Biexponential fluorescence decay of diphenylbutadiene rotational conformers after extreme red edge excitation

After excitation at the extreme red edge (358 nm) of the S-1 <-- S-0 absorption band, the ambient temperature fluorescence decay of all-trans-1,4-diphenyl-1,3-butadiene (DPB) in cyclohexane is biexponential, indicating the presence of two distinct molecular species with substantially different excited-state lifetimes of 0.6 and 9 ns. The enthalpic barrier to nonradiative decay of 3.5 kcal/mol observed for the short-lived s-trans component over the temperature range 20-90 degrees C in methylcyclohexane agrees well with the value measured by Velsko and Fleming [J. Chem. Phys. 76 (1982) 3553] for DPB in cyclohexane. The red-shifted spectral profile of the long-lived emission matches the fluorescence spectrum previously assigned to the s-cis rotamer.