cis–trans photoisomerization of silylene-vinylene-p-phenylene polymers and their model compounds

The emission and photochemical properties of the poly[dimethylsilylene-p-phenylene-dimethylsilylene-(E)-vinylene-p-phenylene-(E)-vinylene] (trans-P) and its model compound (E,E)-{1,4-bis(2-dimethylphenylsilyl)ethenyl}benzene (trans-ST) were investigated in a quantitative manner. It was shown that both compounds exhibit similar properties indicating that the 2-dimethylphenylsilylethenylbenzene chromophore is responsible for the absorption and emission processes and for the reactivity in the lowest excited singlet states. The fluorescence quantum yields measured in dichloromethane were found to be small and equal to 0.032 and 0.008 for trans-P and trans-ST, respectively. On the other hand the cis-ST isomer was shown to be a nonfluorescent species. The model compounds and the polymer undergo the same type of efficient photochemical reaction, i.e. the cis–trans isomerization leading to the photo-stationary state depending on the excitation wavelengths. The photoisomerization reaction was studied quantitatively using UV and HPLC analyses. The initial isomerization quantum yields were relatively large (in the range of 0.14–0.49 depending on the excitation wavelengths and the solvent used). Quantum yields Φtrans→cis for the polymer trans-P and its model compounds trans-ST were found to be similar within the experimental errors (0.23 and 0.26, respectively, for irradiation with the monochromatic light λexc = 313 nm in dichloromethane). During prolonged irradiations, a photodegradation reaction of low efficiency (the quantum yield <1 × 10−2) was also observed in dichloromethane. It was shown that the UV absorption spectroscopy and the procedure applied can successfully be used for a quantitative description of the cis–trans photoisomerization mechanism of silylene-vinylene-p-phenylene polymers and its model compounds.