Lijun Guo

Photoresponsive structure transformation and emission enhancement based on a tapered azobenzene gelator

A new low molecular mass organic gelator bearing 1,3,4-oxadiazole and azobenzene groups, namely N-(3,4,5-tributoxyphenyl)-N′-4-[(4-hydroxyphenyl)azophenyl] 1,3,4-oxadiazole (AOB-t4), was designed and synthesized in this work. The organogelator shows a great ability to gel moderate polar solvents and form stable organogels with the critical gelation concentration as low as 2.0 mg mL−1, which thus can be considered as a supergelator. The morphology of xerogels demonstrates a strong dependence on the nature of gelling solvents. Due to the photo-induced isomerization of azobenzene unit, the transformations from fiberous to porous structure in AOB-t4 dichloromethane gel, and from fiber to nanoparticle in ethanol solution have been successfully achieved with UV 365 nm irradiation, respectively. Meanwhile, an enhanced fluorescence via J-aggregate molecular arrangement can be observed with the trans-to-cis photoisomerization in both dichloromethane gel and ethanol solution of AOB-t4, and the emission quantum yield can be increased from 10.7 × 10−5 to 18 × 10−2 in ethanol, corresponding to 1682 times enhancement. The obtained results would be of significance in developing novel photo-controllable luminescence molecular device and broadening the application fields of azobenzene derivatives.

Light-driven fluorescence enhancement and self-assembled structural evolution of an azobenzene derivative

Producing a fluorescent azobenzene derivative and investigating the relationship between the self-assembled structure and photophysical properties is a challenging topic, which is of significance in material science. Here, we report the light-driven fluorescence enhancement and self-assembled structural characteristics of an azobenzene derivative N-(3,4,5-octanoxyphenyl)-N′-4-[(4-hydroxyphenyl)azophenyl]1,3,4-oxadiazole (AOB-t8). A highly fluorescent self-assembled aggregate of cis-AOB-t8 in organic solution under UV light illumination has been observed; the quantum yield can reach 33.7 × 10−2, which is an enhancement of about 306 times compared with that of trans-AOB-t8, which is negligibly fluorescent in the initial nonirradiated solution. It has been proposed that the fluorescence enhancement originates from the trans-to-cis monomeric isomerization and the aggregation of cis-AOB-t8. Based on the theoretical calculations, we have discussed the mechanism for light-driven fluorescence enhancement of the monomeric cis-AOB-t8, which was commendably applied to interpret the observed spectroscopic result. In addition, the formation of the J-aggregate of cis-AOB-t8 leads to further enhancement of the emission. Moreover, this photocontrolled fluorescence enhancement in a concentrated solution (1 × 10−3 M) could be attributed to the significant structural changes of the aggregates, from fiber-like aggregates to layer structured aggregates. According to the spectroscopy study, it is suggested that the trans-AOB-t8 is inclined to form H-aggregates and the cis-AOB-t8 forms J-aggregates, which could probably lead to the structural variety related to the different molecular conformations of AOB-t8.

Morphology and Wettability Tunable Organogel System Based on An 1,3,4-Oxadiazole Derivative

A new low molecular mass organic gelator (LMOG) bearing 1,3,4-oxadiazole and azobenzene groups, namely N-(3,4,5-octanoxyphenyl)-N’-4-[(4-hydroxyphenyl)azophenyl] 1,3,4-oxadiazole (AOB-t8), was designed and synthesized. The organogelator has shown great ability to gel a variety of organic solvents to form stable organogels with the critical gelation concentration as low as 5.0 × 10−4 M and, therefore, can be considered a supergelator. It has been demonstrated that the aggregation morphology and surface wettability produced by organogels strongly depend on the nature of gelling solvents. A structure of uptwisting fibers on the substrate and superhydrophobicity were observed in the xerogel formed from 1,2-dichloroethane. The wettability of the xerogel could also be tuned by applying a sol-gel process with different solvents. Cooperation of hydrogen bonding, π-π interaction, and Van der Waals force are suggested to be the main contribution for this self-assembled structure. The unique and tunable surface properties such as superhydrophobicity distinguish the obtained organogels as a novel class of functional materials.

Single-step fabrication of large-scale patterned honeycomb structures via self-assembly of a small organic molecule

Microstructures with regular patterns, in particular, honeycomb-patterned structures have attracted extensive attention in various applications, such as photonic crystals, cell cultures, protein patterning, and superhydrophobic coatings. Here we report the successful single-step fabrication of large-scale honeycomb-patterned structures on a variety of substrates via self-assembly of an azobenzene derivative, N-(3,4,5-octanoxyphenyl)-N′-4-[(4-hydroxyphenyl)azophenyl]1,3,4-oxadiazole (AOB-t8). Factors such as concentration, substrate, solvent evaporation temperature and the relative humidity are revealed to be able to control the size of the formed micropores. A model to describe the dynamical formation of honeycomb structures from AOB-t8 has been proposed. The used method in this work demonstrates the potential to open up new possibilities for preparing large-scale, orderly structured materials from low mass organic molecules, which is of significance in a variety of potential applications.

The gelation ability and morphology study of organogel system based on calamitic hydrazide derivatives

The gelation property of a series of LMOG bearing hydrazide and azobenzene groups, namely, N-4-(alkoxyphenyl)-N′-4-[(4- methoxyphenyl)azophenyl] benzohydrazide (BNBC-n, n = 8,12,14), has been systematically studied in this work. The obtained results demonstrate that the gelling ability in organic solvents is significantly influenced by the length of terminal alkoxy chain. In different organic solvents, it is hard to observe the organogel formation for BNBC-8 molecule. On the contrary, the organogelators BNBC-12 and BNBC-14 bearing longer terminal chains have shown great ability to gel organic solvents to form stable organogels. The critical gelation concentration for BNBC-12 reaches as low as 5.3 × 10-3 M, which can be considered as a supergelator. It has been manifested that the aggregation morphology of organogel strongly depends on the nature of the gelling solvents and the length of the terminal alkoxy chain. The gelation of BNBC-n provides an easy method for the preparation of multidimensional structure and manipulation of morphology from ribbons, hollow tube fiber to 3D net-like structure in different solvents. The cooperation of hydrogen bonding, π-π interaction, and Van der Waals force is suggested to be the main contribution to this self-assembled structure.

Probing the inhomogeneity and intermediates in the photosensitized degradation of rhodamine B by Ag3PO4 nanoparticles from an ensemble to a single molecule approach

The photoinduced dynamics related to the degradation of a surface adsorbate by a semiconductive catalyst is critical for understanding the photocatalytic mechanism and improving the catalytic property of nanoscale materials. Herein, we report the investigation of the inhomogeneous interactions between Ag3PO4 nanoparticles and rhodamine B (RhB), and the direct observation of the intermediates generated in the photodegradation of RhB, using ensemble-averaged as well as single-molecule time-resolved fluorescence spectroscopies. The results demonstrate the existence of electron injection from RhB into the conduction band of Ag3PO4 and the formation of a deethylation intermediate before the subsequent degradation process. The fluorescence diversity both in lifetime and intensity fluctuation indicates an inhomogeneous interfacial interaction between the RhB molecule and Ag3PO4 nanoparticle with surface heterogeneity. Based on the lifetime distribution, the duration time of single-molecule events and the related dynamical analysis, it was revealed that the RhB molecule adsorbed on the active site of the Ag3PO4 nanoparticle has a higher injection efficiency and better photocatalytic activity. Moreover, the lifetime evolution derived from a subsection of the single-molecule emission trajectories proved that the electron injection occurred prior to the degradation through the attack of free radical O2˙−. These findings provide new insights into the heterogeneous interactions and dynamical information of the photosensitized degradation in an adsorbate/semiconductor catalyst.

Colorimetry and phase transition characteristics in sensing fluoride anion based on hydrazide organogelators

The fluoride anion sensing properties of BNB-t4 and BNBC-t8 consisting of hydrazide and azobenzene moieties both in solution and gel state, and the involved binding mechanism have been systematically investigated in this work. The remarkable changes in the absorption of receptor BNB-t4 with a terminal hydroxyl group demonstrate a colorimetric chemosensor with a higher sensitivity in sensing fluoride anions than that of BNBC-t8 with a terminal methoxy group. The detection limit of BNB-t4 for the analysis of F− can reach as low as 4.27 × 10−8 M, while this value is 2.02 × 10−6 M for BNBC-t8. The results indicate that the F− ion interacts with the amidic –NH and hydroxyl proton of BNB-t4 via hydrogen-bonding to give the stable 1 : 2 complex at the first equilibrium state, and further addition of F− can induce deprotonation by forming HF2− to establish a second equilibrium state. Meanwhile, the gel–sol transition of BNB-t4 has been successfully applied in sensing fluoride anions and thus makes BNB-t4 a naked-eye sensor. The color change of BNB-t4 induced by binding fluoride anions can be safely switched off with the addition of HSO4−, demonstrating an OFF–ON–OFF colorimetric sensor with a good reversibility.

Controllable wettability and adhesion of superhydrophobic self-assembled surfaces based on a novel azobenzene derivative

Superhydrophobic surfaces with controllable adhesion have received considerable attention due to their potential in numerous applications. Here, we report a controllable adhesion of superhydrophobic surfaces through tuning the self-assembly structure based on a novel Y-shaped molecule AOB-Y8, consisting of azobenzene groups, 1,3,4-oxadiazole moieties, and three octyl chains. The surface hydrophobicity of AOB-Y8 films was successfully regulated by tuning the assembly morphologies through changing the composition of CHCl3/CH3CN mixed solvents, concentration and temperature. Morphological studies of the film prepared from 50 : 50 CHCl3/CH3CN solvents revealed that the self-assembled hierarchical, flower-like structure constructs a lot of grooves to trap the surrounding air, generating the superhydrophobic effect at room temperature. Particularly, the surface adhesion of this superhydrophobic film was further regulated from a low level to a very high level by simply changing the concentration of AOB-Y8 in mixed solvents. Meanwhile, the AOB-Y8 self-assembled surfaces showed an excellent chemical resistance to acid and alkali, which is suitable for applications in many environmental conditions. As examples, the tunable adhesive superhydrophobic AOB-Y8 surfaces demonstrated good features to be used in selective transportation of droplets, self-cleaning and droplet-based microreactors to quantitatively detect NaOH and FeCl3.