Triplet Pair States in Singlet Fission

This account aims at providing an understanding of singlet fission, i.e., the photophysical process of a singlet state (S-1) splitting into two triplet states (2 x T-1) in molecular chromophores. Since its discovery 50 years ago, the field of singlet fission has enjoyed rapid expansion in the past 8 years. However, there have been lingering confusion and debates on the nature of the all-important triplet pair intermediate states and the definition of singlet fission rates. Here we clarify the confusion from both theoretical and experimental perspectives. We distinguish the triplet pair state that maintains electronic coherence between the two constituent triplets, (1)(TT), from one which does not, (1)(T...T). Only the rate of formation of (1)(T...T) is defined as that of singlet fission. We present distinct experimental evidence for (1)(TT), whose formation may occur via incoherent and/or vibronic coherent mechanisms. We discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.