Photo-Switchable Surfactants for Responsive Air–Water Interfaces: Azo versus Arylazopyrazole Amphiphiles

Arylazopyrazoles (AAPs) as substitutes for azobenzene derivatives have gained considerable attention due to their superior properties offering E / Z photoisomerization with high yield. In order to compare and quantify their performance, azobenzene triethylammonium (Azo-TB) and arylazopyrazole triethylammonium (AAP-TB) bromides were synthesized and characterized in the bulk (water) using NMR and UV/Vis spectroscopy. At the air–water interface, complementary information from vibrational sum-frequency generation (SFG) spectroscopy and neutron reflectometry (NR) has revealed the effects of E/Z isomerization in great detail. In bulk water the photostationary states of >89% for E / Z switching in both directions were very similar for the surfactants, while their interfacial behavior was substantially different. In particular, the surface excess Γ of the surfactants changed drastically between E and Z isomers for AAP-TB (maximum change of Γ: 2.15 μmol/m 2 ); for Azo-TB, the change was only moderate (maximum change of Γ: 1.02 μmol/m 2 ). Analysis of SFG spectra revealed that strong nonresonant contributions that heterodyned the resonant vibrational bands were proportional to Γ, enabling the aromatic C–H band to be interpreted as an indicator for changes in the interfacial molecular order. Close comparison of Γ from NR with the SFG amplitude from the aromatic C–H stretch as a function of concentrations and E / Z conformation revealed substantial molecular order changes for AAP-TB. In contrast, only Γ and not the molecular order varied for Azo-TB. These differences in interfacial properties are attributed to the molecular structure of the AAP center that enables favorable lateral interactions at the air–water interface, causing closed-packed interfacial layers and substantial changes during E / Z photoisomerization.