Micrometre-long covalent organic fibres by photoinitiated chain-growth radical polymerization on an alkali-halide surface

Abstract

On-surface polymerization is a promising technique to prepare organic functional nanomaterials that are challenging to synthesize in solution, but it is typically used on metal substrates, which play a catalytic role. Previous examples on insulating surfaces have involved intermediate self-assembled structures, which face high barriers to diffusion, or annealing to higher temperatures, which generally causes rapid dewetting and desorption of the monomers. Here we report the photoinitiated radical polymerization, initiated from a two-dimensional gas phase, of a dimaleimide monomer on an insulating KCl surface. Polymer fibres up to 1 μm long are formed through chain-like rather than step-like growth. Interactions between potassium cations and the dimaleimide's oxygen atoms facilitate the propagation of the polymer fibres along a preferred axis of the substrate over long distances. Density functional theory calculations, non-contact atomic force microscopy imaging and manipulations at room temperature were used to explore the initiation and propagation processes, as well as the structure and stability of the resulting one-dimensional polymer fibres.

Polycyclic aromatic chains on metals and insulating layers by repetitive [3+2] cycloadditions

“…In situ synthesis, on the other hand, provides an alternative pathway towards functional nano-architectures and is expected to decrease costs, favor miniaturization, improve the purity and enhance the performance of polyaromatic frameworks. However, only few reaction types for aryl-aryl coupling have been realized on insulating surfaces [9][10][11][12][13][14][15][16][17][18][19] . The control of covalent coupling reactions on nonmetallic surfaces proves difficult due to the lack of metal-catalyzed carbon-hydrogen or carbon-halogen bond activation 20,21 .…”

“…Such environments commonly lead to impurities and (randomly coupled) side products that remain on the surface, substantially hampering the quality of the fabricated architectures. Creation of atomically precise surface-supported organic materials for devices prompts catalyst-free polymerization 23 and ring-formation reactions with high selectivity and preferably surface-independent reactivity. For instance, we recently introduced PAMY as a powerful halogen-free building block for the surface-assisted dimerization into a conjugated polycyclic hydrocarbon, namely pyrazine-embedded hexa-perihexabenzocoronene (N 2 -HBC) 24 .…”

Two‐Sidedness of Surface Reaction Mediation

“…In addition to single crystalline metal substrates and supported metal catalysts, insulating and semiconducting substrates can also be applied to host surface reactions. KCl(001), an insulating material, has been taken as the substrate for the linear polymerization of N , N ′‐(1,4‐phenylene)dimaleimide . KCl surface enables the precursor molecules to form a 2D gas phase and hence decreases the barrier to diffusion.…”

“…Most of the reactions are realized by thermal annealing, however, recent studies show that external excitation can also initiate the reaction. Para et al have explored the influence of ultraviolet illumination on the growth of polymer fibers using N , N ′‐(1,4‐phenylene)dimaleimide on KCl surface. After ultraviolet illumination for 14 h by a light‐emitting diode, the initiation of the chain‐like polymerization ( Figure b) is about two orders of magnitude more frequent than that without illumination (Figure a).…”

“…The ∆ z scales denote the height differences in the images between the dim and the bright colors. a,b) Reproduced with permission . Copyright 2018, Springer Nature.…”