Electromodulation of fluorescence in a crystalline organic photoconductor (thionaphthenindole)

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Abstract
The influence of electric fields on the fluorescence of polycrystalline layers of thionaphthenindole has been studied by an electromodulation technique that allows internal electric fields to be accounted for the effect. The in-phase first harmonic (1ω) and out-of-phase second harmonic (2ω) of the fluorescence response to a modulating (sinusoidal) electric field of frequency ω reveal different electric-field behavior, the 2ω response providing nearly internal field-free signal suitable for verification of theoretical models. Fluorescence quenching with this signal has been observed and attributed to electric-field modulation of the probability of charge separation within an excited state as a precursor. It was found that fluorescence quenching data could not be well explained using theoretical models formulated by Onsager, Poole–Frenkel, and Noolandi–Hong–Popovic for the charge separation via delocalized charge-transfer excitons. The macrotrap model which reconciles both the experimental data and their physical interpretation is proposed. The model attributes the fluorescence quenching to carrier photogeneration by the field-assisted thermal dissociation of a trapped charge-transfer exciton which has a higher located molecular singlet S1 state as a precursor. An analysis of 1ω fluorescence signals provides information on the internal fields evaluated on ≂105 V/cm and attributed to spontaneous polarization effects in ordered crystalline layers of this polar compound, in accordance with conclusions drawn previously from electroabsorption data.