Theoretical Study of Singlet Fission in Carotenoids

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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.