Zhenqiang Yu

High efficiency luminescent liquid crystal: aggregation-induced emission strategy and biaxially oriented mesomorphic structure

Rational combination of aggregation-induced emission active luminogens and mesogens generates high solid-state efficiency luminescent liquid crystals, thus resolving the problem of aggregation-caused quenching normally occurs in the fabrication of luminescent mesomorphic films.

Synthesis and self-assembly of tetraphenylethene and biphenyl based AIE-active triazoles

Self-assembly of fluorescent functional materials has attracted increasing interest in the fabrication of optoelectronic and biological nanodevices. Tetraphenylethene (TPE) is a typical dye molecule with aggregation-induced-emission (AIE) characteristics. Melding TPE carrying triple-bond functionality with diazide-containing biphenyl through “click” chemistry generates AIE-active luminogens [1,1′-biphenyl]-4,4′-diyl bis(6-(4-(4-(1,2,2-triphenylvinyl)phenyl)-1H-1,2,3-triazol-1-yl) hexanoate) [1(5)] and [1,1′-biphenyl]-4,4′-diyl bis(11-(4-(4-(1,2,2-triphenylvinyl)phenyl)-1H-1,2,3-triazol-1-yl) undecanoate) [1(10)] with solid state efficiencies up to unity. Slow addition of dilute THF solutions of 1(m) (m = 5, 10) into nonsolvents such as n-hexane and water yields self-assembled white wooly solids. TEM and SEM observations reveal the (helical) nanofibrous structure of the aggregates. Upon cooling from their concentrated hot solutions, 1(m) readily precipitate. Meanwhile, they can also form gels at high concentrations. Both precipitates and gels of 1(m) exhibit structures similar to those of the aggregates formed in nonsolvents. These results indicate that 1(m) can facilely self-assemble into high emission efficiency (helical) nanofibers, thus paving the way for their optoelectronic and biological applications.

Mesogen jacketed liquid crystalline polyacetylene containing triphenylene discogen: synthesis and phase structure

Triphenylene-containing acetylenes with one or three methylene units as spacers and the corresponding mesogen-jacketed liquid crystalline polyacetylenes (MJLCPAs) were designed and synthesized, the mesomorphic properties and phase behaviors of the monomers and novel side-chain liquid crystalline polymers were investigated. The monomers [HCC(CH2)mC18H6(OC6H13)5; m = 1, 3] are prepared by consecutive etherization, coupling, and etherization reactions, and the chemical structures were confirmed by mass spectroscopy and 1H/13C-NMR. The phase behaviors of the monomers were investigated by differential scanning calorimetry (DSC), polarized light microscopy (PLM), and wide-angle X-ray diffraction (WAXD). The results show that both of the monomers form a hexagonal columnar liquid crystal phase at room temperature. The monomers are polymerized using [Rh(nbd)Cl]2 as catalyst and producing soluble polymers in the yields of 55% and 52%, respectively. The chemical structures and phase behaviors of the two polymers are characterized and evaluated by IR, NMR, TGA, DSC, and WAXD analyses. Both of the two polymers show enhanced thermal and chemical stability with thermal decomposition temperatures up to higher than 340 °C due to the protection of the “jacketed effect” of the side-chain triphenylenes wrapped around the rigid polyacetylene main-chain. The polymers adopt a columnar shaped structure and self-organized into hexagonally packed columnar phase. The relative electron density maps of the columnar structure are also reconstructed.

Multiple stimuli-responsive and reversible fluorescence switches based on a diethylamino-functionalized tetraphenylethene

Tetra(4-(diethylamino)phenyl)ethene (TPE-4N), a new derivative of tetraphenylethene (TPE), is facilely prepared in one step from bis(4-(diethylamino)phenyl)methanone in a good yield of 85%. TPE-4N shows aggregation-induced emission (AIE) with high solid-state fluorescence quantum yields up to 63.5%. It has strong proton capture capability, allowing for reversible fluorescence switching in acidic and basic solutions. A good linear relationship between the emission intensity and the pH value ranging from 4.4 to 6.0 is established. By exposing to hydrochloride vapor, the color of TPE-4N powder is changed from yellow-green to white, accompanied by a fluorescence color change from green to sky-blue. The resulting protonated luminogen (p-TPE-4N) can be readily reverted to TPE-4N by fuming with hot triethylamine vapor. The protonation and deprotonation processes are reversible and can be repeated many times without fatigue in the solid state. In addition, p-TPE-4N exhibits reversible thermochromism between 80–120 °C, and reverts to TPE-4N by heating up to 120 °C. Multiple stimuli-responsiveness and reversible fluorescence indicate that TPE-4N is a promising candidate for chemical sensing and environmental monitoring.

Tunable Helical Assemblies of l-Alanine Methyl Ester-Containing Polyphenylacetylene

The self-assemblying behaviors of L-alanine methyl ester-containing polyphenylacetylene (PPA-Ala, in Chart 1 ) were investigated upon the evaporation of its solvent on mica and on air/water interfaces. The introduction of chiral amino acid attachments to the polyphenylacetylene backbone induced a helical conformation of the backbone, which was stabilized by various noncovalent interactions, especially hydrophobic effect and hydrogen bonds. The helicity of the polymer was further amplified in its higher-order self-assemblies as the formation of helical fibers on the surface of mica upon natural evaporation of its THF solution. By LB technique, the polymer chains were guided to form ordered parallel ridges and highly aligned, with their helical conformation still remaining. The reorganization of the chiral polymer chains on air/water interface was associated with the additional hydrophobic effect of PPA-Ala on an air/water interface. The polymer backbones had to adopt different arrangements to minimize their contact with water, and this adjustment led to the formation of aligned polymer ridges under proper surface pressure.

Towards stable deep-blue emission and low efficiency roll-off in OLEDs based on phenanthroimidazole dimers

In this paper, two novel deep-blue emitters, mm-BPPI and mm-CN-BPPI, were obtained by coupling two 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) blocks in a meta–meta pattern and using a central benzene moiety as the bridge. Owing to this meta–meta connection, the dimer conjugation degree of mm-BPPI was effectively limited, and thus mm-BPPI exhibited very similar photophysical and electrochemical properties as PPI. However, the thermal stability of mm-BPPI was improved successfully, overcoming the defects of PPI for practical applications. Non-doped optimized devices fabricated with mm-BPPI exhibited stable deep-blue emission with CIE coordinates of (0.156, 0.050). After inserting cyano groups, higher glass-transition temperature and a quasi-reversible redox process were observed in mm-CN-BPPI; moreover, the lower efficiency roll-off was realized in its non-doped devices. This study demonstrates a strategy towards stable deep blue emitters with higher stability, better efficiency and lower roll-off ratio by chemical modification (meta–meta coupling or cyano group insertion) in non-doped deep-blue-emitting devices based on aryl-substituted phenanthroimidazole blocks.

1-((12-Bromododecyl)oxy)-4-((4-(4-pentylcyclohexyl)phenyl)ethynyl) benzene: Liquid crystal with aggregation-induced emission characteristics

The luminescent liquid crystals (LLCs) are expected to solve the conflicts between the aggregation caused quenching and the requirement of aggregation or self-organization for LCs. Herein, we developed a new strategy of applying aggregation-induced emission (AIE) phenomenon to the molecular design of LCs towards LLCs. In this report, a calamitic liquid crystal based on tolane with AIE characteristics was successfully synthesized and the chemical structure was characterized by 1H, 13C NMR, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) high-resolution mass spectra. The fluorescence behavior was studied by fluorescence spectroscopy and the liquid crystal phase behaviors were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM). The crystal structure was obtained by X-ray diffraction crystallography with P1 space group. Results demonstrated that the sample was AIE active and the LC phases sequence during cooling was nematic, smectic C and smectic B phase.

Electronic effect on the optical properties and sensing ability of AIEgens with ESIPT process based on salicylaldehyde azine

Two novel AIE-active salicylaldehyde azine (SAA) derivatives with a typical excited-state intramolecular proton transfer (ESIPT) process are prepared by introducing electron-withdrawing and donating groups at para-position of phenolic hydroxyl group (CN-SAA and TPA-SAA). The effect of the proton activity in SAA framework on their optical behaviors is investigated spectroscopically. The results from NMR and solvation measurements show that the proton of phenolic hydroxyl group has higher activity when there are electron-withdrawing groups, and the absorption and fluorescence spectra in buffers with different pH also provide the same results. After inviting F− as a nucleophilic probe, this proton activity difference in CN-SAA and TPA-SAA becomes more obvious. The potential application of both molecules is investigated. TPA-SAA exhibits good quantitative sensing ability towards F− with a fluorescence “turn-on” mode, whereas the aggregates of TPA-SAA can selectively and sensitively detect Cu2+ in aqueous solution. From these results, a structure-property relationship is established: the occurrence of ESIPT process will become much easier when linking electron-withdrawing groups at the para-position of phenolic hydroxyl group (e.g., CN-SAA), and it is better to introduce electron-donating groups to enhance the sensing ability towards ions (e.g., TPA-SAA). This work will provide guidance for further design and preparation of AIE-active luminogens with ESIPT process for sensing applications.

Novel optical anisotropy of a liquid crystalline “cubic” phase in a discotic crown ether derivative

To obtain a columnar structure with nucleophilic channels embedded in the column centres, we synthesized a series of new crown ether derivatives (compounds 1, 2, and 3) which are composed of a core of dibenzo-24-crown-8 symmetrically surrounded by four identical substituents. Experimental results indicate that compounds 2 and 3 with smaller substituents around the dibenzo-24-crown-8 core form only crystalline phases, while compound 1 bearing larger dendron substituents on the molecular periphery shows multiple liquid crystalline phase transitions. X-ray diffraction demonstrates that 1 can stack parallel to form a hexagonal columnar liquid crystalline (Colh) phase. A stable phase, which renders the diffraction features of a body-centred cubic (BCC) structure and shows unexpected optical anisotropy, is observed in between the Colh and the isotropic state. On the basis of further support from model calculations, we propose that the optically anisotropic “BCC” structure is an array of ellipsoidal domains placed in the BCC lattice with their long axis along the [111] direction, which results from the periodic density fluctuation of the columns.