Harnessing the charge-transfer-to-solvent state of aqueous triiodide: A strategy to mitigate I2 trapping and enhance hydrated electron yield

Abstract

The charge-transfer-to-solvent (CTTS) states of aqueous halides serve as prototypical systems for probing electron-transfer dynamics. In the photogeneration of hydrated electrons [e−(aq)] from iodide ions [I−(aq)], the concomitant formation of I2 and I3− as primary byproducts severely limits the e−(aq) quantum yield. Although the formation of these byproducts has been extensively studied, the post-photoexcitation dynamics of I3−(aq), particularly the competition between molecular dissociation and electron ejection, remain unclear and warrant further investigation. In this paper, we employ time-dependent density functional theory calculations to confirm that the experimentally observed absorption peak at ∼5.5 eV originates from a CTTS state of I3−(aq). Furthermore, ab initio molecular dynamics simulations in excited states reveal that photoexcited I3−(aq) can generate a short-lived e−(aq) prior to dissociation. Crucially, the nascent I2 fragment efficiently traps the ejected electron via its low-lying σ* molecular orbital (MO). To overcome this bottleneck, we propose a strategic solution: introducing an electron-donating protic solvent (e.g., ethylene glycol). This approach simultaneously suppresses I3− formation and elevates the unoccupied MO energy level of I2, thereby mitigating its electron-trapping capability and ultimately enhancing the e−(aq) quantum yield from I−(aq). This work establishes a novel design principle, modulating solute MO energetics, for optimizing electron injection efficiency in liquid-phase systems.