Synthesis, spectral and electrochemical studies of electron-deficient nitrile porphyrins and their utilization in selective cyanide sensing†

Two series of β-cyano-substituted porphyrins, MTPP(CN)_X (where M = 2H, Co(II), Ni(II), Cu(II), and Zn(II) and X = 1 or 2), were synthesized and thoroughly characterized using UV-visible, fluorescence, and NMR spectroscopic techniques, mass spectrometry, and cyclic voltammetry. One of the investigated compounds, CuTPP(CN)₂ (2-Cu), was structurally characterized using single crystal X-ray diffraction, and its saddle-shape macrocyclic conformation was revealed. Compared to MTPPs, these compounds showed red-shifts of 7–24 nm and 13–46 nm in the Soret and Q_x(0,0) bands, respectively, owing to the resonance and inductive effects of 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 was due to the electron-withdrawing nature of the β-substituent, which made 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) than H₂TPP (0.052 D). This increase was attributed to the high-polarized pull effect of the cyano group. Notably, nickel(II) dicyanoporphyrin (2-Ni) facilitated a selective and reversible visual detection of cyanide ions with a detection limit of 4.97 ppm.