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 S₁→S₀ (2¹A_g^- → 1¹A_g^-) and S₂→S₀ (1¹B_u⁺ → 1¹A_g^-) noted in previous studies of intermediate length polyenes and carotenoids. The ratio of the S₁→S₀ and S₂→S₀ 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 S₁ state. At 77 K, the cis isomer cannot convert to the more stable all-trans isomer in the 2¹A_g^- state, resulting in the striking increase in its S₁→S₀ fluorescence. These experiments imply that the S₁ 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 S₁ 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 S₁ potential energy surfaces may help explain the complicated S₂→S₁ 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.