1
$\begingroup$

Variety of books and literature reviews consider excimer (or exciplex) emission something distinct from through-space charge transfer. Is this actually the case?

This question arises, because the more literature I read on this subject the less sure it seems to have any distinctive characteristics one from another. Mainly because the same ''evidence'' (e.g. the bathochromic shift of emission, lifetime increase, concentration studies) is used to argue for excimer or exiplex emission in one study, while other authors propose that such evidence supports CT emission.

$\endgroup$
2
  • 1
    $\begingroup$ In an excimer, e.g. classical example pyrene ($\ce{M + M^*<=> MM^*}$ ) the ground state is repulsive but the excited state is not and from which broad structureless emission occurs. In exciplexes the situation is similar except there has to be some charge transfer character as the two species are different. This makes the spectra etc sensitive to solvent polarity. CT complexes are different as they are formed directly on photon absorption. $\endgroup$
    – porphyrin
    Sep 23 '20 at 14:07
  • $\begingroup$ The featureles, broad and red shifted emission is observed for both excimers and CT bands. If we look at this from a chemistry stand point then excimers and exciplexes form from already associated ground state complexes or dimers (from pi-pi stacking). CT complexes can form between identical molecules (if these molecules are polarized enough) or different species (this is easier to achieve because one can easily choose electron rich and electron poor molecules). There are even examples of through space CT emission within a single molecule $\endgroup$ Sep 24 '20 at 7:41
1
$\begingroup$

The excimer / exciplex ground state is wholly repulsive, molecules $\ce{M and M^*}$ diffuse together to form excimer $\ce{MM^*}$ the excimer or exciplex if formed from different species. The complex does not exist in the ground state. Emission is observed from both $M^*$ and $MM^*$. ($MM^*$ can also diffuse apart before fluorescence) The reason the excimer/exciplex emission is broad and structureless is because the ground state is repulsive. The exciplex is formed from different species so there has to be some charge transfer character, i.e. potential energy is sensitive to solvent polarity. Intramolecular excimers and exciplexes were first synthesised and studied in the late 1970's.

CT complexes are intimate in the ground state (via $\pi$ stacking etc.) and the CT absorption is generated upon absorption $(DA) + hv\to (D^+A^-)$. Any emission is then from this CT state plus normal fluorescence from either D* or A* should they be isolated in solution (as DA is in equilibrium with D and A) and then excited.

For details of kinetic see J.B. Birks, 'Photophysics of Organic Molecules' 1970, publ Wiley, and Organic Molecular Photophysics' 1975, publ Wiley. M. Gordon & W. Ware 'The Exciplex' 1975, Academic Press.

$\endgroup$
2
  • $\begingroup$ Indeed, this is what books tell us. Any thoughts on: pubs.rsc.org/en/Content/ArticleLanding/2020/TC/… ? My main interest is in understanding the mechanism, because there are some instances where excimer emission provides a net PLQY increase and do not work as a general luminescence quenching route. $\endgroup$ Sep 24 '20 at 11:28
  • 1
    $\begingroup$ Time resolved experiments confirm this model in solution, but being in a crystal makes this very different, however, it must remain true that the ground state is repulsive and the excited state MM* has a minimum otherwise it is going to be hard to call it an excimer. $\endgroup$
    – porphyrin
    Sep 24 '20 at 11:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.