One of the most important features of the parent cuprates such as La2CuO4, which predetermines their unusual behavior under non-isovalent substitution, is high ionic polarizability and proximity to the «polarization catastrophe». We show that at the same time parent cuprates are characterized by a charge transfer instability with the formation of a system of metastable dipole-active «Mott-Hubbard» excitons, or electron-hole (EH) dimers. Non-isovalent substitution shifts the phase equilibrium in the direction of condensation of EH-dimers and the formation of an inhomogeneous EH-liquid, in the simplest model, equivalent to a system of composite bosons. To effectively describe the electronic state of doped cuprates, it is proposed to use the S = 1 pseudospin formalism, which allows one to consider novel charge states of the Anderson RVB-type phase. The recombination of EH-dimers under a critically small value of the energy of local and nonlocal correlations leads to the transition of the system to the Fermi-liquid state.

An analysis of the optical properties of compounds based on 3d elements provides valuable information about the electronic structure of the ground state and low-energy excitations. Thus, we show that the analysis of charge-transfer d-d excitons in the dielectric antiferromagnetic phase of cuprates and metastable low-energy electron-hole EH dimers being a result of their evolution after electron-lattice relaxation, turns out to be very fruitful not only for describing linear and nonlinear optical properties and photoinduced effects, but also to develop a promising model of charge triplets to describe the low-energy electronic structure and T-x phase diagrams of active CuO2 planes in cuprates of the T-La2CuO4 or T’-Nd2CuO4 type, as well as NiO2 planes in nickelates of the RNiO2 type, and their evolution with changes in the main energy parameters.