A combined quantitative experimental and theoretical structure determination shows TCNQ is not bent on Ag(111) as expected from conventional wisdom.

We report the formation of dimethyl sulfoxide (DMSO) molecular complexes on Au(111) enabled by native gold adatoms unusually linking the molecules via a bonding of ionic nature, yielding a mutual stabilization between molecules and adatom(s). DMSO is a widely used polar, aprotic solvent whose interaction with metal surfaces is not fully understood. By combining X-ray photoelectron spectroscopy, low temperature scanning tunneling microscopy, and density functional theory (DFT) calculations, we show that DMSO molecules form complexes made by up to four molecules arranged with adjacent oxygen terminations. DFT calculations reveal that most of the observed structures are accurately reproduced if, and only if, the negatively charged oxygen terminations are linked by one or two positively charged Au adatoms. A similar behavior was previously observed only in nonstoichiometric organic salt layers, fabricated using linkage alkali atoms and strongly electronegative molecules. These findings suggest a motif for anchoring organic adlayers of polar molecules on metal substrates and also provide nanoscale insight into the interaction of DMSO with gold.

Peculiar hydrogen-bonded molecular chains are spontaneously created from the selfassembly on a gold surface of a porphyrin functionalized with four aromatic amine moieties. The molecular chains are formed by a sequence of dyads, where the same molecule behaves alternately as a hydrogen bond acceptor or donor as a whole, at all its four aromatic amino groups. This remarkable bifunctional behavior is due to the conformational flexibility of the functionalizing amino groups, that switch from a planar, aniline-like conformation in donors, to a pyramidal, amine-like one in acceptors. Furthermore, we show that the acceptor porphyrins can trap gold adatoms underneath their center. Combined scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations characterize the structural and electronic modifications suffered by such molecules to establish amino-amino interactions. Notably, scanning tunneling spectroscopy (STS) measurements show that the HOMO-LUMO gaps of the acceptors and donors are respectively larger and smaller with respect to the isolated molecule, according to the reduced extent of conjugation occurring in the acceptors. In summary, experimental and theoretical results reveal a remarkable hydrogen-bonded complex where the amino groups act both as hydrogen bond donors and acceptors and suggest how hydrogen bonding can modify the geometrical and potentially also the electronic structures of highly conjugated molecules.

The replacement of carbon atoms at the zigzag periphery of a benzo[fg]tetracenyl derivative with an NBN atomic triad allows the formation of heteroatom-doped PAHs isosteres, which expose BN mimics of the amidic NH functions. Their ability to form H-bonded complexes has never been touched so far. Herein we report the first solution recognition studies of peripherally NBN-doped PAHs to form doubly H-bonded DD•AA and ADDA•DAAD-type complexes with suitable complementary H-bonding acceptor partners. The first determination of the Ka in solution showed that the 1:1 association strength is around 27 ± 1 M-1 for the DD•AA complexes in C6D6, whereas it rises to 1820 ± 130 M-1 for the ADDA•DAAD array in CDCl3. Given the interest of BNdoped polyaromatic hydrocarbons in supramolecular and materials chemistry, it is expected that these findings will open new possibilities to design novel materials, where the H-bonding properties of peripheral NH hydrogens could serve as anchors to tailor the organizational properties of PAHs. INTRODUCTION Following the vigorous synthetic developments of polycyclic aromatic hydrocarbons (PAHs), 1 the substitution (i.e., doping) of sp 2-carbon atoms with isoelectronic and isostructural BN couples is re-emerging as a versatile approach to tune the optoelectronic properties of these materials. 2-7 In particular, borazines 8,9 and BN-doped PAHs (e.g., azaborines, 10,11 borazapyrenes, 12,13 borazaphenanthrenes 12,14 borazanaphthalenes, 15,16 borazaanthracene 17,18 and boraazaperylene 19) are now increasingly attracting the attention of the physical and chemical community for their use in a broad spectrum of optoelectronic applications. 20-22 When used to decorate a periphery, nonsubstituted BN couples terminate with NH functions that, being more acidic than the CH analogues, could engage into H-bonding interactions as observed with boronic acids. 23 For instance, Liu and co-workers showed in a seminal report that 1,2-dihydro-1,2-azaborine can act as an H-bonding donor in the solid state and engage into a H-bond with the C=O

The E/Z isomerization process of a uracil-azobenzene derivative in which the nucleobase is conjugated to a phenyldiazene tail is studied in view of its ability to form triply H-bonded complexes with a suitably complementary 2,6-diacetylamino-4-pyridine ligand. UV-vis and H NMR investigations of the photochemical and thermal isomerization kinetics show that the thermal Z → E interconversion is 4-fold accelerated upon formation of the H-bonded complex. DFT calculations show that the formation of triple H-bonds triggers a significant elongation of the N═N double bond, caused by an increase of its π* antibonding character. This results in a reduction of the N═N torsional barrier and thus in accelerated thermal Z → E isomerization. Combined with light-controlled E → Z isomerization, this enables controllable fractional tuning of the two configurational isomers.