Unsymmetrical β-Functionalized ‘Push-Pull’ Porphyrins: Synthesis, Photophysical, Electrochemical and Nonlinear Optical Properties
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 such as 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 redshift in the Soret and Qx(0,0) bands, respectively, as compared to MTPPs due to resonance and inductive effect of β-substituents on the porphyrin π-system. CuTPP(TPA)2NO2 and CuTPP(TPA)2CHO exhibited the first reduction potential was 0.44 and 0.36 V anodically shifted as referenced to CuTPP, respectively, due to the electronic nature of β-substituents (NO2 and CHO) which made them easy to reduce than CuTPP. MTPP(TPA)2NO2 and MTPP(TPA)2CHO (M = 2H and Zn(II)) exhibited largest resultant dipole moment (7.66 D and 4.55 D, respectively) as compared to H2TPP (0.052 D) due to 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 H2TPP(TPA)2NO2 and H2TPP(TPA)2CHO were invetigated in a broad spectral range (680-850 nm) using the Z-scan technique with femtosecond, 80 MHz pulses. In open aperture Z-scan experiments, we have obtained two-photon absorption coefficients () along with the third-order NLO susceptibility [(3)] by fitting the experimental data fits whereas closed aperture Z-scan data revealed the magnitude as well as the sign of the nonlinear optical refractive index (n2). These materials demonstrated strong nonlinear optical coefficients finding prominent photonic applications.