Unsymmetrically β-Functionalized π-Extended Porphyrins: Synthesis, Spectral, Electrochemical Redox Properties, and Their Utilization as Efficient Two-Photon Absorbers.

Two new series of unsymmetrically β -functionalized porphyrins, MTPP(NO 2 )MA ( 1M ), (MA = methyl acrylate) and MTPP(NO 2 )MB ( 2M ) (MB = mono-benzo) (where M = 2H, Co(II), Ni(II), Cu(II) and Zn(II)), were synthesized and characterized by various spectroscopic techniques. The saddle shape conformation of ZnTPP(NO 2 )MAPy and ZnTPP(NO 2 )MB was confirmed by single-crystal X-ray analysis. Density functional theory (DFT) calculation revealed that NiTPP(NO 2 )MB has a severe nonplanar geometry possessing a high magnitude of ΔC β = ±0.727 Å and Δ24 = ±0.422 Å values among all other porphyrins. Synthesized β -substituted porphyrins exhibited red-shifted B- and Q-bands corresponding to their parent molecule due to the electron-withdrawing peripheral substituents. Notable redshift (Δλ max = 50-60 nm) in electronic spectral features and with weak-intensity emission spectral features were observed for the free-base porphyrins and Zn(II) complexes compared to H 2 TPP and ZnTPP, respectively. The first-ring reduction potential of MTPP(NO 2 )MA ( 1M ) exhibited 0.21-0.5 V anodic shift, whereas 0.18-0.23 V anodic shift was observed in the first-ring oxidation potential compared to the corresponding MTPPs due to the presence of electron-withdrawing β -substituents at the periphery of the macrocycle. Interestingly, NiTPP(NO 2 )MA ( 1Ni ) has shown an additional Ni II /Ni III oxidation potential observed at 2.05 V along with two ring-centered oxidations. The first-ring reduction and oxidation potentials of MTPP(NO 2 )MB ( 2M ) have shown 0.39-0.46 and 0.19-0.27 V anodic shifts with respect to their corresponding MTPPs. The nonlinear optical (NLO) properties of all of the porphyrins were investigated, and the extracted nonlinear optical parameters revealed intense reverse-saturable absorption (RSA) behavior and the self-focusing behavior with positive nonlinear refractive index in the range of (0.19-1.75) × 10 -17 m 2 /W. Zn(II) complexes exhibited the highest two-photon absorption coefficient ( β ) and cross section (σ TPA ) of ∼95 × 10 -12 m/W and 19.66 × 10 4 GM, respectively, among all of the metal complexes.