Fig. 1: Schematic illustration for the cycloaddition of C₆₀ and BCPM on Au(111).

Two-dimensional (2D) on-surface synthesis (OSS) is based on the dissociation/formation of organic precursors’ σ bonds at the planar active sites of precursors, allowing only lateral covalent bonding within a single molecular layer thus confining the synthesis to be 2D. In addition to the lateral covalent bonding, to extend OSS from 2D to 3D, covalent bonding perpendicular to the surface is also required. In this work, 3,5-bis(carboxylic acid)-phenyl-3-maleimide (BCPM, C₁₂H₇NO₆) molecules assemble into a 2D ordered honeycomb network on Au(111), where carbon, nitrogen, oxygen, and hydrogen atoms are in grey, blue, red, and white, respectively. The C₆₀ layer is constructed on the BCPM layer. One C₆₀ sits on top of the phenyl ring of one BCPM, providing appropriate steric configuration for the coupling between the lowest unoccupied molecular orbital of C₆₀ and the highest occupied molecular orbital of BCPM. Upon thermal activation, [4 + 2] cycloaddition between the phenyl ring of BCPM and[6,6] bond of C₆₀ is triggered. Thanks to the multiple reactive sites of C₆₀, the resultant C₆₀-BCPM molecules can laterally bond with one another by [2 + 2] cycloaddition between[6,6] bonds of their C₆₀ heads. In this way, both lateral and vertical covalent bonding is realized, representing a prototype for 3D synthesis on surfaces.