Long-lived probes to study the dynamics of liquid crystals by ultrafast infrared vibrational spectroscopy [electronic resource]
Keywords: Chemistry.

Nematic liquid crystals have been widely studied in the context of practical applications and the nature of phase transitions. Above the nematic-isotropic phase transition temperature (TNI), orientational fluctuations are correlated; the existence of these pseudonematic domains in the isotropic phase dominate the dynamic properties of the bulk liquid. To examine the influence of pseudonematic ordering on the local dynamics, ultrafast infrared (IR) spectroscopies were employed. Two dimensional (2D) vibrational echo spectroscopy supplies information on the spectral diffusion of a vibrational probe molecule. Spectral diffusion is the process by which the frequency of an oscillator changes as the liquid structure about it evolves; it is characterized by the frequency-frequency correlation function (FFCF). The FFCF directly relates the frequency fluctuations of the probe molecule to the dynamics of the liquid structure surrounding it. The model nematic liquid crystal system selected is 4-cyano-4'-pentylbiphenyl (5CB); 5CB contains a nitrile group which can act as the vibrational probe. Examination of the natural abundance carbon-13 nitrile stretch allows for thicker samples, mitigating heating effects. Unfortunately, the vibrational lifetime of the carbon-13 stretch is short, and spectral diffusion is not nearly complete in the experimental window. Two novel extended lifetime vibrational probes similar in structure to 5CB were synthesized. A small amount of either molecule can be added to 5CB to probe the local structure dynamics. OHD-OKE experiments were performed on the doped solutions confirm that the addition of vibrational probe molecule does not significantly perturb the pseudonematic domains of 5CB. Temperature dependent 2D IR experiments were conducted on 5CB and 4-pentylbiphenyl (5B). 5B is molecularly very similar to 5CB, but is not a liquid crystal. The spectral diffusion dynamics in the two liquid are remarkably similar far removed from the liquid crystal phase transition. Close to TNI, the dynamics in 5CB slow down dramatically. The time constants for spectral diffusion are shown to diverge as a[T*/(T-T*)]^1/2. The temperature dependence of the time constants matches that of the size of the pseudonematic domains, not the orientational relaxation time for the domains. Two more liquid crystals are shown to have the same behavior approaching their respective phase transitions, suggesting the divergence may be universal for rod-shaped nematogens. Even at temperatures very close to TNI, spectral diffusion in liquid crystals is complete on the order of a few nanoseconds. This is drastically different than the time for total orientational relaxation which is multiple orders of magnitude slower. The complete randomization of pseudonematic domains is not required for the vibrational probe to sample all structures that contribute to the inhomogeneous line shape. A mode coupling model is constructed for the FFCF to describe spectral diffusion in terms of coupling to large wave vector density fluctuations and orientational fluctuations. The source of spectral diffusion is proposed to be small wave vector density fluctuations, which critically slow as TNI is approached.