Singlet Fission in Carotenoid Dimers -- The Role of the Exchange and Dipolar Interactions

TL;DR

This paper presents a theoretical model explaining singlet fission in carotenoid dimers, emphasizing the roles of exchange and dipolar interactions, and supports findings with simulated EPR spectra.

Contribution

It introduces a new theory detailing the mechanism of singlet fission in carotenoid dimers, highlighting the influence of exchange and dipolar interactions.

Findings

Ultrafast internal conversion to dark triplet-pair state

Simulation of EPR spectra shows residual exchange coupling

Complete singlet fission results in unentangled triplets

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" $n^1B_u^+$ state, there is ultrafast internal conversion to the intrachain "dark" $1^1B_u^-$ 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.