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

Abstract

Self-assembled monolayers (SAMs) with photoswitchable arylazopyrazole (AAP) silanes on SiO 2 surfaces that exhibit fluorinated terminal groups (CF 3 -AAP) were studied on a molecular level with time-dependent sum-frequency generation (SFG) spectroscopy as well as on a microscopic level with dynamic contact angle measurements. Through photoswitching of the CF 3 -AAP moieties in the SAM from their predominant E state to their predominant Z state, the molecules are driven from a linear to a bent configuration, which exposes the more hydrophilic azo group. This causes changes in surface wettability and a change in the static contact angle of about 10°. We analyzed the molecular kinetics of the photoswitching in the SAMs in detail by using both vibrational SFG and dynamic contact angle measurements. For this, we systematically changed the surface coverage of the AAP photoswitches 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 the drop dynamics. In particular, it was found that the packing density of the SAMs and the possibility of incorporating water molecules in the SAM can have drastic influences on the wetting dynamics of CF 3 -AAP monolayers.