Excited-State Dynamics of all-trans-1,3,5,7-Octatetraene in Solution. Direct Observation of Internal Conversion from the S2 to S1 State and Relaxation Processes in the S1 State

Abstract

Femtosecond transient absorption spectroscopy was used to study the excited-state dynamics of the S2 (11Bu) state all-trans-1,3,5,7-octatetraene in solution. The sample was excited at 267 nm and probed at eleven different wavelengths from 340 to 540 nm. Combined with the picosecond fluorescence spectroscopy, this study allows direct observation of the initial excited-state dynamics of all-trans-1,3,5,7-octatetraene and the subsequent relaxation process to the ground state. Transient absorption signals decaying on time scales of about 0.4 ps were seen at wavelengths longer than 480 nm. These absorptions are assigned to an Sn-S2 transition, indicating that internal conversion from the S2 to S1 state takes place on a 400-fs time scale. The transient absorption signals observed at shorter wavelengths, which correspond to the Sn-S1 transition, decay on a picosecond to subnanosecond time scale. A weak fluorescence at about 300−350 nm, originating from the S2 state, and a strong fluorescence at about 350−500 nm,...