Theoretical Study of Singlet Fission in Carotenoids

单线态裂变是一种光物理过程，由于其复杂的机制和提高光伏电池效率的潜力而受到人们的极大关注。经历单重态分裂(或其他多激子产生过程)的分子可以用于串联光伏电池。从理论上讲，这些电池的最大光电转换效率最高可达5%。本项目旨在使用两个不同的理论模型来探索类胡萝卜素和多烯中的单线态裂变：1)类胡萝卜素的含时密度矩阵重整化群(TD-DMRG)模拟，使用量子声子来模拟原子核，使用UV哈密顿量来模拟电子。2)考虑扩展的三重态-三重态和激子基团的多烯二聚体偶联模型。这个模型将探索一个由一个或两个多烯链组成的系统中的束缚三重态对基态的三重态-三重态的退相干。这项研究遵循了最近对多烯和类胡萝卜素中激发态的有序性的理论和实验兴趣，以及对发生单重态裂变所必需的三重态-三重态退相干的研究。第一个目标是探索类胡萝卜素中如何光产生具有单态分裂能力的单态三线态。我们将通过将Ehrenfest模拟与使用量子声子的模拟进行比较，来检验Ehrenfest近似在模拟从亮态到这些单重态三重态对的内部转换方面的有效性。第二个目标是探索三联体如何从束缚的三联体对解码成同一链或不同链上的独立的、非成对的三联体状态。这一过程被认为是分子间的，因为分子内单态分裂被认为是类胡萝卜素的吸热过程。我们想要探索这个模型是否支持提出的单线态裂变方案，在该方案中，电子相互作用的损失之后是自旋退相干。使用TD-DMRG对多烯和类胡萝卜素系统进行建模是该研究小组独有的一种新方法。我们将在以前工作的基础上，用量子德拜声子对原子核进行量子力学建模，而不是经典的Ehrenfest近似。这将提供更准确的模拟，当演变的波函数不停留在单个绝热板上，并且当波函数通过圆锥形交点时。通过将两条多烯链耦合在一起，耦合的乙烯二聚体多烯模型将能够探索分子间单重态裂变中三重态的退相干。以前的工作只集中在分子内单重态裂变过程。该项目属于计算化学和理论化学EPSRC研究领域。该项目还纳入了量子光学研究领域的部分内容，并支持物理科学和量子技术的战略重点。我们要感谢我们的实验合作者珍妮·克拉克。

Singlet fission is a photophysical process that is gaining lots of interest because of its complex mechanism and its potential to improve the efficiency of photovoltaic cells. Molecules that undergo singlet fission (or other multiple-exciton generation processes) could be used in tandem photovoltaic cells. Theoretically, these cells have a greater maximum photon-to-current efficiency by up to 5%. This project aims to explore singlet fission in carotenoids and polyenes using two different theoretical models: 1) Time-dependent density matrix renormalisation group (TD-DMRG) simulations of carotenoids, using quantum phonons to model the nuclei and the UV Hamiltonian for electrons. 2) Coupled ethene dimer model of polyenes, considering an expanded triplet-triplet and exciton basis. This model will explore triplet-triplet decoherence from a bound triplet pair ground state in a system of either one or two polyene chains. This research follows much recent theoretical and experimental interest into the ordering of the excited states in polyenes and carotenoids, as well as research into triplet-triplet decoherence that is necessary for singlet fission to occur. The first aim is to explore how singlet fission capable singlet triplet pair states are photogenerated in carotenoids. We will examine the effectiveness of the Ehrenfest approximation in modelling internal conversion from the bright state to these singlet triplet pair states, by comparing Ehrenfest simulations to simulations using quantum phonons. The second aim is to explore how triplets decohere from a bound triplet pair into separate, non-geminate pair of triplet states on the same chain or different chains. It is believed this process is intermolecular since intramolecular singlet fission is thought to be endothermic in carotenoids. We want to explore if this model supports a proposed scheme of singlet fission where loss of electronic interactions is followed by spin decoherence. Using TD-DMRG to model polyene and carotenoid systems is a novel approach unique to this research group. We will build upon previous work by modelling nuclei quantum mechanically with quantum Debye phonons, rather than classically with the Ehrenfest approximation. This will provide more accurate simulations when the evolving wavefunction does not stay on a single adiabat and when the wavefunction passes through a conical intersection. The coupled ethene dimer polyene model will be able to explore the decoherence of triplets in intermolecular singlet fission, by coupling two polyene chains together. Previous work has only focussed on intramolecular singlet fission processes. This project falls within the computational and theoretical chemistry EPSRC research area. This project also incorporates parts of the quantum optics research area and supports the physical sciences and quantum technologies strategic priorities. We would like to thank our experimental collaborator Jenny Clark.