The Macrocycle versus Chain Competition in On‐Surface Polymerization: Insights from Reactions of 1,3‐Dibromoazulene on Cu(111)

Abstract

Ring/chain competition in oligomerization reactions represents a long‐standing topic of synthetic chemistry and was treated extensively for solution reactions but is not well‐understood for the two‐dimensional confinement of surface reactions. Here, the kinetic and thermodynamic principles of ring/chain competition in on‐surface synthesis are addressed by scanning tunneling microscopy, X‐ray photoelectron spectroscopy, and Monte Carlo simulations applied to azulene‐based organometallic oligomers on Cu(111). Analysis of experiments and simulations reveals how the ring/chain ratio can be controlled through variation of coverage and temperature. At room temperature, non‐equilibrium conditions prevail and kinetic control leads to preferential formation of the entropically favored chains. In contrast, high‐temperature equilibrium conditions are associated with thermodynamic control, resulting in increased yields of the energetically favored rings. The optimum conditions for ring formation include the lowest possible temperature within the regime of thermodynamic control and a low coverage. The general implications are discussed and compared to the solution case.

On‐Surface Decarboxylation Coupling Facilitated by Lock‐to‐Unlock Variation of Molecules upon the Reaction

“…When relatively high energy barriers are involved, the OSS approach faces a primary issue which is related to a typical dilemma: a firmly anchored adsorption is necessary for maintaining the molecules on the surface sufficiently long to overcome the reaction barrier; [25, 26] on the other hand, the molecular mobility is in itself important for the subsequent molecular coupling and arrangement [27, 28] . In addition, due to the known competition between the chain and ring coupling forms on surfaces, [13, 29–32] another issue for OSS relates to the difficulty in controling the structural homogeneity of the final product.…”

On‐Surface Decarboxylation Coupling Facilitated by Lock‐to‐Unlock Variation of Molecules upon the Reaction

“…When relatively high energy barriers are involved, the OSS approach faces a primary issue which is related to a typical dilemma: a firmly anchored adsorption is necessary for maintaining the molecules on the surface sufficiently long to overcome the reaction barrier; [25,26] on the other hand, the molecular mobility is in itself important for the subsequent molecular coupling and arrangement. [27,28] In addition, due to the known competition between the chain and ring coupling forms on surfaces, [13,[29][30][31][32] another issue for OSS relates to the difficulty in controling the structural homogeneity of the final product.…”

On-Surface Synthesis of Unsaturated Carbon Nanostructures with Regularly Fused Pentagon–Heptagon Pairs

“…Considering the large distance (∼1 nm) and protrusions between most of the neighboring 1,1′-biazulenyl-3,3′-diyl moieties (indicated by red arrows in Figure 2c−e), the observed polymer 3 is attributed to a 1,1′-biazulenyl-3,3′-diyls-Au organometallic polymer. 36,37 Although C−Au−C bonds are rarely formed on Au(111), they seem to occur where steric hindrance between debrominated species hampers subsequent C−C bond formation. 45 Interestingly, despite abundant C−Au−C motifs in 3, some C−C bonds were already formed, as evidenced by the existence of oligo(1,3-azulenediyl) moieties in the polymer chain (Figure 2c−e).…”

“…On-surface synthesis is emerging as a new method with remarkable success in fabricating novel sp 2 -carbon nanostructures that are often inaccessible by conventional organic synthesis. − Typically, molecular precursors are sublimed onto metal surfaces under ultrahigh vacuum (UHV) conditions, followed by thermally induced surface-catalyzed reactions. ,, Syntheses of PAHs containing two embedded pentagon–heptagon pairs in their skeletons have been sporadically explored employing cyclodehydrogenation of precursors composed mainly of hexagons on Au(111) at elevated temperature. − On-surface polymerizations − and strain-induced or thermally activated rearrangements of azulene derivatives have also been reported. Polymers with multiple azulene units could serve as potential precursors for polycyclic sp 2 -carbon nanostructures with abundant pentagon–heptagon pairs.…”