Charge-Transfer Exciton States in Aromatic Molecular Crystals

Abstract

In this paper we consider some of the consequences of the mixing of neutral exciton states (neutral excitation waves or Frenkel excitons) with charge‐transfer exciton states (ion‐pair excitons) in the oriented gas model of aromatic molecular crystals. The lowest‐energy charge‐transfer exciton is described by suitable symmetry‐adapted functions built on the basis functions of a near‐neighbor positive and negative ion pair. It is then shown that the energy of this state is not greatly different from the p states of anthracene and naphthalene, and therefore that the ion‐pair exciton and Frenkel exciton states can mix. For the cases of anthracene and naphthalene, such configuration interaction leads to the following contributions to the Davydov splitting of the triplet manifold: $$\begin{array}{c}\text{Δε=26/ΔE\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{naphthalene},\\ \text{Δε=31/ΔE\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{anthracene},\end{array}$$ where ΔE is the energy difference (in electron volts) between the triplet level and the near‐neighbor charge‐transfer level. In making the computations, it is necessary to consider carefully the effects of vibronic coupling and of configuration changes in the excited state. It is concluded that charge‐transfer excitons can make an important contribution to the Davydov splitting of a triplet manifold. This conclusion is supported by preliminary estimates of the interaction matrix elements for crystalline benzene.