Severin Schneebeli | Columbia University (original) (raw)
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Papers by Severin Schneebeli
Recent experiments have shown that use of tri-methyl tin (SnMe3) link groups results in the forma... more Recent experiments have shown that use of tri-methyl tin (SnMe3) link groups results in the formation of alkane single molecule junctions with measured conductance ˜100 times higher than found for any other link group previously used. Further evidence points to the in-situ formation of direct Au-C bonds to the electrode. In this work we use Density-Functional Theory based calculations to study the formation and structure of junctions based on direct Au-C link bonds. Transport calculations based on Non-Equilibrium Green's Functions for benzene and alkane molecules anchored through Au-C bonds show that the alkane backbone couples more strongly to the leads, resulting in a higher transmission as compared with other link groups. In the case of benzene, however, transport is primarily through the σ system, yielding a smaller conductance increase. Finally, we discuss corrections to the position of molecular resonances found in the DFT-based calculations and the implications for conductance.
Organic Letters, 2010
The effect of the partial antiaromaticity of biphenylene on its substitution chemistry, its oxida... more The effect of the partial antiaromaticity of biphenylene on its substitution chemistry, its oxidation potential, and its single-molecule conductance is explored. Biphenylene and fluorene molecules with linkers of two amino groups or two cyclic thioether groups were synthesized and their conduction properties were investigated using scanning tunneling microscopy (STM) break-junction techniques and DFT calculations. Despite the partial antiaromaticity of biphenylene, which causes the biphenylenes to be much more easily oxidizable, no significant increase in molecular conductance was found.
We present a novel method to directly link single alkane chains to gold electrodes using trimethy... more We present a novel method to directly link single alkane chains to gold electrodes using trimethyl tin (SnMe3) linkers. We characterize electron transport through single molecule junctions using the STM-based break-junction technique, where a gold point contact is repeatedly formed and broken in a solution of the SnMe3-alkanes while conductance is measured. Based on analysis of more than 10,000 individual junctions, we find that we create single molecule junctions which are ˜100 times more conducting than those with alkanes terminated with any other linker previously studied. The contact resistance, determined by extrapolating to zero carbons, is 4kφ, two orders of magnitude lower than analogous values found using amine linkers. Strong evidence supports the hypothesis that in situ cleaving of the SnMe3 end groups facilitates the formation of a direct bond between the carbon backbone and gold leads, thereby enhancing conductance. We corroborate this result by comparing the conductance of junctions formed from SnMe3- and Ph3PAu-terminated benzenes.
Journal of The American Chemical Society, 2009
Herein we report the first fully quantum mechanical study of enantioselectivity for a large data ... more Herein we report the first fully quantum mechanical study of enantioselectivity for a large data set. We show that transition state modeling at the UB3LYP-DFT/6-31G* level of theory can accurately model enantioselectivity for various dioxirane-catalyzed asymmetric epoxidations. All the synthetically useful high selectivities are successfully "predicted" by this method. Our results hint at the utility of this method to further model other asymmetric reactions and facilitate the discovery process for the experimental organic chemist. Our work suggests the possibility of using computational methods not simply to explain organic phenomena, but also to predict them quantitatively.
We compare experimentally measured single-molecule conductances with Density-Functional Theory (D... more We compare experimentally measured single-molecule conductances with Density-Functional Theory (DFT) based calculations for a series of ``double-decker'' molecules in which two parallel backbones are held between a common contact group on each end, consisting of S(CH2)2 units. In the simplest example, the two backbones are both phenyl groups, but an extensive series has been studied in which one or both parallel backbones are varied to include either short alkane segments, phenyl groups with substituents or fluorenes. Single molecule transport measurements using the STM-break-junction technique reveal that these molecules have multiple conducting states, suggesting that the single molecule junction geometry plays an important role in the outcome of these experiments. Density Functional Theory based calculations are used to explore the role of the link geometry on the conductance and to understand the relationship between the double-decker structure and the conductance.
Journal of The American Chemical Society, 2011
We measure electronic conductance through single conjugated molecules bonded to Au metal electrod... more We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ resulting in formation of a direct covalent sigma bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated pi-system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the pi-system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls with two to four phenyl units show a hundred-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling as they exhibit an exponential dependence of conductance with oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission with a cross-over to tunneling for the longer oligomers.
Recent experiments have shown that use of tri-methyl tin (SnMe3) link groups results in the forma... more Recent experiments have shown that use of tri-methyl tin (SnMe3) link groups results in the formation of alkane single molecule junctions with measured conductance ˜100 times higher than found for any other link group previously used. Further evidence points to the in-situ formation of direct Au-C bonds to the electrode. In this work we use Density-Functional Theory based calculations to study the formation and structure of junctions based on direct Au-C link bonds. Transport calculations based on Non-Equilibrium Green's Functions for benzene and alkane molecules anchored through Au-C bonds show that the alkane backbone couples more strongly to the leads, resulting in a higher transmission as compared with other link groups. In the case of benzene, however, transport is primarily through the σ system, yielding a smaller conductance increase. Finally, we discuss corrections to the position of molecular resonances found in the DFT-based calculations and the implications for conductance.
Organic Letters, 2010
The effect of the partial antiaromaticity of biphenylene on its substitution chemistry, its oxida... more The effect of the partial antiaromaticity of biphenylene on its substitution chemistry, its oxidation potential, and its single-molecule conductance is explored. Biphenylene and fluorene molecules with linkers of two amino groups or two cyclic thioether groups were synthesized and their conduction properties were investigated using scanning tunneling microscopy (STM) break-junction techniques and DFT calculations. Despite the partial antiaromaticity of biphenylene, which causes the biphenylenes to be much more easily oxidizable, no significant increase in molecular conductance was found.
We present a novel method to directly link single alkane chains to gold electrodes using trimethy... more We present a novel method to directly link single alkane chains to gold electrodes using trimethyl tin (SnMe3) linkers. We characterize electron transport through single molecule junctions using the STM-based break-junction technique, where a gold point contact is repeatedly formed and broken in a solution of the SnMe3-alkanes while conductance is measured. Based on analysis of more than 10,000 individual junctions, we find that we create single molecule junctions which are ˜100 times more conducting than those with alkanes terminated with any other linker previously studied. The contact resistance, determined by extrapolating to zero carbons, is 4kφ, two orders of magnitude lower than analogous values found using amine linkers. Strong evidence supports the hypothesis that in situ cleaving of the SnMe3 end groups facilitates the formation of a direct bond between the carbon backbone and gold leads, thereby enhancing conductance. We corroborate this result by comparing the conductance of junctions formed from SnMe3- and Ph3PAu-terminated benzenes.
Journal of The American Chemical Society, 2009
Herein we report the first fully quantum mechanical study of enantioselectivity for a large data ... more Herein we report the first fully quantum mechanical study of enantioselectivity for a large data set. We show that transition state modeling at the UB3LYP-DFT/6-31G* level of theory can accurately model enantioselectivity for various dioxirane-catalyzed asymmetric epoxidations. All the synthetically useful high selectivities are successfully "predicted" by this method. Our results hint at the utility of this method to further model other asymmetric reactions and facilitate the discovery process for the experimental organic chemist. Our work suggests the possibility of using computational methods not simply to explain organic phenomena, but also to predict them quantitatively.
We compare experimentally measured single-molecule conductances with Density-Functional Theory (D... more We compare experimentally measured single-molecule conductances with Density-Functional Theory (DFT) based calculations for a series of ``double-decker'' molecules in which two parallel backbones are held between a common contact group on each end, consisting of S(CH2)2 units. In the simplest example, the two backbones are both phenyl groups, but an extensive series has been studied in which one or both parallel backbones are varied to include either short alkane segments, phenyl groups with substituents or fluorenes. Single molecule transport measurements using the STM-break-junction technique reveal that these molecules have multiple conducting states, suggesting that the single molecule junction geometry plays an important role in the outcome of these experiments. Density Functional Theory based calculations are used to explore the role of the link geometry on the conductance and to understand the relationship between the double-decker structure and the conductance.
Journal of The American Chemical Society, 2011
We measure electronic conductance through single conjugated molecules bonded to Au metal electrod... more We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ resulting in formation of a direct covalent sigma bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated pi-system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the pi-system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls with two to four phenyl units show a hundred-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling as they exhibit an exponential dependence of conductance with oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission with a cross-over to tunneling for the longer oligomers.