Kinetics of Fluorescence Quenching by Inorganic Anions
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Kinetics of Fluorescence Quenching by Inorganic Anions A.
R. Watklns
Max-Planck-institut fur Biophysikalische Chemie, D 3400 Gottingen-Nikobusberg, West Germany (Received March 1 1, 1974; Revised Manuscript Receivsd July 2, 1974) f’ubiication costs assisted by the Max-Planck-lnstitut fur Biophysikalische Chemie
The rate constants for the fluorescence quenching of a series of aromatic hydrocarbons by inorganic anions in acetonitrile are reported. For those systems where efficient quenching occurs, a correlation can be observed for any one anion between the measured rate constants k and AG‘, the excited state reduction potential of the aromatic. A similar correlation is also found between k and the calculated redox potentials of the anion quenchers for any one aromatic. In the region where AG‘ becomes large and the electron-donating ability of the anion is small, k becomes small and these correlations disappear. The free-energy change accompanying a possible electron transfer process in the excited state can also be calculated; this is, with three exceptions, positive, showing that in the majority of cases an electron transfer mechanism is energetically not favored. The other commonly accepted mechanism, a heavy-atom perturbation of the spin-orbit coupling of the excited aromatic, appears to be relatively unimportant for quenching by these anions, and probably has a slight influence only in the region of weak quenching and for systems quenched by iodides.
There has been much speculation about the mechanism by which fluorescent molecules are quenched by inorganic ions, in particular by inorganic anions. In reviewing the work which had been carried out on such systems up to 1950, Forster’ came to the conclusion that the quenching process consisted essentially of electron transfer from the anion to the excited molecule with the formation of the corresponding radical anion. This has received support from more recent work”:
