Optical properties of films built up by polar molecules

N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) was considered. The formation of charge transfer excitonstates in the absorption and luminescence spectra by increase of the thicknesses of DMABI films was observed. Theevidence of a strong exciton-phonon interaction and its effects on the exciton absorption and luminescence spectra wasdiscovered. The coexistence of free and self-trapped excitons at elevated temperatures was found in DMABI films. Thedynamics of both shallow short-lived and deeply trapped long-lived self-trapped exciton states in various films andcrystal of DMABI was discussed.Keywords: molecular crystals, N,N-dimethylaminobenzylidene 1 ,3 -indandione, spectroscopy, luminescence,self-trapped excitons, charge transfer1. INTRODUCTIONThe dipole moment of the electronic transition is the main parameter determining the excitonic feature of thespectrum of molecular crystals. The presence of steady-state dipole moments in the ground and excited molecularstates creates the interaction with other molecules of the surrounding. By means of the exciton theory, the latter causesan essential increase of the displacement gy'2 in comparison with the classical crystals and, moreover, strongelectron-phonon or electron-vibrational modes interaction cannot be treated perturbatively anymore. The dispersiveinteraction is responsible for absorption band shifts depending to the polarity of solvents or by modifying the crystalstructure, while strong electron-phonon interactions can create a distinct Stokes-shift of the fluorescence band. Due tothose interactions the spectral properties of polar systems become very sensitive to the structural changes, differenciesin temperature and excitation conditions, and these systems can express optically non-linear behaviour in somespectral regions.3 Therefore, new possibilities of their application to non-linear optics and molecular electronics are