Femtosecond transient absorption studies of trans- and cis-1,3,5-hexatriene in solution

The electronic, vibrational and conformational relaxation of trans-and cis-1,3,5-hexatriene in solution have been investigated by femtosecond transient absorption spectroscopy. Upon excitation to the 1B state, the transient absorption is observed in the visible region, which mainly consists of an instantaneous rise (<150 fs) and a subpicosecond decay. For the trans isomer, the decay of the transient absorption signals becomes faster at longer wavelengths. On the other hand, for the cis isomer, the decay of the signals does not depend on the probe wavelength significantly. For both isomers, these temporal profiles of signals show that the internal conversion from the S-2 to S-1 state occurs within 100 fs and the lifetime of the SI state is less than 500 fs. The observed dependence of the decay on the probe wavelengths for the trans isomer is ascribed to vibrational energy redistribution in the S-1 state. We also measured the bleach recovery signals by the one-color pump-probe method in the UV region. The bleach recovers in several time scales. The temporal profiles of trans isomer in the first 20 ps are similar to those of cis isomer, which come from the vibrational relaxation in the ground state. However, on a longer time scale, these signals are quite different to each other. While the trans isomer takes 150-300 ps to fully recover the bleach, the cia isomer requires no more than 100 ps. These slow components are attributed to the conformational relaxation around the C-C single bond to the original forms. The different behavior between the trans and cis isomers reflects different product distributions and dynamics among the isomers around the C-C single bonds.