376Results of quantum-chemical studies of the nile red (NR) molecule and its protonated structures by the INDO/S method are presented. It is demonstrated that the best agreement between the calculated and experimental data is obtained for the flat molecule in the ground electron state. Energies of the strongest singlet and triplet electronic states, molecular nature of these states, transition oscillator force, dipole moments in the ground and excited states, electron density distribution around atoms and molecular fragments in the S 0 and S 1 states, and rate constants of radiative, internal, and intercombination conversion are presented for the NR molecule and its protonated structures. The most probable NR protonation centers are analyzed using the molecular electrostatic potential (MESP) method. It is established that the reaction of proton addition to the NR molecule proceeds at the cyclic nitrogen atom. As demonstrated the results of quantum-chemical calculations, low fluorescent properties of the protonated NR structures (with a quantum yield of 4%) are due to a high rate of the S 1 -T 4 intercombination conversion.Keywords: nile red, protonated nile red structure, quantum-chemical investigations, INDO/S method, electron absorption spectra, fluorescence, photophysical parameters.Nile red (NR) is a widely known yellow-red laser dye that possesses clearly pronounced solvatofluorochromic properties [1]. These properties allow NR to be used to determine polarities of biological objects and polymeric and organic-inorganic systems, including proteins and ormosils, to synthesize optical sensor materials to detect the presence of vapors of various organic solvents, etc. [2][3][4].Owing to the presence of proton-acceptor oxygen and nitrogen atoms in the NR molecule ( Fig. 1) in acidic media, the protonated structure possessing the spectral and luminescent properties that differ from the properties of the neutral molecule can be formed.In the present work, the spectral and luminescent properties of NR and its protonated structures are investigated both theoretically and experimentally. EXPERIMENTALThe spectral and luminescent properties of NR and its protonated structures were theoretically investigated using the quantum-chemical software package based on the method of intermediate neglect of differential overlap (INDO) with special spectroscopic parameterization [5]. The software package allowed calculations of the following characteristics of the electronically excited states of polyatomic molecules: the energy and nature of molecular orbitals, energy of singlet and triplet electronically excited states, oscillator force and polarization of electron transitions, electron density distribution around atoms and bonds of the molecule, and dipole moments in the ground and excited

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