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

Abstract

The isotropic phase of nematogenic liquid crystals has nanometer length scale domains with pseudonematic ordering. As the isotropic to nematic phase transition temperature (TNI) is approached from above, the orientational correlation length, ξ, of the pseudonematic domains grows as (T - T(*))(-1/2), where T(*) is 0.5-1 K below TNI. The orientational relaxation, which is a collective property of the pseudonematic domains, was measured with optical heterodyne detected-optical Kerr effect (OHD-OKE). The orientational relaxation obeys Landau-de Gennes theory, as has been shown previously. To examine the environmental evolution experienced by molecules in the pseudonematic domains, two-dimensional infrared (2D IR) vibrational echo experiments on the CN stretching mode of the non-perturbative vibrational probes 4-pentyl-4(')-selenocyanobiphenyl (5SeCB) and 4-pentyl-4(')-thiocyanobiphenyl (5SCB) in the nematogen 4-cyano-4(')-pentylbiphenyl (5CB) were performed. The 2D IR experiments measure spectral diffusion, which is caused by structural fluctuations that couple to the CN vibrational frequency. Temperature dependent studies were performed just above TNI, where the correlation length of pseudonematic domains is large and changing rapidly with temperature. These studies were compared to 2D IR experiments on 4-pentylbiphenyl (5B), a non-mesogenic liquid that is very similar in structure to 5CB. The time constants of spectral diffusion in 5CB and 5B are practically identical at temperatures ≥5 K above TNI. As the temperature is lowered, spectral diffusion in 5B slows gradually. However, the time constants for spectral diffusion in 5CB slow dramatically and diverge as T(*) is approached. This divergence has temperature dependence proportional to (T - T(*))(-1/2), precisely the same as seen for the correlation length of pseudonematic domains, but different from the observed orientational relaxation times, which are given by the Landau-de Gennes theory. The data and previous results show that spectral diffusion in 5CB has no contributions from orientational relaxation, and the structural dynamics responsible for the spectral diffusion are likely a result of density fluctuations. The results suggest that the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length, ξ. The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length. The results presented here show that there is another divergent dynamical process, likely associated with density fluctuations.

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 ,…”

“…Our interest in LCs is to study the orientational dynamics of its rod shaped molecules around an excited solute dipole. Outcome of this study will help researchers in rational designing of optical switchable windows, optical devices for telescope and other liquid crystal devices where efficiencies of devices are largely controlled by the orientational dynamics of the LCs molecules ,. Reorganization time of LCs molecules, obtained in this study, can be correlated with the response time of rearrangements of LCs molecules against an external electric/magnetic field in a birefringence LC cell .…”

“…In a seminal work by Fayer and his coworkers, using 2D IR vibrational echo spectroscopy, they observed the presence of long range isotropic phase within the anisotropic nematogenic liquid crystalline phase. They used heterodyne detected‐optical Kerr effect (OHD‐OKE) to measure the orientational relaxation as a result of association of pseudo‐nematic domains near isotropic to nematic phase transition temperature . Among the other phases of LCs, chiral nematic phase has appealed profound interest of the researchers .…”

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

“…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.…”