Symmetry Control of Radiative Decay in Linear Polyenes: Low Barriers for Isomerization in the S_1 State of Hexadecaheptaene

摘要

The room temperature absorption and emission spectra of the 4-cis and all-trans isomers of 2,4,6,8,10,12,14-hexadecaheptaene are almost identical, exhibiting the characteristic dual emissions S_1→S_0 (2~1A_g~- → 1~1A_g~-) and S_2→S_0 (1~1B_u~+ → 1~1A_g~-) noted in previous studies of intermediate length polyenes and carotenoids. The ratio of the S_1→S_0 and S_2→S_0 emission yields for the cis isomer increases by a factor of ～15 upon cooling to 77 K in n-pentadecane. In contrast, for the trans isomer this ratio shows a 2-fold decrease with decreasing temperature. These results suggest a low barrier for conversion between the 4-cis and all-trans isomers in the S_1 state. At 77 K, the cis isomer cannot convert to the more stable all-trans isomer in the 2~1A_g~- state, resulting in the striking increase in its S_1→S_0 fluorescence. These experiments imply that the S_1 states of longer polyenes have local energy minima, corresponding to a range of conformations and isomers, separated by relatively low (2-4 kcal) barriers. Steady state and time-resolved optical measurements on the S_1 states in solution thus may sample a distribution of conformers and geometric isomers, even for samples represented by a single, dominant ground state structure. Complex S_1 potential energy surfaces may help explain the complicated S_2→S_1 relaxation kinetics of many carotenoids. The finding that fluorescence from linear polyenes is so strongly dependent on molecular symmetry requires a reevaluation of the literature on the radiative properties of all-trans polyenes and carotenoids.