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 initial photoexcitation of a carotenoid chain to a "bright" $n^1{B}_u^{+}$ state, there is ultrafast internal conversion to the intrachain "dark" $1^1{B}_u^{-}$ triplet-pair state. This strongly exchanged-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 spectra for lycopene dimers shows a distinct spectral signal due to the residual exchange coupling between the triplet-pairs on seperate carotenoid chains.