Chemical Identification and Bond Control of π-Skeletons in a Coupling Reaction

Abstract

Highly unsaturated π-rich carbon skeletons afford versatile tuning of structural and optoelectronic properties of low-dimensional carbon nanostructures. However, methods allowing more precise chemical identification and controllable integration of target sp-/sp 2 -carbon skeletons during synthesis are required. Here, using the coupling of terminal alkynes as a model system, we demonstrate a methodology to visualize and identify the generated π-skeletons at the single-chemical-bond level on the surface, thus enabling further precise bond control. The characteristic electronic features together with localized vibrational modes of the carbon skeletons are resolved in real space by a combination of scanning tunneling microscopy/spectroscopy (STM/STS) and tip-enhanced Raman spectroscopy (TERS). Our approach allows single-chemical-bond understanding of unsaturated carbon skeletons, which is crucial for generating low-dimensional carbon nanostructures and nanomaterials with atomic precision.

On-Surface Synthesis of 2D Porphyrin-Based Covalent Organic Frameworks Using Terminal Alkynes

“…44 Fortunately, recent work using the tip-enhanced Raman spectroscopy (TERS) technique has already proved that the coupling of terminal alkynes on Ag(111) is nondehydrogenative coupling, not dehydrogenative Glaser coupling. 31 These findings overturned the previous view, which was based on the STM images that the Glaser coupling is dominant on the Ag(111) surface. 45 So based on the latest TERS experiment, the linear coupling of terminal alkynes on Ag(111) in our experiment was most likely to be nondehydrogenative coupling.…”

“…The only structural difference between the enyne bond and the diyne bond is that the enyne bond has obvious offsets between neighboring units, but due to the flexibility of the sp carbon skeleton, the stress can also cause the nonstraightness of the diyne bond . Fortunately, recent work using the tip-enhanced Raman spectroscopy (TERS) technique has already proved that the coupling of terminal alkynes on Ag(111) is nondehydrogenative coupling, not dehydrogenative Glaser coupling . These findings overturned the previous view, which was based on the STM images that the Glaser coupling is dominant on the Ag(111) surface .…”

“…However, dehalogenation reactions produce halogens as byproducts around the nanostructures, thereby hampering the formation and electronic properties of low-dimensional nanostructures with a large area. − On the other hand, the covalent linking of acetylene compounds generates only excess hydrogen as byproducts, which makes it a powerful tool for the synthesis of low-dimensional materials such as carbyne chains, 2D graphyne, or graphdiyne networks . Moreover, alkynes have several reaction pathways on the surface, including Glaser coupling, nondehydrogenative coupling, , cyclotrimerization, cycloaddition, and radical cyclization . Therefore, it is important to investigate the effect of different surfaces on the reaction of alkyne for the precise regulation of nanostructures at a molecular level …”

Scanning probe microscopy in probing low-dimensional carbon-based nanostructures and nanomaterials

“…One promising way is the incorporation of n-membered rings with π conjugation based on C(sp 2 ), for example, preparation of 1D π-conjugated polymers with indenofluorene units where five-membered rings were incorporated [147]. Alternatively, C(sp)-containing skeletons have been successfully embedded in the sp 2 -hybridized GNRs and GNR-like nanostructures for better electronic tailoring, such as ethynylene-bridged [148][149][150][151], diacetylene-bridged [152,153], and cumulene-bridged [154,155] polymers. Besides, 1D cumulene-linked polymers with the involvement of five-and seven-membered rings (based on a tribenzoazulene building block) [156] was also achieved.…”

Interactions between water and organic molecules or inorganic salts on surfaces

“…Nevertheless, there remain many open questions to be solved in the near future in terms of developing experimental techniques and theoretical models and further establishing methodologies in precise visualization and interpretation of detailed hydration processes for modeling the dynamics. In this sense, combination of various atomic-scale topographic characterization techniques (such as bond-resolved STM [14,62] and nc-AFM [63][64] ) and precise spectroscopic techniques with single-chemical-bond sensitivity (such as tipenhanced Raman spectroscopy [65][66] ) would give valuable and complementary indications involving both skeleton and chemical information in water-incorporated dynamic processes. Moreover, most of studies discussed here have been devoted to the well-defined solid surfaces under ultrahigh vacuum conditions as simplified model systems, while significant differences exist between the model systems and the ambient conditions of the real world.…”