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Silver-based heterogeneous catalysts, modified with a range of elements, have found industrial application in several reactions in which selectivity is a challenge. Alloying small amounts of Pt into Ag has the potential to greatly enhance the somewhat low reactivity of Ag while maintaining high selectivity and resilience to poisoning. This single-atom alloy approach has had many successes for other alloy combinations but has yet to be investigated for PtAg. Using scanning tunneling microscopy (STM) and STM-based spectroscopy, we characterized the atomic-scale surface structure of a range of submonolayer amounts of Pt deposited on and in Ag(111) as a function of temperature. Near room temperature, intermixing of PtAg results in multiple metastable structures on the surface. Increasing the alloying temperature results in a higher concentration of isolated Pt atoms in the regions near Ag step edges as well as direct exchange of Pt atoms into Ag terraces. Furthermore, STM-based work function measurements allow us to identify Pt rich areas of the samples. We use CO temperature programmed desorption to confirm our STM assignments and quantify CO binding strengths that are compared with theory. Importantly, we find that CO, a common catalyst poison, binds more weakly to Pt atoms in the Ag surface than extended Pt ensembles. Taken together, this atomic-scale characterization of model PtAg surface alloys provides a starting point to investigate how the size and structure of Pt ensembles affect reaction pathways on the alloy and can inform the design of alloy catalysts with improved catalytic properties and resilience to poisoning.

We introduce a new pentacene-based organic semiconductor, 5,6,7-trithiapentacene-13-one (TTPO). TTPO is a small-molecule organic semiconductor that is simple to synthesize and purify, readily crystallizes, melts in air from 386−388 °C without decomposition, and is indefinitely stable against degradation in acid-free solution. TTPO has a high molar absorptivity, optical and electrochemical HOMO− LUMO gaps of 1.90 and 1.71 eV, respectively, and can be thermally evaporated to produce highly uniform thin films. Its cyclic voltammogram reveals one reversible oxidation and two reversible reductions between +1.5 and −1.5 V. The crystal structure for TTPO has been solved and its unique parallel displaced, and head-to-tail packing arrangement has been examined and explained using high-level density functional theory. High-resolution scanning tunneling microscopy (STM) was used to image individual TTPO molecules upon assembly on a pristine Au(111) surface in ultrahigh vacuum. STM images reveal that vapor-deposited TTPO molecules nucleate in a unique stacked geometry with a small acute angle with respect to Au(111) surface. Preliminary TTPO-based bilayer photovoltaic devices show increases in short circuit current density upon heating from 25 to 80 °C with a concomitant 4−160-fold increase in power conversion efficiencies. TTPO has the potential to be used in thinfilm electronic devices that require operation over a wide range of temperatures such as thin-film transistors, sensors, switches, and solar cells.

Surface-bound molecular rotors provide a useful way to study the structure and dynamics of molecular motion at the single-molecule level. However, when most molecules adsorb on a metal surface, their interaction with the metal changes their properties dramatically, making a priori design impossible. We report a case in which gas-phase predictions of the stable orientations of a class of molecular rotors hold true when they are attached to a surface. This transferability is achieved by mounting the molecular rotor moiety on a metal–organic complex formed as an intermediate in the surface-catalyzed Ullmann coupling reaction of 1-bromo-4-ethylbenzene versus 1-bromo-4-methoxybenzene. Gas-phase calculations predict that, while the ethyl molecular rotor is most stable when oriented perpendicular to the phenyl ring, the methoxy rotor’s stable orientation is in plane with the phenyl ring. Our STM imaging results confirm this behavior, with the methoxy rotor exhibiting switching in plane with the surface versus the ethyl rotor, which switches out of plane with respect to the surface. Furthermore, the two rotors exhibit different rotational excitation characteristics. Action spectra measurements reveal that, while the threshold voltage for direct excitation of the rotational process of the ethyl rotor is identical to the rotational barrier (45 meV), the methoxy rotors require a significantly larger applied voltage (300 mV) than the 128 meV torsional barrier calculated for methoxybenzene in the gas phase. Density functional theory (DFT) calculations of a methoxybenzene molecule on Cu(111) reveal that, while interaction with the Cu(111) surface does not change the preferred orientations of the methoxy rotor, the barrier for rotation is raised to 246 meV, which is much closer to that observed experimentally. This study offers insight into the factors determining the dynamics of molecular rotors based on both the chemical nature of the rotor and its interaction with the surface.

The delicate balance between hydrogen bonding and van der Waals interactions determines the stability, structure, and chirality of many molecular and supramolecular aggregates weakly adsorbed on solid surfaces. Yet the inherent complexity of these systems makes their experimental study at the molecular level very challenging. In this quest, small alcohols adsorbed on metal surfaces have become a useful model system to gain fundamental insight into the interplay of such molecule-surface and molecule-molecule interactions. Here, through a combination of scanning tunneling microscopy and density functional theory, we compare and contrast the adsorption and self-assembly of a range of small alcohols from methanol to butanol on Au(111). We find that longer chained alcohols prefer to form zigzag chains held together by extended hydrogen bonded networks between adjacent molecules. When alcohols bind to a metal surface datively via one of the two lone electron pairs of the oxygen atom, they become chiral. Therefore, the chain structures are formed by a hydrogen-bonded network between adjacent molecules with alternating adsorbed chirality. These chain structures accommodate longer alkyl tails through larger unit cells, while the position of the hydroxyl group within the alcohol molecule can produce denser unit cells that maximize intermolecular interactions. Interestingly, when intrinsic chirality is introduced into the molecule as in the case of 2-butanol, the assembly changes completely and square packing structures with chiral pockets are observed. This is rationalized by the fact that the intrinsic chirality of the molecule directs the chirality of the adsorbed hydroxyl group meaning that heterochiral chain structures cannot form. Overall this study provides a general framework for understanding the effect of simple alcohol molecular adstructures on hydrogen bonded aggregates and paves the way for rationalizing 2D chiral supramolecular assembly.