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.