A Supramolecular Dual-Lock Strategy for Retarding Prompt Back Electron Transfer

Abstract

Prompt back electron transfer (BET) limits the lifetime and chemical utility of photogenerated radical pairs in solution. While proton-coupled electron transfer (PCET) can alter the recombination coordinate, effective proton-coupled stabilization requires simultaneous control of electron-transfer energetics, proton-transfer geometry, and radical-pair recombination. Here, we introduce a supramolecular “dual-lock” strategy distributing redox and proton-transfer functions across two molecular components. As a proof of principle, acridine (Acr) and citrazinic acid (CAH) self-assemble through ground-state proton transfer and ion pairing into supramolecular assemblies containing locally H-type-coupled CAH-derived domains. Photoexcitation generates an AcrH•²⁺-containing radical-ion pair, and a persistent transient accumulates only in the CAH-rich assembled regime. Assembly-dependent stabilization of this radical pair provides a “spatial lock” against prompt BET. An approximately 190 ps aggregate-associated relaxation may involve charge distribution, whereas a later 970 ps conversion contains a substantial solvent-friction-sensitive contribution together with residual isotope sensitivity and is most consistent with proton transfer coupled to structural reorganization. Together with potential-dependent spectroelectrochemical evolution from AcrH•²⁺-like toward Acr•⁺-like absorption, these observations support proton transfer to an Acr•⁺-containing, proton-reorganized aggregate radical state. The resulting state remains detectable into the sub-microsecond regime. These observations support a kinetic-relay model in which early assembly-dependent stabilization creates the time window required for slower proton-gated stabilization of a charge-separated state.