Catassembly in the Host-Guest Recognition of 2D Metastable Self-Assembled Networks

Abstract

Catassembly is a newly developed concept concerning the process of molecular assembly improved by a catalyst-assembler (catassembler). However, it has not been visualized in detail at the molecular level. To achieve the formation of highly complex structures with high efficiency and selectivity, a deeper understanding of catassembly is essential. In this study, we present the scanning tunneling microscopy (STM) characterization of a catassembly process within host-guest assembly. We utilize a metastable self-assembled network of 1,3,5-tris(4-carboxyphenyl)-benzene (BTB) at the liquid-solid interface between 1-octanoic acid and highly oriented pyrolytic graphite (HOPG). Different adsorption behaviors of low-concentration guest molecules (copper phthalocyanine (CuPc), and coronene (COR)) are contrastively analyzed during the host-guest assembly in both single-guest (COR/BTB or CuPc/BTB) and multi-guest molecule (COR&CuPc/BTB) systems. The spontaneous phase transition from a hexagonal to an oblique structure of BTB monolayers (high concentration, approximately 500 μmol∙L⁻¹ in octanoic acid) provides an ideal metastable phase for studying the dynamic assembly process. In the host-guest assembly, the metastable BTB hexagonal phase serves as a host network and can be stabilized by co-assembling guest molecules under a negative bias voltage. However, the stability of the metastable phase varies with different guest molecules. We observe that the BTB metastable phase is more robust with COR guest molecules than with CuPc. In the CuPc/BTB system, we find that low-concentration CuPc (approximately 1.5 μmol∙L⁻¹ in octanoic acid) can hardly co-assemble with BTB, leading to the gradual collapse of the metastable BTB networks into the oblique phase. The different stability of BTB metastable phase in the host-guest assembly is attributed to differences in the kinetics of trapping guest molecules. Guest COR molecules exhibit kinetic advantages over CuPc when assembling with host BTB networks under a negative sample bias. The lower trapping rate of CuPc hinders the formation of co-assembled BTB/CuPc networks. These differences in the dynamic behavior of the guest molecules are further explored in the research of catassembly. In a multi-guest molecule system (COR&CuPc/BTB), COR molecules are preferentially trapped by BTB hexagonal networks and can gradually be replaced by CuPc during continuous scanning. The more energetically stable structure of CuPc/BTB compared to COR/BTB rationalizes the exchange of the guest molecule and the evolution of the assembly phase. The involvement of COR significantly increases both the efficiency and quality of the CuPc/BTB assembly, serving as a catassembler. This observation provides insights into a complete catassembly process at the molecular level, enabling further investigations into the selectivity and efficiency of host-guest phenomena for potential applications in analysis and separation. Additionally, this work serves as a prototype for constructing highly complex 2D assembled monolayers.

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