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 ΔCβ 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.