Fluorescent Hexacatenar Liquid Crystals
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a unique columnar hexagonal phase (0h). It is conjectured that the columns in the (Dh phase are composed of disk-like units made up of two to four mesogens which may or may not have average internal packing order within the disks (Fig. 1) [2-4]. Although a number of polycatenar LCs have been studied in order to determine fundamental structure-property relationships[2-51, the incorporation of functional properties into these systems is unprecedented. Herein, we present the first example of a functional phasmidic LC with intrinsic luminescence properties (1).[6] The synthesis and physical properties of homologs la-i are described, and the effect of the solid-state structure on the optical properties of these materials is discussed. EXPERIMENTAL Instrumentation. 1H NMR spectra were obtained on a Bruker AMX-300 (300 MHz), AMX400 (400 MHz), or DRX-500 (500 MHz) FT spectrometer. Elemental analyses were performed by M-H-W Laboratories, Phoenix, AZ. Optical textures under crossed polarizers were obtained on a Leica DMRXP POL microscope equipped with a Linkam THMSE 600 hot stage controlled by a Linkam TP 92 temperature controller (accuracy ± 0.1 °C). DSC traces were obtained on a Perkin-Elmer DSC-7. Low-angle X-ray diffraction profiles were obtained on an Inel CPS 120 powder diffraction system using Ni-filtered Cu Ka (self absorption radiation. This system was equipped with an Inel programmable capillary oven (accuracy ± 3 'C) for variable-temperature studies. The absorption spectra are measured with a Perkin-Elmer Lambda-9 spectrometer. The 305 Mat. Res. Soc. Symp. Proc. Vol. 558 © 2000 Materials Research Society
absorption spectra are corrected for the optical properties of the solvent material. For emission and excitation spectroscopy, a Shimadzu RF-5301 PC spectrofluorophotometer was used. The luminescence lifetimes have been measured using an I.S.S. Greg Multifrequency crosscorrelation phase and modulation fluorometer. In this method, the excitation beam is modulated by Pockels-cell in a frequency range between 2 MHz and 200 MHz. Based on the modulation depth of the emitted radiation compared to the modulation depth of the exciting beam the total lifetime of excited species was calculated [7-9]. The integral photoluminescence was detected with the wavelength range only limited by an edge filter. Sample preparation. Compounds la-i (Fig. 1) were synthesized from l,4-bis([2-(3,5dimethoxy-4-hydroxyphenyl)ethenyl]benzene and 3,4,5-tris(n-alkoxy)benzoic acid derivatives of different length tails [10]. To compare the photophysical properties of the isolated hexacatenar LC's with those in the bulk material, compounds lb, If, and lh were studied in dilute hexane solution at a concentration of 0.01 mg/mL. Such a low concentration was chosen to reduce the absorption of the emitted radiation inside the solution before reaching the detector). For producing bulk films, three different film processing procedures were employed: "* Method A: The compound was put on a glass slide, heated to 125 'C in air (isotropic phase with high mol
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