It is known that relaxation of Excimers produce spectral bands which are of greater wavelength and broader than relaxation bands of monomer excited state.
Why is the band broadening? My argument would be that the Exciplex is formed by collision of the monomer (say A*) with solvent (say B). B molecules will have different energies associated with them as well as collisions between A and B will have different amounts of energies lost during the collision. As a result, there will be a wider band of energy for Exciplex excited state than for the monomer excited state, and hence a broader band would be seen.
Is the reasoning correct or is there a different reason behind the broadening? Any reference regarding this question is most welcome.
Thank you.
Spectral width of absorption/fluorescence bands of typical organic molecules is determined by selection rules based on Frank-Condon principle for bound electronic-vibrational states. Excimer and exciplex fluorescence take place between bound upper state and unbound ground state. No quantization for vibrational states exist for unbound electronic state. It means that no spectral limitation in accordance to Frank-Condon rules exist. So that is reason for spectral broadening of excimer (exciplex) fluorescence.
Spectral width of absorption/fluorescence bands of typical organic molecules is determined by selection rules based on Frank-Condon principle for bound electronic-vibrational states. Excimer and exciplex fluorescence take place between bound upper state and unbound ground state. No quantization for vibrational states exist for unbound electronic state. It means that no spectral limitation in accordance to Frank-Condon rules exist. So that is reason for spectral broadening of excimer (exciplex) fluorescence.
Thank you very much for the quick reply sir. It made a lot of sense. Thank you again.
The clearest explanation I found to this effect is because the different energy between the ground state dimer (AA) and the excited stated dimer (AA)*. You can read about this in the next book: Molecular Fluorescence: Principles and Applications, 2nd Edition. Bernard Valeur (page 94). Here is the link: http://physweb.bgu.ac.il/~bogomole/Books/Valeur%20B%20-%20Molecular%20Fluorescence%20Princeples%20And%20Applications%20-%20(Wiley-Vch%202001_%20399%20P).pdf
Justo
I searched for it in the book an did not find the proper answer as given by dr Tikhonov above. Also, the book mentions that an excimer is formed in the collision of an exited molecule with a ground-state molecule but should also have mentioned that it is a non-elastic collision i.e. that the 2 molecules stay attached after the collision
my explanation goes for ,The excited dimers will have many different vibrational levels each of them allows to make the exciplex with different energies which makes wide band of spectral of the emitted pulse .theoretically one can prove that via calculating the excited exciplex and compare it with the increments of excited dimers .
Yes
and what does this mean for the difference in the spectra of dimers and monomers, as the questioner asks?
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