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Relaxations in Poly(vinyl alcohol) and in Poly(vinyl acetate) Detected by FluorescenceEmission of 4-Aminophthalimide and Prodan

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Steady-state and time-resolved emission spectroscopy (TRES) of the medium-sensitive probes 4-aminophthalimide (4-AP) and 6-propionyl-2-(dimethylamino)naphthalene (Prodan) were performed at 77 and 298 Kin vacuum-sealed thin films of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc). The two probesshow similar red-edge effect in steady state emission and a red shift with time in TRES in PVA. In PVAc thered shifts are much smaller and the spectral shift for 4-AP is slower. 4-AP locates in highly polar environmentsin PVA, where H-bond interaction with the polymer is important. Prodan locates in less polar environments,as evidenced by the position of the emission maximum with respect to reference solvents. Consequently, theobserved monoexponential spectral red shift with time of 4-AP in PVA and in PVAc is attributed to relaxationof the interaction of the probe with the hydroxy and acetate moieties, respectively. The more intense interactionof the lighter −OH moiety with the probes explains the greater and faster spectral shift observed in PVAcompared to PVAc. The lifetime of this monoexponential spectral shift is independent of temperature inPVA and takes place with a highly negative activation entropy. This fact is attributed to a collectiverearrangement of −OH groups to better interact with the excited state. This relaxation nevertheless does notaccount for the complete accommodation of the excited state. Prodan shows a linear variation of the spectralshift with time that can be explained by microheterogeneity. In PVA, the width at half-maximum of theemission spectra does not change with time for Prodan and it decays with a lifetime similar to the lifetimeof the spectral shift in the case of 4-AP. The differences in the behavior of the probes are attributed to theirdifferent average location in the polymer matrix.

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