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. 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