Colorless High Dielectric Compounds for Low Voltage Liquid Crystal Application
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dopants is restrained to -10% or less, they do not necessarily need to have a liquid crystal phase. The push-pull effect has been incorporated in liquid crystal dyes [6] that exhibit a huge dielectric anisotropy. For example, some nitro-amino azo and tolane dyes [7] show a modest melting point, small heat fusion enthalpy and their As is greater than 50. The bicyano polyene
dyes [8,9] exhibit not only large dielectric anisotropy (AE-65), but also small elastic constant. These dyes cause a significant reduction in threshold voltage. However, the absorption of these dyes is quite strong in the visible spectral region. For full-color displays, the absorption in the visible region reduces the display brightness and should be avoided. In this paper, we present the structures and physical properties of several colorless high AE compounds. We start with the polar polyene, ester, fluorine and then dioxan compounds. POLYENE COMPOUNDS We have synthesized and evaluated following colorless cyano polyene compounds:
CN (I)
H3C
HC)
CHCN 3
cH3
CH3 .. ••c
(I
Hr3 NN
lI 235
Mat. Res. Soc. Symp. Proc. Vol. 559 0 1999 Materials Research Society
In structure (I), we have synthesized two homologues with n=l and 6. Their melting point (Trp, in 'C) and heat fusion enthalpy (AH, in kcal/mol) were measured by DSC to be [84.0, 5.93] and [3.4, 5.32], respectively. Both compounds have relatively low melting temperatures and small AH that assure a good solubility with most LC hosts. The n=6 homologue, designated as I-b, is a clear liquid at room temperature. Figure 1 shows the UV absorption spectra of compounds I-b and III. In the absorption measurement~s, we dissolved 1% of each compound in an UV transparent LC mixture ZLI-2359 (from Merck). A homogeneous-aligned quartz cell of 6-jtm gap was used for such measurements. From Fig. 1, the absorption peak of compound I-b and III occurs at 305 and 320nm, respectively, and then falls off quickly beyond 360nm. In the visible spectral region, they are transparent. 1.4 I-b 1.2 1.0 IL. a 0.8 0.6 I0)0.4 _
0.2 0.0
" '
200
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WAVELENGTH, nm
Fig. 1 Absorption spectra of compounds (I-b) and (III). Concentration: 1%; LC host = ZLI-2359. Cell gap = 6 gtm. To evaluate the dielectric anisotropy, we mixed 3% of compound (I-b) by weight in a binary diphenyl-diacetylene mixture, PTTP-24/36. [3] The voltage-dependent capacitance method was used for determining its dielectric constants through a single cell experiment. [10] By comparing the measured dielectric constants of the guest-host system to the host LC, we could extrapolate the Ac value of the guest compound studied. The measured Ac for compound (I-b) is -50. Two factors contribute to the observed large dielectric anisotropy: 1. The compound still exhibits a considerable charge transfer that is confined along the quasi one-dimensional it-conjugated bridge; [8] and 2. The bicyano groups provide a large effective dipole moment along the principal molecular axis. The dielectric anisotropy of compound (I-b) is -77% of tha
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