Temperature and viscosity sensitive S1 emission from a highly substituted triene

Fluorescence from a highly substituted triene, called mini-3, studied at cryogenic temperatures, originates from the lowest excited state (S1). Compared to the longer compounds homologous to all-trans-β-carotene, the emission is more Stokes-shifted. Another contrast is that the measured fluorescence quantum yield of mini-3 shows a distinct temperature/viscosity dependence: in 3-methylpentane decreasing from 0.61 at 77 K to 0.047 at 110 K. In room-temperature liquids it is virtually non-fluorescent. The discrepancies between mini-3 and the longer mini-carotenes are best explained by the involvement of out-of-plane vibrational modes in the coupling between S1 and S0 in mini-3, while only the CC stretching vibrations act as accepting and promoted modes in the longer ones. It is likely that the low-frequency modes in mini-3 are caused by a nonplanar equilibrium conformation, probably caused by steric hindrance. An Arrhenius plot resulted in an activation energy of 610 ± 200 cm−1, probably mainly governed by solvent viscosity. The fluorescence excitation anisotropy (rs) is about 0.39, indicating that the absorption and emission transition dipoles are parallel.