Raman Imaging of Nematic and Smectic Liquid Crystals

Abstract

In this article, we demonstrate that confocal Raman microscopy can be utilized to image the director fields of liquid crystal molecules, revealing the spatial patterns of the molecular orientational order both in lateral and axial directions. Here we report on the polarized confocal Raman imaging of the electric field-aligned uniaxial nematic liquid crystal and the in-depth director profiling of the focal conic domains of smectic A liquid crystal. The CN stretching vibrational band [n(CN) ¼ 2228 cm À1 ] and the CÀC stretch of aromatic rings [n(CC) ¼ 1606 cm À1 ] are used to probe the spatial director orientations of 4-pentyl-4 0 -cyanobiphenyl (5CB) and 4-octyl-4 0 -cyanobiphenyl (8CB).

Chitosan as matrix for bio-polymer dispersed liquid crystal systems

“…In the Raman spectra of this compound four bands can be evidenced at 1180 cm −1 , assigned to aromatic CH in-plane deformation, 1286 cm −1 to C C stretching of the biphenyl link, 1606 cm −1 to C C stretching of the aromatic rings and 2228 cm −1 to C N stretching (Buyuktanir, Zhang, Gericke, & West, 2008;Kachynski, Kuzmin, Prasad, & Smalyukh, 2008). The assignment of such modes has been performed by taking into account the fact that biphenyl belongs to the C2v point group and their skeletal vibrational modes can be described in four symmetry species (Castriota et al, 2011).…”

Determination of 3D molecular orientation by concurrent polarization analysis of multiple Raman modes in broadband CARS spectroscopy

“…They analyze a series of slightly defocused images and reconstruct the 3D orientated image of molecules or particles of interest. These methods were applied to various samples by various spectroscopic techniques, including single molecule spectroscopy of fluorophores [8], second harmonic generation of single nanocrystals [9], confocal Raman [10] and coherent anti-Stokes Raman scattering (CARS) of liquid crystals [11]. However, these methods are only applicable for isolated molecules or defects that can be defocused, and not applicable for measuring molecular orientations of continuous samples.…”

Three-dimensional imaging of chemical bond orientation in liquid crystals by coherent anti- Stokes Raman scattering microscopy

“…The frequency/wavelength shifts from the monochromatic pump wavelength correspond to the natural frequencies of vibration of individual chemical groups within the molecule. For example, Ofuji et al [10] used Raman microscopy to image solid films of orientationally ordered fibrils in polyacetylene films with contrast based on the C=C bond stretching frequency, while Buyuktanir et al [11] used the cyano (CN) group stretching frequency to image defects in liquid crystal samples. Blach et al [12] developed a theory of polarized Raman microscopy in a birefringent medium and probed the director by using the vibrational mode at 2226 cm -1 .…”