Order parameters of some nematic p, p′ substituted tolanes as determined by polarized Raman scattering

Abstract

The order parameters <P ,> and <P .> of three nematic compounds have been determined by polarized Raman scattering. Results obtained with p-pentyl-p'-cyanobiphenyl (5CB) agree well with published data. Next the influence of a strongly polar cyano end group on the order parameters was investigated. This was done by comparing two nematic liquid crystals of the tolane class, namely p-heptyl-p' -cyanotolane and p-hepty1-p'-methoxyto1ane, which have an almost identical geometrical shape. In spite of the difference in polarity hardly any difference in the order parameters <P,> and <p.> of both compounds couId be observed. <P,) and <p.> of both tolanes show approximately mean field behavior in contrast to the low <p.> of SCB.

Comparative Study of Orientational Order of Some Liquid Crystals from Various Homologous Series

“…Nevertheless, the nontypical behaviour of this parameter was observed by many other authors studying the orientational order in the nematic phase using Raman scattering [8,9,[22][23][24][25] and fluorescence depolarization [3 ,5-7] methods. Although many attempts were undertaken in order to explain the deviation of the experimentally obtained P 4 values from the theories [22][23][24][25][26][27], up to now no satisfactory explanation was found. In our present study we checked the influence of the angle between the absorption transition moment and the long axis of a dye molecule, ϕ, on P 4 values obtained from fluorescence measurements as well as on P 2 A and P 2 F .…”

Time resolved fluorescence depolarization in a nematic liquid crystal

“…There has been a lot of controversy in the literature [15,18,21,49,50] as to which kind of corrections to apply. In particular anisotropic internal field effects have been considered in an approximate and different way by some authors [15,49].…”

Orientational Behaviour of the Guest-Host Systems in the Smectic A and Nematic Phases

“…Nevertheless the non-typical behaviour of the (Ρ4 ) parameter for some nematics had been noticed by many authors studying the orientational order by Raman scattering [27,[29][30][31][32] as well as by depolarization of fluorescence [7,14]. If all systematic experimental errors are estimated (as it was done in this work using the appropriate geometry and incorporating the optical corrections), then three reasons of the deviation of the experimentally obtained (Ρ4) values from the theories are possible: 1) molecular flexibility, especially due to the alkyl chain of the liquid crystal [27,29,30], 2) the anisotropy of the molecular local field [28][29][30] and 3) the molecular association ("dimerization") due, for example, to the strongly polar-CN group of the nematic molecules [29][30][31]. Dalmolen et al [26] studying the order parameters of homologous series of alkylcyanobiphenyls gave some evidences that the anomalously low values observed for (Ρ4) of some nematics might be related to the dimerization.…”

“…Both (P2) and (Ρ4) can be simultaneously obtained on the basis of X-ray diffraction [19][20][21][22][23], magnetic resonance [25], Raman spectroscopy [25][26][27][28][29][30][31][32] as well as fluorescence depolarization of the dye dissolved in liquid crystalline matrix [7,[10][11][12][13][14][15].…”