Self-Assembled Monolayers of Arylazopyrazoles on Glass and Silicon Oxide: Photoisomerization and Photoresponsive Wettability

Abstract

Surface coatings that respond to external influences and change their physical properties upon application of external stimuli are of great interest, with light being a particularly desirable choice. Photoswitches such as azobenzenes have been employed in a range of photoresponsive coatings. One striking change in physical property of many photoresponsive coatings is their responsive wettability upon illumination. In this work, we present photoswitchable self-assembled monolayers based on arylazopyrazoles (AAPs). In solution, AAPs offer significant improvements in terms of the photostationary state, thermal stability, and fatigue resistance. The AAP photoswitch is coupled to triethoxysilanes for an easy, one-step functionalization of glass and silicon oxide surfaces. We show the synthesis of AAP-based silanes and the successful surface functionalization, and we confirm the excellent photoswitchability of the AAPs in a self-assembled monolayer upon alternating irradiation with UV (365 nm) and green (520 nm) light. The self-assembled monolayers are investigated by UV/vis spectroscopy, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and contact angle goniometry. We furthermore investigate the effect of substitution of the AAPs on the photoresponsive wetting behavior and compare this with density functional theory (DFT) calculations of the dipole moments of the AAPs.

Toward real-world applications: promoting fast and efficient photoswitching in the solid state

“…The photoresponsive molecules are introduced on the surface of the substrate either by chemical anchoring or physisorption. Surfaces with various chemical environments, including glass 57 and silicon wafers, 58 metal 59 and metallic oxide, 60 polymers 61 and two-dimensional materials 62 have been widely used as the substrates to reveal the isomerization property of photoswitches. Meanwhile, the hydrophilicity, optical property 63 and conductivity 64 of the surface could be controlled by light.…”

“…Functional groups on the surface are important for the introduction of photoresponsive molecules. Metal 65 or non-metal oxides, including SiO 2 , 57 Al 2 O 3 and CeO 2 60 have been used as substrates for anchoring and depositing photoswitches, because of the abundant hydroxyl on the surface. For example, Ravoo and coworkers reported introducing a monolayer of arylazopyrazole onto the SiO 2 surface through the reaction of silane.…”

“…In Ravoo's work, the water contact angle was controlled between 82.31 and 75.71 under UV and visible light irradiation due to the isomerization of arylazopyrazole between trans and cis. 57 On the other hand, Braunschweig and coworkers successfully fabricated an arylazopyrazole monolayer on the surface of a-Al 2 O 3 through a reaction with phosphonic acid. 66 Gold as a chemically inert metal shows a stable surface, which is attractive for the immobilization of photoswitches.…”

“…For example, Ravoo and coworkers reported introducing a monolayer of arylazopyrazole onto the SiO 2 surface through the reaction of silane. 57 Silane with the functional groups of alkenyl, alkynyl, amino and isocyanate could be commercially obtained, which are used for structures extending through clicking and condensation. In Ravoo's work, the water contact angle was controlled between 82.3° and 75.7° under UV and visible light irradiation due to the isomerization of arylazopyrazole between trans and cis .…”

“…When the Azo is in trans , the hydrophobic group heads up and generates a hydrophobic surface, which hides into the monolayer and increases the hydrophilicity when Azo is in cis . 57 Moreover, choosing SP and DASA with zwitterionic photostationary states is also useful.…”