Synthesis and 2D self-assembly at the liquid–solid interface of novel H-bonding linear π-conjugated oligomers terminated by uracil and melamine units

Abstract

We report here on the synthesis and 2D self-assembly of novel H-bonding linear π-conjugated oligomers terminated at both ends by either two uracil moieties (compound GR1) or by two melamine groups (compounds GR2 and GR3). The bis(uracil) GR1 compound was prepared by Sonogashira coupling of an ethynyluracil with a dihalobenzene and characterized spectroscopically. The synthesis of GR2 and GR3 was performed by Suzuki coupling of the dihalogenated benzene derivatives leading to the respective bisadducts which were then converted into the target bis-melamine derivatives by treatment with dicyanodimide. Self-assembly of the GR1 derivative is investigated at the liquid/graphite interface using scanning tunneling microscopy (STM). When deposited from n-tetradecane, GR1 forms a row-like structure stabilized by multiple terminal H-bonds between neighboring molecules. Co-adsorption of two n-tetradecane molecules per unit cell is also clearly identified. In turn, when deposited from 1-phenyloctane GR1 forms a complex quasi square-shaped open structure having asymmetric sides. One possible arrangement is that four phenyloctane molecules are co-adsorbed in-between GR1 rows similar to those formed from n-tetradecane, thus forming the two other sides of the square-shaped motif. From both solvents, the row-like 2D arrangements are governed by the multiple terminal H-bonds which create long-range alignments in one direction and by the van der Waals interdigitation of the dodecyl chains of the GR1 molecules that stabilize the network in a nearly perpendicular direction.