A Photoresponsive Polymeric Actuator Topologically Cross-Linked by Movable Rings Based on a [2]Rotaxane Structure

Abstract

A photoresponsive polymeric material, in which [2]rotaxane units with α-cyclodextrin (αCD) threading onto an azobenzene (Azo) derivative is used as a topological cross-link for the main chains of the polymeric material in aqueous media, was achieved. As shown in Figure 1a, A polycondensation reaction between Lys-αCD/Am2Azo inclusion complex and Su2PEG gave polymeric material cross-linked by [2]rotaxane (αCD-Azo hydrogel). [2]Rotaxane structures in the αCD-Azo hydrogel were found to act as movable links in the polymer network, and the mechanical properties of the material were enhanced to show a rupture strain of 2800%. The materials were reversibly deformed by irradiation with UV or visible lights in aqueous media, which caused photoisomerization of the azobenzene moiety and changed the structure of the [2]rotaxane linker, leading to deformation of the polymer network (Figure 1b). Surprisingly, the dry material (αCD-Azo xerogel), which had been uniaxially extended in air, showed a faster response than the αCD-Azo hydrogel. The orientation of the polymeric network in the materials enables the efficient response. As shown in Figure 1c, the αCD-Azo xerogel quickly bent towards the light source side within 3 seconds upon irradiation with UV light. 5.6 mg of the αCD-Azo xerogel also performed 5.6 μJ of mechanical work within 10 seconds by the UV-irradiation, which is approximately 50 times higher than that achieved in our previous work1. Figure 1. The preparation of the αCD-Azo hydrogel as a topological material (a), schematic illustration of the αCD-Azo hydrogel before and afeter the irradiation with UV (b), and lateral views of the αCD-Azo xerogel showing deformations driven by UV irradiation. [1]. Iwaso, K.; Takashima, Y.; Harada, A. Nat. Chem. 2016, 8, 625-632. Figure 1