3RD-ORDER NONLINEAR-OPTICAL PROPERTIES IN THE EXCITED-STATE OF WELL-DEFINED THIOPHENE-DIMETHYLSILYL CO-OLIGOMERS

A new series of oligomers for nonlinear optics (NLO), the thiophene-di methylsilyl co-oligomers, (TSi)(n)-T with n = 1-3, have been synthesiz ed in order to combine third-order NLO response and transparency in th e visible region. Stationary-state absorption spectra, fluorescence sp ectra, and time-resolved absorption (picosecond) have been carried out . In sharp contrast with oligothiophenes, the absorption spectra of (T Si)(n)-T are not red-shifted as n increases, but as for short oligothi ophenes their quantum yield of fluorescence is low because of the comp etitive triplet-state formation and their emission wavelength range is remarkably solvent-independent. These data show slight increase of th e dipole moment between the S-0 ground state and the S-1 excited state , in agreement with theoretical calculations (ZINDO method). Fast inte rsystem crossing is observed between singlet and triplet states and S- 1 --> S-n and T-1 --> T-n spectra have been measured. Optical nonlinea rity (gamma) of the excited state has been measured and compared to th e terthiophene. The large molecular second hyperpolarizability of the silyl derivatives associated with their interesting transparency prope rties shows that such oligomers may be considered, in the excited stat e, as promising molecules for nonlinear applications.