Cathy Ka Wai Jim

Fluorescence enhancements of benzene-cored luminophors by restricted intramolecular rotations: AIE and AIEE effects

Photoluminescence of simple arylbenzenes with ready synthetic accessibility is enhanced by two orders of magnitude through aggregate formation; viscosity and temperature effects indicate that the emission enhancement is due to the restriction of their intramolecular rotations in the solid state.

Luminogenic materials constructed from tetraphenylethene building blocks: Synthesis, aggregation-induced emission, two-photon absorption, light refraction, and explosive detection

Luminogenic molecules [(TPE)3 (1), TPE-C = C-TPE-C = C-TPE (2), and TPE-C≡C-TPE-C≡C-TPE (3)] and their polymers P1–P3 are constructed from tetraphenylethene (TPE) building blocks in high yields by Suzuki, Witting, and Sonogashira coupling reactions. All the compounds are soluble and enjoy high thermal stability, losing little of their weights when they are heated to 290–528 °C under nitrogen or 288–436 °C in air. Analyses by UV spectroscopy and cyclic voltammetry as well as theoretical calculations show that the conjugation of the luminophores is in the order of 2 > 3 > 1, P2 > P3 > P1, and P1–P3 > 1–3. All the molecules and polymers are weakly emissive in solutions. They, however, become strong emitters in the aggregate state with fluorescence quantum yields up to 90%. Both 1–3 and P1–P3 exhibit the feature of aggregation-enhanced two-photon excited fluorescence. Large two-photon absorption cross sections (up to ∼900 GM) are observed in the nanoaggregates of the polymers. Thin solid films of the polymers show high refractive indices (RI = 1.7649 − 1.6873) in a wide wavelength region of 400–1700 nm, high modified Abbe numbers (vD′ up to 3436), and low optical dispersions (D′ down to 2.9 × 10−4). The light emissions of the polymers can be quenched exponentially by picric acid with large quenching constants, suggesting that they can be utilized as efficient chemosensors for explosive detection.

Aggregation-Enhanced Emissions of Intramolecular Excimers in Disubstituted Polyacetylenes

Whereas chain aggregation commonly quenches light emission of conjugated polymers, we here report a phenomenon of aggregation-induced emission enhancement (AIEE): luminescence of polyacetylenes is dramatically boosted by aggregate formation. Upon photoexcitation, poly(1-phenyl-1-alkyne)s and poly(diphenylacetylene)s emit blue and green lights, respectively, in dilute THF solutions. The polymers become more emissive when their chains are induced to aggregate by adding water into their THF solutions. The polymer emissions are also enhanced by increasing concentration and decreasing temperature. Lifetime measurements reveal that the excited species of the polymers become longer-lived in the aggregates. Conformational simulations suggest that the polymer chains contain n=3 repeat units that facilitate the formation of intramolecular excimers. The AIEE effects of the polymers are rationalized to be caused by the restrictions of their intramolecular rotations by the aggregate formation.

An AIE-active hemicyanine fluorogen with stimuli-responsive red/blue emission: extending the pH sensing range by “switch + knob” effect

In this work, a red-emissive zwitterionic hemicyanine dye, named TPE–Cy, containing tetraphenylethene (TPE) and N-alkylated indolium is designed and synthesized. TPE–Cy inherits the aggregation-induced emission (AIE) feature of TPE and displays a large Stokes shift (>185 nm), overcoming the limitations of the concentration-quenching effect and small Stokes shift (from a few to 20 nm) encountered by conventional cyanine dyes. By taking advantage of the photophysical AIE property and chemical reactivity towards OH−/H+, TPE–Cy is able to sense pH in a broad range (the broadest to date) by showing different emission colors and intensities: strong to moderate red emission at pH 5–7, weak to no emission at pH 7–10, and no emission to strong blue emission at pH 10–14. The acid/base-switched red/blue emission transition is reversible and can be repeated for many cycles. By means of NMR and HRMS analyses, we have drawn a mechanistic picture at molecular level to illustrate how this dye works as a pH-sensitive fluorescent probe.

Aggregation-Induced Emission in a Hyperbranched Poly(silylenevinylene) and Superamplification in Its Emission Quenching by Explosives.

A silicon-containing hyperbranched polymer (hb-P1/2) with σ*-π* conjugation was prepared in a good yield and high molecular weight by rhodium-catalyzed alkyne polyhydrosilylation of 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene (1) with tris(4-dimethylsilylphenyl)amine (2). The polymer was thermally stable, losing merely 5% of its weight when heated to ≈445 °C. Whereas hb-P1/2 was weakly luminescent when molecularly dissolved, it became highly emissive when supramolecularly aggregated, showing an aggregation-induced emission (AIE) phenomenon. A superamplification effect was observed when the AIE nanoaggregates were used as fluorescent chemosensor for explosive detection: the quenching efficiency was greatly increased in a nonlinear fashion with increasing quencher concentration.

Using tetraphenylethene and carbazole to create efficient luminophores with aggregation-induced emission, high thermal stability, and good hole-transporting property

Tetraphenylethene (TPE) is an archetypal luminogen that exhibits a phenomenon of aggregation-induced emission (AIE), while carbazole is a conventional chromophore which shows the opposite effect of aggregation-caused quenching (ACQ) of light emission in the condensed phase. Melding the two units at the molecular level generates a group of new luminescent materials that suffer no ACQ effect but depict high solid-state fluorescence quantum yields up to unity, demonstrative of the uniqueness of the approach to solve the ACQ problem of traditional luminophores. All the TPE–carbazole adducts are thermally and morphologically stable, showing high glass-transition temperatures (up to 179 °C) and thermal-degradation temperatures (up to 554 °C). Multilayer electroluminescence devices with configurations of ITO/NPB/emitter/TPBi/Alq3/LiF/Al are constructed, which exhibit sky blue light in high luminance (up to 13 650 cd m−2) and high current and external quantum efficiencies (up to 3.8 cd A−1, and 1.8%, respectively). The devices of the luminogens fabricated in the absence of NPB or hole-transporting layer show even higher efficiencies up to 6.3 cd A−1 and 2.3%, thanks to the good hole-transporting property of the carbazole unit.

Pyrazine luminogens with “free” and “locked” phenyl rings: Understanding of restriction of intramolecular rotation as a cause for aggregation-induced emission

Nitrogen-containing heterocyclic pyrazine luminogens with “free” (1) and “locked” (2) phenyl rings were readily prepared in high yields. The propeller-shaped luminogen 1 shows aggregation-induced emission feature, whereas the disk-shaped chromophore 2 shows conventional aggregation-caused quenching effect, offering a direct evidence to support our hypothesis that restriction of intramolecular rotation is the main cause for the AIE phenomenon.

Conjugated Hyperbranched Poly(aryleneethynylene)s: Synthesis, Photophysical Properties, Superquenching by Explosive, Photopatternability, and Tunable High Refractive Indices

Triphenylamine (TPA)-based conjugated hyperbranched poly(aryleneethynylene)s (PAEs), hb-P1/2, hb-P1/3, and hb-P1/4, were synthesized with high molecular weights and good solubilities through Sonogashira coupling reactions. These PAEs exhibited outstanding thermal stabilities and different emission behaviors. Tetraphenylethene (TPE)-containing hb-P1/2 fluoresced faintly in THF, although its light emission was enhanced by aggregate formation in aqueous media or in thin films, thereby exhibiting an aggregation-induced emission-enhancement (AIEE) effect. Whereas 1,1,2,3,4,5-hexaphenylsilole (HPS)-bearing hb-P1/3 showed no significant change in emission intensity with increasing water content in aqueous media, hb-P1/4, which consisted of TPA-fluorenone donor-acceptor groups, presented almost identical absorptions, but both positive and negative solvatochromic emissions in various solvents. A superquenching effect was observed in the picric-acid-detection process by using nanosuspensions of hb-P1/2. All of the polymers possessed good film formability. UV irradiation of the thin films induced simultaneous photobleaching and cross-linking, thus making them applicable in the fabrication of 2D and 3D patterns. Furthermore, the polymer films also showed high refractive indices, which were tunable upon exposure to UV light.

Synthesis and Curing of Hyperbranched Poly(triazole)s with Click Polymerization for Improved Adhesion Strength

We successfully synthesized hyperbranched poly(triazole)s by in situ click polymerization of diazides 1 and triyne 2 monomers on different metal surfaces (copper, iron, and aluminum) and characterized their adhesive properties. Optimizations were performed to obtain high adhesive strength at different temperatures by analyzing the effects of curing kinetics, annealing temperature and time, catalyst, monomer ratio, surface conditions, alkyl chain length of diazides 1, etc. The adhesive bonding strength with metal substrate is 2 orders of magnitude higher than similar hyperbranched poly(triazole)s made by click polymerization and clearly higher than some commercial adhesives at elevated temperatures. With the same conditions, adhesives prepared on aluminum and iron substrates have higher adhesive strength than those prepared on copper substrate, and an excess of triyne 2 monomer in synthesis has greater adhesive strength than an excess of diazide 1 monomer. Tof-SIMS experiment was employed to understand these phenomena, and the existence of an interphase between the polymer and metal surface was found to be critical for adhesive bonding with thicker interphase (excess of triyne 2 monomer) and the higher binding energy between polymer atoms and substrate atoms (e.g., aluminum substrate) generating the higher bonding strength. In addition, the light-emitting property of synthesized polymers under UV irradiation can be used to check the failure mode of adhesive bonding.

Thiol–bromo click polymerization for multifunctional polymers: synthesis, light refraction, aggregation-induced emission and explosive detection

New multifunctional polymers PI and PII are synthesized from 4,4′-thiodibenzenethiol and 1,2-bis[4-(bromoalkoxy)phenyl]-1,2-diphenylethene through facile thiol–bromo click polymerization. The resulting polymers exhibit high refractive indices over a wide spectral region (300–1700 nm) and large Abbe numbers due to the presence of sulfur atoms and a large fraction of aromatic building blocks. The polymers also show high optical transparency in the visible region, rendering them ideal candidates for optical applications. Additionally, they demonstrate typical aggregation-induced emission (AIE) characteristics owing to the incorporation of propeller-like tetraphenylethene (TPE) moieties. The polymers are nonemissive in good solvents; however, they become highly fluorescent as aggregated suspensions whose emission is further effectively quenched by a representative explosive of picric acid (PA), exhibiting a significantly amplified superquenching effect, with the detection limit as low as 0.5 ppm. These results suggest a great promise of preparing multifunctional polymers by thiol–bromo click polymerization with a rational molecular design.