Self-assembly is a ubiquitous and important process in nature,as well as the most promising and representative bottom-up approach to build up nanostructures. Adsorption and self-assembly at surface and interface is a key physical chemistry subject due to its close relationship to many fundamental and application scientific problems like catalysis,chirality, electron and energy transfer, single molecule science, and organic electronics. The invention of scanning tunneling microscopy (STM) provides us a powerful tool to study the surface self-assembly.With the help of STM at sub-molecular resolution, the research activities of our group are focused on exploring the rules that governs the formation of molecular self-assembly, and controlling the self-assembled nanostructures. Besides, we have managed to fabricate the well-ordered surface covalent organic frameworks(SCOFs) by covalent bondings.Our study address the basic problem of physical chemistry science and methodology to build nanostructures.
Main contents include:
1.Fabrication and modulation of hierarchical self-Assembled nanostructures
2.Formation and transition of chiral nanostructures
3.Manipulation and fabrication of nanostructures by external conditions
4.Nanostructures constructed by polycyclic aromatic conjugated molecules.
5.Construction of 2D covalent organic frameworks
1.Fabrication and modulation of hierarchical self-Assembled nanostructures
We have managed the construction of a series of 2D chiral hierarchical structures by trinary molecular self-assembly with copper phthalocyanine (CuPc), 2,3,7,8,12,13-hexahexyloxy-truxenone (TrO23), and 1,3,5-tris(10-carboxydecyloxy) benzene (TCDB). A series of flower-like chiral hierarchical molecular architectures with increased generations are formed, and the details of these structures are investigated by high resolution scanning tunneling microscopy (STM).The flower-like hierarchical molecular architectures could be described by a unified configuration in which the lobe of each architecture is composed of a different number of triangular shape building units (TBUs). The off-axis edge-to-edge packing of TBUs confers the organizational chirality of the hierarchical assemblies. On the other hand, the TBUs can tile the surface in a vertex-sharing configuration, resulting in the expansion of chiral unit cells, which thereby further modulate the periodicity of chiral voids in the multilevel hierarchical assemblies. The formation of desired hierarchical structures could be controlled through tuning the molar ratio of each component in liquid phase. The results are significant for the design and fabrication of multicomponent chiral hierarchical molecular nanostructures.
The structural evolution of hierarchical structures of different generations.
2.Formation and transition of chiral nanostructures
With the help of weak interaction between molecules as well as halogen bonding, we manage to control the two-dimentional assembled structures of oligo(p-phenylenevinylene) (OPV) and its derivatives on HOPG, changing from chiral helix domains to linear ones. Onthe other hand, by a multiple hydrogen bonding between co-assembly molecules,we have modulated 2D global homochirality in molecular-assembled networks on highly oriented pyrolytic graphite.Globally homochiral network assembly with preferred handedness is successfully achieved. Moreover, it is the first time to demonstrate the nonlinear chirality amplification in 2D molecular assemblies from achiral molecules. Such an induction and nonlinear chirality amplification effect promises a new approach towards 2D homochirality control and may reveal important insights into asymmetric heterogeneous catalysis, chiral separation and chiral crystallization.
Dense windmill structure of OPV derivatives before annealing. (A) and (B) High resolution STM images showing clockwise and counterclockwise fashion.
Linear structure of OPV derivatives after annealing.
3.Manipulation and fabrication of nanostructures by external conditions
The research involves the manipulation of self-organized molecular adlayers by external stimuli such as electrode potential. The relationship between intermolecular interaction and structure has been revealed. From the self-assembly to the fabrication of oriented 1D organic chains of nitroaromatic compounds on Au(111), a breakthrough has been achieved.Moreover,we find that TCAS interact with the substrate by -SO3-, forming three sorts of trimer by Van der waals in different domains, such as Kagome.
STM images of TCAS adlayers on Au(111) surface. (left) Kagome network, and (right) snub trihexagonal network.
4.Assembled structure constructed polycyclic aromatic conjugated molecule.
The assembly rules of Janus-type double-concave graphene molecules are investigated by STM and density functional theory calculation. With the increase of the alkyl chain length, van der Waals interaction becomes stronger than hydrogen bonds interaction, resulting in the structure transformation from "up-down" to hexagonal honeycomb; Two Layer host-guest arrays of shape-persistent arylene-ethynylene-butadiynylene macrocycle and macrocycle/metallacycles have been obtained on HOPG by their π-π interaction. Hybrid supramolecular architectures have been fabricated with acceptor PBP and donor DTT on HOPG. And their structures and molecular conductance are characterized by adjusting the acceptor/donor molar ratio and STS measurements.
Hybrid nanostructures of DTT and PBP in different relative molecular ratios.
DTT:PBP(a) 1:2，(b) 1: 20，(c) 1:30，(d) 1:100
5. Construction of 2D covalent organic frameworks
We proposed a new method to synthesize highly ordered 2D covalent networks by chemical equilibrium regulation. By introducing a small amount of water into a closed reaction system, the chemical equilibrium of the boroxine ring formation reaction could be regulated to favor the occurrence of the reverse reaction and therefore improved the abilities of self-adjustment or self-healing of the SCOFs. This strategy offered a universal strategy to fabricate 2D covalent-bonded network with controlled chemical functionality.
Synthesis route to 2D covalent organic frameworks via the dehydration of di-borate aromatic molecules
