Solvent Effect on Halogen-Bonded Self-Assembled Nanostructures of a 2,9-Dibromo-Phenanthridine Derivative at the Liquid–Solid Interface

Abstract

Probing the halogen-bonded nanostructures of two-dimensional (2D) supramolecular self-assembly is essential to understand and engineer the nature of halogen bonding. However, exploring the formation mechanism of the halogen bond still remains a challenge because it is a new weak noncovalent interaction. Herein, we introduce nitrogen atoms and Br groups in the conjugated core to synthesize 2,9-dibromo-6-(hexadecyloxy)­phenanthridine (2,9-DHP) in order to fabricate multiple action sites of halogen bonds. We investigate the solvent effect on the 2D molecular self-assembly at the liquid–solid interface by using scanning tunneling microscopy. 1-Octanoic acid, n-tridecane, n-tetradecane, and n-hexadecane are chosen as the solvents. The coadsorption of 1-octanoic acid molecules at low concentrations by the molecule–solvent Br···O halogen bonds could change the molecular arrangement. n-Tridecane, n-tetradecane, and n-hexadecane also coadsorb in the adlayers; however, the molecular packing and intermolecular interactions of 2,9-DHP remain the same at different solution concentrations. The results demonstrate that the molecule–molecule N···Br···H halogen bonds are strong enough to overcome the transformation of intermolecular interactions.

Solvent Co-adsorption Changing the Type of Halogen-Based Bonds Associated with the Position of Bromine Substituents Revealed by Scanning Tunneling Microscopy

“…Compared with our previous work, , the bonding energy of dimer-I in 3,8-BHP adlayers (−5.59 kcal/mol) is higher than that of dimer-I in 2,9-BHP adlayers (−7.12 kcal/mol), indicating that the Br···N···H bonds in 3,8-BHP adlayer are weaker. Moreover, the bonding energy of dimer-I (−8.77 kcal/mol) involved in H···Br hydrogen bonds is the lowest in all the dimers, which is the main reason for the structural transformation once the solvents participate in the formation of nanostructures.…”

“…The solvent effect on halogen-bonded self-assembly nanostructures of 2,9-dibromo-6-(hexadecyloxy)­phenanthridine was also explored. Although solvents take part in the self-assembly process, the intermolecular interaction and molecular arrangement remain the same at different solution concentrations, resulting in the strong intermolecular N···Br···H halogen bonds and H···Br hydrogen bonds, so the vdW interactions are not competitive with them . In common, once the position of bromine substituent changes, it may directly affect the formation of the intermolecular interactions and nanostructure resulting from the directionality of halogen and hydrogen bonds. − Thus, it is necessary to investigate the solvent and solution concentration effects on the molecular self-assembly for phenanthridine derivatives with the position of the bromine substituent.…”

Solvent Coadsorption Effect on I···O Halogen-Bonded 2D Self-Assembled Nanostructures

“…The strength of “σ-hole” decreases with the electronegativity and increases with the polarizability of the halogen atom, which affects the directionality and tunability of the halogen bond. Generally, the strength of “σ-hole” follows the order I > Br > Cl > F = 0. , In homo-halogen bonds, halogen atoms act concurrently as both the donor and acceptor, and the electron-deficient “σ-hole” is toward the central “belt” of an electron-rich contiguous halogen atom. , Alternatively, other atoms (N, O, S...) act as acceptor sites in hetero-halogen bonds. − …”

Odd-Even Effect on Supramolecular Co-Assemblies: Control over the Two-Dimensional Self-Assemblies of a Fluorenone Derivative with Asymmetrically Substituted Alkyl Chains

“…Researchers have explored the self-assembly polymorphs of π-conjugated molecules via changing the solvent, , concentration, , temperature, , scanning tunneling microscopy (STM) bias, − molecular functional group, , the length of the alkyl chains, , and the substrate. , The used substrate contains highly oriented pyrolytic graphite (HOPG), Au (111), Ag (111), Si (111), Cu (110), and so on. − In particular, STM is the widely used visualization tool due to its atomic-level resolution. The driving forces that stabilize the self-assembled nanostructures include the weak hydrogen bonds, van der Waals interaction, dipolar interaction, halogen bonds, and π–π attractions. − …”