Aggregation-induced emission (AIE) is one of the innovative concepts that was initially described by the group Ben Zhong Tang in 2001. It defies the folklore in the field of photochemistry, whose expectation is that such aggregation of dye molecules usually causes the dye molecule to quench its light emission, a phenomenon referred to as aggregation-caused quenching (ACQ). By contrast, AIE luminogens (AIEgens) are molecules that do not emit when in solution in a good solvent but emit strongly when the molecules are confined in aggregates, solid films, or nanocrystals.
Fundamental Mechanisms: Restriction of Intramolecular Motion (RIM)
The dominant process in the AIE effect is the Restriction of Intramolecular Motion (RIM) that includes the restriction of intramolecular rotation (RIR) and the restriction of intramolecular vibration (RIV). AIEgen molecules are allowed to rotate and vibrate in a good solvent. Such dynamic movements offer non-radiative processes in which the excited state may be de-excited to the ground state, and thus the absorbed energy is dissipated as heat, and therefore, weak or no fluorescence is emitted. When, however, the molecules are tightly packed together in the condensed phase, in the state of aggregation. Such physical confinement seriously limits their internal rotations and vibrations, hindering the non-radiative decay modes.
Key Advantages of AIE Luminogens
Their finest benefit is their excessive quantum yield of strong-kingdom fluorescence, which makes them properly applicable to use in thin-film optoelectronic applications along with OLEDs, in which the ACQ effect has usually been the source of great efficiency roll-off at excessive brightness.
Common Applications and Uses
These special characteristics of AIEgens have resulted in their application in a broad range of advanced applications. They are also utilized in optoelectronics as efficient emitters in non-doped OLEDs, where the ACQ issue that afflicts conventional dyes is overcome. Still in chemical sensing and biosensing, AIEgens also act as (so-called) light-up probes to a wide variety of targets, such as explosives, heavy metal ions, proteins, and enzymes, in which the binding process leads to aggregation and a subsequent fluorescence increase. AIEgens are useful in bioimaging and theranostics.
Limitations and Design Challenges
Their extraordinary benefits notwithstanding, there is no trouble in the development and use of AIEgens. The first weakness is the possibility of weakened or quenched emission in aggregates that are packed in excessively compact packs, which allow pi-pi stacking interactions to occur, recreating ACQ-like behavior. The synthesis and design of new AIEgens may be complicated, and sometimes multistage organic synthesis is necessary to make molecules that have the optimal ratio of rotatable bonds and electronic structure to facilitate RIM.
Conclusion
The process of aggregation-induced emission has become one of the most powerful concepts in the contemporary studies of luminescence. The ease with which it allows the elimination of the ancient issue of aggregation-induced quenching has enabled AIE luminogens to open the door to the application of organic substances in their solid form. The mechanistic insight offered by the restriction of intramolecular motion (RIM) has led to the rational design of a library of new AIE-active molecules with desired properties in a large number.
