Benzo-fused low symmetry metal-free tetraazaporphyrin and phthalocyanine analogs: synthesis, spectroscopy, electrochemistry, and density functional theory calculations

Abstract

A whole series of low-symmetry, metal-free tetrapyrrole analogs 1–11 ranging from tetraazaporphyrin H2TAP(tBu)4 to napthalocyanine H2Nc(tBu)4via H2Pc(tBu)4 have been designed and prepared. Their electronic structures have been spectroscopically and electrochemically investigated. Through the preparation of a series of tetrapyrrole compounds with just the same number of four tert-butyl substituents at similar positions of the macrocyclic ring, the effect intrinsic to the enlargement of the ring system has been easily extracted and understood. Along with the increase in the size of central conjugated system, the half-wave potentials of the first oxidation for 1–11 are shifted significantly to the negative direction. This is also true for the half-wave potentials of the first reduction process of this series of tetrapyrrole derivatives but in a slight manner. These results indicate that along with the ring extension, the energy of the HOMO increases significantly while that of the LUMO only slightly increases. The effect of the ring size of these tetrapyrrole compounds on the electronic absorption and fluorescence spectra is also clear: both the Q band and fluorescence shifts to longer wavelength with the increase in the number of fused benzene rings but with a diminishing increase due to the extending of the central conjugated system. These results were reasonably explained by considering the energy levels of frontier molecular orbitals of the series of compounds obtained by the calculations using DFT method at the B3LYP/6-31G(d) level.