1 Photoswitchable Imidazolium based Spiropyrans- A physicochemical study Simon Coleman 1 , Robert Byrne 2 , Nameer Alhashimy 2 , Kevin J Fraser 3 , Douglas R MacFralane 3 , and Dermot Diamond *1,2 1 Biomedical Diagnostics Institute, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland 2 CLARITY: Centre for Sensor Web Technologies, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland 3 School Of Chemistry, Monash University, Clayton, Victoria 3800 Melbourne, Australia Abstract We investigate the photo, solvate and thermochromic properties of a novel photoswitchable spiropyran derivative; SP Im in imidazolium based ionic liquids (ILs). SP Im was prepared by alkylation of the photoswitchable compound to an imidazolium cation and is added to imidazolium based ILs with increasing chain length to examine the stability of funtionalised cations 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 x10 -3 s -1 and 1.0 x10 -3 s -1 for SP Im and BSP respectively in [C 6 mIm][NTf 2 ]. Since ILs are believed to form nano-structured domains it is proposed that the covalent attachment of the imidazolium side group of SP Im 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 stablised 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 had upon the ionic liquid structure. Ionic Conductivity was found to decrease by up to 23% for SP Im with effects increasing with cation side-chain length. Unlike BSP, the photoswitching of the SP Im 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 evolution of this physical property.