Structural, photophysical, electrochemical and spintronic study of first-row metal Tetrakis(meso-triphenylamine)-porphyrin complexes: A combined experimental and theoretical study

Abstract

Different metal ions in the center of 3d-metalloporphyrins have subtle influences on their structural, photophysical, electrochemical, and spintronic properties. A new series of symmetrically meso-substituted porphyrins with six first row transition metals, namely TPA-MPs (M = 2H, Mn, Fe, Co, Ni, Cu, and Zn) have been synthesized and characterized by various spectroscopic techniques. The electrostatic potential energy surface (ESP) calculated by density functional theory (DFT) exhibits the concentrated positive electrostatic potential on the substituted triphenylamine group, while the negative electrostatic potential localized on the porphyrin ring varies depending on metal ions. All compounds were crystallographically characterized and showed a coplanar porphyrin macrocyclic ring except for TPA-MnP and TPA-ZnP, which are distorted due to their occupied axial sites by Cl− and H2O, respectively. Furthermore, the X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) confirmed the valence states of the metal ions in the TPA-MPs. Electronic spectra revealed a gradual bathochromic shift following the order of TPA-CoP < TPA-CuP < TPA-P = TPA-NiP < TPA-FeP < TPA-ZnP < TPA-MnP by appending different metal ions in the porphyrin skeleton. TPA-MnP exhibited the most significant bathochromic shift (Δλmax = 49 nm) in the Soret band compared to TPA-P. The electrochemical redox potentials of the TPA-MPs decrease with the stepwise filling of d orbitals of the central metal ions from 0.86 to 0.81 V and the completely filled 3d orbital of Zn endows TPA-ZnP with the lowest oxidation onset. Meanwhile, TPA-MnP and TPA-FeP exhibited dramatically reduced HOMO-LUMO gaps of 1.78 eV and 1.58 eV, respectively, compared to other TPA-MPs. The excellent thermal stability, panchromatic absorption in the whole visible range, and suitable energy levels of TPA-MPs indicate huge potential in the application as dyes.