Hydroxy-Group Topology as a Molecular Trigger Between Antioxidant and Photosensitizing Properties in Dihydroxynaphthalenes

Dihydroxynaphthalenes (DHN) provide a simple molecular platform to probe how hydroxy-group topology governs the interplay between antioxidant activity and photoinduced reactive oxygen species generation. Four regioisomeric dihydroxynaphthalenes (1,3-, 1,5-, 1,8-, and 2,7-DHN) have been investigated by combining steady-state spectroscopy, time-correlated single-photon counting, and femtosecond transient absorption with functional assays. The results show that intramolecular hydrogen bonding and π-electron delocalization control excited-state relaxation pathways, determining the balance between nonradiative energy dissipation and intersystem crossing. Peri-hydrogen-bonded 1,8-DHN exhibits the highest radical-scavenging efficiency while suppressing triplet formation and singlet-oxygen generation, whereas isomers lacking intramolecular hydrogen bonding display enhanced photosensitizing behavior. These findings establish hydroxy-group topology as a key structural parameter linking ground-state redox chemistry with excited-state photophysics.