Single Pyrrole Unit Functionalized Electron Deficient Porphyrins and Their Metal Complexes: Synthesis, Structural, Spectral and Electrochemical Properties

Abstract

AbstractA family of electron‐deficient regioselective β‐disubstituted meso‐tetraphenylporphyrins, H2TPP(R)2 (where R=trifluoromethyl (CF3) and cyano (CN) and its metal complexes (Co(II), Ni(II), Cu(II) and Zn(II)) were synthesized from versatile precursor H2TPPBr2 by palladium‐catalyzed trifluoromethylation and nucleophilic aromatic cyanation respectively. The absorption spectrum of H2TPP(CF3)2 and H2TPP(CN)2 exhibited a notable shift towards longer wavelengths both in the Soret (Δλmax=16–17 nm) and longest wavelength band, Qx(0,0) (Δλmax=34–52 nm) as compared to precursor porphyrin, H2TPPBr2. The first ring reduction potential of H2TPP(CF3)2 and H2TPP(CN)2 are centred at −0.69 and −0.68 respectively which show drastic anodic shift as compared to precursor H2TPPBr2 (−1.01 V) and parent porphyrin H2TPP (−1.25 V). The anodic shift trend for first ring reduction potentials follows as H2TPP(CN)2>H2TPP(CF3)2>H2TPP(Br)2>H2TPP. Introducing two highly electron withdrawing substituents (CN and CF3) at the single pyrrole unit of the porphyrin macrocycle via regioselective substitution has led to a significant reduction in the HOMO‐LUMO energy gap (0.15 V–0.39 V) in comparison to the precursor H2TPPBr2 (2.08 V). The analysis of the crystal structures for ZnTPP(CN)2 and H2TPP(CF3)2 revealed nearly planar and distinct saddle conformations respectively which are further supported by DFT calculations.