Date Published: February 21, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Qianqian Li, Zhen Li.
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.
Organic light‐emitting materials, generally bearing the π‐conjugated structures, have been a hot topic for their wide applications, such as data storage, photoswitches, organic light‐emitting diodes (OLEDs), organic light‐emitting transistor (OLET) devices, and sensors.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 Since the performance heavily depends on the inherent physicochemical properties of materials, especially the emitting efficiency in solid state, it is badly needed to develop excellent luminogens with strong emissions in the aggregated state. Actually, most of the π‐conjugated molecules demonstrate strong emissions in dilute solutions (almost in the single molecule state), but partially or even completely quenched emission in the solid state. This phenomenon is mainly ascribed to the intense intermolecular π–π stacking interactions with the famous name of the concentration quenching effect termed more than half a century ago,13 rather than their inherent electronic structures. And the noncovalent intermolecular interactions are considered to be determined by the distances among adjacent molecules and their packing status.
AIEgens are a special class of luminogens, completely opposite to aggregation‐caused quenching (ACQ) ones. Their emissions are much more brilliant in the practically useful solid state than in solution, exhibiting the academic value and practical applications in life science and biomedical engineering.24, 25, 26, 27, 28, 29 Basically, the emission of AIEgens in the aggregated state can be derived from two ways with different packing modes. In most cases, AIE is realized by the restriction of intramolecular motions (RIM) in the single molecule state, which blocks the nonradiative pathway for the excitons to decay (knr,F in Figure1A), with the activated radiative transition (kr,F in Figure 1A).30 The structural feature of these AIEgens is the highly twisted configuration, to avoid the possible intermolecular π–π interactions. However, some planar conjugated molecules also exhibit AIE characteristic with the high‐efficiency excimer fluorescence from the pairwise (or sandwich herringbone) stacking state (Figure 1B).31 The excimer emission with the red‐shifted fluorescent spectrum comes from an excited‐state molecule close to another ground‐state molecule, or the formed dimer in the ground state. For example, pyrene exhibits weak emission at 350–400 nm in solution, but much stronger fluorescence peaked at about 480 nm, indicating the formation of intramolecular excimers.32
Unlike fluorescence, phosphorescence is a spin‐forbidden radiative transition from the lowest triplet excited state (T1) to the ground state (S0) (Figure20). Basically, without precious metals, the rate of phosphorescence is very slow, thus, the RTP of pure organic molecules is hardly observed, mostly due to the presence of triplet oxygen, thermal and vibrational relaxation, inaccessible triplet states, weak spin–orbit coupling, and highly forbidden triplet–singlet transitions. Excitedly, in recent years, thanks to the well‐organized alignments of molecules in the solid state, some pure organic RTP luminogens with relatively long lifetimes have been reported.82, 83, 84, 85, 86
In this paper, based on some selected examples and the hidden thoughts of the authors, we try to disclose the influence of the molecular packing status, in addition to the electronic structures, on their emissive properties in the aggregated state. Mainly, two kinds of luminogens are involved, pure organic AIEgens and RTP luminogens, for their abnormal and shining characteristics. All the elaborately chosen cases exhibit the key role of molecular packing in aggregated state, often leading to the much different emissive behavior from that in solution. Taking AIEgens as examples, the dramatically enhanced emission in the aggregated state, in some degree, subverts the traditional thought lost in the concentration quenching effect, and arouses the deep thinking of solid‐state emission, providing an alternative approach to develop strong emitters in solid state, in addition to the conventional strategies against aggregation. Actually, in pursue of good luminogens for practical applications, the essential point is the high emissive efficiency in aggregated state, but not whether with AIE characteristic or not, just as the cases of 21 and 22. However, in some cases like sensors, the AIE characteristic, especially nonemissive in solution but emissive in aggregate state, possesses inherent and unique advantages. Undoubtedly, the packing status affects the emission largely, even with the formation of excimer for pyrene. However, the previous summarized design principles for luminogens, perhaps, have emphasized heavily on their electronic properties, ignoring, in a large degree, the packing status in aggregates and the possibly new born energy levels. Thus, the present structure–property relationships focus mainly on the structure of single molecule and the performance of molecular aggregates, lacking the important information of molecular packing. As partially demonstrated in this review paper, scientists have strengthened the researches on how packing status affecting the performance. However, for the further and rapid development of the investigation on the packing mode, perhaps, at least three issues should be considered seriously: the defined structure with packing status, the related theory, and the accurate structure–packing–performance relationship.