Photo-labile lamellar phases

Photoresponsive Self-Assembly of Surface Active Ionic Liquid

A novel photoresponsive surface active ionic liquid (SAIL) 1-(4-methyl azobenzene)-3-tetradecylimidazolium bromide ([C14mimAzo]Br) with azobenzene located in the headgroup was designed. Reversible vesicle formation and rupture can be finely controlled by photostimuli without any additives in the aqueous solution of the single-tailed ionic liquid. The photoisomerization of the azobenzene derivative was investigated by (1)H NMR and UV-vis spectroscopy. Density functional theory (DFT) calculations further demonstrate that trans-[C14mimAzo]Br has less negative interaction energy, which is beneficial to aggregate formation in water. The incorporation of trans-azobenzene group increases the hydrophobicity of the headgroup and reduces the electrostatic repulsion by delocalization of charge, which are beneficial to the formation of vesicles. However, the bend of cis-azobenzene makes the cis-isomers have no ability to accumulate tightly, which induces the rupture of vesicles. Our work paves a convenient way to achieve controlled topologies and self-assembly of single SAIL.

Aggregation Behavior of Imidazolium-Based Surface-Active Ionic Liquids with Photoresponsive Cinnamate Counterions in the Aqueous Solution

Two imidazolium-based surface active ionic liquids (SAILs) with photoresponsive cinnamate aromatic counterions, viz. 1-dodecyl-3-methylimidazolium cinnamate ([C12mim][CA]) and 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C12mim][PCA]), were newly synthesized, and their self-assembly behaviors in aqueous solutions were systematically explored. Results of surface tension and conductivity measurements show that both [C12mim][CA] and [C12mim][PCA] display a superior surface activity in aqueous solutions compared to the common imidazolium-based SAIL, 1-dodecyl-3-methylimidazolium bromide (C12mimBr), which implies the incorporation of cinnamate aromatic counterions can promote the micellar formation. Furthermore, [C12mim][CA] shows higher surface activity due to the higher hydrophobicity of its counterion in comparison to [C12mim][PCA] that has a hydroxyl group. Both hexagonal liquid-crystalline phase (H1) and cubic liquid-crystalline phase (V2) were constructed in the [C12mim][CA] aqueous solutions. In contrast, the [C12mim][PCA]/H2O system only exhibits a single hexagonal liquid-crystalline phase (H1) in a broad concentration region. These lyotropic liquid crystal (LLC) phases were comprehensively characterized by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheometer. Investigation on the temperature-dependent self-assembly nanostructures demonstrates that the higher temperature leads to a looser arrangement. Under UV irradiation, trans-cis photoisomerization of the phenylalkene group results in inferior surface activity of the prepared SAILs in aqueous solution with higher cmc values. Moreover, UV light irradiation induces obvious change of the structural parameters without altering the LLC phases. This work is expected to enrich the investigations of phase behaviors formed in SAILs systems and receive particular attention due to their unique properties and potential applications in drug delivery, biochemistry, materials science, etc.

Wormlike micelles with photoresponsive viscoelastic behavior formed by surface active ionic liquid/azobenzene derivative mixed solution

The UV-light-stimulated self-assembly behavior of a surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), with an azobenzene derivative, sodium azobenzene 4-carboxylate (AzoCOONa), was investigated in aqueous solution. The properties and structures of the aggregates, formed at a concentration ratio equal to 2:1 ([C16mimBr]:[AzoCOONa]), were comprehensively characterized by rheometer and cryogenic transmission electron microscopy. Initially, viscoelastic wormlike micelles with a viscosity of 0.65 Pa·s were constructed in the C16mimBr/AzoCOONa system. Upon irradiation by UV light (365 nm), particularly fascinating is that the wormlike micelles become much longer and more entangled, exhibiting a high viscosity of 6.9 Pa·s. This can be attributed to photoisomerization of the AzoCOONa molecule from trans to cis form. It is the first time that, with exposure to UV or visible light, the aggregate type of the photoresponsive system has remained unchanged, with only a change of internal property parameters. The cation-π interaction prevailing over the hydrophobic interaction and electrostatic interaction between C16mimBr and AzoCOONa molecules is supposed to be responsible for this peculiar phase behavior. The wormlike micelles constructed with the SAIL and photosensitive additive exhibit controllable viscoelastic behavior in the photoresponsive process. In addition, the average contour length of wormlike micelles was found to slightly decrease with the increase of temperature. We expect this system will receive particular attention due to its unique properties and potential applications in drug delivery, biochemistry, and materials science, etc.

Advanced molecular self-assemblies facilitated by simple molecules

Advanced materials are often based on smart molecular self-assemblies that either respond to external stimuli or have hierarchical structures. Approaches to this goal usually stem from complicated molecular design and difficult organic synthesis. In this invited feature article, we demonstrate that desired molecular self-assemblies can be made conveniently by introducing simple functional molecules into amphiphilic systems. We show that upon introducing specific small molecules which serve as responders, modulators, or even building blocks, smart supramolecular architectures can be achieved which avoid complicated organic synthesis. We expect that this could be a general and economical way to produce advanced materials in the near future.

Novel spiropyran amphiphiles and their application as light-responsive liquid crystalline components

Light-responsive materials formed by liquid crystalline lipids in water have potential application to drug delivery through inclusion of photochromic additives such as spiropyran. A series of novel analogues of spiropyran (SP) have been synthesized with an SP headgroup that possess a C8 (SP-OC), C12 (SP-L), and C16 (SP-P) tail to probe the influence of the length of the hydrophobic tail on their physicochemical properties and effect on behavior in liquid crystal matrices with a view to application as stimulus-responsive elements on ultraviolet irradiation. In addition, compounds possessing an oleyl (SP-OL) and phytanyl (SP-PHYT) tail, to mimic those of the "parent" reverse bicontinuous cubic (V2) phase forming lipids, glyceryl monooleate (GMO) and phytantriol, were also prepared. The photochromic compounds were characterized by their melting points and photophysical behavior in solution using techniques including hot stage microscopy (HSM), differential scanning calorimetry (DSC), and UV-visible spectroscopy. Their effect on the equilibrium nanostructure of bulk V2 phases and phase-switching kinetics after exposure to UV light was assessed using small-angle X-ray scattering (SAXS). The melting point of the SP derivatives decreased linearly with increasing chain length, which suggests that interactions between the head groups governed their melting point, rather than the van der Waals interactions between the tails. Changing the R group did not influence the equilibrium rate constants for the isomerization of SP. Phase transition temperatures of liquid crystalline (LC) matrices were influenced significantly by incorporation of the SP derivatives and were greatest when the photochromic compound possessed an intermediate tail length substituent compared to the short alkyl or bulkier moieties. The level of disruption of lipid packing, and hence phase structure, were dependent on the duration of UV exposure.

Photofoams: remote control of foam destabilization by exposure to light using an azobenzene surfactant

We report evidence for photocontrolled stability and breakage of aqueous foams made from solutions of a cationic azobenzene-containing surfactant over a wide range of concentrations. Exposure to UV or visible lights results in shape and polarity switches in the surfactant molecule, which in turn affects several properties including critical micelle concentration, equilibrium surface tension, and the air-water interfacial composition (cis isomers are displaced by trans ones). We demonstrate that the trans isomer stabilizes foams, whereas the cis isomer forms unstable foams, a property that does not correlate with effects of light on surface tension, nor with total surfactant concentration. Achieving in situ breakage of foam is accordingly ascribed to the remote control of the dynamics of adsorption/desorption of the surfactant, accompanied by gradients of concentrations out of equilibrium. Photomodulation of adsorption kinetics and/or diffusion dynamics on interfaces is reached here by a noninvasive clean trigger, bringing a new tool for the study of foams.