Aggregation-induced emission luminogens (AIEgens) have attracted extensive interest for their outstanding luminescence properties in the aggregated state and even in the solid state. In this work, we developed a series of novel AIEgens based on 2,5-bis(4-alkoxycarbonylphenyl)-1,4-diaryl-1,4-dihydropyrrolo[3,2-b]pyrrole (AAPP). The AIEgens can be facilely synthesized through a single-step reaction under mild conditions with satisfactory yields. Interestingly, AAPP was found to have multiple luminous mechanisms that result in variable fluorescence properties. The propeller-like structure of AAPP enables a restricted intramolecular rotation (RIR) process which significantly enhances its fluorescence in the aggregated state (i.e. AIE fluorescence). In addition, there is a donor-acceptor interaction between the heterocycle center and the alkoxycarbonyl units in AAPP which allows a typical twisted intramolecular charge transfer (TICT) process in the dispersed state, resulting in strong fluorescence emissions in non-polar or low-polarity solvents but fluorescence quenching in high-polarity solvents. Due to the tunable RIR and TICT processes and the multiple fluorescence, AAPP compounds exhibit multifunctional applications: (1) as a reversible fluorescent thermometer, AAPP exhibited excellent fatigue resistance. There was a good linear relationship between its fluorescence intensity and temperature from 10 °C to 60 °C. (2) The desethyl AAPP derivative (CAPP) was successfully applied in the detection of Cd(ii) in aqueous solution at neutral pH, and showed a 500-fold fluorescence "turn-on" response to Cd(ii) with good selectivity.

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2,5-bis(4-alkoxycarbonylphenyl)-1,4-diaryl-1,4-dihydropyrrolo[3,2-b]pyrrole (AAPP) AIEgens: tunable RIR and TICT characteristics and their multifunctional applications.

Ammonia nitrogen (ammonia or ammonium) is widely present in natural waters and can significantly affect aquatic flora and fauna. It plays an extremely important role in human production and health as well as ecosystem stability. Many detection methods, including spectrophotometric methods and ion-selective electrodes, have been developed for the analysis of ammonia nitrogen in water. Herein, recent developments in optical detection, electrochemical detection, and biological enzyme detection systems for aqueous ammonia nitrogen detection are reviewed. The detection principles are described, advantages and disadvantages of each method are highlighted, and reagent optimization, technological innovations, and novel sensitive materials to improve the detection limits and reduce interference are discussed in detail. For oceanographers and related researchers, this review will serve as a reference of promising detection techniques.

Detection methods of ammonia nitrogen in water: A review

In principle, mechanochromic fluorescent (MCF) materials can be classified into two categories: turn-off and turn-on types. The latter is superior to the former as it is more sensitive and less likely to induce false-positive signals. In this study, we report the synthesis and photoluminescence (PL) behaviour of a biplane molecule, 2-amino-3-((E)-((2-hydroxynaphthalen-1-yl)methylene)amino)maleonitrile (AHM), which consists of an electron donor (D) plane and an acceptor (A) plane. AHM undergoes both aggregation-enhanced emission (AEE) and intramolecular charge transfer (ICT) mechanisms. It is weakly emissive in the crystalline phase but shows a remarkable emission enhancement with a large bathochromic-shift (67 nm) upon applying mechanical force. As confirmed by both spectroscopic methods and fluorescence microscopy, MCF arises from crystal defects, where the molecules would twist their conformation to break D–A coupling, inducing strong fluorescence. This defect-induced emission (DIE) endows the material with ultra-high sensitivity toward pressure. The detection limit (DL) of an AHM-based sensing film is as low as 1.1 N (0.62 MPa). Additionally, the DIE phenomenon of AHM further enables a new mechanical printing technique. Handwriting and imprinted letters exhibit a distinct emission change which is readily detected by the naked eyes. We believe that the unique turn-on MCF and high sensitivity make this material well-suited to tackle the challenges faced by conventional MCF materials.

A turn-on type mechanochromic fluorescent material based on defect-induced emission: implication for pressure sensing and mechanical printing

Synthesis of new tetraphenylethene (TPE) conjugates via an innocuous route led to the revelation of a unique TPE-based aggregation-induced emissive fluorogen 3 (TPEN), which showed an interesting mechanochromic property when the emission was changed from blue to green upon grinding and green to blue upon fuming. The mechanochromic property of TPEN has been explored to prepare ink-free rewritable paper for security documentation. A detailed photophysical investigation of the TPE-naphthyridine scaffold led to the discovery of its high sensitivity to silver ions (Ag+) over other metal ions with a detection limit of 0.25 μM in an aqueous system. The stoichiometry of the complex of TPEN and silver ion was established to be 2:1 (TPEN:Ag+) on the basis of photophysical studies, mass analysis, and high-resolution mass spectrometry analysis.

A Tetraphenylethene-Naphthyridine-Based AIEgen TPEN with Dual Mechanochromic and Chemosensing Properties

For optical data storage (ODSes), fluorescence may be an ideal computer-readable signal on account of its scalability, correction capability, and potentially enhanced capacity. In this study, we report the synthesis and photoluminescence (PL) behaviour of a class of fluorophores with electron donor and acceptor units. They exhibit AIE properties with long-wavelength emission colour. Owing to the reactive electron withdrawing group, their emission spectra have shown either “on–off” switching or a remarkable blue-shift upon alkali treatment. The changes in their fluorescence are due to TICT–LE transition as confirmed by their absorption spectra and DFT calculation. The fluorescent response can be recovered or not, depending on the chemical stimulant used for encoding. Based on the stimuli-responsive nature of the fluorophores, we have further developed both rewritable and write-once read-many (WORM) ODSes with single or dual-channel outputs. “0” and “1” state of the encoded binary data, corresponding to the TICT and LE emission, can be read easily using a spectrofluorophotometer. In addition, the WORM ODSes enjoy the advantage of long-term stability and the rewritable ODSes can be reused many times without fatigue.

Stimuli-responsive fluorophores with aggregation-induced emission: implication for dual-channel optical data storage

In this study, we report the synthesis and photoluminescence (PL) properties of a new luminogen, (E)-4-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzoic acid (HNMA), which undergoes both aggregation-induced emission (AIE) and intramolecular charge transfer (TICT) mechanisms. The crystalline powder emits intense green fluorescence under UV illumination, whereas after grinding, the emission decreases dramatically with a red-shift in the emission wavelength. The changes in its fluorescence are due to crystalline–amorphous transition as confirmed by XRD analysis and the microscopic technique. Furthermore, the emission of the amorphous phase can be readily converted back into its original state by a recrystallization process, which can be realized by either immersion or fumigation in common organic solvents. Importantly, the switching between two states can be repeated for over 25 cycles without fatigue. Based on indefatigability intrinsic to mechanochromic luminogens, a new strategy to realize rewritable optical data storage is developed. “0” and “1” states of the binary data, corresponding to the two emission states, can be encoded on a substrate by mechanical force and erased by fumigation treatment, demonstrating good repeatability and potential for bulk storage.

One-pot synthesis of a mechanochromic AIE luminogen: implication for rewritable optical data storage

In this study, we synthesized a class of positional isomers by attaching a carboxylic group in different sites (para-, meta- and ortho-position) of an aromatic core. The isomers undergo both aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) mechanisms, as proved by their photoluminescence (PL) behaviour. Changing the position of the carboxylic group will adjust the dipole–dipole direction, which further controls the self-assembled architecture: The morphology undergoes a transition from 1D nanowire to 2D microsheet, and even to 3D microcube. The benzoic acid moiety of the AIE isomers has strong affinity to amines, and adsorbing amines leads to spectral changes including hypochromatic shift and emission quenching. The sensing mechanism was studied by density functional theory (DFT), which reveals that the amine-responsive fluorescence can be ascribed to the weakened ICT effect. A prototype of amine sensor based on m-DB self-assemblies was developed accordingly, showing a remarkable quenching efficiency as high as 85.4%. A linear relation was obtained from the quantitative determination of amine, offering a low detection limit of 2.02 Pa. It can also respond to other organic amines such as diisopropylamine (DIPA), triethylamine (TEA), isopropylamine (IPA) and cyclohexylamine (CHA). Finally, the practicality of the sensor was also demonstrated: When placing the sensing film beside the spoiled meat, the spectrum shows a remarkable fluorescence quenching; in contrast, fresh meat samples have induced a much lower quenching efficiency. This demonstrates that the AIE isomers have a great potential to serve as indicators for meat spoilage.

To direct the self-assembly of AIEgens by three-gear switch: Morphology study, amine sensing and assessment of meat spoilage

Solvent-assistant self-assembly of an AIE+TICT fluorescent Schiff base for the improved ammonia detection.

Advances in sensors benefit tremendously from the stimuli-responsive self-assembly technique. Aggregation-induced emission (AIE) and Aggregation-enhanced emission (AEE) luminogens have emerged as reliable building blocks for optical sensor as they overcome many of the limitations of conventional fluorescent probes. In this report, we describe the first synthesis and photophysical properties of DBIA, a donor-acceptor (D-A) type AEE luminogen with red solid-sate emission. It shows stimuli-responsive self-assembly behavior upon fumigation with water steam. Our systematic investigation using microscopy has observed various supramolecular architectures with different relative humidity (RH) values at room temperature (25 °C), including rough-surfaced flocs structure (RH = 20%), nanorods (RH = 60%) and rectangular stratified microsheets (RH = 85%). In addition, there is a sharp increase in emission intensity when DBIA molecules are assembled in the presence of water steam, as the AEE mechanism is further reinforced in this case. Hydrogen bonding between DBIA and water molecule was detected by dynamic spectral analysis, and further proved by DFT calculation. Thus the mechanism for the stimuli-responsive self-assembly toward water and the consequent emission enhancement is illustrated. Taking advantage of the water-sensitivity, a water sensor was developed, affording a detection limit of 2.64 mM as a liquid water probe, and RH 22.5% as a humidity sensor. It also shows fast response, easy visualization, broad detection region and the capacity of resisting disturbance.

Yaping Li

Papers

One-pot synthesis of a mechanochromic AIE luminogen: implication for rewritable optical data storage

To direct the self-assembly of AIEgens by three-gear switch: Morphology study, amine sensing and assessment of meat spoilage

Solvent-assistant self-assembly of an AIE+TICT fluorescent Schiff base for the improved ammonia detection.

Water-directed self-assembly of a red solid emitter with aggregation-enhanced emission: Implication for humidity monitoring

A turn-on mechanochromic luminescent material serving as pressure sensor and rewritable optical data storage