Unsymmetrical β-functionalized 'push-pull' porphyrins: synthesis and photophysical, electrochemical and nonlinear...

Two new series of β-triphenylamine-appended porphyrins (MTPP(TPA)₂X, (where M = 2H, Co(ii), Ni(ii), Cu(ii), Zn(ii) and X = NO₂/CHO) have been synthesized and characterized by various spectroscopic techniques, namely, UV-vis, fluorescence, NMR spectroscopy, mass spectrometry, cyclic voltammetry, density functional theory and ultrafast nonlinear optical (NLO) studies. They exhibited 16-22 nm and 39-58 nm red-shifts in the Soret and Q_x(0,0) bands, respectively, as compared to MTPPs due to the resonance and inductive effects of β-substituents on the porphyrin π-system. The first reduction potential of CuTPP(TPA)₂NO₂ and CuTPP(TPA)₂CHO exhibited an anodic shift by 0.44 and 0.36 V, respectively, as referenced to CuTPP, due to the electronic nature of β-substituents (NO₂ and CHO), which led to their easier reduction compared with CuTPP. H₂TPP(TPA)₂NO₂ and H₂TPP(TPA)₂CHO exhibited the largest resultant dipole moments (7.66 D and 4.55 D, respectively) as compared to H₂TPP (0.052 D) due to the cross-polarized push-pull effect of β-substituents (NO₂/CHO and triphenylamino groups) and the nonplanarity of the macrocyclic core. Third-order nonlinear optical properties of MTPP(TPA)₂NO₂ and MTPP(TPA)₂CHO (M = 2H and Zn(ii)) were investigated in a broad spectral range (680-850 nm) using the Z-scan technique with femtosecond 80 MHz pulses. These materials demonstrate strong nonlinear optical coefficients, endowing them with potential for prominent photonic applications.