Two-Dimensional Self-Assembled Molecular Structures Formed by the Competition of van der Waals Forces and Dipole-Dipole Interactions

The self-assembly of 2,7-bis(10-ethoxycarbonyl-decyloxy)-9-fluorenone (BEF) has been investigated by scanning tunneling microscopy at the liquid/solid interface. The coexistence and reversible transformation of linear structure and cyclic network were observed. Solvents with different polarity had little effect on the self-assembly of BEF on the HOPG surface. The observed structural transformation was found to be driven by voltage pulses applied to the STM tips. The influence of dipole-dipole interactions and van der Waals (vdWs) interactions on the different packing patterns was scrutinized by comparing the two-dimensional self-assembly of 2,7-bis(10-ethoxycarbonyldecyloxy)-9-fluorene, 2,7-ditetradecyloxy-9-fluorenone, and 2,7-ditetradecyloxy-9-fluorene with that of BEF molecule. The formation of different morphologies depended critically not only on the molecular conformation, but the different dipole-dipole interactions of the fluorenone unit and two ester alkoxy chains. The computer simulation and calculation of different morphologies are useful tools to dissect and explain the formation mechanism. On the basis of the comparative experiments and calculation, we could conclude that for the linear structure, the vdWs forces between the molecules were more important than the dipole-dipole interactions; on the contrary, the intermolecular dipole-dipole interactions induced the cyclic network.