The time-resolved fluorescence behavior of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) has been investigated by synchrotron radiation. Solutions of different concentrations of PEN dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (10-4 and 10-2 M) have been excited at a wavelength of 340 nm, and the emission has been monitored in the wavelength range from 360 to 480 nm in 10 nm steps. The measurements have been performed at two different temperatures (288 and 298 K). Nearly all decay curves can be fitted by a sum of three exponential functions. These three exponential decay time components seem to be parameters which give the main values of a distribution of decay times. We obtained such distribution functions by assuming a simple model on the basis of excitation energy migration from monomer units (donors) to excimer sites (traps) by a Förster type mechanism. These distribution functions can be fitted by a sum of three exponential functions with reliable confidence parameters. The fits on the calculated distribution functions are in good agreement with the fits obtained from the measured data. We compared these results with data obtained on amorphous PEN films excited at the same energy. Again three exponential functions are needed to fit the measured data. The measured fluorescence decay curves can also be explained by our model.

Charge transfer (CT) interactions of the dibutyl ester of N-(4-carboxyphenyl)trimellitimide (NCTD) dissolved in organic solvents have been investigated by means of UV absorption and fluorescence spectroscopy and time-resolved fluorescence measurements. The spectroscopic data are interpreted in connection with quantum mechanical calculations using the AM1 and ZINDO/S method. The electronic transition lowest in energy, observed as an absorption tail in the UV spectra, is interpreted as a direct intramolecular CT excitation from the N-(4-carboxyphenyl) moiety (donor) to the trimellit moiety (acceptor). Independent of the excitation wavelength, a dominant red-shifted CT emission is observed in the fluorescence spectra. Thus, in addition to the direct CT excitation, indirect methods of CT formation exist. In concentrated solutions ground-state stable dimer formation is observed. Direct monomeric CT excitation and ground-state stable dimer excitation takes place in an overlapping energy range. The ground-state stable dimer fluorescence appears blue-shifted with respect to the monomeric CT fluorescence. Ground-state stable dimers exist that have decay times longer than those of the monomeric CT species. As a distribution of decay times is observed in the fluorescence decay curves, dimers with different decay times exist. According to the performed quantum mechanical calculations, dimers with intermolecular CT character are formed.

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Dimethyl 2,6-naphthalene dicarboxylate (DMN) was embedded in a poly(methyl methacrylate) (PMMA) matrix. According to the molecular weight of the repeating unit of PMMA different samples containing 0.02 up to 8 mol % DMN have been prepared. Starting from 1 mol % DMN content, excimer formation is observed. From time-resolved fluorescence experiments the fluorescence lifetime of the monomer was derived to be 15 ns and the excimer lifetime amounts to 30 ns. It was shown that Förster excitation energy transfer takes place among the DMN monomers and the Förster radius of the monomer was determined by time-resolved fluorescence depolarization experiments to be 23 ± 3 Å. Excitation energy transfer is assumed to be the dominant process for the excimer formation. Using a model of three-dimensional transport among DMN donors until an excimer trap is reached, dimensionless trap concentrations of 0.005 and 0.014 were derived for the samples with 1 and 2 mol % DMN, respectively. For larger DMN concentration the excitation energy transfer between DMN molecules becomes anisotropic.