TY - JOUR
T1 - Symmetry control of radiative decay in linear polyenes
T2 - Low barriers for isomerization in the S1 state of hexadecaheptaene
AU - Christensen, Ronald L.
AU - Galinato, Mary Grace I.
AU - Chu, Emily F.
AU - Fujii, Ritsuko
AU - Hashimoto, Hideki
AU - Frank, Harry A.
PY - 2007/2/14
Y1 - 2007/2/14
N2 - 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 (21Ag- → 11Ag -) and S2→S0 (11B U+ → 11Ag-) 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 21Ag- 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→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.
AB - 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 (21Ag- → 11Ag -) and S2→S0 (11B U+ → 11Ag-) 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 21Ag- 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→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.
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U2 - 10.1021/ja0609607
DO - 10.1021/ja0609607
M3 - Article
C2 - 17284007
AN - SCOPUS:33846946516
SN - 0002-7863
VL - 129
SP - 1769
EP - 1775
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 6
ER -