The application potential of photochromic molecules as tiny light-driven switches, manipulators, actuators or memories depends critically on how the 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 fs time-resolved fluorescence up-conversion spectroscopy after excitation at pump = 475 nm. In terms of the molecular dynamics in solution and in the polymer matrix, the excited-state relaxations were found to be substantially different. For the AB dyes in polymers, significantly longer excited-state lifetimes were observed. Towards practical applications, we will investigate AB-doped polymer films by means of transient absorption spectroscopy.

For the investigation of electronic coupling and chromophore interactions in molecular systems, two multichromophore AB derivatives were synthesized and characterized by NMR and UV/Vis spectroscopy. To probe and compare the photodynamics of these molecules, first femtosecond time resolved fluorescence measurements were performed. Furthermore, we will focus on molecular photoswitches based on the excited-state intramolecular hydrogen atom transfer process (electron-driven proton transfer, EDPT).

Die Magnetresonanztomographie (MRT) ist eines der wichtigsten bildgebenden Verfahren der Medizin. Da sie im Gegensatz zu den meisten anderen gängigen Methoden ohne ionisierende Strahlung auskommt, steigt ihre Bedeutung stetig. Ein wichtiger Aspekt bei der MRT ist der Einsatz von Kontrastmitteln, die bei ca. 35 % aller Untersuchungen verwendet werden.[1] Ein bisher unerschlossenes Gebiet sind photoschaltbare Kontrastmittel. Ein Grund hierfür ist, dass es bisher nicht möglich war den Magnetimus von isolierten Molekülen mit Licht zu schalten. Die Entdeckung des LD-CISSS eröffnet neue Möglichkeiten auf diesem Gebiet.[2] Die Funktionstüchtigkeit der verwendeten Nickel(II)porphyrine wurde in vitro bereits gezeigt.[3] Um einen Schritt in Richtung Anwendung zu machen, sollen wasserlösliche Versionen des gezeigten Moleküls synthetisiert werden. Damit die Funktion im Medium Wasser nicht verloren geht, muss außerdem die Donorstärke des axialen Liganden durch die Verwendung elektronreicher Pyridinderivate gesteigert werden.

Photoswitchable surfaces offer an effective control of cell adhesion. The use of light allows not only for a precise timing but also an effective spatial application. The photoswitchable layer consists of azobenzene derivatives mounted on glass, quartz or silicon surfaces.

Employed strategies include:
1. Switching of wettability
2. Fast constant oscillations
3. Integrin mediated adhesion

We aim at switching the adhesion of eukaryotic and prokaryotic cells in a spatially und temporally defined way by developing photoswitchable functional surfaces. On one hand, cell adhesion will be switched independently from any adhesion receptors by using azobenzene molecules. On the other hand, we will employ functionalized azobenzene molecules, which also make use of mechano-sensitivity in cell adhesion. First experiments on gold-nanostructured surfaces have shown switchable hydrophobicity. The surfaces have been fabricated using block-copolymer micelle nanolithography. The switching will be carried out using functionalized azobenzene molecules.

Photoswitchable functional surfaces have attracted considerable research interest in the past years owing to their high potential for numerous novel applications [1-3]. Detailed investigations of the specific factors and mechanisms which affect the photoisomerization efficiencies of surface-attached molecules are therefore of great importance.

Here, we report on the synthesis and spectroscopic investigation of a series of gold nanoparticles (AuNPs) functionalized with mixed self-assembled monolayers (mSAMs) containing different ratios of 4-(11-mercapto-undecanoxy)azobenzene (C-11 AB) and a photochemically inert background ligand (decanethiol). The obtained AB-covered AuNPs were characterized using transmission electron microscopy (TEM) and NMR, IR, and UV/Vis spectroscopy. The measured TEM images showed the production of monodisperse nanoparticles with diameters of ≈ 4 ± 0.8 nm (1σ). NMR, FTIR and UV/Vis spectroscopy confirmed the attachment of the ligands to the NP surface. Furthermore, it could be demonstrated that all ABfunctionalized AuNP systems studied so far show efficient, reversible trans-cis photoisomerisation. Upon irradiation in the ππ*-band with UV light at λ = 365 nm and in the nπ*-band with visible light at λ = 455 nm, respectively, the systems could be photoswitched back and forth several times. Investigations of the ultrafast dynamics of the AB isomerization processes on these AuNPs via fs time-resolved transient absorption spectroscopy are in progress. Moreover, the influence of the distance between chromophore and Au surface will be studied. AuNPs functionalized with 4-(7-mercapto-heptanoxy)azobenzene (C-7 AB) and 4-(3-mercapto-propanoxy)- azobenzene (C-3 AB) have been synthesized and are currently under investigation.

[1] S. D. Evans, S. R. Johnson, H. Ringsdorf, L. M. Williams, H. Wolf, Langmuir 1998,14, 6436.
[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.

1. Spincrossover on Surfaces
Spincrossover (SCO) systems on Au(111) surfaces for information storage are promising synthetic targets. The SCO complex units are based on Fe(II) metal ions in an octahedral coordination environment. Bispyrazolpyridine (BPP) Fe(II) complexes are interesting, because they exhibit an abrupt thermal spincrossover in the solid state. To generate a SCO complex on Au(111) surfaces, we are using a platform approach. The platform has a high affinity to the gold surface.

2. Azimuthal Rotors on Surfaces
Azimuthal rotors on Au(111) surfaces based on Porphyrin-TATAs are interesting targets, which can be used as molecular compasses. Those molecules can be orientated in magnetic fields due to their high and strong anisotropic, diamagnetic susceptibility. Their alignment can be detected by the polarisation of their absorption or fluorescence.

The norbornadiene-quadricyclane system as a reversely photoswitchable system has been investigated in terms of storing sunlight as chemical energy (cyclic stress). [1-3] We decided not only to use the energy that can be obtained from the cyclic stress but also the change in geometry, caused by the transformation of four sp2- to four sp3-carbon atoms. If these centers are substituted with sidechains containing donor atoms, the light-induced [2+2]-cycloaddition or cycloreversion should result in a drastic change of the complexation properties for metal ions. Therefore, this system could be used as an light-driven active ion carrier.

This “molecular excavator” was formerly substituted with polyglycols or amide-type “arms”. [4-7] Since the switching properties of these systems were not optimal, they should be enhanced by other types of substituents, especially heteroaromatic amides or ketones.

Recent experiments with pyrazolide substituents failed due to the easy hydrolysis of the amide bond, promoted even by weak acids and bases. In future systems, the heterocycles will be linked by C-C-bonds for increased stability.

References
1. Maruyama, K.; Tamiaki, H.; Kawabata, S. J. Org. Chem. 1985, 50, 4742-4749.
2. Scharf, H.; Fleischhauer, J.; Leismann, H.; Ressler, I.; Schlenker, W.; Weitz, R. Angew. Chem. 1979, 91, 696-707.
3. Nagai, T.; Takahashi, I.; Shimada, M. Chem. Lett. 1999, 9, 897-898.
4. Herges, R.; Reif, W. Liebigs Ann. 1996, 761-768.
5. Starck, F.; Jones, P.G.; Herges, R. Eur. J. Org. Chem. 1998, 2533-2539.
6. Herges, R.; Winkler, T. Eur. J. Org. Chem. 2001, 4419-4422.
7. Winkler, T.; Dix, I.; Jones, P.G.; Herges, R. Angew. Chem., Int. Ed. 2003, 42, 3541-3544.

Due to the ongoing trend to miniaturization the interest in molecular scale switches is growing fastly. Fulgides are promising systems for future applications. The contribution is going to present recent theoretical results on the photochemical properties and dynamics of E-iPr-Furylfulgide. After careful evaluation by comparison to high-level ab-initio data (CASSCF(14,12)/CASPT2 in a mixed double and triple zeta basis), semi-empirical GUGA-CI-OM3 has been used in direct molecular dynamics simulations[1] to get a better understanding of the ultrafast switching processes. Transient spectra covering a wavelength range of 500 nm and a time period of 1500 fs have been simulated, including 5 electronically excited states. The excellent agreement of these simulated spectra with the experimental ones allows for an interpretation in terms of mechanistic details. The analysis of the calculated trajectories leads us to the identification of three major deexcitation pathways and indicates how seemingly minor modifications of the functional groups play major roles in the control of the switching dynamics.

[1] Fabiano E. and Thiel W., J. Phys. Chem. A 112 (2008) 6859.

Functionalized triazatriangulenium (TATA) molecules adsorbed on a metal surface, e.g. Au(111) have been developed by Herges et al. as a general, new concept to obtain well ordered monolayers. STM, TDS and XPS experiments on a large number of TATA derivatives conclusively demonstrate the potential of the molecular system to achieve a variety of functionalized surfaces. We now performed DFT calculations to elucidate the bindung mode resonsible for the high affinity of the TATA platform to Au(111). Moreover, we gain inside into the influence of the gold surface on geometry and electronic structure. Calculations were performed on the PBE/SVP level including an additional dispersion correction term by Grimme et al. (PBE-D3) using a highly symmetric, four-layer cluster of 170 gold atoms cut from a Au(111) surface as a bulk gold model. The calculated large adsorption energies notwithstanding we predict a rather flat hypersurface for the lateral dislocation of the platform on the substrate. This strongly suggests a non-directional binding mode dominated mainly by disperse interactions.

The increasing interest in nanosciences in the last decades lead to the exploration of functionalized nanoparticles. Compared to the bulk material the nano scaled particles exhibit exceptional electronic, optical and electrochemical properties.[1] Gold nanoparticles (AuNPs) covered with thiol ligands are probably the most investigated systems. Various molecules bound to the gold surface thoroughly by the strong gold-sulfur bond have been used to stabilize gold colloids from aggregation.[2]

Our research in functionalizing surfaces with switchable molecules resulted in the development of the platform approach.[3] In this concept the azobenzene as switchable molecule is connected by an ethinyl spacer to a triazatriangulene platform which allows to control the lateral distance between the platforms.[4] To combine the topics of switchable molecules and nano technology, we transferred our established platform approach to gold nanoparticles.

[1] R. Sardar et al., Langmuir 2009, 25, 13840-13851.
[2] M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293-346.
[3] B. Baisch et al., J. Am. Chem. Soc. 2009, 131, 442-443.
[4] J. Kubitschke et al., Eur. J. Org. Chem. 2010, 5041-5055.

The coordination of axial ligands to porphyrins is of fundamental importance in many areas of chemistry, biology and material science.[1]

Ni(II)-porphyrins with their rigid quadratic planar coordination framework provide an excellent model to study the coordination-induced spincrossover effect upon binding of one or two axial ligands to the metal center which leads to a spin transition from S = 0 to S = 1.

In an ideal system the first association constant K1 should be considerably larger than K2 to provide the possibility to switch the spin state by adding/removing a single ligand. K1S and K2 are determined by NMR spectroscopy, monitoring the paramagnetic shift of the pyrrol protons as a function of the concentration of added ligand. By measuring the temperature dependence of K1S and K2, bonding enthalpies and entropies are determined.[2]

Aiming at the realization of a light-controlled, reversible system we use photoisomerizable azopyridines as axial ligands of the Ni-porphyrin. The photochromic ligands are designed in such a way that they associate/dissociate upon switching. Alongside with the change of the coordination number of Ni(II) the spin state switches between dia- and paramagnetic.[3]

In contrast to the classical spin crossover and LIESST which are solid state phenomena the LD-CISSS approach achieves bistability without coorperative effects in isolated molecules at room temperature. A similar approach with the photochromic ligands covalently attached to the porphyrin has recently been published.[4]

References
1. „The Porphyrin Handbook“, K. M. Kadish, K. M. Smith, R. Guilard Eds. Academic Press, London, 2000.
2. S. Thies, C. Bornholdt, F. Köhler, F. D. Sönnichsen, C. Näther, F. Tuczek, R. Herges, Chemistry Eur. J. 2010, 16, 10074–10083.
3. S. Thies, H. Sell , C. Schuett , C. Bornholdt , C. Näther , F. Tuczek and R. Herges JACS, 2011, accepted 02. sept.
4. S. Venkataramani, U. Jana, M. Dommaschk, F. D. Sönnichsen, F. Tuczek, R. Herges, Science 2011, 331, 445-448.

Azobenzenes and other photochromic molecules have gained great attention due to their use as molecular switches. Even though, these systems are well investigated in solution most promising applications require immobilization of the switches on a surface. In this project we are developing methods to attach azobenzene derivatives to single crystalline Au(111) surfaces.

To minimize the electronic coupling of the azobenzen with the gold surface and to prevent steric interaction with neighbouring molecules the azobenzene units have to be anchored in an upright position with a well defined orientation and distance to the surface and to each other. This approach should provide higher quantum yields for photochemical switching (trans to cis and cis to trans) then azobenzenes physisorbed flat on the surface. Moreover with polarized light it should be possible to control the switching direction. Thus, more advanced functions such as directed motion should be possible. A promising method to achieve the above mentioned properties is the use of TATA (triazatriangulenium) and TOTA (trioxatriangulenium) as the platform. The azobenzene derivative molecules can be covalently attached to the central carbon atom and are therefore perpendicular to the plane of the platform. [1]

Both the unfunctionalized and functionalized platforms form hexagonally ordered self assembled monolayers on Au(111) surfaces. [2,3,4] The intermolecular distance can be controlled by changing the length of the platforms side chains.

More advanced lateral structures will be attempted by substitution of the platforms with hydrogen-bond donor and acceptor domains. Via supramolecular interactions the platforms should form well-defined and highly ordered mixed monolayers. Using functionalized platforms with different functional groups attached to the central carbon atom, also control of vertical growth could be possible.

[1] J. Kubitschke, C. Näther, R. Herges, Eur. J. Org. Chem. 2010, 5041- 5055.
[2] S. Kuhn, B. Baisch, U. Jung, T. Johannsen, J. Kubitschke, R. Herges, O. Magnussen, Phys. Chem. Chem. Phys. 2010, 12, 4481-4487.
[3] B. Baisch, D. Raffa, U. Jung, O. M. Magnussen, C. Nicolas, J. Lacour, J. Kubitschke, R. Herges, J. Am. Chem. Soc. 2009, 131, 442-443.
[4] S. Kuhn, U. Jung, S. Ulrich, R. Herges, O. Magnussen, ChemComm 2011, 47, 8880-8882.

Regarding the literature on learning and interest, central findings can be summarized as following:

• Students often agree on the relevance of science, but do not see a likewise relevance or perspective for themselves in the area of science (e.g. the ROSE-study). Parents seem to have the highest impact of students´ career choices in the field of science (Taskinen, 2010).
• Students´ perspectives on scientists´ fields of activities and their personal characteristics are rather stereotype (e.g. Taskinen, 2010; Wentorf et al., in prep.).
• Teacher education does hardly include modern scientists´ areas of work, consequently, classroom teaching often represents and old fashioned “Nature of Science” (e.g. HLG, 2004; Blonder, 2010) Hence, the goals of this outreach and education project focus on applying the research on functional switches for the development of “educational switches”, both regarding interest and attitudes as well as understanding of core principles.

To enable long-term developments, students – especially student teachers – shall be involved in the program. Learning environments will be developed for secondary school students, accompanied by outreach activities for a greater public. Regarding teacher education, an international co-operation has been started and already led to a joint symposium at the most important European Conference on Science Education (ESERA) in August 2011. Teacher training workshops have been developed and piloted, as well as presentations for students, for example at the MNU student congress in September 2011. Existing materials such as experiments and models are collected and categorized for their possible use in class and/or in a laboratory for students at the university (in prep.). Additionally, a co-operation with the publisher “Spektrum” has been started, aiming at producing a “Spektrum neo” magazine on NanoScience in the near future. The next important step will be the enlargement of an exchange with all research areas and exemplary research groups of the Collaborative Research Centre which should be discussed during the meeting.