Hydrogen bonding network of truxenone on a graphite surface studied with scanning tunneling microscopy and theoretical computation.

The self-assembly of truxenone on highly oriented pyrolytical graphite (HOPG) was investigated with scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Truxenone assembled into a dense hexagonal adlayer with large and near-perfect domains on the graphite surface. Two types of topography image of truxenone were observed in experiments. DFT modeling indicates the center-hole image represents the contour of the lowest unoccupied molecular orbital (LUMO) of truxenone while the propeller-like image is more complicated and can't be assigned simply to an individual molecular orbital. Truxenone trimer was proposed as the basic unit of the adlayer through analysis of STM images and molecular structure. The optimized structure of the truxenone trimer by DFT calculation revealed that the hydrogen bonding network exists in the adlayer, which is responsible for the excellent stability and perfect domains of the whole assembly. In addition, the preferred adsorption site of truxenone on graphite was determined on the basis of DFT calculations and STM images. The results are of significance in supramolecular engineering by self-assembly and device fabrication based on truxenone and its derivatives.