Sevryukova M. Exciton processes in polar and centrosymmetric aggregates and crystals with intramolecular charge transfer.

In this thesis, the electronic properties of molecules and crystals of two donor-acceptor compounds which differ only in the replacement of the Cl atom for Br and the "model" of the merocyanine compounds in a wide temperature range have been studied. Excited states in these compounds with optical excitation, in particular at low temperatures, are investigated. The objects of research in work are solutions, films and crystals of a number of new merocyanine dyes with intramolecular charge transfer. It is experimentally found that the slight differences in the molecular structure of the two donor-acceptor compounds associated with the replacement of atoms with different electric negative charge (Cl with Br) lead to the following changes: one of the compounds containing the atom Cl, exhibits high cubic nonlinearity, and in the aggregated state forms J-type structures. Another compound, containing the Br atom, shows the generation of the second harmonic and the two-photon photoluminescence, as well as forms H-type structures in the aggregated state. The influence of conformational and electronic structure of investigated molecules on their aggregation in organic solvents and liquid crystal matrix is studied. It has been experimentally proved that a polymethine π-conjugate chain contributes to the formation of one-dimensional J-type aggregates (with a long-wave spectral shift) and a centrosymmetric crystalline structure of a substance. The polyene structure of the chain contributes to conformational changes in the molecule, which lead to formation of the H-type aggregates and polar crystalline structure of the substance. Thus, the π-conjugate system participates in the process of charge transfer in such compounds as an independent functional fragment. The change in the state of the π-conjugate chain enables to control the hyperpolarizability of the molecules and to influence the structure of formed aggregates. Investigation of nonlinear optical properties of the crystalline state of these compounds allow classifying its symmetry and the manifestation of excitonic states with charge transfer. The dynamics is studied and the scheme of electron excitation transfer and the ways of its relaxation in the molecular and crystalline states of the investigated compounds is proposed.