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

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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.