TEMPERATURE AND VISCOSITY SENSITIVE S-1 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 (S -1). Compared to the longer compounds homologous to all-trans-beta-car otene, the emission is more Stokes-shifted. Another contrast is that t he measured fluorescence quantum yield of mini-3 shows a distinct temp erature/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 vibra tional modes in the coupling between S-1 and S-0 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 a re caused by a nonplanar equilibrium conformation, probably caused by steric hindrance. An Arrhenius plot resulted in an activation energy o f 610 +/- 200 cm(-1), probably mainly governed by solvent viscosity. T he fluorescence excitation anisotropy (r(s)) is about 0.39, indicating that the absorption and emission transition dipoles are parallel.