Synthesis, Spectral, and Electrochemical Studies of Electron-Deficient Nitrile Porphyrins and the Utilization in Selective Cyanide Sensing

Abstract

Two series of β-cyano-substituted porphyrins, MTPP(CN)X (where M = 2H, Co(II), Ni(II), Cu(II), Zn(II) and X = 1 or 2, were synthesized and thoroughly characterized using a range of spectroscopic techniques, including UV-visible, fluorescence, NMR, mass spectrometry, cyclic voltammetry and density functional theory. One of the investigated compounds, CuTPP(CN)2 (2-Cu) was structurally characterized with single crystal X-ray diffraction and shown to possess saddle-shape macrocyclic conformation. Compared to MTPPs, these compounds showed red-shifts of 7–24 nm and 13–46 nm in the Soret and Qx(0,0) bands, respectively, due to resonance and inductive effects from the β-substituents on the porphyrin π-system. The first reduction potentials of H2TPP(CN) (1-H2) and H2TPP(CN)₂ (2-H2) showed anodic shifts of 0.25 V and 0.53 V, respectively, compared to H2TPP. This shift is due to the electron-withdrawing nature of the β-substituent, which make these compounds more readily reduced than H2TPP. Additionally, (1-H2) and (2-H2) exhibited significantly higher dipole moments (5.41 D and 9.34 D, respectively) compared to H₂TPP (0.052 D). This increase is attributed to the high-polarized pull effect of the cyano group. The nickel(II) dicyanoporphyrin (2-Ni) exhibited a selective and reversible visual detection of cyanide ions with a detection limit of 4.97 ppm.