Synthesis and optical, magnetic, and electrochemical properties of 5,10,15,20-tetraaryl-5,15-diazaporphyrin - Tertiary amine conjugates

We report on the synthesis and optical, magnetic, and electrochemical properties of nickel(II) and copper(II) complexes of 5,10,15,20-tetraaryl-5,15-diazaporphyrin-tertiary amine (TADAP-TA) conjugates. Metal-templated cyclization reactions of 9-(4-(dimethylamino)phenyl)amino-1-chloro-5-mesityldipyrrin and 9-(4-(diphenylamino)phenyl)amino-1-chloro-5-mesityldipyrrin (mesityl = 2,4,6-trimethylphenyl) with nickel(II) or copper(II) acetate afforded the corresponding metal(II) complexes of TADAP-TA. The 20π, 19π, and 18π oxidation states of the DAP ring in the TADAP - TAs were reversibly interconvertible by redox reactions. NMR spectroscopy of the 20π and 18π Ni-TADAP - TAs revealed their antiaromatic and aromatic characters, respectively, whereas electron paramagnetic resonance spectroscopy of the 19π Ni-TADAP-TAs showed effective delocalization of an unshared electron spin in the DAP ring. The interconversion between the three oxidation states of TADAP - TAs also caused a distinct change in the optical properties of the DAP π-electron system. Notably, all the 18π dications exhibited weak and broad absorption bands in the near infrared region owing to the charge-transfer from the peripheral tertiary amine units (donor) to the cationic DAP center (acceptor). Cyclic voltammetry of TADAP - TAs exhibited the reversible 20π/19π and 19π/18π redox couples and the irreversible amine oxidation at the periphery. The electrochemical oxidation of the Ni-TADAP-triphenylamine conjugate generated reactive ammoniumyl radicals, which underwent intermolecular coupling to form a polymer of TADAP-TA on the electrode surface.