A new insight into the photochemistry of avobenzone in gas phase and acetonitrile from ab initio calculations

Abstract

Avobenzone (4-tert-butyl-4'-methoxydibenzoylmethane, AB) is one of the most widely used filters in sunscreens for skin photoprotection in the UVA band. The photochemistry of AB includes keto-enol tautomerization, cis-trans isomerization, rotation about the single bond and α bond cleavages of carbonyl groups. In this contribution we study chelated and non-chelated enol, rotamers Z and E, and keto tautomers of AB in the ground and excited states in gas phase and acetonitrile by means of a coupled cluster method. Our findings suggest that torsion around the double C2-C3 bond of photoexcited chelated enol leads to internal conversion to the ground state and formation of rotamer E. In addition, opening of the chelated hydrogen ring by torsion of the hydroxyl group creates non-chelated enol. The possible mechanisms of rotamer Z formation are discussed. The solvent dependent photolability is related to the relative order of the lowest triplet ππ* and nπ* states of the keto tautomer.

Unravelling the Keto-Enol Tautomer Dependent Photochemistry and Degradation Pathways of the Protonated UVA Filter Avobenzone

“…This disagreement mirrors the previous reported issues in predicting relative energies values for the neutral keto and enol tautomers of dibenzoylmethane systems without anharmonic methods. 27,69 Many of these previous papers also identified twisted Z and E rotamer geometries. Our conformational search identified many such twisted forms, but their relative energies lay at much higher energies (> 40 kJ/mol and 30 kJ/mol using the ωB97XD and M06-2X functionals, respectively) so they have been omitted from this work.…”

“…1,[23][24] AB is a dibenzoylmethane derivative and a member of a family of molecules known to take multiple isomerization pathways following UV excitation. [25][26][27][28][29][30] Photoexcitation of AB is believed to lead to nonadiabatic population of high-lying S0 vibrational levels, which can lead to isomerization to a less photostable form. 31 Previous photolysis experiments performed in hexane have shown that this photodegradation of AB occurs by a Norrish type 1 mechanism.…”

Unravelling the Keto-Enol Tautomer Dependent Photochemistry and Degradation Pathways of the Protonated UVA Filter Avobenzone

“…Computational work on neutral AB by Kojićet al showed that under both gaseous and solvated conditions (MeCN), the S 1 state of CE has a T 1 state of alternate spin parity available energetically within the zero-point-energy window of S 1 at the Franck−Condon geometry. 27 If T 1 is accessed, there is a flat potential about the out-of-plane OH dihedral coordinate which crosses the contorted S 0 state. An easily accessible T 1 state for Ea/Eb at S 1 Franck−Condon geometries may facilitate the rearrangements shown in Scheme 4.…”

Elucidating Solvent Effects on Strong Intramolecular Hydrogen Bond: DFT‐MD Study of Dibenzoylmethane in Methanol Solution

“…[10,[15][16][17][18][19][20][21][22][23][24][25][26][27][28] The perplexity caused by the relative stability of the asymmetric and symmetric DBM's protic tautomers is due to the inability of experimental scattering and spectroscopic techniques to resolve ultrafast proton motion in the hydrogen bond, but also possibly due to various experimental conditions and aggregate states in which DBM was studied. [10,[15][16][17][18][19][20][21][22][23][24][25][26][27][28] The perplexity caused by the relative stability of the asymmetric and symmetric DBM's protic tautomers is due to the inability of experimental scattering and spectroscopic techniques to resolve ultrafast proton motion in the hydrogen bond, but also possibly due to various experimental conditions and aggregate states in which DBM was studied.…”