Solvent effects on supramolecular networks formed by racemic star-shaped oligofluorene studied by scanning tunneling microscopy

Two-dimensional networks of star-shaped oligofluorene end-capped with carboxylic groups (StOF-COOH3) are investigated using scanning tunneling microscopy (STM) in solvents with different polarities and functionality on graphite surface. The high-resolution STM images show that the assembly of StOF-COOH3 is strongly solvent dependent. Well-ordered porous honeycomb networks are revealed at the octanoic acid/graphite and 1,2,4-tricholrobenzene/graphite interfaces, while an irregular and densely packed structure is observed at the 1-phenyloctane/graphite interface. In n-tetradecane, an intermediate state of the coexistence of a disordered and honeycomb structure is identified on the surface. High resolution STM images revealed two different hydrogen-bonded networks for StOF-COOH3 molecules with 1-octanoic acid as the solvent, including homochiral domains and heterochiral domains. The stabilization of the networks consisting of chiral species at the graphite surface is suggested to be possibly associated with the asymmetrical polar environments because of the ordered arrangements of coadsorbed polar solvent molecules.