Poly(silyleneethynylenephenylene) and poly(silylenephenyleneethynylenephenylene)s: Synthesis and photophysical properties related to charge transfer

Novel poly(dimethylsilyleneethynylenephenylene) and poly(dimethyl (or diphenyl) silylenephenyleneethynylenephenylene)s without diyne defects were synthesized by using the Pd/Cu-catalyzed AB-type coupling reaction. 6's showed small but clear π-to-σ charge-transfer absorption bands at longer wavelengths (>340 nm) than those of π-π* absorption bands (<320 nm), whereas the charge-transfer absorption band was not found in 5. When 6's were excited at the π-π* absorption wavelengths, two type of emission were observed: One is the vibronically structured emission with a maximum around 325 nm due to the locally excited π* state, and the other is the intramolecular charge-transfer emission having a broad and structureless band (emission λ_max ca. 370 nm) with a large Stokes shift. On the other hand, when excited at the charge-transfer absorption wavelengths, 6's exhibited only the intramolecular charge-transfer emission with a maximum above 390 nm. This suggests that the indigo blue light emission of 6's is due to the charge-transfer phenomenon and not to the locally excited π* state. The charge-transfer character of 6's was clearly verified by the shift of emission in polar solvents.