Vibronic spectra of charge-transfer excitons in donor–acceptor molecular crystals

Abstract

The linear absorption spectra of a molecular stack of alternatingly and regularly arranged donor and acceptor (D, A) molecules have been studied theoretically for the vibronic spectral regions. The vibronic excitations consist of charge-transfer excitons (CTEs) plus one or two quanta of the intramolecular vibrational mode of the D (A) molecule. The model of CTEs on a DA molecular stack considered by Haarer, Philpott and Morawitz is extended by including the linear and quadratic coupling between CTEs and intramolecular vibrations and represents the basis for the theoretical calculations of the linear optical susceptibility and of the absorption coefficient using the methods of Green’s functions and of canonical transformations. The parameters of the CTEs and of the vibrations obtained in the experimental studies of the absorption, reflection and electroreflectance spectra of the anthracene-PMDA crystal have been introduced in the numerical simulations of the absorption coefficients in pure excitonic, in one- and in two-phonon vibronic spectra. In the case of a relatively weak transfer of the components of the CTEs (of electrons between acceptors and holes between donors) the vibronic spectra will manifest the bound (one-particle) exciton–phonon states only. In the case of stronger transfer, the vibronic absorption spectra will be modified by the impact of many-particle (unbound) exciton–phonon states and the absorption curve will be asymmetrical and broadened (non-Lorentzian). The calculated derivative of the absorption coefficient gives information on vibronic states in both cases of weak and strong transfer.