Interfacial Nanoarchitectonics of Stimuli-Responsive Ortho-Fluorinated Alkoxy Azobenzene Monolayers

Uniform, stable, and stimuli-responsive monolayer films are of significant interest for photopharmacology, targeted drug delivery, and electro-optical device applications. Here, we report the self-assembly and photoswitching kinetics of an ortho-fluorinated alkoxy azobenzene (F-AAB) amphiphile at the air-water interface. The surface pressure-area per molecule isotherm and Brewster angle microscopy revealed that both the trans and cis isomers of F-AAB form uniform and stable monolayer films, each exhibiting two liquid expanded phases upon compression. The cis-rich system is characterized by a larger area per molecule and a higher collapse pressure, owing to the bent structure and the enhanced dipole moment of the cis isomer. Comparison of F-AAB and its non fluorinated analog, AAB (alkoxy azobenzene), indicated that ortho-fluorination suppresses intermolecular hydrogen bonding, leading to significant changes in the monolayer properties. In situ photoswitching of the Langmuir monolayer films demonstrates light induced reversible modulation of surface pressure for both molecules. Following first-order kinetics, the rate constants of photoisomerization upon UV light irradiation were determined to be 1.7 × 10^-2 s^-1 and 2.18 × 10^-2 s^-1 for F-AAB and AAB, respectively for an initial surface pressure of ≈1.8 mN/m. This study unlocks the importance of molecular engineering on the photoisomerization and self-assembly of such stimuli-responsive systems.