Functional interfaces play an important role in material science. Whereas more basic applications like corrosion inhibition or lubrication have been crucial for technological advances in past centuries more recently the spectrum of desired interface properties has expanded significantly. In addition to applications e.g. in the field of organic solar cells, where a rather complex, yet static architecture of rather heterogeneous components is employed (Graetzel cell), presently the rendering of dynamics to interfaces is becoming a challenge. In particular it is very attractive to realize interfaces which can be switched between two or more states by electrical signals – or by illumination with light.

Many of these approaches require a grafting of suitable molecules to solid substrates; e.g. in case of electrical switching clearly a contact to electrodes is needed.

In this talk we will describe a general strategy to prepare structurally well-defined model systems for organic thin layers supported on solid substrates based on organothiols [1,2]. A number of case studies will be presented, illustrating the general approach to obtain a desired organic surface by first choosing a target molecule, which performs the desired function, and then attaching a thiol anchor group (-SH) to it. The second step consists of simply immersing an Au-substrate in an ethanolic solution of the thiolated target molecule, thus yielding thiolate-based self-assembled monolayers, SAMs.

An important goal of the talk is to demonstrate that this apparently straightforward approach which has in the past decade been adopted by a huge number of groups is prone to problems which are frequently difficult to identify. Often it is tacitly assumed that the grafting of thiol-based anchors to an Au-substrate is a rather trivial process, leading to two-dimensional arrangements of the target molecules exhibiting more or less the same properties already known from bulk studies (either bulk or solution). A few selected case studies will demonstrate that frequently SAMs with unexpected properties are obtained. First, the preparation conditions occasionally require a systematic optimization (see [3] for the case of a larger, aromatic target mode molecule, anthracene). Quite often also other, non-trivial complications can occur. These phenomena include the presence of structures within SAMs different from those predicted assuming the bulk geometry of the target molecules, reactivities which are substantially lower than those in the liquid phase, or packing-induced changes of relative conformational energies. The latter aspect is particularly important for light-induced conformational changes in organic molecules and illustrates that a systemic approach is needed for building sophisticated functional interfaces.

[1] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Self-assembled monolayers as a form of nanotechnology, Chem. Reviews 105, 1103 (2005) [2] M. Kind, Ch. Wöll, Dünnstschichten - Maßgeschneiderte Organische Oberflächen, Chemie in unserer Zeit 2, 128-141 (2008) [3] D. Käfer, G. Witte, P. Cyganik, A. Terfort, Ch. Wöll, A comprehensive study of self-assembled monolayers of anthracenethiol on gold: solvent effects, structure, and stability, J.Am.Chem.Soc. 128, 1723, 2006

Nanocomposites consisting of metallic nanoparticles in a dielectric polymer matrix exhibit very interesting functional properties with applications ranging from optical filters [1] and high frequency soft magnetic materials [2] to antibacterial coatings [3]. In the present talk, we focus on nanocomposites near the percolation threshold, where the electronic properties change from metallic to insulating, and the electrical conductivity changes by many orders of magnitude [4]. Below percolation, electronic conductivity is governed by thermally activated hopping between neighboring nanoparticles. Since the tunneling probability depends exponentially on the particle separation this regime is very interesting for sensing applications [5]. Moreover, switchable devices can be obtained if suitable switches such as photosensitive molecules are incorporated into the structure [6].

The sensing ability of polymer based metal nanoparticle containing composites is demonstrated by sensors for organic vapors, where the swelling due to vapor sorption increases the nanoparticle separation. Selectivity is achieved via a fingerprinting approach taking advantage oft the different vapor solubilities in different polymers. Concerning the switching by chromophores, azobenzene derivate molecules were incorporated in PMMA and other polymers. In order to stabilize the molecules against crystallization and migration branched side groups were attached. Optical switching proved to be fully reversible. The resulting changes in local free volume were also studied by means of positron annihilation lifetime spectroscopy at the positron beam in Munich.

For switching of the electrical resistivity, gold nanoparticles were either incorporated in the chromophore containing polymers or the nanocomposites were applied as a coating. In the ongoing investigations, optical switching of the resistivity was demonstrated, but the effect turned out to be small and to involve a drift which is still under study. In contrast, large and fully reversible effects were seen for optical switching of the capacity. In a separate approach we also explore switching of the resistivity in nanocomposites by a magnetic field using the large hysteresis and large strains in ferromagnetic shape memory alloys [7].

1. A. Biswas, O. C. Aktas, U. Schürmann, U. Saeed, V. Zaporojtchenko, T. Strunskus and F. Faupel, Appl. Phys. Lett., 84, 2655, (2004). 2. H. Greve, C. Pochstein, H. Takele, V. Zaporojtchenko, F. Faupel, A. Gerber, M. Frommberger, and E. Quandt, Appl. Phys. Lett. 89, 242501 (2006). 3. V. Zaporojtchenko, R. Podschun, U. Schürmann, A. Kulkarni and F. Faupel, Nanotechnology, 17, 4904, (2006). 4. 5. 6. F. Faupel, V. Zaporojtchenko, T. Strunskus H. Greve, U. Schürmann, H. Takele, C. Hanisch, V. S. K. Chakravadhanula, N. Ni, A. Gerber, E. Quandt, and R. Podschun, Polymers & Polymer Composites , 16, 471 (2008). 7. Bechtold, C.; Gerber, A.; Wuttig, M.; Quandt, E.; Magnetoelastic hysteresis in 5 M NiMnGa single crystals, Scripta Materialia 58 (2008), 1022-1024

Due to the very regular and variable nano-sized pores, SBA-15 is a suitable host material for anchoring photoswitchable molecules (azobenzenes) on the surface of the pores. Light induced trans-cis isomerization of the azobenzene changes the free space in the pores. The final goal of the project is the development of a nanofilter allowing the separation of small molecules. The functionalization of SBA-15 was achieved applying a two-step post-synthesis (grafting) approach. In contrast to a one-step synthesis, the two-step approach offers a good control of the switch density in the pores. In the first step, the hydroxyl groups on the surface of the SBA-15 pores reacted in a condensation reaction with (3-aminopropyl)-triethoxysilane (APTES). In the second step, the amine groups of APTES were reacted with 4 phenylazobenzoylchloride yielding the final azobenzene switch. The results of chemical analysis, X-ray diffraction, nitrogen sorption and solid state NMR evidence covalent bonding between the azobenzene molecules and the silica surface. The more stable trans-form of azobenzene requires more space than the cis-form. By irradiation with UV-light (wave length = 365 nm), isomerization from the trans- to the cis-form is induced. Switching back from the cis- to the trans-form is achieved with light at 440 nm or by heating to 100 °C. The trans-form shows a strong absorption at 325 nm, whereas the cis-form exhibits only a weak absorption. Switching from trans to cis, the absorption located at 325 nm significantly decreases. The reversibility of this switching process can be simply monitored by the decrease and increase of this significant absorption. The stability of the switched state is much longer in the solid than in solution.

Research on dye molecules has been continuing to be at the forefront of new developments in Chemistry owing to their versatile functional properties associated with pi-conjugation. On a supramolecular level, appropriately controlled spatial arrangement of dyes enables pivotal functions in nature, the most intriguing examples being provided by the light-harvesting systems of purple and green bacteria which contain a large number of chlorophyll and carotene chromophores organized in cyclic arrays or tubular architectures by non-covalent interactions.

During the last few years, we have intensively investigated the organization of merocyanine, porphyrin, and perylene bisimide dyes by non-covalent forces into desirable nanoscale architectures as well as liquid-crystalline and crystalline solid state materials.

In this lecture, I will provide a brief overview on our achievements in the preparation of defined dye assemblies and their functional properties that originate from proper pi-pi-stacking. Our results reveal efficient energy and charge transport along artificial dye assemblies. Thus, smart electronic and photofunctional materials based on self-assembled synthetic dyes became available that have prospect to be embedded in technical devices or to be applied in biological environments.

Exposing functional compounds to light, heat, metal ions, and/or an electric potential may result in a (reversible) process that transforms the system into another state characterized by different physicochemical properties. Integration of compounds that are selectively responsive to such external triggers in various matrices (e.g., polymers and plastics, polyelectrolytes, and thin films) is of much current interest. These stimuli responsive materials (SRMs) are expected to become important components in organic/polymer-based devices. Possible applications range from targeted drug delivery, sensing materials, artificial muscles, molecular electronics to Boolean logic.

We used surface-bound Os and Ru polypyridyl complexes to demonstrate ppm-level sensing of water, hexavalent chromium, and Boolean logic. These systems are electrochromic and can exist in one of two oxidation states (M2+/3+). Reaction of an oxidizing or reducing agent with the covalently immobilized complexes on glass substrates can convert the metal centers from one oxidation state to the other. The absorption of light is coupled with the oxidation state of the sensors, logic gates and circuits and provides the output for these monolayer-based devices. The set-up is highly robust, accurate, and can be reset. In addition, the formation and electrochromic properties of self-propagating molecular-based assemblies will be discussed.

In order to build up miniaturized devices, one of the key challenges is to control molecular function at surfaces. Switches represent a prototype of such functional molecules. In particular, azobenzene undergoing trans-cis isomerization of the N=N double bond has been well investigated in solution as well as in the gas phase. However, molecular switches on active, i.e. metallic, substrates have thus far been poorly explored and for this purpose scanning tunneling microscopy (STM) at low temperature constitutes a powerful tool since it allows imaging of single molecules as well as manipulation of discrete atoms and molecules.

We have recently shown that isomerization of 3,3',5,5'-tetra-tert-butylazobenzene (TBA) on Au(111) can be induced either by the STM-tip via a tunneling mechanism at small distances or via an electric field mediated process at large distances. Thereby, the choice of the substrate turns out to be crucial as TBA cannot be isomerized neither on Cu(111) nor on Au(100). Several other azobenzene derivatives have been prepared to elucidate the role of intrinsic electronic properties as well as the coupling to the substrate.

Furthermore, multiple azobenzene switches have been connected either covalently or non-covalently. Using the latter approach, discrete supramolecular azobenzene assemblies have been prepared and could be successfully manipulated using the STM-tip as well as periodic switching could be accomplished in a self-assembled monolayer of another derivative depending on the commensurability with the substrate. Here, we will discuss our most recent results in these and related areas.

Using a rotaxane as backbone, transport of protons from one end of the rotaxane to the other shall be enabled by irradiation with light. For such a pump, the rotaxane must be equipped (i) with a ring containing a protonable site, (ii) with one photoacidic stopper (iii) with a positive charge in vicinity to the acid and (iv) with two different stations along the axle. One station consists of an amide which will bind the basic site of the ring by a hydrogen bond. Next to this station, an acid will be formed upon irradiation which will then protonate the ring. The positive charge in its vicinity will repulse the protonated ring and it will diffuse to the second station from which the proton can be liberated to the other side of the rotaxane. In total, light will induce an active transport of protons along the rotaxane.

We have investigated the switching behaviour of single M-TBA (methoxy-tetra-tert-butylazobenzene) molecules, which are based on an azobenzene core. Upon deposition at room temperature, these molecules, which are very similar to the previously studied TBA molecules [1,2], but exhibit an additional methoxy group, diffuse on the Au(111) surface and form highly ordered islands. In contrast to the TBA molecules, various molecular structures are found. Switching experiments (by applying voltage pulses) reveal that, although all molecules are chemically identical and adsorbed planar on the surface, their isomerization ability depends strongly on the surface structure. While molecules in some structures can be switched efficiently, no isomerization process could be induced in other structures. This shows that the surrounding molecules, i.e. direct atomic-scale environment, of each molecule determines its switching capability.

Furthermore, we could show that not only the lateral environment, but also the substrate underneath plays a fundamental role for the isomerization. After switching many molecules, the cis isomers can form a regular switching lattice. Thus, some molecules are efficiently isomerized (probability > 85%), while others are rarely switched (probability < 6%). Our results show that this unexpected behaviour is due to the commensurability between the surface structure and the molecular layer. Hence, the exact adsorption site of each molecule on the Au(111) surface precisely determines its ability to be switched [3].

Ultrathin insulating films on metal substrates are unique systems to use the scanning tunneling / atomic force microscope to study the electronic properties of single atoms and molecules, which are electronically decoupled from the metallic substrate.

Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film [1]. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. For example in the particular case of Ag adatoms even three different charge states could be observed [2]. Employing a low temperature tuning fork AFM the different charge states can be observed directly in the force signal.

In the case of molecules on ultrathin NaCl films the electronic decoupling allows the direct imaging of the unperturbed molecular orbitals. This will be shown for individual pentacene molecules [3]. Scanning tunneling spectroscopy of these double-barrier tunneling-junctions reveals strong electron-phonon coupling to NaCl phonons.

Using atomic/molecular manipulation a covalent bond between an individual pentacene molecule and a gold atom can be formed. This bond formation is reversible and different structural isomers can be produced. Direct imaging of the orbital hybridization upon bond formation provides insight into the energetic shifts and occupation of the molecular resonances.

Molecular switches will be an essential part of possible future molecular devices. The bistability in the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules constitutes a two-level system that can be manipulated and probed by low-temperature scanning tunnelling microscopy [4]. When adsorbed on an ultrathin insulating film, the molecules can be switched in a controlled fashion between the two states by excitation induced by the inelastic tunnelling current. The tautomerization reaction can be probed by resonant tunnelling through the molecular orbitals. Coupling of the switching process such that charge injection in one molecule induced tautomerization in an adjacent molecule will be shown.

References:
1. J. Repp, G. Meyer, F.E. Olsson, M. Persson, Controlling the Charge State of Individual Gold Adatoms, Science 305, 493 (2004) 2. F. E. Olsson, S. Paavilainen, M. Persson, J. Repp, G. Meyer, Multiple Charge States of Ag Atoms on Ultrathin NaCl Films, Phys. Rev. Lett. 98, 176803 (2007) 3. J. Repp, G. Meyer, S. M. Stojkovic, A. Gourdon, C. Joachim, Molecules on Insulating Films: Scanning-Tunneling Microscopy Imaging of Individual Molecular Orbitals, Phys. Rev. Lett. 94 026803 (2005) 4. P. Liljeroth, J. Repp, G. Meyer, Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules, Science 317, 1203 (2007)

Some of the principal issues in molecular conduction junctions are associated with their characterization and control. Presently, the main tool for characterizing such systems is their current response to an external (source drain or gate) voltage. An external voltage is also the principal real time control instrument available at present. In principle, light can also be used for these tasks. Several successful experiments involving light in molecular junctions, including surface enhanced Raman scattering (SERS) from such systems, show the feasibility and potential power of this approach. This is further underscored by recent observations of "giant" (SERS) that were attributed to molecules positioned in narrow gaps between metal particles - another type of nanojunction. I will describe our recent work on the optical phenomena in non-equilibrium molecular conduction junctions, focusing on the following subjects: (a) Bias effects on the linear response of molecular junctions to external radiation fields.

[M. Galperin and A. Nitzan, J. Chem. Phys. 124, 234709 (2006)] (b) Light induced current in unbiased junctions. [M. Galperin and A. Nitzan, Phys. Rev. Lett. 95, 206802 (2005), B. D. Fainberg, M. Jouravlev and A. Nitzan, Phys. Rev. B 76, 245329 (2007), B. Fainberg and A. Nitzan, Phys. Stat. Sol. 2009] (c) Raman scattering in biased molecular junctions. [M. Galperin, M.A. Ratner and A. Nitzan, Nano Letters, 9, 758 (2009), J. Chem. Phys. 130, 144109 (2009)] (d) Electron transmission induced by circularly polarized light. [S. S. Skourtis, D. N. Beratan, A. Nitzan, R. Naaman and D. Waldeck, Phys. Rev. Lett. 101, 238103 (2008)].

Using a cryogenic scanning tunneling microscope, reversible switches of intermolecular bonds, molecule conformations and orientations were induced. Azobenzene molecules located at the end of azobenzene chains were controllably translated back and forth by reversibly forming and breaking hydrogen bonds to the adjacent azobenzene molecule. A conformational modification of Tin-Phthalocyanine was performed via the reversible vertical up and down movement of the central tin ion. This switch enables writing of patterns in ordered and densely packed molecule arrays [1]. Passing ballistic electron currents through C60 molecules, which was achieved by moving the tip of the microscope into contact with an individual molecule, was used to reversibly interconvert molecule orientations [2]. A first step to control the molecular magnetic moment was performed by adsorbing Iron-Phthalocyanine on ferromagnetic cobalt islands. The islands exhibit a moiré pattern with strongly localized Co d states, which organize the arrangement of the molecules [3]. Single-molecule spectroscopy provided evidence that the iron-phthalocyanine molecules are magnetized.

[1] Y. F. Wang, J. Kröger, R. Berndt, W.A. Hofer, J. Am. Chem. Soc. 131, 3639 (2009). [2] N. Néel, L. Limot, J. Kröger, R. Berndt, Phys. Rev. B 77, 125431 (2008). [3] T.G. Gopakumar, N. Néel, J. Kröger, R. Berndt, submitted to Phys. Rev. Lett. (2009).

The ring-opening reaction of spiropyran (SP) to form merocyanine (MC) is a photochromic processes, which can be reversibly induced in solution. The interest in this basic reaction stems from the different properties of each isomer, regarding to structure (SP is chiral), electrical dipole moment (MC has a large dipole) and electronic structure (MC is coloured). In solution SP is the thermally stable isomer. Here, we report that on a metal surface, MC turns out to be more stable. We investigate the thermally induced ring-opening transitions from the organic photo switch 6-nitro-spriropyran to its merocyanine isomer on a Au(111) surfaces by means of low temperature scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS) and force-field simulations. Under submonolayer coverage, each isomer presents a very distinct behaviour in terms of electron properties and structure of the self-assembled molecular domains. Once that the merocyanine domains are created, they remain stable, suggesting that interaction of the conjugated planar backbone with the metal surface helps to stabilize this isomer on the metal. The ring opening reaction can be also induced by local electric currents conducted through the molecule by the STM tip.
co-authors: Gunnar Schulze, and Katharina Franke

The processing, nanopatterning, manipulation and quantitative study of the physico-chemical properties of multifunctional materials across multiple length scales are crucial for technological applications in organic electronics. Solvent vapour annealing (SVA) post-treatments of (macro)molecular thin films physisorbed on surfaces can be successfully employed to form crystalline electroactive architectures such as millimetre long crystalline fibers of the semiconducting n-type perylene-bis-dicarboximide (PDI)[1a] and p-type hexabenzocoronene[1b]. The mechanism of this process complies with an Avrami-type nucleation governed growth. [1a] Fibers formed using such a strategy revealed a 10-fold increase in conductivity compared to the pristine PDI thin layer.[1a]

Supramolecular recognition between pre-programmed building blocks incorporating electrically and optically active units can be successfully used to generate multifunctional bi- and multi-component nanopatterns with controlled geometries. To this end, highly directional non-covalent interactions among suitable molecular modules have been successfully employed, including metal-ligand among molecular tectons embedding anthracene moieties [2] and H-bonds among guanosine derivatives exposing oligothiophene side-groups,[3] to generate 2D pattern with a precision on the sub-nm scale. Furthermore we demonstrated for the first time that by applying a supramolecular approach to the development of chemisorbed bi-component SAMs on Au(111), it is possible to form multicomponent crystalline domains, thereby achieving sub-nm control over the molecular patterning of a surface. This represents the first, yet fundamental, step towards the controlled spatial confinement of single molecules or functional groups on a surface. This supramolecular multicomponent array allows potential recognition of target functional groups and could therefore lead to the detection of single-molecule properties. [4]

External stimuli can be used to manipulate single molecules and aggregates embedded in a supramolecular ensemble. Prototypes of light-powered mechano-chemical switches can be developed: Significantly, the photochemical isomerization of a new terminally thiolated azobiphenyl rigid rod, forming a single component and tightly packed SAM on metallic surfaces, was found by STM to be highly cooperative and to be complete over a molecular 2D crystal.[5a] Such an azobenzene SAM has been successfully used to modulate the current through metal-organic-metal junctions. By incorporating the azobenzene SAM between a Au(111) support and a metal coated AFM tip, we could detect a 30-fold difference in the current through the junction, providing the first example of conducting AFM measurement on a bi-stable system. [5b] When incorporated into a macroscopic Hg drop based junction the SAM could also operate as a current photoswitch; interestingly the light induced vertical displacement of the Hg drop revealed that our SAMs acts as light-powered cargo lifter generating a force per unit area as high as 1x105 N/m2..[5b] This result unambiguously demonstrates that our azobenzene SAM represents a prototype of a molecular machine able to transport mass, and in particular to act as a cargo lifter. [5c] Noteworthy, our SAMs undergoing cis-trans isomerization represents the first molecular switch/motor operating on a surface according to a cooperative process. Our responsive system can be used to gate optical signals, and therefore has potential for implementing logic operations on arrays of switching elements, and ultimately for high density data storage based on artificial molecular systems.

Change in pH can also be employed to trigger conformational transition in 2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridine (BTP) molecules physisorbed on surfaces, as observed with a sub-nm resolution by STM at the solid-liquid interface. Upon addition of trifluoroacetic acid two different BTP molecules, each forming a highly ordered physisorbed monolayer, underwent significant conformational changes from their "rosette" to their "tetragon" forms, as reflected in dramatically altered 2D self-assembly over large areas extending over hundreds of nanometers.[6]

Beyond imaging,[7] Scanning Probe Microscopies make it possible to study quantitatively various physico-chemical properties of architectures based on organic molecules. We have focused our attention to exploration of the electronic properties of mono-[8] and multi-component functional nanostructures by Kelvin Probe Force Microscopy (KPFM).[9]. By KPFM the photovoltaic activity in electron acceptor/donor blends was quantitatively studied both on the hundreds of nanometers[10] and, for the first time, on the few nanometers scale.[11] These results are of paramount for both achieving a full understanding over the fundamental processes of exciton split and electron/hole diffusion in blends for photovoltaics, but also for the optimization of organic solar cells.

[1] (a) Adv. Funct. Mater. 2007, 17, 3791. (b) Small 2009, 5, 112. [2] (a) Angew. Chem. Int. Ed.., 2007, 46, 245. (b) Adv. Mater. 2009, 21, 1131 [3] (a) Org. Lett. 2006, 8, 3125. (b) Chem. Eur. J. , 2007, 13, 3757. (c) Adv. Mater. 2008, 20, 2433 [4] Angew. Chem. Int. Ed. 2008, 47, 2484 [5](a)PNAS 2007, 104, 9937. (b)J. Am. Chem. Soc. 2008, 130, 9192. (c)Angew.Chem.Int.Ed. 2008, 47, 3407 [6] Chem. Eur. J. 2009, 15, 4788 [7] (a) J. Mater. Chem. 2004, 14, 1353. (b) Chem. Soc. Rev. 2005, 34, 551 (c) Scanning probe microscopies beyond imaging (Guest Editor: P. Samori) Wiley-VCH (2006). [8] Adv. Funct. Mater. 2006, 16, 1407-1416. (b) J. Phys. Chem. C 2008, 112, 17368. (c) Adv. Funct. Mater. 2008, 18, 907 [9] (a) Adv. Mater 2006, 18, 145-164. (b) Chem. Commun. , 2007, 3326-3337. [10] (a) Adv. Funct Mater. 2007, 17, 472. [11] (a) J. Am. Chem. Soc. 2008, 130, 780-781. (b) J. Am. Chem. Soc. 2008, 130, 14605

Molecular switches such as azobenzenes and other photochromic molecules are well investigated in solution. In our project we are developing methods to attach azobenzene derivatives on metals, e.g. Au(111) to obtain functional surfaces. To achieve high switching quantum yields with UV (trans/cis-) and visible light (cis/trans-isomerization) and efficient functionality (e.g. data storage, surface property switching, directed motion) the azobenzene has to be mounted in a well defined orientation and distance with respect to the surface and to each other.

To achieve that goal, we attach the azobenzene in an upright position to a molecular platform, the so called TATA (triazatriangulenium) platform.[1] The azobenzene is attached covalently to the central carbon atom and perpendicular to the plane of the platform (like a lantern on a supporting stand). The TATA platform itself as well as the functionalized platform form hexagonally ordered monolayers on Au(111) surfaces by self assembly.[2] By increasing the length of the side chains of the platform the intermolecular distance can be tuned.[2] The cis/trans switching quantum yields of the azobenzene units are by far superior to previously investigated alkane-thiol fixed azobenzenes.[3]

[1] B. W. Laursen, F. C. Krebs, Chem. Eur. J. 2001, 7, 1773-1783. [2] 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. [3] S. D. Evans, S. R. Johnson, H. Ringsdorf, L. M. Williams, H. Wolf, Langmuir 1998, 14, 6436-6440.

The geometric structure and optical properties of self-assembled monolayers of azobenzene-functionalized alkanethiols have been investigated with respect to UV/Visible and near-edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl endgroup to the chromophore both the molecular tilt and twist angle of the azobenzene moiety were determined. Based on a detailed structural analysis the energetic shifts observed in optical reflection spectroscopy and the formation of J and H aggregates can be qualitatively described within an extended dipole model. This substantiates sizeable excitonic coupling among the azobenzene chromophores as an important mechanism that strongly suppresses trans to cis isomerization in densely-packed self-assembled monolayers.

This talk gives an overview of the solid-state photoemission activities in the Kiel SFB 677. It is divided into two parts, which focus, on the one hand, on the ultrafast dynamics of a molecular switching process and, on the other hand, on the effect of layered substrate materials on the switching efficiency. The first part concentrates on tin-phthalocyanine (SnPc). This non-planar (shuttlecock-shaped) molecule can adsorb on metallic surfaces in two different orientations due to its specific geometry. Wang et al. have demonstrated switching between these geometries using the tip of a scanning tunneling microscope [1]. The question arises if also light-induced switching of the adsorbed molecule is possible. In a Two-Photon Photoemission study we monitored changes in the spectral signature of SnPc molecules adsorbed on Ag(111) in response to the illumination with femtosecond light pulses. Reversible changes are observed, which depend on the intensity of the illumination light. Time-resolved 2PPE measurements show, that the characteristic dynamics of the light-induced processes take place on a femto- to picosecond time scale. This ultrafast behavior indicates that mainly the electronic excitation and relaxation of the molecules are probed in the 2PPE study. In the second part, the adsorption and possible photoswitching of azobenzene and some derivatives on the surfaces of layered transition-metal dichalcogenides were investigated with angle-resolved photoemission spectroscopy (ARPES). The key motivation is that layered materials may turn out as ideal substrates because their inert surfaces should lead to weak molecule-substrate coupling and thus more efficient switching. Thin molecular films were prepared on metallic as well as semiconducting compounds under ultrahigh vacuum conditions. The characterization by ARPES provides some insight into the growth modes and molecular ordering. Most notably, however, significant and reproducible photoinduced changes in the photoemission spectra are found for certain molecule/substrate combinations. These are tentatively attributed to photoswitching of the molecules. [1] Y. Wang, J. Kröger, R. Berndt, W.A. Hofer, J. Am. Chem. Soc. 131, 3639 (2009).

Many ortho-substituted nitroarenes are prone to photo-induced intramolecular redox reactions. The redox process can result in the release of a leaving group turning such nitroarenes into important photo-labile protecting groups. The photo-redox reaction of o-nitrobenzaldehyde yielding o- nitrosobenzoic acid is prototypical for this class of photo-reactions. We have performed a series of femtosecond experiments aiming at the mechanism of this reaction. These experiments will be discussed with regard to the following aspects: (i) How can the structure of short-lived intermediates be elucidated? (ii) How does the photon energy of the excitation light affect the reaction? (iii) What is the multiplicity of the reactive state? (iv) What is the impact of the vibrational excitation on reaction kinetics?

The photo-induced reactions of molecular optical switches are the basis for numerous applications such as optical memory and light-driven molecular machines. For an optimized rational design of functional optical devices, a detailed knowledge of the underlying molecular ultrafast processes and their affecting factors is required. We have studied prototypical photochromic furylfulgides and severely constrained azobenzene molecular switches by femtosecond time-resolved spectroscopy and by quantum-chemical calculations.

Our broadband transient absorption data of the parallel E - C ring closure and E - Z isomerisation after excitation of the thermally irreversible photochromic furylfulgide (E)-2-[1-(2,5-dimethyl-3-furyl)-ethylidene]-3-isopropylidene succinic anhydride (1E) are consistent with conformer-specific photoreactions of 1E, where the s-cis conformer forms the (C)-isomer and the s-trans conformer reacts to the (Z)-isomer. The distinct isomerisation times of 0.1 and 0.25 ps for the E - C ring closure and E - Z isomerisation and TDDFT calculations suggest that the reactions proceed via distinctive conical intersections. The slower time scale for the E - Z isomerisation might indicate a pathway that is not barrierless or involves an optically dark intermediate state. An alternative explanation would imply photo-excitation to a common origin in the Franck-Condon region of the -conformer and a subsequent ultrafast branching of the excited-state wave-packet. A similar, but sterically more demanding furylfulgide showed E - C ring closure also with 0.1 ps, but only negligible E - Z isomerisation. This indicates a strong steric effect on the photoreactions, which we are currently investigating further. We have also turned to the study of the related benzofurylfulgides, where the benzo-annulation provides steric hindrance for the furyl moiety.

The bridged azobenzene (AB) derivative 5,6-dihydrodibenzo[c,g][1,2]diazocine (2) offers unique opportunities to study the Z - E and E - Z photoisomerizations of AB switches in the presence of restricted conformational freedom and ring strain. The severe intramolecular constraints lead to a clear separation of the S1 absorption bands with maxima at 405 nm (2Z) and 490 nm (2E), i.e., improved photochromic properties, and a reversed thermodynamic stability of both isomers compared to the unbridged AB. Our transient absorption measurements gave isomerisation times of 0.28 ps (2Z) and 0.35 ps (2E) and suggest barrierless excited-state isomerisation reaction pathways via conical intersections. From TDDFT and CASSCF calculations, a photoisomerisation mechanism on an isomerisation coordinate involving torsion of the CNNC dihedral angle and a conical intersection could be inferred. The improved photochromic properties, high quantum yields and ultrafast photoisomerisations make 2 a promising candidate as molecular optical switch especially at low temperatures.

Thorough understanding photo-induced reactions of molecular optical switches in isolation and in complex environments can only arise from a tight interplay of experiment and theory. Therefore, in project A1, we are conducting both femtosecond time-resolved spectroscopy and theoretical simulations (both static electronic structure theory and dynamics calculations) in close cooperation, for the same set of prototype molecular switches.

As backbone model for furylfulgide switches, we have examined the photochemical ring opening of cyclohexadiene (CHD). With a systematic series of CASSCF/CASPT2 calculations, we have established a global excited-state potential energy surface in the two degrees of freedom that drive the reaction, including all other degrees of freedom via full relaxation. Reactive dynamics on this surface have been simulated using quantum-mechanical wavepacket propagations, resulting in good agreement with experimental data on the ultrafast photochemical dynamics of CHD. However, our theoretical data also indicate that CHD has certain defects as a model for furylfulgide. Therefore, we are currently examining several more elaborate molecular systems in order to establish a better furylfulgide model.

As a cheaper alternative to this expensive ab-initio approach, we are employing full-dimensional classical-mechanical surface-hopping trajectory simulations with on-the-fly calculation of forces, which are generated from a semiempirical configuration-interaction method. System-specific re-adjustments of the semiempirical parameters to CASSCF/ CASPT2 calculations for the molecule under study are performed with our global optimization algorithms, leading to ab-initio-quality results at a fraction of the cost. As a demonstration of this approach, we present the first photodynamics simulations for the bridged azobenzene derivative that is also in the focus of the experimental work within A1.

These initial investigations set the stage for future work, in particular for a systematic development of better model systems and new photoswitchable molecules, and for simulations of molecular switching against/by external forces and in confined spaces. Via QM/MM embedding, the latter avenue will be extended towards simulating the dynamics of molecular switches in polymer matrices, again in close cooperation with experimental work on such systems.

Molecules with properties that due to external stimuli may be reversibly changed between defined states, or so called 'molecular switches', are envisioned as a key component of a future molecular nanotechnology. In this context, switches affixed at solid surfaces are of particular interest. We have in previous work studied the Azobenzene (C12H10N2) switch adsorbed at coinage metal surfaces, using Density Functional Theory (DFT) in the Generalized Gradient Approximation (GGA). At DFT-GGA level of theory, the switch - surface interaction ranges from weak chemisorption to pure physisorption, which combined with the adsorbate size, implies that known inadequacies of DFT in the description of the dispersive molecular van der Waals interactions significantly influence the results. The state of the art remedy for these shortcomings are semi-empirical pair potentials added as a correction to the DFT total energy. We have extended our previous study to include several such schemes in the description of Azobenzene adsorption at coinage metals. The esulting changes to both geometric structure and adsorption energies are substantial, in some cases significantly altering the pure DFT picture. Differences between schemes are also large. We discuss these results and varying performance of published schemes in the greater context of molecular adsorption at metal surfaces.

Molecular switches represent a fascinating class of functional molecules, whose properties can be reversibly changed between different molecular states by excitation with light or other external stimuli. Using surface science concepts like self assembly to align such molecules in a well defined geometry at solid surfaces, new functional properties may arise, which are relevant for different fields like, e.g., molecular electronics, sensing or biocompatible inter­faces. For a microscopic understanding of molecular switching at surfaces, it is essential to obtain detailed knowledge on the underlying elementary processes, for instance the excitation mechanism in photoinduced switching.

We will present a case study of a specifically designed azobenzene derivative on a metal surface, namely tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111), which is so far one of the best studied system for which reversible conformational changes have been demon­strated. Using two-photon photoemission (2PPE) [1-3] enabled us to follow the photoinduced and thermally activated reversible switching of TBA in direct contact with a Au(111) surface. The trans/cis-isomerization of TBA is accompanied by reversible changes in the electronic structure of the molecules, allowing to gain mechanistic and quantitative insight into the switching process. Our results demonstrate the feasibility of molecular switching at metal surfaces, but also indicate that the switching properties of the surface-bound species are strongly modified by the interaction with a metal substrate.

[1] S. Hagen et al., Appl. Phys. A, 93 (2008) 253. [2] S. Hagen et al., J. Chem. Phys., 129 (2008) 164102. [3] M. Wolf and P. Tegeder, Surf. Sci. in press, 2009.

The biconformational switching of single cyclooctadiene molecules chemisorbed on a Si(001) surface was explored by quantum chemical and quantum dynamicalcalculations and low-temperature scanning tunneling microscopy experiments. The calculations rationalize the experimentally observedswitching driven by inelastic electron tunneling (IET) at~5 K. At higher temperatures, they predict a controllable crossover behavior between IET-driven and thermally activated switching, which is fully confirmed by experiment.

The fast developments in advanced electronic technology call for new compounds showing a bistability behaviour on the nanometer scale. Spin crossover (SCO) molecular materials belong to an appealing class of switchable coordination compounds with spin state that can be reversibly triggered by temperature, pressure or electromagnetic radiation [1]. Most of the investigations have been carried out for coordination compounds incorporating 3d5, 3d6 and 3d7 transition metal ions in octahedral surrounding,[2] but the first crystal structures of a 3d4 system solved in both high-spin (HS) and low-spin (LS) states was recently communicated [3]. This phenomenon implies a variation of the ML6 geometry as well as of the unit cell, which can be tracked by X-ray diffraction. Although the origin of the SCO phenomenon is molecular, its cooperative manifestation depends on an efficient coupling between the SCO species in the crystal lattice through covalent and supramolecular interactions [4].

In this context, iron(II) 1,2,4-triazole and 1-R-tetrazole coordination polymers have attracted particular interest as their abrupt spin transition is generally associated with both hysteretic and thermochromic effects, thus providing a basis for their potential use in thermal display, memory devices and sensors [4]. We will demonstrate that the nature and mechanical character of the bridging ligand play a crucial role on the elastic transmission of molecular distortions associated to the thermally induced SCO of Fe(II) ions in 1D systems. We will also highlight the importance of the anionic sublattice dynamics on the spin transition of these compounds [5]. The importance of supramolecular interactions on the existence of the switching mechanism itself will be addressed too for 2D coordination polymers [6]. Recent developments on the use of a new range of thermochromic and photochromic switches will also be highlighted [7].

We acknowledge financial support from the IAP-VI INANOMAT (P6/17) and the FNRS (FRFC).
[1] P. Gütlich, Y. Garcia, H. Spiering, Magnetism: From Molecules to Materials IV, Wiley-VCH 2003, 8, 271. [2] Y. Garcia, P. Gütlich, Top. Curr. Chem. 2004, 234, 49. [3] (a) P. Guionneau, M. Marchivie, Y. Garcia, J. A. K. Howard, D. Chasseau, Phys. Rev. B 2005, 72, 214408. (b) Y. Garcia, H. Paulsen, V. Schünemann, A. X. Trautwein, J. A. Wolny, Phys. Chem. Chem. Phys. 2007, 9, 1194. [4] Y. Garcia, V. Niel, M. C. Muñoz, J. A. Real, Top. Curr. Chem. 2004, 233, 229. [5] (a) Y. Garcia, S. J. Campbell, J. S. Lord, Y. Boland, V. Ksenofontov, P. Gütlich, J. Phys. Chem. B 2007, 111, 11111. (b) Y. Garcia, V. Ksenofontov, S. Mentior, M. M. Dîrtu, C. Gieck, A. Bhatthacharjee, P. Gütlich, Chem. Eur. J. 2008, 14, 3745. [6] Y. Garcia, G. Bravic, D. Chasseau, C. Gieck, W. Tremel, P. Gütlich, Inorg. Chem. 2005, 44, 9723. [7] F. Robert, A. D. Naik, B. Tinant, R. Robiette, Y. Garcia, Chem. Eur. J. 2009, 15, 4327.

Transition-metal complexes with photoswitchable spin-states are an important class of molecular switches. One possibility to achieve photoinduced spin-state switching is the use of photoswitchable ligands that change the coordination number and/or the ligand field strength of the transition-metal complex upon irraditation with light. The talk addresses three topics which are centered around this theme:

(1) Ni-porphyrins with pyridine and azopyridine ligands: The electronic ground state of Nickel(II) porphyrins is S=0 without axial ligands and S=1 with one or two axial ligands. This can be exploited for the synthesis of Ni(II) complexes the spin state of which is switched by coordination and decoordination of axial ligands. With this goal in mind various azopyridine ligands have been synthesized. Coordination and light-induced decoordination of these ligands have been studied in detail. Preliminary investigations have been devoted to the determination of association constants for pyridine and amine ligands to Ni(II) porphyrins.

(2) Iron Complexes with azopyridine ligands. A “classic” theme of transition-metal complexes with photoswitchable electronic ground states are Fe(II) and Fe(III) spin-crossover complexes. In the present project a Fe(III) spin-crossover complex with an axial azopyridine ligand is investigated. The ligand field of the complex can be switched between high-spin and low-spin a by a photoinduced cis-trans isomerization of the ligand.

(3) Surface fixation of complexes with photoswitchable ligands: In order to become addressable at a molecular scale transition-metal complexes with switchable ligands have to be attached to surfaces. Synthetic methods to achieve this goal are presented. The surface-fixed complexes are investigated with IRRAS (infrared reflection absorption spectroscopy), XPS, NEXAFS, AFM and STM.

The talk will present two kinds of iron(ii) complex, data relating to their spin crossover behaviour in the solid state, and strategies to anchor ordered arrays on surfaces (gold, graphite). An ensemble showing unique spin characteristics will be discussed.