Efficient ultraviolet-light energy dissipation by an aromatic ketone

Abstract

Experimental evidence on the efficiency of 2,2 0 ,4,4 0 -tetramethoxybenzil for UV-light energy dissipation is provided. This non-phenolic aromatic ketone has a low energy triplet which quickly decays to the ketone ground state, thus avoiding the generation of undesirable reactive species.Compounds showing an optimal ultraviolet (UV) spectral performance and low capacity as photosensitisers can dissipate UV-light energy, thereby protecting living organisms and materials from the harmful effects of UV radiation of sunlight.1 These compounds, termed photostabilisers, can provide photoprotection by competitive energy absorption of UV photons. They should have an efficient mechanism for light energy dissipation, i.e. exceptionally low quantum yield of photochemical reaction, 2-4 bypassing potential reactive triplet excited states. However, their triplet excited state can be generated by means of energy transfer from an electronically excited triplet donor.5 As a consequence, those compounds show a diminished photoprotective capacity.Aromatic ketones are good UV-absorbers, but they are usually photolabile due to an enhanced spin-orbit coupling between the pp* and np* states, resulting in an efficient singlet-triplet intersystem crossing. In addition, the usually high energy of their triplet excited state allows them to photosensitise the transformation of many compounds by triplet energy transfer. 6 Moreover, the triplet excited state can efficiently abstract hydrogen via a direct hydrogen abstraction process (from good hydrogen donors such as methanol and alkanes) or via a coupled electron-proton transfer process (fenol and indoles); in both cases, reactive radicals are generated. Furthermore, aromatic ketone triplets can promote degradation of materials due to photosensitised generation of singlet oxygen.

Crystal structure of 1,2-bis(3,5-difluorophenyl)ethane-1,2-dione

“…1) crystallized in the orthorhombic space group Pbcn. Benzil (1,2-diphenylethane-1,2-dione) and similar ,-diketones crystallize in trigonal or monoclinic space groups, respectively (Charpe et al, 2020;El Moncef et al, 2010;Fun et al, 2008). The title compound crystallizes with one half-molecule per asymmetric unit (Z 0 = 0.5), and exhibits the expected bond lengths and angles for -diketone sp 2 hybridized atoms.…”

Ultrafast Excited‐State Deactivation Dynamics of Cyclotrisazobenzene—A Novel Type of UV‐B Absorber

“…The completely non-photochemical relaxation of the ES on the femtoseconds cale makes CTAa ne xtraordinarily efficient molecular UV light-to-heate nergy converter without chemical transformation or molecular degradation (see Figure S8). Comparisonw ith natural and synthetic UV absorbers/light stabilizers [6] (cinnamic acid derivatives, [25] b-diketones, [26] benzophenone derivatives, [27] menthyl anthranilate, [28] octocrylene), [29] showst hat the ES relaxation kinetics is faster or at least equally fast in CTA, but most importantly that it proceeds without secondary,l onger-livedp hotoproducts due to keto-enol tautomerization, radical formation, or triplet formation and ensuing singlet oxygen generation. Consequently,C TA might be of interest for applications that demand high UV resistance and filtering.…”