Excited-state energy dynamics of conjugated polycarbogermane oligomers:: Introduction effects of germanium atom into π-conjugated molecular system

Excited-state energy dynamics of the conjugated polycarbogermane oligomers, poly[ [1,4-bis(thiophenyl)buta-1,3-diyne]-alt-(dimethylgermane)] (PBTBD-DMG; n = 33), poly{[1,4-bis-(thiophenyl)buta-1,3-diyne]-alt-(diphethylgermane)} (PBTBD-DPG; n = 12), polyl[1,4-bis(phenyl)buta-1,3-diyne]-alt-(dimethylgermane)} (PBPBD-DMG; n = 36), and polyl[1,4-bis(phenyl)buta-1,3-diyne]-alt-(diphenylgermane)} (PBPBD-DPG; n = 2), were investigated by steady-state and picosecond time-resolved fluorescence spectroscopies in liquid solution. The introduction effect of a germanium atom into pi-conjugated oligomer backbones and the substitution effect of a methyl or phenyl group on the germanium atom are discussed from solvent polarity-dependent studies. Steady-state and time-resolved fluorescence studies on the thiophene-containing polycarbogermane (PBTBD-DMG and PBTBD-DPG) oligomers revealed considerable solvent polarity-dependent characteristics, whereas those of the phenylene-containing polycarbogermane (PBPBD-DMG and PBPBD-DPG) oligomers do not significantly show such characteristics. As the solvent polarity increased from n-hexane to tetrahydrofuran, the steady-state fluorescence spectra of PBTBD-DMG and PBTBD-DPG oligomers were significantly redshifted, and their fluorescence lifetimes seemed to change from similar to624 to similar to46 ps. These results suggest that the excited-state dynamics of PBTBD-DMG and PBTBD-DPG oligomers are related to an intramolecular charge transfer (ICT) emission process through (d-p) pi conjugation between the pi-conjugated system and unoccupied 4d orbitals of the germanium atom. These results are supported by quantum chemical (AM1 and CNDO/2) calculations. (C) 2002 Wiley Periodicals, Inc.