An Adjacent Dibenzotetraazaporphyrin: A Structural Intermediate between Tetraazaporphyrin and Phthalocyanine

An adjacent dibenzotetraazaporphyrin, in which two benzene units are fused to the adjacent pyrrole rings of tetraazaporphyrin skeleton, has been synthesized as a copper complex for the first time and characterized by electronic absorption and magnetic circular dichroism spectroscopy, by cyclic voltammetry and spectroelectrochemistry using an optically transparent thin layer electrochemical cell. The results were compared with those of tetraazaporphyrin and phthalocyanine analogues. The title compound shows intermediate characteristics between those of normal tetraazaporphyrins and phthalocyanines; the Q-band shifts to the red and becomes more intense while the Soret band broadens with increasing the size of the pi-system. With the expansion of the pi-system, the first reduction potential does not change significantly, while the first oxidation potential shifts negatively, indicating that the LUMO level remains almost constant while the HOMO level destabilizes significantly. These spectroscopic properties and redox potentials were reproduced by molecular orbital calculations within the framework of the Pariser-Parr-Pople approximation. Thermogravimetry analysis suggests that the skeleton of the adjacent dibenzotetraazaporphyrin complex decomposes at about 95 degrees lower than that of the tetra-tert-butylated phthalocyanine analogue.