Free and self-trapped charge-transfer excitons in crystals of dipolar molecules of N,N-dimethylaminobenzylidene 1,3-indandione

In this paper we consider the spectral properties of exciton transitions of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) films and crystals, which are typical examples of polar molecular compounds. A large dipole moment in the electronic ground state and the considerable change (up to 12 D) of this value by excitation of a free DMABI molecule induce strong polarization of the surroundings, which causes the remarkable self-trapping effects in the crystal. The exciton-phonon coupling value g = 1.87 is estimated for the crystal. Charge-transfer (CT) exciton formation is observed in the absorption and luminescence spectra by analyzing the spectral behavior at various thicknesses of DMABI films. The CT excitons created along molecular stacks dominate in the spectra of the DMABI crystals. Coexistence of free and self-trapped CT excitons is discovered at elevated temperatures. The exciton band parameters are estimated, and the adiabatic energy potential energy for two self-trapped excitons is suggested. Relaxation paths of shallow short-lived and deeply trapped long-lived self-trapped CT excitons are discussed. Mechanism of two-photon absorption and photoconductivity of DMABI crystals and films are considered.