Polarized ATR-IR Studies of Surface-Field-Induced Anchoring of Alkyl Cyanobiphenyls

Abstract

We report initial studies of the use of polarized ATR-IR spectroscopy to obtain information on molecular alignment, on the mesoscopic scale, in the interfacial region of alkyl cyanobiphenyl liquid crystals in contact with self-assembled monolayers (SAMs). Specific mesoscopic alignment was induced by variation of the surface field, via the ω-functional group of self-assembled monolayers of alkanethiol derivatives. Our experimental approach involved the deposition of a thin gold layer onto a ZnSe prism. The presence of this layer strongly attenuates the s-polarized field leading to an “effective surface selection rule”, which can be used to determine the average orientation of the liquid crystal molecules within the attenuation depth of the evanescent field. A frequency shift of the C⋮N stretching band was seen in the case of homeotropic anchoring.

Surface Plasmon Raman Scattering Studies of Liquid Crystal Anchoring on Liquid-Crystal-Based Self-Assembled Monolayers

“…Self-assembled monolayers (SAMs) have been used to promote alignment of nematic liquid crystals (LCs). − Many of these studies have concentrated on the use of SAMs to generate “high”- and “low”-energy surfaces to achieve alignment. − Few studies have dealt with the case when the functional group of the SAM is an LC look-alike. In this situation, the strong interactions between the functional group and LC allow one to control the alignment of the LC simply by selecting the orientation of the functional group of the SAM.…”

“…The use of SAMs of alkanethiols for the study of LC/surface interactions has been pioneered by Abbott − and Evans. − The work of Abbott has concentrated mainly on polarized light microscopy and interferometer studies of the alignment of 4′-pentylbiphenyl-4-carbonitrile (5CB) in SAM-functionalized planar cells (2−30 μm thick). In cells functionalized with single-component SAMs of alkanethiols, Drawhorn and Abbott found planar anchoring .…”

“…Evans has concentrated on the study of the LC/SAM interface using an ensemble of evanescent wave techniques: evanescent Brewster angle ellipsometry, − surface plasmon resonance (SPR) imaging, and attenuated total reflection infrared (ATR-IR) spectroscopy . Using evanescent Brewster angle ellipsometry with SAM-modified gold substrates, Evans and co-workers were able to obtain a phase diagram showing both the presence of an anchoring transition and orientational wetting. − In all cases of homeotropic anchoring, complete orientational wetting was seen as T → T IN .…”

Photoswitching of Azobenzene Derivatives Formed on Planar and Colloidal Gold Surfaces

“…While the polymeric (LB) and silane (SAM) approaches produce films which display reversible “photoswitching”, the alkanethiol-based systems have, to date, not shown switchable behavior. However the potential advantage of having such functionalized moieties attached to metallic substrates that can used as electrodes is evident, leading to application in electrochemical investigations or, and of more relevance to us, for the facilitating control over the anchoring of liquid crystals. , Detailed scanning probe microscopy investigations have shown that azobenzene-derivatized alkanethiols form SAMs in which the chromophore units are close packed and present little free volume to allow switching between the cis and trans conformers. , …”

“…However the potential advantage of having such functionalized moieties attached to metallic substrates that can used as electrodes is evident, leading to application in electrochemical investigations 8 or, and of more relevance to us, for the facilitating control over the anchoring of liquid crystals. 9,10 Detailed scanning probe microscopy investigations have shown that azobenzene-derivatized alkanethiols form SAMs in which the chromophore units are close packed and present little free volume to allow switching between the cis and trans conformers. 11,12 In this paper we present results obtained from SAMs formed from a variety of azo moieties, on planar and colloidal substrates.…”