Switchable ionic conductivity and viscoelasticity of ionogels containing photo- and thermo-responsive organometallic ionic liquids

Abstract

Ionic liquid gels (ionogels) are useful as flexible solid electrolytes owing to their high ionic conductivity and non-volatility. Ionogels possessing photoswitchable ionic conductivity have versatile electronic applications, although the fabrication of such materials remains a challenge. Herein, we synthesized photoresponsive ionogels by adding a low-molecular-weight gelator, 12-hydroxystearic acid, to photoreactive organometallic ionic liquids. UV photoirradiation of an ionogel containing an ionic liquid [Ru(C5H5)(C6H5OC3H6CN)][N(SO2F)2] (Gel-1) decreased its ionic conductivity while increasing its viscoelastic modulus. This is owing to the photochemical formation of coordination bonds between the ionic liquid cations in the gel. The reaction was reversed upon heating, hence the physical properties were reversibly controlled by the application of light and heat. An ionogel containing an ionic liquid [Ru(C5H5){C6H3(OC6H12CN)3}][N(SO2F)2] with three substituents (Gel-2) was also prepared, which was transformed into a rubber-like solid having a coordination polymer structure upon photoirradiation. The photoproduct exhibited much lower ionic conductivity and higher viscoelastic modulus compared with the photoproduct of Gel-1.