Enabling Efficient Intermolecular Singlet Fission in a Peropyrene Derivative through Dipole Tuning

Singlet fission (SF), where one high-energy singlet exciton converts into two low-energy triplet excitons, offers a promising route for surpassing the Shockley–Queisser limit in photovoltaics. Intermolecular electronic coupling plays a pivotal role in determining the SF performance. The molecular dipole moment provides an effective handle for tuning this coupling and thereby controlling the SF process. Here, we synthesized two peropyrene (PP) derivatives, 1,10,11,14-peropyrene diimide (PPDI) and 3,8-dicyano-PPDI (PPDI-2CN), with systematically varied dipole moments (6.8 D vs 0.9 D) to probe whether dipole-moment engineering can modulate intermolecular packing and thereby switch SF behavior. It is demonstrated that PPDI, with its relatively large dipole moment (6.8 D), exhibits strong intermolecular packing and overstabilization of charge transfer (CT)/excimer-like states, which suppresses SF. In contrast, the small-dipole derivative PPDI-2CN (0.9 D) adopts a looser packing motif, allowing a CT/excimer-like state to serve as a transient intermediate rather than a competing trap. Transient spectroscopy reveals that PPDI-2CN undergoes efficient SF with a time constant of τSF = 26.1 ps and a triplet yield of 172%. Our work demonstrates that introducing substituents to tune the molecular dipole in peropyrene derivatives provides an effective strategy for regulating intermolecular packing, thereby enabling the SF pathway.