Surface [4+2] Cycloaddition Reaction of Thymine on Si(111)7x7 Observed by Scanning Tunneling Microscopy

The early stage of thymine (Thy) adsorption on Si(111)7x7 surface is studied with scanning tunneling microscopy (STM) and density functional theory (DFT)-based computational method. Bright protrusions corresponding to the adsorbed thymine molecules are observed in both empty-state and filled-state STM images. These bright protrusions in the empty-state images exhibit three different degrees [lower (L), medium (M), and higher (H)] of intensities. The L and M protrusions are found on the adatom-restatom pairs in the 7 x 7 unit cell, indicating bidentate configurations for Thy adsorption. A free Thy molecule has been found to interact with the Si surface by a two-step process: Thy first undergoes keto-enol tautomerization to form the more stable dienol tautomer, which then binds to the Si adatom-restatom pair via the [4 + 2] cycloadclition reaction, leading to two different adproducts. Our DFT/B3LYP/6-31++G(d,p) calculations show three plausible cycloaddition products, with the 1,4-cydohexadiene adproduct being more stable than 3,6- and 2,5-cydohexadienes. Our calculations also suggest the viability of formation of a Thy molecule hydrogen-bonded with the bidentate Thy adproducts already on the surface. Statistical analysis for three different exposures of Thy on the 7 x 7 surface reveals that the L protrusion has the highest relative surface concentration (80%), with the M (16%) and H features (496) being significantly less popular. These results lead us to attribute the L and M protrusions to the 1,4- and the 3,6-cycloaddition products, respectively, with the least popular H protrusion assigned to a Thy molecule hydrogen-bonded to the bidentate Thy, all attached to an adatom-restatom pair. The observation of [4 + 2] cycloaddition products on the Si surface confirms the formation of precursor dienol Thy tautomer. This surface-mediated two-step reaction pathway for Thy is unique for surfaces, in contrast to keto-enol tautomerization that is mainly observed in the solution phase either by acid- or base-catalyzed pathways. Our STM study coupled with our separate XPS work have demonstrated that this type of tautomerization can also be observed on Si surfaces, and this can initiate the subsequent cycloaddition reactions of Thy molecule with the surface.