Principles of Aggregation‐Induced Emission: Design of Deactivation Pathways for Advanced AIEgens and Applications

Abstract

Twenty years ago, the concept of aggregation‐induced emission (AIE) was proposed, and this unique luminescent property has attracted scientific interest ever since. However, AIE denominates only the phenomenon, while the details of its underlying guiding principles remain to be elucidated. This minireview discusses the basic principles of AIE based on our previous mechanistic study of the photophysical behavior of 9,10‐bis(N,N‐dialkylamino)anthracene (BDAA) and the corresponding mechanistic analysis by quantum chemical calculations. BDAA comprises an anthracene core and small electron donors, which allows the quantum chemical aspects of AIE to be discussed. The key factor for AIE is the control over the non‐radiative decay (deactivation) pathway, which can be visualized by considering the conical intersection (CI) on a potential energy surface. Controlling the conical intersection (CI) on the potential energy surface enables the separate formation of fluorescent (CI:high) and non‐fluorescent (CI:low) molecules [control of conical intersection accessibility (CCIA)]. The novelty and originality of AIE in the field of photochemistry lies in the creation of functionality by design and in the active control over deactivation pathways. Moreover, we provide a new design strategy for AIE luminogens (AIEgens) and discuss selected examples.

Microfluidic Assisted Flash Precipitation of Photocrosslinkable Fluorescent Organic Nanoparticles for Fine Size Tuning and Enhanced Photoinduced Processes

“…Such observation echoes the recent theoretical developments on fluorescence quenching in solution and fluorescence recovery in AIEgene nanoparticles. Such studies point out the dramatic energy shift of the conical intersection (CI) between the S 1 excited state and S 0 ground state undergone along the potential energy surface for self‐assembled fluorophores, thereby reducing the radiationless deactivation pathway [12b] . Based on previous transient absorption spectroscopy experiments carried out on a similar compound, we herein suspect that such CI shift may operate to explain the considerable dye emission amplification observed for VIN fluorophores embedded in ALD FONs compared to dyes dissolved in THF (Figure S5) or self‐assembled in “monocompound” FONs, which would result not from steric effects but from strongly exacerbated polarity effects.…”

The Promise of Aggregation-Induced Emission Luminogens for Detecting COVID-19

“…Aggregation-induced emission (AIE), first proposed by Tang in 2001, is a diametrically opposite phenomenon to ACQ ( 14 ). The luminogen with AIE characteristics is named AIEgen, which has no/low luminescence in the molecularly dispersed state but enhanced luminescence in the aggregated state ( 14 – 18 ). Further mechanistic studies indicate that strong luminescence can be also achieved by restricting the intramolecular motions (RIM) of the AIEgens in the molecularly dispersed state ( 16 – 18 ).…”

“…The luminogen with AIE characteristics is named AIEgen, which has no/low luminescence in the molecularly dispersed state but enhanced luminescence in the aggregated state ( 14 – 18 ). Further mechanistic studies indicate that strong luminescence can be also achieved by restricting the intramolecular motions (RIM) of the AIEgens in the molecularly dispersed state ( 16 – 18 ). After 20 years of outstanding development, a great family of AIEgens has been established, ranging from twisted conjugated AIEgens to planar conjugated AIEgens and irregular non-conjugated AIEgens ( 14 – 21 ).…”

Chemiluminescence Resonance Energy Transfer Efficiency and Donor–Acceptor Distance: from Qualitative to Quantitative

“…Accordingly, the corresponding CRET efficiencies are quantitatively measured. However, the conventional acceptors inside the micelle often suffer from aggregation‐caused quenching (ACQ) problem, leading to a dramatic drop in their luminescence performances [19–21] . Supposing that if aggregation‐induced emission (AIE)‐active acceptors can be fixed at different positions inside the micelles, their intense luminescence properties in the aggregated state could be exhibited without ACQ problem.…”