Time-Resolved Electron Paramagnetic Resonance Study of Photoexcited Fullerenes in Ionic Liquids

Molecular-level properties of ionic liquids (ILs) draw an increasing interest. Several informative experimental approaches for investigation of nano/miscrostructuring phenomena and local viscosity/rigidity of ILs use probe molecules sensitive to microenvironment along with suitable detection techniques. In this work, we for the first time investigate capabilities of photoexcited triplet fullerenes to probe local properties of ILs, with time-resolved electron paramagnetic resonance (TR EPR) being a sensitive detection tool. We have selected C₆₀ and its derivative phenyl-C₆₁-butyric acid methyl ester (PCBM) as probes and ILs [Bmim]BF₄ and [C₁₀mim]BF₄ as solvents. C₆₀ and PCBM demonstrate different sensitivities to microenvironment in ILs. Spin dynamics of photoexcited C₆₀ is strongly contributed by pseudorotation of the Jahn-Teller axis, making its use as a probe for microenvironment challenging. This behavior is strongly suppressed in PCBM, which, in addition, is more soluble in ILs than C₆₀. The in-depth analysis of variable-temperature two-dimensional TR EPR data shows that spectral shapes are sensitive to the restricted mobility of PCBM in ILs. In this way, the information on local environment and heterogeneities in ILs can be obtained by TR EPR. PCBM usefully complements the other spin probes previously implemented for EPR studies in ILs. It is larger in size and in addition allows high-sensitivity TR EPR measurements up to a room temperature, which is an important improvement for characterization of heterogeneities in room-temperature ILs.