J. MATER. CHEM., 1991, 1(3), 485-486 485 Synthesis and Optical Spectroscopy of Platinum-metal-containing Di- and Tri-acetylenic Polymers Brian F. G. Johnson," Ashok K. Kakkar," Muhammad S. Khan," Jack Lewis,*a Ann E. Dray,b Richard H. Friend,*b and Felix Wittmand a University Chemical Laboratory, Lensfield Road, Cambridge, UK Ca vendish Laboratory, Madingley Road, Cambridge, UK Optical absorption and photoluminescence measurements on the title polymers, -f-Pt(PBu~),-(C-C),,,-C~C+, (m= 1, 2) show that there is appreciable n-electron conjugation extending through the metal sites on the chain, with a lower n-n* energy gap for the triacetylenic than for the diacetylenic compound. Keywords: Optical absorption; Photoluminescence; Polyyne polymers; Vibronic Transition-metal a-acetylide complexes of the Ni triad ~M(PBu?)),-C=C-(R),-C=C+~ (where M =Ni, Pd, Pt; R=p-C,H,; rn=0, 1; Bu=butyl), first developed' by Hagihara's group, are of great interest owing to their potential applications in the new materials industry.We have recently reported that bistrimethyltin(SnMe3) derivatives of H-C-C-R-C-C-H (R =P-C6H4; P-C6H4-P-C6H4) are versatile reagents to a diverse range of transition-metal polyalkyne polymer^.^-^ Using this new synthetic route, we have now prepared the di- and tri-acetylenic platinum (Pt) polymeric complexes, fPt(PBu",), -(CZC),,,- C=C +n (rn=1, 2). Although, the diacetylenic Pt polymers have been reported previously' by the reaction of trans-di(butadiynyl)bis(tri-n-butylphosphine)platinum(II) [(PBu?)),Pt(-C-C-CfC-H)),] with dichlorobis(tri-n-but ylp hosp hine)platinum(xI) [P t( PBu:)~ Cl J, the correspond- ing triacetylenic polymers are unknown.The latter have been suggested6 to be of great importance in the studies related to non-linear optical properties of these systems. There is also considerable interest in the electronic structure of the ground and excited states of the polyyne chain7v8 and we consider that these compounds, especially the triacetylenic polymers, provide an important experimental realisation of the delocal- ised polyyne n-electron system. In this communication we wish to describe synthesis and a preliminary investigation of the optical spectra of these polymeric materials. A general synthetic route (Scheme 1) to the polymeric species requires the addition of 1 equivalent of platinum complex, [Pt(PBu",),Cl,] (1) to 1 equivalent of bis(SnMe,) reagents 2 or 3.t A systematic characterization1 of these complexes was achieved by analytical and spectroscopic methods (IR; 'H, 13C, and 31PNMR) and molecular weight determinations.The weight-average molecular weight (M,) values§ for the polymeric compounds (130 000 for 4 and 160 000 for 5) show a high degree of polymerisation. t The bis(SnMe,)acetylides [Me3Sn -C C-C=C-SnMe, (2) and Me,Sn -CrC-C= C -Cr C -SnMe, (3)] were synthesised by adaptation and modification of the literature procedure^.^ $ (i) Selected data for 4. Calc. for CzsH54PzPt: C, 51.92, H, 8.40; P, 9.56%. Found: C, 51.89; H, 8.47; P, 9.47%.M,= 130000 (n,= 200). v,,, 1999 cm-' (C=C); "P{'H} (CD,Cl,, 162 MHz) 6, 136.6 (J,-,=2381 Hz); 'H(CD,C12, 400 MHz) dH 0.92, 1.44, 1.51, 2.01. (ii)Selected data for 5. Calc. for C,,H,,P,Pt: C, 53.63; H, 8.10%. Found: C, 52.78; H, 8.12%. Mw=160000 (nw=238).v,,, 2096cm-' (C-C); 31P('H} (CDZCl2, 162 MHz) 6, 136.3 (Jpt-p=2300 Hz);13C{1H} (CD2Cl2, lOOMHz) 6, 13.8, 24.1, 24.5, 26.5, 59.5, 81.5, 89.4; 'H (CDZCIZ, 400 MHz) 6H 0.93, 1.43, 1.51, 2.00. 0 Molecular weights were determined by Gel Permeation Chroma- tdgraphy (GPC) method. For GPC procedural details see ref. 10. High solubility of these polymers in common organic solvents allows preparation of samples suitable for physical measurements. Optical absorption spectra were obtained from dilute solutions in dichloromethane, and the photoluminesc- ence (PL) measurements were performed on thin films of the polymers which were spin-coated onto spectrosil substrates from solutions in toluene.Excitation was provided by the quadrupled output (266 nm) from a Q-switched Nd :YAG laser, and measurements were made at low temperatures (<30 K) in order to maximise the PL output and to maximise the sharpness of the PL features. The results of this study are presented in Table 1. Both polymers show strong absorption with well resolved structure which we ascribe to the n-n* transitions associated with acetylenic units of the chain. The minimum excitation energy is lower for the triacetylenic polymer (2.7 eV) than for the diacetylenic polymer (3.0 eV).This provides direct evidence for the role of the length of the polyyne chain in determining the energy gap. One of the particular advantages of working with the polyyne spacers between the metal sites (in distinction with earlier work".'2 on more complicated spacers such as -C~C-c6H4-C-C-) is that the vibrational spectrum of the chain is particularly simple, with a single vibrational mode coupling to the bond dimerisation amplitude; this is the -C=C-stretch frequency at ca. 2250cm-' (= 0.27 eV).t This accounts for the well resolved structure in the absorption spectra, which is due to transitions from ground to excited electronic state coupled with transitions to the various vibronic levels of the excited state.As listed in Table 1, many of the energy spacings for all materials studied here are close to 0.27 eV. We expect that the excited states of the n-electron system will be excitons, and that these will be localised, probably to within no more than one or two repeat units of the hai in,^.^ and we can expect to see radiative decay of the singlet excitons. The measured PL spectra are characteristic of the decay of such excited states, and we see very well resolved bands corresponding to the different levels of the vibrational modes of the ground electronic state; we find again that the energy separation of the PL bands is close to 0.27 eV in most cases. The strong vibronic coupling to the electronic transitions, and substantial Stokes' shift between the strongest bands in the absorption and emission spectra (ca.0.75 eV) indicates that there is strong coupling between the n-electron system and the carbon-carbon bond dimerisation amplitude. We t CEC triple-bond stretch, see ref. 13. J. MATER. CHEM., 1991, VOL. 1 SnMe3-C=C-C=C-SnMe3(2) Pt~PBU~~,C,, SnMe,-C=C-C=C-C=C-SnMe, (3) 1 equiv., toluene, 30"C, 1 h. (11 PBu; PBu; fPt-C=C-C=C* fpt-c~c-c=c-c=c+ Scheme 1 Table 1 Optical absorption and photoluminescence results for polymers 4 and 5 maxima in absorption maxima in luminescence spectrum/eV spectrum/eV polymer 4 ~P~(PBU;)~-(C=C),+~ 2.96, 3.22, 3.48 2.07, 2.34, 2.61 polymer 5 fP~(PBU;)~-(C=C), -3n 2.74, 3.11, 3.38, 3.66 1.88, 2.09, 2.37 infer from this that the dimerisation amplitude for the excited 2 S.J. Davies, B. F. G. Johnson, M. S. Khan and J. Lewis, J. Chem. state is considerably weaker than that for the ground SOC., Chem. Commun., 1991, 187. state (butatrienic, C=C=C=C, or hexapentaenic, 3 S. J. Davies, B. F. G. Johnson, M. S. Khan and J. Lewis, J. Organomet. Chem., 1991, 401, C43.c=c=c=c=C=C). 4 B. F. G. Johnson, A. K. Kakkar, M. S. Khan and J. Lewis, In conclusion, Pt-metal-containing di- and tri-acetylenic J. Organomet. Chem., in the press. polymeric complexes are now accessible in good quantities, 5 K. Sonogashira, S. Takahashi and N. Hagihara, Macromolecules, and their high solubility in common organic solvents allows 1977, 10, 879. one to examine their physical properties.The electronic 6 S. Guha, C. C. Frazier, P. L. Porter, K. Kang and S. E. Finberg, Optics Lett., 1989, 14, 952.properties of these complexes are determined by the delocal- 7 G. R. Williams, J. Phys. C., 1988, 21, 1971.ised n-electron system along the polymeric chain. A detailed 8 S. R. Phillpot, M. J. Rice, A. R. Bishop and D. K. Campbell,investigation of the electronic structure, conductivity and non- Phys. Rev. B, 1987,36, 1735. linear optical properties of these and related polymers is 9 G. Zweifel and S. Rajagopalan, J. Am. Chem. SOC., 1985, 107, currently in progress. 700; H. Bock and H. Seidl, J. Chem. SOC. (B), 1968, 1158. 10 Organometallic Polymers, ed. S. Takahashi, M. Kariya, T. Yatake, We would like to thank NSERC of Canada for a postdoctoral K. Sonogashira and C. U. Pittman Jr., Academic Press, New York, 1978.fellowship to A. K. K., SERC for support to M. S.K. and 11 A. E. Dray, F. Wittmann, R. H. Friend, A.M. Donald, M. S.A. E. D., and Dhaka University (Bangladesh) for study leave Khan, J. Lewis and B. F. G. Johnson, Proc. Znt. Conf: Synth. (M. S.K.). We would also like to thank Professor Todd B. Met., Tubingen, Sept., 1990. Marder of University of Waterloo (Canada) for helpful dis- 12 A. E. Dray, F. Wittmann, R. H.Friend, A.M. Donald, M. S. cussions and Dr. Elizabeth Meehan (Polymer Laboratories, Khan, J. Lewis and B. F. G. Johnson, Synth. Met., in the press. UK) for molecular-weight determinations. 13 R. M. Silverstein, G. C. Bassler and T. C. Morrill, Spectroscopic Identification of Organic Compounds, Wiley, New York, 198 1. References Communication 1/01499D Received 28th March, 1991 N. Hagihara, K. Sonogashira and S. Takahashi, Adu. Polym. Sci., 1980, 41, 149.