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. 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