Nematogen liquids in the isotropic phase have pseudonematic domains with temperature-dependent correlation lengths that grow as the isotropic-nematic phase transition (TNI) is approached from above. This provides a unique opportunity to study nanoscopic length scales in a one-component system. The dynamics of the nematogen 4-cyano-4'-pentylbiphenyl (5CB) were studied using optical heterodyne detected optical Kerr effect (OHD-OKE) measurements and 2D IR vibrational echoes. The use of these two techniques allows the local orientational correlation (measured by OHD-OKE) to be related to the spectral diffusion experienced by a 2D-IR vibrational probe within a pseudonematic domain. The OHD-OKE data were fit with schematic mode coupling theory (MCT), which provided both the density and orientational correlation functions. For 5CB, the orientational correlation function decays on too slow of a time scale to significantly contribute to spectral diffusion. Additionally, the time dependence of the spectral diffusion in 5CB is similar to the time dependence of the density correlation function. Thus, the source of spectral diffusion at temperatures at least 5 K above TNI is probably density fluctuations. Data was also collected for a 2.5 mol% solution of 4-pentyl-4'-thiocyanobiphenyl (5SCB) in 5CB, a 2.5 mol% solution of 4-pentyl-4'-selenocyanobiphenyl (5SeCB) in 5CB, and a structurally similar non-nematogen, 4-pentylbiphenyl (5CB).

Pseudonematic domain effects on the dynamics and spectral diffusion of a nematogen in the isotropic phase: An optical Kerr effect and 2D IR vibrational echo study

248th National Meeting (2014)

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