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

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• 作者：Kathleen P. Sokolowsky ; Heather E. Bailey ; David J. Hoffman ; Hans C. Andersen ; Michael D. Fayer
• 刊名：Journal of Physical Chemistry B
• 出版年：2016
• 出版时间：July 21, 2016
• 年：2016
• 卷：120
• 期：28
• 页码：7003-7015
• 全文大小：531K
• 年卷期：0
• ISSN：1520-5207

文摘

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 (T_NI). 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 T_NI. 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 T_NI, precisely where the rate of spectral diffusion in 5CB was observed to deviate from that of a similar nonmesogenic liquid.