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Measurements with linearly polarized exciting and fluorescent light show that the excimerlike fluorescence of pyrenebutyric acid in alkaline solution is polarized. The degree of polarization r depends on temperature and, very pronounced, on the wavelength of excitation. This occurs in the spectral region where the excitation spectrum of the excimer differs from the corresponding monomer spectrum. The value of r, after correction of rotational depolarization, is 0.36 and indicates that single transition moments of absorption and emission coincide nearly in their direction. These results, together with the fluorescence time decay laws, strongly support the conclusion that, at least in an hydrophilic surrounding, not excimers but ground-state stable dimers of pyrene are formed.
As far as indicated by the spectra and the fluorescence time decay law, the properties of the
fluorescence of poly(butylene terephthalate) (PBT) are almost identical to those of PET. The groundstate-
stable sandwich dimers in the noncrystalline regions of uniaxially-drawn PBT films were used as chainintrinsic
fluorescent labels for studying the orientation distribution function. The dimers are formed only
in the amorphous regions; concerning the fluorescence signal, "amorphous" includes all the material outaide
perfect crystallites. During deformation, the orientation coefficient <P2>F of the dimers follows approximately a superposition curve of crystallite-like orientation, easily separable in the initial range of stretching ratio λ < 2.5, and of true-amorphous orientation of the affine network type that becomes noticeable at λ > 3. The total fluorescence intensity of PBT, if excited at the proper wavelength, is proportional to the concentration of the dimers; it remains constant in the initial range of stretching, i.e. λ < 1.5. At higher λ, correlated to the increase of amorphous orientation, a strong increase of the dimer concentration, up to a factor of 6, is
observed.