Hello all,
Can we detect excimer formations with steady state emission anisotropy (not excitation anisotropy)? In one of my experiments with a fluorophore I could dectect the formation of a possible excimer at longer wavelengt in the emission spectrum. However I would like to confirm this by anisotropy measurements also.
When I measured emission anisotropy in DCM (low viscosity) as a solvent I could find a change (increase) in anisotropy in the range where the excimer was detected in the normal fluorescence spectrum, however when the same anisotropy measurement was done in PEG400, which is a high viscous solvent (chose in order to restrict rotational diffusion) I found there was a decrease in anisotropy in the excimer wavelength region (similar to DCM but opposite in nature).
What could be the reason behind this?
I would be grateful for any valued suggestions.
Hello,
The reason should be sought in changing of the orientation of the radiation oscillator.
(Nonradiative deactivation of the triplet T1 state of 3, 6-diacetylamino-N-methylphthalimide in polar vitrifying solutions.
- January 1987
- Optics and Spectroscopy 62:461-465)
@Vladimir S. Pavlovich, Thank you so much. Do you mean the polariser as the radiation oscillator?
Hello,
You should see a decrease of the emission anisotropy upon the formation of the excimer since the orientation of the new radiation dipole with repect to the monomer one adds a new source of randomization. For the result in DCM that disagree with that prediction, you mention an increase of the anisotropy. But since DCM is a low viscosity solvent this increase start from a low value. Since the excimer emission is weak the measured anisotropy might be contaminated by the anisotropy of some background solvent.
By the way the change of viscosity should change the foramtion rate of the excimer. This might be another indication of the presence of the excimer.
What about the presence of a dimer ?
Thank you for your suggestion Prof. @Robert Bernard Pansu. We are doing our experiment at the initial stage, on steady state emission. You mean to say that that increase could be a solvent effect in DCM, right? But could it be that, since the monomer is of low molecular weight its rotational diffusion might be higher hence lower anisotropy contrary to the excimer which could be of larger size and weight, as a result the rotational diffusion decreases which also increases the anisotropy?
With some references we are assuming it to be an excimer although, as per your view it might be a dimer which could be formed from in-situ photoreaction.
You are right, the excimer volum is twice that of the monomer. This will explain the bigger stationary anisotropy observed in DCM. I am not mentioning a photodimerization. When proposing excimer formation, it is always wise to be sure that dimers are not present. Excitation spectra can be used for that.
Thank you so much for your suggestion, Prof @Robert Bernard Pansu. It would be kind enough if you could provide links (if available) to any such papers based on dimers as you've mentioned done by you and your groups.
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