Photochromism of a surface-active spirobenzopyran moiety in dioxane–water mixtures and self-assembled surfactant aggregates

Abstract

The photochemical and physicochemical properties of 1′-hexadecyl-3′,3′-dimethyl-6-nitrospiro [2H-1-benzopyran-2,2′-indoline](SP-16) in micellar self-assembled surfactant aggregates have been investigated. For comparison the properties of 1′,3′,3′-trimethyl-6-nitrospiro [2H-1-benzopyran-2,2′-indoline](SP-1) in 1,4-dioxane–water mixtures and, to a lesser extent, some neat organic solvents have also been examined. In all the media studied, both SP-16 and SP-1 undergo heterolytic cleavage when irradiated with UV light. This process may be reversed thermally or by irradiation with visible light. The spirobenzopyran open form, which is generated by the UV irradiation, is solvatochromic. A complex interplay of open-form stereoisomer contributions complicates the analysis of the solvatochromism, especially for micellar systems. In all the media, the thermal decoloration of the open forms of SP-16 and SP-1 follows single first-order kinetics. In the micellar systems, the first-order rate constants are interpreted in terms of interfacial solvent and charge stabilization effects and steric restrictions. pH induced UV–VIS spectral changes of the open forms of SP-16 and SP-1, in the low to medium region of the pH scale, are ascribed to the phenoxide moiety functioning as an acid–base group. The apparent pK_a of SP-16 in non-ionic micellar systems reflects solely a solvent effect whilst the apparent pK_a of SP-16 in ionic micellar systems reflects solvent and electrostatic effects as well as either a complexation of anionic species to the indolenium nitrogen or a ‘Debye–Hückel’ screening effect. Light-induced UV–VIS spectral modifications of the protonated open forms of SP-16 and SP-1 are explained in terms of a cis–trans photoisomerization process. pH (adjusted with NaOH) induced UV–VIS spectral changes are indicative of a partially reversible hydroxide degradation of the open forms of SP-16 and SP-1. A photochemical and physicochemical scheme is presented which summarizes the behaviour of 1′-alkyl-3′,3′-dimethyl-6-nitrospiro [2H-1-benzopyran-2,2′-indoline] derivatives in environments that have an aqueous component.