Migliore Agostino, Corni Stefano, Agostini Alessandro, Carbonera Donatella
Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italy.
CNR Institute of Nanoscience, 41125 Modena, Italy.
Phys Chem Chem Phys. 2023 Nov 1;25(42):28998-29016. doi: 10.1039/d3cp03836j.
The influence of carotenoid triplet states on the Q electronic transitions of chlorophylls has been observed in experiments on light-harvesting complexes over the past three decades, but the interpretation of the resulting spectral feature in the triplet minus singlet (T-S) absorption spectra of photosystems is still debated, as the physical-chemical explanation of this feature has been elusive. Here, we resolve this debate, by explaining the T-S spectra of pigment complexes over the Q-band spectral region through a comparative study of chlorophyll-carotenoid model dyads and larger pigment complexes from the main light harvesting complex of higher plants (LHCII). This goal is achieved by combining state-of-the-art time-dependent density functional theory with analysis of the relationship between electronic properties and nuclear structure, and by comparison to the experiment. We find that the special signature in the T-S spectra of both model and natural photosystems is determined by singlet-like triplet excitations that can be described as effective singlet excitations on chlorophylls influenced by a stable electronic triplet on the carotenoid. The comparison with earlier experiments on different light-harvesting complexes confirms our theoretical interpretation of the T-S spectra in the Q spectral region. Our results indicate an important role for the chlorophyll-carotenoid electronic coupling, which is also responsible for the fast triplet-triplet energy transfer, suggesting a fast trapping of the triplet into the relaxed carotenoid structure. The gained understanding of the interplay between the electronic and nuclear structures is potentially informative for future studies of the mechanism of photoprotection by carotenoids.
在过去三十年中，通过对光捕获复合物的实验观察到类胡萝卜素三重态对叶绿素Q电子跃迁的影响。但是，对于光系统三重态减单重态（T-S）吸收光谱中产生的光谱特征的解释仍存在争议，因为对这一特征的物理化学解释一直难以捉摸。在这里，我们通过对叶绿素-类胡萝卜素模型二元体以及高等植物主要光捕获复合物（LHCII）中更大的色素复合物进行比较研究，解释了色素复合物在Q波段光谱区域的T-S光谱，从而解决了这一争议。这一目标是通过将最先进的含时密度泛函理论与电子性质和核结构之间关系的分析相结合，并与实验进行比较来实现的。我们发现，模型和天然光系统的T-S光谱中的特殊特征是由类单重态三重态激发决定的，这种激发可以描述为受类胡萝卜素上稳定电子三重态影响的叶绿素上的有效单重态激发。与早期对不同光捕获复合物的实验比较证实了我们对Q光谱区域T-S光谱的理论解释。我们的结果表明叶绿素-类胡萝卜素电子耦合起着重要作用，这也导致了快速的三重态-三重态能量转移，表明三重态快速捕获到松弛的类胡萝卜素结构中。对电子结构和核结构之间相互作用的深入理解可能为类胡萝卜素光保护机制的未来研究提供信息。
