Monday, 20.08.2012
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09:30
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introduction, welcome
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Session 1: Chair: Gernot Friedrichs
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09:40
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Majed Chergui
Lausanne, Switzerland
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Ultrafast switching of spin and structure
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show abstract
We will present recent results concerning ultrafast intramolecular relaxation processes in metal-based molecular complexes, involving changes of spin and structure. Using a broad range of ultrafast optical and X-ray tools we present studies of the ultrafast electronic/vibrational relaxation at sub-vibrational time scales, ultrafast intersystem crossing processes and ultrafast structure changes. We will stress the points that allow a rationale to be developed, that describes these processes. The systems we investigate consist of Fe, Ru, Re, Pt and Cu complexes in solutions.
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10:25
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Friedrich Temps
Kiel
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Ultrafast Dynamics of Molecular Photoswitches with Intramolecular and Intermolecular Constraints
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show abstract
The development of rational design strategies for optimized photochromic molecular switches requires detailed knowledge about the dynamics of the ensuing ultrafast photo-induced chemical transformations and the competing radiationless electronic deactivation processes that determine the switching efficiencies and the quantum yields. Moreover, the application potential of the photoswitches as tiny light-driven switches, manipulators, actuators or memories depends critically on how the surrounding matrix affects the photo-induced transformations. In order to elucidate these topics, we have investigated the influences of severe intramolecular or intermolecular constraints on the dynamics of selected photoswitches in solution and in polymeric micronetworks. For a diazocine (an azobenzene derivative with an eight-membered ring) and for a set of furylfulgides, we observed strongly accelerated photoreactions in only ≈ 50 fs by intramolecular constraints leading to directed movement on the excited-state potential energy hypersurfaces to the conical intersection regions that control the molecular transformations. Strong intermolecular mechanical forces, on the other hand, acting on a cross-linked trans-azobenzene switch in the main-chain of a polybutylmethacrylate matrix were found to impede the photoreaction (τ ≈ 180 ps). In multi-azobenzene compounds, we have observed strong chromophore-chromophore interactions and electronic coupling between two resp. three addressable azobenzene units connected by a central nitrogen atom. The talk will give an overview over recent results obtained in the SFB.
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11:00
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coffee break
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Session 2: Chair: Richard Berndt
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11:30
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Andreas Hauser
Geneve, Switzerland
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Persistent Bidirectional Optical Spin State Switching in the 2D High-Spin Polymer {[Fe(bbtr)3](BF4)2}∞
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show abstract
The 2D coordination network [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) is known for its abrupt high-spin (HS) ⇔ low-spin (LS) transition at around 100 K and a thermal hysteresis of 13 K due to an accompanying crystallographic phase transition. [1] In the corresponding dilute mixed system [Zn1-xFex(bbtr)3](ClO4)2 (x = 0.02), the iron(II) centres stay essentially in the HS state down to 10 K. For increasing values of x, the LS fraction at 10 K increases. We report the HS⇔LS relaxation dynamics over a wide temperature range using wavelength selective excitation for the light-induced population of the LS state through irradiation in the near IR. Thus for instance for x = 0.02, the LS→HS relaxation rate constant increases by 14 orders of magnitude, [2,3] that is, from less than 5x10-7 s-1 (τ = 25 days) below 40 K to 3x107 s-1 (τ = 30 ns) at 220 K.
On the other hand, the neat BF4 - analogue stays in the HS state down to 10 K (see Fig. 1). It can, however, be quantitatively converted to the LS state by irradiating into the near-IR spin allowed 5 dd band and back again by irradiating into the visible 1 dd band. Depending upon the initial light-induced LS/HS ratio the system either returns to the HS state or goes fully to the LS state. In the latter case it only returns to the HS state above 100 K. Therefore, it shows true light induced bistability below 100K, [4] thus having the potential for persistent bidirectional optical switching at elevated temperatures. This behaviour is attributed to strong cooperative effects of elastic origin.
The variation of elastic interactions and the study of the dynamics in [Zn1-xFex(bbtr)3](ClO4)2 mixed crystals as well as in the BF4 - derivative using LIESST and reverse-LIESST could thus help to design highly cooperative systems for memory devices.
Figure 1: The HS fraction as function of temperature: on cooling from room temperature down to 10 K at a rate of 0.2 K/min (black dots), after irradiation at 12050 cm-1 at 10 K to the steady state high-spin fraction of 15% and subsequent warming to 120 K at a rate of 0.2 K/min (red dots) or stopping at 60 K and recooling to 10 K (yellow dots), after a short irradiation time at 12050 cm-1 and10 K resulting in an initial high-spin fraction of 85% and subsequent warming to 120 K at a rate of 0.2 K/min (green dots)
[1] I. Krivokapic, C. Enachescu, R. Bronisz, and A. Hauser, Chem. Phys. Lett. 2008, 455, 192.
[2] I. Krivokapic, P. Chakraborty, C. Enachescu, R. Bronisz and A. Hauser, Angew. Chem. Int. Ed. 2010, 49, 8509.
[3] I. Krivokapic, P. Chakraborty, C. Enachescu, R. Bronisz and A. Hauser, Inorg. Chem. 2011, 50, 1856.
[4] P. Chakraborty, R. Bronisz, C. Besnard, L. Guénée, P. Pattison, A. Hauser, J. Am. Chem. Soc. 134 (2012) 4049.
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12:15
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Nadine Hauptmann
Kiel
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Contacting single molecules: Force and conductance
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show abstract
Electron transport through single molecule contacts is the basis of designing molecular electronics and is capable of inducing switching processes. To understand the geometry of the junction that underlies its transport properties, the forces arising during conductance measurements in such contacts are important. We use low-temperature scanning tunneling and atomic force microscopy to simultaneously measure the force and conductance in single molecule contacts and show that the detailed electrode geometry and molecular structure strongly affect the conductance.
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12:50
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lunch (13:15-14:00), check-in
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Session 3: Chair: Ulrich Lüning
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15:05
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Marcel Dommaschk
Kiel
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Spin State Switching in Isolated Molecules at Room Temperature
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show abstract
Magnetic properties such as magnetic bistability (e.g. magnetization in ferromagnetic materials) are solid state properties. Numerous attempts notwithstanding, (mainly with miniaturization of magnetic storage media in mind) switchable magnetic systems are either restricted to very low temperatures (~4K) or to solid state materials. We present the first system which is bistable as an isolated molecule at room temperature. The operating principle is based on the fact that some transition metals are either dia- or paramagnetic depending on their coordination number. We designed and synthesized a molecular machine-like transition metal complex whose coordination number, and hence whose magnetic state can be reversibly switched by irradiation with green and blue light. Potential applications are smart contrast agents for magnetic resonance imaging (MRI), switchable spin labelling for NMR, and light-controlled magnetic levitation.
1. Light-induced Spin-Change by Photodissociable External Ligands: A New Principle for Magnetic Switching of Molecules. S. Thies, H. Sell, C. Schuett, C. Bornholdt, C. Naether, F. Tuczek and R. Herges, J. Am. Chem. Soc. 2011 , 133, 16243–16250.
2. Magnetic Bistability of Molecules in Homogeneous Solution at Room Temperature, S. Venkataramani, U. Jana, M. Dommaschk, F.D. Sönnichsen, F. Tuczek and R. Herges, Science 2011 , 331, 445-448.
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15:40
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Rainer Adelung
Kiel
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Hierarchical Materials for Photoswitchable Adhesives
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show abstract
Changes in color, magnetic moment, electrical conductivity or occupied volume are only a view of the fascinating properties of switchable molecules. These properties inspire to transfer them into applications with macroscopically visible effects. Often, this is a challenge for the used materials and requires new developments of material solutions that allow the integration of switchable molecules. Mainly on the example of the development of a bioinspired light switchable dry adhesive material the challenges and novel material solutions will be introduced. These include novel concepts to join mostly unjoinable polymers like PDMS (Silicone) and PTFE (Teflon) and ultralightweight materials like the recently developed Aerographite which is currently the least dense material.
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16:15
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coffee break
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Session 4: Chair: Stefan Heinze
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16:45
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Nicolae Atodiresei
Jülich
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Ab Initio Exploration of Hybrid Organic-Ferromagnetic Interfaces
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show abstract
Merging the concepts of molecular electronics with spintronics opens one of the most exciting avenue in designing and building future nanoelectronic devices. The ability to reliably describe the electronic properties of molecules adsorbed on magnetic surfaces is essential to understand and design the functionality of hybrid organic molecular devices. This strongly depends on the accuracy of the state-of-theart theoretical methods used to assess the interaction between a molecule or a molecular layer and a substrate of choice. Our theoretical studies [1-6] show that, with its predictive power, the density functional theory (DFT) provides a framework where a realistic description of these hybrid systems can be expected and can be further used to tune both the electronic and magnetic properties at hybrid organometallic interfaces. Besides this, our studies demonstrate the decisive role played by the van der Waals forces in correctly describing the interaction between aromatic molecules and metallic surfaces.
I will present conceptual studies performed to understand how to tailor the magnetic properties at a hybrid organic-ferromagnetic interface by adsorbing organic molecules containing π-electrons onto a magnetic substrate. For such hybrid systems, the magnetic properties like molecular magnetic moments and their spatial orientation can be specifically tuned by an appropriate choice of the chemical substituents. Our ab initio calculations demonstrate that, by employing an appropriate chemical functionalization of organic molecules adsorbed on a ferromagnetic surface, a fine tuning of the spin-unbalanced electronic structure can be achieved. For example, by using molecular substituents with different electronegativities attached to π- conjugated systems adsorbed on a ferromagnetic surface, the electrons with a specific spin [i.e. up (↑) and down (↓)] can selectively be injected at the molecular site from the same ferromagnetic substrate. Even more important, we show that there is direct correspondence between the substituent’s electronegativity and the size of the induced molecular magnetic moment. As regarding the stability of the magnetization direction of the hybrid organic-ferromagnetic system, we demonstrate that the adsorbed hydrogenated molecules destabilize more the out-of-plane magnetization of the ferromagnetic surface as compared to molecules containing more electronegative atoms as Cl and F which could also enhance it. Ultimately, this allows us to precisely engineer the magnetic properties of the hybrid organic-ferromagnetic interfaces which can be further exploited to design more efficient spintronic devices based on organic molecules.
References:
[1] N. Atodiresei, P. H. Dederichs, Y. Mokrousov, L. Bergqvist, G. Bihlmayer, S. Blügel, Phys. Rev. Lett. 100, 117207 (2008)
[2] N. Atodiresei, V. Caciuc, P. Lazic, S. Blügel, Phys. Rev. Lett. 102, 136809 (2009)
[3] N. Atodiresei, J. Brede, P. Lazic, V. Caciuc, G. Hoffmann, R. Wiesendanger, S. Blügel, Phys. Rev. Lett. 105, 066601 (2010)
[4] J. Brede, N. Atodiresei, S. Kuck, P. Lazic, V. Caciuc, Y. Morikawa, G. Hoffmann, S. Blügel, R. Wiesendanger, Phys. Rev. Lett. 105, 047204 (2010)
[5] C. Busse, P. Lazic, R. Djemour, J. Coraux, T. Gerber, N. Atodiresei, V. Caciuc, R. Brako, A. T. N'Diaye, S. Blügel, J. Zegenhagen, T. Michely, Phys. Rev. Lett. 107, 036101 (2011)
[6] N. Atodiresei, V. Caciuc, P. Lazic, S. Blügel, Phys. Rev. B 84, 172402 (2011)
The π-electrons of the cobalt-phthalocyanine molecule hybridize strong with the d-electrons of a magnetic metal and, as a consequence, a complex energy dependent magnetic structure is formed. Therefore, near the Fermi level, at the molecular site an inversion of the spin-polarization with respect to the ferromagnetic surface occurs. Our studies demonstrate that electrons of different spin [i.e. up (↑) and down (↓)] can selectively be injected from the same ferromagnetic surface by locally controlling the inversion of the spinpolarization.
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17:30
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Jan Boyke Schönborn
Kiel
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Shedding light on photochemistry
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show abstract
For use as motor units in any conceivable application, photochemically switching molecules have to show very specific properties. A rational design process of these properties requires a detailed unterstanding of the underlying ultrafast dynamical processes. This can only be achieved by close co-operation of experimental and theoretical investigations. Therefore, the Hartke group complements the state-of-the-art experimental work in SFB677 with high-end theoretical investigations. These include highly accurate multi-referential ab-initio electronic structure methods combined with direct dynamics simulations of the molecules of interest.
This talk will highlight examples of recent work in our group: Furylfulgides can act as molecular switches via interconversion between their E- and C-forms, by photochemically induced electrocyclic ring opening/closure at the central system of 6 pi-electrons. This interconversion is perturbed by the ability of the E-form to also interconvert to the Z-form. Additionally, all of these photochemical steps can either successfully form their respec- tive products or lead back to the reactants. In experiments, a strong sensitivity of this photochemical reaction network to seemingly minor substituent changes was found. Our theoretical investigations uncovered an intricate balance between the desired ”concerted” reaction of all 6 pi-electrons as a single chromophor and ”individual” photochemical steps of only 2 pi-electrons acting as in an ethylenic chromophor [1]. This balance is changed substantially by small alterations in the substituent pattern. Additionally, this picture is complicated further by the Z-form exhibiting electronic characteristics that differ from that of the E- and C-forms. Again, evidence for this may be visible in the experimental findings.
Current work focuses on coupled multi-chromophoric azobenzene-based systems. Simu- lations of static UV-spectra are in good agreement with experiment. Initial studies of a detailed balance between the individual chromophors acting on their own or as a strongly coupled, larger chromophor, will be presented in the talk.
References
[1] J. B. Sch ̈ onborn, A. Koslowski, W. Thiel, and B. Hartke. Phys. Chem. Chem. Phys., 2012. in press.
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18:15
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Mehdi Keshavarz Hedayati
Kiel
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Photoswitchable plasmonic matertials
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show abstract
Smart materials that react to a stimulus or their environment to produce a dynamic and reversible change in decisive properties are drawn the attention recently. Photochromic molecule is one of those materials which can turn any composite system into smart materials provided the host matrix is soft enough (e.g. polymer) to let the molecule rotate upon illumination. The exclusive properties of these molecules can be even more advantageous once it is integrated onto the devices whose optical properties are the matter of interest (e.g. plasmonic devices) and make them smart.
Here we highlight our recent works on optoelectronic devices that explore the interaction of plasmon resonance of the metals and dipoles, i.e. nanoparticles, molecules [1-4]. As specific highlight we introduce the first photoswitchable transparent conducting metal composed of Spirophenanthrooxazine (SPO) molecules and metallic film [5].
1. M. Elbahri, M.K. Hedayati, F. Faupel, T. Strunkus, V. Zaporojtchenko, V.S.K. Chakravanadhanula, “Metall-Komposit-Beschichtung mit hoher optischer Transmissivität im visuellen Spektrumpatent,” DE 102010050110.7 (Patent pending).
2. M. Elbahri, M.K. Hedayati, V.S.K. Chakravanadhanula, M. Jamali, T. Strunkus, V. Zaporojtchenko, F. Faupel, Adv. Mater., Vol. 23, 1993-1997, 2011.
3. M. Elbahri, M.K. Hedayati, F. Faupel, T. Strunkus, V. Zaporojtchenko, “Absorberschicht für den VIS- und/oder NIR-Spektralbereich, ” DE 10 2011 113 571.9 (Patent pending).
4. M. K. Hedayati, M. Javaherirahim, B. Mozooni, R. Abdelaziz, A. Tavassolizadeh, V. S. K. Chakravadhanula, V. Zaporojtchenko, T. Strunkus, F. Faupel, M. Elbahri, Adv. Mater. 23, 5410–5414 (2011).
5. M. Jamali, M.K. Hedayati, B. Mozooni, M. Javaherirahim, R. Abdelaziz, A.U. Zillohu, M. Elbahri, Adv. Mater., Vol. 23, 4243-4247, 2011.
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19:30
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dinner
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20:15
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poster session
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show list of posters
Julia Bahrenburg
Ultrafast Dynamics of Molecular Photoswitches by Femtosecond Spectroscopy
show abstract
The application potential of photochromic molecules depends critically on how a surrounding matrix affects the ultrafast photo-induced transformations. To elucidate these effects, we performed a comparative study of the ultrafast E-Z photoisomerization of two azobenzene (AB) derivatives in solution and in cross-linked polymer colloids. The photo-induced isomerization of the covalently attached or cross-linked AB molecules was probed by means of femtosecond time-resolved fluorescence up-conversion spectroscopy. In terms of the molecular dynamics in solution and in the polymer matrix, the excited-state relaxations were found to be drastically different. For the AB dyes in polymers, much longer excited-state lifetimes were observed.
For the investigation of electronic coupling and chromophore interactions in molecular systems, two multichromophore AB derivatives were investigated systematically by means of NMR and UV/VIS spectroscopy. The variance of the thermal back-reaction lifetimes demonstrates a dependence of the Z-E isomerisation on the chromophore size and number of AB units. To probe and compare the photodynamics of these molecules, first femtosecond time-resolved absorption and fluorescence measurements were performed.
A molecular photoswitch based on the excited-state intramolecular hydrogen atom transfer (ESIHT) process was investigated with the aims to obtain bistable, photoreversible systems and to develop proton-transfer cascades. First femtosecond time-resolved absorption measurements were performed in a protic and an aprotic solvent. As expected, the molecular dynamics are substantially different, reflecting the inter- and intramolecular hydrogen bonding environment.
Vijayanand Chandrasekaran
Switchable cell adhesion by azobenzene glycosides
show abstract
Conformational changes are important in many biological events, such as in the vision process, in receptor signalling or for ATPase action, among others. Recently, it has become our goal to investigate the role of conformational control within the molecular recognition processes occurring at the glycosylated cell surface [1]. The glycocalyx of eukaryotic cells is involved in many essential biological process like cell-cell communication, cell signalling, and cell adhesion. [2] It has been evident that spatial distribution and relative orientation of carbohydrate ligands on the cell surface plays a key role in their biological function. To address and to understand the importance of conformational control of glycoconjugates in biological events, we have designed and synthesised photoswitchable azobenzene glycosides as functional glycomimetics.
Azobenzene derivatives are well suited for biological applications as they show very good photochemical stability. Thus azobenzene glycoconjugates can be promising tools to manipulate the ligand orientation of a carbohydrate array, thereby fine-tuning carbohydrate-protein interactions. The E/Z isomerizaiton of the azobenzene moiety considertably changes the relative spatial orientation of groups conjugated at opposite ends of the molecule. [3-4] Switching of azobenzene glycosides from trans to cis configuration can be achieved by irradiation at 365 nm, whereas back-isomerization occurs at 440 nm or be thermal equilibration (Fig. 1).
We will report syntheses, photophysical properties as well as biological applications in the context of type 1 fimbriae-mediated bacterial adhesion [5].
Figure 1: Photoswitching of azobenzene glycosides changes the orientation and relative distance of the conjugated groups.
[1] A, J. Petrescu, M. R. Wormald, R. A. Dwek, Curr. Op. Struct. Biol. 2006, 16, 600
[2] Crocker, P; Paulson, J. C; Varki, A. Nat. Rev. Immunol. 2007, 7, 255-266.
[3] Srinivas, O; Mitra, N; Surolia, A; Jayaraman, N. Glycobiology, 2005, 15, 861-873.
[4] Kleinert, M; Winkler, T; Terfort, A; Lindhorst, Th. K. Org. Biomol. Chem., 2008, 6, 2118-2132.
[5] Chandrasekaran, V; Lindhorst, Th. K. Chem. Commun. 2012, 48, 7519-7521.
Marcel Dommaschk
Switchable cell adhesion by azobenzene glycosides
show abstract
Magnetic Resonanz Imaging (MRI) is an important tool in diagnostic medicine. MRI is going importance because it’s working without ionizing radiation unlike Computer Tomography (CT). Contrast Agents (CA) are used for a numbers of applications and amplify the power of MRI. All single molecule CAs are based paramagnetic metal complexes. We investigate in nickel(II)porphyrins which can be switched between the dia- and paramagnetic state by irradiation with light. Alongside the magnetic state we also control MRI contrast which allows a number of novel applications.
Nickel(II)porphyrins can exist in two different spin states. A square planar geometry always gives rise to the diamagnetic low-spin configuration. Axial coordination of one ligand leads to a square pyramidal geometry. In case of electron-deficient porphyrin and strong axial ligand the complex has a high-spin configuration. This process was coined Coordination-Induced-Spin-State-Switch (CISSS). [1]
In solution there is always an equilibrium between dia- and paramagnetic species dependent only on the concentrations. Using photoswitchable azopyridines as axial ligands, the magnetic equilibrium can be varied by irradiation of light of proper wavelengths. This process is called Light-Driven-Coordination-Induced-Spin-State- Switch (LD-CISSS). But still the magnetism is a function ligand concentration. [2]
To avoid concentration dependence, which is required for application as CA, the azopyridine is linked covalently to the nickel(II)porphyrin. Only cis-configuration can coordinate intramolecular and therefor is a CA. The thermodynamic stable trans- Isomer is always diamagnetic and therefor invisible in MRI pictures. For all medical applications the intramolecular LD-CISSS has to be performed in aqueous solution which is now the challenge. [3]
Figure 1: Photoswitching of azobenzene glycosides changes the orientation and relative distance of the conjugated groups.
Literature:
[1] S. Thies, C. Bornhold, F. Köhler, F. D. Sönnichsen, C. Näther, R. Herges, F. Tuczek, Chem. Eur. J. 2010, 16, 10074-10083.
[2] S. Thies, H. Sell, C. Schütt, C. Bornhold, C. Näther, F. Tuczek, R. Herges, J. Am. Chem. Soc., 2011, 133, 16243- 16250.
[3] S. Venkataramani, U. Jana, M. Dommaschk, F. D. Sönnichsen, F. Tuczek, R. Herges, Science 2011, 331, 445-448.
Lars Heepe
Microscopic detachment behavior of mushroom-shaped adhesive microstructure at 180000 fps
show abstract
To shed light on the underlying mechanisms of enhanced adhesion of mushroom-shaped adhesive microstructure (MSAMS), the microscopic contact behavior was investigated. Therefore, a new experimental arrangement was developed. Using high-speed video recordings the detachment process of individual MSAMSs was resolved with a temporal resolution of 5.5 microseconds. In order to describe and analyze this process a simple but general model was proposed which allows one to determine the temporal course of the important contact mechanical quantities like contact area, crack growth behavior and local crack propagation velocities. Based on the data obtained, it was shown that the separation of individual MSAMS in contact with a smooth surface is a two-phase process. Under a continuously increasing applied load the contact area of individual MSAMS contracted by up to 25 per cent prior an interfacial crack was nucleated. Once the crack was nucleated, it propagated towards the outer edges in accordance with previous observations. Local crack propagation velocities reached up to 12 per cent of the shear sound velocity. These results suggest that the complex and highly dynamic detachment behavior may play a crucial role in the understanding of the enhanced adhesion of MSAMS. Moreover, it is assumed that this experimental technique may also be applied to study different contact geometries and the contact behavior biological attachment structures, even in living animals.
Gernot Heitmann
Intramolecular LD-CISSS in azoimidazole-functionalized Nickel(II)porphyrins for development of photoswitchable MRI contrast agents
show abstract
Magnetic Resonance Imaging (MRI) is one of the most important imaging methods in modern medicine. As it works with non-ionizing irradiation this technique will probably even gain in importance as compared to Computer Tomography (CT) which uses x-radiation. The quality of an MR image can be affected by contrast agents. These are paramagnetic substances which lower the relaxation time of the surrounding protons and thus lead to a brighter contrast. There is a great interest in developing so called „smart contrast agents“ which respond to external stimuli such as temperature, pH, or certain ion concentrations. However, to this date there is no contrast agent which can be reversibly turned on and off on request.
In 2010, HERGES et al. reported on the synthesis of a Nickel(II) complex which shows magnetic bistability in solution at ambient temperature. The concept presented is based on intramolecular LD-CISSS (light-driven coordination-induced spin state switch) in a Nickel(II)porphyrin covalently linked to an azopyridine as photoswitchable ligand. In trans-configuration, an intramolecular coordination of the pyridine to the diamagnetic low spin Nickel(II) is impossible. If switched to the cis-isomer by irradiation with green light (495 nm) the pyridine is able to coordinate to the Nickel(II) and causes a spin state switch to the paramagnetic high spin state. Backswitching to the diamagnetic trans-isomer is achieved with blue light (435 nm).[1][2][3][4]
First MRI investigations show a significant contrast change between the two stable magnetic states of this complex. However, for medicinal application the concept has to be realized in aqueous solution which is to date not possible. In this matter we work on several enhancements of the existing pyridine prototype such as replacing the photoswitchable ligand pyridine by the stronger donor ligand methylimidazole.[5][6]
[1] R. Herges et al., Chem. Eur. J. 2010 , 16, 10074-10083.
[2] R. Herges et al., J. Am. Chem. Soc. 2011, 133, 16243-16250.
[3] R. Herges et al., Chem. Eur. J. 2012 , submitted.
[4] R. Herges et al., Science 2011 , 331, 445-448.
[5] R. Herges et al., The Journal of Organic Chemistry 2012 , 77, 3284-3287.
[6] G. Heitmann, diploma thesis 2012, unpublished.
Michelle Holz
Photoswitchable Monolayers: A Dynamic Control over Cell Adhesion
show abstract
Photoswitchable monolayers of azobenzene derivatives mounted on glass, quartz or silicon surfaces offer an effective control over cell adhesion. The light induced reversible cis- / trans-isomerization of azobenzenes permits a precise temporal and spatial control of surface properties.
We employed three strategies:
1. Switching of wettability
Upon isomerization of the azobenzene derivative, the surface wettability changes due to different dipole moments of the azobenzene isomers. This is especially the case when using polar head groups.
2. Fast oscillations
Upon irradiation with white light, azobenzenes with push/pull substitution patterns are known to undergo a fast continuous switching between their cisand trans-isomers. [1] We intend to utilize this effect to create a fast oscillating monolayer on surfaces.
3. Integrin mediated adhesion
The establishment of an arginine-glycine-aspartate (RGD) tripeptide squence cell binding domain on surfaces offers the opportunity to target specific integrins of cells. We have chosen the c(-RGDfK-) peptide [2] as our future head group for this purpose, due to its specific binding to αvβ3 integrin.
[1] T. Asano J. Am. Chem. Soc. 1980, 102, 1205; S. Hvilsted et al. J. Mater. Chem. 2009, 19, 6641; B. Schmidt et al. J. Phys. Chem. A. 2004, 108, 4399.
[2] R. Haubner et al. J. Am. Chem. Soc. 1996, 118, 7461.
Hanne Jacob, Sven Olaf Schmidt
Iron-complexes for spin switching in solution, fixation on gold and in thin films
show abstract
In the present work new FeII and FeIII -SCO compounds were synthesized for spin switching in solution, fixation on gold and in thin films and characterized by Mößbauer, IR, RAMAN, UV/Vis, NMR-spectroscopy. To study the LD-LISC-effect on [Fe III (salten)L] + -complexes phenylazopyridines were used as photoswitchable ligands. [1]
Furthermore transition metal complexes with suitable linker molecules should be fixed to gold(111) surfaces and the spectroscopic and structural characterization of the resulting, chemically modified surfaces will be investigated by IRRAS, XPS and NEXAFS. In the present work self-assembling technique out of solution is used to fix the transition metal complexes on to the surface.
On the other hand thin films of vacuum deposited SCO-complexes [Fe II (H2Bpz)2(L)] (L = phen, bipy) were synthesized and characterized by UV/VIS, IR, RAMAN, AFM, XPS, NEXAFS, UPS & STM. It was possible to observe the first light-induced spin change in vacuum deposited thin films. [2] With a STM tip induced electron-transfer to [Fe II (H2Bpz)2(phen)] leads to a spin crossover on a gold surface. This new SCO phenomenon was called Electron-induced Excited Spin State trapping (ELIESST). [3]
[1] A. Bannwarth, Sven Olaf Schmidt, Gerhard Peters, Frank D. Sönnichsen, Wulf Thimm, Rainer Herges, Felix Tuczek, Eur. J. Inorg. Chem. 2012, 2776.
[2] H. Naggert, A. Bannwarth, S. Chemnitz, T. van Hofe, E. Quandt, F, Tuczek, Dalton Trans., 2011, 40, 6364.
[3] T. G. Gopakumar, F. Matino, H. Naggert, A. Bannwarth, F. Tuczek, R. Berndt, Angew. Chem. 2012, 124, 6367.
Laith Kadem
Nanostructured surfaces as platforms for photoswitchable cell adhesion
show abstract
Cell signaling, differentiation and apoptosis are strongly affected by cell adhesion. In an attempt to mediate specific cell adhesion, we are developing surfaces where light-induced switchable cell adhesion is feasible with azobenzene molecules, which are functionalized with RGD peptides. In order to switch cell adhesion in a non-specific way, we employ functional groups, which switch the wettability of the surfaces. The big advantage of azobenzenes compared to other light-sensitive molecules is the reversibility of their photo-induced switching between two isomeric states. This would give us the possibility to switch cell adhesion in a spatially and temporally fashion by relatively short light illumination. The main platforms for our adhesion experiments are gold nanostructured surfaces that have been fabricated using block-copolymer micelle nanolithography. The nanostructure is represented by gold nano-dots (size: 5-10 nm) arranged in a quasi-hexagonal patterns with a defined and controllable inter-dot spacing in the range of a few tens of nanometers. The areas between the gold dots are passivated against non-specific cell adhesion with polyethylene glycol and the gold dots themselves are functionalized with azobenzenes. Here we show first results of cell adhesion on our nanostructured surfaces.
Emre Kizilkan
Underwater Adhesion of Mushroom Shaped Adhesive Microstructure: An Air-Entrapment Effect
show abstract
In this work, we revisited the underwater adhesion of mushroom-shaped adhesive microstructure (MSAMS) which has come a long way from the inspiration of attachment devices evolved in beetles to a large-scale industrial production. It was shown that MSAMS has an about twice higher pull-off force compared to smooth control made from the same material measured in air. Enhanced adhesion was found to be a combination of intermolecular van der Waals forces and particular crack-trapping geometry. Recently, it was shown that underwater adhesion of MSAMS was even higher than that in air (M. Varenberg, and S. N. Gorb, J. R. Soc. Interface 5, 383, 2008). It was a surprising result, since van der Waals interactions should actually be strongly reduced, when the surfaces are submerged in water. It was hypothesized that each individual MSAMS acts as a passive suction device. The present study aimed at an experimental investigation of an alternative hypothesis, namely the effect of air-entrapment. For this purpose, we compared pull-off forces of MSAMS, measured under water with a layer of entrapped air with those completely wetted by water. In the presence of an air layer pull-off forces were distinctly higher compared to measurements in air. When MSAMS was completely wetted by water, lower pull-off forces compared to those measured in air were obtained. These data suggest that air retaining capabilities of MSAMS, when submerged in water, may also explain their enhanced underwater adhesion.
Hauke Kobarg, Anne Müller
Photoswitchable Antifreeze Proteins
show abstract
Antifreeze Proteins can be found in many organisms like fish, insects, plants or mushrooms and polar subzero regions. Those proteins bind with high selectivity, specificity and affinity to ice crystals. They affect the speed of growth as well as the morphology of ice. This leads to a freezing point depression and protects organisms of death by freezing.
Our aim is to develop antifreeze proteins whose activity can be switched. As it strongly depends on an undisturbed tertiary structure, a small structural change can lead to a decrease in the proteins ice affinity. This is achieved by the attaching of a photo switchable crosslinker to the protein’s sidechains. Via irradiation and isomerization of an azobenzene crosslinker, we can control the helical content of the protein and thus we can switch between an ice-active and an ice-inactive state. This switching makes new mechanistically studies possible, as a spatial and temporal resolution can be introduced to experiments.
We present a new model system to optimize the chemical crosslinking process. This model includes a new, shortened peptide sequence as well as different precursors for the photo switchable moiety.
Isabel Köhl
Molecular switches for AFM measurements
show abstract
Azobenzenes are the prototype of switchable molecules.[1] Therefore, the mechanical work of their photoisomerization shall be determined quantitatively. Azobenzene derivatives shall be attached covalently between a glass surface and a cantilever in an atomic force microscope (AFM).
New azobiphenyl molecules to amplify the effective distance change during the E,Z-isomerization will be presented. This shall lead to a better signal-to-noise ratio due to increased change of length.[2] The azobiphenyls are substituted with methylgroups to improve solubility. All synthesis were done by an oxidative copper(II)-catalyzed coupling reaction in combination with a Suzuki reaction.
In solution, the switching behavior of azobenzenes and azobiphenyles was examined by NMR and UV/Vis spectroscopy. The amount of cis isomer was determined by NMR spectroscopy. The reversibility of switching was investigated by UV/Vis spectroscopy. All investigated molecules showed E,Z-isomerization and sufficiently long half-life times.
[1] G. S. Hartley, Nature 1937, 140, 281.
[2] C. Glockner, Dissertation 2011, CAU Kiel.
Anja Köhntopp
Photoswitching Properties of Azobenzene-Functionalized Gold Nanoparticles
show abstract
For the design of photoswitchable functional surfaces, which have attracted much interest in the past years [1-3], detailed information on the specific factors and mechanisms influencing the photophysical properties of functional molecules on surfaces is of great importance.
Towards this end, this work is concerned with the influence of the distance between chromophore and the substrate. We use gold nanoparticles (AuNPs) functionalized with mixed self-assembled monolayers (mSAMs) of an azobenzene (AB) and a photochemically inert co-ligand (pentanethiol or decanethiol). The AB ligands possess different linker chain lengths (C11, C7 and C3), resulting in different chromophore-to-surface distances. UV/Vis spectroscopy is used to analyze the photoswitching properties of the different systems. All investigated systems exhibit trans-cis isomerisation upon irradiation with UV light at λ = 365 nm as well as cis- trans isomerisation upon subsequent irradiation with visible light at λ = 455 nm. In this manner, they can be repeatedly switched between two photostationary states (PSSs). While all the systems show only minor differences in the trans state, they behave differently when brought into the cis state. The Surface Plasmon Resonance Band (SPRB) of the AuNPs is broadened and decreases in intensity compared to the trans state, when the C7 linker is combined with pentanethiol as coligand. When switched back to the trans state, the SPRB complete regains its previous shape. This change suggests that a reversible aggregation of the AuNPs is taking place in solution. No such behavior is observed for C3 and C11 linkers combined with longer decanethiol co-ligand. The results show that the distance between chromophore and surface has a much smaller effect on the photoswitching properties of the investigated AB ligands than previously thought. However, the chain length apparently influences the polarity of the cis-AB functionalized AuNPs, making reversible aggregation possible.
[1] U. Jung, S. Kuhn, U. Cornelissen, F. Tuczek, T. Strunskus, V. Zaporojtchenko, J. Kubitschke, R. Herges and O. Magnussen, Langmuir 2011 , 27, 5899.
[2] R. Klajn, J.F. Stoddart, B.A. Grzybowski, Chem. Soc. Rev. 2010, 39, 2203.
[3] J. Zhang, J. K. Whitesell, M. A. Fox, Chem. Mater. 2001, 13, 2323.
Sonja Kuhn
In situ STM Investigations of Platform Adlayers on Au(111) Surfaces
show abstract
Functionalization of metal surfaces by self-assembly of appropriate molecules from solution is of great interest in current research. We have introduced the so called platform approach for preparation of highly-ordered adlayers, where Triazatriangulenium (TATA) [1,2,3] or Trioxatriangulenium (TOTA) [4] molecules are employed, which can be functionalized vertically at the central carbon atom as well as by alkyl side chains. These systems have been extensively characterized by microscopic, spectroscopic and electrochemical methods [1, 3-5], which showed that the platforms adsorb flat on the Au(111) surface and exhibit intermolecular distances controlled by the length of the side chains. Furthermore, adlayers of compounds with azobenzene derivatives as central functional group revealed a perpendicular orientation of the functionalities with respect to the substrate surface.
Here we present in situ STM studies of adlayers of the octyl-TATA platforms. Since within our studies extensive photoisomerization experiments performed in electrochemical environment it is of great interest to investigate the potential-dependent structural behavior of these platform molecules. Near the open circuit potential of about 0.5 VSCE the same commensurate superstructure (√19x√19)R23.4° as in air was found. In contrast, at potentials more negative than 0.4 VSCE a transition to a (4x4) superstructure with an about 15% higher packing density was found, which could be explained by desorption of anions from the electrolyte or by electrostatic effects. If the potential is decreased further to potentials more negative than 0.1 VSCE an order-disorder transition was observed, where the platform molecules form a partial bilayer. Both structural transitions occur highly reversible.
Figure 1.Cyclic voltammogram and STM images in 0.1M HClO4 of an Octyl-TATA adlayer at potentials positive and negative of the potential of the order-disorder transition.
References
[1] Baisch et al., J. Am. Chem. Soc. (2009), 131, 442.
[2] Kuhn et al., PCCP (2010), 12, 4481.
[3] Kubitschke et al., Eur. J. Org. Chem. (2010), 5041.
[4] Kuhn et al., Chem. Commun., 2011, 47, 8880-8882.
[5] Jung et al,. Langmuir, 2011, 27, 5899-5908.
Matthias Müller
Photoisomerization of Azobenzene-Functionalized Alkanethiols and Platform Adlayers on Au
show abstract
Functional self-assembled monolayers (SAMs) are of major interest because of future applications as e.g. (bio)sensors, data storage, or molecular machines. Very often, such adlayers are prepared by self-assembly of the functional molcules onto the substrate. A typical approach towards functionalized surfaces via self-assembly are thiol-based molecules. Because of their densely-packed structure, intermolecular interactions lead to steric hindrances. Another crucial aspect are electronic substrate-adsorbate interactions. Both types of interactions may strongly affect or even completely suppress the function. In common approaches mixed monolayers with short chain spacer molecules or special designed molecules are employed. However, these approaches still have the disadvantage of low structural order of the SAMs and tend to phase separation [1]. For preparation of highly-ordered SAMs we have recently introduced a novel concept based on triazatriangulenium (TATA) platforms. In extensive microscopy (STM), spectroscopy (SERS, XPS and NEXAFS), and electrochemical studies (cyclic voltammetry) [2-4], we clearly demonstrated that the platform molecules form hexagonally ordered adlayers with large intermolecular spacings and the functionalities oriented perpendicular to the substrate. Here, we report UV/VIS and localized surface plasmon resonance (LSPR) spectroscopy studies of the photoswitching of azobenzene-functionalized thiol and platform adlayers. For the UV/VIS measurements two experimental setups were designed, one for transmisson and the other for reflection measurements, allowing direct irradiation and hence the monitoring of the photoinduced isomerization kinetics. For LSPR spectroscopy a transmission setup was developed, which allows to monitor photoswitching at gas-substrate and liquid-substrate interfaces. In particular, the experiment can be performed under temperature control from -25°C to room temperature. Reversible photoinduced switching was identified in both kind of experiments. The photo-induced isomerization reaction occurred reversibly on a short time scale, i.e. was highly efficient. In particular, for thiol-based azobenzene-containing SAMs larger signal changes than for TATA-based photoswitches were observed, which can be explained by a higher concentration of cis-azobenzene on the surface. However, distinct differences between TATA and thiol-bound azobenzenes were found with respect to the thermal cis-trans isomeriztation. Whereas for the thiols the cis-isomer is stable in the adlayers, for the TATA platforms the thermal cis-trans isomerizaion proceeds several magnitudes faster than in liquid phase.
References
[1] U. Jung et al., Langmuir, 2010, 26, 13913
[2] B. Baisch et al., J. Am. Chem. Soc. 2009, 131, 442
[3] S. Kuhn et al., PCCP 2010, 12, 4481
[4] U. Jung et al., Langmuir 2011, 27, 5899
Lina Sophie Noll, Bettina Schwager
Iron-complexes as a switchable MRI-contrast-agent
show abstract
Iron(II)-Complexes have suitable characteristics to act as nontoxic switchable MRIcontrast-agents. The spin-difference between the low-spin- and the high-spinconfiguration of an Iron(II)-Complex ist ∆S=2. This exhibits the maximum attainable spin change for SCO-transition metal complexes.
In this project different ligands for switchable Fe(II)-Complexes should be synthesized. The ligands of choice have a Scorpion-Design. The Salapo-Scorpion and the Trident-Scorpion are two different concepts.
In both cases the light induced switchable moiety is a azocompound. In case of the Trident-Scorpion the azopyridine is attached directly to the coordinated nitrogen donor of the equatorial ligand. The Salapo-Scorpion is a funtionalized salen-ligand with a azopyridin moiety. As donor groups amine are used as well as phosphine. The complexes must be watersoluble to use them as MRI-agents.
In addition to the synthesized ligands the twodentate ligand bipyridine will be used to coordinate the iron(II)-ion.
Julia Reverey
Investigation and biomimicry of target cell killing by Acanthamoeba
show abstract
Pathogenic amoebae like Acanthamoeba castellanii, can cause severe diseases such as Acanthamoeba keratitis. 90% of the patients with this painful disease are contact lens users, who are infected with Acanthamoeba due to wrong contact lens care. Through small lesions of the epithelial cells the extremely motile amoebae reach the interior of the eye and start to destroy target cells by an elaborate extracellular killing mechanism. A crucial first step during this killing machinery is the carbohydrate-mediated formation of a close contact between the Acanthamoebae and the host cells. The initial adhesion between amoebae and target-cells is followed by the transport of intracellular granules to the contact site, where pore-forming molecules are released from the granules, which cause the death of the target cell. In order to avoid infections with Acanthamoeba, it is highly essential to understand the first step of contact formation. We are studying this mechanism by mimicking the cell coat with carbohydrate-functionalized surfaces as well as with carbohydrate coated beads. The adhesion of Acanthamoebae to functionalized surfaces is analyzed with live cell imaging. In order to understand the full picture of Acanthamoeba pathogenicity we are also investigating the transport of the granules to the contact site with a combined approach based on live cell imaging, automated image analysis and nonequilibrium systems theory.
Saira Riaz
Studying the Morphology of Azobenzene Containing Mixed Monolayers on Gold by VSFG Spectroscopy
show abstract
The photoswitching efficiency of mixed azobenzene-alkylthiol-/alkylthiol-monolayers – next to diverse energy dissipation processes – is determined by morphological aspects such as phase separation and the available free volume. So far, the potential of surface sensitive vibrational sum-frequency generation (VSFG) spectroscopy to investigate the switching properties of mixed monolayers has not been exploited. In principle, VSFG should be capable of monitoring both the morphology of the monolayer as well as the switching state of the molecular switch.
In this study, pure and mixed monolayers on gold were prepared from (i) either decanethiol, perdeuterated dodecanethiol or perdeuterated hexadecanethiol as lateral spacer molecules and (ii) C11-methyl-azobenzenethiol (MeAB) or C11-azobenzenethiol as molecular switches. Here, the methyl substituent in MeAB serves as a sensitive SFG marker to measure the switching state of the azobenzene moiety.
Detailed simulations of the CH-stretching VSFG spectra revealed spectral shifts indicating well mixed monolayers instead of phase separated molecular domains. First switching experiments with optical excitation at 365 nm revealed monolayer specific but spectrally indifferent reversible intensity alterations. Further experiments are required to clarify if the observed effects are attributable to optical switching or other processes such as thermal heating.
Benjamin Sahlmann
Synthesis of Photoswitchable Ligands for Metal Ions Based on the Norbornadiene-Quadricyclane-System
show abstract
The norbornadiene-quadricyclane system is a well-established backbone for the development of photoswitchable metal ligands. Possible applications include active ion transport and spin switching materials. A new functionalization protocol which exhibits acces to highly functionalized derivatives is being presented along with first results from photoswitching experiments.
Christian Schütt
Light-Driven-Coordination Induced Spin-State Switch: Rational Design of Photodissociable Ligands using Density Functional Theory Methods
show abstract
The coordination of axial ligands to porphyrins is of fundamental importance in many areas of chemistry, biology and material science. [1] Azopyridinederivates which are isomerizing between the cis and trans configuration upon irradiation with UV and visible light were used to realize a Light-Driven-Coordination Induced Spin-State Switch (LD-CISSS) at Ni(II)-porphyrins. [2,3] Upon the association/dissociation of axial ligands the Ni(II) center can be switched between the high spin (HS) and the low spin (LS) state. Whereas the trans isomer binds with its pyridine nitrogen the coordination of the bent cis isomer is sterically hindered. We performed density functional theory calculations to find a substitution pattern at the phenylazopyridine ligand which provided an optimum switching efficiency. A similar approach with the photochromic ligands covalently attached to the porphyrin has recently been published. [4]
[1] „The Porphyrin Handbook“, K. M. Kadish, K. M. Smith, R. Guilard Eds. Academic Press 2000, London.
[2] S. Thies, H. Sell, C. Schütt, C. Bornholdt, C. Näther, F. Tuczek, R. Herges, J. Am. Chem. Soc. 2011, 133, 16243 - 16250.
[3] S. Thies, H. Sell, C. Bornholdt, C. Schütt, F. Köhler, F. Tuczek, R. Herges, Chem. Eur. J. 2012, submitted.
[4] S. Venkataramani, U. Jana, M. Dommaschk, F. D. Sönnichsen, F. Tuczek, R. Herges, Science 2011, 331, 445-448.
Doreen Schütze
Single molecule force-clamp spectroscopy on azobenzene monomers linking PEG spacers
show abstract
With single molecule force-clamp spectroscopy [1] (SMFCS) it is in principle possible to determine the mechanical properties of light-driven molecular switches as well as mechanically induced switching on the single molecule scale. With azobenzenes as molecular switches, conformational changes lead to a length change, which can be measured during clamping. So far, reversible switching of conformational changes have only been shown on oligo-azobenzenes by Gaub, Moroder and coworkers [2]. In our experiments, monomers of molecular switches are covalently anchored between a glass surface and an AFM cantilever tip via EDC/NHS-activated coupling chemistry. Both the cantilever tip and the glass surface are functionalised in the first step with a heterobifunctionalised polyethylene glycol (PEG) acting as spacer molecule to overcome surface forces. The goal of the project is to optically switch the azobenzene molecule by light in the UV region between its E and Z isomer and back at a well-defined, constant clamp-force with an atomic force microscope (AFM) Asylum Research MFP-3D BioTM. Progress towards this goal is shown with three major changes: (1) The influence of acoustic insulation on the accuracy of the force feedback in terms of force control and length measurement; (2) Total Internal Reflection (TIR) illumination for switching; (3) use of custom synthesized azobenzenes [3] with increased length change upon switching.
[1] A.P. Wiita, S.R.K. Ainavarapu, H.H. Huang, J.M. Fernandez, Proc. Natl. Acad. Sci. U. S. A. 103 (2006) 7222.
[2] T. Hugel, N.B. Holland, A. Cattani, L. Moroder, M. Seitz, H.E. Gaub, Science 2002, 296, 1103.
[3] I. Köhl, unpublished diploma thesis "Azobenzole mit verlängertem Hebel als molekulare Schalter", Christian-Albrechts-University, Kiel, 2011
Stefan Schwarzer
Subject matter education support of the SFB 677
show abstract
Contemporary research from the Collaborative Research Center 677 is linked with context of applications and scholar experiments. Additionally, the work of the scientist is going to be presented in a multimedia context and will be educationally reconstructed for secondary school students on a subject-matter base. The aim of the project Public Relations is to communicate main scientific principles and the resulting research products to a broader public in schools and society. Not only is it desirable to reach a broader understanding of the scientific content but also to stimulate the interest in study and work perspectives on topics like synthesis, analysis and technical use of of nano structures and molecular switches. With regards to contents, the different projects of the Collaborative Research Center shall be made accessible by answering the three following central questions: - What does Nano-Science mean and how can the special characteristics of the “nano world” be explained? Which potential/ risk perceptions do we find in this development? - Which principles of chemical and physical switching of matter are used to construct “nano switches” - Which methods and procedures are suitable to investigate and represent nano structures and the functionality of nano materials? We strive for the following educational goals: - Extension of Concept-Based Education - Combination of experiment and multimedia - Insights in everyday research work of scientists The research plan, that is conducted by the IPN is a combination of testing the understanding of structure-property-relationships, collecting questions, conceptions and interests of visitors in scientific outreach activities. This will be accompanied by investigation of beliefs about scientists, their work environment and their interests in natural sciences. To complete the project, we are not only interested in what the public thinks about science but we also want to carve out how scientists experience the usage of media, translating their work to a broader public.
Jan Strüben, Xin Jin
Tetrapodal ZnO Microparticles as Powerful Linkers Between Low Surface Energy Polymers
show abstract
Joining polymer materials is an important challenge in materials engineering. In many cases,the chemical and physical properties of polymers to be connected render them incompatible to conventional methods for facilitating a firm attachment between them. One of the most extreme cases is the combination of polytetrafluoroethylene (PTFE) and cross-linked polydimethylsiloxane (PDMS). PTFE is a thermoplastic material with high chemical inertness and is well known for its non-adhesive properties.[1] Similarly, cross-linked PDMS is, together with PTFE, among the materials with the lowest surface energy known to date.[2] In this work, we present a new approach to effect a strong joint between PTFE and crosslinked PDMS as prime examples for typically non-sticky polymers. We show that tetrapodal ZnO crystals (T-ZnO) can be used as powerful mechanical adhesion facilitators in a straightforward and easily applicable way. Furthermore, we present new methodologies to attatch functional materials, like photo responsive polymers (e.g. the “Ikeda Polymer”)[3] to such low surface energy substrates.
[1] a) W. Georgette, Gregoire, France Patent FR 1 119 221, 1956; b) T. Soc, France Patent FR 1 156 405, 1958.
[2] J. E. Mark, Polymer Data Handbook, Oxford University Press, New York, USA 1999.
[3] Y.Yu, M. Nakano, T. Ikeda, Nature 2003, 425, 145.
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Tuesday, 21.08.2012
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07:00
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breakfast
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Session 5: Chair: Rainer Herges
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09:00
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Stefan Hecht
Berlin
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Azobenzene and Diarylethene Photochromes with Improved Switching Properties
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show abstract
Photochromic molecules are able to translate an incoming light stimulus into property changes of a given material, thereby allowing to control and even power its function in a specific process. The exquisite spatio-temporal control provided by the light input has sparked a considerable interest in such photoswitchable systems with potential application ranging from ”smart” surfaces and polymers[1] over remote-controlled catalysts[2] to light-driven actuators.[3] The interface between the light stimulus and the material is provided by a photosensitive gate – the photochromic unit. While a lot of effort recently has been devoted towards incorporating such photoswitchable gates into macro- and supramolecules and organic-inorganic hybrid materials as well as decorating surfaces, fundamental research to develop and improve photochromic systems remains important. In this lecture, the most recent progress by our group as well as others on improving the switching performance of azobenzene as well as diarylethene photochromes will be described.
References
[1] M.-M. Russew, S. Hecht, Adv. Mater. 2010, 22, 22, 3348-3360.
[2] R. S. Stoll, S. Hecht, Angew. Chem. Int. Ed. 2010, 49, 5054-5075.
[3] D. Bléger, Z. Yu, S. Hecht, Chem. Commun. 2011, 47, 12260-12266.
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09:45
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Dirk Trauner
Munich
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Optochemical Genetics
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show abstract
Transmembrane receptors allow a cell to communicate with its environment in response to a variety of input signals. These can be changes in the concentration of ligands (e.g. hormones or neurotransmitters), temperature, pressure (e.g. via acoustic waves or touch), transmembrane potential, or light intensity. Many important receptors have now been characterized in atomic detail and our understanding of their functional properties has markedly increased in recent years. As a consequence, these sophisticated molecular machines can be reprogrammed to respond to unnatural input signals. Arguably, the most useful of these signals is light. I will show how ligand-gated ion channels, G-protein coupled receptors, as well as voltage-gated ion channels, can be manipulated with synthetic photoswitches to become light-sensitive. The resulting hybrid photoreceptors can be used to optically control neurons with very high precision. They have been used to dissect neural networks and might find applications in the restoration of vision and the control of other sensations (such as pain). This combination of synthetic photoswitches and receptor proteins augments the field of Optogenetics and adds a new functional dimension to Chemical Genetics. As such, we propose to call it “Optochemical Genetics”.
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10:30
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coffee break
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Session 6: Chair: Olaf Magnussen
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11:00
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Ralph Nuzzo
Urbana IL, USA
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Functional Surfaces for Energy Applications
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show abstract
The fabrication of high performance integrated circuits provides examples of the most sophisticated manufacturing methods, as well as the most high performance materials, used in an area of modern technology. The advanced functional systems they provide are ones that are generally characterized by a massive integration of circuit elements within compact, rigid and essentially planar form factor devices. New means of fabrication and enabling materials are beginning to provide a set of means through which it is possible to lift some of these latter constraints—doing so in ways that both retain capacities for high performance while enabling new, low cost opportunities for use in technology. Our collaborative research here at the University of Illinois is providing form factors for devices with interesting but what had been to date difficult to realize features. Examples include: light weight, large area, high performance electronics, optics, and photonics; electro optical systems with curvilinear shapes and capacities for accommodating demanding forms of mechanical flexure; and hybrid systems for lighting, energy storage, and photovoltaic energy conversion that might provide potentially transformational approaches to supplant current technologies with high performance, low cost alternatives. In this lecture I will explore examples of energy technologies enabled by new form factors for devices and the highly functional surfaces used for their construction.
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11:45
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Manfred Buck
St. Andrews, Scotland
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Going Nano at the Liquid/Solid Interface
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show abstract
The extent to which effects emerging at the nanoscale can be exploited is crucially dependent on both the precision at which structures can be controlled and dimensional gaps be bridged. Driven by the limitations of established lithography based approaches, a range of alternative routes is explored to address these issues with concepts involving templating and self-assembly on surfaces playing an important role. The vitually unlimited flexibility in the design of organic molecules in combination with simple processing at the liquid/solid interface make two complementary self-assembly strategies, namely self-assembled monolayers (SAMs) and supramolecular networks (SMNs), appealing components of a toolbox for the nanoscale. SAMs provide unique opportunities for the functionalisation of surfaces and control of interfacial energy but, as part of a top-down approach, their patterning is limited in resolution by the applied lithographic techniques. In contrast, SMNs as prototypical systems for the bottom-up approach provide access to ultraprecise structures on the sub-5 nm scale but are less amenable to chemical functionalisation. Thus, merging both assembly concepts [1] has an obvious potential for the control of interfacial structure and properties on a length scale ultimately down to the level of individual molecules. The scope of SAMs and SMNs is further augmented if combined with electrochemistry as another powerful tool for the nanoscale due to the precise thermodynamic and kinetic control of processes. The talk will discuss various approaches where SAMs and SMNs serve as templates on a length scale ranging from micrometres to the bottom end of the nanoscale. Emphasis will be put on the generation of metal structures by SAM controlled electrodeposition [2] and the use of SMNs as ultraprecise matrix [3].
References:
[1] R. Madueno, M. T. Raisanen, C. Silien, M. Buck, Nature 454, 618 (2008), Functionalizing hydrogen-bonded surface networks with self-assembled monolayers.
[2] Z. She, A. DiFalco, G. Hähner, M. Buck, Beilst. J. Nanotechnol. 3, 101 (2012), Electron-beam patterned self-assembled monolayers as templates for Cu electrodeposition and lift-off. .
[3] M. T. Räisänen, A. G. Slater, N. R. Champness, M. Buck, Chem. Sci. 3, 84 (2012), Effects of pore modification on the templating of guest molecules in a 2D honeycomb network.
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13:15
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lunch
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Session 7: Chair: Rainer Adelung
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14:15
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Ulrich Jung
Kiel
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Photoswitching of Azobenzene-Functionalized Adlayers on Au(111)
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show abstract
Functional surfaces are of particular interest because of future applications (e. g. for data storage devices, lab-on-a-chip, molecular machines, etc.). In many cases, such surfaces are prepared by adsorption and self-organization of appropriate functional molecules from solution on solid substrates. In particular, very often molecules are employed, which bind to a metal via a thiol group. However, such adlayers were typically found to exhibit only a reduced or even completely quenched function, which can be explained by interactions between the substrate and the adsorbate molecules (electronic or excitonic coupling) as well as intermolecular interactions (steric hindrances). Common approaches to overcome these problems employ molecules with additional bulky spacer groups or mixed monolayers with short spacer molecules. Unfortunately, these adlayers often exhibit only a reduced structural order or a tendency to form bilayers. We have introduced a novel concept based on chemically customizable molecular platforms [1,2], which allows to orient molecular functions in a defined manner with respect to the substrate and to precisely control the distance between the surface and the functions as well as the intermolecular distances within the surface plane. In addition, we have extensively characterized adlayers of such compounds on Au(111) surfaces by structure-sensitive (STM) [1,3,4,5], spectroscopy (XPS, SERS, and surface plasmon resonance (SPR) spectroscopy) [4], and electrochemical methods (cyclic voltammetry) [4] and clearly demonstrated that the molecular platforms adsorb intact and form highly-ordered structures.
Here, we discuss the photoswitching of azobenzene-functionalized platform adlayers in detail and compare it to that of conventional adlayers basing on thiols. In particular, we have studied these systems by photoelectrochemical methods (cyclic voltammetry and chronoamperometry) [6,7], (localized) surface plasmon resonance spectroscopy [6,7], and UV-VIS spectroscopy. The trans-cis photoisomerization was identified in all adlayers and follows first-order kinetics with a life time of a few seconds under the experimental conditions. However, distinct differences between platform- and thiol-bound azobenzenes were found in respect of the thermally activated cis-trans backisomerization. Whereas for the latter species the cis isomer is very stable in the adlayers and in solution, the thermal cis-trans isomerization proceeds in the platform adlayers several orders of magnitude faster than in solution. The surprising instability of the cis isomer in the platform adlayers may be explained by a better electronic coupling between the azobenzene functions and the metal substrate.
[1] B. Baisch et al., J. Am. Chem. Soc., 131, 442 (2009)
[2] J. Kubitschke et al., Eur. J. Org. Chem. 5041-5055 (2010)
[3] S. Kuhn et al., PCCP, 12, 4481 (2010)
[4] U. Jung et al., Langmuir, 27, 5899 (2011)
[5] S. Kuhn et al., Chem. Commun., 47, 8880 (2011)
[6] U. Jung et al., Langmuir, 26, 13913 (2010)
[7] U. Jung et al., submitted to Adv. Mat.
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14:50
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Franz Faupel
Kiel
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Photoswitchable polymer nanocomposites
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show abstract
In this work photoswitchable molecules are combined with polymer nanocomposites [1] containing metal nanoparticles or carbon nanotubes to achieve different functional properties. This includes a new concept of light-controlled impedance switching. Photoswitchable metal/polymer nanocomposite films and nanosheet capacitors were prepared by physical vapor deposition of Au and Pt nanoparticles onto spin-coated polymer/chromophore blends with a thickness around 200 nm, followed by embedding of nanoparticles into the polymer. These blends consist of poly(methylmethacrylate) films doped with azo dyes molecules. A second type of nanocomposite films were made from these blends mixed with carboxyl functionalized multiwall carbon nanotubes (MWCNT-COOH). High dye concentrations were achieved by functionalizing the azo molecules with tails and branches thus enhancing solubility. The composites show completely reversible light controlled impedance switching with capacitance changes > 50 % upon alternating irradiation with UV and blue light, respectively. [2]. Reversible light controlled conductance switching is shown for metal/polymer and MWCNT/nanocomposite films near the percolation threshold. For the metal nanoparticle containing composites this is attributed to changes in the metal nanoparticle separations upon isomerization based on model experiments where analogous conductance changes were induced by swelling of the composite films in organic vapors and by tensile stress [3]. In contrast, the behavior of the MWCNT/nanocomposite films arrears to be dominated by electronic effects.
[1] F. Faupel, V. Zaporojtchenko, T. Strunskus, M. Elbahri, Adv. Eng. Mat. 12, 1177 (2010)
[2] V. Zaporojtchenko. C. Pakula, S. W. Basuki, T. Strunskus, D. Zargarani, R. Herges, F. Faupel, Appl. Phys. A. 102, 421 (2011).
[3] C. Pakula, V. Zaporojtchenko, T. Strunskus, D. Zargarani, R. Herges, F. Faupel, Nanotechnol. 21, 465201 (2010).
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15:25
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boat trip excursion
(16:00 - 18:00)
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19:30
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dinner
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Wednesday, 22.08.2012
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07:00
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breakfast
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Session 8: Chair: Thisbe K. Lindhorst
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09:00
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Peter L. Davies
Kingston, Canada
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Design and Engineering of Proteins that Bind to Ice
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show abstract
Antifreeze proteins (AFPs) prevent some organisms from freezing and help others to tolerate freezing. These highly diverse proteins are freely soluble in water and yet irreversibly bind to water in its solid state to inhibit ice crystal growth. The ice-binding site (IBS) of an antifreeze protein is typically flat, extensive, and relatively hydrophobic compared to other surfaces of the protein. There is increasing evidence for a binding mechanism where the IBS organizes surface waters around hydrophobic groups into a clathrate structure that is anchored to the protein by hydrogen bonding. These ice-like surface waters match those of the quasi-liquid layer next to ice and the two ordered layers merge and turn to ice. Thus the AFP helps form the ligand to which it binds. The activity of AFPs can be increased by joining them to other proteins, where the size of the fusion protein is proportional to the increase in activity. Even greater activity is attained by increasing the surface area of the IBS. Ice nucleating proteins (INPs) have the opposite effect to AFPs by promoting freezing at high sub-zero temperatures. Modeling has shown that INPs look like much larger versions of some AFPs. By extension, the mechanism of ice nucleation is thought to be similar to ice binding and involves the INPs ordering sufficient ice-like water molecules on their surface to form a homogeneous ice nucleus that initiates freezing.
Supported by CIHR
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09:45
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Joachim Spatz
Stuttgart
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Spatially and temporally coordinated processes of cells at molecular to cellular scales
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show abstract
Our approach to engineer cellular environments is based on self-organizing spatial positioning of single signaling molecules attached to synthetic extracellular matrices, which offers the highest spatial resolution with respect to the position of single signaling molecules. This approach allows tuning tissue with respect to its most relevant properties, i.e., viscoelasticity, peptide composition, nanotopography and spatial nanopatterning of signaling molecule. Such materials are defined as “nano-digital materials” since they enable the counting of individual signaling molecules, separated by a biologically inert background. Within these materials, the regulation of cellular responses is based on a biologically inert background which does not initiate any cell activation, which is then patterned with specific signaling molecules such as peptide ligands in well defined nanoscopic geometries. This approach is very powerful, since it enables the testing of cellular responses to individual, specific signaling molecules and their spatial ordering. Detailed consideration is also given to the fact that protein clusters such as those found at focal adhesion sites represent, to a large extent, hierarchically-organized cooperativity among various proteins. We found that integrin cluster have a functional packing density which is defined by an integrin-integrin spacing of approximately 68 nanometers. Such critical spacing values vary as matter of transmembrane receptor choice of interest. We have also developed methods which allows the light initiated activation of adhesion processes by switching the chemical composition of the extracellular matrix. This enabled us to identify the frequency of leader cell formation in collective cell migration as a matter of initial cell cluster pattern size and geometry. Moreover, “nano-digital supports” such as those described herein are clearly capable of involvement in such dynamic cellular processes as protein ordering at the cell’s periphery which in turn leads to programming cell responses.
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10:30
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coffee break
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Session 9: Chair: Franz Faupel
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11:00
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Rainer Herges
Kiel
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Mounting Functional Molecules on Surfaces Using Molecular Platforms
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show abstract
The most frequently used method to attach functional molecules to gold surfaces is using thiols as anchor groups and alkyl chains as spacers. Disadvantages are the lack of free volume for molecular isomerization/switching in densely packed monolayers, and the fact that in sub-monolayers these fluffy molecules tend to lie flat on the surface. We avoid these problems by using molecular platforms as anchor groups, and stiff ethynyl or phenyl units as spacers. Functional molecules (switches) are placed in well-organized monolayers upright like lanterns on pedestals. Based on the “platform approach” a number of functional surfaces have been prepared. Currently, azimuthal rotors (a)), and a molecular muscle (b)) are investigated. Platform design also includes the development of volatile molecules for vacuum deposition or suitable substituted platforms for ESI deposition and STM investigation.
Figure. Model calculations: a) Zn-porphyrin mounted on a platform via an ethynyl pivot joint, and b) azobenzene connecting two phenylethynyl posts each mounted on a platform (molecular muscle).
[1] B. Baisch, D. Raffa, U. Jung, O. Magnussen, C. Nicolas, J. Lacour, J. Kubitschke, R. Herges, J. Am. Chem. Soc. 2009, 131, 442-443.
[2] J. Kubitschke, C. Näther, R. Herges, Eur. J. Org. Chem. 2010, 5041-5055.
[3] U. Jung, M. Mueller, N. Fujimoto, K. Ikeda, K. Uosaki, U. Cornelissen, F. Tuczek, C. Bornholdt, D. Zargarani, R. Herges and O. Magnussen, J. Colloid Interface Sci. 2010 , 341, 366-375.
[4] S. Kuhn, U. Jung, S. Ulrich, R. Herges and O. Magnussen, Chem. Commun. 2011 , 47, 8880-8882.
[5] U. Jung, S. Kuhn, U. Cornelissen, F. Tuczek, T. Strunskus, V. Zaporojtchenko, J. Kubitschke, R. Herges and O. Magnussen, Langmuir 2011 , 27, 5899-5908.
[6] S. Kuhn, B. Baisch, U. Jung, T. Johannsen, J. Kubitschke, R. Herges, O. Magnussen, PhysChemPhys 2010, 12, 4481-4487.
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11:45
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Jascha Repp
Regensburg
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Scanning Probe Spectroscopy of Individual Molecules on Thin Insulating Films
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show abstract
Ultrathin insulating films on metal substrates facilitate the use of the scanning tunneling microscope (STM) to study the electronic transport properties of single molecules in the weak coupling limit. The ionic relaxations in a polar insulator lead to a charge bi-stability in some adsorbed atoms and molecules. It is shown that control over the charge-state of individual molecules in such systems can be obtained by choosing a substrate system with an appropriate work function. We study the spatial distribution of the additional charge by suitable subtraction of low-voltage STM images of the adsorbates in their different charge states. These difference images show marked intra-molecular contrast [1]. In addition, we investigated C20S2H12 molecules adsorbed on ultrathin layers of NaCl in our combined STM/AFM setup. These non-planar molecules exist in two stable conformations. By means of excitations from inelastic tunneling electrons we can switch between both conformations. We present atomic force microscopy (AFM) measurements with submolecular resolution directly revealing the conformational changes [2]. From AFM data and taking the chirality of the molecules into account, we could unambiguously determine the pathway of the conformational change. Hence, the AFM channel reveals additional information that is truly complementary to the STM data set.
References:
[1] I. Swart, T. Sonnleitner, and J. Repp, Nano Letters 11, 1580 (2011).
[2] N. Pavliček, B. Fleury, M. Neu, J. Niedenführ, C. Herranz-Lancho, M. Ruben, and J. Repp, Phys. Rev. Lett. 108, 086101 (2012).
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12:30
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Thirunvancheril Gopakumar
Kiel
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Electronic and Magnetic Switching in Single Molecules on Surface
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show abstract
Molecules with switchable magnetic and electronic properties are promising building blocks for spintronics. The magnetic and electronic property of organo-metallic molecules may be manipulated via controlling the ligand field on the metal ion. An electron induced spin switching in a spin cross-over molecule and controlled de-coordination/coordination of ligand from/to the metal center of Fe-pophyrin will be discussed.
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13:05
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closing remarks
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13:15
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lunch
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departure
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