Singlet fission in carotenoid dimers – the role of the exchange and dipolar interactions

Abstract

A theory of singlet fission in carotenoid dimers is presented which aims to explain the mechanism behind the creation of two uncorrelated triplets. Following the excitation of a carotenoid chain “bright” n¹B⁺_u state, there is ultrafast internal conversion to the intrachain “dark” 1¹B⁻_u triplet-pair state. This strongly exchange-coupled state evolves into a pair of triplets on separate chains and spin-decoheres to form a pair of single, unentangled triplets, corresponding to complete singlet fission. The simulated EPR spectrum for parallel lycopene monomers in a dimer (i.e., H-aggregate) shows a distinct spectral signal due to the residual exchange coupling between the triplet-pairs on seperate carotenoid chains.