Cooperation and competition of hydrogen and halogen bonds in 2D self-assembled nanostructures based on bromine substituted coumarins

摘要

Three coumarin derivatives (Co16, 6-Br-Co16 and 6,8-Br-Co16) with ester, ether, and carbonyl groups and different numbers of bromine substituents on the coumarin cores were synthesized. Their two-dimensional (2D) self-assemblies were probed by scanning tunneling microscopy (STM) at the liquid-solid interface. The Co16 molecules formed a zigzag lamellar pattern on the HOPG surface through intermolecular H center dot center dot center dot O hydrogen bonds. The 6-Br-Co16 molecule with one Br substituent was found to form a uniform zigzag linear pattern, owing to intermolecular Br center dot center dot center dot O=C (ester groups) halogen bonds and H center dot center dot center dot Br hydrogen bonds. The dibromo-substituted 6,8-Br-Co16 molecule could form a dislocated linear pattern, in which the molecules are linked through the Br center dot center dot center dot O=C (ester group) halogen bond, and Br center dot center dot center dot Br and H center dot center dot center dot Br bonds. The STM results reveal that the introduction of the Br atom actuates the formation of the Br center dot center dot center dot O=C bond instead of the H center dot center dot center dot O=C bond, which is the key dominant force to form the different 2D adlayers. At the same time, the cooperation of H center dot center dot center dot Br hydrogen bonds leads to the stability of the nanostructures. DFT calculations were performed to unravel the cooperation and competition mechanism for the formation of halogen bonds and hydrogen bonds in 2D molecular self-assembly.