Triplet excitons in organic molecules underscore a variety of processes and technologies as a result of their long lifetime and spin multiplicity Organic phosphorescence, which originates from triplet excitons, has potential for the development of a new generation of organic optoelectronic materials and biomedical agents However, organic phosphorescence is typically only observed at cryogenic temperatures and under inert conditions in solution, which severely restricts its practical applications In the past few years, room-temperature-phosphorescent systems have been obtained based on organic aggregates Rapid advances in molecular-structure design and aggregation-behaviour modulation have enabled substantial progress, but the mechanistic picture is still not fully understood because of the high sensitivity and complexity of triplet-exciton behaviour This Review analyses key photophysical processes related to triplet excitons, including intersystem crossing, radiative and non-radiative decay, and quenching processes, to illustrate the intrinsic structure–property relationships and draw clear and integrated design principles The resulting strategies for the development of efficient and persistent room-temperature-phosphorescent systems are discussed, and newly emerged applications based on these materials are highlighted Advances in molecular-structure design and modulation of the aggregation behaviour have enabled much progress in the observation of room-temperature phosphorescence from organic aggregates This Review analyses key photophysical processes related to triplet excitons, illustrating the intrinsic structure–property relationships and identifying strategies to design efficient and persistent room-temperature-phosphorescent systems

Room-temperature phosphorescence from organic aggregates

Since the introduction of the concept of aggregation-induced emission (AIE) in 2001, many research groups have become involved in AIE research. Aggregation-induced emission luminogens (AIEgens) have emerged as a novel type of advanced material with excellent performance in various fields. Much effort has been devoted to determining the AIE mechanism(s) by theoreticians and experimentalists. Restriction of intramolecular motion has been recognized as the general working mechanism of AIE, but the mechanims of some AIE systems still remain unclear. In this focus article, the progress of the fundamental understanding of the AIE mechanism is reviewed and the future developments in AIE research are discussed. The goal is to provide a brief yet insightful introduction and interpretation of the subject to both new and experienced AIE researchers.

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Aggregation-induced emission: fundamental understanding and future developments

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.

Once considered the exclusive property of metal complexes, the phenomenon of room-temperature phosphorescence (RTP) has been increasingly realized in pure organic luminophores recently. Using precise molecular design and synthetic approaches to modulate their weak spin-orbit coupling, highly active triplet excitons, and ultrafast deactivation, organic luminophores can be endowed with long-lived and bright RTP characteristics. This has sparked intense explorations into organic luminophores with enhanced RTP features for different applications. This Review discusses the fundamental mechanism of RTP in pure organic luminophores, followed by design principles, enhancement strategies, and formulation methods to achieve highly phosphorescent and long-lived organic RTP luminophores even in aqueous media. The current challenges and future directions of this field are also discussed in the summary and outlook.

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Enhancing the performance of pure organic room-temperature phosphorescent luminophores

The strong light emission of organic luminogens in the aggregated state is essential to their applications as optoelectronic materials with good performance. In this review, with respect to the aggregation-induced emission and room-temperature phosphorescence luminogens, the important role of molecular packing modes is highlighted. As demonstrated in the selected examples, the molecular packing status in the aggregate state is affected by many factors, including the molecular configurations, the inherent electronic properties, the special functional groups, and so on. With the consideration of all these parameters, the strong fluorescence and phosphorescence in the aggregated state could be achieved in the rationally designed organic luminogens, providing some guidance for the further development.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201600484

The Strong Light-Emission Materials in the Aggregated State: What Happens from a Single Molecule to the Collective Group.

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.

Two stable, purely organic luminogens exhibit both mechano- (ML) and photoluminescence (PL) with dual fluorescence–phosphorescence emissions at room temperature. Careful analysis of the crystal structures, coupled with theoretical calculations, demonstrate that room-temperature phosphorescence and ML properties are strongly related to molecular packing. In particular, the formation and fracture of molecular dimers with intermolecular charge-transfer properties has a significant effect on intersystem crossing, as well as excited triplet state emissions, in both PL and ML processes.

Elucidating the Excited State of Mechanoluminescence in Organic Luminogens with Room-Temperature Phosphorescence

Six blue AIEgens, including TPE-2Py(p,1), TPE-2Py(m,1), TPE-2Py(p,2), TPE-2Py(m,2), TPE-2Py(p,4) and TPE-2Py(m,4), were designed by combining a TPE core and pyrene side groups. Modifying the linkage modes of TPE (para to meta-position) and pyrene units (1- to 2- to 4-position), their EL emissions in standard OLEDs devices were tuned from sky blue (484 nm) to deep blue (452 nm). Because of the good interaction between the pyrene side groups, the target compounds showed enhanced hole-transporting ability. When fabricated as emitters in OLEDs with or without hole-transporting layers, all the emitters exhibited blue emissions and comparable EL performances. Also, all these AIEgens showed excellent reversible mechanochromism properties with different maximum emission peaks larger than 40 nm from the crystalline to the amorphous state, offering them additional applications in the opto-electronic field.

Pyrene-based blue AIEgens: tunable intramolecular conjugation, good hole mobility and reversible mechanochromism

Three blue luminogens, Cz-3tPE, TPA-3TPP and Cz-3TPP, have been successfully synthesized, in which intramolecular conjugation has been restricted effectively by decreasing the unsaturated bonds and weakening the electron donating ability of the molecular building blocks, which is much different from previous strategies Due to the introduction of triphenylamine and carbazole groups, these luminogens exhibited excellent hole-transporting abilities, with a maximum hole mobility of up to 10−3 cm2 V−1 s−1, which was better than most commercial hole-transporting materials Once fabricated as emitters in OLEDs, as a result of the controlled conjugation, their EL emissions were blue-shifted from pure blue (464 nm) to deep blue (436 nm), and then to blue-violet (397 nm) Excitingly, Cz-3TPP exhibited the best EL performance, with a maximum external quantum efficiency of up to 427% at 397 nm without a hole-transporting layer, which is among the highest reported performances for double-layer OLEDs with deep blue emission

Xuming Gao

Papers

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

Elucidating the Excited State of Mechanoluminescence in Organic Luminogens with Room-Temperature Phosphorescence

Pyrene-based blue AIEgens: tunable intramolecular conjugation, good hole mobility and reversible mechanochromism

New insight into intramolecular conjugation in the design of efficient blue materials: from the control of emission to absorption