Intramolecular Excited State Proton Transfer of 2-(2′-Hydroxyphenyl)benzimidazole in Nonionic Micelles: Tweens

Abstract

Spectral characteristics and prototropic reactions of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) in Tween-20, Tween-40, Tween-60, Tween-80, and dioxane–water mixtures of different compositions have been studied. Comparison of the fluorescence band maxima of the tautomer band in nonionic micelles with the correlation diagram, drawn between the fluorescence band maxima and the dielectric constants of the dioxane–water mixtures, have shown that the effective dielectric constant (ϵeff) at the binding site of HPBI is 18 ± 2 for all the Tweens. Fluorescence lifetimes (τf) and fluorescence quantum yields (φfl) have shown that the hydrophobicity of these micelles is maximum for Tween-80 and at a minimum for Tween-20. Similar results have also been observed from the pKavalues for the deprotonation of HPBI, whereas the protonation reaction of HPBI occurs at the site in nonionic micelles which is more hydrophilic than that where the deprotonation reaction takes place. The protonation reaction in 2-(2′-methoxyphenyl)benzimidazole (MPBI) has shown that the value of ϵeffin Tweens where this reaction occurs is less hydrophobic than the site where the same reaction occurs in HPBI.