Dynamic Wetting of Photoresponsive Arylazopyrazole Monolayers is Controlled by the Molecular Kinetics of the Monolayer

Abstract

Smart surfaces that can change their wettability on demand are interesting for applications such as self-cleaning surfaces or lab-on-a-chip devices. We have synthesized arylazopyrazole (AAP) phosphonic acids as a new class of photoswitchable molecules for functionalization of aluminum oxide surfaces. AAP monolayers were deposited on α-Al2O3(0001) and showed reversible E/Z photoswitching that can trigger contact angle changes of up to ∼10°. We monitored these changes on the macroscopic level by recording the contact angle while the monolayer was switched in situ. On the molecular level, time-dependent vibrational sum-frequency generation (SFG) spectroscopy provided information on the kinetic changes within the AAP monolayer and the characteristic times for E/Z switching. In addition, vibrational SFG at different relative humidity indicates that the thermal stability of the Z configuration is largely influenced by the presence of water which can stabilize the Z state and hinder E → Z switching of the AAP monolayer when it is wetted with H2O. Having established the switching times on the molecular scale, we additionally measured the dynamic contact angle and show that the time scales of the substrate and droplet dynamics can be extracted individually. For that, we report on a relaxation model that is solved analytically and is verified via a comparison with simulations of a Lennard–Jones system and with experimental data. The slower E to Z switching in the presence of the droplet as compared to the vapor phase is rationalized in terms of specific interactions of water with the exposed AAP moieties.

Light-Responsive Conductive Surface Coatings on the Basis of Azidomethyl-PEDOT Electropolymer Films

“…Another possible explanation could be the interaction of water with the polymer surfaces, which contributes to the general increase in hydrophilicity of the surfaces over the measurement time. 67 ■ CONCLUSIONS This work describes the application of PEDOT-N 3 electropolymers as a functional template for post-modification with light-responsive coatings via the CuAAC reaction. The CuAAC click reaction with AAP-alkyne proceeds efficiently both homogeneously for the EDOT-N 3 monomer in solution and heterogeneously for the PEDOT-N 3 films.…”

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

“…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. Molecules with functional groups, including thiol and carboxyl are used as the linkers between the gold surface and photoresponsive molecules.…”

“…57 On the other hand, Braunschweig and coworkers successfully fabricated an arylazopyrazole monolayer on the surface of α-Al 2 O 3 through a reaction with phosphonic acid. 66…”

Advances in the Structural Strategies of the Self-Assembly of Photoresponsive Supramolecular Systems

“…The catalytic activity of peptide-based systems that can be reversibly controlled by light has been used to prepare azobenzene-based molecular assemblies with nanoparticles. For instance, the peptide Azo-Gly-Phe-Gly - His ( Azo -GFGH peptide) shows a structural transition from antiparallel β-sheet to random coil under light irradiation, resulting in the assembly/disassembly of peptide fibrils [ 144 ]. Due to the proximity effect of histidine residues in supramolecular assembly and hydrophobic microenvironment, the assemblies have high catalytic activity for the hydrolysis of 4-nitrophenyl acetate.…”

“…In another instance, under the same success, it has been evidenced by means of arylazopyrazole phosphonic acid molecules ( butyl-AAP -C 18 PA) deposited on α-Al 2 O 3 (0001) single crystals, and the surface density has been shown using the Langmuir−Blodgett technique. This surface density facilitates large changes in molecular structure by applying time-dependent vibrational, namely sum-frequency generation (SFG), spectroscopy [ 144 ]. In this respect, nonlinear optical (NLO) responses and second harmonic generation (SHG) of photochromic molecules are welcome chemical structures in academic and industrial aspects [ 147 ].…”