Mono- and tri-β-substituted unsymmetrical metalloporphyrins: synthesis, structural, spectral and electrochemical properties

Abstract

Mono-/tri-β-substituted metalloporphyrins, viz. MTPP(X)Y₂ (X = CHO, CH₂OH, COOH; Y = H, Br, Ph; M = 2H, Co(II), Ni(II), Cu(II), Zn(II)) have been synthesized and characterized. This work examines the influence of β-substitution on the structural, spectral and electrochemical redox properties of MTPP(X) and MTPP(CHO)Y₂. The redox tunability was achieved by introducing electron donors (CH₂OH and Ph) and acceptors (CHO, COOH and Br) on the MTPP skeleton. Dramatic reduction in the HOMO–LUMO gap with considerable increment in Δa_1u was observed as the number of electron withdrawing groups increased. The spectral and electrochemical redox potentials are influenced by the peripheral β-substituents and electronegativity of the core metal ion. These porphyrins exhibited tunable electronic spectral and redox properties with modulated frontier orbitals by means of mono- and tri-β-substituents which are in direct conjugation with the porphyrin π-system. DFT studies of these porphyrins revealed that mono-substituted porphyrins are nearly planar whereas tri-substituted porphyrins have a moderate nonplanar conformation.