The absorption wavelength and quantum yields of the molecules can be altered by the incorporation of some modifications but I am just curious how we can alter the relaxation time which could be applied for the conversion of a Fluorescent compound to Phosphorescent compound.
Whem you use a good spectroscopy boo9k you will notice that a substitution by a heavy atom, especially iodine can do the job of increasing the ISC rate. There are several textbooks from the sixties, in which prior ISC was intensively studied.
Names, like Azumi McGlynn a.o.
Yes, heavy atoms such as Iodine or Bromide can increase the rate of intersystem crossing. You can either incorporate these atoms to the structure of your dye (a good example of this is the structure of Rose Bengal) or you can add them to the solution in which the dye is dissolved.
Molecular rigidity also effect fluorescence rates. The Spin-orbit coupling effect has to do with increasing the number of possible total magnetic moments (J) in the molecule such that the spin of the system becomes less well defined. This can also be done if the spn density can take a number of orientations due to molecular fluctuations. This is why rigid planar molecules tend to be fluorescent.
Excited state outcomes are all about kinetics, so one also needs to reduce non-radiative decay as well. This will increase both fluorescence quantum yields and phosphorescence. If you want more emission, get rid of floppy side groups or dipolar functions, which easily couple to bath states in polar solvents. You may also change the solvent. Tough question.
Peter
The question is to promote phosphorescence at the cost of fluorescence that means increasing the non-radiative intersystem crossing by increasing spin-orbit coupling.
Thank you all for the nice discussion. Is there any reason to break the conjugation in the fluorescein by the quaternary carbon (spiro) ?
Due to aggregation/dimer of molcules. Take a look on "The exciton model in molcecular spectroscopy" by Kasha
http://www.degruyter.com/view/j/pac.1965.11.issue-3-4/pac196511030371/pac196511030371.xml
It can be converted by using the external heavy atom (solvent) effect, and looking at the emission at low temperature (77 K). See figure 3 in this paper:
Article Photoinduced Intramolecular Electron Transfer in a Bridged C...
a later edition of the Turro book is for free on the web as scanned google-book; pg 191 describes the heavy atom-effect see there
http://books.google.nl/books?id=KPlz1EEDiXsC&pg=PA76&hl=zh-CN&source=gbs_toc_r&cad=3#v=onepage&q&f=false
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