Thermally Tuned Acceleration of Photoinduced Crawling Motion in Azobenzene Crystals and Enhanced Particle Transport

Abstract

Abstract Mechanically responsive organic crystals are attracting increasing attention as soft actuators. However, precise control over the velocity of their continuous translational motion remains a challenge. Crystals of trans-3,3′-dimethylazobenzene (DMAB) exhibit continuous crawling motion on solid surfaces under simultaneous irradiation with ultraviolet (UV) and visible light from opposite directions. In this study, we controlled the velocity of photoinduced crawling motion in DMAB crystals through temperature modulation. A 5 °C increase in substrate temperature markedly changed the crawling velocity. Within the investigated temperature range, the maximum velocity at 30 °C reached 13.4 ± 4.4 μm min–1, more than three times the velocity at 25 °C (4.2 ± 1.7 μm min–1). In contrast, at 35 °C, crawling motion was barely observed, and the maximum velocity decreased significantly to 4.7 ± 3.2 μm min–1. Polarized optical microscopy and microspectroscopy provided insights into the temperature-dependent crawling mechanism by revealing the dynamics of melting and recrystallization under UV and visible light irradiation. Furthermore, the rapid crawling motion at 30 °C was maintained during silica microparticle transport, improving the transport step in cargo capture–carry–release tasks based on the photoinduced crawling motion of DMAB crystals. This represents a notable improvement over previous studies, which did not achieve comparable acceleration of the motion.