Anisotropy of Dye Diffusion in Smectic Phases Studied by Forced Rayleigh Scattering

Forced Rayleigh scattering studies of tracer diffusion in a nematic liquid crystal: The relevance of complementary gratings

“…As noted above, nematic/isotropic diffusion discontinuities have been reported in FRS studies for MR in other NLC systems. [39][40][41][42] Although the current experiment involves only 5CB, it is possible that the nematic/isotropic discontinuities reported for these other systems could also be related to complementary grating effects. In any case, it is FIG.…”

“…A considerably larger positive jump at T NI was reported for MR in MBBA in the same publication, and later papers showed positive jumps at T NI for MR in a number of other mesomorphic solvents, including other nCB compounds (n ϭ6 -9), 4O.8, and 8OCB. [39][40][41][42] To explain the nematic/ isotropic discontinuity, it was proposed 41 that tracer diffusion through the nematic phase could be envisioned as a ''dressed'' process, in which the mobility of the tracer was reduced due to local distortions in the nematic continuum generated by the tracer movement, in a manner somewhat analogous to polaron motion in the solid state. The data submitted in the current report, interpreted in the context of the complementary grating model, largely confirm the important observation by Hara et al of non-Arrhenius diffusion in the 5CB nematic phase; however, in the 5CB isotropic phase we observe a distinctly nonexponential decay, which significantly changes the values obtained for the isotropic diffusion coefficients.…”

“…MR has been frequently chosen as a tracer dye in liquid crystal systems due to its convenient photochromic properties and the similarity between its structure and that of liquid crystal molecules containing a two-phenyl-ring core. 21,22,25,32,[39][40][41][42][43][44] However, here we make no a priori assumptions that tracer diffusion of MR will mimic exactly the self-diffusion of the 5CB solvent molecules, since the chemical properties of MR and 5CB are certainly different. Indeed, using FRS, Urbach et al 32 and, more recently, Ohta and co-workers 45 have reported that MR-tracer diffusion is considerably slower than the self-diffusion of the liquid crystalline solvent molecules in nematic MBBA.…”

Slow Single-Molecule Diffusion in Liquid Crystals

“…Other experimental techniques applied to nematics include single-molecule fluorescence correlation measurements (FCS) and the fluorescence recovery after photobleaching (FRAP) on thin free-standing films of a liquid crystal . Results on the time-dependent concentration spreading of dye-doped particles in 5CB and on forced Rayleigh scattering (FRS) − have been reported as well. Most of these methods, however, detect averaged properties of the liquid crystal from large prealigned sample regions, determined over long periods of time or at large particle ensembles.…”

Electrohydrodynamics of Dye Molecules in an Isotropic Liquid Crystal Studied by the Transient Grating Method

“…We use methyl red (MR) as the dopant dye molecule in MBBA (4-methoxybenzylidene-4- n -butylaniline), which is one of the most widely studied liquid crystal molecules. MR is a widely used probe molecule for the TG method in various liquids, , polymeric solutions 9-11 and matrixes, and liquid crystals. − , MR has the trans form in the dark but converts to the cis form with illumination of wavelengths shorter than 590 nm the isomerization through the electronically excited states and the relaxation to the ground state of each isomer take place within the time resolution of our experiment (∼1 μs).…”