Photo-Switchable Surfactants for Responsive Air-Water Interfaces: Azo vs. Arylazopyrazole Amphiphiles

Arylazopyrazoles (AAPs) as substitutes for azo derivatives have gained considerable attention due to their superior properties offering E/Z photo-isomerization with high yield. In order to compare and quantify their performance, azobenzene tetraethylammonium (Azo-TB) and arylazopyrazole tetraethylammonium (AAP-TB) bromides were synthesized and characterized in the bulk (water) using NMR 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/Z isomers for AAP-TB (maximum change of $Γ$: 2.15 $μ$mol/m$^2$); for Azo-TB the change was only moderate (maximum change of $Γ$: 1.02 $μ$mol/m$^2$). Analysis of SFG spectra revealed that strong non-resonant 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 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 photo-isomerization.

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