Femtosecond studies of electronic relaxation, vibrational relaxation, and rotational diffusion in all-trans-1,8-diphenyl-1,3,5,7-octatetraene

Femtosecond transient absorption studies of the excited singlet state dynamics of all-trans-1,8-diphenyl-1,3,5,7-octatetraene were conducted in various hydrocarbon solvents at room temperature using 390 nm excitation and probe wavelengths between 630 and 820 nm. Linear dichroism measurements show that one of the decay components (100- 150 ps) arises from rotational diffusion. The solvent viscosity dependence is consistent with that interpretation. Transient absorptions of the S-2(1B(u)) and S-1 (2A(g)) states show ultrafast S-2 --> S-1 internal conversion (470-600 fs) in association with possible vibrational relaxation within S-1 and S-2 On similar time scales.