Photoswitching the mechanical properties in Langmuir layers of semifluorinated alkyl-azobenzenes at the air-water interface.

Antigoni Theodoratou,Thomas Geue,René Stangenberg,Dimitris Vlassopoulos,Benoit Loppinet,Rüdiger Berger,Dan Li,Ulrich Jonas

doi:10.1039/c5cp04242a

Abstract

Semifluorinated alkyl-azobenzene derivatives (SFAB) can form stable Langmuir layers at the air-water interface. These systems combine the amphiphobic character of semifluorinated alkyl units as structure-directing motifs with photochromic behavior based on the well-known reversible cis-trans isomerization upon irradiation with UV and visible light. Herein, we report our investigations of the structural and dynamic tunability of these SFAB layers at the air-water interface in response to an external light stimulus. The monolayer structures and properties of [4-(heptadecafluorooctyl)phenyl](4-octylphenyl)diazene (F8-azo-H8) and bis(4-octylphenyl)diazene (H8-azo-H8) were studied by neutron reflectivity, surface pressure-area isotherms with compression-expansion cycles, and interfacial rheology. We find that UV irradiation reversibly influences the packing behavior of the azobenzene molecules and interpret this as a transition from organized layer structures with the main axis of the molecule vertically oriented in the trans form to random packing of the cis isomer. Interestingly, this trans-cis isomerization leads to an increase in surface pressure, which is accompanied by a decrease in viscoelastic moduli. These results suggest ways of tailoring the properties of responsive fluid interfaces.

Highlights

Semifluorinated alkanes (SFAs), which contain fluorinated alkyl chains in addition to hydrogenated alkyl moieties, constitute a specific class of materials that can organize into ordered structures due to the mutual incompatibility of the fluorinated and hydrogenated segments
The semifluorinated alkylazobenzene (SFAB) derivative F8-azo-H8 combines the structureguiding motif of mutual incompatibility between fluorinated and hydrogenated alkyl chains with the photochromic property of switching molecular configuration by external stimulation with light. This water-insoluble compound can be spread at an air–water interface as stable Langmuir layers to control the mechanical behaviour of this interface in response to light. For both the azobenzene derivatives, we investigated the interfacial organization by means of neutron reflectivity and surface pressure isotherms in a Langmuir trough, whereas the interfacial viscoelasticity was investigated using a magnetic-rod interfacial stress rheometer (ISR)
The surface pressure (P)–molecular area (A) isotherms of F8-azo-H8 measured at the air–water interface are shown in Fig. 2 for different illumination conditions

Summary

Introduction

Semifluorinated alkanes (SFAs), which contain fluorinated alkyl chains in addition to hydrogenated alkyl moieties, constitute a specific class of materials that can organize into ordered structures due to the mutual incompatibility of the fluorinated and hydrogenated segments Based on this structure-guiding motif, they have received attention over recent years, for example, due to their ability to form different mesophases in the bulk form upon a decrease in temperature[3] and stable Langmuir layers at an air– water interface via hierarchical self-assembly, despite the hydrophobicity of both fluorinated and alkane moieties.[4] The specific challenge with fluid interfaces coated with SFAs is the manipulation of their properties using external stimuli, light.

Methods

Results

Conclusion