Endothermic singlet fission does not proceed via an excimer intermediate

TL;DR

This study shows that in endothermic singlet fission, the excimer state does not act as an intermediate but instead traps excited states, reducing the efficiency of triplet formation in photovoltaic applications.

Contribution

The paper provides experimental evidence that excimers do not facilitate but hinder endothermic singlet fission, guiding future material design to avoid excimer formation.

Findings

Excimers trap excited states, reducing singlet fission yield.

Singlet fission occurs at longer distances than excimer formation.

Designing materials to prevent excimer formation enhances photovoltaic efficiency.

Abstract

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for maximum energy conversion efficiency, and such systems have been shown to form an excimer-like state with multi-excitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states, to the detriment of singlet fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet fission material must be designed that avoids excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.