Liquid ion association in ionic liquids (ILs) has been examined using a comprehensive series of electronic structure calculations that measure the relative extents of ion association and probe stabilisation for the photochromic dye nitrobenzospiropyran (BSP) in a range of ILs featuring both long-tailed phosphonium cations and short-tailed imidazolium cations, paired with both chloride and NTf2 anions. New physicochemical experiments measured the photochromic properties of BSP in the phosphonium-based room temperature ILs. Taken together, the computed complexation energies and measured spectroscopic properties support recent Walden plots of unusual conductivity–viscosity behaviour obtained for the same ILs and reveal some new features in the atom-scale structure and energetics of local, ion–ion and ion–molecule interactions. Calculations show inter-ion interactions strengthened by between 0.4 and 0.7 eV as stronger constituent ions are used, which contributes to the longer range rigidity of the Cl-based IL structure as reflected in the doubled |zwitterion → closed| probe relaxation time measured for Cl−vs.NTf2− in phosphonium-based ILs. Calculations further reveal a similar, approximately 0.6–0.7 eV maximum “residual” IL headgroup-mediated probe stabilisation potentially available for the anion–probe–cation complexes via the stabilising interaction that remains following the “quenching” interaction between the IL anion and cation. This potential stabilisation, however, is offset by both longer-range charge networks, beyond the scope of the current purely quantum mechanical simulations, and also energetic penalties for disruption of the highly-interdigitated alkyl tail networks in the phosphonium-based ILs which may be estimated from known diffusion data. Overall the electronic calculations of local, individual ion–ion and ion–molecule interactions serve to clarify some of the measured physicochemical properties and provide new data for the development of classical force field-based approaches to measure also the longer range effects that, together with the electronic effects, provide the condensed phase IL structure and properties. More generally, the combined simulation and experimental results serve as a further example of how both the polar hydrophilic headgroup and non-polar hydrophobic tail of the constituent ions serve as distinct targets for IL rational design.
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