Reactivity of Saturated Hydrocarbon Anchoring Arms on Si(100) upon White Light Photoactivation: Experimental Evidence and Theoretical Insights

In the frame of the much debated issue of photostimulated reaction mechanisms on Si-oriented surfaces, the first report is presented here where the presence of saturated anchoring arms in organometallic and organic molecules unexpectedly leads to covalent bonding on H-terminated Si(100). The molecular substrates tested (methyl-, ethyl-, and n-propylferrocene, 4-chlorotoluene, and 1-chloro-4-ethylbenzene) were chosen considering that the carbon atom directly attached to the benzene ring, C1, sits at a peculiar state of charge also by virtue of sigma-pi hyperconjugation effect and that it can be sterically hindered in its interaction to a surface if it becomes part of a longer chain. The anchoring reaction was conducted via exposure to low-intensity white light, and the resulting hybrid species were characterized by means of X-ray photoelectron spectroscopy (XPS), electrochemical measurements by cyclic voltammetry (CV), atomic force microscopy (AFM), and density functional theory (DFT) calculations. At variance with the so far proposed theories on the light-induced hydrosilylation of Si surfaces, which is invariably considered as necessary for a covalent bonding in the presence of unsaturated carbon groups as the anchoring moiety to the surface, all the aromatic molecules tested here successfully anchored to HSi(100). As a countercheck, attempts to anchor onto HSi(100) two species with a sterically hindered C1 atom, n-butylferrocene, and a nonaromatic structure, 1-chlorodecane, led to a surface coverage close to zero. XPS, CV, and AFM results showed that the molecular monolayer coverage on the Si surface decreases as the sterical hindrance of C1 increases. DFT calculations proved that the adhesion ability of the different species diminishes as the length of the alkyl chain increases, while the breakage of the C1H bond is always favored, irrespective of chain length. In light of our previous theoretical studies, a reaction mechanism is envisaged as involving radical species via a homolytic cleavage of the CH bond at the C1 atom of the molecule, induced through white light irradiation.