Exploring the photodynamical landscape of biomimetic lumichrome-ephedrine-class amine complexes across femtosecond to millisecond regimes.

Flavin-amine interactions are central to neurotransmitter metabolism, yet how H-bonding and amine structure govern excited-state pathways remains unclear. Using lumichrome (LC) as a biomimetic model, we examine interactions and photobehaviour with sympathomimetic amines, norephedrine (Neph), ephedrine (Eph), and pseudoephedrine (Peph). In water, LC with amines readily forms anions. In dichloromethane, H-bonding drives ultrafast single-proton transfer (SPT) in the LC-Neph complex (~800 fs), followed by population redistribution (~14 ps) and a long-lived anion (τ ~5.4 ns), while LC-Eph and LC-Peph complexes show shorter lifetimes (~4.3-4.8 ns). Control experiments with phenethylamine confirm that the amine group drives SPT-mediated anion formation. Computational calculations support excitation-induced charge redistribution and SPT-mediated zwitterionic stabilisation. Nanosecond-millisecond transient absorption shows minor triplet quenching by Neph (τ ~0.74 → 0.66 μs), but Eph and Peph enhance quenching (τ ~0.45-0.55 μs) and photoproduct formation. This study unravels LC-sympathomimetic amine interactions, offering an avenue to interrogate excited-state pathways in flavin photochemistry.