σ,π* Charge-Transfer Excited States of Substituted (Phenylethynyl)pentamethyldisilanes

A series of substituted (arylethynyl)pentamethyldisilanes was prepared by the palladium(0)-catalyzed coupling of aryl halides and ethynylpentamethyldisilane. The absorption and emission spectra of these acetylenic disilanes were measured over the temperature range from 296 to 77 K in both polar and nonpolar solvents and rigid organic glasses. At 77 K normal (π,π*) fluorescence and ³(π,π*) phosphorescence are observed for those (phenylethynyl)pentamethyldisilanes (1) bearing electron-donating substituents (e.g. 4-MeO). At 77 K those (phenylethynyl)pentamethyldisilanes bearing electron-withdrawing substituents (e.g. 4-NC, 4-CO₂Me) show only a unique intramolecular ¹(σ,π*) charge-transfer (CT) fluorescence in addition to the ³(π,π*) state phosphorescence. Neither ground-state CT complex absorption bands nor exciplex emissions were observed in any of the systems studied. The singlet character of these emissive σ,π*CT states was confirmed by picosecond lifetime measurements. The previously reported photochemical rearrangement of (phenylethynyl)pentamethyldisilanes to silacyclopropenes and silapropadienes as well as the observed photochemical cleavage of (phenylethynyl)pentamethyldisilanes to (phenylethynyl)trimethylsilanes are fully consistent with the π,π* CT state assignment. The general involvement of analogous σ→π*CT or electron transfer in photoexcited polysilanes is suggested by the demonstration of intermolecular fluorescence quenching of electron-deficient benzenes and aryl acetylenes by hexamethyldisilane and dodecamethylcyclohexasilane. The general implications of this type of σ→π* CT to polysilane photophysics and photochemistry are discussed.