| Summary: | <i>Cis</i> isomers of carotenoids play important roles in light harvesting and photoprotection in photosynthetic bacteria, such as the reaction center in purple bacteria and the photosynthetic apparatus in cyanobacteria. Carotenoids containing carbonyl groups are involved in efficient energy transfer to chlorophyll in light-harvesting complexes, and their intramolecular charge–transfer (ICT) excited states are known to be important for this process. Previous studies, using ultrafast laser spectroscopy, have focused on the central-<i>cis</i> isomer of carbonyl-containing carotenoids, revealing that the ICT excited state is stabilized in polar environments. However, the relationship between the <i>cis</i> isomer structure and the ICT excited state has remained unresolved. In this study, we performed steady-state absorption and femtosecond time-resolved absorption spectroscopy on nine geometric isomers (7-<i>cis</i>, 9-<i>cis</i>, 13-<i>cis</i>, 15-<i>cis</i>, 13′-<i>cis</i>, 9,13′-<i>cis</i>, 9,13-<i>cis</i>, 13,13′-<i>cis</i>, and all-<i>trans</i>) of β-apo-8′-carotenal, whose structures are well-defined, and discovered correlations between the decay rate constant of the S<sub>1</sub> excited state and the S<sub>0</sub>−S<sub>1</sub> energy gap, as well as between the position of the <i>cis</i>-bend and the degree of stabilization of the ICT excited state. Our results demonstrate that the ICT excited state is stabilized in polar environments in <i>cis</i> isomers of carbonyl-containing carotenoids and suggest that the position of the <i>cis</i>-bend plays an important role in the stabilization of the excited state.
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