Third-Order Nonlinear Optical Properties in the Excited State of Well-Defined Thiophene−Dimethylsilyl Co-oligomers

Abstract

A new series of oligomers for nonlinear optics (NLO), the thiophene−dimethylsilyl co-oligomers, (TSi)n−T with n = 1−3, have been synthesized in order to combine third-order NLO response and transparency in the visible region. Stationary-state absorption spectra, fluorescence spectra, and time-resolved absorption (picosecond) have been carried out. In sharp contrast with oligothiophenes, the absorption spectra of (TSi)n−T are not red-shifted as n increases, but as for short oligothiophenes their quantum yield of fluorescence is low because of the competitive triplet-state formation and their emission wavelength range is remarkably solvent-independent. These data show slight increase of the dipole moment between the S0 ground state and the S1 excited state, in agreement with theoretical calculations (ZINDO method). Fast intersystem crossing is observed between singlet and triplet states and S1 → Sn and T1 → Tn spectra have been measured. Optical nonlinearity (γ) of the excited state has been measured and co...