Aggregation-Induced Emission
Aggregation-Induced Emission (AIE) refers to the phenomenon in which the luminescence of organic compounds is enhanced in the aggregated state. This effect was first discovered and termed in 2001 by Ben Zhong Tang and co-workers during their investigation of the photophysical properties of 1-methyl-1,2,3,4,5-pentaphenylsilole. 1-Methyl-1,2,3,4,5-pentaphenylsilole is an organic compound with the chemical formula C₃₅H₂₈Si, exhibiting aggregation-induced emission (AIE) properties. It can be synthesized via the reaction of diphenylacetylene with metallic lithium in tetrahydrofuran (THF), followed by reaction with phenylmethyl dichlorosilane.Materials exhibiting the AIE effect are almost non-emissive in the molecularly dispersed state but display markedly intensified fluorescence upon aggregation, demonstrating photophysical characteristics distinct from conventional fluorophores. As a pioneering research direction initiated by Chinese scientists, AIE has become a prominent focus in chemistry and materials science, attracting worldwide attention.
Figure 1. Mechanism of AIE
Traditional fluorescent dyes or π-conjugated molecules often suffer from aggregation-caused quenching (ACQ) at high concentrations or in aggregated states due to strong intermolecular π–π stacking interactions and energy transfer processes, leading to significant attenuation of fluorescence. In contrast, AIE-active molecules operate through the unique mechanism of restriction of intramolecular motion (RIM): they are nearly non-emissive in dilute solution but exhibit intense fluorescence upon aggregation, nanoassembly, or in the solid state.
Aggregation-Caused Quenching (ACQ)
♦︎ Phenomenon: Conventional fluorophores exhibit reduced or even quenched emission at high concentrations or in the aggregated state.
♦︎ Mechanism: Strong intermolecular π–π stacking, energy migration, and excimer formation dissipate the excited-state energy through non-radiative pathways.
♦︎ Consequence: Weakened fluorescence severely limits the applicability of traditional dyes under solid-state or high-concentration conditions.
Restriction of Intramolecular Motion (RIM)
♦︎ Phenomenon: AIE luminogens display enhanced fluorescence in the aggregated or solid state.
♦︎ Mechanism: Upon aggregation, intramolecular rotations and vibrations (RIR/RIV) are restricted, thereby suppressing non-radiative decay channels.
♦︎ Consequence: Excited-state energy is preferentially released through radiative pathways, resulting in boosted emission intensity.
Figure 2. Comparison between ACQ (Aggregation-Caused Quenching) and AIE (Aggregation-Induced Emission).
References
1. J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu, B. Z. Tang, Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem. Commun. 2001, 1740–1741.
2. Han, Z., Yang, Z., Sun, H., Xu, Y., Ma, X., Shan, D., Chen, J., Huo, S., Zhang, Z., Du, P., & Lu, X. (2019). Electrochemiluminescence platforms based on small water‐insoluble organic molecules for ultrasensitive aqueous‐phase detection. Angewandte Chemie International Edition, 58(18), 5915-5919.
3. Han Nie, Kun Hu, Yuanjing Cai, Qian Peng, Zujin Zhao, Rongrong Hu, Junwu Chen, Shi-Jian Su, Anjun Qin, Ben Zhong Tang. Tetraphenylfuran: aggregation-induced emission or aggregation-caused quenching?. Materials Chemistry Frontiers. 2017, 1 (6): 1125–1129.
4. Abdulhamid Umar, Mustaffa Shamsuddin. Preparation and Aggregation-Induced Emission of New 1,3,5-Triazine-2,4,6-tricarboxamide with Liquid Crystal Properties. Oriental Journal of Chemistry. 2018-08-28, 34 (4): 1741–1748.
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