β-Methoxyphenyl substituted porphyrins: synthesis, characterization and comprehensive spectral, structural, electrochemical and theoretical analysis

Abstract

A new series of β-functionalized meso-tetraphenylporphyrins bearing 4-methoxyphenyl, 3,5-dimethoxyphenyl and 3,4,5-trimethoxyphenyl groups appended selectively to the single pyrrole unit of the porphyrin macrocycle, H₂TPPR₂ (where R = p-CH₃O-Ph, m-CH₃O-Ph and m,p-CH₃O-Ph), and their Co(II), Ni(II), Cu(II) and Zn(II) metal complexes were synthesized, characterized and meticulously examined for their adjustable electronic spectral, electrochemical and structural attributes. A gradual bathochromic shift of absorption bands (Δλ_max = 5–8 nm) was observed in these porphyrins relative to the unsubstituted parent porphyrin, H₂TPP. A progressive cathodic shift in the first ring oxidation potential was observed in the series. Among all free base porphyrins, H₂TPP(p-CH₃O-Ph)₂ showed the maximum red shifted absorption and the largest cathodic shift in the first ring oxidation, unveiling the effective electron donation via the +R effect of methoxy groups placed at the para position of β-phenyl rings. Within this framework, fine-tuning of the HOMO–LUMO gap accompanied by a gradual reduction in energy was observed which followed the trend H₂TPP(m,p-CH₃O-Ph)₂ (2.22 V) > H₂TPP(m-CH₃O-Ph)₂ (2.13 V) > H₂TPP(p-CH₃O-Ph)₂ (2.08 V). Single crystal X-ray analyses of H₂TPP(p-CH₃O-Ph)₂, ZnTPP(m-CH₃O-Ph)₂ and CuTPP(m-CH₃O-Ph)₂ unfolded planar, quasi-planar and saddle conformations respectively. Hirshfeld surface and 2D fingerprint plot analysis were also performed to see the significant intermolecular interactions. Further, DFT and TDDFT calculations were performed to gain a deeper understanding of the observed experimental results.