Preparation of Liquid Crystalline Subnaphthalocyanines As Optoelectronic Materials

Abstract

Chloroboron(III) hexachloro- hexabromo-, and hexaiodosubnaphthalocyanines were prepared by cyclotrimerization of 6,7-dihalo-2,3-naphthalenedicarbonitrile in the presence of BCl3, They were converted to fluoroboron(III) hexahalosubnaphthalocyanine by the reaction with AgBF4. Their solubility, electronic absorption spectroscopy, fluorescence emission spectroscopy and HOMO/LUMO energy levels were evaluated. Fluorescence quantum yields of chloroboron(III) hexafluoro-, hexachloro-, hexabromo-, and hexaiodosubnaphthalocyanine were 0.22, 0.20, 0.11, 0.05, respectively, indicating that heavy atom effects of Br and I were effectively deactivate the singlet excited states. These subnaphthalocyanines showed lower HOMO/LUMO energies than the parent subnaphthalocyanine by 0.13 – 0.39 eV. The optical band gap decreased in the order: hexafluoro- > hexachloro- > unsubstituted > hexaiodo-subnaphthalocyanine. Hexaiodosubnaphthalocyanines were converted to chloroboron(III) and fluoroboron(III) hexa(1-alkynyl)- and hexa(2-arylethynyl)subnaphthalo-cyanines by Sonogashira coupling of hexaiodosubnaphthalocyanines with substituted acetylenes. Introduction of butyl or longer alkyl groups via ethynylene linkages on the periphery resulted in lower melting points, and higher solubility in dichloromethane. X-ray diffraction (XRD) patterns of the cast film indicated that hexa(2-(4-hexylphenyl)ethynyl)- and hexa(2-(4-hexyloxyphenyl)ethynyl)subnaphthalocyanines with a B-F bond are packed in a discotic hexagonal columnar phase with lattice constants a = 59-64 Å and stacking distance c = 4.7-4.8 Å. The Q-band in visible spectra of the thin film of these B-F derivatives was blue-shifted, supporting the formation of H-aggregate in stacked columns. Polarized optical microscope showed that these subnaphthalocyanines with a B-F bond exhibited mesophase at 180oC. XRD of these subnaphthalocyanines at 180 oC confirms a two dimensional hexagonal phase. XRD of the corresponding derivatives with a B-Cl bond showed that they were either amorphous or less crystalline. Figure 1