Electronic coupling of multiexcitonic states: Development of quantum chemical methods and their application to singlet fission and triplet fusion

多激子态的电子耦合：量子化学方法的发展及其在单线态裂变和三线态聚变中的应用

• 批准号: 442935252
• 负责人: Professorin Dr. Christel M. Marian
• 金额: --
• 依托单位: Institut für Theoretische Chemie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/442935252?language=en
• 关键词: Electronic; coupling; multiexcitonic; states; Development

Conventional photovoltaic devices harvest only fractions of the incident solar light. Owing to the importance of renewable energy, substantial effort has been put into improving the efficiency of solar cells. Two closely related strategies for harvesting those parts of the solar light spectrum, which are typically lost by heat-conversion, are the fusion of two lowenergy triplet excitons and upconversion to a higher-energy singlet exciton (triplet–triplet annihilation up conversion, TTA-UC) and singlet fission (SF), the fission of a high-energy singlet exciton into two lower-energy triplet excitons. Although there is consensus regarding the general mechanisms of TTA-UC and SF, the details are still heavily debated and an active field of research. It turned out, for example, that the faster SF process is not necessarily the more efficient one. Moreover, deviations of the quantum yield from the spin-statistical limit have been observed in several SF and TTA-UC systems. This means either that the formation of the (T: : :T) triplet pair states do not follow spin statistics or that fine-structure interactions couple the (T: : :T) states. The here proposed project is set in the field of theoretical and computational chemistry. The triplet-pair states are multiexcitonic states, i.e., they are doubly excited with respect to the electronic ground state, which prevents the application of Förster- and Dexter-type models for describing the excitation energy transfer. The main objectives of the proposed research are to put in place powerful tools for the computational evaluation of spin-allowed and spin-forbidden couplings of multiexcitonic states beyond minimal active space models and to apply these tools to a series of challenging case studies on covalently linked polyacence dimers or tightly bound encounter complexes of polyacences. Herein, we aim to understand and thereby get a handle on the factors and structural parameters that steer the efficiency of SF and TTA-UC in such systems. In particular, we intend to design and parameterize a new semiempirical DFT/MRCI Hamiltonian which tolerates a one-particle basis of fragment-localized, noncanonical orbitals, thus easing the determination of nonadiabatic coupling matrix elements via diabatization of the DFT/MRCI eigenstates. Moreover, we will develop computer codes and approximations enabling the calculation of electronic spin–spin coupling between singlet, triplet, and quintet states in large molecular systems. Application of these codes will provide access to the magnitude of fine-structure interactions between the aforementioned (T: : :T) states. The complexity of these phenomena, the sheer size of the molecular systems and the different natures of the electronic states potentially involved — in particular of multiexcitonic states — represent real challenges for any given excited-state electronic structure method but we are confident that we can tackle these problems in the present project.

传统的光伏设备只能收集入射太阳光的一小部分。由于可再生能源的重要性，在提高太阳能电池的效率方面已经投入了大量的努力。两种密切相关的方法来收集太阳光谱中那些通常因热转换而丢失的部分，它们是两个低能三重态激子的融合并上转换为一个高能量的单重态激子（三重态湮灭上转换，TTA-UC）和单重态裂变（SF），即一个高能量的单重态激子裂变成两个低能的三重态激子。虽然关于ta - uc和SF的一般机制已经达成共识，但其细节仍然存在激烈的争论和活跃的研究领域。例如，事实证明，更快的顺丰过程并不一定更有效。此外，在一些SF和ta - uc体系中，已经观察到量子产额偏离自旋统计极限。这意味着（T:::T）三重态的形成不遵循自旋统计，或者精细结构相互作用耦合（T:::T）态。本课题设置在理论与计算化学领域。三对态是多激子态，即它们相对于电子基态是双重激发的，这使得Förster-和dexter型模型无法用于描述激发能转移。所提出的研究的主要目标是建立强大的工具来计算超过最小活性空间模型的多激子态的自旋允许和自旋禁止耦合，并将这些工具应用于一系列具有挑战性的案例研究，包括共价连接的多聚二聚体或紧密结合的多聚配合物。在这里，我们的目标是理解并因此得到一个处理的因素和结构参数，引导在这种系统的SF和ta - uc的效率。特别是，我们打算设计和参数化一个新的半经验DFT/MRCI哈密顿量，它可以容忍片段局域化的非正则轨道的单粒子基础，从而通过DFT/MRCI特征态的非绝热耦合矩阵元素的非绝热耦合矩阵元素的确定。此外，我们将开发计算机代码和近似，以便计算大分子系统中单重态，三重态和五重态之间的电子自旋-自旋耦合。这些代码的应用将提供对上述（T:::T）状态之间精细结构相互作用幅度的访问。这些现象的复杂性、分子系统的庞大规模和可能涉及的电子状态的不同性质——特别是多激子状态——对任何给定的激发态电子结构方法都是真正的挑战，但我们有信心在目前的项目中解决这些问题。