Emissivity and Photoconductivity of Organic Molecular Crystals

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Abstract
The materials investigated in this work were of three extreme types with regard to their fluorescence characteristics: (a) Materials which are strongly fluorescent at room temperature in both the solid and the dilute neutral solution, and in which quenching of all sorts is small; (b)Materials which are nonfluorescent in both solid and dilute neutral solution at room temperature and also at —190°C, but which are strongly phosphorescent at —190°C, and in which the intersystem crossing-rate constant is very large (kIS∼1012 sec—1); (c) Materials which fluoresce strongly in dilute neutral solution at room temperature, but in which the fluorescence efficiency decreases markedly with increasing concentration until in the single crystal it may be absent or weak, and in which the self-quenching rate constants of a bimolecular or higher-order process must be very large. Many materials intermediate in their luminescence behavior between these extremes were investigated, and, in particular, materials in which kIS was increased by a number of different ways were studied. These methods all function by increasing the spin—orbital coupling in the molecule and are: (a) Appending a heavy atom, as a substituent, to the molecular framework, (b) Decreasing the S′↔T energy split, (c) Forming charge-transfer complexes, (d) Designing the molecule such that η,π* levels intervene between Sπ,π′ and Tπ,π states. The conclusions to be drawn are as follows, and it is to be emphasized that the reservations appended to them in the text are not to be forgotten here: (1) Photogeneration of carriers proceeds with significantly higher probability from the lowest singlet excited state than from the lowest triplet state. (2) As a corollary of (1) it may be concluded that the better photoconductors will be found among those species which have simultaneously the highest quantum yield of fluorescence and the lowest fluorescence rate constant. (3) The difference in probability quoted in (1) is such that the intersystem crossing process leading to triplet-state population may be considered in a kinetic sense to be competitively concurrent with charge carrier formation. (4) Photoconductivity increases in a fashion which corresponds roughly to the extent of increasing intermolecular interaction in the solid. (5) As one corollary of (4) it is not surprising that the best photoconductor investigated by us, namely rubrene, shows complete self-quenching of fluorescence in the solid. (6) The process which provides energy for the carrier generative act is predominately, or perhaps even completely, communal; it requires the participation of two or more molecules. It ma be first order, but it is not monomolecular. It is possible that conclusion (2) is relevant only because the singlet exciton is more mobile than the triplet exciton and can more readily attain the surface or an appropriate defect where it then degrades to a triplet, or because a biphotonic bimolecular process is involved, one molecule being a S′ species, the other a triplet species. The simple biphotonic process, in which the second photon causes excitation entirely within the triplet manifold of a previously excited molecule, is not excluded either.