Influencing singlet fission via coordination chemistry in 2,5-di(2-pyridyl)pyrrole-linked bipentacenes

Abstract

By converting one singlet exciton into two lower energy triplet states, singlet fission has garnered renewed attention as a possible means to more efficiently convert high energy photons into elec. current in next-generation photovoltaic materials. There is also growing interest in the coherent spin dynamics of triplet pair states obsd. in singlet fission processes for potential applications in quantum information science and spintronics. Recently, mol. 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 diverse structures and rigorous examn. of the relationship between such structures and the interpentacene electronic coupling within the ground and excited states. 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 (DPP-Pent), which allows for p-contact between the pentacene moieties. Addnl., deprotonation of the pyrrole moiety permits coordination of DPP-Pent to alkali metal cations (Li+ and K+), leading to distinct morphologies that can influence the interpentacene interaction. Ultrafast transient absorption spectroscopy reveals singlet to triplet conversions on the femtosecond timescale, with triplet features that can be obsd. for several microseconds. By comparing the photophys. dynamics between DPP-Pent and the alkali metal pyrrolide complexes, we elucidate design principles that demonstrate how coordination chem. can be used to modulate the electronic interactions between pentacenes and map how these perturbations affect the microscopic properties of singlet fission.