π-Extended Nonplanar NIR Absorbing Porphyrins: Synthesis, Spectral, Electrochemical, and DFT Studies and Their Excellent Nonlinear Optical Properties

Two new series of nonplanar π-extended difused porphyrins were synthesized; one bearing two antipodal phenylethynyl (PE) units, M(VCN)2(PE)2, (VCN = vinyl cyanide), and the other with one PE and one tetracyanobutadiene (TCBD) unit, MTCBD, where M = 2H, Zn(II), Cu(II), Ni(II), and Co(II). All compounds were fully characterized by various spectroscopic techniques and single-crystal XRD analysis. Ni(VCN)2(PE)2 and ZnTCBD adopted ruffled geometries with high twist angles of 28.46° and 20.25°, respectively. The synthesized porphyrins display a panchromatic absorption spectral feature with low-energy bands extending into the near-infrared (NIR) region, attributed to the extended π-conjugation. H2(VCN)2(PE)2 and Zn(VCN)2(PE)2 exhibit pronounced emission red shifts compared to parent TPPs (tetraphenylporphyrins), whereas H2TCBD and ZnTCBD are nonemissive, indicating strong fluorescence quenching and excited-state processes. Cyclic voltammetry reveals facile four-electron reductions and impressively narrowed HOMO–LUMO gaps compared to TPPs. Frontier molecular orbital analysis suggests a charge-transfer or charge-separated state in MTCBD, and all optimized structures display high ground state dipole moments (10.79–15.18 D). Third-order nonlinear optical (NLO) properties evaluated by Z-scan reveal exceptionally high two-photon absorption coefficients and cross sections, even at low laser power, outperforming recently reported macrocycles. These findings underscore their immense potential in optical limiters, advanced optoelectronic devices, and cutting-edge biomedical technologies.