Anatomy of On-Surface Synthesized Boroxine Two-Dimensional Polymers

Synthetic two-dimensional polymers (2DPs) obtained from well-defined monomers via bottom-up fabrication strategies are promising materials that can extend the realm of inorganic 2D materials. The on-surface synthesis of such 2DPs is particularly popular, however the pathway complexity in the growth of such films formed on solid surfaces is poorly understood. In this contribution, we present a straightforward experimental protocol which allows the synthesis of large-area, defect-free 2DPs based on boroxine linkages at room temperature. We focus on unravelling the multiple pathways available to the polymerizing system for the spatial extension of the covalent bonds. Besides the anticipated 2DP, the system can evolve into self-assembled monolayers of partially fused monodisperse reaction products that are difficult to isolate by conventional synthetic methods or remain in the monomeric state. The access to each pathway can be controlled via monomer concentration and the choice of the solvent. Most importantly, the unpolymerized systems do not evolve into the corresponding 2DP upon annealing, indicating the presence of strong kinetic traps. Using high-resolution scanning tunneling microscopy, we show reversibility in the polymerization process where the attachment and the detachment of monomers to 2DP crystallites could be monitored as a function of time. Finally, we show that the way the 2DP grows depends on the choice of the solvent. Using UV-vis absorption and emission spectroscopy, we reveal that the dominant pathway for 2DP growth is via in-plane self-condensation of the monomers, whereas in the case of an aprotic solvent, the favored growth mode is via π stacking of the monomers.