Aggregation-induced emission (AIE) describes a photophysical phenomenon in which molecular aggregates exhibit stronger emission than the single molecules. Over the course of the last 20 years, AIE research has made great strides in material development, mechanistic study and high-tech applications. The achievements of AIE research demonstrate that molecular aggregates show many properties and functions that are absent in molecular species. In this review, we summarize the advances in the field of AIE and its related areas. We specifically focus on the new properties of materials attained by molecular aggregates beyond the microscopic molecular level. We hope this review will inspire more research into molecular ensembles at and beyond the meso level and lead to the significant progress in material and biological science.

Aggregation-Induced Emission: New Vistas at the Aggregate Level

Organic luminogens with persistent room temperature phosphorescence (RTP) have attracted great attention for their wide applications in optoelectronic devices and bioimaging. However, these materials are still very scarce, partially due to the unclear mechanism and lack of designing guidelines. Herein we develop seven 10-phenyl-10H-phenothiazine-5,5-dioxide-based derivatives, reveal their different RTP properties and underlying mechanism, and exploit their potential imaging applications. Coupled with the preliminary theoretical calculations, it is found that strong π-π interactions in solid state can promote the persistent RTP. Particularly, CS-CF3 shows the unique photo-induced phosphorescence in response to the changes in molecular packing, further confirming the key influence of the molecular packing on the RTP property. Furthermore, CS-F with its long RTP lifetime could be utilized for real-time excitation-free phosphorescent imaging in living mice. Thus, our study paves the way for the development of persistent RTP materials, in both the practical applications and the inherent mechanism.

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The influence of the molecular packing on the room temperature phosphorescence of purely organic luminogens.

ConspectusOptoelectronic material properties are governed by the whole collective of organic moieties, and these aggregate states present the characteristic performance of extended assemblies with different molecular packing, not only of single molecules themselves. Thus, controlling molecular packing is an essential issue for obtaining the optimized optical and electronic properties. It is also a great challenge because of the unclear structures and complicated intermolecular interactions, including dispersion forces, electrostatic interactions and hydrogen bonding. Moreover, upon the introduction of some external force as the stimulus source, dynamic optical properties can be achieved with the transformation of molecular packing in some cases, such as the photoinduced room temperature phosphorescence (RTP) effect, mechanochromic luminescence, the thermal treatment-dependent mechanoluminescence effect, and the optimized nonlinear optical (NLO) property achieved after electric poling. Therefore, it is essential to understand the relation between characteristics of molecular packing and the resultant optoelectronic performance at the molecular level, which becomes increasingly demanding for the further development of functional materials for their applications in organic light-emitting diodes (OLEDs), chemo- and biosensors, organic solar cells, data storage, and anticounterfeiting devices.This Account gives a summary of our research on the molecular design of optoelectronic materials, with the consideration of a molecular uniting effect in different aggregated states, such as crystalline states, thin films, and nanoparticles. Through the systematical investigation of structure-packing-performance relationships, some strategies are afforded to partially control the molecular packing via the tunable size, shape, and configuration of aromatic moieties with different electronic and steric effects, together with different types of substituents as functional units to adjust the intermolecular interactions. The utilization of π-π interactions and hydrogen bonding by rational molecular design is considered as the key point to achieve the bright emission of organic materials, including the RTP and mechanoluminescence effects. Also, the dynamic optoelectronic properties are highlighted with different kinds of stimuli, including light irradiation, mechanical force, thermal treatment, and electric field, which are mainly related to the subtle molecular motions under external force and the changeable molecular packing as the metastable state. These selected examples will not only open a window for further development of organic and polymeric optoelectronic materials by the adjustable molecular packing and noncovalent interactions, but also prompt further advances for more interesting and exciting properties.

Molecular Packing: Another Key Point for the Performance of Organic and Polymeric Optoelectronic Materials.

Mechanochromic fluorescence (MCF) materials are a sort of smart material whose photophysical properties are sensitive to mechanical stimulation, such as photoluminescence color, fluorescence quantum yield and emission lifetime. Recently, an increasing number of studies have shown that these photophysical properties can be affected greatly by the molecular packing and conformation, enabling the rapid development of functional materials with mechanochromic fluorescence properties. In this review, we focus on MCF materials with distinct emission properties and various molecular arrangements, especially the inherent correlation between molecular packing modes and emissive behaviors. Many of the selected representative examples possess polymorphism, offering the possibility of exploring different emissions from the exact molecular packing in single crystals. Correspondingly, some remarks are made on the outlook for the next developments in MCF materials and the required thinking about the structure–packing–performance relationship.

Molecular conformation and packing: their critical roles in the emission performance of mechanochromic fluorescence materials

Luminescent materials with thermally activated delayed fluorescence (TADF) can harvest singlet and triplet excitons to afford high electroluminescence (EL) efficiencies for organic light-emitting diodes (OLEDs). However, TADF emitters generally have to be dispersed into host matrices to suppress emission quenching and/or exciton annihilation, and most doped OLEDs of TADF emitters encounter a thorny problem of swift efficiency roll-off as luminance increases. To address this issue, in this study, a new tailor-made luminogen (dibenzothiophene-benzoyl-9,9-dimethyl-9,10-dihydroacridine, DBT-BZ-DMAC) with an unsymmetrical structure is synthesized and investigated by crystallography, theoretical calculation, spectroscopies, etc. It shows aggregation-induced emission, prominent TADF, and interesting mechanoluminescence property. Doped OLEDs of DBT-BZ-DMAC show high peak current and external quantum efficiencies of up to 51.7 cd A−1 and 17.9%, respectively, but the efficiency roll-off is large at high luminance. High-performance nondoped OLED is also achieved with neat film of DBT-BZ-DMAC, providing excellent maxima EL efficiencies of 43.3 cd A−1 and 14.2%, negligible current efficiency roll-off of 0.46%, and external quantum efficiency roll-off approaching null from peak values to those at 1000 cd m−2. To the best of the authors' knowledge, this is one of the most efficient nondoped TADF OLEDs with small efficiency roll-off reported so far.

Achieving High-Performance Nondoped OLEDs with Extremely Small Efficiency Roll-Off by Combining Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence

Two crystalline polymorphs of TMPE, with the space groups P21(c) and C2, are cultured from different solvent mixtures and display apparent blue fluorescence with the characteristic of aggregation induced emission (AIE). Excitedly, the P21(c) crystal exhibits easily observed mechanoluminescence (ML), while there is no mechanoluminescence for the C2 crystal. Careful investigation of their crystal structures and three analogues demonstrates that the special molecule packing of TMPE in the P21(c) crystal accounts for its exciting efficient ML performance, providing some information to understand the structure–property relationship of efficient organic ML materials.

A stable tetraphenylethene derivative: aggregation-induced emission, different crystalline polymorphs, and totally different mechanoluminescence properties

As a reactive oxygen species (ROS), hypochlorite (OCl−) plays a crucial role in oxidative stress and signal transduction, controlling a wide range of physiological functions. In addition, the wide use of OCl− in the treatment of food and water might possibly threaten human health if the residual quantity was out of limits. Currently, sensitive methods employed to selectively monitor OCl− in aqueous samples in situ are still scarce and badly needed. Boron esters or acids are considered to be suitable functional groups for the detection of hydrogen peroxide due to their reliable reactivity. In this work, we try to develop a highly sensitive and selective OCl− probe (TPE2B) based on the mechanism of aggregation induced emission (AIE). Due to the distinct increase in water solubility of TPE2OH, which is generated from the reaction between TPE2B and OCl−, the strong emission of TPE2B is quenched dramatically. The response speed was as fast as 30 seconds with a detection limit as low as 28 nM. Additionally, test papers were also fabricated and exhibited a highly sensitive response to 0.1 mM OCl−.

A highly sensitive and selective fluorescent probe for hypochlorite in pure water with aggregation induced emission characteristics

As a promising option out of all of the well-recognized candidates that have been developed to solve the coming energy crisis, polymer solar cells (PSCs) are a kind of competitive clean energy source. However, as a convenient and efficient method to improve the efficiency of PSCs, the inherent mechanism of the interfacial modification was still not so clear, and interfacial materials constructed with new units were limited to a large degree. Here we present a new kind of interfacial material consisting of AIE units for the first time, with an efficiency of 8.94% being achieved by inserting TPE-2 as a cathode interlayer. This is a relatively high PCE for PC71BM:PTB7-based conventional PSCs with a single-junction structure. Different measurements, including TEM, AFM, SEM, GIXRD, UPS, SKPM, and SCLC, were conducted to investigate the properties in detail. All of the obtained experimental results confirmed the advantages of the utilization of new interfacial materials with AIE characteristics in polymer solar cells, thus providing an additional choice to develop new organic cathode interfacial layers with high performances.

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The marriage of AIE and interface engineering: convenient synthesis and enhanced photovoltaic performance.

The desirable air cathode in Zn-air batteries (ZABs) that can effectively balance oxygen evolution and oxygen reduction reactions not only needs to adjust the electronic structure of the catalyst but also needs a unique physical structure to cope with the complex gas-liquid environment. In this work, first-principles calculations were carried out to prove that oxygen-terminated Nb2CO2 MXene played an active role in enhancing the sluggish reaction of oxygen intermediates. Nb2CO2 MXene could also stimulate the spatial accumulation of discharge products, which was beneficial to improve the stability of secondary ZABs. Molecular dynamics simulation was used to show that the confinement effect of COF could effectively regulate the concentration of O2 on the surface of Nb2CO2@COF, which was conducive to an efficient and durable reaction. COF-LZU1 was self-assembled on the interface of Nb2CO2 MXene (Nb2CO2@COF) for the first time. The Nb2CO2@COF electrode had excellent OER/ORR overpotentials with the potential difference (ΔE) of 0.79 V. When applied to the configuration of ZABs, Nb2CO2@COF showed a power density of 75 mW cm-2 and favorable long-term charge/discharge stability, so it could be used as a potential candidate cathode for noble-metal-based catalysts. This idea of combining MXenes and COFs sheds some light on the design of ZABs.

Oxygen-Terminated Nb2CO2 MXene with Interfacial Self-Assembled COF as a Bifunctional Catalyst for Durable Zinc-Air Batteries.

A two-dimensional MXene (Ta4C3) was innovatively used herein to modulate the space group and electronic properties of vanadium oxides, and the MXene/metal-organic framework (MOF) derivative VO2(B)@Ta4C3 with 3D network cross-linking was prepared, which was then employed as a cathode to improve the performance of aqueous zinc ion batteries (ZIBs). A novel method combining HCl/LiF and hydrothermal treatments was used to etch Ta4AlC3 to obtain a large amount of accordion-like Ta4C3, and the V-MOF was then hydrothermally grown on the surface of the stripped Ta4C3 MXene. During the annealing process of V-MOF@Ta4C3, the addition of Ta4C3 MXene liberates the V-MOF from agglomerative stacking, allowing it to show additional active sites. More significantly, Ta4C3 prevents the V-MOF in the composite structure from converting into V2O5 of space group Pmmn but into VO2(B) of space group C2/m after annealing. A considerable advantage of VO2(B) for Zn2+ intercalation is provided by the negligible structural transformation during the intercalation process and the special tunnel transport channels, which have an enormous area (0.82 nm2 along the b axis). According to first-principles calculations, there is a strong interfacial interaction between VO2(B) and Ta4C3, which deliver remarkable electrochemical activity and kinetic performances for the storage of Zn2+. Therefore, the ZIBs prepared with the VO2(B)@Ta4C3 cathode material exhibit an ultra-high capacity of 437 mA h·g-1 at 0.1 A·g-1 while showing good cycle performance and dynamic performance. This study will offer a fresh approach and a reference for creating metal oxide/MXene composite structures.

Mengshu Li

Papers

A stable tetraphenylethene derivative: aggregation-induced emission, different crystalline polymorphs, and totally different mechanoluminescence properties

A highly sensitive and selective fluorescent probe for hypochlorite in pure water with aggregation induced emission characteristics

The marriage of AIE and interface engineering: convenient synthesis and enhanced photovoltaic performance.

Oxygen-Terminated Nb2CO2 MXene with Interfacial Self-Assembled COF as a Bifunctional Catalyst for Durable Zinc-Air Batteries.

Ta4C3-Modulated MOF-Derived 3D Crosslinking Network of VO2(B)@Ta4C3 for High-Performance Aqueous Zinc Ion Batteries.