An azanorbornadiene anchor for molecular-level construction on silicon(100)

N-trimethylsilyl-7-azanorbornadiene (TMSAN) is synthesized and chemisorbed on the silicon(100)-2 × 1 surface under ultra-high vacuum conditions and the resulting structure is determined using scanning tunnelling microscopy (STM). The binding exhibits poor short-range order, similar to that for norbornadiene. Patterning of the adsorbate is demonstrated following STM electron-stimulated depassivation of a silicon(100)-2 × 1-H surface, indicating that the placement of TMSAN on the surface can be controlled. Density-functional theory (DFT) calculations verify the close analogy between the binding of TMSAN and its much studied parent compound, norbornadiene. This analogue is novel, however, in that it can provide anchor points for construction at the molecular level above the silicon surface. How such construction could proceed is controlled by the topology of the nitrogen atom and the torsional potential for rotation about the N-Si bond. While these key features are not readily apparent from the STM results, DFT predicts that TMSAN above silicon(100) adopts a structure containing an azimuthal rotor: the nitrogen atom is in a planar configuration so that the N-Si bond is normal to the silicon surface, there being also nearly free rotation about the N-Si bond. Further, variants of TMSAN are considered in which a double-well potential for nitrogen inversion is predicted, suggesting that chemical control can be established over the architectural function of this class of compounds.