Meso-β, β–β′ trifused porphyrins: synthesis, spectral, electrochemical and DFT studies and their femtosecond third-order nonlinear optical properties†

Meso-β, β–β′ trifused porphyrins incorporating two distinct active methylene groups (MN = malononitrile and IND = 1,3-indanedione) and their corresponding metal complexes with Cu(II) and Zn(II) have been synthesized with good to excellent yields and characterized by various spectroscopic techniques and spectrometric methods. Single crystal X-ray analysis of the Zn(II) complex ZnTFPMB(MN)₂ (where TFP = trifused porphyrin and MB = mono benzo) revealed a ruffled nonplanar ‘armchair’ type conformation with a twist angle of 24.10°. The absorption spectra showed a significant bathochromic shift in both the B- and Q-bands, extending into the near-infrared (NIR) region, particularly for π-extended trifused porphyrins. The cyclic voltammograms of MN-appended trifused porphyrins revealed unusual redox behavior, likely due to chemical reactions occurring at the electrode surface during electroreduction. The HOMO–LUMO energy gap for the π-extended porphyrins (MTFPMB(VCN)₂) was effectively reduced to ≤1.5 V, compared to ∼2.23 V for the parent porphyrins. Additionally, the femtosecond third-order nonlinear optical properties of the synthesized trifused porphyrins and reported Ni(II) complexes were investigated using the Z-scan technique. Most of the studied porphyrins exhibited promising three-photon absorption coefficients and cross-section values, suggesting their potential applications in optical limiting, bio-imaging, and advanced optoelectronics.