Molecular Approaches and Structural Insights into Correlated Triplet Pair Dynamics in Intramolecular Singlet Fission

Abstract

Singlet Fission (SF) is a bichromophoric process whereby a singlet exciton is converted to two triplet excitons through an overall spin-conserved process. The last decade has seen an upsurge in SF research driven by its potential application in photovoltaics to exceed the power conversion efficiency of solar cells beyond the Shockley-Quiesser (S-Q) limit. Singlet fission proceeds through a correlated triplet pair state, 1(TT), with an overall singlet character allowing the singlet state to access the triplet surface in ultrafast timescales. The formation of a coherent exciton pair makes these materials promising candidates in quantum technology and as polarized spin generators. As a result, understanding the triplet pair dynamics and their correlation with molecular structure attracts widespread attention. It has been established that the primary governing factor that dictates the singlet fission dynamics is the electronic coupling between the two chromophores. Interestingly, the intrinsic duality of the SF process is that a strong electronic coupling leads to efficient triplet pair formation and enhanced recombination, reducing the spatial separation into free triplets. This demands the development of design approaches to optimise electronic coupling. In my thesis, I have rationally designed molecular approaches to control triplet pair dynamics, coherences and triplet separation in molecular dimers. We have designed an array of pentacene dimers, particularly looking into the effects of molecular geometry and conformations on SF. Transient absorption measurements show that not only the static molecular geometry but dynamic molecular conformations also play a significant role in the outcome of the SF process. I have also studied the singlet fission dynamics in dimers with a resonant bridge where I have established that singlet fission is possible in systems where the initial excitation is delocalised over the two chromophores. Our observations show that the molecular structure and conformations heavily impact the dynamics within the triplet pair manifold.