Electron deficient β-trisubstituted porphyrins: synthesis, structural, spectral, and electrochemical studies and their intensity-dependent third-order nonlinear optical properties

Abstract

A series of electron-deficient β-trisubstituted 12-nitro-5,10,15,20-tetraphenyl-2,3-bis(trifluoromethyl)porphyrins, abbreviated as H₂TPP(NO₂)(CF₃)₂, and their metal complexes MTPP(NO₂)(CF₃)₂ (where M = Co^II, Ni^II, Cu^II, and Zn^II) have been prepared and characterised by various spectroscopic techniques. Single crystal X-ray analysis revealed the saddle-shaped configuration of CoTPP(NO₂)(CF₃)₂, NiTPP(NO₂)(CF₃)₂ and CuTPP(NO₂)(CF₃)₂ with the average deviation of the 24 core atoms from the mean porphyrin plane (Δ24) ranging from ±0.492 to ±0.499 Å and the average displacement of β-pyrrole carbons from the porphyrin mean plane (ΔC_β) ranging from ±0.926 to ±1.005 Å. H₂TPP(NO₂)(CF₃)₂ exhibited a 28 nm red shift in the B-band and a 102 nm red shift in the longest Q-band (Q_x(0,0)) as compared to H₂TPP. In ¹H NMR, the inner imino protons of H₂TPP(NO₂)(CF₃)₂ were observed at −1.72 ppm, which is significantly downfield shifted compared to H₂TPP. The first ring reduction potential of MTPP(NO₂)(CF₃)₂ (M = 2H, Co^II, Ni^II, Cu^II, and Zn^II) is positively shifted by 280–620 mV compared to their corresponding MTPPs. Notably, it is observed that the synthesized MTPP(NO₂)(CF₃)₂ porphyrins are readily reduced compared to their MTPPs. Intensity-dependent third-order nonlinear optical property studies demonstrated that the synthesized asymmetric β-substituted electron-deficient porphyrins exhibit significant two-photon absorption coefficients (β = 0.04–8.10 × 10⁻¹⁰ m W⁻¹) and two-photon absorption cross-section values (σ_2PA = 0.02–1.67 × 10⁶ GM). Additionally, the materials exhibit self-defocusing negative nonlinear refraction (n₂ = (−) 1.08–40.27 × 10⁻¹⁷ m² W⁻¹). The extracted NLO data suggest the potential of the investigated porphyrins for future optoelectronic and advanced material applications.