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Excimer emission lifetime compared with monomer

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blackcat:
In emission spectroscopy, does excimer emission have shorter lifetime than the corresponding monomer? Why is this so?

Corribus:
kamiyu2550, you ask lots of questions without showing any of your own thoughts about a solution, and then we give you answers and you move on to your next question without even acknowledging the answers to the one that came before it. That's no how Chemical Forums is intended to work. Please read the forum rules.

blackcat:

--- Quote from: Corribus on October 15, 2019, 02:01:06 PM ---kamiyu2550, you ask lots of questions without showing any of your own thoughts about a solution, and then we give you answers and you move on to your next question without even acknowledging the answers to the one that came before it. That's no how Chemical Forums is intended to work. Please read the forum rules.

--- End quote ---

I apologize for not following the rules strictly. I have found that the excimer for pyrene has shorter radiative lifetime than its monomer: https://royalsocietypublishing.org/doi/10.1098/rspa.1963.0187.

I think there can be two possible reasons. Firstly, the excited dimer being larger than the monomer is more susceptible to non-radiative deactivating collision of solvent molecules. Secondly, the excimer has lower emission energy which is more susceptible to energy gap law deactivation.

Corribus:
Mostly the latter of your two suggestions. Franck Condon factor is also enhanced due to larger change in nuclear coordinates in the dimer vs monomer (if you will, the dimer explores a larger conformational space due to less restricted motion), and in these kinds of systems intersystem crossing efficiencies usually play a role as well. Which effect dominates in the control of nonradiative decay rates is probably a complicated function of solvent polarity/viscosity, temperature, and pyrene concentration.

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