Photoinduced Phase Transitions in Rod-shaped Azobenzene with Different Alkyl Chain Length

The direct and reversible solid-liquid phase transitions by photochromic reactions at constant temperature are of interest because of potential applications in reusable adhesives, photoresists and so on. We report photoinduced solid-liquid phase transitions in rod-shaped azobenzenes, possessing a methyl group at the 3-position and alkoxy groups with different chain length (Cn = 1-18) at the 4- and 4'-positions. Thermal property, photochemical property, and adhesive properties were investigated. We found that the azobenzenes with alkyl chain length of 6-10 showed relatively fast photoinduced solid-liquid phase transition than that with shorter (Cn = 1-5) and longer (Cn = 11-18) ones. Tensile shear strength were measured by using these azobenzenes as photoresponsive adhesives. Irradiation with UV light (365 nm) induced the phase transition to the liquid phase and the adhesion strength values decreased to almost zero. Then the irradiation with visible light (450 nm) recovered the adhesion strength.