Unlocking Structure-Self-Assembly Relationships in Cationic Azobenzene Photosurfactants.

Azobenzene photosurfactants are light-responsive amphiphiles that have garnered significant attention for diverse applications including delivery and sorting systems, phase transfer catalysis, and foam drainage. The azobenzene chromophore changes both its polarity and conformation (trans-cis isomerization) in response to UV light, while the amphiphilic structure drives self-assembly. Detailed understanding of the inherent relationship between the molecular structure, physicochemical behavior, and micellar arrangement of azobenzene photosurfactants is critical to their usefulness. Here, we investigate the key structure-function-assembly relationships in the popular cationic alkylazobenzene trimethylammonium bromide (AzoTAB) family of photosurfactants. We show that subtle changes in the surfactant structure (alkyl tail, spacer length) can lead to large variations in the critical micelle concentration, particularly in response to light, as determined by surface tensiometry and dynamic light scattering. Small-angle neutron scattering studies also reveal the formation of more diverse micellar aggregate structures (ellipsoids, cylinders, spheres) than predicted based on simple packing parameters. The results suggest that whereas the azobenzene core resides in the effective hydrophobic segment in the trans-isomer, it forms part of the effective hydrophilic segment in the cis-isomer because of the dramatic conformational and polarity changes induced by photoisomerization. The extent of this shift in the hydrophobic-hydrophilic balance is determined by the separation between the azobenzene core and the polar head group in the molecular structure. Our findings show that judicious design of the AzoTAB structure enables selective tailoring of the surfactant properties in response to light, such that they can be exploited and controlled in a reliable fashion

Light-responsive self-assembly of a cationic azobenzene surfactant at high concentration.

The formation of high-concentration mesophases by a cationic azobenzene photosurfactant is described for the first time. Using a combination of polarised optical microscopy and small-angle X-ray scattering, optically anisotropic, self-assembled structures with long-range order are reported. The mesophases are disrupted or lost upon UV irradiation

A single-component photorheological fluid with light-responsive viscosity.

Viscoelastic fluids whose rheological properties are tunable with light have the potential to deliver significant impact in fields relying on a change in flow behavior, such as in-use tuning of combined efficient heat-transfer and drag-reduction agents, microfluidic flow and controlled encapsulation and release. However, simple, single-component systems must be developed to allow integration with these applications. Here, we report a single-component viscoelastic fluid, capable of a dramatic light-sensitive rheological response, from a neutral azobenzene photosurfactant, 4-hexyl-4'butyloxymonotetraethylene glycol (C6AzoOC4E4) in water. From cryo-transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and rheology measurements, we observe that the photosurfactant forms an entangled network of wormlike micelles in water, with a high viscosity (28 Pa s) and viscoelastic behaviour. UV irradiation of the surfactant solution creates a less dense micellar network, with some vesicle formation. As a result, the solution viscosity is reduced by four orders of magnitude (to 1.2 × 10-3 Pa s). This process is reversible and the high and low viscosity states can be cycled several times, through alternating UV and blue light irradiation

Structure-self-assembly-function relationships of azobenzene photosurfactants

Self-organisation of individual molecules into supramolecular systems is an attractive approach to design stimuli-responsive materials. This thesis aims to understand and predict the self-assembly behaviour of surfactants containing a photoresponsive azobenzene chromophore. These photosurfactants (PS) are amphiphilic molecules which demonstrate the combined ability to form supramolecular structures above a critical micelle concentration (CMC) and change their properties, such as size and polarity, upon photoisomerization. The goal is to investigate the structure-function relationships in azobenzene photosurfactants (PS) to design new light responsive systems for micellar catalysis and nanotemplating applications. Chapter 1 introduces key concepts of self-assembly processes, photoisomerisation properties of azobenzenes and recent developments and applications of PS. Chapter 2 provides a theoretical background of the concepts, whilst Chapter 3 gives a detail experimental list of instruments used in this thesis. Chapters 4-6 focus on the design, synthesis and characterisation of a series of cationic azobenzene photosurfactants (AzoTABs) in water and the use of light to modulate the physicochemical properties. The ability to tune the self-assembled nanoscale organisation of AzoTABs is shown by varying the hydrophobic segment of the molecule and the position of the chromophore within the structure. The investigation of the aggregate growth, from dispersed unimers, to micellar assemblies was conducted using a combination of techniques, such as surface tensiometry, dynamic light scattering, small-angle neutron and X-ray scattering. Additionally, light-induced isomerisation was proven to significantly increase the hydrophilicity of the surfactants and to modify the morphology of the micellar aggregates. Conventional non-responsive surfactants are known to form a variety of lyotropic liquid crystal (LLC) phases. Such supramolecular systems are used as sacrificial templates or matrices to synthesise porous materials. However, cationic azobenzene photosurfactants forming LLC phases have never been reported to date. The results presented in Chapter 6 demonstrate that the careful molecular design of the amphiphilic molecules can promote the formation of LLC phases in AzoTABs. Additionally, the concentration-temperature-structure relationships required to access desirable LLC phases have been identified and the stability of these mesophases has been investigated as a function of temperature and photoisomerisation. The current need to identify new paths towards the sustainable synthesis of complex organic molecules has driven chemists to use water as a solvent. Micellar catalysis is an elegant way to perform traditional organic reactions using the self-assembly of surfactants as nanoreactors. In Chapter 7, cationic azobenzene photosurfactants in micellar conditions have been investigated to catalyse an aldol condensation reaction in water. The self-assembly-structure-efficiency of the catalyst was studied as a function of concentration and temperature. The effect of photoisomerisation on the efficiency of the catalysis was also studied. Finally, the ability to recycle the solution of micelles to perform several cycles of catalysis is discussed. This thesis demonstrates that the molecular design of cationic azobenzene photosurfactants drives the self-organisation towards ordered structures. These structures can be rationalised and predicted based on the molecular design of the starting photosurfactant. The self-assembly process is a dynamic event that can be modified by subtle changes of easily controllable external parameters, such as concentration, temperature and photoresponse. The knowledge gathered in this thesis on the structure-property relationships in AzoTABs will be crucial to design new light-responsive surfactants with targeted properties for their intended applications

Unlocking Structure–Self-Assembly Relationships in Cationic Azobenzene Photosurfactants

Azobenzene photosurfactants are light-responsive amphiphiles that have garnered significant attention for diverse applications including delivery and sorting systems, phase transfer catalysis, and foam drainage. The azobenzene chromophore changes both its polarity and conformation (trans–cis isomerization) in response to UV light, while the amphiphilic structure drives self-assembly. Detailed understanding of the inherent relationship between the molecular structure, physicochemical behavior, and micellar arrangement of azobenzene photosurfactants is critical to their usefulness. Here, we investigate the key structure–function–assembly relationships in the popular cationic alkylazobenzene trimethylammonium bromide (AzoTAB) family of photosurfactants. We show that subtle changes in the surfactant structure (alkyl tail, spacer length) can lead to large variations in the critical micelle concentration, particularly in response to light, as determined by surface tensiometry and dynamic light scattering. Small-angle neutron scattering studies also reveal the formation of more diverse micellar aggregate structures (ellipsoids, cylinders, spheres) than predicted based on simple packing parameters. The results suggest that whereas the azobenzene core resides in the effective hydrophobic segment in the trans-isomer, it forms part of the effective hydrophilic segment in the cis-isomer because of the dramatic conformational and polarity changes induced by photoisomerization. The extent of this shift in the hydrophobic–hydrophilic balance is determined by the separation between the azobenzene core and the polar head group in the molecular structure. Our findings show that judicious design of the AzoTAB structure enables selective tailoring of the surfactant properties in response to light, such that they can be exploited and controlled in a reliable fashion

Research data supporting "Light-responsive self-assembly of a cationic azobenzene surfactant at high concentration"

Data files contain the raw data for the Figures in the manuscript and ESI. Figure 1 contains the UV-Vis absorption XY data for both photostationary states as wavelength (nm), trans-PSS absorption, cis-PSS absorption. Figure 2 contains the raw SAXS data for the trans-PSS state at 10, 15 and 20 wt%. Columns are q (A^-1), 20 wt%, yError, 10 wt%, y error, 15 wt%, y error. Figure 3 contains the raw SAXS data for the cis-PSS state at 10, 15 and 20 wt%. Columns are q (A^-1), 20 wt%, yError, 15 wt%, y error, 10 wt%, y error. Figure S1 contains the UV-Vis absorption XY data for the 100% trans-assembly as wavelength (nm) and absorption. Figure S2a contains the UV Vis absorption and kinetics of the trans to cis photoisomerisation XY data, as wavelength (nm) and absorption at fixed intervals over 90 seconds. Figure S2a_inset contains the XY data for the calculated rate constant (trans-cis) as a function of time (s). Figure S2b contains the UV Vis absorption and kinetics of the cis to trans photoisomerisation XY data, as wavelength (nm) and absorption at fixed intervals over 180 seconds. Figure S2b_inset contains the XY data for the calculated rate constant (cis - trans) as a function of time (s). Figure S4a contains the SAXS data for the trans state (20 wt%) as a function of temperature, as q (A^-1), and scattering intensity at T = 20 degrees Celcius, 30 degrees, 40 degrees, 50 degrees, 60 degrees. Figure S4b contains the DSC XY data for the heat flow thermograms at 20 and 30 wt% across 10-90 degrees Celcius

A Single-Component Photorheological Fluid with Light-Responsive Viscosity

Manuscript and supporting information detailing characterisation of photoresponsive viscoelastic fluids prepared from wormlike micelles composed of azobenzene surfactants. Data includes results from rheology, UV/vis absorption, transmission electron microscopy and small-angle X-ray scattering measurements

A Single-Component Photorheological Fluid with Light-Responsive Viscosity

Manuscript and supporting information detailing characterisation of photoresponsive viscoelastic fluids prepared from wormlike micelles composed of azobenzene surfactants. Data includes results from rheology, UV/vis absorption, transmission electron microscopy and small-angle X-ray scattering measurements.</jats:p

Developing an in-situ LED Irradiation System for Small-Angle X-Ray Scattering at B21, Diamond Light Source

Abstract Beamline B21 at the Diamond Light Source synchrotron in the UK is a SAXS beamline that specialises in high-throughput measurements via automated sample delivery systems. A system has been developed whereby a sample can be illuminated by a focussed beam of light coincident with the X-ray beam. The system is compatible with the highly automated sample delivery system at the beamline and allows a beamline user to select a light source from a broad range of wavelengths across the UV and visible spectrum and to control the timing and duration of the light pulse with respect to the X-ray exposure of the SAXS measurement. The intensity of the light source has been characterised across the wavelength range enabling experiments where a quantitative measure of dose is important. Finally, the utility of the system is demonstrated via measurement of several light-responsive samples

Developing an in situ LED irradiation system for small-angle X-ray scattering at B21, Diamond Light Source.

Publication status: PublishedBeamline B21 at the Diamond Light Source synchrotron in the UK is a small-angle X-ray scattering (SAXS) beamline that specializes in high-throughput measurements via automated sample delivery systems. A system has been developed whereby a sample can be illuminated by a focused beam of light coincident with the X-ray beam. The system is compatible with the highly automated sample delivery system at the beamline and allows a beamline user to select a light source from a broad range of wavelengths across the UV and visible spectrum and to control the timing and duration of the light pulse with respect to the X-ray exposure of the SAXS measurement. The intensity of the light source has been characterized across the wavelength range enabling experiments where a quantitative measure of dose is important. Finally, the utility of the system is demonstrated via measurement of several light-responsive samples