Two-dimensional molecular assembly and surface-confined reaction of pyrene-based bifunctional molecules on silver (111) and gold (111)

Abstract

Two-dimensional (2D) materials have attracted significant attention owing to their remarkable properties stemming from their reduced dimensionality. The bottom-up approach is one of the most promising methods to design single layered 2D materials with engineered functionalities or properties. The factors impacting the creation process and the products include the effect of precursor symmetry/shape, the unique nature of bifunctional systems, and the competition between kinetics and thermodynamics of on-surface phenomena. This research reports the creation and characterization of 2D self-assembled molecular networks and their effects on the subsequent on-surface reactions. Low-temperature scanning-tunneling microscopy (STM) at 77.5K was used to explore the molecular arrangements created, and to identify structural changes as a result of on-surface reactions, of the bifunctional molecule 4,4’-(3,8-Bis(4-aminophenyl)pyrene-1,6-diyl)dibenzaldehyde (APPDB), which is a pyrene-based precursor containing two amines and two aldehyde functional groups. The APPDB molecule exhibits a distinct windmill-like arrangement on Ag(111) caused by the grouping of four aldehydes and four amine functional groups into separate centres. The arrangement was additionally exposed to a sequential post-annealing to invoke a reaction. At 100 °C, enantiomeric domains were resolved. At 150 °C, atomically resolved silver atoms appeared in the molecular arrangement, one atom decorated with four functional groups provided by four molecules, indicative of metal-organic structure. Additionally at this temperature, reacted APPDB monomers were observed on the edges and defects of molecular domains. Finally, after annealing to 225 °C, the STM images recording the domains of APPDB no longer resolved the silver atom. In order to highlight the difference between the APPDB-Ag network and the reaction of APPDB monomers, a heated substrate deposition at 200 °C was performed which successfully introduced reacted monomer to the domain centres although the network was overall disordered. The APPDB molecule was also deposited on Au(111) which did not result in an organized molecular arrangement, instead disordered rope-like structures were formed. The structure remained disordered after post-annealing although reacted molecular features were formed. A second bifunctional molecule, 4,4’-(3,8-bis((4-aminophenyl)ethynyl)pyrene-1,6-diyl)dibenzaldehyde (APDB), was studied; it is nearly identical to APPDB but contains a carbon-carbon triple bond between the pyrene core and the aniline substituent. Adsorption of APDB on Au(111) gave disordered small clusters of molecules rather than the rope-like structures of APPDB. Finally, a star-shaped molecule containing three aldehyde groups, 1,3,5-tris(p-formylphenyl)benzene (TFPB), was also studied on Au(111). The molecular structures were disordered but contained some local assemblies such as dog bone-like structures, windmill-like structures, and hexagonal structures. The adsorbed TFPB was post-annealed, displaying no structural changes up to 150 °C.