Alkyl chain length regulated diverse 2D polymorphic self-assembly of benzothiadiazole-based π-conjugated fluorophore

Structural diversity induced by the chain-length effect in the field of two-dimensional molecular self-assembly has attracted lots of attention. In this study, the self-assembly behaviors of benzothiadiazole-based fluorophore (FHP-C12) were investigated in different solvents by scanning tunneling microscopy. The STM observations display that the self-assemblies of FHP-C12 have obvious solvent and concentration dependence. At the long-chain n-alkanes (n-tridecane, n-tetradecane)/graphite interface owing to the space matching, the grid-like and X-shaped structures are formed successively from high concentration to low concentration. Whereas the X-shaped structure is not observed at the n-dodecane/graphite interface due to the weak adsorption of solvents. When using octanoic acid and heptanoic acid as the solvents, a new H-shaped pattern is observed, due to the coadsorption of solvents. Moreover, we compare the self-assembly patterns of molecules with carbon chain length of 14 (FHP-C14) at the n-tetradecane/graphite interface. Although these two molecules only differ in the length of alkyl chains, the STM results show great differences, which indicates that the length of alkyl chains can control the self-assembly of benzothiadiazole-based systems. This work not only provides an efficient method for fabricating complex nanostructures, but also helps to promote the study of the chain-length effect in the field of supramolecular chemistry.