Photoresponsive Hydrogel System Using Molecular Recognition of α-Cyclodextrin

Abstract

Using molecular recognition of α-cyclodextrin (α-CD), we have successfully constructed a photoresponsive hydrogel system by combination of simple components, α-CD, dodecyl (C 12 )-modified poly(acrylic acid) (p(AA/C 12 )), and a photoresponsive competitive guest, 4,4 ‘ -azodibenzoic acid (ADA). An aqueous solution of p(AA/C 12 ) exhibited a gel-like behavior because polymer chains form a network structure via hydrophobic associations of C 12 side chains. When α-CD was added to the gel-like aqueous solution, the gel was converted to a sol mixture because hydrophobic interactions of C 12 side chains were dissociated by the formation of inclusion complexes of α-CD with C 12 side chains. Upon addition of ADA to a binary sol mixture of p(AA/C 12 ) and α-CD, α-CD interacted predominantly with ADA, and then hydrophobic associations of C 12 side chains were restored, resulting in a sol-to-gel transition. When a ternary gel mixture of p(AA/C 12 ), α-CD, and ADA was irradiated with UV light, ADA was isomerized from trans to cis, and the mixture underwent a gel-to-sol transition because α-CD formed inclusion complexes more favorably with C 12 side chains than with cis -ADA. When the ternary sol mixture was irradiated with visible light, ADA was isomerized from cis to trans and the mixture underwent a sol-to-gel transition. Furthermore, these gel-to-sol and sol-to-gel transitions occurred repeatedly by repetitive irradiations of UV and visible light.