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

Abstract

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( 3 P) and phenyl-substituted alkyne bonds by monitoring chemical and structural changes in an oligo(phenylene ethynylene) SAM reacting with O( 3 P) 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( 3 P) 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( 3 P) 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.

Atomic Hydrogen Reactions of Alkanethiols on Au(111): Phase Transitions at Elevated Temperatures

“…There has been significant interest in how alkanethiolate monolayers change when exposed to energetic environments that include reactive species such as hyperthermal rare-gas atoms, atomic radicals, and ions. − Hydrogen-atom reactions with alkanethiolates have received particular attention, starting with the work of Fairbrother and co-workers in 2007, who showed that there are two pathways for the reaction: (1) hydrogenation of the sulfur–gold bond at the monolayer–substrate interface, leading to the desorption of the molecule from the surface, and (2) hydrogen abstraction from the hydrocarbon backbone, producing surface radicals that eventually lead to cross-linking between neighboring carbon chains . For short-chain monolayers (≤12 carbon atoms) the first reaction dominates and hydrogen-atom exposure eventually leads to complete removal of organics from the surface.…”

Hydrogen Abstraction Probability in Reactions of Gas-Phase NO3 with an OH-Functionalized Organic Surface

“…Similar to the experiments described in recent work by Sibener et al, RAIR spectra were provided by a Bruker IFS 66v/S spectrometer interfaced with the UHV chamber. Focused IR radiation from a SiC globar was reflected from the gold surface at ∼86° relative to the surface normal through a differentially pumped KBr window .…”

“…Specifically, the work presented below describes an ultrahigh vacuum (UHV) surface science study into the reaction between a well-controlled flux of NO 3 and well-characterized hydroxyl-terminated self-assembled monolayers (SAMs). The Sibener group (honored in this special Festschrift issue) pioneered the use of alkanethiol SAMs on gold surfaces as model substrates for fundamental studies of gas–surface energy exchange and reactivity. , Their UHV-based work involving molecular beam scattering from SAMs provided detailed insight into the dynamics of interfacial collisions and has motivated numerous other studies into these interesting systems. − The work described below employs many of the methods highlighted in their studies. Specifically, by tracking the rate of bond rupture and formation during the gas–surface collision, we have determined the initial reaction probability and identified the likely initial reaction mechanism for this important interfacial oxidation process.…”

Ordering and dynamics of oligo(phenylene ethynylene) self-assembled monolayers on Au(111)

“…The structural and electronic properties of PPB-S have been extensively investigated by scanning tunneling microscopy (STM). 22,[25][26][27][28][29][30][31][32][33] In two previous studies structural models for the PPB-S SAM have been proposed. 30,31 Both articles reported the formation of densely packed SAMs, where the molecules are ordered in several rotational invariant domains.…”