Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

Abstract

Unusual physicochemical properties of ionic liquids (ILs) open vistas for a variety of new applications. Herewith, we investigate the influence of microviscosity and nanostructuring of ILs on spin dynamics of the dissolved photoexcited molecules. We use two most common ILs [Bmim]PF6 and [Bmim]BF4 (with its close analogue [C10mim]BF4) as solvents and photoexcited Zn tetraphenylporphyrin (ZnTPP) as a probe. Time-resolved electron paramagnetic resonance (TR EPR) is employed to investigate spectra and kinetics of spin-polarized triplet ZnTPP in the temperature range 100-270 K. TR EPR data clearly indicate the presence of two microenvironments of ZnTPP in frozen ILs at 100-200 K, being manifested in different spectral shapes and different spin relaxation rates. For one of these microenvironments TR EPR data is quite similar to those obtained in common frozen organic solvents (toluene, glycerol, N-methyl-2-pyrrolidone). However, the second one favors the remarkably slow relaxation of spin polarization, being much longer than in the case of common solvents. Additional experiments using continuous wave EPR and stable nitroxide as a probe confirmed the formation of heterogeneities upon freezing of ILs and complemented TR EPR results. Thus, TR EPR of photoexcited triplets can be effectively used for probing heterogeneities and nanostructuring in frozen ILs. In addition, the increase of polarization lifetime in frozen ILs is an interesting finding that might allow investigation of short-lived intermediates inaccessible otherwise.

Validation of Structural Grounds for Anomalous Molecular Mobility in Ionic Liquid Glasses

“…Therefore, protons located in imidazolium ring have a negligible impact on the spin relaxation of radical probe. This indirectly confirms the surrounding of alkyl tails around spin probe, which was earlier proposed in the literature [ 35 , 38 , 44 , 78 ], as well as in our previous studies [ 53 , 54 ].…”

“…Recently, extensive studies of heterogeneities in IL glasses were performed using Electron Paramagnetic Resonance (EPR) spectroscopy [ 47 , 48 , 49 , 50 , 51 , 52 ]. We developed and implemented complex approach including continuous wave (CW) EPR, pulse and time-resolved (TR) EPR [ 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 ]. Since studied ILs are naturally diamagnetic, spin probes (stable radicals or photoinduced triplet states) were used for EPR detection.…”

“…Since studied ILs are naturally diamagnetic, spin probes (stable radicals or photoinduced triplet states) were used for EPR detection. Variable-temperature CW EPR data show that mobile and immobile local environments coexist within a broad range of temperatures upon transition from glassy to a liquid state of imidazolium ILs [ 53 , 56 , 61 ]. Pulse EPR allows monitoring small-angle wobbling (stochastic molecular librations) of spin probe that is dictated by a surrounding glassy matrix, as a function of temperature [ 62 ].…”

Probing Laser‐Induced Heterogeneous Microenvironment Changes in Room‐Temperature Ionic Liquids

“…Modulation of the physical chemistry processes in various heterogeneous systems triggered by external optical or electrical control methods is currently a subject of intense research . Many probe molecules have been optimized to improve their responsive properties towards changes in their surrounding microenvironment . A heterogeneous solvent could provide a more versatile media in which simultaneous modulation of the microenvironment of solvent and the associated physical chemistry properties (such as spectral behavior) of the solute may enable many promising applications.…”

“…Porphyrins/porphyrinoids, with their excellent triplet excited state properties, are important candidates for photochemical and photoelectric research related to sensing, solar cells, diagnostics, and therapeutics . Moreover, because the dynamics of their triplet excited states are very sensitive to the microenvironment with respect to oxygen, porphyrin molecules are widely used as a probe to study the microstructure changes associated with oxygen‐related chemical reactions in many heterogeneous systems . Meanwhile, coumarin 153 (C153) has also become a well‐established probe for investigating the structural heterogeneity of RTIL systems …”