Anion-Controlled Dielectric Behavior of Homochiral Tryptophan-Based Metal–Organic Frameworks

Three
homochiral metal-tryptophanate frameworks, {[Zn2(l-trp)2­(bpe)2(H2O)2]­·2H2O·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·NO3}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.