Probing Molecular Nanostructures of Aromatic Terephthalic Acids Triggered by Intermolecular Hydrogen Bonds and Electrochemical Potential

Abstract

Self-assembly provides unique routes to create supramolecular nanostructures at well-defined surfaces. In the present work, we employed scanning tunneling microscopy (STM) in combination with electrochemical techniques to explore the adsorption and phase formation of a series of aromatic carboxylic acids (ACAs) at Au(111)/0.1 M HClO4. Specific goals are to elucidate the roles of electrochemical potential and directional hydrogen-bonding on the structures and orientation of individual ACAs that form nanoarchitectures. ACAs are prototype materials for supramolecular self-assemblies via stereospecific hydrogen bonds between neighboring molecules. In this study, we mainly focus on a special ACA, terephthalic acid (TPA), which is almost insoluble in water, making the assembly of this molecule from aqueous solution challenging. Depending on the applied electric field, TPA molecules form distinctly different, highly ordered adlayers on Au(111) triggered by directional intermolecular hydrogen bonds. At low electrochemical potentials, TPA molecules are planar oriented, forming a potentially infinite hydrogen-bonded adlayer without any observed domain boundaries. The increase of the electrode potential triggers the deprotonation of one carboxylic acid functional group of TPA; additionally, this is accompanied by an orientation change of molecules from planar to perpendicular. In contrast, structural “defects” and multiple domain boundaries were found at this positively charged surface. The assembled nanostructures of TPA are compared with other ACAs (trimesic acid, benzoic acid, and isophthalic acid), and corresponding adsorption models were built for all molecular adlayers, showing that intermolecular hydrogen-bonding plays a determining role in the formation of two-dimensional ACA nanostructures.

Single-Molecule Sensing of Interfacial Acid–Base Chemistry

“…The conductance value of 10 −4.60 G 0 (Figure S3), almost 2 orders of magnitude lower than 10 −2.90 G 0 , could be assigned to molecular junctions formed by intermolecular hydrogen bonds of two molecules individually adsorbed at the tip and substrate. 31,32 As proven above, the formation of single-molecule junctions of carboxylic acids relies on the -COO − −Au bond. Therefore, we turn to the acid−base chemistry of 4-MTBA assembled on Au (111) and probe the probability of junction formation depending on the solution conditions (schematically illustrated in Figure 2a).…”

“…According to the solvent polarity order of dodecane < dimethyl sulfoxide, it may infer that the 4-MTBA almost remains protonated in dodecane. The conductance value of 10 –4.60 G 0 (Figure S3), almost 2 orders of magnitude lower than 10 –2.90 G 0 , could be assigned to molecular junctions formed by intermolecular hydrogen bonds of two molecules individually adsorbed at the tip and substrate. , …”

“…43,44 ρ surf is ∼0.2 nm 2 /molecule for the tilted adsorption of 4-mercaptobenzoic acid (4-MBA) with a molecular structure similar to that of 4-MTBA 45,46 and 0.6 nm 2 /molecule for the flat-lying adsorption of TPA. 31 Thus, we take the average r value of 20 nm (Figure S8) and introduce these values into eq 5. pK a surf values of 6.1 for TPA and 6.6 for MTBA were found, which are almost close to the reported values of 6.0 for 2-mercaptobenzoic acid (2-MBA) immobilized on a polycrystalline silver surface 7 and 7.0 for the interfacial pK a of 4-mercaptobenzoic acid (4-MBA) immobilized on a Au surface. 47 It is worth pointing out that the logarithm of ρ surf /πr 2 is used to obtain the value of pK a surf , and changes in ρ surf /πr 2 of several orders of magnitude will not make a great difference in pK a surf .…”

The self-assembly and pyridine regulation of a hydrogen-bonded dimeric building block formed by a low-symmetric aromatic carboxylic acid

“…10–15 For example, trimesic acid (TMA) is a typical building block with three symmetric distributed carboxylic acid groups which can form a varies of networks under different temperature and concentration conditions. 16–18 Other C 3 -symmetric tricarboxylic acid molecules with different aromatic rigid skeletons can also form typical hexagonal honey-comb porous networks, such as triphenylene-2,6,10-tricarboxylic acid (H 3 TTCA) and 1,3,5-tris(4-carboxyphenyl) benzene (BTB). 19,20 In addition, symmetrically distributed aromatic tetracarboxylic acid molecules such as NN4A and H 4 ETTC have been proved to be able to form uniform Kagomé or quadrilateral networks, respectively.…”

Molecular Imaging of Viologen Adlayers and In Situ Monitoring Structural Transformations at Electrode–Electrolyte Interfaces

“…Design and control of functional molecular assembly on a surface into an ordered two-dimensional (2D) supramolecular architecture are essential because the ordered molecular layers, or adlayers, form the basis of molecular devices, − metal–organic frameworks (MOFs), sensors, − surface templates, and liquid crystal materials . Organic adlayers are generally generated via molecular self-assembly on well-defined surfaces such as a single crystal substrate, − highly oriented pyrolytic graphite (HOPG), , mica, − and atomically flat metal films. , The interplay of molecule–substrate and molecule–molecule interactions allows the creation of a variety of nanoscale architectures. Compared with noncovalent interactions (hydrogen-bonding and van der Waals interactions), the strong Au–S interaction provides a unique opportunity to construct molecular adlayers using thiolated molecules.…”