Jutta Schwarz

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.

Improving the Fatigue Resistance of Diarylethene Switches

When applying photochromic switches as functional units in light-responsive materials or devices, an often disregarded yet crucial property is their resistance to fatigue during photoisomerization. In the large family of diarylethene photoswitches, formation of an annulated isomer as a byproduct of the photochromic reaction turns out to prevent the desired high reversibility for many different derivatives. To overcome this general problem, we have synthesized and thoroughly investigated the fatigue behavior of a series of diarylethenes, varying the nature of the hetaryl moieties, the bridging units, and the substituents. By analysis of photokinetic data, a quantification of the tendency for byproduct formation in terms of quantum yields could be achieved, and a strong dependency on the electronic properties of the substituents was observed. In particular, substitution with 3,5-bis(trifluoromethyl)phenyl or 3,5-bis(pentafluorosulfanyl)phenyl groups strongly suppresses the byproduct formation and opens up a general strategy to construct highly fatigue-resistant diarylethene photochromic systems with a large structural flexibility.

Polymerization on Stepped Surfaces: Alignment of Polymers and Identification of Catalytic Sites†

Stepped surfaces have widely been used as model catalysts as they provide active sites which trigger many catalytic processes. Molecular-dissociation processes on surfaces are expected to be sensitive to the surface structure and occur preferentially at defects, such as step edges and kinks. Various surface-science techniques that average over the entire surface have been employed for the investigation of catalytic properties. Although such studies demonstrate the catalytic activity of a surface in general, they do not offer sitespecific information about the exact location of the reaction. Scanning tunneling microscopy (STM) provides a local probe, allowing site-specific information to be obtained about the structure and location of active sites on a catalytically active surface with atomic resolution. STM studies have allowed the direct observation of the active sites of heterogeneous catalysts, as shown for the dissociation of nitric oxide on a ruthenium (0001) surface, where the dissociation process was found to occur along steps, but the difference in reactivity between step edges and kinks is not clear. The majority of STM studies in catalysis have focused on small molecules, such as diatomic species, with relatively few studies investigating the reactivity of larger organic molecules. The largest molecule investigated to date is thiophene on MoS2 nanoclusters. The deposition of complex molecules, potentially carrying an intrinsic function, onto stepped surfaces has attracted much attention, especially with regard to templated supramolecular structures. Laterally ordered structures have been studied on various stepped gold surfaces with (111) terraces, but oligomers or polymers have not been investigated to date. Herein we study the adsorption of a,wdibromoterfluorene (DBTF) molecules on a stepped gold surface. Low-temperature STM under ultrahigh vacuum (UHV) conditions is used to locally determine both the molecular adsorption geometry and the site-specific catalytic activity of the step edges. We identify the step-edge kinks as the active sites in the catalytic process, because these defects promote the selective C Br bond dissociation at specific locations within the DBTF molecule. Thermally induced polymerization leads to the formation of chains, which align along the step edges in a predefined orientation. This system thus provides catalytically active sites as well as an anisotropic structure for the alignment of both the monomer precursors and the polymer products. For our studies, a gold surface with (10,7,7) orientation was used (experimental details in Supporting Information). This vicinal Au(111) surface has a misorientation angle of approximately 98 and {100}-orientated microfacets. It provides straight step edges along the [011] direction, separate flat terraces with (111) orientation, terrace widths (Wt) of around 1.4 nm [18] (Figure 1a), and is thus suitable for the adsorption of one row of molecules along each terrace. STM images of the clean surface show these terraces with an average width of 1.48 0.07 nm (Figure 1b). In agreement

Quantifying the atomic-level mechanics of single long physisorbed molecular chains

Significance Mechanical properties of biopolymers such as DNA and proteins have been studied to understand the details of complex processes in living systems via systematic statistical analyses of repeated measurements. However, the mechanical behavior of a single molecular chain pulled off a surface has never been investigated with atomic-scale resolution. Herein, we present such a study on in situ polymerized fluorene chains by pulling individual chains with the tip of an atomic force microscope at 4.8 K. The measured variations of the force gradient provide detailed insights into the detachment process of fluorene units and the role of near-incommensurability with the substrate structure. Individual in situ polymerized fluorene chains 10–100 nm long linked by C–C bonds are pulled vertically from an Au(111) substrate by the tip of a low-temperature atomic force microscope. The conformation of the selected chains is imaged before and after manipulation using scanning tunneling microscopy. The measured force gradient shows strong and periodic variations that correspond to the step-by-step detachment of individual fluorene repeat units. These variations persist at constant intensity until the entire polymer is completely removed from the surface. Calculations based on an extended Frenkel–Kontorova model reproduce the periodicity and magnitude of these features and allow us to relate them to the detachment force and desorption energy of the repeat units. The adsorbed part of the polymer slides easily along the surface during the pulling process, leading to only small oscillations as a result of the high stiffness of the fluorenes and of their length mismatch with respect to the substrate surface structure. A significant lateral force also is caused by the sequential detachment of individual units. The gained insight into the molecule–surface interactions during sliding and pulling should aid the design of mechanoresponsive nanosystems and devices.

Electrocatalytic Z → E Isomerization of Azobenzenes

A variety of azobenzenes were synthesized to study the behavior of their E and Z isomers upon electrochemical reduction. Our results show that the radical anion of the Z isomer is able to rapidly isomerize to the corresponding E configured counterpart with a dramatically enhanced rate as compared to the neutral species. Due to a subsequent electron transfer from the formed E radical anion to the neutral Z starting material the overall transformation is catalytic in electrons; i.e., a substoichiometric amount of reduced species can isomerize the entire mixture. This pathway greatly increases the efficiency of (photo)switching while also allowing one to reach photostationary state compositions that are not restricted to the spectral separation of the individual azobenzene isomers and their quantum yields. In addition, activating this radical isomerization pathway with photoelectron transfer agents allows us to override the intrinsic properties of an azobenzene species by triggering the reverse isomerization direction (Z → E) by the same wavelength of light, which normally triggers E → Z isomerization. The behavior we report appears to be general, implying that the metastable isomer of a photoswitch can be isomerized to the more stable one catalytically upon reduction, permitting the optimization of azobenzene switching in new as well as indirect ways.

Enantioselective Preparation of (2R)- and (2S)-Azetidine-2-carboxylic Acids

The enantiomerically pure amino ketones 13 and 31 were prepared starting from the commercially available amino diol 9 and D-serine (21), respectively. Irradiation afforded highly functionalized azetidinols 15 and 33 in a fully stereoselective manner and in high yields, whereas N-phenacylglycine 5 gave only the secondary products of a Norrish-Type-II cleavage. Compounds 15 and 33 were converted into (2R)- and (2S)-azetidine-2-carboxylic acids 20 and 37, respectively, in several steps. The influence of H-bonds on efficiency, chemo-, and stereoselectivity of the photochemical cyclization of 5, 13, and 31 was discussed. It was shown that conformational analysis of corresponding triplet biradicals is often valuable in understanding the photochemistry of amino ketones.

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.

Light‐Controlled Reversible Modulation of Frontier Molecular Orbital Energy Levels in Trifluoromethylated Diarylethenes

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their π-electronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge- as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energy levels. Here, we present a comprehensive study detailing how variation of the substitution pattern of DAE compounds, in particular using strongly electron-accepting and chemically stable trifluoromethyl groups either in the periphery or at the reactive carbon atoms, allows for the precise tuning of frontier molecular orbital levels over a broad energy range and the generation of photoinduced shifts of more than 1 eV. Furthermore, the effect of different DAE architectures on the transduction of these shifts to an adjacent functional group is discussed. Whereas substitution in the periphery of the DAE motif has only minor implications on the photochemistry, trifluoromethylation at the reactive carbon atoms strongly disturbs the isomerization efficiency. However, this can be overcome by using a nonsymmetrical substitution pattern or by combination with donor groups, rendering the resulting photoswitches attractive candidates for the construction of remote-controlled functional systems.

Making Nonsymmetrical Bricks: Synthesis of Insoluble Dipolar Sexiphenyls

A versatile synthesis of nonsymmetrical, terminally substituted p-sexiphenyl (6P) derivatives has been developed. The synthesis makes use of a nonsymmetrical starting material as well as modular functionalization using Suzuki cross-coupling to yield a soluble precursor, which finally is converted to the insoluble target 6P derivatives. These derivatives display similar electronic and optical properties to the parent 6P, yet the permanent dipole along their molecular axis allows for tuning of their self-assembly on various substrate surfaces.