Directed Assembly of Nonplanar TPE-Based Esters via C-H···O═C Hydrogen Bonds at the Solution/Solid Interface

Nonconventional hydrogen bonds are attractive molecular interactions that can generate self-assembled, dynamic, yet robust, multifunctional hydrogen-bonded organic 2D and 3D frameworks (HOFs). To effectively create and manipulate such HOF structures requires detailed knowledge of their molecular geometrical ordering properties. Here, we employ scanning tunneling microscopy (STM) and X-ray crystallography to probe the differential control of C-H···O═C hydrogen bonding in the ordering of nonplanar tetraphenylethene (TPE)-based methyl ester derivatives at the solution/solid interface and in the solid state. The esters include the core structures of tetraphenylethene (Me₄TPE), tetraphenylethene biphenyl (Me₂TPDC), and tetraphenylethene tetrakis-phenyl (Me₄ETTC), both meta- and para-substituted. At the solution/HOPG (highly ordered pyrolytic graphite) interface, the esters form surprisingly robust monolayer assemblies stabilized by a combination of multiple intermolecular C-H···O═C hydrogen bonds, π-π interactions, and molecule-substrate interactions. The proposed dominant C-H···O═C bonds involve cyclic motifs that show cooperative recognition together with bifurcated or three-centered hydrogen bonding geometries. Mostly similar conformations of C-H···O═C bonds exist in the solid state. Both molecular geometry and energetic parameters are found to complement each other in determining the extent and type of hydrogen bonding present in the tetraphenylethene-based methyl ester organizations at the solution/solid interface and in the solid state.