Syntheses, characterization and non-linear optical properties of nickel complexes of multi-sulfur 1,2=dithiolene with strong near-IR absorption Jing-Lin ZUO,'Tian-Ming Yao,' Fei YOU,' Xiao-Zeng YOU,"'Hoong-Kun Funb and Boon-Chuan Yipb aCoordination Chemistry Institute and the State Key Laboratory of Coordination Chemistry, Nanjing University, Center for Advanced Studies in Science and Technology of Microstructure, Nanjing, 210093, P.R. China bX-Ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia A series of new nickel complexes with multi-sulfur 1,2-dithiolene ligands have been synthesized and characterized by electrochemical measurements, EPR, IR and UV-VIS spectroscopies. One of the typical structures for the neutral complex of [Ni( phdt),] 2 (phdt=5,6-dihydro-5-phenyl-l,4-dithiin-2,3-dithiolate)was obtained by X-ray structure determination.Electrical conductivities measured for compacted pellets range from 1.4 x lo-* to 1.4 x S cm-l. These complexes show intense absorptions in the near-IR region. The third-order non-linear optical susceptibilities, f3), for three of the complexes have been determined to be of the order of lo-'' esu. In 1972, Drexhage and co-workers discovered that transition- metal-dithiolene complexes are particularly suited for Q-switching lasers (such as Nd-YAG lasers, operating at 1064nm).' Their choice was the class of square-planar tran- sition-metal dithiolene complexes, especially those of Ni, because they possess delocalized n-electron systems and strong near-IR absorption bands, which can be tuned by altering the metal ion and substituents.The second important property of the dithiolenes is their ability to exist in several clearly defined oxidation states which are connected through reversible redox steps. The third useful characteristic is their high thermal and photochemical stabilities, even at their absorption maxima. An intense research effort is presently aimed at producing materials with low response times and favourable ratios of the non-linear refractive index (n,) to the linear (a) and the non- linear absorption coefficients (p).,-' In near-resonant systems tuning into the low-energy side of an electronic transition leads to an increase in the absorptive contribution to the linear loss, and also to a commensurate increase in the non-linear refrac- tive index.This would result in a figure of merit (W)increasing until the absorptive contribution was at least the size of other loss mechanisms. This suggests that one way to improve the performance of organic materials is to study near-resonant systems. For these reasons, a series of metal dithiolenes have featured in third-order non-linear optical studies and f3) values were found in the range 7.16~lo-', to 3.8~lo-'' esu.6 Polymeric metal-dithiolene complexes may be candidate mate- rials for all-optical signal-processing devices for their sub- picosecond response times and reasonable off-resonant non- linearity.'.' Metal complexes with sulfur-rich dithiolene ligands are also very interesting from the viewpoint of their conductive behav- iour, owing to the intermolecular sulfur-sulfur interactions of the ligands.8 Dmit-metal complexes (dmit = 1,3-dithiole-2-thione-4,Sdithiolate; C3S5,-) were reported to be molecular inorganic conductors and even superconductor^.^ Following this, researches aimed at the search for new multi-sulfur 1,2-dithiolene ligands have We report here the syntheses and properties of several new complexes based on the multi-sulfur dithiolene ligand.One of the typical structure of the series, the neutral salt of [Ni(phdt),] (phdt =5,6-dihydro-5-phenyl-1,4-dithiin-2,3-dithiolate) is2, presented. The third-order non-linear optical susceptibilities (x'~))for three of the complexes have also been measured using the Z-scan method.1 R = 4H(Ph)CHr n = 1 2 R = -CH(Ph)CHr n = 0 3 R = -CH(Me)CHr n= 1n=2 n= 15 R = S=C< 7 R = 4H2CH2-n= 1 4 R = -CH(Me)CHr n = 0n=l 6 R=S=Cc n=O8 R Z-CH~CHZ- 9 R = ~H~CH~CHT Experimental Reagents 2,3-Dihydro-2-phenyl- 173-dithiolo-[ 4,5-e]- 1,4-di thiin-6-one (phedo) was prepared by a literature method and recrystallized from ethanol-chloroform (3 :l).I4 All solvents were dried by standard techniques prior to use. [Bu,N][Ni(medt),] 3 (medt =5,6-dihydr0-6-methyl-1,4-dithiin-2,3-dithiolate),'~ [Bu,N], [Ni(dmit),] 5,16 [Bu,N][Ni(dmit),] 6,16 [Bu,N][Ni(dddt),] 7 (dddt=5,6-dihydro-1,4-dithiin-2,3-dithi-olate)," [Ni(dddt),] 8," [Bu,N][Ni(pddt),] 9 (pddt =6,7- dihydro-5H-1,4-dithiepin-2,3-dithiolate)13were prepared according to the literature methods. Syntheses All reactions were carried out under N,.Elemental analyses were performed using a Perkin-Elmer 240C analytical instru- ment. Ni element analysis was performed on a Jarrell-Ash ICP quantimeter. Preparation of [Bu,N][Ni(phdt),] 1. A mixture of ethanol (20 ml), potassium hydroxide (2.0 g) and phedo (1.0 g, 3.5 x lop3 mol) was stirred for 1 h at 40°C under N,. The resulting pure yellow microcrystals of potassium 5,6-dihydro- 5-phenyl-1,4-dithiin-2,3-dithiolate(K,phdt) were isolated by centrifugation. The salt was then dissolved instantly in meth- anol (20 ml). A solution of 0.39 g (1.7 x mol) NiC1, -6H20 in 20ml methanol was added dropwise to the solution under N,.After standing for 15 min, the solution was then stirred in air for 30 min. The colour of the solution changed from amber to dark green as the reaction proceeded. After filtration, to the filtrate was added 1 equiv. of tetrabutylammonium bromide, and dark green solids were precipitated immediately. The green J. Muter. Chew., 1996, 6(lo), 1633-1637 1633 solids were then collected by filtering and recrystallized from acetone. The yield was 0.99 g (70%). Anal. Calc. for C,,H,,NNiS8: C, 53.12; H, 6.44; N, 1.72; Ni, 7.21. Found: C, 52.92; H, 6.44; N, 1.58; Ni, 7.37. IR (KBr)~/cm-~:3026(w), 1468(m), 1450(m), 1365(s), 1179(m), 858(m), 696(m). Preparation of [Ni(phdt),] 2. The black powder of neutral [Ni(phdt),] was prepared by I,-oxidation of 1.Black opaque rectangular crystals were obtained by recrystallization from carbon disulfide. Anal. Calc. for C,,H,,S,Ni: C, 42.03; H, 2.82; Ni, 10.27. Found: C, 42.30; H, 3.12; Ni, 9.99. IR (KBr)v/cm-': 1234(s), 1174(s), 1134(m), 886(m), 469(m). Preparation of [Ni(medt),] 4. The preparation of the neutral complex 4 was similar to 2. The product was washed with acetonitrile and then recrystallized from benzene or carbon disulfide. Anal. Calc. for C,,H,,S,Ni: C, 26.85; H, 2.70; Ni, 13.12. Found: C, 27.02; H, 2.75; Ni, 13.07. IR (KBr)v/cm-': 1257(s), 1214(s), 1144(s), 1076(m), 873(m), 483(m). Physical measurements IR spectra were recorded on a Nicolet FTIR 170sx spectropho- tometer.UV-VIS spectra were recorded on a UV-3100 spectro- photometer. The cyclic voltammetry was performed by a model 79-1 V-A analyser with a electrochemical cell using a platinum wire as the working electrode, a platinum plate as the auxiliary electrode and an SCE as the reference electrode. Nitrogen was passed through the solution in the cell for 15 min prior to each measurement. The EPR spectrum was recorded on a Bruker ER 200-D-SRC spectrometer. Electrical conductivities for com- pacted pellets were measured on a ZC-43 high resistance meter by a two-probe technique at 25°C. Structure determination A representative crystal of [Ni(phdt),] was surveyed. A data set was collected using a Siemens P4 diffractometer eguipped with graphite-monochromated Mo-Ka (2=0.71073 A) radi-ation.The empirical (uia t,b scans) absorption correction was applied.18 The crystal data, data collection and structure refinement are summarized in Table 1. The structure was solved by direct methods using SHELXL86l'" and refined by full-matrix least-squares methods on F2 using SHELXL93.19' The H atoms were located from difference maps and refined isotropically. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Table 1 Crystal data and details of data collection and refinement formula M* crystal system monoclinic space group p2, /CTlF 293 (2) a14 8.9310( 10) bl4 6.9770(10) CIA 19.583( 3) PldFgrees 102.7 10( 10) VIA3 1190.3( 3) Z 2 DJg cmP3 1.595 p/cm-' 15.23 8 range for data collection 2.13-27.50 scan mode 8-28 maximum, minimum transmission 1.000, 0.641 reflections collected 3756 independent reflections 2724 data/restraints/parameters 2723/0/150 goodness of fit on F2 1.059 Crystallographic Data Centre (CCDC). See Information for Authors, J.Mater. Chem., 1996, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/9. Results and Discussion Crystal structure of [Ni( phdt),] 2 Fig. 1 shows the structure and labelling scheme of [Ni(phdt),] 2. The interatomic distances and bond angles are given in Table 2. The four sulfurs surrounding the nickel atom yield a square- planar coordination with both S-Ni-S angles being 91.57( 3)".In the five-membered ring containing the nickel atom, the average S-Ni, S-C and C=C bond lengths are 2.126, 1.706 and 1.384 A, respectively. The correspondjng values in [Bu,N][Ni(dddt),] are 2.148, 1.735 and 1.339 A." The shorter C-S bond lengths are indicative of double-bond character. This electron delocalization in transition-metal dithi- olenes creates a difficulty in assignment of the oxidation state to the metal. The outer phenyl groups are almost perpendicular to the least-squares plane formed by the rest of the molecule. A stereoview of the unit-cell packing of complex 2 viewed down the c axis (Fig. 2) shows the molecules forming a layered structure in the ab plane. Fig. 3 shows that the molecules are arranged as two different oriented uniform 'stacks' approxi- mately aloFg the b axis.The closest Ni-..Ni distance is 6.9770( 10) A, which is the length of tee b axis. The shortest interstack S-..S contact is 3.6876( 14) A and occurs between S(3) and S(4), which is about the sum of the van der Waals radii (3.70 A). The large separation between stacks is due to the phenyl substitution on the external part of the ligand. This is consistent with the semiconductive property of the complex as described later. Fig. 1 ORTEP plot of the molecule of [Ni(phdt),] Table 2 Selected bond distances/A and bond angles/degrees in "i(Phdt),l 2 Ni-S( 1) 2.1246(8) Ni-S( la) 2.1246( 8) Ni-S(2) 2.1278(8) Ni-S( 2a) 2.1278( 8) S( 1 )-C( 1 ) 1.703(3) S(2)-C(2) 1.7 10( 3) S(3 )-C( 1) 1.736( 3) s(3 )-C (4) 1.756(4)s(4)-C( 2 ) 1.738(3) S(4)-C(3 1 1.765(4) C( 1)-C(2) 1.384( 4) C( 3 )-C(4) 1.285( 6) C(41-C ( 51 1SO8(5) S( 1)-Ni-S( la) 180.0 S( l)-Ni-S(2) 91.57(3) S( 1a)-Ni-S(2) 88.43( 3) S(l)-Ni-S(2a) 88.43( 3) S( 1a)-Ni-S (2a) 91.57(3) S(2)-Ni-S (2a) 180.0 C(1)-S( 1)-Ni 104.95( 11) C (2)-S (2)-Ni 105.06( 11) C( 1)-S(3)-C(4) 1043 2) c(2)-s (4)-c ( 3) 1033 2) C(2)-C( 1)-S( 1) 119.6(2) C(2)-C( 1)-s(3) 126.7 (2) S( 1)-C( 1)-S( 3) 113.7(2) c( 1)-c(2)-s (2) 118.7(2) C( l)-c(2)-s(4) 127.3 (2) S(2)-C( 2)-s(4) 113.9(2) C( 4)-C( 3)-S( 4) 127.7(4) c(3)-c (4)-c (5) 121.5 (4) C(3)-C(4)-S( 3) 127.7( 4) c(5)-C(4)-S( 3) 107.9( 3) C( 10)-C(5)-C(6) 118.0( 3) maximum shift error 0.001 Final R indices [1/20(1)] R(F)=0.0445, wR(~z)=0.1 190 Symmetry transformations used to generate equivalent atoms: a-x+l, --y, -z.1634 J. Mater. Chew., 1996, 6(lo), 1633-1637 Fig. 2 Packing diagram of the unit cell of [Ni(phdt),] looking down the c axis Fig. 3 Packing diagram of the unit cell of [Ni(phdt),] looking down the a axis IR, cyclic voltammetry, EPR and NIR spectra [Bu,N)[Ni(phdt),] 1 displays a very rich IR spectrum with the characteristic absorptions of monoanionic nickel dithiol- enes.20 vl, C=C at 1445cm-'; v2, C=S at 1179cm-l; v3, R-C (ring) at 858 cm-l, while the neutral complex 2 shows these absorptions at 1234, 1174 and 840 cm-l, respectively. Such a noticeable low-frequency shift of the C=C stretching band suggests that oxidation occurs essentially on the phdt ligand rather than on the metal centre.This is the same as for the complexes of [Bu,N][Ni(medt),] 3 and [Ni(medt),] 4, which show the C=C stretching band at 1443 and 1257 cm-', respectively. Cyclic voltammetry of [Bu,N] [Ni( phdt),] in acetonitrile (0.1 mol dmb3 Bu,NClO,, 250 mV s-l) reveals two reversible waves, 1/-2)= -0.65 V and E1,,(O/-1)=0.06 V. The corresponding values for [Bu,N][Ni(dddt),] are -0.69 and 0.17 V," and -0.70 and 0.05 V for [B~,N][Ni(medt),].'~ This trend follows the electron-withdrawing ability of Ph >H >Me. The greater the electron-attracting ability of the ligand, the more electron density the complex can accommo- date and the higher the redox potentials. The frozen-glass EPR spectrum of [Bu,N][Ni(phdt),] 1in DMF at 128 K shows three peaks with gl=2.104, g2=2.048 and g3=2.004.This spectrum is similar to those of [Ni(dddt),] -lo and [Ni(pddt),] -which show a rhombic g tensor. Fig. 4 shows the NIR spectra of complexes 1and 2. Complex 1 in acetonitrile solution shows a strong broad absorption at 1172 nm (E= 15000), while complex 2 in benzene solution exhibits this absorption at 1028 nm (E =43 000). Table 3 lists the near-IR absorption maxima and absorbances of the com- plexes. The intensity of the absorption is unmatched in any other transition-metal compound, where low-energy bands usually are of d+d character and thus considerably weaker. The EHMO calculations assign this strong electronic absorp- tion band to a n+n* transition (2A,, 2B,, 3B3,, 2B3,-+3B,,) of the delocalized ligand.,, The electron delocalization in the coordination ring and the central metal is made possible by the stronger overlap involving d-orbitals of sulfur and thus will lead to bathochromic shifts of the multi-sulfur 1,2-dithi- olene complexes.By inspection of the data in Table3, it can be noted that complexes 1, 3 and 7 exhibit NIR absorption bands at 1172, 1177 and 1175 nm, respectively. This is due to the weak donor property of the methyl group and the weak acceptor property of the phenyl group, which are in accordance with the results from the values of cyclic voltammetry discussed above. But this effect is not significant in [Ni(medt),] 4 and [Ni(phdt),] 2.Attachment of bulky substituents, such as phenyl and methyl A Inm Fig.4 Absorption spectra of 1 in acetonitrile (-) and 2 in benzene (---) (25°C) Table3 Absorbance and absorption maxima of the complexes of dithiolenes (25 "C) complex solvent I,,,/nm &/dm3 mol-' cm-' 1 [Bu,N][Ni(phdt),] acetonitrile 1172 15000 3 2 [Bu,N][Ni(medt),]"i(Phdt),l benzene acetonitrile 1028 1177 43000 15000 4 5 6 7 [Ni(medt),] [Bu,N],[Ni(dmit),] [Bu,N][Ni(dmit),] [Bu,N][Ni(dddt),] benzene acetonitrile acetonitrile acetonitrile 1029 1219 1137 1175 36000 6500 45000 15600 9 [Bu,N][Ni(pddt),] acetonitrile 938 11000 J. Muter. Chem., 1996,6(lo), 1633-1637 1635 groups, will increase solubility, which is favourable for use as -10 -5 0 5 10 candidates for Q-switching dyes 23 Complex 1, 3 and 7 with the outer six-membered ring show bathochromic shifts of cu 240 nm compared to that of complex 9 with the outer seven-membered ring, which has been used as an NIR absorbing dye 24 This means that structural factors 12-contribute to the absorption band shift significantly ...,...j::,1 0..*: =,....;..:..*..-...*....;-..,...,..-* a.Electrical properties Table 4 lists the electrical conductivities measured for the compacted pellets at 25°C Complexes 1, 3, 6 and 9 exhibit small conductivities due to the weak interactions between the anions However, in the neutral complexes 2 and 8, the interaction between the molecules may be strengthened as revealed in the crystal structures of [Ni(phdt),] and [Ni(dddt),] l7 The increase of conductivity is of the order of 10' Comparing 7 with 1 and 3, the face-to-face overlapping of the molecules may be prevented by the introduction of bulky phenyl and methyl groups into [Ni(dddt),]-, and thus decreased the conductivities significantly Non-linear optical properties The third-order susceptibility, x(~),at 532 nm, was determined by Z-scan techniques as described earlier 26 27 The experimental arrangement utilizes a M200 high power mode-locked Nd YAG laser with 200 ps pulse at a frequency of 5 Hz A CH3CN solution of the compound was placed in a 1 mm quartz cell and used for all the optical measurements The NLO properties of these complexes are dominated by non-linear refraction, as illustrated in Fig 5(u) The non-linear absorption is negligible (Fig 4) The valley-peak pattern of the normalized transmittance curve obtained under closed aperture configuration shows characteristic self-focusing behaviour of the progating light in the sample The valley and peak occur at equal distances from the focus with the valley-peak separation [Fig 5(b)] fitting eqn (l), where coo is the laser beam waist radius (33+5 pm) and A is the laser wavelength (532 nm) AZv-p = 1 72n.~r>,'/A (1) This result suggest that the observed optical non-linearity has a third-order dependence on the incident field The difference between normalized transmittance values at valley and peak portions, AT,,, is related to the non-linear refractive index n, (m2 W-') by eqn (2) and (3), where ASP, and I, are the on-axis phase shift and the on-axis irradiance, both at focus, respectively, and a, and L are the linear absorption coefficient and the optical path of the sample ATpp=0 406( 1-S)' 25 IASPO I (2) In our experiment, S=O 3, therefore A TvPp=0 3 7 1IASP0 I IAcDoI =(2n/A)I,[( 1-e-"oL)/a,]nz (3) By inserting the values I, =3 26 GW cm-', L =10 x m, then the n2 value can be calculated Experiments with varied Table 4 Electrical conductivities (u)of the complexes at 25 "C ~~ complex 01s cm-' ref 1 CBu,NICNl(Phdt),l 14x10 15 2 "l(Phdt),l 15x10 this work 3 [Bu,N][Ni(medt),] 2 6 x 15 6 [Bu4N][ Ni(dmit ),I 10 x 25 7 [B~4N][Ni(dddt),] 49x10 this work 8 [Ni(dddt),] 14x10 this work 9 cBu4N 1CWPddt),I 5 4 x this work "At 20 "C 1636 J Muter Chern, 1996, 6(10), 1633-1637 08-~ v)C -10 -5 0 5 10 c-l2 Zlmm Ua, -10 -5 0 5 10 ,.IT(.I 12 10 io 4 -10 -5 0 5 10 Z/mm Fig. 5 The Z-scan data of complex 1 (1 0 x lop3mol dm-') at 532 nm with 1,=3 26 GW cmP2 (a) Collected under the open aperture configuration showing very small NLO absorptions, (b) collected under the closed aperture configuratlon showing the self-confusing effect I, show that the n2 thus measured is indeed independent of I,, consistent with the notion that n =no+n,I, and the observed NLO phenomenon is third order in nature The samples are almost transparent in the green region of the spectrum and dispersion of n2 is expected to be negligible If we ignore the contribution of NLO absorption, the third- order NLO susceptibilities x(3) of the three complexes can be calculated from the n, value by using eqn (4) ~(~'(esu) =cno2nZ/8O~=x(~)~~ f3)(rn2V-') =c~,n,~n~ (4) The calculated figures of merit (W)for the complexes are based on the following formula W=Ansat/an =n21sat/an (5) where Ansat is the maximum change in the refractive index, which is the product of n2 and the damage intensity for most organic materials Isatis taken as 3 GW cm-' for the values given in Table 5 The measured susceptibilities for the three complexes are listed in Table 5, which are in the range 14x lo-'' to 1 6 x lo-'' esu This work was supported by grants for key research project in climbing program from the State Science and Technology Commission and National Nature Science Foundation of China H-K F and B-C Y would like to thank the Malaysia Government and Universiti Sains Malaysia for research grant R&D No 190-9609-2801 The authors also thank Mr Hai-Ming Wu from the National Laboratory of Molecular and Biomolecular Electronics, Southeast University of China, for help with the electrical conductivity measurements Table 5 Third-order non-linear optical susceptibilities f3) for some nickel dithiolene complexes ~ esucomplex i,,,/nm x/m-l x(3)/10-11 ~(~)/10-*Om2 v w ref 1 c Bum “l(Phdt),l 1172 56 16 35 7 6 this work 7 [B~,N][Ni(dddt)2] 1175 47 16 35 76 this work 9 cBu4NI “I( Pddt), 1 938 46 14 30 64 this work 1’“ bis [l-methyl-2-phenylethene-1,2-dithiolato(2-)-S,Y] nickel 770 9 14 90 3 9 bis [1,2-diphenylethene-l,2-dithiolato(2-)-S,S’] nickel 865 88 19 10 3 12’” bis [1,2-bis(4-methylphenyl)-1,2-ethenedithiolato(2-)-900 34 11 19 3 S,S’]nickel -)-13“ bis [1,2-bis(4-methylphenyl)-1,2-ethenedithiolato(2 935 216 34 09 3 S,S’]nickel Of3) was measured at 1064nm and used a degenerate four-wave mixing (DFWM) setup References 14 S Larsen, T Thorsteinsson, S Bowadt, T K Hansen, K S Varma, J Becher and A E Underhill, Acta Chem Scand ,1991,709 1 K H Drexhage and U T Muller-Westerhoff, IEEE J Quantum 15 T-M Yao, J-L Zuo, X-Z You and X-Y Huang, Polyhedron, 1995, Electron, QE-8, 1972, 759, K H Drexhage and U T Muller-14,1487 Westerhoff, US Pat, 3743 964, 1973 16 G Steimecke, J Sieler, R Kirmse, W Dietzsch and E Hoyer,2 P Calvert, Nature (London), 1991,350, 114 Phosphorus Sulfur, 1982,12,237 3 C S Winter, S N Oliver, R J Manning, J D Rush, C A S Hlll 17 H Kim, A Kobayashi, Y Sasaki, R Kato and H Kobayashi, Bull and A E Underhill, J Mater Chem ,1992,2,443 Chem SOC Jpn ,1988,61,5794 A E Underhill, C A S Hill, C S Winter, S N Oliver and 18 XSCANS Users Manual, version 21, Siemens Analytical X-ray J D Rush,Mol Cryst Lzq Cryst, 1993,217,7 Instruments Inc ,Madison, WI, USA, 1994 5 A E Underhill, C A S Hill, A Charlton, S Oliver and 19 (a) G M Sheldrick, Acta Crystallogr, Sect A, 1990, 46, 467, S Kershaw, Synth Met, 1995,71,1703 (b) G M Sheldrick, Program for crystal structure refinement,6 N J Long, Angew Chem, Int Ed Engl, 1995,34,1 University of Gottingen, Germany, 1993 7 C S Winter, C A S Hill and A E Underhill, Appl Phys Lett, 20 J A McCleverty, Prog Inorg Chem ,1968,10,971991,58,107 21 T-M Yao, X-Z You and Q-C Yang, Chin J Chem , 1994,12,2488 L Valade, P P Legros, M Bosseau, P Cassoux, M Garbauskas 22 Q Fang, Y-M Sun and X-Z You, Chin J Chem Phys, 1992,and L V Interrante, J Chem SOC, Dalton Trans, 1985,783 1,1299 M Bousseau, L Valade, J P Legros, P Cassoux, M Garbauskas 23 U T Muller-Westerhoff, D I Yoon and K Plourde, Mol Crystand L V Interrante, J Am Chem SOC,1986,108, 1908, Lzq Cryst, 1990,183,291A Kobayashi, H Kim, Y Sasaki, H Kobayashi, S Monyama, 24 T Hasegawa, Jpn Pat, 03155538,1991Y Nishino, K Kojita and W Sasaki, Chem Lett, 1987,1819 25 F Nusslein, R Peter and H Kisch, Chem Ber , 1989, 122, 1023 C T Vance, R D Bereman, J Bordner, W E Hatfield and 26 J-Y Niu, X-Z You, C-Y Duan, H-K Fun and Z-Y Zhou, InorgJ H Helms, Inorg Chem ,1985,24,2905 Chem ,in press J H Welch, R D Bereman, P Singh and C Moreland, Inorg 27 M Sheik-Bathe, A A Said and E W Van Stryland, Opt Lett, Chzm Acta, 1989,158, 17 J H Welch, R D Bereman and P Singh, Inorg Chem, 1988, 1989,17,955 27,2862 R D Bereman and H Lu, Inorg Chzm Acta, 1993,204,53 Paper 6/020111, Received 22nd March, 1996 J Mater Chem., 1996, 6(lo), 1633-1637 1637