几种代表性的AIE的发光特点和机制（2020-10-11）

幸福侠侣

一、多芳基取代的杂环化合物





Silole衍生物的结构式

这种化合物是螺旋状的非平面构型，四周的苯环与Silole基团以单键相连，单键旋转产生无辐射衰减，导致荧光猝灭。这种结构聚合以后，分子的苯环单键旋转受限，抑制了无辐射衰减，加强了辐射衰减，产生聚集诱导发光。
唐本忠院士团队通过增加溶剂的粘度或者降低溶液的温度，也同样观察到荧光增强的现象，这也进一步反应了分子内单键旋转受限是其放光原理之一。



Silole衍生物结构化合物，Ge、Sn取代了Si

Mullin等人将Silole中心环上的Si换成了Ge、Sn，新合成的化合物在聚集态和低温态都表现出AIE现象。他们认为这类螺旋状的分子有助于抑制分子间的堆积所致的猝灭现象。



多芳基乙烯化合物

另外一种经典的杂环化合物就是四苯乙烯（TPE）衍生物，乙烯基与苯环是以单键相连接，单个分子发光很弱，当分子发生聚集或者溶液温度降低以后，苯环的旋转受限，激发态的电子辐射衰减增强，出现聚集诱导发光现象。
研究者通过在四苯乙烯衍生物中加入位阻基团，可以将ACQ类化合物转化为AIE化合物，因为位阻基团能够避免分子间紧密堆积。比如棒状反式结构的1,4-均二苯乙烯衍生物本身属于ACQ类化合物，但是用甲基取代乙烯的α位的H原子后，化合物则出现与ACQ完全相反的AIE效应。因为这种化合物本身为面-面堆积结构，很容易形成很强的π-π分子间的化学键，形成激基缔合物，产生ACQ现象。加入甲基的位阻基团后，形成了非平面结构，降低了分子间的相互作用，抑制了激基缔合物的产生，从而可以表现出AIE性质。



甲基取代乙烯的α位的H原子

二、分子内电荷转移化合物

以上AIE发光的机制主要和化合物的结构有关，且发光波段有限，基本都在蓝光波段。要想使化合物发光波段红移，主要有两种手段：

• 第一就是改变化合物的构型，使化合物更加平面化，增加共轭程度。但是这种平面结构的缺点前面也有所提及，那就是容易形成面-面堆积，形成激基缔合物，产生ACQ现象。
  
• 第二种方法就是在化合物中掺入原子，让化合物形成推拉电子结构，这样可以产生分子内电荷转移（intramolecular charge transfer，ICT），改变分子的光物理行为，得到红移波段的发射波长。
  

这种通过ICT来达到红移效应的分子如果是刚性平面化结构，那么也会因为聚集发生ACQ现象，因此在利用ICT设计分子的时候需要考虑到避免分子结构的平面化。

氟化硼络合二吡咯甲川衍生物是最早报道具有AIE性质的ICT化合物，分子结构是由三苯胺给体单元和受体单元BODIPY组成。这种化合物在不同极性溶剂中呈现不同的颜色，随着极性的增加，化合物激发态由“local”态向TICT转化，从而导致发光波长红移，而发光强度逐渐降低。



氟化硼络合二吡咯甲川衍生物的结构

另外一类化合物吡喃衍生物中同时接上强吸收电子的氰基和强给电子的氨基形成推拉电子后，分子内产生强烈的推拉作用，导致分子机化，导致结构平面化、刚性化，从而会在聚集状态下产生ACQ现象。如果将分子中的氨基去掉，分子内的推拉电子作用减弱，分子从平面变为扭曲，从而可以表现为AIE。



吡喃衍生物（推拉电子明显）ACQ



吡喃衍生物（推拉电子弱）AIE

三苯胺（TPA）本身不发光，但是能够形成螺旋结构，非常有利于构建AIE化合物，而三苯胺本身也具有很强的给电子能力，通过三苯胺与吸电子基团相结合，就可以调节分子内的推拉电子的作用，达到对发光调节的目的。
三、含氢键的化合物

有机分子之间如果能够形成氢键，就可以使分子结构更加刚性，抑制分子内旋转，降低无辐射衰减，增加发光强度。
化合物分子在光、热、电等作用下，分子由基态变为激发态，分子内基团上的氢质子通过分子内氢键转移到临近的N、O、S等杂原子上，形成相应的异构体，这种现象称为激发态分子内质子转移（ESIPT）。这一类分子具有E-E*-K*-K-E四能级跃迁规律，E代表醇式，E*表示激发态，K代表酮式，这种多变性使这种化合物有着非常广泛的应用。
比如苄叉连氮化合物在溶液和固体状态都不发光，但是其羟基ESIPT衍生物在溶液中不发光，但是在固体状态可以发光，表现出AIE效应。



苄叉连氮羟基ESIPT衍生物

因为在溶液中分子内的苯环能够C-C和N-N单键旋转，体现ACQ特性。
而聚集的状态，羟基和N之间相处分子内氢键，导致化合物结构更加刚性，抑制了分子内旋转，体现AIE特性。
四、聚合物

聚合物的单体一般也是具有AIE特征的，聚合物的优势是聚聚合物合成简单、容易加工，并且有独特的理化性质。很多AIE聚合物在溶液中和聚集态下都会发光，但是由于聚合物链的空间位阻效应，分子内的旋转受限制，导致聚合物在聚集态的发光要强于溶液状态，这种现象称为聚集发光增强（AIEE）。

<hr/>英文版：
1. Polyaryl substituted heterocyclic compounds

Such compounds are non-planar helical configuration around the benzene ring to single bonds Silole group, a single bond rotation nonradiative decay, leading to fluorescence quenching. After polymerization of such a structure, a single bond restricted rotation benzene molecule inhibits nonradiative decay, enhanced radiative decay, the AIE is generated.
By increasing the viscosity of the solvent or lowering the temperature of the solution, the team of Academician Tang Benzhong also observed the phenomenon of enhanced fluorescence, which further reflected that the limited rotation of single bonds in molecules is one of its emission principles.
Mullin et al. replaced the Si on the central ring of Silole with Ge and Sn, and the newly synthesized compound showed AIE phenomenon in both the aggregate state and the low temperature state. They believe that such helical molecules can inhibit the quenching phenomenon caused by the accumulation of molecules.
Another classic heterocyclic compound is tetraphenylethene (TPE) derivative. The vinyl group and the benzene ring are connected by a single bond. A single molecule emits weakly. When the molecules aggregate or the solution temperature decreases, the benzene ring rotates. The attenuation of the electron radiation in the excited state increases, and the phenomenon of aggregation-induced luminescence occurs.
Researchers can convert ACQ compounds into AIE compounds by adding sterically hindered groups to tetraphenylethene derivatives, because sterically hindered groups can avoid close packing between molecules. For example, 1,4-stilbene derivatives with a rod-shaped trans structure belong to ACQ compounds, but after replacing the H atom at the α position of ethylene with a methyl group, the compound exhibits an AIE effect that is completely opposite to ACQ. Because this compound itself has a face-to-face stacking structure, it is easy to form a strong π-π molecular chemical bond to form an excimer, resulting in ACQ phenomenon. After adding the steric hindrance group of the methyl group, a non-planar structure is formed, which reduces the interaction between molecules and inhibits the generation of excimer complexes, which can exhibit AIE phenomenon.
2. Intramolecular charge transfer compounds

The above-mentioned AIE emission mechanism is mainly related to the structure of the compound, and the emission band is limited, and the emission spectrum is in the blue band. In order to redshift the luminescent band of the compound, there are two main methods:

• The first is to change the configuration of the compound to make the compound more planar and increase the degree of conjugation. However, the shortcomings of this planar structure have also been mentioned earlier. It is easy to form surface-to-surface accumulation, form excimer complexes, and produce ACQ phenomenon.
  
• The second method is to dope atoms into the compound to form a push-pull electronic structure, which can generate intramolecular charge transfer (ICT), change the photophysical behavior of the molecule, and obtain the emission wavelength in the red shift band.
  

If the molecule that achieves the redshift effect through ICT is a rigid planar structure, ACQ phenomenon will also occur due to aggregation. Therefore, when designing molecules using ICT, it is necessary to consider avoiding the planarization of the molecular structure.
Boron fluoride complexed dipyrromethene derivative is the first reported ICT compound with AIE properties, and its molecular structure is composed of triphenylamine donor unit and acceptor unit BODIPY. This compound presents different colors in different polar solvents. As the polarity increases, the excited state of the compound transforms from the &#34;local&#34; state to TICT, which results in a red shift of the emission wavelength, while the emission intensity gradually decreases.
Another type of compound pyran derivatives is connected to the strong electron-absorbing cyano group and the strong electron-donating amino group at the same time to form push-pull electrons, which produces a strong push-pull effect in the molecule, which leads to molecular organization, resulting in a flattened and rigid structure. As a result, ACQ phenomenon occurs in the aggregate state. If the amino group in the molecule is removed, the push-pull electron effect in the molecule is weakened, and the molecule changes from a plane to a twist, which can be expressed as AIE.
Triphenylamine (TPA) itself does not emit light, but can form a helical structure, which is very conducive to the construction of AIE compounds. Triphenylamine itself also has a strong electron donating ability. Combining triphenylamine with electron-absorbinggroups can regulate intramolecular function of pushing and pulling electrons achieves the purpose of adjusting light emission.

3. Compounds containing hydrogen bonds

If hydrogen bonds can be formed between organic molecules, it can make the molecular structure more rigid, inhibit intra-molecular rotation, reduce non-radiative attenuation, and increase luminous intensity.
Under the action of light, heat, electricity, etc., the molecule changes from the ground state to the excited state. The hydrogen protons on the intramolecular group are transferred to the adjacent N, O, S and other heteroatoms through the intramolecular hydrogen bond to form the corresponding Isomers, this phenomenon is called excited state intramolecular proton transfer (ESIPT). This type of molecule has a four-level transition law of E-E*-K*-K-E. E represents the enol form, E* represents the excited state, and K represents the ketone form. This variability makes this compound have a very wide range of applications.
For example, the benzylidene azide compound does not emit light in the solution or solid state, but its hydroxyl ESIPT derivative does not emit light in the solution, but can emit light in the solid state, showing the AIE effect.
Because the benzene ring in the molecule can rotate with C-C and N-N single bonds in the solution, it reflects ACQ characteristics.
In the state of aggregation, the intramolecular hydrogen bond between hydroxyl and N leads to a more rigid compound structure, inhibits intramolecular rotation, and reflects the characteristics of AIE.
4.  polymer

The monomer of the polymer generally also has the characteristics of AIE. The advantage of the polymer is that the synthesis of the polymer is simple, easy to process, and has unique physical and chemical properties. Many AIE polymers emit light in the solution and in the aggregated state, but due to the steric hindrance effect of the polymer chain, the rotation of the molecule is restricted, causing the polymer to emit light stronger in the aggregated state than in the solution state. This phenomenon is called aggregation induced enhanced emission (AIEE).

原文地址：https://zhuanlan.zhihu.com/p/265033543