Steady-State and Dynamic Fluorescence Measurements of a Perylene-Labeled Triblock Copolymer:  Evidence for Ground-State Dye Aggregate Formation

Steady-state fluorescence and fluorescence decay measurements were carried out on a perylene-labeled triblock copolymer, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), both in toluene solution and in a thin film. For the polymer solution, the data fit well to a double-exponential function, and the same two lifetimes τ short = 4.4 ns and τ long = 9.2 ns, but with different relative intensities, are recovered at different emission wavelengths. The longer lifetime, τ long = 9.2 ns, is attributed to the presence of dye aggregates, with τ short = 4.4 ns assigned to unassociated dye. This interpretation of decay profiles is supported by steady-state fluorescence measurements: the onset of aggregate emission corresponds to a sharp decrease in the ratio of preexponential factors, A short /A long. Excitation spectra of the labeled polymer are found to be dependent on the emission wavelength, suggesting that the population of aggregated dye molecules exists in the ground state prior to excitation. The fractional contribution of unimer fluorescence in the polymer film (f short = 0.36−0.10) is much smaller than that in solution (f short = 0.97−0.84), suggesting a higher degree of aggregate formation in the film.