Journal of Physical Chemistry B, Vol.124, No.31, 6913-6923, 2020
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 Gamma of the surfactants changed drastically between E and Z isomers for AAP-TB (maximum change of Gamma: 2.15 mu mol/m(2)); for Azo-TB, the change was only moderate (maximum change of Gamma: 1.02 mu mol/m(2)). Analysis of SFG spectra revealed that strong nonresonant contributions that heterodyned the resonant vibrational bands were proportional to Gamma, enabling the aromatic C-H band to be interpreted as an indicator for changes in the interfacial molecular order. Close comparison of Gamma 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 Gamma 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.