Photochromic imidazolium based ionic liquids based on spiropyran.

We investigate the physicochemical properties of a novel imidazolium benzospiropyran derivative, SPIm, in imidazolium based ionic liquids (ILs). SPIm was prepared through alkylation of an imidazole to the photoswitchable compound and this derivative was characterised in imidazolium based ILs with increasing chain length to examine the stability of its merocyanine (MC) and spiropyran (SP) forms and compared to standard spiropyran, BSP. The rate of thermal relaxation of the new derivative is found to be about ten times faster than that of BSP as reflected in rates of 13.9 x 10(-3) s(-1) and 1.0 x 10(-3) s(-1) for SPIm and BSP, respectively, in [C(6)mIm][NTf2]. Since ILs are believed to form nano-structured domains it is proposed that the covalent attachment of the imidazolium side group of SPIm fully integrates the photoswitchable moiety into the non-polar region through side-chain association. In contrast, unbound BSP is relatively free to migrate between both polar and non-polar regions and the MC form is more readily stabilised by the IL charge via through space interactions and spontaneous movement to charged nano-domains leading to enhancement of the MC lifetime. At higher concentrations, rheological and transport properties were investigated to determine the impact of covalent attachment of the BSP fragment to an imidazolium cation on the ionic liquid structure. Ionic conductivity was found to decrease by up to 23% for SPIm with effects increasing with cation side-chain length. Unlike BSP, the photoswitching of the SPIm did not affect conductivity or viscosity values. This may indicate that the mobility of the photoswitchable compound and the resulting disruption of such movement may be critical to the control of this physical property.