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 a bsorption band, the ambient temperature fluorescence decay of all-tran s-1,4-diphenyl-1,3-butadiene (DPB) in cyclohexane is biexponential, in dicating the presence of two distinct molecular species with substanti ally 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-l ived s-trans component over the temperature range 20-90 degrees C in m ethylcyclohexane agrees well with the value measured by Velsko and Fle ming [J. Chem. Phys. 76 (1982) 3553] for DPB in cyclohexane. The red-s hifted spectral profile of the long-lived emission matches the fluores cence spectrum previously assigned to the s-cis rotamer.