Layer-by-Layer Deposition of Organic Molecules Controlled by Selective Click Reactions

Layer-by-layer deposition of organic molecules on Si(001) was realized by the sequential application of two orthogonal click chemistry reaction steps in solution. The Si(001) surface was prepared and prefunctionalized via chemoselective adsorption of the bifunctional molecule ethynyl­cyclopropyl­cyclooctyne (ECCO) in UHV. Via a two-step reaction cycle in solution, ordered multilayers of organic molecules were then realized on this ECCO-prefunctionalized Si(001) surface. First, 1-(azidomethyl)-4-(2-azidopropan-2-yl)­benzene was coupled in acetonitrile via a copper-catalyzed azide–alkyne click reaction; second, a layer of ECCO molecules was coupled via a catalyst-free, strain-promoted azide–alkyne click reaction. Without contact to ambient conditions, the samples were analyzed by means of X-ray photoelectron spectroscopy in UHV after each reaction step in solution; the N 1s spectra clearly indicated in the first step the selective click reaction of the primary azido group of the diazide molecule, whereas the tertiary azido group stayed intact. In the second step, this tertiary azido group was reacted selectively with the strained triple bond of the ECCO molecule in solution, forming a third layer of organic molecules on Si(001) with the terminal triple bond of ECCO available for further reactions according to this cyclic reaction scheme. Alternating application of the two orthogonal reaction steps then led to a well-controlled layer-by-layer growth; it opens the possibility for the controlled synthesis of layers with physical or physicochemical properties that alternate on the molecular scale.