FREE AND SELF-TRAPPED CHARGE-TRANSFER EXCITONS IN CRYSTALS OF DIPOLARMOLECULES OF N,N-DIMETHYLAMINOBENZYLIDENE 1,3-INDANDIONE

In this paper we consider the spectral properties of exciton transitio ns of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) films and cr ystals, which are typical examples of polar molecular compounds. A lar ge dipole moment in the electronic ground state and the considerable c hange (up to 12 D) of this value by excitation of a free DMABI molecul e induce strong polarization of the surroundings, which causes the rem arkable self-trapping effects in the crystal. The exciton-phonon coupl ing value g = 1.87 is estimated for the crystal. Charge-transfer (CT) exciton formation is observed in the absorption and luminescence spect ra 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 par ameters are estimated, and the adiabatic energy potential energy for t wo self-trapped excitons is suggested. Relaxation paths of shallow sho rt-lived and deeply trapped long-lived self-trapped CT excitons are di scussed. Mechanisms of two-photon absorption and photoconductivity of DMABI crystals and films are considered.