Unsymmetrical zinc azaphthalocyanines, peripherally substituted with thiophen-2-yl and 2-functionalized phenoxy groups

Abstract

Four targeted octasubstituted zinc azaphthalocyanines (ZnAzaPc), substituted with thiophen-2-yl groups and ortho-substituted phenoxy groups, were obtained by cyclotetramerization of 5-aryloxy-6-(thiophen-2-yl)pyrazine-2,3-dicarbonitriles. Thiophen-2-yl substituents are known to extend the macrocyclic conjugation, and thereby cause red-shifted UV–Vis Q-bands. Peripheral phenoxy groups with bulky ortho substituents are expected to suppress aggregation and thereby improve solubility of these compounds. The reagent Zn(quinoline) 2Cl 2 was used for one-step syntheses of these ZnAzaPc. Four tetrasubstituted ZnAzaPc, with phenoxy, (2-isopropyloxy)phenoxy, (2-isopropyl)phenoxy or (2- tert-butyl)phenoxy substituents, were obtained as controls from 5-aryloxypyrazine-2,3-dicarbonitriles. The tetra- and octa-substituted ZnAzaPc, 5 and 6, were obtained in 30–50% yields after purification by chromatography on silica. UV–Vis Q-bands with high molar extinction coefficients (100 000–160 000), were observed at 635 nm for compounds 5, and at 660–665 nm for 6. Grass-green solutions were obtained of compounds 6 in most organic solvents, whereas the less soluble compounds 5 gave blue-green solutions. 2D NMR methods were applied in analyses of DMSO- d 6 solutions of ZnAzaPc 5 and 6. Broad and partly overlapping 1H NMR signals for some of the compounds indicate some aggregation as well as presence of two or more structural isomers. Molecular ions of ZnAzaPc 5 and 6 were determined by mass spectrometry (MALDI-TOF). Structure analyses of 5-phenoxy-6-(thiophen-2-yl)pyrazine-2,3-dicarbonitrile ( 4a), and 5-(2-isopropyloxyphenoxy)-6-(thiophen-2-yl)pyrazine-2,3-dicarbonitrile ( 4b), show the impact on conformation exerted by the bulky isopropyloxy ortho-substituent of the phenoxy group in 4b, and indicates how this substituent hinders the type of disorder found in 4a.