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arXiv · 2007.08120

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

Marco Schnurbus·Richard A. Campbell·Jörn Droste·Christian Honnigfort·Dana Glikman·Philipp Gutfreund·Michael Ryan Hansen·Björn Braunschweig
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Abstract

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 $\Gamma$ of the surfactants changed drastically between E/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 non-resonant 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 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 photo-isomerization.

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Marco Schnurbus, Richard A. Campbell, Jörn Droste, Christian Honnigfort, Dana Glikman, Philipp Gutfreund, Michael Ryan Hansen, Björn Braunschweig. 2020-07-16. Photo-Switchable Surfactants for Responsive Air-Water Interfaces: Azo vs. Arylazopyrazole Amphiphiles. https://doi.org/10.1021/acs.jpcb.0c02848

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