Controlled and Reversible Photomodulation of the Alkoxy Azobenzene Monolayer: Combined Experimental and Molecular Dynamics Simulation Study

Constant-area photoswitching of Langmuir monolayer films of an alkoxy azobenzene (AAB) molecule was carried out at different phases of the monolayer, and the effect on surface pressure change was studied. The surface pressure rise upon UV light irradiation was found to decrease with the increased initial packing within the monolayer film. First-order kinetics were employed to calculate the rate constants of photoisomerization in the Langmuir monolayer. To elucidate the molecular origin of these observations, atomistic molecular dynamics (MD) simulations were performed. The simulation successfully reproduced the surface pressure–area isotherm, demonstrating the reliability of the computational model. Detailed analyses of density distributions, alkyl-chain ordering, and hydrogen-bonding networks revealed that the cis and trans isomers impact the monolayer distinctly. In addition, the number of H-bonds between AAB molecules and water is higher in the case of PSS. These findings provide a molecular-level explanation for the reduced surface pressure response observed experimentally in densely packed monolayers following UV irradiation.