By converting one singlet exciton into two lower energy triplets, singlet fission has garnered renewed attention as a possible means to more efficiently convert high energy photons into electrical current in next-generation photovoltaic materials. There is also growing interest in the coherent generation of triplet pair states observed in singlet fission for potential applications in quantum information science. Recently, molecular pentacene dimers have emerged as a powerful means to study the structural and electronic parameters that influence singlet fission. Despite these advances, there are still unknowns regarding the coupling pathways governing the microscopic rates of fission, the ultimate rate of triplet decay, or the role of vibrational coherences in these processes. As such, there is need for a rigorous examination of the relationship between structure and singlet fission dynamics. Here we examine how ligand design can be applied to pentacene dimers to control interchromophore orientation and through-space interactions. We have employed a joint synthetic, spectroscopic, and computational approach to study a bipentacene system bridged by a flexible 2,5-di(2-pyridyl)pyrrole linker (HDPP-Pent). Additionally, deprotonation of the pyrrole moiety permits coordination of DPP-Pent to K+ or Li+ cations, leading to monomeric (KDPP-Pent) and dimeric (Li2(DPP-Pent)2) structures, respectively. Ultrafast transient absorption spectroscopy reveals the rate of singlet fission to be 7-fold faster in Li2(DPP-Pent)2 than the parent HDPP-Pent, with a modest change in eventual triplet lifetimes. By comparing within the DPP-Pent series, we elucidate design principles that demonstrate how coordination chemistry can be used to generate diverse structures using the same ligand scaffold to control interpentacene interaction and thus the photophysical properties of singlet fission.

Influencing singlet fission via coordination chemistry in dipyridyl pyrrole bipentacenes

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.