Theoretical Study of Singlet Fission in Carotenoids

单线态裂变是一个物理过程，由于其复杂的机制和提高光伏电池效率的潜力而引起了人们的广泛兴趣。经历单线态裂变（或其他多激子产生过程）的分子可以用于串联光伏电池。理论上，这些电池具有高达5%的更大的最大光子-电流效率。该项目旨在使用两种不同的理论模型探索类胡萝卜素和多烯的单线态裂变：1）类胡萝卜素的时间依赖密度矩阵重整化组（TD-DMRG）模拟，使用量子声子模拟原子核和电子的UV哈密顿量。2)考虑扩展三重态-三重态和激子基的多烯耦合乙烯二聚体模型。这个模型将探讨从一个或两个多烯链的系统中的束缚三重态对基态的三重态-三重态退相干。这项研究遵循最近的理论和实验兴趣到多烯和类胡萝卜素的激发态的排序，以及三重态-三重态退相干的研究，这是必要的单线态裂变发生。第一个目标是探索如何在类胡萝卜素中光生单重态裂变能力的单重三重态对。我们将研究的有效性，埃克塞特近似建模内部转换从明亮的状态，这些单重三重态对状态，通过比较埃克塞特模拟模拟使用量子声子。第二个目标是探索三重态如何从束缚的三重态对退变为同一链或不同链上单独的、非成对的三重态对。据信该过程是分子间的，因为分子内单线态裂变被认为在类胡萝卜素中是吸热的。我们想探索这个模型是否支持一个单重态裂变的方案，在这个方案中，电子相互作用的损失之后是自旋退相干。使用TD-DMRG来模拟多烯和类胡萝卜素系统是该研究小组独有的一种新方法。我们将建立在以前的工作，通过模拟原子核量子力学与量子德拜声子，而不是经典的埃克塞特近似。这将提供更准确的模拟时，演变的波函数不停留在一个单一的交点，当波函数通过一个圆锥形的交叉。通过将两个多烯链耦合在一起，耦合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线态裂变过程。这个项目属于计算和理论化学EPSRC研究领域的福尔斯。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.