Redox and ionic-binding switched fluorescence of phenosafranine and thionine included in Nafion® films

This report describes electrochemical and optical properties of azine-loaded Nafion() film electrodes that generate redox-activated optical signal. The apparent charge diffusion coefficient in the film decreased abruptly as the solution pH increased from 0.4 to 3, probably because deprotonation of the dyes lowered the rate of intermolecular electron hopping. Reversible switching of the fluorescence of phenosafranine was performed by electrochemical oxidation/reduction cycles. Both of the redox couple were isolable and stable; the reduced form was non-fluorescent, whereas the oxidized form was strongly fluorescent. Since the photonic output depended on chemical binding, the electrode system could operate as a dual-mode switching device. The second protonation of the dye molecule changed the fluorescence intensity sufficiently for chemical signaling of the photonic output, because the fully protonated oxidized-form was non-fluorescent; however, a pK(a1) of 0.8 was too low to change the pH rapidly. 1,1'-ferrocenedimethanol quenched the fluorescence of the oxidized form efficiently to cause the 'switching off' of fluorescence. In the presence of the guest, however, the dye-loaded film did not recover the fluorescent state at any potential, because the cationic dye formed a nonfluorescent complex with the guest incorporated into the film. (C) 2002 Elsevier Science Ltd. All rights reserved.