Fluorescence behavior of a pyrene-end-capped poly(ethylene oxide) in organic solvents and in dioxane-water mixtures.
A poly(ethylene oxide) polymer end-labeled with two pyrene units (M(w) = 9500 g/mol and abbreviated as Py(2)PEO) was investigated in dioxane-water mixtures and several organic solvents by steady-state and time-resolved fluorescence techniques. In dioxane-water mixtures, the fluorescence emission spectra indicate that with the gradual addition of dioxane to the solvent mixture, the maximum of the pyrene monomer band remains practically constant, whereas the excimer band displays a blue shift of approximately 8-10 nm. From time-resolved fluorescence data, two- and three-exponential decays at the pyrene monomer and excimer emission wavelengths, respectively, were obtained. Two of the decay times (tau(1) and tau(2)) are identical at all emission wavelengths. The additional shorter decay time (tau(3)), which is only observed at the excimer emission wavelength, was attributed to the component responsible for the blue-shifted excimer maximum, that is, to a second excimer, uncoupled to the other two species. Thus, the time-resolved fluorescence data suggests that one pyrene monomer is able to form either one or two excimers (E(1) and E(2), coupled and uncoupled, respectively). The sum of the pre-exponential factors, associated with the coupled species, at the excimer emission wavelength in dioxane-water mixtures always differs from zero. This, together with differences in the excitation spectra when collected at the monomer and excimer wavelengths, revealed that a static route is partially responsible for E(1)* formation. In the case of pure organic solvents, the proposed kinetic scheme can be simplified. In this case, the sum of the pre-exponential factors at the excimer emission wavelength is practically zero, which is a direct consequence of the dynamic mechanism being the only route for E(1)* formation. The kinetic scheme has been solved, and the rate constants for excimer formation (k(a)), dissociation (k(d)), and excimer decay (k(E)) are presented.
Costa
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Department of Chemistry, University of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal.
Seixas de Melo
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J
Burrows
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HD