Fluorescent 2-phenyl-2H-benzotriazole dyes with intramolecular N–H···N hydrogen bonding: Synthesis and modulation of fluorescence properties by proton-donating substituents

Abstract

Listen

Translate article

A series of 2-(2-phenyl)benzotriazole derivatives having amino, N-methyl amino, and acetamido groups on the ortho position of the phenyl group were synthesized to elucidate the effects of amino/amido substitutions on the molecular structures and photophysical properties. 1H NMR spectra and DFT calculations of the present dyes revealed the formation of N–H∙∙∙N intramolecular hydrogen bonds between benzotriazole and amino/amide groups in the ground state. Amino-functionalized dyes showed absorption bands that were assigned to HOMO-LUMO transition with a partial charge-transfer feature. The dye with an acetoamido group in which the acidity of NH proton was increased by the effect of electron-withdrawing acetyl groups exhibited absorption bands attributable to intramolecular hydrogen-bonded “closed” conformers in CHCl3 and non-hydrogen-bonded “open” conformers in DMSO. In comparison to 2-phenylbenzotriazole, which had no acidic proton involved in intramolecular hydrogen bonding, the present derivatives exhibited weak fluorescence emissions with quite low quantum yields, suggesting that non-radiative deactivation of ESIPT states also operated in the N–H∙∙∙N hydrogen bonded 2-phenylbenzotriazoles just as in O–H∙∙∙N intramolecular hydrogen-bonded 2-hydroxyphenylbenzotriazoles, which are known as UV absorbers. Taking a closer look at the fluorescence spectra, dyes with amino and N-methylamino groups showed faint but dual emissions from charge-transfer excited states in CHCl3 and blue-shifted single emission in DMSO. Fluorescence emissions of the acetamido-functionalized dye were unimodal in each solvent and stronger than those of amino-functionalized dyes. Thus, the fluorescence properties of these dyes depended on substitutions at amino groups. The luminescence of these derivatives was enhanced in the polymer matrix in which conformation changes and molecular motions were suppressed. Dye-doped polymer films have absorption bands in the UV range (360–400 nm) and emitted at the visible region (450–510 nm), and thus could be used as organic materials for light conversion films that convert UV light into visible light.