Determination of the Hydrogen-Bonding Induced Local Viscosity Enhancement in Room Temperature Ionic Liquids via Femtosecond Time-Resolved Depleted Spontaneous Emission

The fluorescence depletion dynamics of Rhodamine 700 (R-700) molecules in room temperature ionic liquids (RTILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF(4)]) and 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate ([HOemim][BF(4)]) were investigated to determine the local viscosity of the microenvironment surrounding the fluorescent molecules, which is induced by strong hydrogen bonding interaction between cationic and anionic components in RTILs. The solvation and rotation dynamics of R-700 molecules in RTILs show slower time constants relative to that in conventional protic solvents with the same bulk viscosity, indicating that the probe molecule is facing a more viscous microenvironment in RTILs than in conventional solvents because of the strong hydrogen bonding interaction between cationic and anionic components. In addition, this effect is more pronounced in hydroxyl-functionalized ionic liquid than in the regular RTIL due to the presence of a hydroxyl group as a strong hydrogen bonding donor. The hydrogen-bonding-induced local viscosity enhancement effect related to the heterogeneity character of RTILs is confirmed by the nonexponential rotational relaxation of R-700 determined by time-correlated single photon counting (TCSPC). The geometry of hydrogen bonding complexes with different components and sizes are further optimized by density functional theory methods to show the possible hydrogen-bond networks. A model of the hydrogen-bonding network in RTILs is further proposed to interpret the observed specific solvation and local viscosity enhancement effect in RTILs, where most of the fluoroprobes exist as the free nonbonding species in the RTIL solutions and are surrounded by the hydrogen-bonding network formed by the strong hydrogen-bonding between the cationic and anionic components in RTIL. The optimized geometry of hydrogen bonding complexes with different components and sizes by density functional theory methods confirms the local viscosity enhancement effect deduced from fluorescence depletion and TCSPC experiments. The calculated interaction energies reveal the existence of the stronger hydrogen bonding network in RTILs (especially in hydroxyl-functionalized ionic liquid) than that in conventional protic solvent, which leads to the enhancement effect of local microviscosity, and therefore leads to the slow solvation and rotation dynamics of probe molecules observed in RTILs.