Comparison of the photochromism of a spirobenzopyran derivative in unilamellar surfactant vesicles and solvent-cast surfactant films

Abstract

The thermal decolouration of the photoinduced merocyanine form of 1′-hexadecyl-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline](SP-16) has been examined in a number of different aqueous unilamellar surfactant vesicle solutions and solvent-cast surfactant film matrices. The thermal decolouration of SP-16 in solvent cast dihexadecylphosphate (DHP) films and unilamellar dimyristoyl phosphatidylcholine (DMPC) vesicles exhibits single first-order kinetics. In DHP vesicles and solvent-cast DMPC films the thermal decolouration deviates from single first-order behaviour. The thermal decolouration in both these systems is well described by the super-position of two first-order rate processes. Dual first-order kinetics in the vesicular DHP media has been attributed to the merocyanine moieties of SP-16 partitioning between two residential sites within the vesicles, probably the surfactant/water interfacial region and the hydrocarbon chain domain. Dual first-order kinetics in the DMPC films has been ascribed to part of the merocyanine population being complexed with phosphatidylcholine headgroups and the other part of the population being located in hydrocarbon chain domains. In solvent-cast poly(diallyl dimethylammonium) dihexadecylphosphate (DHPP) films the thermal decolouration of SP-16 deviates from single first-order behaviour. The thermal decolouration in DHPP films is inadequately described as a dual first-order process but is well described as a dispersive process.