Reflection and absorption spectra of singlet charge transfer excitons in anthracene–PMDA crystals

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Abstract
The polarized reflection spectra of single crystals of the weak charge transfer (CT) complex anthracene–pyromellitic dianhydride (A–PMDA) have been measured at 2 K. The oscillator strengths and energies of the singlet charge transfer transitions have been determined from the frequency dependent dielectric function derived from the reflection spectrum by means of a Kramers–Kronig transformation. A theory of exciton–phonon coupling appropriate for CT transitions in a mixed stack system is briefly described and used to interpret the derived absorption spectrum. The absorption spectrum consists of a purely electronic zero phonon line (ZPL) and a set of zero lattice phonon lines (ZLPL) corresponding to the excitation of high frequency intramolecular modes of the ions. Each ZLPL, like the ZPL, acts as the origin for several librational modes and for a progression in a lattice mode of approximately 30 cm−1. Exciton–phonon coupling is weakest for the vibrationless ZPL and increases with the energy of the intramolecular vibrational mode of the corresponding ZLPL. Approximate values for the exciton–phonon coupling parameter S are found for the ZPL and each ZLPL using the local excited state model.

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