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

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 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 non-polar solvent cyclohexane, ~98%of DHHB photoexcited at 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 S0 state, but excited state absorption (ESA) bands seen in TEAS are assigned to both the S1-keto and S1-enol forms, indicating a role for ultrafast intramolecular excited state hydrogen transfer (ESHT). This ESHT is inhibited by polar solvents. The two S1 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 S1-enol energy, quenches the SE, and is proposed to lead to a conical intersection with the S0 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 (T1) enol production in polar solvents, and for T1 quenching by octocrylene, a common UVB absorber sunscreen additive. The T1 keto form is observed in cyclohexane solution.

Elucidating the photoprotective properties of natural UV screening agents: ZEKE–PFI spectroscopy of methyl sinapate

“…In view of the attention that nowadays is given to protection against solar radiation and the potential of photothermal materials, it is not surprising that over the years increasingly more studies have been reported that focus on these aspects. The development of spectroscopic techniques that enable such studies over a wide range of wavelengths, encompassing time scales ranging from femtoseconds to days and longer, and under a large variety of environmental conditions [18][19][20][21][22][23][24][25][26][27] in combination with advanced quantum chemical calculations [28][29][30][31][32][33] has from this point of view contributed significantly to the body of knowledge that is currently available.…”

Determining the photostability of avobenzone in sunscreen formulation models using ultrafast spectroscopy

“…[20][21][22] Transient electronic absorption spectroscopy (herein abbreviated as TEAS), is an ultrafast pump-probe technique that has been utilised previously to explore the photoprotection mechanisms of many individual UV filters in solution, including avobenzone; additional examples include benzophenone-3, ethylhexyl methoxycinnamate, homosalate, ethylhexyl salicylate, octocrylene and diethylamino hydroxybenzoyl hexyl benzoate. [23][24][25][26][27][28] An ideal sunscreen filter should be able to dissipate excited state energy safely (ideally non-radiatively) and rapidly (within femtoseconds, 1 fs = 10 À15 s, to picoseconds, 1 ps = 10 À12 s). This reduces the likelihood of harmful side reactions such as forming triplet states, or fragmentation of the molecule.…”

“…Computational methods such as density functional theory (DFT), including time-dependent DFT (TD-DFT), can also be used to guide the assessment of a molecule's suitability for sunscreen use. 28,34,35 As an example, Wang et al include both experimental (including ultrafast) data and computational methods in their study of UV-induced photochemical reactions in diketo avobenzone derivatives. 36 However, TEAS studies have not been widely used in conjunction with industry-standard efficacy tests that sunscreen formulations are subject to, such as sun protection factor (SPF) and UVA protection factor (UVA-PF) determination.…”

Illuminating the Effect of the Local Environment on the Performance of Organic Sunscreens: Insights From Laser Spectroscopy of Isolated Molecules and Complexes

“…This work complements the gas-phase work described above since it provides insight into how bulk solvation impacts on the intrinsic sunscreen photophysics. A study of diethylamino hydroxybenzoyl in four different solvents (i.e., methanol, dimethyl sulfoxide, acetonitrile, and cyclohexane) by Orr-Ewing and co-workers provides a recent example ( Kao et al, 2021 ). Work in this area has been previously reviewed by Stavros and co-workers ( Rodrigues and Stavros, 2018 ; Holt et al, 2020 ) so we refer the reader to the reviews for further information.…”