Photoinduced multi-directional deformation of azobenzene molecular crystals

Developing biomimetic stimuli-responsive materials to imitate movements found in plants and animals has been a very exciting research field in materials science and engineering. However, achieving multiple forms of motion with a mono-component material is still extremely challenging. Here, photoinduced multi-directional deformations were firstly achieved with plate-like crystals of 4-cyano-4'-pentyloxy azobenzene (AZ). Upon irradiation of UV light, three separate types of photomechanical motions are observed in different temperature regimes: Firstly, a rapid and reversible photomechanical bending away from the light source was achieved when the temperature was kept below the melting point of cis-AZ (MPCA). Secondly, photoinduced bending towards the light source was observed when the temperature was higher than the melting point of MPCA. Thirdly, fully bidirectional motions were achieved in the temperature regime below but near the melting point of MPCA, due to a competition between photoisomerization-induced volume expansion and phase-transition caused volume contraction, aided by a photothermal effect. The light-directed multi-directional motions of the same crystal sample under the same photoirradiation conditions enabled by temperature tuning demonstrated here signify a new and versatile class of photocontrollable mono-component molecular crystals and provide a novel level of control of operation for photomechanical actuators by combination of photoisomerization and photothermal effects.