Controlling the Fluorescence Performance of AIE Polymers by Controlling the Polymer Microstructure

Aggregation-induced emission (AIE) polymers with expected emission wavelength/color and fluorescence efficiency are valuable in applications. However, most AIE polymers exhibit irregular emission wavelength/color changes compared to the original AIE monomers. Here, we report the synthesis of AIE polymers with unchanged emission wavelength by ring-opening (co)polymerizations of 4-(triphenylethenyl)phenoxymethyloxirane (TPEO) and other epoxides or phthalic anhydride. The chemical structures/physical properties of all (co)polymers were characterized by NMR, SEC, MALDI-TOF, and DSC. The co-polyether microstructures were revealed by calculating the reactivity ratios and visualized by Monte Carlo simulation. The photoluminescence quantum yields of all the (co)polymers were determined in the solid state. We systematically correlated the fluorescence performance with molecular weights, crystallinity, monomer compositions, glass transition temperatures, side lengths, and flexibility/rigidity.

Stimuli-Responsive Electrospun Fluorescent Fibers Augmented with Aggregation-Induced Emission (AIE) for Smart Applications

This review addresses the latest advancements in the integration of aggregation-induced emission (AIE) materials with polymer electrospinning, to accomplish fine-scale electrospun fibers with tunable photophysical and photochemical properties. Micro- and nanoscale fibers augmented with AIE dyes (termed AIEgens) are bespoke composite systems that can overcome the limitation posed by aggregation-caused quenching, a critical deficiency of conventional luminescent materials. This review comprises three parts. First, the reader is exposed to the basic concepts of AIE and the fundamental mechanisms underpinning the restriction of intermolecular motions. This is followed by an introduction to electrospinning techniques pertinent to AIE-based fibers, and the core parameters for controlling fiber architecture and resultant properties. Second, exemplars are drawn from latest research to demonstrate how electrospun nanofibers and porous films incorporating modified AIEgens (especially tetraphenylethylene and triphenylamine derivatives) can yield enhanced photostability, photothermal properties, photoefficiency (quantum yield), and improved device sensitivity. Advanced applications are drawn from several promising sectors, encompassing optoelectronics, drug delivery and biology, chemosensors and mechanochromic sensors, and innovative photothermal devices, among others. Finally, the outstanding challenges together with potential opportunities in the nascent field of electrospun AIE-active fibers are presented, for stimulating frontier research and explorations in this exciting field.

Reaching Nearly 100% Quantum Efficiencies in Thin Solid Films of Semiconducting Polymers via Molecular Confinements under Large Segmental Stresses

Quantum efficiencies remain a critical issue for general applications of semiconducting polymers in optoelectronics and others. In this work, we demonstrate that nearly 100% quantum efficiencies (η's) in thin solid films can be reached when the polymer molecules are mechanically stretched into molecular confinement. We selected three conjugated polymers of varied backbone stiffness and interchain coupling, prepared in both diluted and pristine states. All of the polymers when highly diluted (c = 0.1 wt %) exhibited massive η increases after stretching to very large strains (∼300-500%) via micronecking, with the rigid polyfluorene (PFO) and semirigid MEH-PPV both manifesting η ≈ 90%, while the most flexible yet regioregular polythiophene (P3HT-rr) exhibited a 10-fold increase to ∼21%. In the pristine state, molecular aggregation and interchain coupling curtail development of the molecular confinement, but the large-strain deformation still enhances η's significantly, to ∼90% (PFO) and ∼55% (MEH-PPV) despite no increases for the crystalline P3HT-rr. Moreover, upon substitution by a bulkier side-group to reduce interchain coupling, the pristine films of polythiophene (P3EHT) exhibited a ∼3-fold increase of η after the stretching. The nearly 100% of η's in fully stretched molecules indicates that the in situ self-trapping occurring via sub-picosecond backbone interactions can be mostly responsible for energy dissipations and quite suppressible by segmental stress control. The mechanical confinement effects also indicate the fundamental role of molecular mechanics during stabilization and migration of photoexcited charges.

Synthesis and electrochromic properties of polyamines containing a 4,4′-diaminotriphenylamine-N,N′-diyl unit in the polymer backbone: Ru-catalyzed N–H insertion polycondensation of 1,4-phenylenebis(diazoacetate) with 4,4′-diaminotriphenylamine derivatives

Polycondensation of a bis(diazocarbonyl) compound and diaminotriphenylamine derivatization via Ru-catalyzed N–H insertion to afford electroactive polyamine.

Colorless-to-Black Electrochromism from Binary Electrochromes toward Multifunctional Displays

Cyclohexane-1,2,4,5-tetracarboxylic diimide with a nonconjugated core has been incorporated to bridge two conventional triphenylamine units. The obtained monomer has successfully hypsochromically shifted the maximum absorption wavelength by 10 nm in comparison to the one with a pyromellitic diimide bridge. Consequently, a colorless electrochromic (EC) polymer poly(bis(N,N-diphenyl-4-aminophenyl)cyclohexane-1,2,4,5-tetracarboxylic diimide) (PTPA-HDI) was electropolymerized on indium tin oxide (ITO)-coated glass. The morphology, absorption, and spectroelectrochemistry properties of polymer PTPA-HDI films electropolymerized by different scan cycles have been systematically investigated. It is found that comprehensive properties, such as color contrast and initial transparence, can be achieved for the polymer film electropolymerized by 15 scan cycles. Moreover, to realize colorless-to-black electrochromism, an asymmetric viologen derivative 1-(4-cyanophenyl)-1'-hexyl-4,4'-bipyridinium dihexafluorophosphate (HVCN) has been designed and straightforward synthesized. With the introduction of a cyanophenyl group and a hexyl chain on the two pyridinium units, colorless-to-green electrochromism can be realized for this processible viologen derivative. The absorption band at 495 nm of colorated PTPA-HDI compensates well for the valley in the absorption spectrum of colorated HVCN. Therefore, different types of colorless-to-black electrochromic devices (ECDs) are fabricated using polymer PTPA-HDI-deposited ITO electrode and HVCN-based gel electrolyte. Such a supporting electrolyte-free ECD with binary electrochromes exhibits fast coloration, high color contrast, and excellent reversibility. Furthermore, an encryption ECD is demonstrated by switching a black two-dimensional code. In addition, an autodigital display is integrated on a smart window and hence different functions can be realized in a single ECD. Overall, this study may facilitate the understanding of the EC behaviors of binary electrochromes and present a new path to design multifunctional displays.

Preserving High-Efficiency Luminescence Characteristics of an Aggregation-Induced Emission-Active Fluorophore in Thermostable Amorphous Polymers

Luminophores usually suffer from luminescent quenching when introduced into a polymer backbone or side chain, which leads to the inefficient luminescence or even no luminescence of the polymer. In this work, alicyclic imide rings were found to be capable of balancing the donor-acceptor properties between the rigid spacer and the aggregation-induced emission-active fluorophore in light-emitting polymers. Along with the nonplanar and rigid emitter, the suppressed intramolecular charge-transfer effect and interchain disturbance can efficiently preserve the luminescence characteristics of the active center, resulting in high solid-state photoluminescence quantum yields of up to 89%. The amorphous polyimides exhibit excellent thermal properties, such as high glass transition temperature (T_g) values (398 °C) and high thermal decomposition temperature (T_d) values (538 °C). As far as we know, these luminescent polymer materials are of excellent heat resistance with the highest luminescence efficiency reported. The results have significant impact for the precise prediction of the optical properties of light-emitting polymers by appropriate monomer design, providing controllable ways for synthesizing high thermal stability polymeric materials with efficient fluorescence properties.

Color-Tunable and Stimulus-Responsive Luminescent Liquid Crystalline Polymers Fabricated by Hydrogen Bonding

Luminescent liquid crystalline polymers (LLCPs) show extensive application potentials, such as liquid crystal displays and circularly polarized luminescence. In this work, we employ a hydrogen-bonding strategy different from the traditional covalent-bonding method to fabricate LLCPs. First, the acceptor and donor of hydrogen bonding, (4,4'-dibutanoxy tetraphenylethylene)-1-pyridine (PTPEC₄) and poly(2-vinyl terephthalic acid) (PPA), respectively, are successfully synthesized. Then, mixtures with different molar ratios ( x's) of PTPEC₄ to PPA are used to prepare a series of LLCPs [denoted as PPA(PTPEC₄) _x]. The resultant LLCPs show a smectic A phase ( x ≥ 0.8), a columnar nematic phase (0.6 ≤ x ≤ 0.05), and an amorphous state ( x = 0.025), depending on the x value. Meanwhile, all polymers exhibit typical aggregation-induced emission behavior. More interestingly, with the variation of the PTPEC₄ content, the series of LLCPs show different colors, that is, the emission peak red shifts from 510 nm ( x = 1.0) to 551 nm ( x = 0.025). Furthermore, because of the reversible protonation effect of the N atom of pyridine in PTPEC₄ by the strong proton acid, PPA(PTPEC₄) _x shows reversible color transformation. This work provides a new method to construct LLCPs with different emission colors and reversible color transformation.

Design, Synthesis, and Electrofluorochromism of New Triphenylamine Derivatives with AIE-Active Pendent Groups

Triphenylamine (TPA) and 4,4'-dimethoxy-triphenylamine (TPAOMe) derivatives were successfully linked with two high-performance AIEgens, triphenylethylene (TPE) and benzo[ b]thiophene-1,1-dioxide (BTO), to obtain four aggregation-induced emission and electro-active materials, TPETPA, BTOTPA, TPETPAOMe, and BTOTPAOMe. The effects on photoluminescence characteristics and electrochromic (EC) and electrofluorochromic (EFC) behaviors in cross-linking gel-type devices derived from the prepared materials were systematically investigated. Furthermore, heptyl viologen was introduced into the EFC devices to enhance EC performance including lower working potential, faster switching time, and superior stability.

Mechano-fluorochromic behavior of AEE polyurethane films and their high sensitivity to halogen acid gas

Three polyurethanes with different contents of tetraaryl-buta-1,3-diene derivatives in the soft segment (STMPU-25/STMPU-50/STMPU-75) have been synthesized and found to present aggregation-enhanced emission features. The fluorescence intensity of polymer films was greatly enhanced with increasing tensile stress. Also, polyurethanes with higher aggregation-induced emission fluorogen content had stronger mechano-fluorochromic behavior in the same tension state. Moreover, the resulting polyurethane films possessed high sensitivity for halogen acid gas, suggesting their potential applications in environmental monitoring fields.

(a) Polyurethanes with higher content of TABDAA2 presented better positive correlation between fluorescence and strain. (b) Sensing mechanism of STMPU-50 for detection of HCl.

Hollow-Core-Photonic-Crystal-Fiber-Based Miniaturized Sensor for the Detection of Aggregation-Induced-Emission Molecules

A miniature sensor for detection of aggregation-induced-emission (AIE) molecules is proposed in this work. The sensing head is fabricated by use of hollow-core photonic crystal fiber with a core diameter of about 4.8 μm. The cladding holes are sealed with a fusion splicing technique, and the central hole remains open to allow the filtration of solution with AIE molecules. When the solution is excited by an ultraviolet lamp, the fluorescence is received by a fiber-optic spectrometer. The fluorescence intensity is associated with the concentration of AIE molecules and the infiltrated-core length. In the whole process of the experiments, the output-peak wavelength is stable, which indicates that the existing forms of AIE particles are stable, and the fluorescence reabsorption can be neglected. The experimental results obtained are in accordance with traditional microplate-spectrophotometer methods. The most exciting result is that the amount of sample measured can be as low as 0.36 nL, which allows the detection of AIE molecules at only 0.02 pmol. In addition, the miniature sensor was successfully applied to the detection of an AIE-based bioprobe for evaluating the activity of the dipeptidyl-peptidase 4 (DPP-4) inhibitor sitagliptin with an IC₅₀ of 59.80 ± 3.06 nM. The advantages of small device size and nanoliter-scale sample volumes suggest that the proposed sensor is promising for many biosensing applications.

Novel Organic Phototransistor-Based Nonvolatile Memory Integrated with UV-Sensing/Green-Emissive Aggregation Enhanced Emission (AEE)-Active Aromatic Polyamide Electret Layer

A novel aggregation enhanced emission (AEE)-active polyamide TPA-CN-TPE with a high photoluminesence characteristic was successfully synthesized by the direct polymerization of 4-cyanotriphenyl diamine (TPA-CN) and tetraphenylethene (TPE)-containing dicarboxylic acid. The obtained luminescent polyamide plays a significant role as the polymer electret layer in organic field-effect transistors (OFETs)-type memory. The strong green emission of TPA-CN-TPE under ultraviolet (UV) irradiation can be directly absorbed by the pentacene channel, displaying a light-induced programming and voltage-driven erasing organic phototransistor-based nonvolatile memory. Memory window can be effectively manipulated between the programming and erasing states by applying UV light illumination and electrical field, respectively. The photoinduced memory behavior can be maintained for over 10⁴ s between these two states with an on/off ratio of 10⁴, and the memory switching can be steadily operated for many cycles. With high photoresponsivity ( R) and photosensitivity ( S), this organic phototransistor integrated with AEE-active polyamide electret layer could serve as an excellent candidate for UV photodetectors in optical applications. For comparison, an AEE-inactive aromatic polyimide TPA-PIS electret with much weaker solid-state emission was also applied in the same OFETs device architecture, but this device did not show any UV-sensitive and UV-induced memory characteristics, which further confirmed the significance of the light-emitting capability of the electret layer.

Flexible Multifunctional Aromatic Polyimide Film: Highly Efficient Photoluminescence, Resistive Switching Characteristic, and Electroluminescence

We report a flexible multifunctional aromatic polyimide (BTDBPI) that shows yellow-green fluorescence with high photoluminescence quantum yield (PLQY) of 30% in the film state. The nonvolatile "write once-read many" (WORM) characteristic in a memory device with the configuration of ITO/BTDBPI/Au indicates that BTDBPI possesses organic semiconductor behavior. Moreover, polymer light-emitting diodes (PLEDs) with the structure of ITO/PEDOT:PSS/BTDBPI/TPBI/Mg-Ag exhibits an interesting dual-emission phenomenon that originates from the electroluminescence (EL) of the BTDBPI nanometer film (yellow-green, 525 nm) and TPBI (deep blue, 380 nm), demonstrating that BTDBPI shows both the charge-transporting and EL properties.

Cyanotriphenylamine-based polyimidothioethers as multifunctional materials for ambipolar electrochromic and electrofluorochromic devices, and fluorescent electrospun fibers

New luminescent and electrochromic polyimidothioethers were synthesized and fabricated as ambipolar electrochromic and electrofluorochromic devices, and fluorescent electrospun fibers.

High-performance electrofluorochromic devices based on aromatic polyamides with AIE-active tetraphenylethene and electro-active triphenylamine moieties

Novel electrochromic (EC) and aggregation-enhanced emission (AEE)-active triphenylamine (TPA)-based polyamides were prepared and fabricated into electrofluorochromic (EFC) devices.

Enhanced fluorescence of phthalimide compounds induced by the incorporation of electron-donating alicyclic amino groups

A significant difference in fluorescence properties between 3- and 4-substituted phthalimides demonstrates the formation of excited TICT and planar ICT states, respectively.

Preparation of Palladium/Silver-Coated Polyimide Nanotubes: Flexible, Electrically Conductive Fibers

A simple and practical method for coating palladium/silver nanoparticles on polyimide (PI) nanotubes is developed. The key steps involved in the process are silver ion exchange/reduction and displacement reactions between silver and palladium ions. With the addition of silver, the conductivity of the PI nanotubes is greatly enhanced. Further, the polyimide nanotubes with a dense, homogeneous coating of palladium nanoparticles remain flexible after heat treatment and show the possibility for use as highly efficient catalysts. The approach developed here is applicable for coating various noble metals on a wide range of polymer matrices, and can be used for obtaining polyimide nanotubes with metal loaded on both the inner and outer surface.

Polyurethanes with aggregation-enhanced emission characteristics: preparation and properties

An amino-terminated poly(propylene glycol)-modified tetraaryl-buta-1,3-diene derivative (TABDAA) was introduced to synthesize polyurethanes with different ratios of soft/hard segments. A mixture of TABDAA and poly(tetrahydrofuran) 1000 as the soft segments was reacted with 4,4-diphenylmethane diisocyanate and 1,4-butanediol as the hard segments in molar ratios of 1 : 2 : 1, 2 : 3 : 1, and 3 : 4 : 1 to give the desired polyurethanes named TMPU-211, TMPU-321 and TMPU-431, respectively. The three polyurethanes exhibited different aggregation-enhanced emission (AEE) behaviors because of their different soft/hard segment ratios. The polyurethanes with a higher soft segment content tended to form bigger particles in a DMF/water mixture solution, thus causing a sharper increase in their fluorescence intensity. In addition, the polyurethane films exhibited different fluorescence intensities after different heat treatments. After a quenching treatment of the soft segments in the polyurethane films, the fluorescence intensity dropped greatly. When these quenched polyurethane films were thermally annealed at 60 °C for 24 hours, their fluorescence intensity exceeded the initial intensity of the as-prepared films. Differential scanning calorimetry results showed that the polyurethane films in the quenched condition did not present the endothermal melting peak of the soft segments, and the melting peaks appeared again after thermal annealing. AFM experiments showed that an ordered arrangement was achieved after the heat treatment of these AEE polyurethane films. These results demonstrated that the polymer structure had a significant effect on the AEE properties of the polyurethane films, and more importantly, it is of great significance in improving the fluorescence emission of the AEE polymers and also for their potential application in fluorescent probes, stimuli-responsive materials, PLED devices and so on.

Polyurethane foam functionalized with an AIE-active polymer using an ultrasonication-assisted method: preparation and application for the detection of explosives

We have fabricated PU foams modified with AIE-active polymer PSiTPE nanoparticles on the flexible sponge surface. The as-prepared foams show a high sensitive to PA solution and DNT vapor.

Highly efficient ultraviolet light-emitting organosoluble polyimide

A non-conjugated siloxane-containing polyimide without fluorescent dyes exhibited a high fluorescence quantum yield.

Effects of cyano groups on the properties of thiazole-based β-ketoiminate boron complexes: aggregation-induced emission and mechanofluorochromism

The triphenylamine-functionalized thiazole-based β-ketoiminate boron complexBF2-TT-CNexhibited strong cyano-dependent aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties.

High-efficiency fluorescent polyimides based on locally excited triarylamine-containing dianhydride moieties

Three fluorescent polyimides (PIs) were readily synthesized from the polycondensation of triarylamine-based tetracarboxylic dianhydride monomers with aliphatic diamine. The PL intensity of solid films and nanofibers revealed high PL quantum yields of up to 32% and 35%, respectively.

Phosphorescence within benzotellurophenes and color tunable tellurophenes under ambient conditions

The zirconium-mediated syntheses of pinacolboronate (BPin) appended benzo[b]tellurophenes and two phenyl/BPin substituted tellurophene isomers with different colors of emission have been achieved.

Switchable fluorescent AIE-active nanoporous fibers for cyclic oil adsorption

A porous fiber of AIE-active PMMA fabricated through the electrospinning technique shows exceptional fluorescence. The green fluorescence of porous fibers can be switched off and on by the adsorption and desorption of silicon or bean oil.

Aggregation-induced emission block copolymers based on ring-opening metathesis polymerization

AIE amphiphilic block copolymers were developed from ROMP for the first time. By self-assembly in selective solvents, water soluble fluorescent nano-objects were prepared with varied structures including micelles and vesicles.

AIE macromolecules: syntheses, structures and functionalities

A comprehensive review of macromolecules with aggregation-induced emission attributes is presented, covering the frontiers of syntheses, structures, functionalities and applications.