Free Radical Chemistry of Phosphasilenes.

Understanding the characteristics of radicals formed from silicon-containing heavy analogues of alkenes is of great importance for their application in radical polymerization. Steric and electronic substituent effects in compounds such as phosphasilenes not only stabilize the Si=P double bond, but also influence the structure and species of the formed radicals. Herein we report our first investigations of radicals derived from phosphasilenes with Mes, Tip, Dur, and NMe2 substituents on the P atom, using muon spin spectroscopy and DFT calculations. Adding muonium (a light isotope of hydrogen) to phosphasilenes reveals that: a) the electron-donor NMe2 and the bulkiest Tip-substituted phosphasilenes form several muoniated radicals with different rotamer conformations; b) bulky Dur-substituted phosphasilene forms two radicals (Si- and P-centred); and c) Mes-substituted phosphasilene mainly forms one species of radical, at the P centre. These significant differences result from intramolecular substituent effects.

Biradical-Featured Stable Organic-Small-Molecule Photothermal Materials for Highly Efficient Solar-Driven Water Evaporation.

With recent progress in photothermal materials, organic small molecules featured with flexibility, diverse structures, and tunable properties exhibit unique advantages but have been rarely applied in solar-driven water evaporation owing to limited sunlight absorption resulting in low solar-thermal conversion. Herein, a stable croconium derivative, named CR-TPE-T, is designed to exhibit the unique biradical property and strong π-π stacking in the solid state, which facilitate not only a broad absorption spectrum from 300 to 1600 nm for effective sunlight harvesting, but also highly efficient photothermal conversion by boosting nonradiative decay. The photothermal efficiency is evaluated to be 72.7% under 808 nm laser irradiation. Based on this, an interfacial-heating evaporation system based on CR-TPE-T is established successfully, using which a high solar-energy-to-vapor efficiency of 87.2% and water evaporation rate of 1.272 kg m-2 h-1 under 1 sun irradiation are obtained, thus making an important step toward the application of organic-small-molecule photothermal materials in solar energy utilization.

Dragonfly-shaped near-infrared AIEgen with optimal fluorescence brightness for precise image-guided cancer surgery.

Organic near-infrared (NIR) emitters with simultaneously high absorption coefficient and photoluminescence quantum yield (PLQY) are highly desirable for biomedical imaging yet seldom reported because these two aspects are usually contradictory. The conjugated planar structures exhibit strong absorption but the emission is seriously quenched in aggregate state, whereas the twisted unplanar molecules display opposite phenomena. Herein, we report a kind of dragonfly-shaped NIR aggregation-induced emission luminogen (AIEgen) with both high absorption coefficient (6.24 × 104 M-1 cm-1) and superior PLQY (51.2%) for precise image-guided cancer surgery. The compound possessing a conjugated structure with vibrational substitutes has been synthesized, in which the good conjugation enables strong absorption, and the molecular vibration affords AIE signature. Moreover, the nonfluorescent processes are significantly suppressed, making every effort to boost fluorescence. The highly bright and stable AIE nanoparticles warrant efficient in vitro cellular imaging and in vivo tumor imaging. Moreover, the fluorescence imaging-guided cancer surgery helps to precisely delineate tiny tumor nodules, significantly improving the cancer surgery outcome. This work will inspire more insights into the development of organic NIR emitters with high brightness for biomedical applications.

Simultaneously boosting the conjugation, brightness and solubility of organic fluorophores by using AIEgens.

Organic near-infrared (NIR) emitters hold great promise for biomedical applications. Yet, most organic NIR fluorophores face the limitations of short emission wavelengths, low brightness, unsatisfactory processability, and the aggregation-caused quenching effect. Therefore, development of effective molecular design strategies to improve these important properties at the same time is a highly pursued topic, but very challenging. Herein, aggregation-induced emission luminogens (AIEgens) are employed as substituents to simultaneously extend the conjugation length, boost the fluorescence quantum yield, and increase the solubility of organic NIR fluorophores, being favourable for biological applications. A series of donor-acceptor type compounds with different substituent groups (i.e., hydrogen, phenyl, and tetraphenylethene (TPE)) are synthesized and investigated. Compared to the other two analogs, MTPE-TP3 with TPE substituents exhibits the reddest fluorescence, highest brightness, and best solubility. Both the conjugated structure and twisted conformation of TPE groups endow the resulting compounds with improved fluorescence properties and processability for biomedical applications. The in vitro and in vivo applications reveal that the NIR nanoparticles function as a potent probe for tumour imaging. This study would provide new insights into the development of efficient building blocks for improving the performance of organic NIR emitters.

Non-aromatic annulene-based aggregation-induced emission system via aromaticity reversal process.

Aggregation-induced emission (AIE) is a photophysical phenomenon correlated closely with the excited-state intramolecular motions. Although AIE has attracted increasing attention due to the significant applications in biomedical and optoelectronics, an in-depth understanding of the excited-state intramolecular motion has yet to be fully developed. Here we found the non-aromatic annulene derivative of cyclooctatetrathiophene shows typical AIE phenomenon in spite of its rotor-free structure. The underlying mechanism is investigated through photoluminescence spectra, time-resolved absorption spectra, theoretical calculations, circular dichroism as well as by pressure-dependent fluorescent spectra etc., which indicate that the aromaticity reversal from ground state to the excited state serves as a driving force for inducing the excited-state intramolecular vibration, leading to the AIE phenomenon. Therefore, aromaticity reversal is demonstrated as a reliable strategy to develop vibrational AIE systems. This work also provides a new viewpoint to understand the excited-state intramolecular motion behavior of lumiongens.

Synthesis of an efficient far-red/near-infrared luminogen with AIE characteristics for in vivo bioimaging applications.

A selenium-containing FR/NIR AIE luminogen with efficient solid-state emission is reported. Its AIE dots exhibit high brightness, large Stokes shift, good biocompatibility and satisfactory photostability, making them the first selenium-containing FR/NIR nanoprobes with AIE characteristics for in vivo bioimaging applications with high contrast and a high penetration depth.

Highly efficient photothermal nanoagent achieved by harvesting energy via excited-state intramolecular motion within nanoparticles.

The exciting applications of molecular motion are still limited and are in urgent pursuit, although some fascinating concepts such as molecular motors and molecular machines have been proposed for years. Utilizing molecular motion in a nanoplatform for practical application has been scarcely explored due to some unconquered challenges such as how to achieve effective molecular motion in the aggregate state within nanoparticles. Here, we introduce a class of near infrared-absorbing organic molecules with intramolecular motion-induced photothermy inside nanoparticles, which enables most absorbed light energy to dissipate as heat. Such a property makes the nanoparticles a superior photoacoustic imaging agent compared to widely used methylene blue and semiconducting polymer nanoparticles and allow them for high-contrast photoacoustic imaging of tumours in live mice. This study not only provides a strategy for developing advanced photothermal/photoacoustic imaging nanoagents, but also enables molecular motion in a nanoplatform to find a way for practical application.

Facile access to deep red/near-infrared emissive AIEgens for efficient non-doped OLEDs.

Notwithstanding the huge demand in bio-imaging and optoelectronics, the construction of highly emissive deep red/near infrared (DR/NIR) organic luminogens is still a big challenge because a narrow energy gap generally leads to low photoluminescence quantum yield. It is even more difficult to afford DR/NIR emitters in the solid state due to the aggregation caused quenching (ACQ) effect. In this work, we found that the direct attachment of a tetraphenylethylene substituted arylamine to the electron accepting 2,1,3-benzothiadiazole produces DR/NIR AIE luminogens with bright emission facilely and efficiently. And the emission wavelengths could be tuned from the red to the DR/NIR region by regulating the variety of the substituents. The long emission wavelength and high photoluminescence quantum yield of these AIEgens are ascribed to the effective intramolecular charge transfer and the suppressed intramolecular motion. Furthermore, non-doped OLEDs based on one of the AIEgens showed an EL emission at 684 nm with a large radiance of 5772 mW Sr-1 m-2 and an impressive external quantum efficiency (EQE) of 1.73%.

Deciphering the working mechanism of aggregation-induced emission of tetraphenylethylene derivatives by ultrafast spectroscopy.

Aggregation-induced emission (AIE) is the long-sought solution to the problem of aggregation-caused quenching that has hampered efficient application of fluorescent organic materials. An important goal on the way to fully understand the working mechanism of the AIE process was, for more than a decade, and still remains obtaining more comprehensive insights into the correlation between the ultrafast excited-state dynamics in tetraphenylethylene (TPE)-based molecules and the AIE effect in them. Here we report a number of TPE-based derivatives with varying structural rigidities and AIE properties. Using a combination of ultrafast time-resolved spectroscopy and computational studies, we observe a direct correlation between the state-dependent coupling motions and inhibited fluorescence, and prove the existence of photocyclized intermediates in them. We demonstrate that the dominant non-radiative relaxation dynamics, i.e. formation of intermediate or rotation around the elongated C[double bond, length as m-dash]C bond, is responsible for the AIE effect, which is strongly structure-dependent but not related to structural rigidity.

Ultrabright red AIEgens for two-photon vascular imaging with high resolution and deep penetration.

Reported here is a successful strategy for the design of ultrabright red luminogens with aggregation-induced emission (AIE) features, donor-acceptor structures, and intense charge transfer effects. These luminogens show no aggregation caused emission quenching in the solid state and have high quantum efficiency. They can be fabricated into AIE dots by a simple nanoprecipitation procedure. The AIE dots exhibit high brightness, a large Stokes shift, good biocompatibility, satisfactory photostability, and a high two-photon absorption cross section. The AIE dots can be utilized as highly efficient fluorescent probes for in vivo deep-tissue imaging by a two-photon technique, which outperforms the one-photon technique under the same experimental conditions, in terms of penetration depth and image contrast. This is the first report of using highly emissive AIE dots for the accurate measurement of capillary diameters in mouse ears. Such a strategy sheds light on the development of efficient solid state red/NIR emitters for biological applications.

An Easily Accessible Ionic Aggregation-Induced Emission Luminogen with Hydrogen-Bonding-Switchable Emission and Wash-Free Imaging Ability.

Ionic fluorophores are powerful tools for the study of environmental science and bio-imaging. However, traditional ionic dyes usually require long synthetic steps and suffer from a quenching effect caused by aggregation. A water-soluble ionic aggregation-induced emission luminogen called DBTA is presented, which is readily accessed by a one-step reaction. The switchable emission manipulated by hydrogen bonding provided solid evidence for the restriction of intramolecular motions as the mechanism of aggregation-induced emission. DBTA can not only differentiate solvents with different H-bond donor acidities but also capable of wash-free imaging in living HeLa cells and fish larva.

Highly Efficient Nondoped OLEDs with Negligible Efficiency Roll-Off Fabricated from Aggregation-Induced Delayed Fluorescence Luminogens.

Purely organic emitters that can efficiently utilize triplet excitons are highly desired to cut the cost of organic light-emitting diodes (OLEDs), but most of them require complicated doping techniques for their fabrication and suffer from severe efficiency roll-off. Herein, we developed novel luminogens with weak emission and negligible delayed fluorescence in solution but strong emission with prominent delayed components upon aggregate formation, giving rise to aggregation-induced delayed fluorescence (AIDF). The concentration-caused emission quenching and exciton annihilation are well-suppressed, which leads to high emission efficiencies and efficient exciton utilization in neat films. Their nondoped OLEDs provide excellent electroluminescence efficiencies of 59.1 cd A-1 , 65.7 lm W-1 , and 18.4 %, and a negligible current efficiency roll-off of 1.2 % at 1000 cd m-2 . Exploring AIDF luminogens for the construction of nondoped OLEDs could be a promising strategy to advance device efficiency and stability.

Furan Is Superior to Thiophene: A Furan-Cored AIEgen with Remarkable Chromism and OLED Performance.

Furan-cored AIEgen namely tetraphenylethylene-furan (TPE-F) is developed by diyne cyclization and its fluorescent and chemical properties are investigated and compared with its thiophene analogue. Results show that furan is superior to thiophene in terms of fluorescence, chromism, and charge transport. The mechanism of chromism of TPE-F is investigated and its efficient solid-state photoluminescence and good charge-transporting property enable it to serve as light-emitting material for the construction of electroluminescence devices with excellent performance. This work not only demonstrates an efficient strategy for constructing furan-cored AIEgens but also indicates that they are promising as advanced optoelectronic materials.

AIEgens for dark through-bond energy transfer: design, synthesis, theoretical study and application in ratiometric Hg2+ sensing.

A novel dark through-bond energy transfer (DTBET) strategy is proposed and applied as the design strategy to develop ratiometric Hg2+ sensors with high performance. Tetraphenylethene (TPE) derivatives with aggregation-induced emission (AIE) characteristics are selected as dark donors to eliminate emission leakage from the donors. The TBET mechanism has been adopted since it experiences less influence from spectral overlapping than Förster resonance energy transfer (FRET), making it more flexible for developing cassettes with large pseudo-Stokes shifts. In this work, energy transfer from the TPE derivatives (dark donor) to a rhodamine moiety (acceptor) was illustrated through photophysical spectroscopic studies and the energy transfer efficiency (ETE) was found to be up to 99%. In the solution state, no emission from the donors was observed and large pseudo-Stokes shifts were achieved (>280 nm), which are beneficial for biological imaging. Theoretical calculations were performed to gain a deeper mechanistic insight into the DTBET process and the structure-property relationship of the DTBET cassettes. Ratiometric Hg2+ sensors were rationally constructed based on the DTBET mechanism by taking advantage of the intense emission of TPE aggregates. The Hg2+ sensors exhibited well resolved emission peaks. >6000-fold ratiometric fluorescent enhancement is also achieved and the detection limit was found to be as low as 0.3 ppb. This newly proposed DTBET mechanism could be used to develop novel ratiometric sensors for various analytes and AIEgens with DTBET characteristics will have great potential in various areas including light harvesting materials, environmental science, chemical sensing, biological imaging and diagnostics.

Recent developments in the field of photoluminescent organically modified cyclosiloxanes.

Herein, we present a brief overview of the recent developments in the field of photoluminescent cyclosiloxanes, with a special focus on the synthesis and unique photophysical properties of newly reported silole-based cyclosiloxanes with pronounced aggregation-induced emission (AIE) behaviour. Comparisons of their photophysical data as well as the results of computational studies of various types of silole-based cyclosiloxanes are presented and their potential applications are briefly discussed.

An acidic pH independent piperazine-TPE AIEgen as a unique bioprobe for lysosome tracing.

Lysosomes are involved in a multitude of cellular processes and their dysfunction is associated with various diseases. They are the most acidic organelles (pH 3.8-6.6, size 0.1-1.2 µm) with the highest viscosity (47-190 cP at 25 °C) in the cell. Because of their acidity, pH dependent non-AIE active fluorescent lysosomal probes have been developed that rely on protonation inhibited photoinduced electron transfer (PET). In this work, an acidic pH independent lysosome targetable piperazine-TPE (PIP-TPE) AIEgen has been designed with unique photophysical properties making it a suitable probe for quantifying viscosity. In a non-aggregated state PIP-TPE shows deep-blue emission as opposed to its yellowish-green emission in the bulk. It possesses high specificity for lysosomes with negligible cytotoxicity and good tracing ability due to its better photostability compared to LysoTracker Red. In contrast to most known lysosome probes that rely solely on PET, restriction of intramolecular motion (RIM) due to the larger viscosity inside the lysosomes is the mechanism responsible for PIP-TPE's fluorescence. PIP-TPE's high selectivity is attributed to its unique molecular design that features piperazine fragments providing a perfect balance between lipophilicity and polarity.

AIE-active theranostic system: selective staining and killing of cancer cells.

Cancer is the leading cause of death worldwide. With the advantages of low cost, high sensitivity and ease of accessibility, fluorescence imaging has been widely used for cancer detection in the scientific field. Aggregation-induced emission luminogens (AIEgens) are a class of synthesized fluorescent probes with high brightness and photostability in the aggregate state. Herein, a new positively-charged AIEgen, abbreviated as TPE-IQ-2O, is designed and characterized. TPE-IQ-2O not only can distinguish cancer cells from normal cells with high contrast with the aid of the difference in mitochondrial membrane potential as well as the quantity of mitochondria, but it also works as a promising photosensitizer to kill cancer cells through generation of reactive oxygen species upon white light irradiation, thus making it a promising AIE theranostic system.

Polyyne bridged AIE luminogens with red emission: design, synthesis, properties and applications.

Construction of a donor-acceptor (D-A) structure and extension of π-conjugation are the commonly used strategies to shift the emission of luminophores to the red region. However, molecules with high conjugation and a strong D-A effect tend to show weak emission due to the severe π-π interactions and twisted intramolecular charge transfer (TICT) effects. The turn-on characteristic of AIE luminogens (AIEgens) will also be lost due to the conjugation-enhanced emission in the solution state. Herein, a polyyne-bridged AIE luminogen (2TPE-4E) with long wavelength absorption and red emission has been afforded. Despite its large π-conjugation, 2TPE-4E suffers from no emission quenching resulted from strong π-π interactions and twisted intramolecular charge transfer effects. The strong red emission and the high photostability of 2TPE-4E inspired us to use it for targeted-imaging of cancer cells and monitoring the receptor mediated endocytosis process.

Introductory lecture: recent research progress on aggregation-induced emission.

Since the discovery of the aggregation-induced emission (AIE) phenomenon in 2001, research on AIE molecules has drawn much attention, and this area has been expanding tremendously. This brief review will focus on recent advances in the science and application of AIE molecules, including new mechanistic understanding, new AIE molecules for sensing and imaging, stimuli-responsive AIE molecules and applications of AIE molecules for OLEDs. Moreover, this review will give a perspective on the possible opportunities and challenges that exist in the future for this area.

A Luminescent Nitrogen-Containing Polycyclic Aromatic Hydrocarbon Synthesized by Photocyclodehydrogenation with Unprecedented Regioselectivity.

We present a nitrogen-containing polycyclic aromatic hydrocarbon (N-PAH), namely 12-methoxy-9-(4-methoxyphenyl)-5,8-diphenyl-4-(pyridin-4-yl)pyreno[1,10,9-h,i,j]isoquinoline (c-TPE-ON), which exhibits high quantum-yield emission both in solution (blue) and in the solid state (yellow). This molecule was unexpectedly obtained by a three-fold, highly regioselective photocyclodehydrogenation of a tetraphenylethylene-derived AIEgen. Based on manifold approaches involving UV/Vis, photoluminescence, and NMR spectroscopy as well as HRMS, we propose a reasonable mechanism for the formation of the disk-like N-PAH that is supported by density functional theory calculations. In contrast to most PAHs that are commonly used, our system does not suffer from entire fluorescence quenching in the solid state due to the peripheral aromatic rings preventing π-π stacking interactions, as evidenced by single-crystal X-ray analysis. Moreover, its rod-like microcrystals exhibit excellent optical waveguide properties. Hence, c-TPE-ON comprises a N-PAH with unprecedented luminescent properties and as such is a promising candidate for fabricating organic optoelectronic devices. Our design and synthetic strategy might lead to a more general approach to the preparation of solution- and solid-state luminescent PAHs.