Effects of structural disorder on the optical properties of molecular crystals

In this paper we advance a model for absorption line shapes of Frenkel excitons in molecular crystals, considering the effects of static disorder and of weak exciton–phonon coupling. Our treatment is limited to single-particle excitations located at the bottom of the exciton band. The effects of disorder scattering at zero temperature are handled by applying the average t matrix, single-site approximation to a system characterized by a Gaussian distribution of site excitation energies. The simultaneous effects of disorder scattering and of phonon scattering were treated by the introduction of an effective exciton–phonon Hamiltonian, which was subsequently utilized within the framework of the single-site approximation for disorder scattering at finite temperatures. Model calculations of the optical absorption line shapes were performed for a quasi-one-dimensional system. The zero-temperature line shapes are asymmetrically broadened, the high energy edge being close to a Lorentzian, while the low energy edge decreases fast with decreasing energy. This asymmetry disappears at higher temperatures where phonon scattering effects dominate. We propose that the asymmetric low-temperature absorption band in 1, 4-dibromonaphthalene originates from the effects of structural disorder. The asymmetric optical line broadening experimental data for this quasi-one-dimensional system are well accounted for in terms of our theory.