Influence of the Solvent Environment on the Ultrafast Relaxation Pathways of a Sunscreen Molecule Diethylamino Hydroxybenzoyl Hexyl Benzoate
Min-Hsien Kao, Ravi Kumar Venkatraman, Mahima Sneha, Matthew Wilton, Andrew J. Orr‐Ewing
Abstract
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 in four different solvents. In the polar solvents methanol, dimethyl sulfoxide (DMSO), and acetonitrile, strong stimulated emission (SE) is observed at early time delays after photoexcitation at a near-UV wavelength of λ ex = 360 nm, and decays with time constants of 420 fs in methanol and 770 fs in DMSO. The majority (∼95%) of photoexcited DHHB returns to the ground state with time constants of 15 ps in methanol and 25 ps in DMSO. In the nonpolar solvent cyclohexane, ∼ 98% of DHHB photoexcited at λ ex = 345 nm relaxes to the ground state with a ∼ 10 ps time constant, and the SE is weak. DHHB preferentially adopts an enol form in its ground S 0 state, but excited state absorption (ESA) bands seen in TEAS are assigned to both the S 1 -keto and S 1 -enol forms, indicating a role for ultrafast intramolecular excited state hydrogen transfer (ESHT). This ESHT is inhibited by polar solvents. The two S 1 tautomers decay with similar time scales to the observed recovery of ground state population. For molecules that avoid ESHT, torsion around a central C–C bond minimizes the S 1 -enol energy, quenches the SE, and is proposed to lead to a conical intersection with the S 0 state that mediates the ground state recovery. A competing trans -enol isomeric photoproduct is observed as a minor competitor to parent recovery in polar solvents. Evidence is presented for triplet (T 1 ) enol production in polar solvents, and for T 1 quenching by octocrylene, a common UVB absorber sunscreen additive. The T 1 keto form is observed in cyclohexane solution.