Investigations of Triplet-Pair States in π-Conjugated Singlet Fission Materials

Singlet fission is the process by which a singlet exciton splits into two ‘free’ triplet excitons. This was the definition of singlet fission up to the last few years when advances in the understanding of singlet fission in acene and heteroacene materials sparked a change in nomenclature in the field. Techniques such as electron paramagnetic resonance spectroscopy have allowed researchers to improve the understanding of singlet fission by visualizing the intermediate states in the singlet fission process.

Currently in acene literature, it is common for the initial step of a singlet exciton converting to a singlet character triplet-pair state to be considered singlet fission. This change has interesting ramifications when it is applied to polyenes, another class of singlet fission material, where internal conversion from the absorbing state is thought to occur to a singlet state of triplet-pair character. We contribute to the above discussion by investigating the intermediate triplet-pair states in three systems using innovative techniques.

We start by investigating the well-studied TIPS-tetracene system using strong light-matter coupling to manipulate the character of the intermediate states. The results here point to a whole field of research, manipulating state energies and radiative character to enhance triplet-fed emission. In Chapters 5 and 6 we investigate the question posed above by studying two polyene systems. First, we measure a series of oligo(thienylene-vinylenes) which allow us to study the conjugation length dependence of singlet fission. Then in the final chapter we use an exciting new design philosophy of man-made proteins to form carotenoid aggregates. Through this chapter we are able to show experimental evidence for the triplet-pair nature of the 2Ag- singlet state in polyenes. Furthermore, we find the surprising result that singlet fission is incredibly robust in carotenoid aggregates being invariant with protein environment and intramolecular structure. We finish by discussing the implications of the results presented here and point to possible future avenues to further demystify the decay processes of singlet fission materials.