Ionic liquids (ILs) are presented as novel solvents for the replacement of organic solvents and the formation of smart liquids. The following thesis investigates the proposed nano- and atom-scale structuring of ionic liquids, a feature that appears to totally underpin their unique behavioural characteristics and facilitate accurate predictions of trends. Solvent organisation is one of the most fundamental properties of any liquid as it determines more complex processes such as solvation and reaction dynamics. It is believed that unique ordering in ionic liquids results form a balance between anion-cation, cation-cation and most importantly, ion-pair formation. The final parameter has been found to be critical to the ‘ionicity’ or transport properties of the liquids and these atom-atom interactions mediate the dissociation of the ions and thus the ability to form solvation shells associated with the ‘ionic liquid effect’. To probe ionic liquid behaviour, the effect of cation changes were examined experimentally and compared to that of model systems drawn for conventional molecular solvents. Initial studies (Chapter 2) involved the addition of photochromic spirocyclic compounds to ionic liquids which interact with both nano-domains within the liquid structure and report the environment present based upon the rates of thermal relaxation of the compounds from their open merocyanine (MC) form to their closed (SP) form and the equilibrium effects upon the MC-SP inter-conversion and the dependence of cation and anion choice while optimising the choice of probe molecule by establishing whether spiropyran (BSP) or spirooxazine (SO) met the required sensitivity to effectively probe the ionic liquids. xiii Subsequent studies (Chapter 3-4) focus upon the effects of nano-structuring in imidazolium and phosphonium based ionic liquids through kinetic and thermodynamic analysis and attempt to rationalise the formation for these distinct nano-domains. Such formations produce complex solvent systems which current methods of charactertisation appear unable to quantify sufficiently, in particular, the ‘polarity’ of ionic liquids. The dynamic nature of the BSP/SO-MCBSP/SO interconversion allows for the unique ability of the compound to examine both regions through migration and solvent reorganisation and thus report parameters previously incomprehensible to traditional probe dyes. Derivatives of spiropyran were subsequently added to ionic liquids to preposition them within the specific regions of the liquids defined by previous experiments to allow for more specific characterisation of the properties of each nano-domain. Final investigations (Chapter 5) examine the novel theory that the unique properties ionic liquids and indeed the nano-structuring observed may be due to liquid organisation occurring at the atomic level. Studies involved the comparison of experimental data based upon thermodynamic and kinetic parameters to quantum mechanical models of ionic liquid ion pairs and their interaction with the MC form of the probe molecule. The formation of Liquid Ion Pairs (LIPs) is believed to be the precursor to the nano-structures which in turn mediate the unique bulk properties of the liquids such as their transport properties. Future work outlines the integration of spiropyran into the ionic liquid nanostructure and the possibility of manipulating the ion pair interactions to produce ionic liquids with photoswitchable rheological properties and possible application in light mediated sensing.