Melnyk V. Spectral and Luminescence Study of Triplet States in Molecular Systems.

In this work, the laws and mechanisms of photophysical processes taking place in molecular systems with participation of triplet states are presented. For the purpose of this work, benzophenone of different structural modifications, naphthalene and impurity systems based on these compounds are employed. A comprehensive program of experimental studies included wide-scale temperature dependent measurements of the phosphorescence spectra (together with time-resolved phosphorescence spectra), phosphorescence decay kinetics, delayed fluorescence spectra, Raman and IR absorption spectra, differential scanning calorimetry, and measurements of acoustic properties for the objects under investigation. Based on the obtained results, the behavior of triplet excitons in different crystal modifications of the investigated molecular systems was proposed. For the first time among the family of molecular crystals, exciton phosphorescence from benzophenone crystal was discovered. The behavior of triplet excitons for benzophenone in the bulk and thin films was analyzed, and effective triplet quenching in the thin films was observed. It was proven that nano-sized clusters are the basic structure elements of glassy benzophenone. In a case of naphthalene crystal, the nature of structural defects and the mechanism of spin-lattice relaxation for impurity triplet state were identified. For this crystal, the correlation between the spectral-kinetic characteristics of the delayed fluorescence and relative position of impurity molecules was proposed. For heterogeneous systems consisting of benzophenone and TiO2 nanoparticles, the two-phase benzophenone structure was observed containing both the crystal and the amorphous phases. The energy transfer and relaxation processes for the triplet states were investigated. The corresponding mechanisms were found to relate to thermal delocalization of the triplet states into the triplet exciton absorption zone of the matrix. These results can be applied to the development of procedures for synthesis of organic luminophores with controllable properties.