Electrostatic forces are long-range interactions that play a key role in most chemical systems in nature. Reactions involving charge-separated processes are considered to be electric field responsive. 1,2 Thus, electric field effects make it possible to manipulate the kinetics and/or thermodynamics of chemical reaction processes. We reasoned that electric field effects could affect boronic acid-based dynamic covalent chemistry (DCC). For this end, the use of scanning tunneling microscopy (STM) was considered a good choice, as it combines localized control of a switchable electric field, and high-resolution imaging. So far, most of the studies about electric-field-induced phase transitions are based on the dynamics of non-covalent interactions. 3,4 To the best of our knowledge, electric-field-induced switchable surfaces based on reversible covalent bonds are not explored yet. Herein, we have designed a surface model system to demonstrate the bidirectional guidance of a DCC system by an external electric field, illustrated in Figure 1. By reversing the direction of the electric field that exists between the STM tip and a conductive solid substrate, one can locally control the on-surface polymerization/depolymerization at a liquid/solid interface. Consequently, the reversible transformation between self-assembled monolayers (SAMs) and covalent organic frameworks (COFs) can be monitored at molecular level.
The quality of crystalline two-dimensional polymers (2DPs) 1-6 is intimately related to the elusive polymerization and crystallization processes. Understanding the mechanism of such processes at the (sub)molecular level is crucial to improve predictive synthesis and to tailor material properties for applications in catalysis 7-10 , and (opto)electronics 11, 12 , among others [13][14][15][16][17][18] . We characterize a model boroxine 2D dynamic covalent polymer, by using in situ scanning tunneling microscopy, to unveil both qualitative and quantitative details in the nucleation-elongation processes in real time and under ambient conditions. Sequential data analysis allows for the observation of the amorphous-to-crystalline transition, the timedependent evolution of nuclei, the existence of "nonclassical" crystallization pathways and importantly, the experimental determination of essential crystallization parameters including critical nucleus size, nucleation rate and growth rate with excellent accuracy. The experimental data has been further rationalized by atomistic computer models that altogether provide a detailed picture of the dynamic on-surface polymerization process. Furthermore, we show how two-dimensional crystal growth can be affected by abnormal grain growth (AGG). This finding provides support for the use of AGG -a typical phenomenon in metallic and ceramic systems -to convert a polycrystalline structure into a single crystal in organic and 2D material systems. Two-dimensional polymers (2DPs) -covalently linked networks of monomers in orthogonal directions-can be found as individual monolayers, as part of few-layer stacks, or as part of multilayered crystals, the latter known as 2D covalent organic frameworks (2D COFs). Over the past decade, research has focused on exploring efficient and controlled synthetic strategies to produce highly crystalline 2DPs [19][20][21][22][23][24][25][26] . Yet little is known about the mechanistic and kinetic aspects of the dynamic processes involving bond formation/breakage, nucleation,
The in situ on-surface conversion process from boroxine-linked covalent organic frameworks (COFs) to boronate ester-linked COFs is triggered and catalyzed at room temperature by an electric field and monitored with scanning tunneling microscopy (STM). The adaptive behavior within the generated dynamic covalent libraries (DCLs) was revealed, providing in-depth understanding of the dynamic network switching process.
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