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

摘要

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-EtO2CC6H4-NiDAP showed that the beta-aryl group was tilted toward the NiDAP ring, with a dihedral angle of 21.7 degrees. 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-Ph2NC6H4-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 Ph2N-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 beta-carbon is a promising strategy for enhancing the light-harvesting and light-emitting abilities of diazaporphyrin-based pi-systems in the visible-near-infrared regions.