Conception of Constructing Tetrahedron by Molecular Tension Engineering

Multi-walled nanostructures represented by multi-walled carbon nanotubes have long been a research hotspot in the field of supramolecular chemistry and nanoscience due to their unique structure and potential application value
.

Generally, the methods for constructing multi-wall nanostructures are the Russian doll model (the nesting combination of multiple single-layer structures) and the parchment model (the continuous curling of a single single-layer structure) (Figure 1)

Figure 1.
Cross-sectional schematic diagram of the matryoshka model, parchment model, and hybrid model

In response to the above problems, the Liu Zhichang research group of West Lake University recently adopted a hybrid model of a Russian doll model and a parchment model (Figure 1), using molecular-strain engineering (Molecular-Strain Engineering, MSE) to use structural strain Tension is applied inside the molecule, so that the molecule itself can produce precise and adjustable strain configurations; these strain configurations are expected to be different in terms of their physical and chemical properties, controllable supramolecular assembly, regulation of reaction processes, and selectivity.
The unique performance of the strain configuration-successfully realized the concept of constructing a double-walled tetrahedron through supramolecular self-assembly starting from the angled double-panel molecule
.

The research results were recently published in Chem

First, based on the molecular tension engineering strategy, Liu Zhichang's group designed a molecular bow with a certain tension energy in the molecule (Chem.
Lett.
, 2020, 49, 1329–1336) (Figure 2)
.

In the bowstring and bow arms, a smaller benzene ring plane and a larger porphyrin ring plane are introduced respectively

Figure 2.
Using a hybrid model to build multi-walled nanostructures

On this basis, the research group further studied the process of assembly and formation of tetrahedrons in the solution phase and the solid phase
.

NMR characterization results show that when the double-sided supramolecular building unit (MB-2) is assembled into a double-walled tetrahedron, it is concentration-dependent: at low concentrations, it is in the form of monomer (Cs symmetry, 17 signals in 1H NMR) Exist, at high concentrations, it exists as a double-walled tetrahedron (C2 symmetry, 34 signals in 1H NMR) (Figure 3)

　　

　　Figure 3.
Self-assembly of double-walled tetrahedrons in solution phase

　　Subsequently, the research team further analyzed and studied the solid-phase double-walled tetrahedron by using single crystal X-Ray diffraction and scanning electron microscopy techniques (Figure 4)
.

In the crystal structure of DWT-2, it can be seen that the porphyrin plane of the monomer molecule MB-2 has been significantly bent, indicating that its structure has a certain tension energy

　　

　　Figure 4.
The crystal structure and superstructure of a double-walled tetrahedron

　　Finally, the author used these porphyrin double-walled tetrahedra to carry out the photodegradation catalysis of dyes (rhodamine B and crystal violet) (Figure 5)
.

Experiments have found that under the same illumination conditions, the efficiency of the double-walled tetrahedron to catalyze the degradation of rhodamine B and crystal violet is significantly higher than that of ordinary planar porphyrins

　　

　　Figure 5.

　　This study shows that using molecular tension engineering strategies, through the interaction between self-stressing rigid building units with tunable conformations, complex supramolecular assembly systems can be constructed and the physical and chemical properties of their constituent units can be changed