Electronic coupling of multiexcitonic states: Development of quantum chemical methods and their application to singlet fission and triplet fusion
Final Report Abstract
Conventional silicon-based photovoltaic devices harvest only fractions of the incident solar light. Two closely related strategies for harvesting those parts of the solar light spectrum, which are typically lost by heat-conversion, are the fusion of lowenergy triplet excitons and up-conversion to a higher-energy singlet exciton (TTA-UC) and singlet fission (SF), the fission of a high-energy singlet exciton into two lower-energy triplet excitons. SF preferentially occurs in pentacene derivatives in the solid state and in covalently linked pentacene dimers. Experimental observations indicate that the relative orientation of the monomer units plays a key role for the rate and efficiency of the SF process. The reported project is set in the field of theoretical and computational chemistry. It aimed at developing methods for a balanced quantum chemical description of all states involved in SF and TTA-UC. Case studies of selected examples should contribute to an understanding of the related mechanisms. Contrary to our expectations, none of the DFT/MRCI variants known so far provided a reliable treatment of doubly excited triplet-pair states. In 2022, we succeeded in developing a DFT/MRCI Hamiltonian with improved description of double excitations. Until then, the MRCI program and the program for computing electronic spin–spin dipole interactions were sharedmemory parallelized. Following its assessment and validation, the new Hamiltonian was applied to investigate the SF mechanism in pentacene and TIPS-pentacene crystals and in 3 regioisomers of a covalently linked TIPS-pentacene dimer. The herringbone structure of the pentacene crystal facilitates a charge-transfer mediated SF mechanism whereas the brickwork structure of the TIPS-pentacene crystal promotes the direct transition from the primarily excited singlet state to the antiferromagnetically coupled triplet-pair state. The different rise times of the triplet formation in the 3 regioisomers of the covalently linked TIPS-pentacene dimer may be explained by the varying electronic coupling strength. We attribute the high triplet quantum yield of the SF in the weakly coupled meta isomer to the positive energy balance of the process which slows down the TTA-UC back reaction. Spin-forbidden channels involving the ferromagnetically coupled triplet-pair state do not contribute decisively to the SF mechanism in the examples studied.
Publications
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Shared-Memory parallelisierte Version der DFT/MRCI-Programms
Marian, Christel M.
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Development and Parameterization of a DFT/MRCI Hamiltonian based on Different Density Functionals, Dissertation, Heinrich-Heine-Universität Düsseldorf
Dombrowski, Dennis R.
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Electron affinities and lowest triplet and singlet state properties of para-oligophenylenes (n = 3–5): theory and experiment. Physical Chemistry Chemical Physics, 25(43), 29850-29866.
Schulz, Timo; Konieczny, Paul; Dombrowski, Dennis R.; Metz, Simon; Marian, Christel M. & Weinkauf, Rainer
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R2022: A DFT/MRCI Ansatz with Improved Performance for Double Excitations. The Journal of Physical Chemistry A, 127(8), 2011-2025.
Dombrowski, Dennis R.; Schulz, Timo; Kleinschmidt, Martin & Marian, Christel M.
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Evaluation of the DFT/MRCI Method in the Contexts of Singlet Fission and Photodetachment– Photoelectron Spectroscopy, Dissertation, Heinrich-Heine-Universität Düsseldorf
Schulz, Timo
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Multiexcitonic and optically bright states in subunits of pentacene crystals: A hybrid DFT/MRCI and molecular mechanics study. The Journal of Chemical Physics, 160(14).
Schulz, Timo; Hédé, Simon; Weingart, Oliver & Marian, Christel M.
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Simulating the full spin manifold of triplet‐pair states in a series of covalently linked TIPS‐pentacenes. Journal of Computational Chemistry, 45(32), 2727-2738.
Schulz, Timo & Marian, Christel M.
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Indications for a Direct Singlet Fission Mechanism in TIPS-Pentacene Crystals from Hybrid DFT/MRCI and Molecular Mechanics Studies. The Journal of Physical Chemistry Letters, 16(11), 2887-2895.
Schulz, Timo; Hédé, Simon & Marian, Christel M.
