1,1-Dimethyl-2,3,4,5-tetraphenylsilole as a Molecular Rotor Probe to Investigate the Microviscosity of Imidazolium Ionic Liquids

Room temperature ionic liquids (ILs) have attracted interest for a wide variety of applications, yet many details regarding their physicochemical properties remain unclear, including how their bulk properties differ from those on the microscopic scale. In this work, 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS) was employed as a molecular rotor probe to investigate the microviscosities of three imidazolium ILs: butylmethylimidazolium tetrafluoroborate, butylmethylimidazolium hexafluorophosphate, and octylmethylimidazolium tetrafluoroborate. The photoluminescence quantum yields (PL QYs) for DMTPS in these ILs were compared to those measured for the same probe in nonpolar viscous (hexanes–mineral oil) and polar viscous (glycerol–ethanol) solvent systems and the microviscosities calculated using the Förster–Hoffmann equation. The PL QY of DMTPS was found to be higher in ILs than in low viscosity solvents but not as high as in nonpolar solvents of similar bulk viscosity. These results indicate that the microviscosity experienced by the silole in the ILs is less than the measured bulk viscosity, suggesting that the siloles occupy a “domain” within the IL matrix that allows enough free volume for the silole to deactivate rotationally. The stability of DMTPS was also shown to be greater in the ILs than in molecular solvents, suggesting that the IL medium might permit the construction of a robust optoelectronic device.