Influence of the Solvent Environment on the Ultrafast Relaxation Pathways of a Sunscreen Molecule Diethylamino Hydroxybenzoyl Hexyl Benzoate

The excited-state dynamics of photoexcited diethylamino hydroxybenzoyl hexyl benzoate (DHHB), a UVA absorber widely used in sunscreen formulations, are studied with transient electronic and vibrational absorption spectroscopy methods four different solvents. In the polar solvents methanol, dimethyl sulfoxide (DMSO), acetonitrile, strong stimulated emission (SE) is observed at early time delays after photoexcitation near-UV wavelength λex = 360 nm, decays constants 420 fs methanol 770 DMSO. majority (∼95%) DHHB returns to ground state 15 ps 25 nonpolar solvent cyclohexane, ∼ 98% 345 nm relaxes 10 constant, SE weak. preferentially adopts an enol form its S0 state, but excited (ESA) bands seen TEAS assigned both S1-keto S1-enol forms, indicating role for ultrafast intramolecular hydrogen transfer (ESHT). This ESHT inhibited by two S1 tautomers decay similar scales recovery population. For molecules that avoid ESHT, torsion around central C-C bond minimizes energy, quenches SE, proposed lead conical intersection mediates recovery. A competing trans-enol isomeric photoproduct as minor competitor parent Evidence presented triplet (T1) production solvents, T1 quenching octocrylene, common UVB additive. keto cyclohexane solution.