Symmetry control of radiative decay in linear polyenes:: Low barriers for isomerization in the S1 state of hexadecaheptaene

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(1)A(g)(-) -> 1(1)A(g)(-)) and S-2 -> S-0 (1(1)B(u)(+) -> 1(1)A(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 similar to 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(1)A(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.