Utilizing scanning tunneling microscopy (STM), bond-resolved scanning tunneling microscopy (BR-STM) and non-contact atomic force microscopy (nc-AFM), we distinguish three distinct carbon-carbon bond types within polymers induced by annealing at elevated temperatures. Density-functional-theory (DFT) calculations unveil the pivotal role of the electron-withdrawing cyano group in activating neighboring methylene to form C(sp³)–C(sp³) bonds, and in facilitating subsequent stepwise HCN eliminations to realize the transformation across three carbon-carbon bond types.

Figure 1a present the reaction scheme with three steps. Step (I): The –CN groups facilitate dehydrogenative homocoupling between monomers, forming linkages of –CH(CN)–CH(CN)–, named as L(C–C), including C(sp³)–C(sp³) bonds. Step (II): One –H and one –CN groups are eliminated from the linkage after thermal annealing, transforming the linkages to be –CH=C(CN)–, named as L(C=C), including C(sp²)=C(sp²) bonds. Step (III): The remaining –H and –CN groups at the linkage further eliminate, transforming the linkages to be –C≡C–, named as L(C≡C), including C(sp)≡C(sp) bonds.

Fig. 1 Schematic illustration of the stepwise evolution in carbon hybridizations from sp³ to sp² and to sp states.

Reaction behavior on Au(111)

In our study, reaction behaviors of methylcyano-functionalized molecules on Au(111) have been first investigated. Deposition of 1,6-di[2-(4-cyanomethylphenyl)ethynyl]pyrene (M1) onto Au(111) held at room temperature results in the self-assembled structure. Dehydrogenative polymerization of M1 molecules was triggered after annealing to 520 K, leading to the formation of polymer 1A. The covalent nature of polymer 1A was confirmed by lateral tip manipulation experiments. A significant challenge we faced was to determine whether the newly formed carbon-carbon bond is a single bond or a double bond. Both high-resolution BR-STM and nc-AFM imaging unveil the structural details of as-formed polymers, wherein the newly formed L(C–C) between molecules are clearly observed (Fig. 2). The sp³-hybridized L(C–C) is evidenced by the brighter contrast in the nc-AFM image (white dotted circle in Fig. 2h), which is attributed to the non-planar tetrahedral configurations.

Fig. 2 Dehydrogenative polymerization of M1 on Au(111) surface.

We utilized DFT calculations to compare the dehydrogenation barriers and differential charge distributions of phenylacetonitrile and ethylbenzene molecules on the Au(111) surface, confirming that the electron-withdrawing –CN group plays a unique role in the dehydrogenative reaction by activating the C–H bonds of the adjacent saturated methylene groups.

Stepwise evolution of carbon hybridization state

Considering the readily dissociable nature of the cyano group during thermal annealing and the elimination reaction it induces, we performed further step-by-step annealing treatments on polymer 1A to realize the stepwise evolution of carbon hybridization state. Annealing the sample to 550 K for 20 mins leads to the dissociation of one side –CN group at the linkage of polymer 1A. The newly-formed C(sp²)=C(sp²) bond in L(C=C) of polymer 1B is further determined by the bond-length comparison with that of the C(sp³)–C(sp³) bond in L(C–C) of polymer 1A. It is important to emphasize that the elimination reaction type in this work is assigned to β-elimination, which involves the dissociation of the –CN group on one carbon atom and the H atom on another carbon atom within the linkage site. Moreover, we have discerned significant alterations in the bright contrast of linkages emanating from the partially eliminated intermediate polymer. This observation reinforces the notion that structural planarization is indeed facilitated by the presence of sp²-hybridized carbon atoms.

Further annealing to 620 K for 20 mins triggered the elimination of remaining HCN species and resulted in the formation of polymer 1C, containing linkages of L(C≡C), namely, –C≡C–. The distinct bright spots observed in the nc-AFM images (Fig. 3) offer compelling evidence for the existence of carbon-carbon triple bonds.

Both DFT calculated result and experimental observation reveal that the polymers are straightened and elongated along with the stepwise transition of carbon hybridization, which is attributed to variations in bond angles arising from the three distinct carbon hybridization states.

Fig. 3 Stepwise transition of carbon hybridization across all three states at the linkage via annealing to elevated temperatures.

We utilized DFT calculations to compare the energy levels of all potential intermediate products, and the energies of the structures observed in the experiments are all optimal.

The evolution of carbon hybridization across three states on silver surface

We also achieved the transition of carbon hybridization in one-dimensional/two-dimensional polymers on silver substrates using the molecules 1,4-di[2-(4-cyanomethylphenyl)ethynyl]benzene (M2) and 2,2'-(5'-(4-(cyanomethyl)phenyl)-[1,1':3',1''-terphenyl]-4,4''-diyl)diacetonitrile (M3).

It is important to emphasize that methylcyano-functionalized molecules do not form polymers containing L(C–C) linkages on silver substrates. However, during the synthesis of polymers containing L(C=C) and L(C≡C) linkages, the selectivity is higher compared to that on Au(111), which is attributed to the superior catalytic activity of the silver substrate.

Fig. 4 The transition of carbon hybridization within 1D linear polymer and 2D COF fabricated by M2 and M3 on Ag(110) and Ag(111) surfaces, respectively.

Conclusion

In summary, this paper realizes in-situ visualizations of the atomic configurations and the transformation process of as-formed polymers with distinct carbon linkages in real space by using STM, BR-STM, and nc-AFM imaging measurements, wherein the carbon hybridization evolves across all three states. The proposed strategy provides an effective approach to regulating carbon hybridization through elaborately designed –CH₂–CN groups, paving the way for the development of new carbon materials for a wide range of applications.

Visualizing stepwise evolution of carbon hybridization from sp3 to sp2 and to sp | Nature Communications