Interfacial assembly of amphiphilic styrylquinoxalines: alkyl chain length tunable topochemical reactions and supramolecular chirality

A series of styrylquinoxaline alkoxy derivatives, 3-(4-(alkoxy) styryl)quinoxalin-2(1H)-one substituted with different alkyl chain lengths (SQC(n), n = 12, 14, 16, 18), have been synthesized. The air/water interfacial assemblies of these compounds fabricated using the Langmuir-Blodgett technique were studied. The assemblies transferred on the solid substrate showed photochemical reactions and supramolecular chirality, which depend on the length of the alkyl chain. The increment of the alkyl chain length caused the SQC(n) assemblies to change from an unstable molecular film to disordered and ordered arrangement in the LB films. Specifically, SQC(12) could not form stable molecular films. Although SQC(14) could form stable molecular films, the disordered packing of the molecules in the LB films caused the absence of supramolecular chirality as well as photodimerization. When the alkyl chain is increased to 16 or above, the SQC LB films show supramolecular chirality and topochemical photodimerization upon irradiation. These results indicated that the hydrophobic interaction between the long alkyl chains also played an important role in regulating the packing of the functional groups at the air/water interface, which would subsequently affect the photodimerization and the emergence of supramolecular chirality.