Maike V. Peters

Quantum Chemical Investigation of Thermal Cis-to-Trans Isomerization of Azobenzene Derivatives: Substituent Effects, Solvent Effects, and Comparison to Experimental Data

Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis --> trans isomerization. The effects of substituents, in particular their type, number, and positioning, on activation energies have been systematically studied with the ultimate goal to tailor the switching process. Trends observed for mono- and disubstituted species are discussed. A polarizable continuum model is used to study, in an approximate fashion, the cis --> trans isomerization of azobenzenes in solution. The nature of the transition state(s) and its dependence on substituents and the environment is discussed. In particular for push-pull azobenzenes, the reaction mechanism is found to change from inversion in nonpolar solvents to rotation in polar solvents. Concerning kinetics, calculations based on the Eyring transition state theory give usually reliable activation energies and enthalpies when compared to experimentally determined values. Also, trends in the resulting rate constants are correct. Other computed properties such as activation entropies and thus preexponential rate factors are in only moderate agreement with experiment.

Spatial periodicity in molecular switching

The ultimate miniaturization of future devices will require the use of functional molecules at the nanoscale and their integration into larger architectures. Switches represent a prototype of such functional molecules because they exhibit characteristic states of different physical/chemical properties, which can be addressed reversibly. Recently, various switching entities have been studied and switching of single molecules on surfaces has been demonstrated. However, for functional molecules to be used in a future device, it will be necessary to selectively address individual molecules, preferentially in an ordered pattern. Here, we show that azobenzene derivatives in the trans form, adsorbed in a homogeneous two-dimensional layer, can be collectively switched with spatial selectivity, thus forming a periodic pattern of cis isomers. We find that the probability of a molecule switching is not equally distributed, but is strongly dependent on both the surrounding molecules and the supporting surface, which precisely determine the switching capability of each individual molecule. Consequently, exactly the same lattices of cis isomers are created in repeated erasing and re-switching cycles. Our results demonstrate a conceptually new approach to spatially addressing single functional molecules.

Photoswitchable Catalysts: Correlating Structure and Conformational Dynamics with Reactivity by a Combined Experimental and Computational Approach

Photocontrol of a piperidines Brønsted basicity was achieved by incorporation of a bulky azobenzene group and could be translated into pronounced reactivity differences between ON- and OFF-states in general base catalysis. This enabled successful photomodulation of the catalysts activity in the nitroaldol reaction (Henry reaction). A modular synthetic route to the photoswitchable catalysts was developed and allowed for preparation and characterization of three azobenzene-derived bases as well as one stilbene-derived base. Solid-state structures obtained by X-ray crystal structure analysis confirmed efficient blocking of the active site in the E isomer representing the OFF-states, whereas a freely accessible active site was revealed for a representative Z isomer in the crystal. To correlate structure with reactivity of the catalysts, conformational dynamics were thoroughly studied in solution by NMR spectroscopy, taking advantage of residual dipolar couplings (RDCs), in combination with comprehensive DFT computational investigations of conformations and proton affinities.

Adatoms underneath Single Porphyrin Molecules on Au(111)

The adsorption of porphyrin derivatives on a Au(111) surface was studied by scanning tunneling microscopy and spectroscopy at low temperatures in combination with density functional theory calculations. Different molecular appearances were found and could be assigned to the presence of single gold adatoms bonded by a coordination bond underneath the molecular monolayer, causing a characteristic change of the electronic structure of the molecules. Moreover, this interpretation could be confirmed by manipulation experiments of individual molecules on and off a single gold atom. This study provides a detailed understanding of the role of metal adatoms in surface-molecule bonding and anchoring and of the appearance of single molecules, and it should prove relevant for the imaging of related molecule-metal systems.

Molecules with multiple switching units on a Au(111) surface: self-organization and single-molecule manipulation

Three different molecules, each containing two azobenzene switching units, were synthesized, successfully deposited onto a Au(111) surface by sublimation and studied by scanning tunneling microscopy at low temperatures. To investigate the influence of electronic coupling between the switching units as well as to the surface, the two azo moieties were connected either via π-conjugated para-phenylene or decoupling meta-phenylene bridges, and the number of tert-butyl groups was varied in the meta-phenylene-linked derivatives. Single molecules were found to be intact after deposition as identified by their characteristic appearance in STM images. Due to their mobility on the Au(111) surface at room temperature, the molecules spontaneously formed self-organized molecular arrangements that reflected their chemical structure. While lateral displacement of the molecules was accomplished by manipulation, trans-cis isomerization processes, typical for azobenzene switches, could not be induced.