Solvent-Dependent Molecular Isomerization and 2D Self-Assembled Phase Transitions of Benzothiadiazole-Based π-Conjugated Fluorophore

Two-dimensional molecular assemblies of benzothiadiazole-based fluorophores with liquid crystalline properties (FHP-C14) have been investigated on a graphite surface in different solvents (n-tetradecane, n-tridecane, and n-heptanoic acid) by scanning tunneling microscopy. The FHPC14 molecule shows cis-trans isomerization due to different orientations of two flanked thiophene groups relative to acetylene units. The solvent and solution concentration effects on the adlayers are observed in the systems. With the concentration gradually decreasing, FHP-C14 molecules successively self-assemble into the linear structure, the box-like structure, and the double-C structure at the n-tetradecane/graphite interface. The molecular cis-trans-cis isomerization is observed, due to different arrangement fashions of the side chains. When using n-tridecane and n-heptanoic acid as the solvents, the double-C structure could not be observed resulting from the mismatch of side chains in FHP-C14 molecules and solvent molecules. The coadsorption of solvent molecules influences the assembly of FHP-C14 molecules and promotes the conformer transformation. Our work provides guidance for the design of benzothiadiazole-based fluorophore from the molecular packing point of view.