The Hidden Role of Through-Bond Energy Transfer in FRET-Dyes: A Quantum Dynamics Perspective

Energy transfer in fluorescent donor-acceptor pairs is typically governed by one of two mechanisms: Förster Resonance Energy Transfer (FRET) or Through-Bond Energy Transfer (TBET). In contrast to FRET, TBET has been widely assumed to require π-conjugation between the donor and acceptor units. In this work, we introduce a versatile strategy for simulating nonadiabatic quantum dynamics in large molecular systems and apply it to a dye <b>CouXan</b> featuring a coumarin donor, a xanthene acceptor and a non-π-conjugated diazinane linker. We demonstrate that strong vibronic coupling drives highly efficient TBET with a remarkable rate constant of 6.8 × 10¹² s^-1 even in the absence of π-conjugation, surpassing the known FRET rate (8.0 × 10¹⁰ s^-1) by 2 orders of magnitude. This finding challenges the present understanding of energy transfer in donor-acceptor pairs and reveals that classical FRET architectures can harbor ultrafast through-bond channels of exciton deactivation.