Investigating nano-structuring within imidazolium ionic liquids: A thermodynamic study using photochromic molecular probes

Following previous studies involving the thermal relaxation of spirocyclic compounds we extended our studies to investigate the formation of nano-structured domains in ionic liquids (ILs). Two compounds, spiropyran (BSP) and spirooxazine (SO) were added to imidazolium cation based ionic liquids with increasing chain lenghts (C2 –C12). Increasing side-chain length was found to have only minor effects on the rate of thermal relaxation of BSP and SO. BSP was found to be a suitable probe molecule as linear correlations in parameters were observed for this compound. This is believed to be due to the fact that BSP-IL interactions were based on hydrogen bonding between MCBSP and the cation compared to MCSO which is limited to electrostatic interactions thus enhancing the sensitivity of MCBSP to the charged polar regions. Increasing the side-chain of the cation resulted in slight increases in MC-BSP activation energy from 96.93 kJ.mol-1 in [C4mIm][NTf2] to 105.27 kJ.mol-1 in [C12mIm][NTf2]. MC-BSP S‡ and H‡ values also increased with increasing side-chain. Expansion and dispersion of polar regions due to increasing non-polar interactions may be enhanced by introduction of the bulky probe molecule. The resulting reorganisation of the system produced positive entropies of activation, 13.79 J.K- 1.mol-1 for C4mIm to 46.15 J.K-1.mol-1 for C12mIm, following an increase in disorder due to probe dye closure from MC to BSP and migration of dye to regions of preferential solvation. The ability for spirocyclic compounds to form both polar and non-polar isomers resulted in the ability to analyse both solvent regions using a single probe dye. Ground state equilibrium, Ke, examined non-polar regions of the IL while equilibrium of activation, K‡, examined the polar regions. A linear response to side chain length to equilibrium of activation was believed to be due to the fact that polar regions were possibly expanding due to increasing influence of non-polar side chain interactions upon the over solvent structure. The result of such reordering and dispersion of polar regions reduces solvent-solute interactions which increases rate of MC-BSP relaxation