ChemInform Abstract: Photoinduced Intramolecular Proton Transfer as the Mechanism of Ultraviolet Stabilizers: A Reappraisal.

Abstract

A general mechanism is widely accepted to explain the photostability of an important class of UV absorbers (or light screeners) which have a phenolic group intramolecularly bonded to a heteroatom.

Excited-State Intramolecular Proton Transfer and Rotamerism of 2-(2‘-hydroxyvinyl)benzimidazole and 2-(2‘-hydroxyphenyl)imidazole

“…26,[40][41][42][43][44][45][46] A large Stokes shift from the keto tautomer followed by a normal emission from the enol form is usually registered in the emission spectrum of the species shown in Scheme 1. 11,14,16,18,19,[24][25][26] Particularly, a strong normal emission along with a tautomer emission was reported for HPBI in polar solvents. [11][12][13][14]24,47 This can be explained by the coexistence of two intramolecularly hydrogen-bonded rotamers in the ground state, the enol (cis) and the enol (trans) forms (see Scheme 2).…”

“…11,14,16,18,19,[24][25][26] Particularly, a strong normal emission along with a tautomer emission was reported for HPBI in polar solvents. [11][12][13][14]24,47 This can be explained by the coexistence of two intramolecularly hydrogen-bonded rotamers in the ground state, the enol (cis) and the enol (trans) forms (see Scheme 2). The rotamer interconversion involves breaking of the intramolecular hydrogen bond in the enol (cis) form and a rotation by 180°around the C-C bond that links the benzimidazole and phenol moieties.…”

“…The former differs from HPBI by the lack of the phenol ring while the phenyl ring of HPBI has been removed in the latter. Comparison of the results for these two systems, in combination with previous theoretical results of 1-amino-3-propenal and salicylaldimine 49 and theoretical 50,51 and experimental [11][12][13][14]24,47 results of HPBI, has allowed the elucidation of the role that each molecular fragment of HPBI plays in the intramolecular proton transfer in the ground and excited electronic states. Moreover, since the enol (trans) is responsible for the strong normal emission in HPBI, 12,13 the rotamerism between the enol (cis) and the enol (trans) of HVBI and HPI in the ground state and the first singlet excited state has been also analyzed.…”

“…Photoexcitations in these systems usually lead to radical changes in the electron density of their acidic and basic centers 13,[31][32][33][34][35][36] that facilitate the transference of the H atom from the OH group to the adjacent N atom and the formation of phototautomers. This is usually a very fast process since it occurs within a few picoseconds even in molecules deposited in rigid matrixes at 4 K. 16,18,19 The above-mentioned molecules can be used as effective photoprotecting agents, 1,11,37,38 triplet quenchers, 39 and laser dyes. 26,[40][41][42][43][44][45][46] A large Stokes shift from the keto tautomer followed by a normal emission from the enol form is usually registered in the emission spectrum of the species shown in Scheme 1.…”

“…The importance of the intramolecular proton transfer in the photophysical and photochemical properties of systems such as 2-(2′-hydroxyphenyl)benzoxazole (X ) O, HPBO), 1-6 2-(2′-hydroxyphenyl)benzothiazole (X ) S, HPBT), 6-10 2-(2′-hydroxyphenyl)benzimidazole (X ) NH, HPBI) [11][12][13][14] (see Scheme 1), and related compounds has been well established. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] An important common feature shared by these compounds is the phenol group which is hydrogen bonded to a nitrogen atom of the same molecule.…”