Solution and solid state photochromism in a family of shape persistent azobenzene tetramers functionalized with alkyloxy substituents

Shape-persistent azobenzene tetramers functionalized at the periphery with alkyloxy substituents of different lengths have been synthesized and their photochemical behaviour has been investigated. Efficient E → Z photoisomerization of the azobenzene units takes place both in solution and in the solid state, a highly desirable yet uncommon property for azobenzene-type photochromic compounds. The solid state E → Z photoisomerization is accompanied by an isothermal crystal-amorphous phase transformation; successively, anisotropic crystals can be grown upon promoting the Z → E isomerization by thermal annealing of the irradiated samples. These results validate the strategy of engineering multiphotochromic architectures with a rigid star-shaped geometry to preserve the solution-based photoreactivity also in the solid state. The observed unexpected photoinduced alignment makes these materials potentially attractive for the development of photo-patternable and photo-responsive surfaces.