Modulating Endothermic Singlet Fission by Controlling Radiative Rates in Perylene Dimers

TL;DR

This study demonstrates how controlling radiative rates in perylene dimers influences endothermic singlet fission, revealing a method to extend triplet state lifetimes and improve triplet yields for photovoltaic applications.

Contribution

It provides experimental evidence that radiative rate manipulation can control the equilibrium and lifetime of singlet fission states in perylene dimers, a novel approach for optimizing triplet generation.

Findings

Slower radiative rates lead to longer-lived 1(T1T1) states.

Endothermic singlet fission can be modulated by molecular design.

Triplet yields can exceed 100% with additional chromophores.

Abstract

Endothermic singlet fission (SF), an exciton multiplication process that produces a pair of high-energy triplet excitons (T1T1), is appealing for photovoltaic or photoelectrochemical applications, as it allows the conversion of entropy into electronic or chemical energy. The mechanistic aspects of this process are not entirely known, and strategies for improving the yield of triplets via endothermic SF have not been developed. In this work we provide experimental evidence that in photoexcited dimers of perylene, S1 is initially in equilibrium with 1(T1T1), and that the lifetime of this equilibrium can be controlled through strategic changes in the radiative rate. Through careful molecular design we fine-tune both the degree of endothermicity and excited state lifetimes in four perylene dimers. Using transient absorption and time resolved fluorescence, we reveal that the dimer with the slowest radiative rate constant produces the most prolonged 1(T1T1). However, in the dimers, the annihilation of the 1(T1T1) state results in a single long-lived triplet rather than a pair, and increasing the free triplet yield above 100% would require additional chromophores.