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1.  Laser-Induced Acoustic Desorption of Natural and Functionalized Biochromophores 
Analytical Chemistry  2015;87(11):5614-5619.
Laser-induced acoustic desorption (LIAD) has recently been established as a tool for analytical chemistry. It is capable of launching intact, neutral, or low charged molecules into a high vacuum environment. This makes it ideally suited to mass spectrometry. LIAD can be used with fragile biomolecules and very massive compounds alike. Here, we apply LIAD time-of-flight mass spectrometry (TOF-MS) to the natural biochromophores chlorophyll, hemin, bilirubin, and biliverdin and to high mass fluoroalkyl-functionalized porphyrins. We characterize the variation in the molecular fragmentation patterns as a function of the desorption and the VUV postionization laser intensity. We find that LIAD can produce molecular beams an order of magnitude slower than matrix-assisted laser desorption (MALD), although this depends on the substrate material. Using titanium foils we observe a most probable velocity of 20 m/s for functionalized molecules with a mass m = 10 000 Da.
doi:10.1021/acs.analchem.5b00601
PMCID: PMC4455108  PMID: 25946522
2.  Single-Photon Ionization of Organic Molecules Beyond 10 kDa 
The volatilization and soft ionization of complex neutral macromolecules at low energies has remained an outstanding challenge for several decades [1]. Most volatilization techniques in mass spectrometry produce ions already in the source and most of them lead to particle velocities in excess of several hundred meters per second. For many macromolecules, post-ionization is inefficient since electronic or optical excitations can be followed by competing non-ionizing internal conversion, electron recapture, or fragmentation processes. Here, we explore the laser-assisted volatilization of neutral perfluoroalkyl-functionalized tetraphenylporphyrins as well as their single-photon ionization using vacuum ultraviolet (VUV) light at 157 nm. A systematic investigation of the ionization curves allows us to determine the molecular velocity distribution and ionization cross sections. We demonstrate the detection of single photon ionized intact organic molecules in excess of 10 kDa from a slow molecular beam.
doi:10.1007/s13361-012-0551-3
PMCID: PMC3622019  PMID: 23444050
Single-photon ionization; VUV ionization; Slow molecular beams; Tailored porphyrin derivatives
3.  Direct monitoring of opto-mechanical switching of self-assembled monolayer films containing the azobenzene group 
Summary
The potential for manipulation and control inherent in molecule-based motors holds great scientific and technological promise. Molecules containing the azobenzene group have been heavily studied in this context. While the effects of the cis–trans isomerization of the azo group in such molecules have been examined macroscopically by a number of techniques, modulations of the elastic modulus upon isomerization in self-assembled films were not yet measured directly. Here, we examine the mechanical response upon optical switching of bis[(1,1'-biphenyl)-4-yl]diazene organized in a self-assembled film on Au islands, using atomic force microscopy. Analysis of higher harmonics by means of a torsional harmonic cantilever allowed real-time extraction of mechanical data. Quantitative analysis of elastic modulus maps obtained simultaneously with topographic images show that the modulus of the cis-form is approximately twice that of the trans-isomer. Quantum mechanical and molecular dynamics studies show good agreement with this experimental result, and indicate that the stiffer response in the cis-form comprises contributions both from the individual molecular bonds and from intermolecular interactions in the film. These results demonstrate the power and insights gained from cutting-edge AFM technologies, and advanced computational methods.
doi:10.3762/bjnano.2.93
PMCID: PMC3257510  PMID: 22259768
AFM; azobenzene; elastic modulus; molecular dynamics; nanomechanics; photoswitch; quantum mechanics computation; self-assembled monolayer
4.  Quantum interference of large organic molecules 
Nature Communications  2011;2:263-.
The wave nature of matter is a key ingredient of quantum physics and yet it defies our classical intuition. First proposed by Louis de Broglie a century ago, it has since been confirmed with a variety of particles from electrons up to molecules. Here we demonstrate new high-contrast quantum experiments with large and massive tailor-made organic molecules in a near-field interferometer. Our experiments prove the quantum wave nature and delocalization of compounds composed of up to 430 atoms, with a maximal size of up to 60 Å, masses up to m=6,910 AMU and de Broglie wavelengths down to λdB=h/mv≃1 pm. We show that even complex systems, with more than 1,000 internal degrees of freedom, can be prepared in quantum states that are sufficiently well isolated from their environment to avoid decoherence and to show almost perfect coherence.
Observing superposition states of mesoscopic quantum systems is an ongoing challenge. Gerlich et al. report quantum interference of large tailor-made organic compounds, demonstrating delocalization and the quantum wave nature of entire molecules composed of up to 430 atoms.
doi:10.1038/ncomms1263
PMCID: PMC3104521  PMID: 21468015
5.  Temperature and magnetic field dependence of a Kondo system in the weak coupling regime 
Nature Communications  2013;4:2110.
The Kondo effect arises due to the interaction between a localized spin and the electrons of a surrounding host. Studies of individual magnetic impurities by scanning tunneling spectroscopy have renewed interest in Kondo physics; however, a quantitative comparison with theoretical predictions remained challenging. Here we show that the zero-bias anomaly detected on an organic radical weakly coupled to a Au (111) surface can be described with astonishing agreement by perturbation theory as originally developed by Kondo 60 years ago. Our results demonstrate that Kondo physics can only be fully conceived by studying both temperature and magnetic field dependence of the resonance. The identification of a spin 1/2 Kondo system is of relevance not only as a benchmark for predictions for Kondo physics but also for correlated electron materials in general.
A lot of theoretical work on the Kondo effect has focused on spin 1/2 systems, but the characterization of a single-spin 1/2 atom or molecule in the weak coupling regime has been missing. Here, the authors close this gap with a scanning tunneling spectroscopy study of an organic radical on a gold surface.
doi:10.1038/ncomms3110
PMCID: PMC3730050  PMID: 23817525
6.  Stability of high-mass molecular libraries: the role of the oligoporphyrin core 
Journal of Mass Spectrometry  2015;50(1):235-239.
Molecular beam techniques are a key to many experiments in physical chemistry and quantum optics. In particular, advanced matter-wave experiments with high-mass molecules profit from the availability of slow, neutral and mass-selected molecular beams that are sufficiently stable to remain intact during laser heating and photoionization mass spectrometry. We present experiments on the photostability with molecular libraries of tailored oligoporphyrins with masses up to 25 000 Da. We compare two fluoroalkylsulfanyl-functionalized libraries based on two different molecular cores that offer the same number of anchor points for functionalization but differ in their geometry and electronic properties. A pentaporphyrin core stabilizes a library of chemically well-defined molecules with more than 1600 atoms. They can be neutrally desorbed with velocities as low as 20 m/s and efficiently analyzed in photoionization mass spectrometry. Copyright © 2015 John Wiley & Sons, Ltd.
doi:10.1002/jms.3526
PMCID: PMC4322477  PMID: 25601698
macromolecular beams; laser desorption; photoionization; TOF-MS; molecular libraries

Results 1-6 (6)