Coexistence of two molecular environments in liquid cis-decalin revealed by spin probe

We used a nitroxide radical as a spin probe in electron paramagnetic resonance (EPR) to study the highly fragile glass-forming molecular liquid cis-decalin and the similar liquid cis-perfluorodecalin. The EPR spectra of the radical dissolved in cis-decalin were measured upon cooling from its equilibrium liquid state at 373 K to its supercooled state below 229 K. The analysis of the EPR spectra revealed the anomalous temperature behavior of the EPR parameters and rotational diffusivity of the radical in cis-decalin, which was not found in cis-perfluorodecalin. The anomalous behavior in cis-decalin led us to conclude that a distinct slow environment appears around the radical upon cooling through the equilibrium state and that the exchange rate between fast and slow radical environments decreases upon cooling into the supercooled state. Because of the slowing down of exchange, we were able to simulate the radical's low-temperature EPR spectra by two spectral components corresponding to the two radicals' environments. By using the theory of environment exchange in EPR, we performed the fitting of EPR parameters to determine temperature dependences of environment-exchange quantities and rotational radical diffusivities in each environment. The fraction of the slow environment was found to continuously grow on cooling, implying that cis-decalin does not exhibit liquid-liquid transition and fluid polyamorphism. However, the coexistence of two local liquid environments in cis-decalin deserves further examination as it may be closely related to very high fragility and other properties of this material. We broadly discuss our spin-probe results and their relations with the literature data for cis-decalin.