Photoresponsive and Fluorescence Behaviors of Azobenzene-Containing Amphiphilic Block Copolymers

The effects of solvency and mole fraction of azobenzene moieties (f(PAzoMA)) on the photoresponsive and fluorescence behaviors of poly(acrylic acid)-block-poly(6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) (PAA-PAzoMA) amphiphilic diblock copolymers were investigated using UV-vis spectroscopy and fluorescence spectroscopy. The photoresponsive behavior depended strongly on the solvency and f(PAzoMA). When dissolved in a PAA-selective solvent, PAA-PAzoMA formed micelles with PAzoMA in the micelle core. The confinement of azobenzene moieties caused a steric hindrance, thereby markedly reducing the kinetics of photoisomerization compared with that of the unconfined PAA-PAzoMA in a nonselective solvent. Additionally, PAA-PAzoMA dissolved in the PAA-selective solvent caused a blue shift of the maximum absorbance, suggesting the formation of H-aggregates of azobenzene mesogens. The high H-aggregate content substantially reduced the fluorescence emission. Consequently, the fluorescence emission of PAA-PAzoMA in the nonselective solvent was more intense than that in the PAA-selective solvent. Upon UV irradiation, the enhanced bent-shaped cis isomers disturbed the compact packing of azobenzene mesogens, which substantially enhanced the fluorescence emission. Both the photoisomerization rate and fluorescence emission decreased with an increase in f(PAzoMA). (c) 2017 Wiley Periodicals, Inc.