Measurements of polarized excitation and fluorescence spectra were carried out with thin films of PPV and with films of oligomer model compounds in a polystyrene matrix. The degree of fluorrescece polarization r and its spectral dependence are discussed with respect to the transfer of excitation energy and the mutual alignment of the flurophores. In contrast to the behaviour of conventional aromatic polymers, the value of r is non-zero in isotropic PPV and depends on preparation (up to r = 0.25 at λexc = 460 nm and λflu = 555 nm). r can be nearly as high as in the dilute solid solution of the oligomers. This is easily understood if the PPV segments are highly aligned in the distance range of energy transfer.
If the intramolecular distance between the two isoenergetic chromophores of a bichromophoric compound is much smaller than the Förster radius of excitation energy transfer (EET), the probability to fluoresce is equal for both chromophores, because the EET takes place at a shorter time compared with the nanosecond timescale of the fluorescence lifetime. Then, the fluorescence depolarization of diluted solid solutions of such bichromophoric molecules can be measured with stationary excitation and used to evaluate the orientation between the two chromophores; in the most simple case the angle between their main axes is determined. Model calculations for bichromophoric alkane compounds with various conformational freedom reveal the amount of depolarization and thus the limits of the method. The results are compared to experimental data of 1,3-di(N-carbazolyl)-propane and 1,2-trans-di(N-carbazolyl)cyclobutane.