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。
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 "local" 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.
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).
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