I've got a solution of pyrene and I excite it with a laser. If I measure the emission spectrum 1 ns after absorption, a well‐structured emission with a small Stoke’s shift is observed.  However, if the emission is measured with a delay of 100 ns, a broad, featureless spectrum with a much larger Stoke’s shift is obtained.  

Is this purely because pyrene has had some time to undergo solvent reorganization and thus the individual excited pyrene molecules have larger differences or is there something else at play?

  • $\begingroup$ Did you observe any dependence on the pyrene concentration? $\endgroup$ – Klaus-Dieter Warzecha Nov 2 '16 at 10:10

I think that you have rediscovered the emission of pyrene exciplexes :)

As you have correctly assigned, the structured fluorescence spectrum shortly after the pulse is the emission of pyrene monomers.

Given sufficient time (100 ns), an excited molecule of pyrene diffuses towards a pyrene molecule in its ground state and forms an exciplex.

Fluorescence emission from these exciplexes is known to be broad and unstructured.

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    $\begingroup$ awesome. it's a 10^-3 M solution of pyrene. 100 ns should be enough for the excited molecule to meet another pyrine molecule. $\endgroup$ – Arno van der Weijden Nov 2 '16 at 10:55
  • $\begingroup$ At this (high) concentration the emission to the red part of the spectrum (relatively speaking) is due to pyrene excimer (one excited pyrene in a face to face dimer with a ground state pyrene). You should be able to see the rise-time of the emission as the excimer is formed if you choose the wavelength carefully. Excimers have a repulsive ground state but a potential well in the excited state. see J. Birks, Rep. Prog. Phys. 1975, 38, 903-974. Excimers refers to the case when both molecules are of the same type, exciplexes when they are different. $\endgroup$ – porphyrin Nov 2 '16 at 23:06

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