Two-Dimensional Nanoarchitectonics with Fluorination and Alkylation on Photoresponsive Azobenzene Molecules Self-Assembled at the Solid Surface

The development of stimuli-responsive molecular electronic devices relies on a comprehensive understanding of the nature and strength of interactions that govern their self-assembly on solid surfaces. Here, we report spatially resolved two-dimensional self-assemblies of fluorinated azobenzene derivatives using scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite substrate. STM images show that fluorine-substituted azobenzene derivatives form large-scale monolayers stabilized by intermolecular hydrogen and halogen bonding, unlike their nonfluorinated counterparts. Incorporating an alkoxy chain into the molecule changes the balance of molecule–molecule and molecule–substrate interactions by introducing stronger molecule–substrate coupling due to van der Waals interactions. High-resolution STM images enable us to observe alkylation-induced changes in the atom-specific intermolecular interactions among polar atoms F, O, and N in the azobenzene cores of the molecules. Our results clearly highlight the significance of minor chemical modifications such as fluorination and alkylation in adjusting the intermolecular and molecule–substrate interactions to achieve precise molecular packing on solid supports.