Theoretical evaluation of the frequency and intensity of low-energy charge-transfer transitions in aromatic hydrocarbon crystals. I. Anthracene

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Calculations are reported for the low-energy part of the charge-transfer (CT) spectrum of anthracene. It is shown that the CT states previously obtained from an electrostatic model are split by intracell and intercell electron and hole transfer integrals. The model chosen for detailed calculations consists of a quasilinear crystal with two molecules per unit cell and only nearest-neighbour interactions. Parameter values are derived from earlier electrostatic and quantum-chemical calculations and from spectroscopic observations. The resulting energy levels and band intensities are compared directly with the experimental electromodulation spectra of anthracene. It is shown that, contrary to an earlier analysis, these spectra show evidence for four rather than three CT bands in the range 3.50–3.80 eV, located at 3.528, 3.590, 3.658 and 3.760 eV. Calculated CT energies and relative intensities compare favourably with these experimental results, leading to unambiguous assignments of the bands as components of the ( 1 2 1 2 0) and (010) CT states. These assignments imply predictions about the polarization of these bands which remain to be tested experimentally.