A Charge Transfer Mediated Sequential Triplet Energy Transfer Model Beyond the Dexter Limit in Organic Photocatalysis

Classical distance-dependent Dexter energy transfer models fail to account for the complex, geometry-sensitive kinetics observed in flexible and dynamically disordered organic photocatalytic systems. To resolve these persistent inconsistencies, particularly in photosensitized [2 + 2] cycloaddition reactions, we investigate how charge-transfer (CT) intermediates mediate triplet-triplet energy transfer (TTEnT) beyond the conventional Dexter framework. Through a combined approach integrating high-level multistate CASPT2 calculations, ultrafast transient absorption spectroscopy, and electrochemical analysis, we uncover two distinct and previously unrecognized TTEnT pathways: a CT trap mechanism and a CT-mediated sequential TTEnT mechanism in which electron and hole transfer steps bridge the donor and acceptor triplet states. Both pathways are governed by nonadiabatic electron transfer processes through CT intermediates and are highly sensitive to the redox potential and steric environment. Using alkenylboronates and N-sulfonylimines as representative acceptors, we demonstrate that favorable redox matching and geometrically confined donor-acceptor interfaces promote long-range TTEnT via CT bridges, effectively bypassing the spatial limitations of Dexter exchange. Compared to the classical model, this CT-bridged sequential TTEnT mechanism exhibits reduced dependence on orbital overlap and stronger correlation with redox thermodynamics, rendering it particularly suitable for sterically hindered or conformationally flexible systems. Based on these findings, we propose a unified TTEnT framework that integrates Dexter-type exchange with dynamic CT-mediated pathways, offering a new conceptual foundation and design principles for next-generation photosensitizers and photocatalytic systems.