Intermolecular Hydrogen-Bond Preorganization Enables Concerted 2e−/2H+ Transfer in the Franck–Condon Region

Concerted proton-coupled electron transfer (CPET) is central to biological and synthetic energy conversion, yet most studies focus on 1e−/1H+ reactions. The principles governing concerted multi‑proton multi‑electron transfer, as illustrated by water oxidation and CO2 reduction, remain poorly understood. Here, we use femtosecond transient absorption spectroscopy to investigate a water-soluble fluoflavine. Following photoexcitation, we resolve an ultrafast branching (<120 fs) from the Franck‑Condon region. One pathway corresponds to concerted 2e−/2H+ transfer (2CPET) from water to fluoflavine and forms a long-lived reduced product; the other produces a short-lived two-electron-reduced intermediate. Kinetic isotope experiments (H2O vs. D2O) show that deuteration suppresses the 2CPET channel, shifting the population toward the two‑electron transfer pathway, further confirming the competitive branching between the two channels. pH-dependent measurements show that the 2CPET channel is progressively suppressed as the intermolecular hydrogen bonds are disrupted. DFT calculations provide a molecular-level explanation for this observation, revealing pre‑organized intermolecular hydrogen bonds in ground state with a short donor-acceptor distance (~2.0 Å) that serve as the structural basis for the ultrafast 2CPET. Our findings establish a direct spectroscopic view of 2CPET and identify hydrogen-bond preorganization as a key molecular prerequisite for efficient multi-equivalent proton-electron transfer, with H2O2 as the oxidation product, offering potential implications for photoelectrochemical energy storage.