Anion-Controlled Dielectric
Behavior of Homochiral
Tryptophan-Based Metal–Organic Frameworks
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Abstract
Three
homochiral metal-tryptophanate frameworks, {[Zn<sub>2</sub>(l-trp)<sub>2</sub>(bpe)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]·2H<sub>2</sub>O·2NO<sub>3</sub>}<sub><i>n</i></sub> (<b>1a</b>, l-Htrp = l-tryptophan,
bpe = 1,2-bis(4-pyridyl)ethylene),
{[Co(l-trp)(bpe)(H<sub>2</sub>O)]·H<sub>2</sub>O·NO<sub>3</sub>}<sub><i>n</i></sub> (<b>1b</b>), and {[Co(l-trp)(bpa)(H<sub>2</sub>O)]·H<sub>2</sub>O·NO<sub>3</sub>}<sub><i>n</i></sub> (<b>2</b>, bpa = 1,2-bis(4-pyridyl)ethane), were constructed
from Zn<sup>2+</sup> or Co<sup>2+</sup> ions, bipyridyl ligands, and
the amino acid l-tryptophan (l-Htrp), respectively.
Compounds <b>1a</b>, <b>1b</b>, and <b>2</b> were
characterized by single-crystal X-ray diffraction analysis. All of
the compounds crystallize in the monoclinic space group <i>P</i>2<sub>1</sub> 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 <b>1a</b> exhibited a green
emission at 546 nm. Dielectric studies of <b>1a</b> 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 <b>1a</b> 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 <b>1a</b> had a good SHG intensity response that was about
twice that of SiO<sub>2</sub>
