THE INVERTED REGION OF ELECTRON-TRANSFER IN INTRAMOLECULAR FLUORESCENCE QUENCHING OF QUINOLINIUM BETAINES

Fluorescence quenching through intramolecular electron transfer (ET) was studied in N-(omega-sulfonatoalkyl)- and N-(omega-carboxyalkyl)-quinolinium betaines by stationary and time-resolved fluorescence spectroscopy in the solvents water and acetonitrile. These substances are zwitterionic in the ground and local excited state of the quinolinium chromophore. In the sulfonato compounds fluorescence quenching through ET does not occur in water, whereas quenching is efficient in acetonitrile. A surprising opposite behaviour is observed for the carboxylato compounds: quenching is efficient in water whereas it is inefficient in acetonitrile. Semiempirical calculations were used to derive the parameters needed for the description of the quenching process by non-adiabatic Bixon-Efrima-Jortner-theory. These parameters include the solvent (lambda(s)) and the intrinsic (lambda(v)) contributions to the total reorganisation energy lambda and the approximate free energy of reaction Delta G degrees. The vibrational motion of an aromatic C-H bond influenced by the interaction with the negatively charged donor was taken as the major intramolecular accepting mode. ET fluorescence quenching in the present compounds and solvents may be interpreted in terms of the ''normal'' and the ''inverted region'' of Marcus theory.