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The influence of electric fields on the fluorescence of several organic photoconductors was studied. The materials investigated were two cyan pigments, the X and β polymorphs of metal-free phthalocyanine (H2Pc), one magenta pigment (NRB), and one yellow pigment (DFC). In both H2Pc polymorphs and NRB, fluorescence quenching was observed and attributed to carrier generation with the first excited singlet state as a direct precursor. In DFC, fluorescence enhancement, rather than quenching, was observed which is believed to be due to a strong coupling of the external electric field to the dipole moment in the excited state. For DFC, no definite conclusion about which state is a precursor to photogenerated carriers could be made. It was found that the fluorescence quenching data could not be fitted well by theories based on an isotropic continuum approximation. A simple empirical formula which fits the experimental data remarkably well is proposed.
Popović et al. (Sat,) studied this question.