LCICD

Liquid Crystal-Induced Circular Dichroism (LCICD)... 

In Section 3.4.6 it was noted that chiral mesophases behave as chiral perturbation moieties for achiral molecules inserted to the mesophases. Here, only relations between the observed signs in LCICD and the pitch-dependent CD in liquid crystalline systems, and the helical sense of the liquid crystalline phases are described. 

We arrive at this conclusion from the lack of more than a slight atropisomeric excess (ca. 0.1% in all but one anomalous experiment) after equilibration of racemic BN in the cholesteric phases at several temperatures (TablelV). The lack of change in the ratio of atropisomers in the cholesteric phases is consistent with our observation that liquid-crystal induced circular dichroism spectra (67) of ISN in cholesteric mixture D are due to a macroscopic property of the solvent the LCICD spectra disappear when mixture D is heated to an isotropic temperature. 

That both phenomena arise as a consequence of macroscopic solvent order and not Intimate solvent-solute Interactions Is clear Saeva and 01In (75) have shown that solute LCICD spectra can be observed In twisted nematic phases only Nakazaki et al. (76) find an excess of one enantiomer of hexahelicene Is produced photochemlcally from achiral precursors In twisted nematic phases no LCICD spectra or optical Induction occurs In untwisted nematic phases and the handedness of the twist can be correlated with the sign of the LCICD and the preferred product enantiomer. Furthermore, Isotropic phases of cholesteric mixtures display no discernible LCICD spectra (12, 67) and the enantiomeric excesses In products of photolablle reactants In Isotropic phases are near zero (51). 

LCICD liquid-crystal-induced circular dichroism... 

The influence of temperature on magnitude of the side-chain LCICD was examined in dioxane. An independent measurement under a polarizing microscope showed that the cholesteric half-pitch in this solvent increased with temperature. Therefore the LCICD magnitude was expected to increase according to the theoretical... 

Fig. 9. Temperature dependence of the LCICD magnitude of the side chain phenyl band at 254 nm in the PBLG-dioxane (c = 20.0 %) liquid crystal...

Fig. 10. LCICD spectra of a PBLG-EDC-acridine orange system at various temperatures. The polymer concentration is 15.1 % and temperatures are (—) 31 °C, (—. —) 42 °C, 50 °C, (—...—)...

Fig. 26a and b. Temperature dependence of CD spectra of a colored PBLG film a cholesteric pitch bands around 500 nm b LCICD of the side-chain phenyl groups... 

As with most research dealing with ordered media, a satisfactory understanding of the system will require data from diverse experimental techniques. While the primary tool employed here is dynamic fluorescence measurements, more LCICD spectroscopy (especially of PnP in CN/CCl and QQ.) would be useful, and other techniques (e.g., NMR spectroscopy) would help elucidate the details of how solutes sit within specific cholesteric environments. In essence, the work completed has served to answer several questions and to identify many more. 

The u.v. and c.d. spectra of (364 R = Me, Et, Pr, Bu , or PhCH2) in EtOH and in iso-octane have been reported.The chiroptical properties of the oxaziridine chromophore were generally characterized by a positive pattern at 190—350 nm. The notable exception to this was a clear negative Cotton effect for (364 R = Pr ). The seemingly anomalous behaviour of this compound was interpreted in terms of the possible existence of solvation equilibria and conformational rotamers about the N—CHMe2 bond. The relatively new technique of Liquid-Crystal-Induced Circular Dichroism (LCICD) has been used in the... 

Up to now, the chirality transfer from the level of molecules (second level) to the phase (third level) has been taken into consideration. An example of the transfer of chirality from the liquid crystal phase (third level) to the molecule (second level) is the liquid crystal induced CD (LCICD) [40], [41]. 

Liquid crystal induced circular dichroism (LCICD) examines the differential absorption of circularly polarized radiation by an achiral solute oriented in a cholesteric liquid crystal. The circular dichroism results from an induced Cotton effect in the achiral solute due to the macroscopic chirality of its ordering in the helical solvent matrix. The effect has been shown theoretically to arise... 

Linearly polarized fluorescence (LPF) and circularly polarized fluorescence (CPF) provide complementary techniques to LCLD and LCICD for the assignment of solute electronic transition moments and for investigating solute orientational ordering and liquid crystalline properties [1,2, 304, 326-328]. Additionally, these techniques can provide information on the electronic structure of excited state complexes (excim-ers and exciplexes) [329, 330]. Time-re-solved luminescence depolarization experiments have been used by various workers to study the ordering and mobility of molecules in liquid crystalline phases [317-319, 331, 332] the information obtainable from these studies in analogous to that obtained by NMR relaxation experiments. Since luminescence depolarization is the main result of probe molecular motions and is consequently very rapid, it leads to complica-... 

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