β-Diformyl porphyrins: Synthesis, structural, spectral and electrochemical properties

Antipodal β-diformyl porphyrin (H2TPP(CHO)2) and its metal complexes (MTPP(CHO)2, M = [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] are synthesized and characterized by various spectroscopic techniques. Diformyl porphyrins possess two electron-withdrawing formyl substituents at their β-pyrrole positions making them electron-deficient macrocycles. UV-Vis absorption spectra of MTPP(CHO)2 are red-shifted as compared to β-monoformyl/hydroxymethyl porphyrins and a further large anodic shift ([Formula: see text][Formula: see text] = 0.15 - 0.52 V) was observed in their redox potentials. Hammett plots of MTPP(X)(Y) (X = CHO, CH2OH; Y = H, CHO) showed that the HOMO-LUMO energy gap decreases with the increment in the Hammett parameter ([Formula: see text] of the substituents. DFT studies are performed for different positional isomeric diformyl porphyrins which reveal that the position of β-formyl groups affects the geometry of the porphyrin core while the symmetrical nature of 2,13-diformyl porphyrin and longer distance between two formyl groups bring quasiplanar conformation of the diformyl porphyrin macrocycle.