Water-Soluble Arylazoisoxazole Photoswitches.

Azoheteroarenes are emerging as powerful alternatives to azobenzene molecular photoswitches. In this study, water-soluble arylazoisoxazole photoswitches are introduced. UV/vis and NMR spectroscopy revealed moderate to very good photostationary states and reversible photoisomerization between the E- and Z-isomers over multiple cycles with minimal photobleaching. Several arylazoisoxazoles form host-guest complexes with ß- and γ-cyclodextrin with significant differences in binding constants for each photoisomer as shown by isothermal titration calorimetry and NMR experiments, indicating their potential for photoresponsive host-guest chemistry in water. One carboxylic acid functionalized arylazoisoxazole can act as a hydrogelator, allowing gel properties to be manipulated reversibly with light. The hydrogel was characterized by rheological experiments, atom force microscopy and transmission electron microscopy. These results demonstrate that arylazoisoxazoles can find applications as molecular photoswitches in aqueous media.

Azo bond formation on metal surfaces.

The formation of azo compounds via redox cross-coupling of nitroarenes and arylamines, challenging in solution phase chemistry, is achieved by on-surface chemistry. Reaction products are analyzed with a cryogenic scanning tunneling microscope (STM) and X-ray photoelectron spectroscopy (XPS). By using well-designed precursors containing both an amino and a nitro functionality, azo polymers are prepared on surface via highly efficient nitro-amino cross-coupling. Experiments conducted on other substrates and surface orientations reveal that the metal surface has a significant effect on the reaction efficiency. The reaction was further found to proceed from partially oxidized/reduced precursors in dimerization reactions, shedding light on the mechanism that was studied by DFT calculations.

Aryl Triflates in On-Surface Chemistry.

The reactivity of aryl triflates in on-surface C-C coupling is reported. It is shown that the triflate group in aryl triflates enables regioselective homo coupling with preceding or concomitant hydrodetriflation on Cu(111). Three different symmetrical π-systems with two and three triflate functionalities were used as monomers leading to oligomeric conjugated π-systems. The cascade, comprising different intermediates at different reaction temperatures as observed for one of the molecules, proceeds via initial removal of the trifluoromethyl sulfonyl group to give an aryloxy radical which in turn is deoxygenated to the corresponding aryl radical. Thermodynamically driven regioselective 1,2-hydrogen atom transfer leads to a translocated aryl radical which in turn undergoes coupling. For a sterically more hindered bistriflate, where one ortho position was blocked, dehydrogenative coupling occurred at remote position with good regioselectivity. Starting materials, intermediates as well as products were analyzed by scanning tunneling microscopy. Structures and suggested mechanism were further supported by DFT calculations.

α-Diazo Ketones in On-Surface Chemistry.

Polymerization of a biphenyl bis α-diazo ketone on Cu(111) and Au(111) surfaces to provide furandiyl bridged poly-para-phenylenes is reported. Polymerization on Cu(111) occurs via initial N2 fragmentation leading to Cu-biscarbene complexes at room temperature as polymeric organometallic structure. At 135 °C, carbene coupling affords polymeric α,ß-unsaturated 1,4-diketones, while analogous alkene formation on the Au(111) surface occurs at room temperature. Further temperature increase leads to deoxygenative cyclization of the 1,4-diketone moieties to provide alternating furandiyl biphenyl copolymers on Cu(111) (165 °C) and Au(111) (240 °C) surfaces. This work shows a new approach to generate Cu-biscarbene intermediates on surfaces, opening the pathway for the controlled generation of biphenyl copolymers.

Reaction Selectivity in On-Surface Chemistry by Surface Coverage Control-Alkyne Dimerization versus Alkyne Trimerization.

This work reports the influence of molecular coverage in on-surface C-C-bond formation on reaction outcome. 6-Ethynyl-2-naphthoic acid (ENA) was chosen as organic component and Ag(111) as substrate. The alkyne moiety in ENA can either react by dimerization to ENA dimers (Glaser coupling or hydroalkynylation) or cyclotrimerization to generate a benzene core as connecting moiety. Dimer formation is preferred at high surface coverage whereas trimerization is the major reaction pathway at low coverage. Mechanistic studies are provided.

Alternating copolymerization by nitroxide-mediated polymerization and subsequent orthogonal functionalization.

A novel method for the preparation of functionalized alternating copolymers is presented. Nitroxide-mediated polymerization of hexafluoroisopropyl acrylate with 7-octenyl vinyl ether provides the corresponding alternating polymer, which can be chemically modified using two orthogonal polymer-analogous reactions. A thiol-ene click reaction followed by amidation provides dual-functionalized alternating copolymers. The potential of this method is illustrated by the preparation of a small library (15 examples) of functionalized alternating copolymers.

Highly Selective On-Surface Reactions of Aryl Propiolic Acids via Decarboxylative Coupling.

Aryl propiolic acids are introduced as a new class of monomers in the field of on-surface chemistry to build up poly(arylenebutadiynylenes) through decarboxylative Glaser coupling. As compared to aryl alkynes that are routinely used in the on-surface Glaser coupling, it is found that the decarboxylative coupling occurs at slightly lower temperature and with excellent selectivity. Activation occurs through decarboxylation for the propiolic acids, whereas the classical Glaser coupling is achieved through alkyne CH activation, and this process shows poor selectivity. The efficiency of the decarboxylative coupling is documented by the successful polymerization of bis(propiolic acids) as monomers. It is also found that the new activation mode is compatible with aryl bromide functionalities, which allows the formation of unsymmetric metal-organic polymers on the surface by chemoselective sequential reactions. All transformations are analyzed by a scanning tunneling microscope and are further studied by density functional theory calculations.

Conformational evolution following the sequential molecular dehydrogenation of PMDI on a Cu(111) surface.

Molecular spatial conformational evolution following the corresponding chemical reaction pathway at surfaces is important to understand and optimize chemical processes. Combining experimental and theoretical methods, the sequential N-H and C-H dehydrogenation of pyromellitic diimide (PMDI) on a Cu(111) surface are reported. STM experiments and atomistic modeling allow structural analysis at each well-defined reaction step. First, exclusively the aromatic N-H dehydrogenation of the imide group is observed. Subsequently, the C-H group at the benzene core of PMDI gets activated leading to a dehydrogenation reaction forming metalorganic species where Cu adatoms pronouncedly protruding from the surface are coordinated by one or two PMDI ligands at the surface. All reactions of PMDI induce conformational changes at the surface as confirmed by STM imaging and DFT simulations. Such conformational evolution in sequential N-H and C-H activation provides a detailed insight to understand molecular dehydrogenation processes at surfaces.

Polymerization of silanes through dehydrogenative Si-Si bond formation on metal surfaces.

Element-element double bonds of group 14 elements can be formed in solution, but generally only by applying harsh reductive conditions using sterically highly shielded tetryl halides as precursors. The two-dimensional confinement in surface-assisted polymerization represents a valuable alternative to access such reactive compounds, as it allows shielding of the labile entities without requiring bulky residues and catalytic activation of the reactive groups. Here, we demonstrate Si-Si bond formation in on-surface chemistry. Polymerization upon multiple Si-H bond dissociation and subsequent Si-Si bond formation was achieved on Au(111) and Cu(111) surfaces by using two different monomers, each containing two silicon functional groups (CH3SiH2 or SiH3) attached to an aromatic backbone, leading to polymeric disilenes that interact with the surface. A combination of experimental and theoretical studies corroborates the formation of covalent Si-Si bonds between the long, highly ordered polymer chains with high diastereoselectivity. The reactive Si=Si bonds formally generated via double dehydrogenative coupling are stabilized via covalent Si-surface interaction.

Intermolecular coupling and intramolecular cyclization of aryl nitriles on Au(111).

The on-surface dimerization reaction of an organic nitrile on Au(111) is reported. The formation of the product, which contains five newly formed σ-bonds and a diazapyrene core structure, was investigated and characterized by scanning tunneling microscopy. Experimental and computational studies of reference compounds support our findings.