Regulating carbon hybridization states lies at the heart of engineering carbon materials with tailored properties but orchestrating the sequential transition across three states has remained elusive. Here, we visiualize stepwise evolution in carbon hybridizations from sp³ to sp² and to sp states via dehydrogenation and elimination reactions of methylcyano-functionalized molecules on surfaces. Utilizing scanning probing microscopy, we distinguish three distinct carbon-carbon bond types within polymers induced by annealing at elevated temperatures. Density-functional-theory 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. We also demonstrate the applicability of this strategy on one-dimensional molecular wires and two-dimensional covalent organic framework on different substrates. Our work expands the scope of carbon hybridization evolution and serves as an advance in flexibly engineering carbon-material by employing cyanomethyl-substituted molecules.
Dehydrogenative polymerization of M1 on Au(111) surface.
a Schematic representation of step (I) of M1, dehydrogenative homocoupling to form polymer 1A on Au(111). b Large-scale STM image (200 mV, 200 pA) showing selfassembled precursors M1 after room temperature (RT) deposition onto Au(111). c Zoom-in BR-STM image (2 mV, tip-sample distance offset Δz = 180 pm) of selfassembled precursors M1 on Au(111). d nc-AFM image (2 mV, Δz = 220 pm) corresponding to (c). The images of (c, d) are overlaid with the chemical structures of M1. e Large-scale STM image (200 mV, 100 pA) showing polymers 1A after annealing to 520 K for 20 min. The depressions on both sides of linkages are marked by white solid arrows. f–i Zoom-in STM image (200 mV, 100 pA) (f), BRSTM image (2 mV, Δz = 170 pm) (g), nc-AFM image (2 mV, Δz = 200 pm) (h) and Laplace-filtered image (i) of polymer 1A derived from the blue dashed box in (e).
The transition of carbon hybridization within 2D COF fabricated by M2 and M3 on Ag(110) and Ag(111) surfaces, respectively.
g Schematic illustration showing the synthesis of 2DCOF 3B and 3Cwith the linkages L(C=C) and L(C≡C) after annealing M3 to 460 K and 520 K for 20min on Ag(111), respectively. h, i High-resolution STM (100mV, 150 pA) (h) and BR-STM images (2mV, Δz = 220 pm) (i) showing the COF 3B. j, k High-resolution STM (200mV, 150 pA) (j) and BR-STMimages (2mV,Δz= 220pm) (k) showing the COF 3C obtained via annealing to 520 K. Scale bars, 0.6 nm (h–k).
