Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов
Читать онлайн книгу.
AIEgens: Synthesis and Applications
Ming Chen1, Anjun Qin3, and Ben Zhong Tang2,3,4
1College of Chemistry and Materials Science, Jinan University, Guangzhou, China
2Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
3State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, Center for Aggregation‐Induced Emission, South China University of Technology, Guangzhou, China
4Shenzhen Institute of Aggregate Science and Technology, School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong, China
1.1 Introduction
Luminescent materials have been widely applied in display, illumination, and information transfer, etc [1–3]. However, in real‐word applications, they are most employed in the aggregate state (e.g. solid‐state‐emissive thin film in organic light‐emitting diodes (OLEDs) and nanoimaging materials) [4–8]. The traditional chromophores show extremely strong emissions in the solution but exhibit almost quenched emission behavior upon aggregation. For such a reason, scientists are in enthusiastic pursuit of highly efficient luminescent materials in the solid state. One would consider it to be better if the aggregation could be utilized to play a positive instead of a negative role in enhancing luminescence. This idea had come true until 2001 when Tang had a beautiful encounter with aggregation‐induced emission (AIE) [9–12]. AIE luminogens (AIEgens) possess the luminescence behavior opposite to the traditional luminogens as their twisted and flexible molecular conformation allows them to dissipate the excited‐state energy nonradiatively by molecular motion in the solution, while such motion is suppressed in the aggregate state to open up the radiation channel [13–16]. Thus, the problem of aggregation‐caused quenching (ACQ) effect perplexed in traditional dyes has been overcome thoroughly by AIE, and more than 10 000 works aiming at mechanism study, molecular design, and functionality exploitation have been published based on this hot topic.
It is crucial to develop a great variety of AIEgens because molecules with different structures may cause different properties and functions. By choosing suitable AIE archetypal molecules for structural decoration, such a target can be achieved readily. Up to date, dozens of archetypal AIEgens have been developed. Among them, hydrocarbon AIEgens like tetraphenylethene (TPE), triphenylethene, tetraphenyl‐1,4‐butadiene (TPBD), and distyrylanthracene (DSA) are very popular because they are competent in designing materials with high luminescence efficiencies Chart 1.1 [17–20].So far, TPE is regarded as the most famous AIEgen because it is very easy to synthesize and modify, making that the majority of AIE researches are established based on this system. The mechanism for elaborating AIE phenomenon of TPE is still in debate [21]. The early studies show that the restriction of rotation of peripheral phenyl rings against the central double bond is the essential cause. Zheng reveals that photoisomerization of the central double bond in TPE plays an important role in quenching its emission in the solution based on the studies of photophysical behaviors of TPEs with its adjacent phenyl rings chemically locked at different sides [22]. In other words, the double bond in TPE is unstable and easy‐to‐form free radicals under irradiation and heat. On the other hand, Chi observed a photochromism of Cl‐substituted triphenylethene as a result of the cyclization reaction taking place between the phenyl rings in the molecule under UV light stimuli [23]. Thus, the stability of AIE materials based on these hydrocarbon AIEgens needs to be considered and improved.
Chart 1.1 Molecular structures of AIEgens of tetraphenylethene (TPE), triphenylethene, tetraphenyl‐1,4‐butadiene (TPBD), distyrylanthracene (DSA), hexaphenylsilole (HPS), pentaphenylpyrrole (PentaPP), phenyl‐substituted oxidized benzothiophene (DP‐BTO), and tetraphenylpyrazine (TPP).
By contrast, no central double bond exists in the heterocycle‐based AIEgens, making them free of such trouble. The typical heterocycle‐based AIEgens are hexaphenylsilole (HPS), pentaphenylpyrrole (PentaPP), phenyl‐substituted oxidized benzothiophene (DP‐BTO), and so on Chart 1.1 [24–30]. The introduction of heteroatoms in these AIEgens obviously endows them with different electronic properties. For example, in HPS, the interaction of σ* orbital of the silicon atom and π* orbital of the carbon atom enables it to possess low‐lying LUMO energy level. It imparts HPS with high electron affinity, which can act as an electron‐accepting unit in molecular design and increase the electron‐transporting property as material [31]. On the other hand, the orbitals of the nitrogen atom in PentaPP is sp2‐hybridized, while the lone pair electrons occupy the p orbital, which arrays parallelly with the p orbitals from adjacent carbon atoms. It remarkably increases the p–π interaction and makes the central pyrrole ring electron‐rich. Some electron‐donating and hole‐transporting properties of PentaPP are thus obtained in molecular and material designs [32]. However, although these heterocycle‐based AIEgens show good photo‐ and thermal stabilities, their chemical stability should be improved. For example, the silole ring in HPS is easy to decompose under basic atmosphere. Besides, their synthesis is always tedious, the reaction condition is rigorous, and the purification is difficult. Thus, it is urgent to develop new AIEgens in combination with the advantages from the above hydrocarbon and heterocycle ones.
In 2015, Tang reported a heterocycle‐based AIEgen, namely, tetraphenylpyrazine (TPP), with its structure possessing four phenyl rings attaching to the central pyrazine heterocycle ring Chart 1.1 [33–37]. The rotation of the peripheral phenyl ring against the pyrazine ring makes the molecule less emissive in the solution. The twisted conformation of TPP prohibits the formation of π–π stacking in the aggregate state, while the multiple intermolecular C–H⋯π interactions exist to lock the molecular motions. These factors collectively contribute to the AIE effect. A more detailed mechanism is still under investigation because the central pyrazine ring is possible to participate in the molecular motions in the excited state. The preparation of TPP is very simple even when compared with that of TPE, and many routes are alternative according to various requirements. The stability of TPP is pretty good due to the whole aromatic structure of the molecule and the high bond energy of the C–N bond in the central heterocycle. Different from pyrrole, the nitrogen atom forms a double bond with one of the neighboring carton atoms in pyrazine. The lone pair electrons, therefore, occupy one of the sp2 orbitals of the nitrogen atom, which parallels to the pyrazine plane. The interaction of lone pair electrons and π electrons will never occur, whereas the strong electronegativity of the nitrogen atom in C–N bonds induces the polarization of an electron cloud in the ring, which makes the pyrazine ring in TPP electron‐deficient [38, 39]. Indeed, by decorating TPP with phenyl rings or weakly electron‐donating methoxyl groups, the resulting donor–acceptor (D–A) effect can finely tune the emissions of derivatives in the whole blue light region. These features indicate that the AIE‐active TPP is a promising candidate utilized for further development of high‐performance materials.
With the development in the last five years, more and more works with TPP as motif have emerged. Firstly, besides the conventional methods, the new methodologies in the preparation of TPP and its derivatives have been established. By using the new catalytic systems, rather high yields (>98%) are obtained in the preparation. A recent work shows that TPP can even be prepared efficiently by direct heating of the starting materials in the solid phase. Secondly, many luminescent applications (e.g. highly efficient OLEDs,