External electric field effects on fluorescence of methylene-linked D-A systems in polymer films: (Carbazole)-(CH2)(n)-(terephthalic acid methyl ester)

External electric field effects on intermolecular exciplex fluorescence of (carbazole)-(CH2)(n)- (terephthalic acid methyl ester), denoted by D-(n)-A, have been examined at various concentrations in PMMA polymer films. At low concentrations, exciplex fluorescence of D-(3)-A is markedly enhanced by an external electric field, whereas the field-induced enhancement for D-(20)-A is very small. For both compounds, the magnitude of the fluorescence enhancement decreases with increasing concentration, and quenching occurs at high concentrations. The enhancement observed at low concentrations is interpreted in terms of the electric field effects on the charge recombination process of an ion pair, which occurs at zero field and leads to fluorescence quenching. The methylene-chain length dependence indicates that such a recombination dominantly occurs as a through-bond intramolecular process. The field-induced quenching of exciplex fluorescence at high concentrations is attributed to a field-assisted dissociation into free carriers. In contrast with the exciplex fluorescence, monomer fluorescence of carbazole chromophore is quenched by an electric field at all the concentrations with the tendency that the efficiency of quenching monotonically increases with increasing concentration. The quenching of the monomer fluorescence is attributed to a field-induced increase of the rate of intermolecular electron transfer. A large difference in polarizability between the ground state and the emitting state is shown to be the dominant factor of the Stark shift of the exciplex fluorescence.