Comparison of electronic effects of β-aryl substituents on optical and electrochemical properties of 5,15-diazaporphyrin π-systems

Abstract

The syntheses and optical/electrochemical properties of 3-aryl-10,20-dimesityl-5,15-diazaporphyrin-metal complexes (MDAPs; mesityl = 2,4,6-trimethylphenyl; M = Ni , Zn ) are reported. Treatment of 3-bromo-MDAPs with arylboronic acids in the presence of a Pd catalyst and a bulky phosphine ligand in a dioxane-water mixed solvent afforded the corresponding 3-aryl-MDAPs in moderate to good yields. X-ray crystallographic analysis of p- EtO 2 CC 6 H 4- NiDAP showed that the β-aryl group was tilted toward the NiDAP ring, with a dihedral angle of 21.7°. In the UV-visible absorption spectra, all the Ar-MDAPs showed intense Q-bands attributable to HOMO-to-LUMO transitions. The para substituents were found to influence the HOMO energies, which eventually resulted in fine tuning of the HOMO–LUMO gaps of the diazaporphyrin chromophores. It is worth noting that the p- Ph 2 NC 6 H 4- ZnDAP showed broad absorption and emission bands in the visible-near-infrared regions. The large Stokes shifts and their linear solvation energy relationships vs. orientation polarizability show that this Ph 2 N -substituted derivative has intrinsically high charge transfer from the triphenylamine (donor) to the ZnDAP (acceptor) unit. These experimental observations were supported by theoretical calculations for model Ar-ZnDAP compounds. These results confirm that the introduction of a highly electron-donating aryl group at the peripheral β-carbon is a promising strategy for enhancing the light-harvesting and light-emitting abilities of diazaporphyrin-based π-systems in the visible-near-infrared regions.