Halide effect in electron rich and deficient discotic phthalocyanines

Abstract

A series of discotic octa- and tetra-alkylthio substituted phthalocyanines containing four Cl, Br, I atoms or twelve fluorine atoms have been prepared. All compounds display columnar mesomorphism as confirmed by polarized optical microscopy, thermal analysis, and variable temperature X-ray diffraction. Phthalocyanines containing halide atoms do not crystallize but form glassy or partially crystalline hexagonal columnar phases. Glass transition temperatures increase with increasing size of the halide atoms and with decreasing length of the alkyl chains. In contrast, all octa- and tetra-alkylthio substituted phthalocyanines crystallize and octa-substituted derivatives with aliphatic chain lengths of C_5–7 exhibit tilted (rectangular) columnar mesophases. Cyclic and differential pulse voltammetry, UV-Vis spectroscopy, and quantum chemical calculations at the DFT level have been employed to determine frontier orbital energies of all synthesized and some reference phthalocyanines. Octa- and tetra-alkylthio substituted phthalocyanines are typical p-type semiconductors and the introduction of four Cl, Br, or I atoms lowers frontier orbital energies by only up to 0.1 eV and the optical gap by up to 0.03 eV. A significant decrease in LUMO energy by 0.5 eV to about −4 eV is observed for the fluorinated phthalocyanine, which is a value right at the border of organic n-type semiconductors that may be stable in air.