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

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

Organic and organometallic molecules, which change their luminescence properties through a mechanical stimulus, such as emission wavelength and quantum yield, are called as mechanochromic or piezochromic materials. These functional molecules may be used as smart materials in the field of luminescent switches, mechanosensors, security papers, data storage, biological and healthcare applications. These materials are classified into two types, fluorescent and phosphorescent molecules. In this review, the recent progress in the phosphorescent mechanochromic materials, including pure organic molecules and metal complexes is highlighted. Their luminescence behaviors under mechanical force stimuli are shown and some critical concepts of molecular design for mechanoresponsive phosphorescent materials are also presented.

Recent progress in the mechanochromism of phosphorescent organic molecules and metal complexes

Since the realization in 2011 that most aggregation-induced emission (AIE) molecules exhibit mechano-responsive luminescence (MRL), research regarding the MRL of AIE molecules has drawn much attention, and this area has been expanding tremendously. As one of the most extensively studied AIE cores, tetraphenylethylene (TPE) has been widely used to construct MRL molecules. This review will focus on recent advances in MRL of TPE derivatives with AIE properties, including a brief history of mechano-responsive AIE-active materials, mechanistic studies on MRL, mechano-responsive luminogens based on TPE, mechano-responsive luminogens containing multiple AIE-active units, mechano-memory chromism and mechanoluminescence of TPE derivatives. Moreover, this review will give a perspective on the possible opportunities and future challenges that exist in this research area.

Recent advances in mechano-responsive luminescence of tetraphenylethylene derivatives with aggregation-induced emission properties

Very bright mechanoluminescence and remarkable mechanochromism using a tetraphenylethene derivative with aggregation-induced emission

The invention discloses an asymmetrical thermal-activation-delayed aggregation-induced emission material based on a diphenyl sulfone phenoxazine, as well as a synthesis method and application of the material. A core structure of the material comprises a diphenyl sulfone unit and a phenoxazine group, and further comprises an electronic structure unit made from aryl heterocyclic groups except a phenoxazine substituent. The synthesis method and the purification process of the material are simple, the thermal performance, luminous performance and the like of an end product can be adjusted according to the connection of different groups, the obtained emission material has thermal-activation-delayed fluorescent and aggregation-induced emission performance at the same time, is good in heat stability, higher in glass transition temperature and excellent in luminous performance. A non-doped OLED device manufactured by using the emission material as the luminous layer is high in luminous brightness and good in stability, so that the luminous efficiency and the service life of the OLED device can meet the practical requirements.

Asymmetrical thermal-activation-delayed aggregation-induced emission material based on diphenyl sulfone phenoxazine, as well as synthesis method and application of material

金崇君

Papers

Very bright mechanoluminescence and remarkable mechanochromism using a tetraphenylethene derivative with aggregation-induced emission

Asymmetrical thermal-activation-delayed aggregation-induced emission material based on diphenyl sulfone phenoxazine, as well as synthesis method and application of material