Dimitris Katsis

Circularly polarized fluorescence from chiral nematic liquid crystalline films: theory and experiment

A theory was formulated for the description of circularly polarized fluorescence (CPF) from a chiral nematic film in the spectral region outside the selective reflection band. The CPF theory incorporates: (1) the ability of a chiral nematic film to accomplish both circular dichroism and circular polarization and (2) linearly polarized fluorescence from chromophores unaxially aligned in the nematic sublayers comprising the film. Chiral nematic fluid films consisting of a nematic fluid, BDH 18523, and a chiral dopant, cholesteryl oleate, were prepared for hosting 1,6-diphenylhexatriene as a fluorescent dye. The experimentally determined dissymmetry factor using both left- and right-handed circularly polarized excitations was found to be in good agreement with the theoretical prediction with all the system parameters determined a priori. The theory was also employed to furnish insight into the effects of the concentration of the fluorescent dye and chiral nematic film thickness on the dissymmetry factor.

MECHANISTIC INSIGHT INTO CIRCULARLY POLARIZED PHOTOLUMINESCENCE FROM A CHIRAL-NEMATIC FILM

Circularly polarized photoluminescence (CPPL) was characterized for rod-like molecules of Exalite 428 helically arranged in a chiral-nematic liquid crystalline film. With an unpolarized excitation at 370nm, CPPL intensities measured at 428nm, and a selective reflection wavelength ranging from 7.2 to 72.6mum, the observed dissymmetry factor, g, was found to be in excellent agreement with theoretical prediction, without resorting to adjustable parameters. As a result, new insight into liquid crystal-induced CPPL has emerged. Specifically, it was found that circular dichroism and circular polarization of the excitation beam prior to inducing linearly polarized photoluminescence (LPPL) at quasi-nematic layers play an insignificant role. The relatively large g value in the spectral region far removed from the selective reflection band was attributed to the circular polarization of LPPL emanating from all quasi-nematic layers comprising the chiral-nematic film. In the absence of a fluorescent dye, the propagati...

Novel glass-forming liquid crystals V. Nematic and chiral-nematic systems with an elevated glass transition temperature

The formerly implemented molecular design concept behind glass-forming liquid crystals (gLCs) was generalized by increasing the volume of the non-mesogenic central core, with an attendant increase in the number of nematic pendants, using 5-hydroxyisophthalic acid as the bridging unit. New nematic gLCs were synthesized and characterized, showing an elevation in Tg by 30 to 40°C with no definite trend in Tc over the benzene, cis, cis-cyclohexane, and exo, endo-bicyclo[2.2.2]oct-7-ene base cores. The exo, exo-configured gLC showed a higher Tg and a higher T c than the exo, endo-counterpart. Morphological characterization with X-ray diffractometry revealed the non-crystalline nature of pristine samples and the morphological stability of thermally processed gLC films against recrystallization for six months. Nematic gLC films were prepared for characterization by FTIR linear dichroism, resulting in an orientational order parameter in the range 0.52 to 0.63. A chiral-nematic gLC derived from exo, exo-bicyclo[2.2.2.]oct-7-ene also showed an elevation in Tg by 10 to 20°C over the cyclohexane-based systems reported previously. With (S)-(-)-1-phenylethylamine as the chiral moiety, the left-handed, chiral-nematic gLC film yielded a selective reflection band centred around 375 nm. Tunability of selective reflection from the UV to visible region was demonstrated by mixing the chiral-nematic and nematic gLCs at varying ratios.

Polarized photoluminescence from solid films of nematic and chiral-nematic poly(p-phenylene)s

Thermotropic nematic and chiral-nematic poly(p-phenylene)s were prepared into well-aligned films between fused-silica substrates in which conjugated backbones were uniaxially and helically oriented. With unpolarized photoexcitation at 350 nm, a nematic film produced a degree of linear polarization of 9 near the emission peak at 410 nm with no evidence of excimer formation. With the same photoexcitation of a chiral-nematic film, the degree of circular polarization was found to vary from −1.3 in the 390–430 nm spectral region to between +0.3 and +0.9 beyond the edge of the selective reflection band. The crossover behavior unique to light emission from the selective reflection region remains inexplicable with existing theories on light propagation through periodically structured films.

Processing of alignment layers for glassy liquid crystals

This paper presents the current status of alignment techniques for a new class of liquid crystalline material being developed for both passive optical filtering/polarizing and latching electro-optic applications. This new glassy liquid crystal (GLC) material has the unique property of being electro-optic and fully latching. That is, in one state, the material has the properties of a conventional nematic liquid crystal, capable of being aligned with either an electric or magnetic field; while in its other state, it is an optical quality solid that maintains the molecular alignment set while in the fluid state. Molecular alignment of nematic GLC films is a critical technology necessary to develop high-performance, novel latching devices. The alignment of the nematic pendant component of GLCs directly correlates to the optical contrast, switching speed (turn-on time), and decay speed (turn-off time) of an active latching device. There has been little prior research conducted to assess conventional LC alignment techniques for use with GLCs. The processing and effectiveness of multiple alignment techniques will be discussed.

Circularly polarized photoluminescence from the resonance region of chiral-nematic poly(p-phenylene) films

Thermotropic nematic and chiral-nematic poly(p-phenylenes) were prepared into well aligned solid films. Dilute solution photoluminescence spectroscopy revealed that, with excitation at 350 nm, the conjugated polymer backbone was the predominant light emitter. Linearly polarized photoluminescence, with (lambda) ex equals 350 nm, and FTIR linear dichroism, performed on a uniaxially aligned film, yielded the orientational order parameter, S, pertaining to the conjugated polymer backbone and nematic pendants, at 0.62 and 0.73, respectively. The supramolecular structure in a uniaxially aligned film was also elucidated by these measurements; the polymer backbone and the nematic pendants were found to be collinear and lie predominantly along the buffing direction, i.e. along the director of the film. For emission outside the selective reflection region, also referred to hereafter as the resonance region, S was estimated at 0.67 for the conjugated backbone within the framework of a recent theory. At a decreasing chiral mole fraction, films prepared with chiral- nematic copolymers showed a selective reflection region in the ultraviolet to visible and infrared region. Moreover, the chiral-nematic films were found to consist of a left-handed helical stack of quasinematic layers with (S)-(-)-1- phenylethanol as the chiral moiety. For emission within the resonance region, the highest degree of circular polarization ever reported for neat conjugated polymers was observed. The left-handed component of the emission was found to be strongly suppressed within the resonance region and enhanced at its edges in close agreement with the results obtained when a rod- like luminophore was doped into a chiral-nematic host. In sharp contrast, regardless of the polarization state of incident light, propagation alone through a chiral-nematic film did not result in handedness reversal of the transmitted light. Viewing angle measurements revealed that the right- handed component of the emission follows Lamberts cosine law closely regardless of emission wavelength. The left-handed component of the emission, on the other hand, was found to be strongly dependent on both emission wavelength and viewing angle. This dependence can be explained, at least in part, to the shift of the resonance region to shorter wavelengths with increasing viewing angle.

Novel Vitrified Liquid Crystals and Potential Applications

An overview is presented for a generic approach to low molar mass liquid crystalline materials capable of vitrification into morphologically stable, optically anisotropic thin films. Potential applications as polarization control devices, optical notch filters, and luminescent elements will be explored. Preliminary data for defect formation and annihilation will also be presented to illustrate an advantage of low molar mass materials over polymer analogs.