S-Shaped Conformation of the Quaterthiophene Molecular Backbone in Two-Dimensional Bisterpyridine-Derivative Self-Assembled Nanoarchitecture
Résumé
The conformation and the two-dimensional self-assembly of 4′-(3′,4″-dihexyloxy-5,2′:5′,2″:5″,2‴-quater-thien-2,5‴-diyl)-bis(2,2′:6′,2″-terpyridine) molecules are theoretically and experimentally investigated. This molecular building block forms a hydrogen-bonded chiral supramolecular nanoarchitecture on graphite at the solid/liquid interface. Scanning tunneling microscopy (STM) shows that the molecule adopts an S-shaped conformation in this structure. DFTB+ calculations reveal that this conformation is not the lowest-energy conformation. The molecular nanoarchitecture appears to be stabilized by hydrogen bonding as well as van der Waals interactions. I-, Land nd D-shaped molecular conformations are, however, locally observed at the domain boundary, but these conformations do not self-assemble into organized 2D structures. ■ INTRODUCTION Engineering novel organic/inorganic interfaces thought the self-assembly of functionalized molecules 1−11 is attracting an enormous amount of research interest due to its expected applications in nanotechnology. 12−14 The electronic properties of a self-assembled organic or hybrid layer can be drastically affected by the organization of its building blocks at the interface with a conductive surface. 15,16 Controlling the arrangement of the building nanoblock at the nanoscale is therefore a key parameter governing the properties of the interface. Hydrogen bonding is a particularly appealing interaction governing molecular self-assembly due to the strength, the high selectivity, and the directionality of this binding. 17−27 Carboxylic groups can be used to strengthen molecular self-assembly because these substituents are expected to lead to the formation of double hydrogen bonds (O−H···O) between neighboring molecules. This strategy has been successfully used to achieve the formation of self-assembled porous and compact nanoachitectures. 28−31 An alternative consists of functionalizing the molecular skeleton with pyridine units instead of carboxylic groups. Pyridine is also expected to drive molecular self-assembly through the formation of double hydrogen bonds (C−H···N) between neighboring molecules. Intense effort has recently been devoted to the synthesis of pyridine-based molecular building blocks. 32,33 Hydrogen-bonded densely packed and porous nanoarchitectures have been engineered using pyridine-based molecular building blocks. 34,35 The review of Wild et al. summarized the recent research effort in synthezising π-conjugated 2,2′:6′,2″-terpyr-idine ligands for application in the fields of supramolecular and coordination chemistry and materials science. 32 The pyridine groups are expected to drive molecular self-assembly through hydrogen bonding or metal coordination whereas the spacer unit is carrying the electronic properties or the active part of the molecular building block. Among the possible spacer units, oligothiophenes are very interesting because their chemistry is now well known to tune their electronic, optical, and redox properties for applications in organic electronics.