Polymorphic Self-Assemblies of 2,7-Bis(decyloxy)-9-fluorenone at the Solid/Gas Interface: Role of C-H•••O=C Hydrogen Bond

The self-assembly of 2,7-bis(decyloxy)-9-fluorenone on highly oriented pyrolytic graphite is investigated at the solid/gas interface by scanning tunneling microscopy, which allows us to determine the effect of its molecular structure on the formation of monolayer morphology. By varying the solution concentration in dichloromethane, seven types of supramolecular assemblies can be obtained. The concentration-dependent structural polymorphism is discussed in terms of thermodynamics. In particular, these different patterns are identified to be bound with weak intermolecular C(sp2)-H center dot center dot center dot O=C hydrogen bonds. As discerned by its position in the fluorenone moiety, the C(sp2) H group with larger chemical shift value and stronger acidity displays a higher priority when involving in the formation of a C(sp2)-H center dot center dot center dot O hydrogen bond. Owing to the high density of C(sp(2))-H donors, various hydrogen-bonding configurations arise, further leading to the occurrence of structural polymorphism. Besides, intermolecular van der Waals interactions as well as the dipole dipole interactions act as the complementary roles to stabilize the whole monolayer. The underlying mechanism is further confirmed by the density functional theory calculations.