Length Scales and Structural Dynamics in Nematogen Pseudonematic Domains Measured with 2D IR Vibrational Echoes and Optical Kerr Effect Experiments

Abstract

Nematogen liquids in the isotropic phase are macroscopically homogeneous but on multinanometer length scales have pseudonematic domains with correlation lengths that grow as the isotropic to nematic phase transition temperature (TNI) is approached from above. Orientational relaxation of nematogens in the isotropic phase manifests as two fast power laws and a slow exponential decay when measured by optical heterodyne detected optical Kerr effect (OHD-OKE) experiments. The long time exponential relaxation is associated with complete randomization of pseudonematic domains. We examine the effect of local orientational correlation on spectral diffusion (structural evolution) experienced by a vibrational probe molecule within the pseudonematic domains of 4-cyano-4'-pentylbiphenyl (5CB) using two-dimensional infrared (2D IR) vibrational echo spectroscopy. The addition of low concentration 4-pentyl-4'-thiocyanobiphenyl (5SCB) as a long-lived vibrational probe to 5CB is shown to lower TNI of the sample slightly, but the fast power law dynamics and exponential decays observed by OHD-OKE spectroscopy are unchanged. We compare the complete orientational relaxation and spectral diffusion for samples of 5SCB in 5CB to 5SCB in 4-pentylbiphenyl (5B) at four temperatures above TNI. 5B has a molecular structure similar to 5CB but is not a nematogen. At all but the lowest temperature, the spectral diffusion in 5CB and 5B is described well as a triexponential decay with very similar time constants. The results demonstrate that the presence of local orientational order at temperatures well above TNI does not affect the spectral diffusion (structural evolution) within pseudonematic domains when the correlation lengths are short. However, when the temperature of the sample is held very close to TNI, the spectral diffusion in 5CB slows dramatically while that in 5B does not. It is only as the correlation length becomes very long that its presence impacts the spectral diffusion (structural fluctuations) sensed by the vibrational probes located in pseudonematic domains. The orientational relaxation is modeled with schematic mode coupling theory (MCT). Fitting with MCT provides density and orientational correlation functions. The density correlation decays are similar for 5B and 5CB, but the orientational correlation decays are much slower for 5CB. Additionally, the time dependence of the spectral diffusion in 5CB is strikingly similar to that of the density correlation function decay, while the orientational correlation function decay is far too slow to contribute to the spectral diffusion. Therefore, density fluctuations are likely the source of spectral diffusion at temperatures at least 5 K above TNI.

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy

“…19 2D IR data were collected at four temperatures above T NI for 5SCB in 5CB and for 5SCB in 4pentylbiphenyl (5B), a structurally similar liquid that is nonmesogenic. 20 We previously found that the time constants of the spectral diffusion were virtually identical in 5CB and 5B at temperatures more than 5 K above T NI , indicating that the presence of pseudonematic domains in 5CB had negligible effect on the structural dynamics reported. 20 However, when the temperature was brought close to T NI , the spectral diffusion in 5CB slowed dramatically to the point that it could not be measured given the lifetime of the CN stretch of 5SCB.…”

“…20 We previously found that the time constants of the spectral diffusion were virtually identical in 5CB and 5B at temperatures more than 5 K above T NI , indicating that the presence of pseudonematic domains in 5CB had negligible effect on the structural dynamics reported. 20 However, when the temperature was brought close to T NI , the spectral diffusion in 5CB slowed dramatically to the point that it could not be measured given the lifetime of the CN stretch of 5SCB. 20 An alternate vibrational probe with a longer lifetime was required to probe the effect of pseudonematic domains at temperatures very close to T NI .…”

“…We confirm the retention of liquid crystal behavior upon addition of the small amount 5SeCB through comparison of OHD-OKE decays of the doped sample and neat liquid as was done in previous experiments on 5SCB in 5CB. 20 The measured time derivative of the polarizability-polarizability correlation function, which is equivalent to the time derivative of the orientational correlation function (second Legendre correlation function) of the bulk liquid, 30,31 was found to be almost identical between the samples at temperatures above T NI . Landaude Gennes dynamics for the final exponential relaxation of the pseudonematic domains are preserved, but T * was ∼1 K lower as compared to the neat sample.…”

“…We have previously shown that temperature dependent ultrafast two dimensional infrared (2D IR) vibrational echoes and polarization selective pump-probe experiments combined with the findings from OHD-OKE experiments provide an expanded picture of nematogen dynamics in the isotropic phase. 19,20 The 2D IR experiments yield information on the dynamics experienced by non-perturbative probe molecules within a pseudonematic domain, while the complete reorientation dynamics over a very wide range of time scales are reported by the complementary OHD-OKE experiments.…”

“…20 However, when the temperature was brought close to T NI , the spectral diffusion in 5CB slowed dramatically to the point that it could not be measured given the lifetime of the CN stretch of 5SCB. 20 An alternate vibrational probe with a longer lifetime was required to probe the effect of pseudonematic domains at temperatures very close to T NI .…”