Photo and thermal responsive pseudorotaxane crystals comprising ferrocene-containing ammonium salts and crown ethers

Abstract

Rotaxanes and pseudorotaxanes have garnered increased attention as mechanically interlocked molecular architectures. These interlocked molecules play fundamental roles in the stimuli-responsive properties of a variety of applications. In contrast to studies of these molecules in solutions, aggregates, and films, there is much less information about their operation in crystals. This review summarizes our recent advances in thermal and photoresponsive crystals of pseudorotaxanes and related complexes comprising ferrocene-containing ammonium salts and dibenzo-crown ethers. A total of 33 complexes and eight axle molecules, which were characterized by single-crystal X-ray crystallography, were compared comprehensively. We discuss the relationships between molecular structures and macroscopic crystal motions (e.g., thermal transitions, photoinduced expansion, and bending) of a diverse range of complexations with various axle and ring molecules, mixed crystals, and azobenzene substituents. Additionally, we compare the rotatability of the ferrocenyl group for various interlocked complexes in crystals and conclude the important factors necessary to achieve stimuli-responsive pseudorotaxane crystals.