Sensitized Photoconduction and Sensitized Delayed Fluorescence of Anthracene Single Crystals

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Abstract
The literature on the sensitized photoconduction and the sensitized delayed fluorescence of anthracene single crystals is briefly reviewed. Some peculiarities of the heterogeneous system anthracene single crystal/aqueous dye solution are described which are important for the correct interpretation of experimental data. The mechanism of sensitized hole injection, as proposed by Mulder and de Jonge, is confirmed for the dye rhodamine B: In the absence of strong oxidants and of heavy atoms and at high field strengths free holes are generated by CT-quenching of excited singlet states of the adsorbed dye, 1 D∗, by the anthracene crystal, A. The quantum yield of the reaction 1D∗ + A → D− + A∔ is ∼ 1. Complications arising by the presence of strong oxidants are discussed in detail. Triplet excitons are generated not only by triplet-triplet-ET, but also by recombination of the radical ions: D- + A∔ → D + 3A∗. The often unusual time dependence of the sensitized delayed fluorescence of anthracene crystals is explained by an increase of surface quenching of triplet excitons during constant excitation. With the same heterogeneous systems, but with reversed polarity, a weak sensitized delayed electron injection is observed. It is probably caused by the quenching of triplet excitons by semiquinone radicals D -.