Chiral assembly from achiral rod-coil molecules triggered by compression at the air-water interface

A series of achiral rod-coil molecules consisting of an oligo(p-phenylene) conjugated rod with a poly(ethylene oxide) or poly(propylene oxide) coil laterally attached through an imidazole linkage were synthesized, and their interfacial behaviors were investigated. Compounds 1a and 1b have a similar surface behavior with a shoulder indicating a transformation of rod segments from flat-on to vertical orientation. Interestingly, CD and AFM suggest chiral films are formed when the LB films were deposited after the shoulder. Although compound 1c shows different surface behavior because the PEO chains dissolved into water upon compression, macroscopic chirality was also detected. In contrast, compound 2, based on a reduced conformational rigidity of the aromatic rod segment, does not form chiral assemblies. Combining the data collected, the cooperative interaction of the hydrogen bond between the molecules and pi-pi stacking as well as the steric constraint between the aromatic rod segments are responsible for the macroscopic chirality. Such kind of stacking can be realized through a molecular design and a lateral compression. A deep insight into the relationship between the molecular structure and the chirality was gained.