Fluorescein-bridged Perylene Bisimide Dimer for Use as Liquid Crystal: Studies on Mesomorphic and Fluorescence Properties

A novel fluorescein-bridged perylene bisimide (PBI) dimer for liquid crystal (LC) with geometrically symmetric structure was developed. The mesomorphic results indicated that the energetically stable and unstable conformers of fluorescein fragments could lead to the transformation of mesophases from a hexagonal columnar mesophase to an uncertain phase at 136.9 °C in heating, whilst a stable hexagonal columnar mesophase maintained between 175.6 °C and 58.6 °C in cooling. The selectively excited fluorescence characters in THF solution demonstrated that the fluorescence resonance energy transfer (FRET) effect between fluorescein fragments and PBI unites could provide a means to effectively impose strong fluorescence of the dimeric PBIs modified with suitable chromophore at the N-imide position, which alternatively serves as a platform for the further study of multi-functional PBI-based LCs.

Room-temperature AIE ionic liquid crystals based on diphenylacrylonitrile-imidazole salts

Although various AIE liquid crystals have been studied, AIE ionic liquid crystals (ILCs) are almost unknown to date. In this work, a series of novel AIE ILCs based on diphenylacrylonitrile-imidazole salts bridged by soft spacers with different anions were prepared in yields of 63-80%. The mesomorphic, photophysical and electrochemical properties were investigated systematically to elucidate the relationship between structures and properties. The results implied that they were the first room-temperature AIE ILCs with wide ranges of mesomorphic temperature, good fluorescence emission in both the solid state and mesophase, and stable electrochemical behaviour. The samples with one alkyl chain possessed the SmA₂ mesophase while the samples with two alkyl chains prefered the Col_h mesophase. The larger anions resulted in the bigger layer spacing length for the SmA₂ mesophase and smaller values of n_cell in each slice of Col_h mesophase. The fluorescence quantum yields in the mesophase maintained reasonable values (0.15-0.22), which decreased a little in comparison with that in solid films (0.18-0.26) due to the orderly molecular stacking in the mesophase. The cyclic voltammetry experiments confirmed that all of them possessed similar and stable electrochemical behaviour. This research not only presented the first room-temperature AIE ILCs with excellent mesomorphic, photophysical and electrochemical properties, but also elucidated the relationship between structures and properties to a certain degree, contributing to the further construction of novel AIE ILCs with excellent mesomorphic, photophysical and electrochemical properties.

A first porphyrin liquid crystal with strong fluorescence in both solution and aggregated states based on the AIE-FRET effect

Porphyrins are good near-infrared fluorescent materials, but the strong self-assembly stacking resulted in the aggregation-caused quenching (ACQ) effect, limiting their emissive performance in aggregated states. In this work, a novel diphenylacrylonitrile-porphyrin derivative with multiple polyglycol chains on the periphery was designed and synthesized as an excellent near-infrared-emissive liquid crystalline material in both solution and aggregated states, which was first observed for porphyrin liquid crystals. It exhibited a high self-assembly ability with the ordered hexagonal columnar mesophase between 70 and 120 °C approximately. The strong AIE-FRET effect was produced based on the overlap of the emission wavelength of diphenylacrylonitrile and the absorption wavelength of the porphyrin, resulting in the excellent near-infrared emission in both solution and aggregated states. The pseudo Stokes shift was as large as 210 nm and the fluorescence quantum yield reached 0.12 in the solid state. Moreover, this porphyrin liquid crystal displayed low biotoxicity and excellent fluorescence bio-imaging ability in living cells, opening a new application prospect for porphyrin liquid crystalline materials.

Perylene-Based Liquid Crystals as Materials for Organic Electronics Applications

Columnar phases formed by the stacking of disclike molecules with an intimate π-π overlap forms a 1D pathway for the anisotropic charge migration along the columns. Columnar phases have great potential in organic electronic devices to be utilized as active semiconducting layers in comparison to organic single crystals or amorphous polymers in terms of processability, ease of handling, and high charge carrier mobility. Intelligent molecular engineering of perylene and its derivatives provided access to tune the physical properties and self-assembly behavior. The columnar phase formed by perylene derivatives has great potential in the fabrication of organic electronic devices. There are several positions on the perylene molecule, which can be functionalized to tune its self-assembly, as well as optoelectronic properties. Thus, many liquid-crystalline molecules stabilizing the columnar phase, which are based on perylene tetraesters, perylene diester imides, and perylene bisimides, have been synthesized over the years. Their longitudinal and laterally extended derivatives, bay-substituted derivatives exhibiting a columnar phase, are reported. In addition, several liquid-crystalline oligomers and polymers based on perylene derivatives were also reported. All such modifications provide an option to tune the energy levels of frontier molecular orbitals with respect to the work function of the electrodes in devices and also the processability of such materials. In this feature article, we attempt to provide an overview of the molecular design developed to tune the applicable properties and self-assembly of perylene derivatives as well as recent developments related to their application in the fabrication of organic solar cells, organic light-emitting diodes, and organic field-effect transistors.

Perylene bisimide with diphenylacrylonitrile on side-chain: strongly fluorescent liquid crystal with large pseudo Stokes shift based on AIE and FRET effect

A novel perylene bisimide (PBI) derivative with an AIE-active diphenylacrylonitrile unit positioned at the terminal N-imide position through a flexible spacer has been synthesized and characterized. The DSC, POM and XRD studies confirmed that it could self-assemble into a stable hexagonal columnar liquid-crystalline phase between 56 °C and 160 °C. This PBI derivative also exhibited strong fluorescence in solution, thin film and mesophase based on the cooperative mechanism of AIE and FRET between the diphenylacrylonitrile group and perylene moiety. The pseudo Stokes shift was as large as 283 nm, and the fluorescence quantum yields were as high as 0.62-0.79 in solution and 0.68-0.86 in solid state. This study provides a good strategy for converting the columnar liquid crystal with ACQ effect to one with the AIE effect, successfully filling the gap between the excellent columnar mesomorphic properties and strong fluorescence in solid state.

Diphenylacrylonitrile-connected BODIPY dyes: fluorescence enhancement based on dark and AIE resonance energy transfer

This study focuses on the construction of novel diphenylacrylonitrile-connected BODIPY dyes with high fluorescence in both solution and an aggregated state by combining DRET and FRET processes in a single donor-acceptor system. The first BODIPY derivatives with one, two, or three AIE-active diphenylacrylonitrile groups were designed and synthesized in moderate yields. Strong fluorescence emissions were observed in the THF solution under excitation at the absorption wavelength of non-emissive diphenylacrylonitrile chromophores, implying the existence of the DRET process between the dark diphenylacrylonitrile donor and the emissive BODIPY acceptor. In the THF/H₂O solution, the fluorescence intensity of the novel BODIPY derivatives gradually increased under excitation at the absorption wavelength of diphenylacrylonitrile chromophores, suggesting a FRET process between diphenylacrylonitrile and BODIPY moieties. A greater number of diphenylacrylonitrile units led to higher energy-transfer efficiencies. The pseudo-Stokes shift for both DRET and FRET processes was as large as 190 nm.

Perylene liquid crystals with multiple alkyl chains: investigation of the influence of peripheral alkyl chain number on mesomorphic and photophysical properties

Novel perylene liquid crystals with 2, 4 and 6 peripheral alkyl chains or 6, 8 and 10 peripheral alkyl chains at both the imide and bay-positions were investigated.

Influence of imide-substituents on the H-type aggregates of perylene diimides bearing cetyloxy side-chains at bay positions

A series of perylene-3,4:9,10-tetracarboxylic acid diimides (PDIs, namely TYR-PDI, AEP-PDI, CET-PDI, ANP-PDI and KOD-PDI), comprising long linear cetyloxy side-chains functionalized at the 1,7-bay positions and the different substituents (i.e., hydrophobic/hydrophilic segments) symmetrically linked at the two imide-positions of the perylene core were synthesized to investigate the influence of imide-substituent patterns on the aggregation behaviours of PDIs. The photophysical properties of these PDIs were studied by UV-Vis absorption, fluorescence and time-resolved photoluminescence spectroscopy. The differences in the photophysical properties of the PDIs indicate (i) blue-shifted and broadening absorption properties in both solution and thin-films, (ii) red-shifted and broadening fluorescence behavior at their emission maximum in solution, however, blue-shifted fluorescence behavior in thin-films, and (iii) obviously longer fluorescence life-times corresponding to the existence of rotationally displaced H-type aggregates. The formation of short-range ordered rod-like microstructures through face-to-face alignment of columnar rectangular H-type PDI aggregates was rationalized by scanning electron microscopy. The X-ray diffraction study revealed that the formation of well-defined columnar rectangular (Col_rp) H-type PDI aggregates indicated a nearly constant intracolumnar stacking distance of ∼3.9 Å for all PDIs. All of these findings were consistent with the formation of hydrophobic/hydrophilic interactions between the imide-substituents in addition to the strong hydrophobic π-π stacking interactions between the conjugated perylene cores, which were enforced in the H-type PDI aggregates that spontaneously self-organized into Col_rp structures.

Novel AIE columnar liquid crystals: the influence of the number of diphenylacrylonitrile groups on the mesomorphic and fluorescence properties

Triphenylene derivatives with one or two diphenylacrylonitrile groups exhibited not only an ordered hexagonal columnar mesophase but also good AIE fluorescence properties in both solution and solid state.

Phenylacrylonitrile-bridging triphenylene dimers: the columnar liquid crystals with high fluorescence in both solid state and solution

The phenylacrylonitrile-bridging triphenylene dimers were prepared and exhibited the ordered hexagonal columnar mesophase and high fluorescence in both the solid state and solution.

Synthesis, mesomorphic and photophysical properties of novel triads and pentads of perylene liquid crystals with cholesterol units at the bay-position

Perylene derivatives with cholesterol units at the bay-position exhibited good mesomorphic and photophysical properties.

The synthesis and structural characterization of twin liquid crystalline perylenebisimides

A smectic to nematic LC phase switch is achieved by changing one methylene unit in the central spacer segment.