Visible and near-infrared ultrafast optical dynamics of hexamethylsexithiophene in solution

The femtosecond and picosecond time evolution of the excited-state dynamics in ?-conjugated hexamethyl sexythiophene in solution is measured by pump-probe technique in a wide probe spectral region from 500 to 1500 nm. The transient differential transmission spectrum displays characteristic features depending upon probe time delay. Stimulated emission in the region of cw fluorescence and photoinduced absorption at longer wavelength are present at short probe delay. These spectral features are assigned to S1->S0 and S1->Sn transitions, respectively. A singlet-state lifetime of 160 ps and a radiative decay time of 1.6 ns are evaluated. At large probe delay (?400 ps) the spectrum is dominated by a single photoinduced absorption band peaking at 650 nm. The lack of absorption structures in the near-infrared region of the spectrum (from 1000 to 1500 nm) strongly argues against photogenerated charged states and supports the presence of triplet-state absorption (T1->Tn).