Real-Time Observation of Multiexcitonic States in Ultrafast Singlet Fission Using Coherent 2D Electronic Spectroscopy

Singlet fission (SF) is the spin-allowed conversion of a spin-singlet exciton into a pair of maximally spin-entangled triplet excitons residing on neighbouring molecules. The existence of this process has been known for many decades, but the potential utility of SF in enhancing organic photovoltaic efficiency has created renewed interest in this mysterious process and the essential mechanisms by which it proceeds. To rationalise what is known about this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesised as an intermediate in SF. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. In this talk, I describe an investigation of a series of pentacene derivatives using ultrafast 2D electronic spectroscopy which unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100-fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.