Synthesis and characterization of unsymmetrical β-functionalized push–pull porphyrins: photophysical, electrochemical, and nonlinear optical (NLO) studies

Abstract

A series of β-trisubstituted dimethylaminophenyl-appended push–pull porphyrins, MTPPNO₂R₂ (M = Co(II), Cu(II), 2H, Ni(II) and Zn(II); R = DMAP), were synthesised via a Suzuki–Miyaura cross-coupling reaction and comprehensively characterised using spectroscopic and electrochemical techniques. The free-base porphyrin derivatives exhibited significant 18–22 nm red shifts in the Soret band and 39–58 nm red shifts in the Q_x(0,0) bands relative to their parent MTPP analogues. These variations are ascribed to the combined resonance and inductive effects of β-substituents, which perturb the porphyrin π-electron system. Furthermore, the fluorescence quantum yields were markedly reduced upon nitro substitution, consistent with enhanced intramolecular charge transfer from the electron-donating R group to the strongly electron-withdrawing NO₂ moiety through the porphyrin core. DFT studies revealed large dipole moments ranging from 11.48 to 11.95 D. They confirmed nonplanar saddle-shaped conformations for MTPPNO₂R₂ (M = Co(II), Cu(II), 2H, Ni(II) and Zn(II); R = DMAP), driven by cross-polarised push–pull effects. Electrochemical analysis showed an anodic shift in the first reduction potential of MTPPNO₂R₂ relative to MTPP, facilitating easier reduction due to the β-substituent impacts. Modulation of the HOMO–LUMO energy levels by peripheral substitution was also observed. Ultrafast nonlinear optical (NLO) studies conducted using the femtosecond Z-scan technique (800 nm, 120 fs, 84 MHz) revealed strong third-order NLO responses for the synthesized push–pull porphyrins, with β values of ∼10⁻¹² m W⁻¹ and Im[χ⁽³⁾] ≈ 10⁻¹³ esu. Among the metallated derivatives, CoTPPNO₂R₂ exhibited the highest two-photon absorption (TPA) coefficients. These results demonstrate the promising potential of these push–pull porphyrins for photonic and bio-imaging applications.