Structural Anomalies in Ionic Liquids near the Glass Transition Revealed by Pulse EPR

Abstract

Unusual physical and chemical properties of ionic liquids (ILs) open up prospects for various applications. We report the first observation of density/rigidity heterogeneities in a series of ILs near the glass transition temperature ( T) by means of pulse electron paramagnetic resonance (EPR). Unprecedented suppression of molecular mobility is evidenced near the glass transition, which is assigned to unusual structural rearrangements of ILs on the nanometer scale. Indeed, pulse and continuous wave EPR clearly indicate the occurrence of heterogeneities near T, which exist in a rather broad temperature range of ∼50 K. The two types of local environments are evidenced, being drastically different by their stiffness. The more rigid one suppresses molecular mobility, whereas the softer one instead promotes diffusive molecular rotation. Such properties of ILs near T are of general importance; moreover, the observed density/rigidity heterogeneities controlled by temperature might be considered as a new type of tunable reaction nanoenvironment.

Inherent heterogeneities and nanostructural anomalies in organic glasses revealed by EPR

“…As was demonstrated in our previous work, 30,32,33 the data from CW and pulse EPR fruitfully complement each other and help to reveal the structural dynamics of glassy ILs and their structural anomalies. CW EPR allows disentangling the observed signal vs. temperature into two fractions of mobile (denoted M(T)) and immobile (1 À M) probes.…”

“…1 mM. [30][31][32][33]53 Pulse EPR measurements were performed using a commercial Bruker Elexsys E580 spectrometer at X-band (9 GHz). CW EPR spectra were obtained at X-band Bruker EMX spectrometer.…”

“…Previous studies demonstrated that the difference of the inverse T 2 values measured at two characteristic spectral positions can be expressed as L $ (1/T (II) 2 À 1/T (I) 2 ) ¼ 10 11 ha 2 is c , where ha 2 i is the mean-squared amplitude of librations and s c is their characteristic time. 33,[55][56][57][58] The temperature dependence L(T) essentially characterizes the molecular mobility and local density/rigidity of the matrix surrounding the spin probe. Since L(T) is measured on the basis of relaxation times, it does not reect the amount of pulse-EPR visible spins vs. temperature.…”

“…While pulse EPR is sensitive to the librational motion (wobbling), CW EPR detects the large-scale diffusive rotation of radicals occurring typically in liquid state and sometimes in soened solid IL matrices [59][60][61][62] or nanopores. 63,64 Following previously developed procedure, 30,33 we obtain CW EPR spectra of dissolved probe TEMPO-D 18 temperature close to the local maximum of L(T) (Fig. 1c and d).…”

“…21,22 Recently, we have discovered unprecedented suppression of molecular mobility with temperature in a series of ionic liquids (ILs) in a glassy state near T g , which was assigned to unusual structural rearrangements on the nanometer scale. [30][31][32][33] In particular, the coexistence of two types of IL environments was observed, one of which progressively suppresses the molecular mobility upon temperature increase between ($T g -60 K) and T g . This is a highly uncommon behavior, since most known substances, including the same ILs, become less dense in the bulk upon temperature increase; therefore it was termed a "structural anomaly".…”