Excited state intramolecular proton transfer of 2-(2[prime ]-hydroxyphenyl)benzimidazole in non-ionic micelles:Brijs

Abstract

Excited state intramolecular proton transfer (ESIPT) of 2-(2′-hydroxyphenyl)benzimidazole (2-HPBI) has been studied in four different Brijs, i.e. Brij-35, Brij-58, Brij-78 and Brij-99. In comparison to water, fluorescence maxima of the tautomer band is red shifted by ≈20 nm, whereas that of normal fluorescence is not affected much. The fluorescence quantum yield of the tautomer band increases and that of the normal band decreases. The effective relative permittivity (ε_eff) of the site where neutral 2-HPBI is present is 20±2, whereas ε_eff of the sites where monocation–neutral and neutral–monoanion equilibria are taking place are higher. The critical micelle concentration (c.m.c.) of the Brijs at pH≈7 decreases with increase of Brij number. At pH ≈1.0 (ionic strength 0.3 M), the c.m.c. of the Brijs are larger, by nearly two orders of magnitude than at pH 7. The pK_a values of the prototropic reactions are determined in different Brijs and are discussed.