Porphyrin amino acids–amide coupling, redox and photophysical properties of bis(porphyrin) amides

Abstract

New trans-AB₂C meso-substituted porphyrin amino acid esters with meso-substituents of tunable electron withdrawing power (B = mesityl, 4-C₆H₄F, 4-C₆H₄CF₃, C₆F₅) were prepared as free amines 3a–3d, as N-acetylated derivatives Ac-3a–Ac-3d and corresponding zinc(II) complexes Zn-Ac-3a–Zn-Ac-3d. Several amide-linked bis(porphyrins) with a tunable electron density at each porphyrin site were obtained from the amino porphyrin precursors by condensation reactions (4a–4d) and mono- and bis(zinc(II)) complexes Zn²22-4d and Zn¹1Zn²2-4d were prepared. The electronic interaction between individual porphyrin units in bis(porphyrins) 4 is probed by electrochemical experiments (CV, EPR), electronic absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy in combination with DFT/PCM calculations on diamagnetic neutral bis(porphyrins) 4 and on respective charged mixed-valent radicals 4^+/−+/−. The interaction via the –C₆H₄–NHCO–C₆H₄– bridge, the site of oxidation and reduction and the lowest excited singlet state S₁, is tuned by the substituents on the individual porphyrins and the metalation state.