Formation of Stabilized Ketene Intermediates in the Reaction of O(3P) with Oligo(phenylene ethynylene) Thiolate Self-Assembled Mono layers on Au(111)

We have taken steps to develop a methodology for quantifying the kinetics and dynamics of bimolecular reactions through spectroscopic monitoring of reactants and products during exposure of well-ordered self-assembled monolayers (SAMs) to supersonic beams of atomic reagents. The use of a SAM stabilizes highly energetic intermediates formed from bimolecular reactions at the vacuum/film interface due to rapid thermal equilibration with the SAM matrix that are otherwise not readily observed under single-collision conditions in the gas phase. In this paper, we will discuss the elucidation of the mechanistic details for the fundamental reaction between O(P-3) and phenyl-substituted alkyne bonds by monitoring chemical and structural changes in an oligo(phenylene ethynylene) SAM reacting with O(P-3) under collision conditions having specified initial reaction orientation. Utilizing time-resolved reflection absorption infrared spectroscopy (RAIRS) and scanning tunneling microscopy (STM) under ultrahigh vacuum conditions, we have confirmed electrophilic addition of O(P-3) onto the alkyne moieties, resulting in formation of a ketene intermediate via phenyl migration. Under single-collision conditions in the gas phase, the vibrationally excited ketene intermediate cleaves to release CO. In contrast to this, formation of the condensed-phase stabilized singlet ketene is observed using RAIRS. Moreover, we have observed that the phenyl ring at the vacuum/film interface significantly cants toward the substrate plane as a result of this reaction. STM images of the SAM taken before and after O(P-3) exposure show an expansion of the ordered lattice resulting from formation of the new nonlinear molecular structures within the SAM, and the reaction preferentially propagates along the lattice direction of the monolayer domain. This approach of using preoriented reactive molecules in ordered SAMs in combination with angle- and velocity-selected energetic reagents provides a general approach for probing geometric constraints associated with reaction dynamics for a wide range of chemical reactions.