Reentrant Photoinduced Morphological Transformation and Temperature-Dependent Kinetic Products of a Rectangular-Shaped Amphiphilic Diarylethene Assembly.

An amphiphilic rectangular-shaped photochromic diarylethene bearing two hydrophobic alkyl chains and two hydrophilic tri(ethylene glycol) chains was synthesized, and its photoinduced morphological transformation in water was investigated. Two unexpected phenomena were revealed in the course of the experiments: a reentrant photoinduced macroscopic morphological transformation and temperature-dependent kinetic products of supramolecular assembly. When the pure closed-ring isomer was dispersed in water, its reentrant photoinduced morphological transformation, i.e., photoinduced transition from the hydrated phase to the dehydrated phase and then back to the hydrated phase, was observed by optical microscopy upon irradiation with green light at 20 °C, which was interpreted by the V-shaped phase diagram of the LCST transition. The aqueous assembly of the pure closed-ring isomer was controlled by changing the temperature; specifically, rapid cooling to 15 and 5 °C gave J- and H-aggregates, respectively, as the kinetic products. The thermodynamic product at both temperatures was a mixture of mostly H-aggregate with a small amount of J-aggregate. This behavior was rationalized by the temperature-dependent potential energy surface of the supramolecular assembly.