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Abstract

Fluorescence lifetimes and time-resolved fluorescence anisotropy decays of the voltage-sensitive styrylpyridinium dye RH421 have been measured in various organic solvents and when bound to both anionic and cationic detergent micelles. At least two and often three exponential functions were necessary to fit the decay data. In organic solvents the form of the lifetime and anisotropy decays were dependent on the emission wavelength. In the red part of the emission spectrum an initial rise in fluorescence after the excitation pulse could be observed. This indicates the presence of a precursor-successor mechanism, in which a second fluorescent species is formed in an excited-state reaction. The initial fluorescence anisotropy (at zero time) was also higher for blue-detected emission than at the red edge, indicating a change in the angle of the emission transition moment of about nine degrees during the excited state lifetime. The results are discussed in terms of the formation of a fluorescent twisted intramolecular charge transfer state, occurring simultaneously with the relaxation of the surrounding solvent cage. The results in micellar systems suggest a distribution of dye over different sites within the micelle.