Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom.
Linacre College, University of Oxford, Oxford OX1 3JA, United Kingdom.
J Phys Chem A. 2023 Feb 9;127(5):1342-1352. doi: 10.1021/acs.jpca.2c07781. Epub 2023 Jan 26.
We describe our simulations of the excited state dynamics of the carotenoid neurosporene, following its photoexcitation into the "bright" (nominally 1B) state. To account for the experimental and theoretical uncertainty in the relative energetic ordering of the nominal 1B and 2A states at the Franck-Condon point, we consider two parameter sets. In both cases, there is ultrafast internal conversion from the "bright" state to a "dark" singlet triplet-pair state, i.e., to one member of the "2A" family of states. For one parameter set, internal conversion from the 1B to 2A states occurs via the dark, intermediate 1B state. In this case, there is a cross over of the 1B and 1B diabatic energies within 5 fs and an associated avoided crossing of the S and S adiabatic energies. After the adiabatic evolution of the S state from predominately 1B character to predominately 1B character, there is a slower nonadiabatic transition from S to S, accompanied by an increase in the population of the 2A state. For the other parameter set, the 2A energy lies higher than the 1B energy at the Franck-Condon point. In this case, there is cross over of the 2A and 1B energies and an avoided crossing of the S and S energies, as the S state evolves adiabatically from being of 1B character to 2A character. We make a direct connection from our predictions to experimental observables by calculating the time-resolved excited state absorption. For the case of direct 1B to 2A internal conversion, we show that the dominant transition at ca. 2 eV, being close to but lower in energy than the T to T transition, can be attributed to the 2A component of S. Moreover, we show that it is the charge-transfer exciton component of the 2A state that is responsible for this transition (to a higher-lying exciton state), and not its triplet-pair component. These simulations are performed using the adaptive tDMRG method on the extended Hubbard model of π-conjugated electrons. The Ehrenfest equations of motion are used to simulate the coupled nuclei dynamics. We next discuss the microscopic mechanism of "bright" to "dark" state internal conversion and emphasize that this occurs via the exciton components of both states. Finally, we describe a mechanism relying on torsional relaxation whereby the strongly bound intrachain triplet-pairs of the "dark" state may undergo interchain exothermic dissociation.
我们描述了类胡萝卜素神经紫质激发态动力学的模拟，模拟其在被光激发到“明亮”（名义上为 1B）态后的情况。为了考虑实验和理论上在 Franck-Condon 点时名义上的 1B 和 2A 态的相对能量排序的不确定性，我们考虑了两个参数集。在这两种情况下，从“明亮”态到“暗”单重三重态对态，即“2A”态家族的一个成员，都有超快的内转换。对于一个参数集，从 1B 到 2A 态的内转换是通过暗的中间 1B 态发生的。在这种情况下，1B 和 1B 绝热能量在 5 fs 内交叉，S 和 S 绝热能量发生避免交叉。S 态从主要的 1B 特征演变为主要的 1B 特征后，会发生较慢的非绝热跃迁，从 S 态到 S 态，同时 2A 态的种群增加。对于另一个参数集，2A 能量在 Franck-Condon 点时高于 1B 能量。在这种情况下，2A 和 1B 能量交叉，S 和 S 能量避免交叉，因为 S 态从主要的 1B 特征演变为 2A 特征。我们通过计算时间分辨激发态吸收，直接将我们的预测与实验可观测量联系起来。对于直接从 1B 到 2A 内转换的情况，我们表明，在大约 2 eV 处的主要跃迁，虽然接近但能量低于 T 到 T 跃迁，可以归因于 S 的 2A 成分。此外，我们表明，负责此跃迁（到更高的激子态）的是 2A 态的电荷转移激子成分，而不是其三重态对成分。这些模拟是使用扩展 Hubbard 模型对π共轭电子进行自适应 tDMRG 方法进行的。 Ehrenfest 运动方程用于模拟耦合核动力学。接下来，我们将讨论“明亮”到“黑暗”态内转换的微观机制，并强调这是通过两个态的激子成分发生的。最后，我们描述了一种依赖于扭转松弛的机制，通过该机制，“黑暗”态的强束缚链内三重态对可能经历链间放热解离。
