Wavefunction-Based Analysis of Dynamics Versus Yield of Free Triplets in Intramolecular Singlet Fission

TL;DR

This study uses wavefunction calculations to explore how linker structure affects the formation and dissociation of triplet states in intramolecular singlet fission, revealing linker-dependent behaviors that influence free triplet yield.

Contribution

It provides new insights into the linker-dependent dynamics of triplet states in acene dimers, highlighting unanticipated features that impact free triplet generation.

Findings

Faster biexciton formation correlates with lower free triplet yield.

Slower biexciton formation suggests smaller binding energy and higher free triplet yield.

Experimental results support the link between formation speed and triplet yield.

Abstract

Experiments in several intramolecular singlet fission materials have indicated that the triplet-triplet spin biexciton has a much longer lifetime than believed until recently, opening up loss mechanisms that can annihilate the biexciton prior to its dissociation to free triplets. We have performed many-body calculations of excited state wavefunctions of hypothetical phenylene-linked anthracene molecules to better understand linker-dependent behavior of dimers of larger acenes being investigated as potential singlet fission candidates. The calculations reveal unanticipated features that we show carry over to the real covalently-linked pentacene dimers. Dissociation of the correlated triplet-triplet spin biexciton and free triplet generation may be difficult in acene dimers where the formation of the triplet-triplet spin biexciton is truly ultrafast. Conversely, relatively slower biexciton formation may indicate smaller spin biexciton binding energy and greater yield of free triplets. Currently available experimental results appear to support this conclusion. Whether or not the two distinct behaviors are consequences of distinct mechanisms of triplet-triplet generation from the optical singlet is an interesting theoretical question.