I want to know the mechanism of electronic state & its changes during Aggregation Induced Emission (AIE) and Aggregation Induced Emission Enhancement (AIEE). Some reports says the reason for AIE & AIEE that the free rotation of molecules restricted by addition of another molecule/ions/substituent and changing the state from liquid to solid via disabling non- radiative decay. But I am not clear about the mechanism for the shifting the wavelength (blue shift or red shift) with enhanced intensity while aggregation. It is possible and found in quantum dots/noble metal clusters also. What will happen actually in electronic state transition during aggregation?
Dear Gopi,
AIE is a very uncommon phenomena and in general you can think of it as follows. Imagine that you have a molecule where some of its constituent parts are free to rotate or move when the molecule is in solution. This means that its electronic structure is quite complicated, constantly changing and adapting to the motion of its atoms. Further, most probably the molecule interacts with light in a different manner depending on its configuration/electronic structure. For example, some configurations will be more efficient at light absorption and emission, and the absorption/emission wavelength might also be different.
Now when the this molecule starts to aggregate and changes to a more solid-like environment then the degrees of freedom in its motion are decreased, and in many cases the molecule "freezes" in a particular configuration. If this configuration happens to be one that can efficiently emit, then you obtain AIE. Further changes in wavelength can occur because the emission spectrum (electronic structure) of this particular molecular conformation was hidden from the solution spectra via ensemble average.
Best,
Dear Rafael Camacho,
Thank you for reply, I am understanding your answer. But still I am not clear. The absorbance and emission spectra will be changing while changing the molecules from solution state to solid state. But somewhere, the excitation wavelength is different from the absorbance wavelength for aggregated one. For ex, I have quantum dots (Aggregation confirmed by HRTEM images) with absorbance of 280 nm, excitation wavelength of 370 nm, emission wavelength of 520 nm. Also the absorbance as well as excitation wavelength exist at same wavelength of 340 nm and emission wavelength is 430 nm for quantum dots before aggregation. Why it is happened? and I am searching answers by terms of energy levels of quantum/molecular mechanics may be using Jablonski diagram or Franck Condon principle.
Dear Gopi Kalaiyarasan
In your case, quantum dots and their aggregates, AIE does not seem to be the most appropriate theoretical framework to explain your observations. As we have discussed above AIE is generally applied to molecules where motions are present in solution. Maybe you are getting confused due to the use of AIE to explain the behaviour of organic dots? see for example: https://www.nature.com/articles/srep01150
In QD this is not the case, even at the nano scale they are in fact a small piece of solid material. In QDs the optical transition can be understood from the confinement of the electrons and holes in three dimensions. You can get an intuitive feeling as to how this work via the particle in a box model: https://en.wikipedia.org/wiki/Particle_in_a_box For example, you can understand how the energy gap between HOMO and LUMO, and therefore the wavelength of fluorescence, is dependent on the size of the QD.
While I’m not an expert on QDs, it is well stablished that the surface of the QD is extremely important for defining their optical properties. For example, most QD are encapsulated to avoid the generation of non-radiative surface states. Also, is very common that QD aggregation induces the quenching of QD fluorescence. I think that is within this framework that you have to try to find the answers to your question and not within the theory of AIE.
Best,
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