Dear Saverio, In a far past I started as a photochemist and I recall that excimers or exciplexes were characterised by their aromatic chromophores and that in order for this dimers of excited state-ground state to occur, the chromophores need to be in a planparallel configuration: so they only occur at high concentrations (in case of intermolecular excimer formation) or at low concentrations in case of intramolecular excimer formation provided that the two chromophores are linked by a flexible chain in order for conformational changes to be possible during the lifetime of the excited state. I think you're right that the pi electrons interact: the excimer is in fact a resonance interaction between an excited and a ground state. I don't know anymore if any other criteria should be met and if larger or smaller chromophores should behave differently. Because of the planparallel configuration I suspect that large multichromophoric structures such as phycobiliproteins probably won't satisfy the criteria, but again this is just a gut feeling. I guess that like many of these photophysical processes the way to know if excimer formation is occuring is by checking the fluorescence spectra: intermolecular excimers are evident from the appearance of red shifted structureless fluorescence at high concentrations. Whether it appears or not is probably a result of the combination and relative importance of all ways by which an excited state can relax to its ground state (be it radiative or non-radiative). I hope some physicists can shed more light on it than I can. I'm looking forward to some more information. Best regards, Dirk Prof. Dirk Van Bockstaele, PhD Laboratory of Hematology Head Flow Cytometry Antwerp University Hospital Wilrijkstraat 10 B-2650 Edegem Belgium phone 32 3 821 3900, fax 32 3 825 1148 > ---------- > Van: Saverio Alberti[SMTP:alberti@alpha400.cmns.mnegri.it] > Verzonden: donderdag 23 november 2000 10:11 > Aan: Cytometry Mailing List > Onderwerp: excimer structure > > > A question for physicists and people who really understand fluorescence > (we really only are rough users..). We are familiar with the properties of > pyrene excimer formation, spectra etc. Now, the question is: can excimers > be designed for larger fluorochromes? In other words, what is the > fundamental structural characteristic of a molecule that forms excited > dimers? Pi electron sharing? > > thanks for the teaching.. > > Saverio Alberti > Head, Lab. of Experimental Oncology > Department of Cell Biology and Oncology > Consorzio Mario Negri Sud > 66030 Santa Maria Imbaro (Chieti), Italy > Phone: (39-0872) 570.293 > FAX: (39-0872) 570.412 > E-mail: alberti@cmns.mnegri.it > >
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