Critical Slowing of Density Fluctuations Approaching the Isotropic-Nematic Transition in Liquid Crystals: 2D IR Measurements and Mode Coupling Theory.

TLDR: A remarkable change in coupling occurs at ∼5 K above TNI, precisely where the rate of spectral diffusion in 5CB was observed to deviate from that of a similar nonmesogenic liquid.

Abstract: Two-dimensional infrared (2D IR) data are presented for a vibrational probe in three nematogens: 4-cyano-4′-pentylbiphenyl, 4-cyano-4′-octylbiphenyl, and 4-(trans-4-amylcyclohexyl)-benzonitrile. The spectral diffusion time constants in all three liquids in the isotropic phase are proportional to [T*/(T – T*)]1/2, where T* is 0.5–1 K below the isotropic–nematic phase transition temperature (TNI). Rescaling to a reduced temperature shows that the decays of the frequency–frequency correlation function (FFCF) for all three nematogens fall on the same curve, suggesting a universal dynamic behavior of nematogens above TNI. Spectral diffusion is complete before significant orientational relaxation in the liquid, as measured by optically heterodyne detected-optical Kerr effect (OHD-OKE) spectroscopy, and before any significant orientational randomization of the probe measured by polarization selective IR pump–probe experiments. To interpret the OHD-OKE and FFCF data, we constructed a mode coupling theory (MCT) sc...