Intra- and intermolecular H-bonding of benzotriazole UV stabilizers evidenced using 1D nuclear Overhauser effect experiments

The UV absorber protection mechanism of 2-hydroxyphenylbenzotriazoles is based upon energy dissipation via an excited state proton transfer from the phenolic OH group to the triazole nitrogen(s). Using 1H-NMR NOE experiments we have established that 2-(2’hydroxy-5′- methylphenyl)-benzotriazole (UVA1) exists in chloroform as an intramolecularly H-bonded form whereas in DMSO this bond is disrupted by the formation of intermolecular H-bonding to the solvent. Conversely, for compounds 2-(2′-hydroxy-3′,5′-di(1,1-dimethyl propane))-benzotriazole (UVA2), and 3′-methylene-hydantoin-2-(2′-hydroxy-5′-methylphenyl)-benzotriazole (UVA3) having bulky substituents ortho to the phenolic OH group 1H-NMR NOE experiments indicate that upon changing solvent from DMSO to chloroform the strength of the intramolecular H-bond is not appreciably affected. The implication of the H-bond strength upon the UV stabilizing effectiveness is discussed.