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The electronic spectra of crystalline charge-transfer complexes. Lower, Stephen Kent


Charge-transfer complexes of the aromatic hydrocarbon-trinitrobenzene type are stabilized principally by interaction between a dative- and a "no-bond" state: φN= a ψ₀ (DA) + b ψ₁ (D⁺A¯) . The resulting complexes usually possess a colour which is associated with an electronic transition between the ground state φN and a predominantly dative excited state; the transition therefore corresponds to the partial transfer of an electron from the donor (D- usually the hydrocarbon) to the acceptor (A). Most of the previous spectroscopic work on CT complexes has been concerned with solutions of the complexes; no detailed investigation of a crystalline complex has been made. The somewhat random nature of CT interaction in solution tends to limit the amount of information we can gain from such studies, and this has prompted the present work on solid complexes. With a system of definite composition and structure, both the spectroscopic examination and theoretical treatment can be carried out at a more quantitative level. The results of a detailed spectroscopic study of the crystalline anthracene-trinitrobenzene complex are presented. It is shown that 1) The CT transition moment is strongly polarized parallel to the c-axis of the crystal (approximately perpendicular to the molecular planes), with a polarization ratio greater than that predicted by the simple oriented gas model. 2) A portion of the intensity of the CT state is derived from mixing with the more intense ¹La state of anthracene, confirming the prediction of J. Murrell. 3) The crystal emission spectrum consists almost entirely of CT fluorescence with vibrational structure similar to that of the absorption. The entire spectrum is blue-shifted with respect to that of the rigid-glass solution; as in solution, there is a very wide gap between the absorption and fluorescence origins. 4) The emission intensity is strongly temperature dependent; a portion of this dependence is reversible, while the remainder is apparently connected with the creation of crystal defects on cooling, and cannot be reversed. It is suggested that the CT state is highly delocalized in the crystal, and that the transition dipole moment connecting the ground- and CT states must be polarized along the c-axis of the crystal, rather than between the centers of the individual donor-acceptor pairs. A number of TNB complexes of other hydrocarbons were studied, although in much less detail. Their spectroscopic properties appear to be similar to those of anthracene-TNB. Polarized phosphorescence- and triplet-triplet absorption spectra of several aromatic hydrocarbons dissolved in crystalline benzophenone are given. This is the first reported observation of triplet-triplet absorption in crystals, and it is shown that it is polarized mainly in the planes of the molecules investigated.

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