Light-responsive surfactants: Photochromic properties of water-soluble azobenzene derivatives

Abstract

Light-responsive self-assembly systems have emerged as a promising tool for controlling a multitude of molecular and macroscopic processes. A number of sophisticated applications of surfactants based on the azobenzene moiety have already been demonstrated including drug delivery and induction of functional changes in biologically active compounds. In most of these applications the switching time between the two compound states is substantial. However, a small number of systematic studies on the relationship between the chemical structure of the compound on the kinetics of photoisomerization and the influence of experimental conditions led us to undertake the studies described in this paper. Changes in the spectral characteristics resulting from reversible photoinduced trans-cis isomerization reaction of selected azobenzene-based surfactants, differing in the hydrophobic tail length, were determined in aqueous solutions employing UV–Vis absorption spectroscopy. The effect of the hydrophobic tail length, surfactant concentration, molecular environment and light intensity on the efficiency and kinetics of photoisomerization were investigated. Due to the thermal stability of both photochromic forms, it is possible to reversibly switch between two different photostationary states of the light-responsive surfactant in a controlled manner using light of two wavelengths (ultraviolet and blue). The results demonstrates that both the increasing concentration of azobenzene amphiphile and the increasing hydrophobic pendant tail length slow down the trans-cis photoisomerization, while the photochromic efficiency remains constant.