Self-Assembly of Isomeric Isosceles Triangle Molecules at the Liquid-Solid Interface

We report the formation of self-assembled molecular networks (SAMNs) by isomeric isosceles triangle molecules, dehydrobenzo[14]-annulene ([14]-DBA), and dehydrobenzo[15]-annulene ([15]-DBA) derivatives bearing six decyloxy chains, at the 1-phenyloctane (PO)-graphite interface as investigated by scanning tunneling microscopy (STM). [15]-DBA produces a Trimer+Tetramer structure with a surprisingly large unit cell comprising 12 independent [15]-DBA molecules. In contrast, [14]-DBA forms a simple Dimer structure consisting of two [14]-DBA molecules per unit cell. The lowest solute concentrations for SAMN formation are 2 × 10^-6 mol/L and 1 × 10^-5 mol/L for [14]-DBA and [15]-DBA, respectively, differing by approximately a factor of 5. Thus, even a slight modification of the triangular core shape leads to significant differences in their self-assembly behavior. Molecular mechanics (MM) simulations suggest that these differences arise, at least in part, from differences in intermolecular and molecule-substrate interactions in SAMNs. Finally, mixing the constitutional isomers results in the formation of a regular coassembly, namely a Dimer+Dimer structure. The relative stabilities of the three structures are discussed based on experimental observations and nonbonding interactions estimated by MM simulations. This fundamental information is useful for advancing two-dimensional crystal engineering at the interface.