Anion-controlled dielectric behavior of homochiral tryptophan-based metal-organic frameworks

Three homochiral metal-tryptophanate frameworks, {[Zn₂(L-trp) ₂(bpe)₂(H₂O)₂]·2H ₂O·2NO₃}_n (1a, L-Htrp = L-tryptophan, bpe = 1,2-bis(4-pyridyl)ethylene), {[Co(L-trp)(bpe)(H₂O)] ·H₂O·NO₃}_n (1b), and {[Co(L-trp)(bpa)(H₂O)]·H₂O·NO ₃}_n (2, bpa = 1,2-bis(4-pyridyl)ethane), were constructed from Zn²⁺ or Co²⁺ ions, bipyridyl ligands, and the amino acid L-tryptophan (L-Htrp), respectively. Compounds 1a, 1b, and 2 were characterized by single-crystal X-ray diffraction analysis. All of the compounds crystallize in the monoclinic space group P2₁ and form homochiral two-dimensional (2D) layers with rectangle-like (4,4) topologies. Anion-controlled dielectric, luminescence, and nonlinear-optic (NLO) properties were measured for these chiral metal-organic frameworks (MOFs) in the solid state. Emission spectra confirmed that compound 1a exhibited a green emission at 546 nm. Dielectric studies of 1a revealed that it had very low dielectric constant (κ = 2.53 at 1 MHz), thus verifying that it is a promising candidate for interlayer dielectrics. The anion-controlled dielectric properties of 1a were observed after treatment with solutions of different anions. The results revealed a significant change in κ value in the case of the phosphate anion. Secondary harmonic generation (SHG) studies revealed that 1a had a good SHG intensity response that was about twice that of SiO₂.