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

Two new series of β-triphenylamine-appended porphyrins (MTPP(TPA)X, (where M = 2H, Co(ii), Ni(ii), Cu(ii), Zn(ii) and X = NO/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(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)NO and CuTPP(TPA)CHO exhibited an anodic shift by 0.44 and 0.36 V, respectively, as referenced to CuTPP, due to the electronic nature of β-substituents (NO and CHO), which led to their easier reduction compared with CuTPP. HTPP(TPA)NO and HTPP(TPA)CHO exhibited the largest resultant dipole moments (7.66 D and 4.55 D, respectively) as compared to HTPP (0.052 D) due to the cross-polarized push-pull effect of β-substituents (NO/CHO and triphenylamino groups) and the nonplanarity of the macrocyclic core. Third-order nonlinear optical properties of MTPP(TPA)NO and MTPP(TPA)CHO (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.