VISIBLE AND NEAR-INFRARED ULTRAFAST OPTICAL-DYNAMICS OF HEXAMETHYLSEXITHIOPHENE IN SOLUTION

The femtosecond and picosecond time evolution of the excited-state dyn amics in cr-conjugated hexamethyl sexythiophene in solution is measure d by pump-probe technique in a wide probe spectral region from 500 to 1500 nm. The transient differential transmission spectrum displays cha racteristic features depending upon probe time delay. Stimulated emiss ion in the region of cw fluorescence and photoinduced absorption at lo nger wavelength are present at short probe delay. These spectral featu res are assigned to S-1-->S-0 and S-1-->S-n 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 domin ated by a single photoinduced absorption band peaking at 650 nm. The l ack of absorption structures in the near-infrared region of the spectr um (from 1000 to 1500 nm) strongly argues against photogenerated charg ed states and supports the presence of triplet-state absorption (T-1-- >T-n).