Light-Modulated Morphological Transformation of Spiropyran Derivative from Nanosphere to Nanorod

The construction of tunable morphological systems has important implications for understanding the mechanism of molecular self-assembly. In this study, a spiropyran derivative M1 is reported with light-responsive assembly morphology, which can be tuned from nanosphere to nanorod by ultraviolet light irradiation. The absorption spectra show that M1 molecules are transformed from closed-ring (SP) isomers into open-ring (MC) isomers and start to form H-aggregates with increasing irradiation time. Density functional theory calculations indicate that MC-MC isomers possess stronger binding energy than SP-SP isomers. The MC isomers may thus facilitate the dissociation of the SP-SP aggregates and promote the change of self-assembled morphology with the aid of stronger π-π stackings and dipole-dipole interactions. The research gives an effective method for modulating the morphology of assemblies, with great potential for applications in smart materials.

UV-Light-Induced Morphological Transformation of Spiropyran Assemblies from Irregular Sheet-like Structures to Nanospheres

Studies on self-assembling systems with a controllable morphology responding to light stimulation are significant for revealing the process and mechanism of assembly. Here, a molecule of spiropyran derivative (SP) possessing photoresponsive assembly morphology is constructed. SP self-assembles into irregular sheet-like structures whose morphology can be significantly transformed into regular nanospheres under continuous ultraviolet light stimulation. The UV-vis absorption spectra indicate that 56% of SP are isomerized from closed-ring form (SPC) to open-ring form (SPO) with color changes from colorless to magenta. Furthermore, theoretical calculations demonstrate that SPO-SPO aggregates possess stronger van der Waals forces than do SPC-SPC aggregates and tend to form stable intermediates combined with SPO isomers. Therefore, the isomerization of SP from SPC to SPO and the differences in intermolecular interactions are important factors in the morphological transition. Our study provides an efficient strategy to modulate the assembled morphology, which holds great promise to be applied in the field of smart materials.

Re-entrant Photoinduced Morphological Transformation and Temperature-Dependent Kinetic Products of a Rectangular Amphiphilic Diarylethene Assembly

An amphiphilic rectangular-shaped photochromic diarylethene bearing two hydrophobic alkyl chains and two hydrophilic tri(ethylene glycol) chains was synthesized, and its photoinduced morphological transformation in water was investigated. Two unexpected phenomena were revealed in the course of the experiments: a re-entrant photoinduced macroscopic morphological transformation and temperature-dependent kinetic products of supramolecular assembly. When the pure closed-ring isomer was dispersed in water, a re-entrant photoinduced morphological transformation, that is, a photoinduced transition from the hydrated phase to the dehydrated phase and then back to the hydrated phase, was observed by optical microscopy upon irradiation with green light at 20 °C; this was interpreted by the V-shaped phase diagram of the LCST transition. The aqueous assembly of the pure closed-ring isomer was controlled by changing the temperature; specifically, rapid cooling to 15 and 5 °C gave J and H aggregates, respectively, as the kinetic products. The thermodynamic product at both temperatures was a mixture of mostly H aggregate with a small amount of J aggregate. This behavior was rationalized by the temperature-dependent potential energy surface of the supramolecular assembly.

Photoinduced repetitive separation of a supramolecular assembly composed of an amphiphilic diarylethene mixture

A supramolecular assembly composed of a two-component mixture of amphiphilic diarylethenes, which have octyloxycarbonyl and N-octylcarbamoyl groups, showed a unique macroscopic transformation upon irradiation with UV light and subsequent standing in the dark. Unlike the pure compounds, the assembly was repetitively separated into a blue sphere and a red-purple sparse structure. Both the blue sphere and the sparse structure turned into colorless spheres upon irradiation with visible light and the divided colorless spheres showed the same response to UV and visible light. Phase diagrams based on the change in absorption spectra upon temperature change suggested that the transformation originates from a LCST transition. In the 0.5 : 0.5 mixture, in contrast to the pure compounds, the transition temperature sharply changed at around 50% of the fraction of the closed-ring isomer. TEM imaging showed that the 0.5 : 0.5 mixture with high photoisomerization yield formed a 10 nm-sized network. Judging from the phase diagram and TEM images, the separation is understood as the local phase transition of the regions with a high fraction of the closed-ring isomer.

Photochromism of Amphiphilic Dithienylethenes as Langmuir-Schaefer Films

Surface pressure-area isotherms were recorded under different irradiation conditions for single-component Langmuir films of three photochromic amphiphilic dithienylethenes. Nonirradiated films of these photochromic amphiphiles were mechanically stable. In addition, a shift of the isotherms to larger mean molecular areas was observed for films prepared from UV-light-irradiated dithienylethenes. Unexpectedly, a significant expansion was observed for a film prepared from visible-light-irradiated dithienylethene incorporating large branched alkyl chains. Upon further study, atomic force microscopy and transmission electron microscopy images of Langmuir-Schaefer films revealed that this pronged dialkyl derivative undergoes a photoinduced change in morphology, as circular aggregates coalesce into larger continuous aggregated structures. Nevertheless, its photoisomerization was completely reversible as single-component multilayer thin films upon direct UV or visible light irradiation.

Photoinduced macroscopic morphological transformation of an amphiphilic diarylethene assembly: reversible dynamic motion

Self-assembled microstructures of an amphiphilic diarylethene featuring an alkyl chain and triethylene glycol groups showed a photoinduced reversible morphological change in water. Reversible photoisomerization of the core diarylethene gave rise to a reversible morphological transformation between colorless microspheres and colored fibers. When colorless microspheres were irradiated with UV light, colored fibers were formed, and when the colored fibers were irradiated with visible light, the spheres were restored to their original positions where the spheres originally existed. This system showed reversible morphological change through not only photoirradiation but also temperature change. These behaviors can be interpreted as a phase transition between the sphere and fiber states. The dynamic process of the phase transition was monitored by polarized optical microscopy (POM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). It was revealed that the formation of fibers upon UV irradiation occurred radially at the surface of the sphere and the formation of the spheres upon visible-light irradiation occurred at the middle of the fiber. The unique photoinduced mechanical motion provides useful information for the design of sophisticated photoactuators.

Photocontrol of ion permeation in lipid vesicles with amphiphilic dithienylethenes

Asymmetrical amphiphilic dithienylethenes have been prepared and photocontrol of ion permeation was observed in lipid vesicles.

Light-controlled formation of vesicles and supramolecular organogels by a cholesterol-bearing amphiphilic molecular switch

A new responsive material composed of an amphiphilic light-switchable dithienylethene unit functionalized with a hydrophobic cholesterol unit and a hydrophilic poly(ethylene glycol)-modified pyridinium group has been designed. This unique single-molecule system shows responsive light-switchable self-assembly in both water and organic solvents. Light-triggered reversible vesicle formation in aqueous solutions is reported. The molecule shows a different behavior in apolar aromatic solvents, in which light-controlled formation of organogel fibers is observed. The light-triggered aggregation behavior of this molecule demonstrates that control of a supramolecular structure with light can be achieved in both aqueous and organic media and that this ability can be present in a single molecule. This opens the way toward the effective development of new strategies in soft nanotechnology for applications in controlled chemical release systems.

Electrically conductive hybrid nanofibers constructed with two amphiphilic salt components

Electrical conductivity was obtained in hybrid organic nanofibers fabricated with low-molecular-weight amphiphilic azopyridinium and dodecylbenzenesulfonic acid or its salt.