Synthesis, structural, spectral and electrochemical studies of $\beta$-acetamide appended porphyrins

Mono $\beta$-acetamide appended porphyrins, MTPP(NHCOMe) (M = 2H, Zn${}^{\text{I}\text{I}}$, Cu${}^{\text{I}\text{I}}$, Ni${}^{\text{I}\text{I}}$, Co${}^{\text{I}\text{I}}$, Mn${}^{\text{I}\text{I}\text{I}}$ and V${}^{\text{I}\text{V}}$, TPP = meso-tetraphenylporphyrin) were synthesized and characterized by various spectroscopic techniques. The effect of the $\beta$-substituents and core metal ions on the electronic spectra was evaluated. The free base and Zn${}^{\text{I}\text{I}}$ porphyrins exhibited weak emission and a lower singlet state lifetime than H₂TPP and ZnTPP. The first ring reduction potential of MTPP(NHCOMe) (M = Co${}^{\text{I}\text{I}}$, Ni${}^{\text{I}\text{I}}$, Cu${}^{\text{I}\text{I}}$, Zn${}^{\text{I}\text{I}}$, V${}^{\text{I}\text{V}})$ porphyrins exhibited a 10-70 mV cathodic shift whereas, free base and Mn${}^{\text{I}\text{I}\text{I}}$ derivatives showed 10-100 mV anodic shift than corresponding MTPPs. The slightly saddled-shaped structures of CuTPP(NHCOMe), ZnTPP(NHCOMe), and Mn(Cl)TPP(NHCOMe) were confirmed by the single crystal X-ray diffraction analysis. Density functional theory (DFT) studies revealed that the NiTPP(NHCOMe) has the highest average deviation of the 24-core atom from the mean porphyrin plane, $\mathrm{\Delta }$24 (± 0.278 Å) and the average displacement of $\beta$-pyrrole carbons from the porphyrin mean plane, $\mathrm{\Delta }$C${}_{\beta }$ (± 0.278 Å) values among all the synthesized porphyrins. The HOMO-LUMO gap observed from DFT and Cyclic voltammetry studies are qualitatively comparable. The EPR spectroscopy confirmed the +4 oxidation state of the vanadyl ion, exhibited by the comparable eight-line pattern.