Photoinduced Mechanical Motions of Pseudorotaxane Crystals Composed of Azobenzene and Ferrocenyl Groups on an Axle and a Crown Ether Ring

This work describes the design and characterization of photoresponsive dynamic pseudorotaxane crystals composed of azobenzene and ferrocenyl groups in an ammonium cation axle component threaded through dibenzo[24]crown-8 ether rings. Pseudorotaxanes provide flexibility for cis and trans isomerization of azobenzene groups in a crystal state, enabling reversible bending motions under alternating 360 and 445 nm laser irradiation. For such bending motions, strained azobenzene structures were essential; these motifs were obtained by increasing the bulkiness of the substituents on the axle and ring molecules. In addition, the crystals showed photosalient effects, such as jumping motions, under 445 nm laser irradiation. These motions were assisted by the photoabsorption of the ferrocenyl group, which converted 445 nm laser light into heat. The maximum lifting weight accompanied by the photoinduced mechanical motion of a particular crystal was estimated to be 9600 times the crystal weight. These pseudorotaxane crystals exhibit promising features for applications in micro-nanometer-sized miniature mechanical devices.