Functional control on the 2D self-organization of phenyleneethynylenes

Abstract

Two-dimensional self-organization of a series of phenyleneethynylenes was investigated, at ambient conditions, by varying the length of alkoxy chain and introducing functional groups at the terminal positions using high-resolution scanning tunneling microscopy (STM). The model phenyleneethynylene molecule, which does not possess any functional groups, self-organizes into wire like structures on surface. High-resolution STM imaging revealed that molecules are arranged in a skewed ID fashion. The spacing between the molecular wires was successfully modulated by replacing hexyloxy (C(6)) chains with dodecyloxy (C(12)) chains. The initial step of the formation of all the molecular assemblies involves the alkyl CH center dot center dot center dot acetylenic pi interactions (CH center dot center dot center dot pi) leading to the organization of molecules as two types of strips. These strips further interlock to two types of 2D organizations. The hydroxyl as well as aldehyde groups present at the terminal positions of the phenyleneethynylene molecules play an important role in the interlocking process. An end-to-end assembly was observed in the case of phenyleneethynylene molecule possessing hydroxyl groups at the terminal positions, which is attributed to the intermolecular hydrogen bonding between the strips. The adsorption of molecules with two faces results in enantiomeric 2D structures and these aspects were investigated using molecular modeling studies