The study of positive homotropic allosterism in supramolecular receptors is important for elucidating design strategies that can lead to increased sensitivity in various molecular recognition applications. In this work, the cooperative relationship between tetrathiafulvalene (TTF)-calix[4]pyrroles and several nitroaromatic guests is examined. The design and synthesis of new annulated TTF-calix[4]pyrrole receptors with the goal of rigidifying the system to accommodate better nitroaromatic guests is outlined. These new derivatives, which display significant improvement in terms of binding constants, also display a positive homotropic allosteric relationship, as borne out from the sigmoidal nature of the binding isotherms and analysis by using the Hill equation, Adair equation, and Scatchard plots. The host-guest complexes themselves have been characterized by single-crystal X-ray diffraction analyses and studied by means of UV-spectroscopic titrations. Investigations into the electronic nature of the receptors were made by using cyclic voltammetry; this revealed that the binding efficiency was not strictly related to the redox potential of the receptor. On the other hand, this work serves to illustrate how cooperative effects may be used to enhance the recognition ability of TTF-calix[4]pyrrole receptors. It has led to new allosteric systems that function as rudimentary colorimetric chemosensors for common nitroaromatic-based explosives, and which are effective even in the presence of potentially interfering anions.
In order to realize significant benefits from the assembly of solid-state materials from molecular cluster superatomic building blocks, several criteria must be met. Reproducible syntheses must reliably produce macroscopic amounts of pure material; the cluster-assembled solids must show properties that are more than simply averages of those of the constituent subunits; and rational changes to the chemical structures of the subunits must result in predictable changes in the collective properties of the solid. In this report we show that we can meet these requirements. Using a combination of magnetometry and muon spin relaxation measurements, we demonstrate that crystallographically defined superatomic solids assembled from molecular nickel telluride clusters and fullerenes undergo a ferromagnetic phase transition at low temperatures. Moreover, we show that when we modify the constituent superatoms, the cooperative magnetic properties change in predictable ways.
We
describe a new approach to synthesize two-dimensional (2D) nanosheets
from the bottom-up. We functionalize redox-active superatoms with
groups that can direct their assembly into multidimensional solids.
We synthesized Co6Se8[PEt2(4-C6H4COOH)]6 and found that it forms a
crystalline assembly. The solid-state structure is a three-dimensional
(3D) network in which the carboxylic acids form intercluster hydrogen
bonds. We modify the self-assembly by replacing the reversible hydrogen
bonds that hold the superatoms together with zinc carboxylate bonds
via the solvothermal reaction of Co6Se8[PEt2(4-C6H4COOH)]6 with Zn(NO3)2. We obtain two types of crystalline materials
using this approach: one is a 3D solid and the other consists of stacked
layers of 2D sheets. The dimensionality is controlled by subtle changes
in reaction conditions. These 2D sheets can be chemically
exfoliated, and the exfoliated, ultrathin 2D layers are soluble. After
they are deposited on a substrate, they can be imaged. We cast them
onto an electrode surface and show that they retain the redox activity
of the superatom building blocks due to the porosity in the sheets.
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