variety of techniques including, radiofrequency or magnetron ~puttering,'~~ spray pyroly~is,~atomic layer epitaxy' and metal-organic chemical vapour deposition (MOCVD).6-1' Of these techniques, MOCVD has considerable potential due to its capability for large area growth, precise control of doping and film thickness, and superior conformal step coverage. Traditional methods for the growth of ZnO by MOCVD have involved the pyrolysis of diethylzinc (Et2Zn)6>8.10or dimethylzinc (Me,Zn)' in the presence of oxygen and/or H,O.However, a severe premature reaction results in the deposition of particulates upstream from the susceptor. Consequently, alternative, less reactive oxygen sources have been investigated, including C02,9 N20,9,12NO," and oxygen-containing heterocycles such as furan.', Unfortunately, these have generally proved unsatisfactory, leading to only very low ZnO growth rates. However, ZnO has been grown successfully, without sig- nificant prereaction, by the use of Et,Zn14 or the tetrahydro- furan adduct of dimethylzinc, Me,Zn( THF)15in combination with alcohols such as methanol, ethanol and tert-butyl alcohol. It was propo~ed'~,'~ that zinc alkoxides or alcohol adducts are formed as intermediates in the gas phase, although no evidence of the exact nature of these species was presented.It has been established16 that the addition of alcohols (R'OH) to R,Zn compounds, even when the alcohol is present in excess, leads to the formation of alkylzinc alkoxides of the type RZn(OR'), without displacement of the second alkyl group. In this paper we have confirmed that the reaction between Me,Zn(THF) and isopropyl alcohol (Pr'OH) or tert-butyl alcohol (Bu'OH) leads to the formation of methylzinc isopropoxide [MeZn(OPr')] and methylzinc tert-butoxide [MeZn(OBu')], respectively. Although such compounds are tetrameric in the solid state,16 both MeZn(0Pr') and MeZn(0Bu') have proved sufficiently volatile for use as single- source precursors for the growth of ZnO by MOCVD.Experimental General Techniques Inductively coupled plasma emission spectroscopy (ICP-ES) was carried out on a Thermoelectron Plasma 300 spectrometer and proton nuclear magnetic resonance ('H NMR) data were obtained on a Bruker WM250 spectrometer operating at 250 MHz. Auger electron spectroscopy (AES) was carried out on a Varian scanning Auger spectrometer and scanning electron microscopy (SEM) data were obtained on a JEOL JS35CM scanning electron microscope. Film thicknesses were estimated by the time taken to sputter through the layer using Ar+-ion bombardment. Sheet resistance measurements were obtained using a voltmeter-based four-point probe, and the X-ray powder patterns were measured on a Philips X-ray powder diffractometer mod$ PW 1050/25 using Cu-Ka radiation of wavelength 1.5418 A.Precursor Synthesis and Characterization MeZn(0Pr') and MeZn(0Bu') were synthesized by modifi- cation of literature procedure^'^.'^ from the reaction between Me,Zn(THF) adduct (1mol equiv.) and the respective alcohols, isopropyl or tert-butyl alcohol (2 mol equiv.). Residual volatiles were removed in uucuo to leave. in each case, white crystalline solids which could be sublimed in uucuo (0.1 Torr) at temperatures >80 "C. The methylzinc dkoxides were characterized using 'H NMR and ICP-ES. MeZn(0Pr'): 'H NMR (C6D6) 6: 0.21, 3 H, S, CH,Zn [OCH(CH,),]; 1.2, 6 H, d, CH,Zn[OCH(CH,),]; 3.98, 1 H, multiplet, CH,Zn[OCH(CH,),].ICP-ES Zn conlent (YO): found, 46.0; calc., 46.9. MeZn(OBut): 'H NMR (C6D6) 6: 0.12, 3 H, S, CH,Zn [OC(CH,),]; 1.38, 9 H, s, CH,Zn[OC(CH,),]. ICP-ES Zn content (%): found, 41.9; calc., 42.6. The ICP-ES and 'H NMR data are fully consistent with the proposed formulation MeZn(0R) and the 'H NMR data for both compounds agree well with published ZnO Film Growth ZnO films were deposited from MeZn(0Pr') at low pressure (15 Torr) in a cold-wall horizontal quartz reactor (Electro Gas Systems Ltd.) using radiant substrate heating. Soda lime glass substrates were used and these were cleaned (20% nitric aciddeionized water), degreased with acetone arid dried before use. The MeZn(0Pr') source was contained in a stainless-steel bubbler heated to 70°C using N, carrier gas (10 SCCMT).This led to ZnO growth rates of between 0.2 and 1.1 pm h-'. Under these conditions 'H NMR analysis of the precursor before and after use showed no evidence of decomposition. t 1 SCCM = 1 standard cm3 min-' Growth experiments using MeZn(0Bu') were carried out in a home-made cold-wall low-pressure reactor (lop2Torr) described in detail elsewhere.18 Soda lime glass substrates were used and these were heated by the action of a quartz halogen lamp on a graphite susceptor. The MeZn(0Bu') precursor was held in a tube furnace17 heated at either 80 or 150 "C, and growth rates as high as 4.4 pm-' were obtained at a source temperature of 80°C. 'H NMR analysis of the precursor showed no evidence of any decomposition at 80 "C, but indicated that significant decomposition had occurred at 150°C.A full summary of growth conditions is given in Table 1. Results and Discussion ZnO films were grown successfully from both MeZn(0Pr') and MeZn(0Bu') in the temperature range 250-350 "C, with-out the addition of a separate oxygen source. No growth was observed at 400 "C using MeZn(0Pr'). The films showed strong interference colours, characteristic of a high-refractive-index material and were generally slightly dark and absorbing. The films were hard and scratch-resistant with good adhesive properties and in the 'Scotch Tape Test''' the films remained intact as the tape was peeled away from the film. The atomic composition of the films was determined by AES and the data are summarized in Tables 2 and 3.The values quoted are from the bulk of the film and were obtained by combining AES with sequential Ar +-ion bombardment until comparable compositions were obtained for consecutive data points. Compositions were determined using experimen- tally derived relative sensitivity factors based on a ZnO reference material. These data showed that the ZnO films are non-stoichiometric with a slight excess of Zn present, demon- strating Zn :0 ratios in the range 1.01-1.09. The excess of Zn leads to the dark/absorbing character of the films, which were Table 1 Growth conditions used to deposit ZnO from alkylzinc alkoxides (a) MeZn(0Pr') cell pressure substrates 15 Torr soda lime glass N, carrier gas flow 10 SCCM deposition temperature 250-350 "C MeZn(0Pr') source temperature typical growth rates 70 "C 0.2-1.1 pm h-' (b)MeZn(0Bu')" reactor pressure lop2Torr substrates soda lime glass deposition temperature 250-400 "C MeZn (OBu') source temperature typical growth rates 80 or 150 "C 3.7-4.4 pm h-l (source temperature =80 "C) "Carrier gas not used.J. MATER. CHEM., 1994, VOL. 4 Table 3 AES analysis of ZnO films grown on glass from MeZn(OBut) precursor (source temperature 150 "C) substrate etch sample temperature/"C time/s 1 250 50 2 300 90 3 350 90 4 400 150 composition (AES) (atom YO) Zn 0 C 50.3 49.7 0.0 52.2 47.8 0.0 51.5 48.5 0.0 50.2 49.6 0.2 Zn:O 1.01 1.09 1.06 1.01 generally conducting with sheet resistances in the range 2 x lo6 to 20 x lo6R 0-'and with resistivities of between 300 and 900 R cm.The AES data clearly indicate that both MeZn(0Pr') and MeZn(0Bu') undergo an efficient intramolecular decompo- sition process to deposit ZnO, without the need for an external oxygen source. This is in marked contrast to earlier work2' in which it was not found possible to deposit ZnO from the Me,Zn( 1,4-dioxane) adduct in the absence of an added oxygen source, as the oxygen-containing ligand was lost from the adduct on pyrolysis and Zn metal was deposited. Carbon was either not detected in the ZnO films, or was only present close to the estimated AES detection limit of 0.2% and this can be attributed to the formation of a cyclic six-centre transition state (Fig.1) previously proposed as an intermediate in the thermal decomposition of RM (OR') compounds2' (M =Al, Mg, Zn). This involves the abstraction, by an incipient carbanion, of a P-hydrogen from the alkoxy group to eliminate methane, alkene and form ZnO. This decomposition may occur in the hot boundary layer adjacent to the substrate, or more likely, during heterogeneous pyrolysis of the alkylzinc alkoxide on the substrate surface. Scanning electron micrographs of ZnO films deposited at various substrate temperatures from MeZn(0Pr') and MeZn(0Bu') are shown in Figs. 2 and 3. At 250°C the film grown using MeZn(0Pr') shows well ordered columnar crys- tallites of ca.0.1 pm diameter. At 300°C the film is smooth and relatively featureless, whilst at the higher growth tempera- ture of 350°C the film surface becomes disordered with orange-peel-like crystallites of ca. 0.2 pm dimensions. A variety of surface morphologies are also observed in films grown using MeZn(OBu'), see Fig. 3. At 250°C, the film shows an irregular surface with flake-like crystallites of MeZn(0Pi) (R = H) MeZn(0Bd)(R = CH,) Fig. 1 Mechanism proposed for the heterogeneous pyrolysis of alkylzinc alkoxides Table 2 AES analysis of ZnO films grown on glass from MeZn(0Pr') precursor substrate etch composition (AES) (atom YO) sample temperature/"C time/s Zn 0 C S Zn:O 188 250 90 51.0 49.0 0.0 0.0 1.04 187 300 150 51.1 48.8 0.0 0.0 1.05 190 300 150 50.4 49.3 0.4 0.0 1.02 192 350 150 50.7 49.3 0.0 0.1 1.03 186 3 50 90 51.4 48.4 0.0 0.1 1.06 J.MATER. CHEM., 1994, VOL. 4 Fig.2 Scanning electron micrographs of ZnO films grown on glass using MeZn(0Pr') (source temperature =70 "C).(a)Substrate tempera- ture 250 'C;film thickness 0.19 pm; (b) substrate temperature 300 "C; (c) substrate temperature 350 "C; film thickness 1.65 pm. ca. 0.4 pm diameter [cJ: Fig. 2(c)]. At 300°C the film displays columnar crystallites of ca. 0.1 pm dimension, whilst at a growth temperature of 350 "C, the columnar crystallites have become larger and bulbous with a grain size of ca. 1 pm.The X-ray diffraction (XRD) data for a film grown at 300 "C on soda-lime glass using MeZn(OBut) are illustrated in Fig. 4. The data indicate that the ZnO has crystalliFed in hexagqnal form, wjth a space group P63/mc; a=3.253 A; b= 3.253 A; c =5.213 A; a =90"; fi= 90"; 6 =120". A comparison with standard ASTMS data for ZnO, see Table 4, shows that Fig. 3 Scanning electron micrographs of ZnO films grown on glass using MeZn(OBut) (source temperature =80 "C). (a) SubstTate tem- perature 250 "C; film thickness 5.5 pm; (b) substrate temperature 300°C; film thickness 4.4 pm; (c) substrate temperature 350 C. a number of reflections are absent and indicates that the crystals are oriented in the 110 direction (i.e. c axis parallel to the substrate surface).These data contrast strongly with those obtained for ZnO grown at 300-400°C on Si or glass using Et2Zn/0,10 or Et2Zn/ROH,l4 in which the XRD pattern was dominated by the (002) peak, indicating that the c axis is perpendicular to the substrate. The results presented herein provide a valuable insight into the gas-phase chemistry occurring during the growth of ZnO from R2Zn compounds and alcohols. The present study and previous work16 have shown that the addition of an alcohol (R'OH), even in excess, leads to the elimination of only one alkyl group from R2Zn and to the formation of alkylzinc alkoxides RZn(0R'). Therefore, in the growth of ZnO by MOCVD from mixtures of Et,Zn/RC)H14 or Me,Zn( THF)/ROH" the probable gas-phase intermediates are RZn(OR'), of which the tert-butoxide derivative has been reported2' to exist as an oligomer in the vapour phase.J. MATER. CHEM., 1994, VOL. 4 2@/degrees Fig. 4 X-Ray diffraction data for a 3 pm ZnO film grown at 300 "C on soda-lime glass from MeZn(0Bu') (source temperature 80 "C) Table4 X-Ray diffraction results for ZnO prepared from MeZn(0Bu') compared with standard ASTMS data for ZnO powder experimental ASTMS d/A; intensity (YO) h k 1 d/A; intensity (YO) h k 1 2.80( 5) 2.60(3) 2.46( 11) -1.62( 100) -1.37(3) 1.36(1) 100 002 101 110 112 201 2.82( 55) 2.61(41) 2.48( 100) 1.9 1 (24) 1.63( 36) 1.48 (34) 1.41(5) 1.38( 29) 1.36( 14) 1.30( 2) 10 00 10 10 1 1 10 20 1 1 20 00 0 2 1 2 0 3 0 2 1 4 1.24(5) 1.18( 3) 20 10 2 4 1.09(11) 20 3 1.06( 4) 21 0 1.04( 12) 1.02( 7) 0.99(5) 0.98 (9) 0.96( 2) 21 11 21 1 0 20 1 4 2 5 4 Conversely, associated molecules of the type [MeZn(OR)], (R =Pr', But) can be pre-synthesized, as reported herein, and used directly as precursors to ZnO.It has previously been reported', that trace amounts of water play a crucial role in the deposition of ZnO from Et,Zn/ROH mixtures. The most transparent films were obtained when lop3mol H20 were present in the alcohol and it was proposed that the role of water is not limited to initial nucleation processes, but significantly affects subsequent dis- sociation and reconstruction processes.14 Support for this is provided by the present work in which absorbing films were obtained using the solid crystalline compounds MeZn(0Pr') and MeZn(OBu'), from which trace H20 is clearly absent (see 'H NMR data).Conclusions ZnO films have been grown successfully by MOCVD using the single-source precursors MeZn(0Pr') and MeZn(OBut) without any added oxygen source. Both precursors are stable at source temperatures of 70-80°C. A mechanism for ZnO growth has been proposed which involves the formation, and subsequent decomposition, of a six-centre transition state complex. This work was supported by the DTI under the LINK/ATP initiative and the Teaching Company Scheme. D.J.H. is a Teaching Company Associate (University of Keele). We are grateful to Professor E. W. Williams, Dr. P. W. Haycock (University of Keele) and to Dr.R. J. M. Griffiths (Metals Research Semiconductors Ltd., UK) for useful discussions. References 1 T. Hata, T. Minamikawa, 0. Morimoto and T. Hada, J. Cryst. Growth, 1979,47, 171. 2 J. 0.Barnes, D. J. Leary and A. G. Gordon, J. Electrochem. Soc., 1980,7, 1636. 3 M. S. Raven, M. H. T. Al-Sinaid, S. J. T. Owen and T. L. Transky, Thin Solid Films, 1980, 71, 23. 4 J. Aronovich, A. Oritiz and R. H. Bube, J. Vuc.SL-1'. Technol., 1979, 16,994. 5 M. Tammenmaa, T. Oskinen, L. Hiltunen. M. Leska and L. Ninisto, Thin Solid Films, 1985,124, 125. W. Kern and R. C. Heim, J. Electrochem. Soc., 1970,117,562. F. T. J. Smith, Appl. Phys. Lett., 1983,43, 1108. A. P. Roth and D. F. Williams, J. Appl. Phys., 1981,52,6685. C. K. Lau, S. K. Tiku and K.M. Lakin, J. Electrochem. Soc., 1980, 127, 1843. 10 S. Ghandi, R. J. Field and J. R. Shealy, Appl. Phys. Lett., 1980, 37,449. J. MATER. CHEM., 1994, VOL. 4 1253 11 A. K. Gyani, 0.F. 2. Khan, P. O’Brien and D. S. Urch, Thin Solid 19 W. L. Gladfelter, D. C. Boyd and K. F. Jensen, ChcBm. Muter., Films, 1989, 182, L1. 1989, 1,339. 12 13 14 15 16 R. Solanki and G. J. Collins, Appl. Phys. Lett., 1983,42,662. P. J. Wright, R. J. M. Griffiths and B. Cockayne, J. Cryst. Growth, 1984, 66, 26. S. Oda, H. Tokunaga, N. Kitajima, J. Hanna, 1. Shimizu and H. Kokada, Jpn. J. Appl. Phys., 1985,24, 1607. T. Kaufmann, G. Fuchs, M. Webert, S. Frieske and M. Gackle, Crystallogr. Res. Technol., 1989,24,269. G. E. Coates and D. Ridley, J. Chem. SOC., 1965, 1870. 20 21 22 B. Cockayne, P. J. Wright, A. J. Armstrong, A. C. Jones and E. D. Orrell, J. Cryst. Growth, 1988,91, 57. E. C. Ashby, G. F. Willard and A. B. Goel, J. Org. Chem., 1979, 44, 1221. B. Adler, A. Lachowicz and K. H. Thiele, 2.Anorg. Allg. Chem., 1976,423,27. 17 18 E. A. Jeffery and T. Mole, Aust. J. Chem., 1968,21, 1187. M. A. Malik and P. O’Brien, Adv. Mater. Opt. Electron., 1994, Paper 4/013711; Received 8th Miirch, 1994 3. 171.

ISSN:0959-9428    

DOI:10.1039/JM9940401249

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(8), 1255-1258 Characterization of Ru0,-based Film Electrodes by Secondary Ion Mass Spectrometry Sergio Daolio," Bruno Facchin,a Cesare Pagura: Achille De Battisti,*b Andrea Barbierib and Janos Kristof a lstituto di Polarografia ed Elettrochimica Preparativa, Consiglio Nazionale delle Ricerche, Corso Stati Uniti 4, 1-35020 Padova, Italy Dipartimento di Chimica dell' Universita, via L. Borsari 46, 1-44 100 Ferrara, Italy Department of Analytical Chemistry of the University Egyetem 70,8201 Veszprem, Hungary Concentration depth profiles of Ti-supported Ru0,-TiO, films, prepared by pyrolysis of ruthenium(i1i) chloride hydrate and titanium diisopropoxide bis-pentane-2,4-dionate, have been studied by secondary ion mass spectrometry (SIMS).The '"Ru:~~T~ ion intensity ratio vs. the nominal concentration of ruthenium oxide was followed for samples obtained from precursors dissolved in different solvents. The dependence on bombarding time of ion intensity profiles for other species, such as 0 -(m/z= 16), OH- (m/z= 17), 46TiO+(m/z =62), '%OH+(m/z=63) was also studied. The mixed oxide films were characterized by cyclic voltammetry. The results of the combined research support the idea that microstructural and chemical impurities are concentrated in the outermost part of the film. This phenomenon seems to depend on the composition of the solvent used for the precursor salts. The mechanism of charging of oxide electrodes has been whose energy ranged between 1and 5 keV and current density studied in several papers, for the case of anodically prepared14 between 3 and 30mAcm-2.The rate of sputtering of the or thermally prepared7-12 films. According to the literature, electrode materials was calculated to be 100-200 A min-'. in this process important roles are played both by electron The calibration curve of ruthenium ionic current was obtained and ion transport in the electrode materials. This is common from samples prepared by dropping solutions of precursor to many different oxide systems. From a quantitative point of salts in the chosen ratios onto platinum supports and then view, the transport properties themselves depend on the evaporating the solvent at room temperature. The same microstructure and surface morphology and consequently on bombardment conditions used for the analysis of the electrode a number of parameters of the preparation method.For the films were applied. The electrode films were prepared accord- case of thermally prepared IrO, and RuO,, which are of ing to a well established procedure, allowing good rcproduc- interest for their catalytic properties, correlations between the ibility of the film proper tie^.,^ Layer-by-layer growth of the conditions of the pyrolysis of the precursor salts, the annealing mixed oxide films was carried out by depositing small .imounts of oxide products and the anodic voltammetric charge q*, of precursor salt mixture, firing at 400 "C under oxj gen and For 1r02, the influence of the nature of repeating the procedure a number of times sufficient to allowhave been de~cribed.~ the solvent of the precursor salt on the electrochemical the attainment of the desired loading (corresponding in our properties of the final material has also been considered." case to a thickness of 300-400 nm).Mirror-finished citanium Correlations between different parameters of the preparation supports were used for the oxide films. In order to compare process and physicochemical properties of one-component the influence of the preparative conditions on the final elec- systems have been In particular, it has been trode properties, three groups of samples were prepared. Two shown that enrichment with hydroxylated species occurs of them, A and B, differ in the nature of the precursor salt of across the first tens of nm below the surface,16 involving the ruthenium oxide.In the case of group A, RuCl3*3H2O Isopropylsegregation of microstructural defects in this region. This was chosen and for group B, Ru(NO)(NO~)~. study has been extended to Ru0,- and Ir0,-based mixed alcohol was used as the solvent of the precursor mixtures in In this case, the role of the film composi- both cases. Group C samples were prepared from the same oxide electrode~.'~-~~ tion and of the nature of the precursor salt has been con- precursor salt mixture as group A, using an aqueous organic sidered. Enrichment with Group VA or IVA metal oxide mixture acidified with hydrochloric acid. In all cases titanium species (e.g. TaV,l7 Ti'v18,19 ) could be ascertained, also implying diisopropoxy bis-pentane-2,4-dionate was used as the precur- an increase in the number of defects in the near-surface region.sor of titanium oxide. The cyclic voltammetry experiments This paper presents the results of a further investigation on were carried out making use of a Solartron 1286 electrochemi- this subject, carried out by SIMS on Ru0,-based mixed oxide cal interface. The electrodes were tested in 1 mol dm-3 HC104, electrodes. SIMS has already proved to be a reliable tool for in the potential range 0-1.2OV us. SCE. Different potential depth profiling of main and secondary components in mixed scan rates in the range 0.010-0.300 V s-l were used. oxide electrodes.21,22 An electrochemical study of the electrode coatings has been performed by cyclic voltammetry, comp- Results and Discussion lementary to the SIMS characterization, and a measure of their catalytic activity.8 Fig.1 shows typical SIMS patterns (Ar' beam energy 3 keV) for a sample belonging to group A, containing 40% of RuO,. The positive-ion mass spectrum [Fig. 1(a)] shows the presence Experimental of Ti', TiO', TiOH+ and Ru' ions. Other oxides and The SIMS equipment was custom-built and has been described isotopic clusters are also present, with lower relative abun- The samples of electrode materials were intro- dances. TiO,f, TiOOH', TizO+, Ti,Oi, Ti;, RuO', KuOH+,else~here.~~'~~ +duced by a fast insertion lock and exposed to an Ar' beam Ru: , Ru20 , TiRu+, TiRuO 'and TiRuOl were identified 1256 x 10 80-l I/100 150 200 250 m/u Fig.1 Secondary positive (a) and negative (6)ion mass spectra of a group A sample (40% of RuO,). The spectra were obtained by an 800 nA, 3 keV Ar' primary current bombardment 01 I 0.3 0.6 1 2 3 primary ion energylkev Fig. 2 0-:OH-ion intensity ratio us. primary ion energy (same sample as in Fig. 1) with the aid of an isotopic pattern simulation program.26 The contributions of different ionic species to partially superim- posed isotopic patterns were thus estimated. In the negative- ion mass spectrum [Fig. l(b)], the most interesting peaks are due to the ionic species H-, 0-, OH-, O,, C1-, Ti02-, Ti03-, Ti020H-, Ti02-, Ti04-, Ru0,-and Ru03-, whereas other low-mass negative ions are probably due to the sample treatment.Electrode films with ruthenium concen- trations between 5 and 90mol% gave similar spectra and only the relative abundance of different isotopic patterns changes with the coating composition, the nature of the precursor or the solvent composition. The study of selected ionic species showed the destructive effect of higher-energy primary beams. Fig. 2 shows a plot of the 0-:OH- ion intensity ratio us. primary ion energy. The ion intensity ratio increases with the bombarding energy. This information has to be borne in mind for the choice of the experimental conditions for depth-profile studies. The depth-profiling of ruthenium and titanium ions in the electrode films were studied first. For this purpose, the depen- dence of the lo2Ru :48Ti ion intensity ratio on sputtering time was determined.A calibration curve, allowing the direct J. MATER. CHEM., 1994, VOL. 4 conversion of ion current ratios into Ru atom fractions (RuO, molar fractions), was obtained as described in the Experimental section. Fig. 3 shows the good correlation between the SIMS data and the nominal composition of the samples. Fig. 4(u) shows a typical concentration depth profile for a group A sample (20 mol% Ru02). An enrichment with titanium oxide species is evident, in agreement with results obtained with other methods for other RuO2-based''~'* or Ir0,-based electr~catalysts.'~ The depth profiles of group B electrodes show larger amounts of noble-metal species in the near-surface region, compared with group A electrodes.As shown in Fig. 4(b), for a group C electrode (10mol% Ru02), surface segregation of ruthenium oxide species occurs when an acidified aqueous organic solvent is used for the preparation of the RuCl,.3H20 solutions. As previously mentioned, other authors ascertained important effects of the composition of the solvent of precursor salts on oxide film morphology and electrochemical behav- io~r.'~,,~In our case, the different acidity of the two solutions can change the mechanism of formation of gel precipitates during the stage of solvent evaporation (see Experimental). 0.1J 0.09 -E [r 0.06, 0.03 1 10 300 50 -Ru (mol "/o) Fig. 3 lo2Ru:48Ti ion intensity ratio us. Ru concentration (mol%) for mixed precursor salt deposits.Arf beam energy 3 keV. 0.043 2 0.011cn 1r: 1 .-0 100 200 300 400 500 100 200 300 400 500 t (arb. units) Fig. 4 Typical concentration-depth profiles (Ru+ :Tif). (a) Samples of group A (isopropyl alcohol precursor solution). (b) Samples containing 10 and 20mol% of RuO, from acidic aqueous-organic precursor solution. J. MATER. CHEM., 1994, VOL. 4 1257 t Fig.5 Ion intensity profiles (arbitrary scale for the ordinate) us. bombardment time of: 0-(m/z=16), Ti' (m/z=46), 46TiO+ (m/z= 62), "TiOH' (m/z=67), obtained for an electrode containing 20 mol% of ruthenium dioxide, belonging to group A. Primary Ar+ beam: 4 keV and 25 mA cm-2. During subsequent stages (ageing, pyrolysis), some of the features of the precipitate are retained.Much experimental work is still required to clarify this point. Besides the distribution of the main components, the degree of hydration of the films was followed using the OH-, 0-, TiO', TiOH+, Tif profiles as monitoring parameters. Fig. 5 shows that the 46TiOf :"TiOH' (m/z=62 and 67) ion inten- sity ratio remains constant during the sputtering time and OH-ion current profiles (m/z=17) exhibit an increase towards asymptomatic values. This indicates that hydroxylated metal species are not directly linked with the OH -species which probably originate from trapped water molecules. A systematic study of electrodes of different composition shows that the part of the film containing hydroxylated metal species is larger for electrodes containing 20 and 30 mol% RuO,. The influence of the solvent composi- tion on the TiOH+:TiO+ ion intensity ratio is relatively small.In any case, the shape of concentration depth profiles of metal ions and hydroxylated species in all the samples studied implies a defective microstructure in their outermost part. Further information on this aspect has been obtained, in situ, by cyclic voltammetry. A typical voltammogram is shown in Fig. 6 which was obtained from a sample belonging to group A (30 mol% RuO,), in 1 mol dm-3 HC104. A single pair of peaks (anodic/cathodic) is observed and is due to the solid state redox couple Ru'~/Ru'''.~ Making use of the baseline shown in the figure,20 to a first approach the faradaic contri- bution to the total charge-storage capacity can be separated from the mainly capacitive one.The dependence of the anodic peak charge on the noble-metal nominal concentration in the electrode films (group B), is shown in Fig. 7. From this it is possible to draw the conclusion that the total number of electroactive sites in the oxide films is maximum at intermedi- ate concentrations of noble metal. Analogous results were obtained in a preliminary study on films prepared as those of group C in this work2' and also for IrO,/TiO, films." A tentative explanation for this occurrence has been sought in the larger degree of microstructural defectivity of mixed oxide films with lower noble-metal contents." SIMS results have shown that titanium oxide segregation effects are more pro- nounced for low to intermediate nominal concentrations of noble-metal oxide.This should further enhance the defective character of their surface region. It has been shown18'19 that the composition of mixed-oxide films in the near-surface region is not strongly affected by the nominal bulk composi- 10 1 5 N 'a5 -5 t -1 c I I 1 4 5 0.0 0.5 1.0 1 5 EN vs.SCE Fig. 6 Typical voltammogram obtained with a group A sample (30mol% RuO,). Solution: 1mol dmP3 HClO,; reference electrode SCE; potential scan rate 0.100 V s-'. tion. It is therefore reasonable to admit that, when there is an almost constant number of electroactive sites (in our case Ru ions) in the film surface region, the degree of porosity, a consequence of the microstructural defectivity, becomes the main factor controlling the charge-storage capacity.Making use of the previously mentioned simplifying assumpticm about the contributions to the total voltammetric charges, integral capacities of the different electrodes can be calculated. Maximum values are still found for samples containing 30-50 mol% ruthenium dioxide. The integral capacit Y, calcu-lated from the anodic part of the voltammogramh, varies between a maximum of 160 F rn-' and a minimum of 58 F m-2 (geometric area). Evaluation of roughness factors can be attempted, making use of data of specific capacity available in the literature. A value of 0.6 F rn-' has been proposed for r~tile.~~'~~More recently a value of 0.8 F rn-' was proposed by Trasatti and co-workers31 for RuO,.Using the tatter, a roughness factor of 200 can be estimated for a sample contain- ing 30 mol% of RuO,. The lower limit is then 70 (80 mol% Ru02). In Fig. 7 the results of the normalization of peak charges to the effective electrode areas are reported. Some 20 15 cu E 03 10 h fCr" v 5 a 0 20 40 60 80 100 film composition (Ru) (mol %) Fig. 7 Dependence of the voltammetric (anodic) peak charge on the noble-metal oxide concentration in the electrode film (group A). (qan)ap; peak charge normalized to the geometric electrode area; (qan)=+peak charge normalized to the effective electrode area. residual effect of the nominal noble-metal concentration can still be observed, although they are reduced in comparison with the results referred to the geometric area.Larger surfaces are likely to favour injection/ejection of counter-ions, in our case protons, during changes in oxidation state of the elec- troactive sites in the films. In this case a linear dependence of peak currents on sl', is expected, at least in the case of larger peak charges. This has been verified for some anodic iridium oxide coating^,^,^^ for which the involvement of the volume of the electrode in the charging process has been extensively d~curnented.~.~~A similar situation has been met for ther- mally prepared IrO,-TiO, mixed oxide electrodes containing <50 mol% IrO,.,' In our case, an accurate analysis showed that the peak currents are linear in s.For Ru0,-TiO, films, therefore, no evidence for control of the charging process by proton diffusion in the films has been found. Conclusions The depth-profiling results obtained by SIMS for the two major components, Ru and Ti, agree substantially with those obtained by XPS coupled with Ar+ etching and a non- destructive technique like RBS, for a limited number of electrode compositions. At this level of the study we can therefore exclude artifacts, like preferential sputtering, due to some specific aspect of the interaction between the ion beam and the oxide matrix. The linearity of the calibration curve in Fig. 3 also favours of this assumption.Therefore, the sensitivity of the method can be fully exploited in the study of the distribution and changes of concentration of minor components, residual from incomplete decomposition of pre- cursors. This also holds for the small amount of counter-ions exchanged between electrode and solution under polarization. A study of the type and distribution of ion clusters emitted from the sample during the ion-etching can also supply more information on chemical bond features at different depths in the films. The shape of the concentration depth profiles supports the idea that the evolution from the precursor salt film to the stoichiometric and crystalline microstructure of the final mixed oxide product is slow. Some evidence of this has been discussed already in the case of pure IrO, films on the basis of thermoanalytical and microstructural datal3'l4 and a detailed X-ray diffractometric study.lS Although a larger average crys- tallite size is generally found for RuO,, large carbon and hydrogen contents have been detected by nuclear reaction analysis and elastic recoil detection in Ru0,-TiO, films prepared by the method described in this paper.33 These films also exhibited densities as low as 3.0g~m-~, although the X-ray pattern of the Ru0,-TiO, solid solution could be observed and, from the peak shape, average crystallite sizes of 15-18 nm were estimated. These data, together with the results of the present work, suggest that crystallites of the Ru0,-TiO, solid solution coexist with large intergranular areas.In the latter, besides Ru, Ti and 0,other components are accumulated in relatively large concentrations and the microstructure becomes highly defective. The porosity of these regions must be responsible for the low average density observed in other studies.33 Any further increase in the crystallite size is more or less hindered by the amorphous J. MATER. CHEM., 1994, VOL. 4 microcrystalline region. This can be thought of as a residual of the gel-like structure of the precursor precipitates on the metal support, following the solvent evaporation stage. The large roughness factors obtained from cyclic voltammograms are better explained by assuming large portions of gel-like structure in the electrode films.References 1 L. D. Burke, in Electrodes of Conductive MtJtal Oxides, ed. S. Trasatti, Part A, Elsevier, Amsterdam, 1980, p. 141. 2 L. D. Burke and M. E. G. Lyons, in Modern Aspects of Electrochemistry, ed. R. White, J. O'M Bockris and B. E. Conway, Plenum Press, New York, 1986, p. 109. 3 J. E. D. McIntyre, S. Basu, W. F. Peck Jr., W. L. Brown and W. M. Augustyniak, Phys. Rev. B, 1982,25, 7242. 4 L. D. Burke and D. P. Whelan, J. 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De Asmundis, Mater. Chem., 1976,1,177. 26 C. Pagura and S. Valcher, unpublished results, 1991. 27 F. Hine, M. Yasuda and T. Yoshida, J. Electrochem. Soc., 1977, 124, 500. 28 A. De Battisti, G. Brunoro and F. Pulidori, Est. Abs., n"0318, 34th I.S.E. Meeting, Erlangen G, 18-23 September, 1983. 29 S. Levine and A. L. Smith, Discuss. Faraday SOC..1967,52, 1290. 30 J. O'M. Bockris and T. Otagawa, J. Electrochem. SOC.,1984, 131, 290. 31 P. Siviglia, A. Daghetti and S. Trasatti, Colloids Surf., 1982, 7, 15. 32 B. E. Conway and J. Mozota, Electrochim. Acta, 1983,28,9. 33 M. Guglielmi, P. Colombo, V. Rigato, G. Battaglin, A. Boscolo-Boscoletto and A. De Battisti, J. Electrochem. SOC.,1992, 139, 1655. Paper 3/07638E; Received 31st December, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940401255

出版商:RSC    

年代:1994

数据来源: RSC