Experimental Evidence for Localized Excitons in the Spectra of Charge-Transfer Complex Molecular Crystals

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Abstract
The polarized absorption and emission of the anthracene—TNB complex crystal are shown to provide evidence for localization of low-energy excitation in this crystal. The absorption and fluorescence polarization ratios at 300°K are very different (5.6:1 and about 15:1, ‖c: ⊥c on 110) the former being close to an oriented gas value. The latter corresponds to a charge-transfer complex more closely aligned with the c axis of the crystal. The motion of the excited molecule is in accord with the behavior of a dipole in an electric field which is nearly along the c axis. Possible ramifications of this effect in photoconductive charge-transfer complexes are discussed. The influence of temperature on the luminescence spectrum of anthracene—TNB crystals is discussed, and apart from some artifacts due to cracking of crystals, the expected increase of emission intensity with decreasing temperature is observed.