AVS2016 Session SS-ThM: Chirality and Enantioselectivity on Surfaces; Ionic Liquid Interfaces

Chiral self-assembled structures formed from organic molecules adsorbed on surfaces can be studied at high resolution by Scanning Tunneling Microscopy under Ultra-High Vacuum conditions. Through collaboration between UHV-STM experiments and organic synthesis, we have investigated how molecular conformational flexibility can lead to new chiral effects in the form of chiral switching, chiral accommodation and chiral induction, allowing transfer of chirality from the molecular to the supra-molecular level [1-5]. To generalize these principles, we have recently developed a new system of 3-bit binary conformational switches (Oligo-Naphthalene Ethynylenes) [6, 7] and in particular quantified their dynamic conformational switching and collective effects in ordering using time-resolved STM movies.

[1] Weigelt, S.; Busse. C.; Petersen, L.; Rauls, E.; Hammer, B.; Gothelf, K.V.; Besenbacher, F.; Linderoth, T.R. Nature Materials, 2006, 5 11.

[2] Bombis, C.; Weigelt, S.; Knudsen, M. M.; Nørgaard, M.; Busse, C.; Lægsgaard, E.; Besenbacher, F.; Gothelf, K. V.; Linderoth, T. R. ACS NANO 20104, 297.

[3] Knudsen, M.; Kalashnyk, N.; Masini, F.; Cramer, J.; Lægsgaard, E.: Besenbacher, F.; Linderoth, T. R.; Gothelf, K. .V. Journ. Am. Chem. Soc. 2011, 133, 4896.

[4] Masini, F.; Kalashnyk, N.; Knudsen, M. M.; Cramer, J. R.; Lægsgaard, E.; Besenbacher, F.; Gothelf, K. V.; Linderoth, T. R. Journ. Am. Chem. Soc. 2011,133 13910.

[5] Nuermaimaiti, A.; Bombis, C.; Knudsen, M. M.,; Cramer, J.; Lægsgaard, E.; Besenbacher, F.; Gothelf, K. V.; Linderoth, T. R. ACS Nano 2014,8, 8074.

[6] Cramer, J. R.; Ning, Y.; Shen, C.; Nuermaimaiti, A.; Besenbacher, F.: Linderoth, T. R.; Gothelf, K.V. Eur. J. Org. Chem. 2013 2813.

[7] Ning, Y.; Cramer, J. R.; Nuermaimaiti, A.; Svane, K.; Yu, M.; Lægsgaard, E.; Besenbacher, F.; Xue, Q.-K. ; Ma, X.; Hammer, B.; Gothelf, K. V.; Linderoth, T. R. J. Chem. Phys. 2015142, 101922.

Pentagonal substituted chiral buckybowls form 2D racemate crystals, but chiral hemifullerene restructures a copper surface such that special chiral kinks become stabilized by the enantiomers.²

1 M. Parschau, K.-H. Ernst, Angew. Chem. Int. Ed. 2015, 54, 14422.

2 W. Xiao, K.-H. Ernst et al., Nature Chemistry2016, in press.

There is rapid progress in the design and application of chirally-modified metal particles for heterogeneous catalytic enantioselective reactions. Modification is achieved by adsorbing chiral molecules to create asymmetric active sites. Great advances are being made in operando studies, surface science studies and theoretical studies of such systems. We will describe combined experimental and theoretical work related to the enantioselective hydrogenation of activated ketones on chirally-modified Pt(111). We will present results from combined variable temperature STM and optB88-vdW DFT studies of individual bimolecular docking complexes formed between enantiopure 1-(1-naphthyl)ethylamine, and related molecules, and representative prochiral substrates. The experiments reveal sub-molecularly resolved site-specific and stereospecific data. Single chemisorbed enanantiomers simultaneously present several chiral pockets, each displaying a specific prochiral ratio for a given substrate molecule. A hierarchy of chemisorption and intermolecular interactions is found to control prochiral selection at each pocket. Fine-tuning the structure of the modifier reveals how sensitive the stereochemical outcome is to even minor molecular changes. Time-lapsed STM measurements of individual substrate molecules sampling a set of chiral pockets provide new insight on stereocontrol, and on reaction paths and barriers at individual binding sites.

The diastereomeric interactions and reactivity of co-adsorbed methyl pyruvate (MP) and R-1-(1-naphthyl)ethylamine (NEA) are studied on a Pd(111) model single crystal catalyst using temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM) with the aim of identifying the nature of the interactions and understanding the observed enhancement in hydrogenation activity of MP to methyl lactate on NEA-modified surfaces. The enhancement in hydrogenation activity of chirally modified sites is critical in designing heterogeneous enantioselective catalysts by mitigating the effect of unmodified racemic sites. TPD experiments of coadsorbed MP and hydrogen or deuterium on Pd(111) reveal that NEA accelerates the rates of both MP hydrogenation and H/D exchange. The measured STM images of docking complexes on Pd(111) are classified according to the angles between the long axes of MP and the naphthyl ring of NEA, which fall into well-defined ranges of 10 ± 5, 45 ± 5 and 70 ± 5º. Docking structures are modeled using first-principles density functional theory (DFT) calculations that include van der Waals interactions, and STM image simulations. Excellent agreement is found both between the shapes of the calculated and simulated images and their theoretical and experimental dihedral angle distributions, thereby confirming the validity of the calculations. Diastereomeric interactions between NEA and MP occur predominantly by binding of the carbon-carbon double bond of the enol tautomer of MP to the surface, while simultaneously optimizing C=O····H₂N hydrogen-bonding interactions. The combination of chiral-NEA driven diastereomeric docking with a tautomeric preference enhances the hydrogenation activity since C=C bonds hydrogenate more easily than C=O bonds. This model provides a rationale for the catalytic observations.

Laboratory studies have mainly focused on atomically flat and chemically homogeneous substrates, i.e. on ideal surfaces, to understand fundamental mechanisms. In our current work we are exploring the influence of nanoscale heterogeneities on ionic-liquid interfacial properties. Such heterogeneities can be composed of contamination, roughness or chemical surface groups. The strong molecular interactions still facilitate self-assembly of ILs on the surfaces, but they reveal new aspects of the IL behavior. We address diverse fundamental questions about the interfacial IL structure and the response to shear in the presence of heterogeneities. Further, these studies are of relevance to extend studies performed on ideal systems to real applications.

Ionic liquids are room temperature molten salts comprised entirely of cations and anions. They have an array of unique physico-chemical properties that have led to their use in a wide variety of electrochemical systems.^[1] These include actuators, corrosion inhibitors, energy storage for batteries, supercapacitors, displays and as the electrolyte in photovoltaic devices. Both the function and performance of devices employing ionic liquids are dependent on how the ionic liquid behaves at phase boundaries, interfaces and near interfacial areas. Previous studies have shown that ionic liquids have a tendency to self organize at the IL/solid interface.^[2] Such ordering has been shown to reduce the barrier to charge injection when ionic liquids are used in oxide solar cells.^[3] As such, the ordering and chemistry of ionic liquids at the anatase TiO₂(101) surface is of some interest since this is the dominant surface in mesoporous TiO₂ films used in dye sensitized solar cells.

Although there is a growing body of work on the surface chemistry of bulk ionic liquids studied by photoelectron spectroscopy, studies of their interaction with solid surfaces are still relatively rare. On this basis, in an attempt to address the gap in the work, we adopted a surface science approach in which we utilized a combination of X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. From this we were able to successfully determine the orientation of the cation on the surface of anatase(101), at both high and low coverage, and also the manner in which it adsorbs. Additionally, a surface induced degradation reaction of the anion was observed at room temperature.

[1] K. R. J. Lovelock et al.Chem. Rev., 2010, 110, 5158–90.

[2] E. Binetti et al.J. Phys. Chem. C, 2013, 117, 12923–12929.

[3] B. R. Lee et al. J. Mater. Chem., 2011, 21, 2051.

[Introduction] A solution of the loss of heat by using black dye in a dye-sensitization solar cell is expending the width of the light absorbing wavelength by growing multi-dye with narrow wavelength to reduce the loss of heat. So, we supply and make the molecules grow according to the reaction between the molecules, and apply the multi-dye layer film by using Liquid-Phase Molecular Layer Deposition. By way of LP-MLD, the multi-dye layer forms on the surface of ZnO, and we test the expansion effect of the photocurrent spectrum width.

[Growth of multi-dye layer film]First, adsorb the p-type dye(Rose Bengal)molecules p1 on the surface. Because there is no reaction between the same molecules, and they don't combine each other. So the first layer is completed on the surface of the substrate which the surface is filled with p1.Second, remove the p1 by IPA(isopropyl alcohol), then supply the n1,molecules of n-type dye(Brilliant Green).p1 and n1 react and they combined. The same as step 1,the p1 layer is filled with n1,then the second layer is completed. Step 3 is using p-type dye(Eosin Y) molecules again. Repeat these steps, adsorb the molecules in the array as designed, and complete the multi-dye molecule layer film.

[Measurement of the photocurrent in LP-MLD]LP-MLD is a technology using the combination power between molecules of p-type dye and molecule of n-type dye to make a layer. Use IPA as the solvent to make a solution with the concentration of 1.6 x 10^-2mol/L. Set the substrate in a cell,inject the cell with IPA until the cell is full of it. Then inject the solution of p1 to make the dye molecules adsorbed by the substrate. After 20 minutes, in order to remove p1 solution in the cell, inject the cell with IPA again. After putting the laser on the substrate and measuring the photocurrent, repeat the process with other dye solution. Base on the measurement, the expansion effect of the photocurrent spectrum width was deduced.