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

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) schematic model for the relationships among three correlation functions: ϕ1, a correlator for large wave vector density fluctuations; ϕ2, the orientational correlation function whose time derivative is the observable in the OHD-OKE experiment; and ϕ3, the FFCF for the 2D IR experiment. The equations for ϕ1 and ϕ2 match those in the previous MCT schematic model for nematogens, and ϕ3 is coupled to the first two correlators in a straightforward manner. Resulting models fit the data very well. Across liquid crystals, the temperature dependences of the coupling constants show consistent, nonmonotonic behavior. 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.

Direct observation of dynamic crossover in fragile molecular glass formers with 2D IR vibrational echo spectroscopy

“…In all three systems, the nitrile stretch peak frequency was ∼2155 cm 1 with a FWHM of about 10 cm 1 , consistent with other molecular selenocyanate probes in analogous systems. 55,56 System-specific details are summarized in Table I. Fitting the peaks to a Voigt function shows an increasing inhomogeneous linewidth and proportionately shrinking homogeneous linewidth with decreasing temperature, in accord with the detailed observations using 2D IR spectroscopy (See Tables SI-SIII in the supplementary material).…”

“…This model has had great success in fitting 2D IR data for a variety of systems including organic liquids, water, proteins, liquid crystals, and room-temperature ionic liquids. [55][56][57][58][59][71][72][73][74][75] The multiexponential model was chosen as our starting point, as above T m , these glass forming liquids are simple organic liquids. At these higher temperatures, the CLS(T w ) decay curves for all three samples fit well to biexponential decays.…”

“…The development of longlived selenocyanate probes has been an important advance, as their nitrile stretches can have lifetimes of many hundreds of picoseconds, permitting the collection of data from <1 ps to ∼1 ns. These probes have previously been used in 2D IR spectroscopy to probe dynamics in liquid crystals, 55,56 in solvation shells of water and ionic liquids, [57][58][59] and in proteins. 60 It has been shown that the spectral diffusion reported by probes of similar shape and size as the molecules that make up the bulk liquid is nearly identical to that felt by natural vibrational probes that are intrinsic to the liquid itself.…”

Probing of Reorganization Dynamics within the Different Phases of Themotropic Liquid Crystals

“…Therefore reorganization dynamics of LC molecules around a C153 dipole would not influence the fluorescence quantum yield of C153. In Figure , the midpoint of the fluorescence intensity transition of C153 clearly reflecting the liquid crystalline‐to‐liquid phase transition temperature, which is highly congruent with earlier reports ,. The actual reason of fluorescence intensity enhancement of C153 in liquid phase of LCs is a matter of debate.…”

“… Plot of emission intensity (in arbitrary unit) of C153 in 5CB (A) and 8CB (B) as a function of temperature (λ ex ∼405 nm). The corresponding liquid crystalline‐to‐liquid phase transition temperature is shown by the black vertical line, which accurately matches with reported value ,…”