What Are AIEgens And Their Application?

Organic luminescent solid materials have attracted a great deal of attention from the scientific community and the industry due to their benefits of a flexible structure and functional modification.

However, traditional organic luminescent materials show strong emission in dilute solution, but weak or even no emission in the aggregated state (nano-particle, micelles, solid films, and powder), which is the phenomenon of Aggregation-Caused Quenching (ACQ), the “Achilles’ heel” of organic luminescent materials. Researchers have made great efforts to use chemical, physical or industrial methods to avoid or minimize the ACQ effect, but they did not work well.

Fig.1 ACQ and AIE phenomenon (Reproduced with permission from Science China Chemistry)

The aggregation behavior of organic molecules in the solid state is an inevitable spontaneous process in terms of enthalpy and entropy, therefore deliberately inhibiting the molecular aggregation cannot fundamentally solve the ACQ problem.

In 2001, our group proposed a new concept of ” Aggregation-Induced Emission (AIE)” in the field of photophysics. This is based on the observed phenomenon that propeller-shaped luminogens (Silole) show a relatively high efficiency of fluorescent quantum yield in their aggregated states. AIE phenomenon offers a radically novel and astoundingly convincing solution for the ACQ problem of organic luminescent molecules.  All the while they break the shackles of light-emitting efficiency and constraints of traditional thinking urging researchers to rethink the classical optical law and transition processes (Fig. 1).

Fig.2 AIEgens’ applications (Reproduced with permission from Science China Chemistry)

Compared to the traditional organic fluorescent materials, the outstanding contributions of AIE is that it solves the problem of insufficient luminescence of organic materials in the aggregated state, which is the common form of luminescent materials in practical applications. For example, flexible display and lighting based on OLED is the breakthrough technology. While the preparation of organic thin films with highly efficient luminescence is the key point to the commercial market, the AIE materials applied in this area have displayed potential advantages.

After seventeen years of development, the AIE materials have expanded their applications in almost all fields such as:

  • Responsive materials to stimuli (pH, temperature, solvent, pressure etc.)
  • Reversible sensor intelligent materials
  • Highly efficient OLED display and lighting materials
  • Optical waveguide materials
  • Selective chemical sensing materials
  • Trace tracing materials and bioimaging materials used in biological systems such as organelles, virus, bacterial and blood vessels.

Among these, the application of AIE fluorescent probes in specific organelle bioimaging and long-term tracking is highly anticipated (Fig. 2).

Fig.3 The luminescence mechanisms of AIEgens (Reproduced with permission from Science China Chemistry)

Developing AIE’s from concept to science lies in precise molecular design and functional modification of AIE molecules. The combination of experimental results and computational simulation indicates that the RIM (Restriction of Intramolecular Motion) theory is the working mechanism of AIE molecules, which includes RIR (Restriction of Intramolecular Rotation) theory proposed in 2003 and RIV (Restriction of Intramolecular Vibration) theory proposed in 2014. The RIM theory is popular and easy to understand, and thus brings instructive significance to the idea of AIE molecular design. The mechanism has greatly facilitated the development of AIE research field.

The quantity and number of citations of literature related to AIE have grown exponentially. There are more than 1500 institutes of scientists working in the AIE field in over 80 countries and regions around the world. In 2015, this topic of AIE ranked no. 2 in the areas of Chemistry and Materials Science by Thomson Reuters in its report on Research Fronts 2015. In 2018, our groups won the National Natural Science Award (China, 1st) for our original and leading work in AIE area.

Fig.4 AIEgens example for their ion sensor characteristics based on SAA-structure (Reproduced with permission from Science China Chemistry)

Salicylaldehyde azine (SAA) is a typical molecule with excited-state intramolecular proton transfer (ESIPT) process, which consists of hydrazine and salicylaldehyde Schiff base, and its derivatives exhibited obvious AIE characteristics with highly distinguishable fluorescence performance and have been widely applied in chemosensors, bioprobes, and bioimaging etc. In fact, the active hydrogen of phenolic hydroxyl in SAA derivatives plays an important role in the ESIPT process. Both its form and transferability determine the fluorescence “turn-on/turn-off” and the quantum yield of the keto form in the aggregation process. However, the detail studies on the structure-properties relationship between the activity of hydrogen proton (or the acidity of the phenolic hydroxyl group) are relatively few.

Here, we constructed two SAA derivatives with different groups at the para-position of a phenolic hydroxyl group and investigated their effect on the ESIPT process of the generated luminogens (CN-SAA and TPA-SAA). By comparing the difference in NMR spectra, solvation effect, the absorption/fluorescence spectra in the aggregation process, and different pH buffers, we found that the substituents significantly impacted the excited state.

After inviting F as a probe in MeCN solution, this activity difference became more obvious, and TPA-SAA exhibits great potential of quantitative F analyzing. Meanwhile, its emissive nanoparticles in aqueous solution show better selectivity and sensitivity to Cu2+ than CN-SAA.

These findings are described in the article entitled Electronic effect on the optical properties and sensing ability of AIEgens with ESIPT process based on salicylaldehyde azine, published in the journal Science China Chemistry. This work was led by Zhiming Wang, Fan Zhou & Ben Zhong Tang from the South China University of Technology.

About The Author

ZW
Zhiming Wang

Zhiming Wang is a researcher affiliated with the South China University of Technology.

FZ
Fan Zhou

Fan Zhou is a researcher affiliated with the South China University of Technology.

Ben Zhong Tang

Dr. Ben Zhong Tang is a researcher affiliated with the South China University Of Technology. Tang has published > 1,200 papers.  His publications have been cited > 58,085 times, with an h-index of 118.  He has been listed by Thomson Reuters as a Highly Cited Researcher in both areas of Chemistry and Materials Science. Tang received B.S. and Ph.D. degrees from South China University of Technology and Kyoto University, respectively.

 

Speak Your Mind!

READ THIS NEXT

The Deadly Algae Bloom Of Lake Erie

To celebrate an upcoming holiday, cities have their own tradition that they carry out. Some places turn their rivers green, like Chicago, for St. Patrick’s Day. Some places go all out for Christmas and decorate from top to bottom, like New York. But for some places, nature decides to decorate for a nefarious reason. The […]

Aqueous Biphasic Systems: The Greener Approach For Separation Of Biomolecules

The separation and recovery of biological molecules from biomass or fermentation media with minimum environmental effect is a tedious task, and developing such efficient and cost-effective downstream processes remains a main challenge. Traditional liquid-liquid extractions are effective in various stages of downstream processes; however, they raise environmental concerns due to the use of volatile organic solvents that […]

Where Do You Come From? Tracking Sediment Provenance Through Lead Isotopes

The quest to understand where something has come from pervades many avenues of science. One important field of geological research is tracking the parenthood of sedimentary rocks through sedimentary provenance studies. When rocks are exposed at the Earth’s surface, they are subject to both physical and chemical weathering, where they are broken down into finer-grained […]

In Search For The True Relationship Between General Relativity And Quantum Mechanics

Despite huge research efforts, the relation between the two big theories of the 20th century – General Relativity and Quantum Mechanics is still not clearly understood. The idea that all interactions are fundamentally quantum, albeit extremely successful with all non-gravitational interactions, seems to fail in case of gravitation as we are still far away from […]

Electron Configuration Chart Of Elements

An electron configuration chart is a tabular representation of patterns in the electron configuration of elements as one goes down the periodic table of elements. An electron configuration chart gives information about the orbital structure of the elements and how those orbitals are filled with electrons. Electrons exist in shells that surround the nucleus of an atom. These […]

What Is Molarity? With Examples

Molarity is how chemists measure the concentration of a solution, allowing them to relate concentrations to one another when calculating chemical reactions and working with chemical solutions. A concentration is what chemists use to refer to the amount of substance dissolved into a given amount of solution. Molarity refers to the number of moles within a […]

Hypothermia And The Little Bird In Winter

The winter ecology of small passerine birds at Northern latitudes provides an example in which counteracting selective forces can be studied in action. Small birds at Northern latitudes face a considerable challenge in winter. The smaller an animal is, the harder it will be to maintain a high body temperature in a cold environment. The […]