Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia.
J Phys Chem A. 2020 Jul 16;124(28):5790-5803. doi: 10.1021/acs.jpca.0c01678. Epub 2020 Jul 6.
Knowledge about excited states of carotenoids is essential for understanding photophysical processes underlying photosynthesis. However, due to the presence of a large number of optically dark states, experimental study of the excited-state manifold is limited to a significant extent. In this paper, we apply high-level quantum chemical methods to study the low-lying excited states of polyenes containing from 8 to 13 conjugated double bonds, which serve as a model for natural carotenoids. Vertical and adiabatic excitation energies from the ground 1A state to the excited 2A, 1B, and 1B states were evaluated by means of density matrix renormalization group (DMRG) with NEVPT2 perturbative correction. The energies of all excited states are highly sensitive to nuclear geometry, especially the 2A state. Thus, the 2A and 1B states interchange their relative positions upon geometry relaxation, while the vertical excitation energy to the 2A state is rather high. At the same time, the 1B state energy is shown to be higher than other studied excited states at any geometry. With relaxed geometries of the excited states, absorption and transient absorption spectra were calculated within the Franck-Condon approximation bridging the gap between experimental spectroscopic data and computational results.
关于类胡萝卜素激发态的知识对于理解光合作用的光物理过程至关重要。然而，由于存在大量的光学暗态，激发态系综的实验研究在很大程度上受到限制。在本文中，我们应用高级量子化学方法研究了含有 8 到 13 个共轭双键的聚烯的低能激发态，这些聚烯作为天然类胡萝卜素的模型。通过密度矩阵重整化群（DMRG）与 NEVPT2 微扰校正，评估了从基态 1A 态到激发态 2A、1B 和 1B 态的垂直和绝热激发能。所有激发态的能量对核几何形状高度敏感，特别是 2A 态。因此，在几何弛豫时，2A 和 1B 态会交换它们的相对位置，而 2A 态的垂直激发能相当高。同时，在任何几何形状下，1B 态的能量都显示出高于其他研究的激发态。对于激发态的弛豫几何形状，我们在 Franck-Condon 近似范围内计算了吸收和瞬态吸收光谱，以弥合实验光谱数据和计算结果之间的差距。
