Ethyl acetoacetate and acetylacetone appended hexabromo porphyrins: synthesis, spectral, electrochemical, and femtosecond third-order nonlinear optical studies

Abstract

Asymmetrically substituted porphyrins possessing ethyl acetoacetate or acetylacetone (EAA or acac) with six bromine atoms at β-positions were synthesized and then characterized by various spectroscopic techniques, such as UV-Vis, fluorescence and NMR, and also by CV, DFT, MALDI-TOF-MS and elemental analysis. The mechanistic pathway followed the nucleophilic substitution reaction (nucleophile: EAA and acac) with MTPP(NO₂)Br₆ (M = 2H, Cu(II), and Ni(II)), and the resultant β-heptasubstituted porphyrins exhibited keto–enol tautomerism, as supported by ¹H NMR spectroscopy. The six bulky bromo and EAA/acac groups made the macrocyclic ring highly electron deficient and nonplanar such that the quantum yield and fluorescence intensity for H₂TPP[EAA]Br₆ and H₂TPP[acac]Br₆ were severely reduced in contrast to those values for H₂TPP. The poor electron density and nonplanarity over the porphyrin ring shifted the first oxidation potential from 11 to 521 mV anodically for MTPP[X]Br₆ [M = 2H, Cu(II), and Ni(II); X = EAA or acac] as compared to corresponding MTPPs. Notably, density functional theory proved the nonplanarity of the synthesized porphyrins as Δ24 spans from ±0.546 to ± 0.559 Å while ΔC_β stretches from ±0.973 to ±1.162 Å. The third-order nonlinear optical measurements were performed using the femtosecond pulsed laser Z-scan technique at 800 nm and 1 kHz repetition rate to acquire insights into nonlinear absorption and nonlinear refraction of the porphyrins. The three-photon absorption coefficients (γ) are in the range of 2.2 × 10⁻²³–2.8 × 10⁻²³ cm³ W⁻² and the nonlinear refractive index values were in the range of 3.7 × 10⁻¹⁶–5.1 × 10⁻¹⁶ cm² W⁻¹. The higher-order nonlinear absorption exhibited by porphyrins helps improve resolution at depth for various photonic and optoelectronic applications.