Molecular Kinetics and Wetting Dynamics of Self-Assembled Monolayers with Fluorinated Arylazopyrazoles

Self-assembled monolayers (SAMs) with photoswitchablearylazopyrazole(AAP) silanes on SiO2 surfaces that exhibit fluorinatedterminal groups (CF3-AAP) were studied on a molecular levelwith time-dependent sum-frequency generation (SFG) spectroscopy aswell as on a microscopic level with dynamic contact angle measurements.Through photoswitching of the CF3-AAP moieties in the SAMfrom their predominant E state to their predominant Z state, the moleculesare driven from a linear to a bent configuration, which exposes themore hydrophilic azo group. This causes changes in surface wettabilityand a change in the static contact angle of about 10 & DEG;. We analyzedthe molecular kinetics of the photoswitching in the SAMs in detailby using both vibrational SFG and dynamic contact angle measurements.For this, we systematically changed the surface coverage of the AAPphotoswitches by changing the deposition time of the fluorinated AAP.The molecular kinetics of the wetted monolayer are surprisingly slow,and the wetting dynamics are even slower when the SAM is closely packed,which we relate to a coupling between the molecular kinetics and thedrop dynamics. In particular, it was found that the packing densityof the SAMs and the possibility of incorporating water molecules inthe SAM can have drastic influences on the wetting dynamics of CF3-AAP monolayers.