The experimental and theoretical study of electronic spectra of paracetamol in water is presented. Two isomers of the molecule are studied. Quantum-chemical interpretation of absorption and fluorescence spectra of paracetamol is performed by the INDO method using averaged and optimized geometric parameters. It is shown that the calculation results are in satisfactory agreement with the experimental spectra. Two centers of interaction of paracetamol with water are revealed, which allowed us to reasonably construct the geometry of complexes with H-bonds of the paracetamol molecule with water.

The absorption and fluorescence spectra of sulfaguanidine, its complexes with hydrogen bonds, and doubly charged cationic forms have been studied experimentally and theoretically. The orbital nature of electronically excited states is established and a theoretical interpretation of the absorption and fluorescence spectra is given. It was shown that the main reason for the anomalously large Stokes shift of fluorescence is the rearrangement of the benzoid structure of the phenyl fragment of sulfaguanidine into a quasi-quinoid one. The influence of the formation of hydrogen bonds and the addition of a proton on the amount of charge transfer between weakly bound fragments of the molecule has been established.

Sulfonamides are one of the oldest groups of veterinary chemotherapeutic agents. Physico-chemical properties, the concentration and the nature of the environment are the factors responsible for the distribution of sulfonamides in the living organism. Although these drug compounds have been in use for more than half a century, knowledge about their behavior is still limited. Physiological activity is currently attributed to the sulfanyl radical. Our study is devoted to the spectral properties of aqueous solutions of sulfaguanidine, in which the formation of complexes with an H-bond and a protonated form takes place. The nature of the fluorescent state of sulfaguanidine was interpreted using computational chemistry, the electronic absorption method and the luminescence method. The structure of sulfaguanidine includes several active fragments: aniline, sulfonic and guanidine. To reveal the role of fragments in the physiological activity of the studied antibiotic, we calculated and compared the effective charges of the fragments of aniline and sulfaguanidine molecules. Chromophore groups were identified in molecules, which determine the intermolecular interaction between a molecule and a proton-donor solvent. The study also revealed the impact of sulfone and guanidine groups, as well as complexation, on the effective charge of the antibiotic fragment responsible for physiological activity and luminescent ability.

The phototransformation of sulfaguanidine in water under ultraviolet (UV) radiation was studied experimentally and theoretically. Absorption, fluorescence and fluorescence excitation spectra of the studied substance before and after irradiation were obtained. Under the influence of KrCl excylamp radiation, stained photoproducts were formed. Quantum-chemical analysis of the orbital nature and localization of electronic transitions of complexes of photoproducts with water 1 : 3 reveals great similarity with the spectrum of the complex of the parent compound. The energy of electronic transitions of primary photoproducts decreases that is there is a low-energy shift of transitions S0→S1(ππ) and S0→S3(ππ) of the original molecule to long-wave region of spectrum (260-315 nm) and decrease of intensity of transition S0→S3(ππ) of sulfaguanidine coplex. In the process of irradiation under the action of KrCl excylamp, the transformation of sulfaguanidine, its primary photoproducts and their subsequent interaction with each other and the solvent occur, which leads to the appearance of a colored photoproduct absorbing at λmax = 560 nm.

The absorption and fluorescence spectra of charged anionic and cationic forms of bisphenol A (BPA) have been studied. The nature of electronically excited states and the photolysis of the BPA molecule have been interpreted based on results of quantum chemical calculations. The BPA spectra in an aqueous solution are compared during transition from the neutral to the ionic form. Results of calculations have shown that the small value of the fluorescence quantum yield in all examined molecular structures is determined by the high efficiency of the singlet-triplet conversion. In the anionic BPA form, a decrease in the efficiency of radiation decay channel by two orders of magnitude has been recorded caused by a change in the orbital nature of the fluorescent state compared to the neutral and cationic forms. It is shown that the probability of photolysis of the molecule under study increases due to the overlap of the absorption spectra of the anionic form in an aqueous solution under exposure to sunlight.

The stages of development of molecular photonics at Tomsk State University are presented. It was A. N. Terenin’s student, N. A. Prilezhaeva, who pioneered the Russian photochemistry and fostered the development of a Tomsk school of spectroscopists, at that time the only research team in the Asian part of Russia. In 1950, the research in the field of molecular spectroscopy and quantum chemistry of complex molecules was initiated by a Prilezhaeva’s student, V. I. Danilova, who became a TSU Professor at a later date. Fundamental problems were posed and solved, which related the spectra and the electron shell structure with the physical-chemical properties of several classes of complex organic molecules. The generalizations made by Danilova were quite valuable for the theory of molecular spectra and helped forming her own scientific school in 1970−80s.

The absorption and fluorescence spectra of sulfaguanidine, its complexes with hydrogen bonds, and doubly charged cationic forms have been studied experimentally and theoretically. The orbital nature of electronically excited states is established and a theoretical interpretation of the absorption and fluorescence spectra is given. It was shown that the main reason for the anomalously large Stokes shift of fluorescence is the rearrangement of the benzoid structure of the phenyl fragment of sulfaguanidine into a quasi-quinoid one. The influence of the formation of hydrogen bonds and the addition of a proton on the amount of charge transfer between weakly bound fragments of the molecule has been established

The absorption spectra of chloramphenicol aqueous solution were obtained experimentally. The absorption spectra of chloramphenicol isomers and the effect on the spectra of the formation of H-bonded complexes were calculated and interpreted using quantum chemistry methods. Calculation results showed that the absorption spectrum of chloramphenicol by position of bands and their nature is largely determined by the nitrobenzene fragment with little participation of propanol and dichloroacetamide fragments of chloramphenicol. The proton acceptor properties of individual chloramphenicol fragments and the effect of the formation of H-bonded complexes on them have been analyzed.

The absorption spectra of chloramphenicol aqueous solution were obtained experimentally. The absorption spectra of chloramphenicol isomers and the effect on the spectra of the formation of H-bonded complexes were calculated and interpreted using quantum chemistry methods. Calculation results showed that the absorption spectrum of chloramphenicol by position of bands and their nature is largely determined by the nitrobenzene fragment with little participation of propanol and dichloroacetamide fragments of chloramphenicol. The proton acceptor properties of individual chloramphenicol fragments and the effect of the formation of H-bonded complexes on them have been analyzed. Keywords: chloramphenicol, enantiomers, electronic absorption spectrum.

The spectral luminescent properties and photolysis of the ionic forms (neutral, cationic and anionic) of bisphenol A have been studied experimentally and by methods of quantum chemistry. Calculations and experiment have shown that, in comparison with a neutral form, no new absorption bands appear in the absorption spectra of ionic form in the range 200–600 nm. The polar solvent (water) shifts the absorption spectrum bands of ionic forms towards low energies. In this case, the shift of the absorption spectrum of the cation is insignificant, and the shift of the anion is significant with an increase in the intensity of the absorption bands. The low quantum yield of fluorescence of ionic form is explained by the prevalence of the efficiency of singlet-triplet conversion over the efficiency of the radiation channel of decay of the fluorescent state. The low quantum yield of fluorescence of the anionic form is due not only to the effective singlet-triplet conversion, but also to the low efficiency of the radiative decay of the fluorescent state of the anion caused by a change in its orbital nature. Calculations have shown that the potential curves of the excited states of bisphenol A and its ionic forms have a significant potential barrier to photolysis. The increase in the efficiency of the process of photodissociation of the bisphenol A anion is caused by a noticeable decrease in the potential barrier and an increase in the overlap of the absorption spectra of the bisphenol A anion and solar radiation.

The paper presents the stages of development of photonics of molecules at Tomsk State University. Student of Academician A.N. Terenina - N.A. Since 1935 Prilezhaeva stood at the origins of the birth and development of domestic photochemistry, laid the foundation for the Tomsk school of spectroscopists, the only one beyond the Urals at that time. Research in the field of molecular spectroscopy and quantum chemistry of complex molecules began in 1950 by a student of N.A. Prilezhaeva - V.I. Danilova, later a professor at TSU. Fundamental questions related to the establishment of a relationship between the spectra, the structure of the electron shell and the physicochemical properties of a number of classes of complex organic molecules were posed and resolved. The generalizations made were of great importance for the theory of molecular spectra and practical application and allowed V.I. Danilova in the 1970-80s. create your own scientific school.

The study investigated the absorption and fluorescence spectra of charged forms of bisphenol A or BPA: anion and cation. The interpretation of the nature of electronically excited states and photolysis of the BPA molecule was made using quantum chemical calculations. Comparison of the BPA spectra in an aqueous solution during the transition from the neutral to ionic form is carried out. Calculations have shown that the small value of the fluorescence quantum yield in all considered molecular structures is determined by the high efficiency of singlet-triplet conversion. In the anionic form of BPA, a decrease by two orders of magnitude in the efficiency of the radiation decay channel was recorded, caused by a change in the orbital nature of the fluorescent state in comparison with the neutral and cationic forms. It is shown that due to the overlap of the absorption spectra of the anionic form in an aqueous solution and sunlight, the probability of photolysis of the studied molecule increases.

The spectral-luminescent properties and photolysis of phenol and some of its derivatives have been considered experimentally and by methods of quantum chemistry. It has been shown that the substitution with a methyl group (p-cresol) and the introduction of the second benzene ring (bisphenol A) lead to a shift of absorption bands toward longer wavelengths and some changes in their intensity. The non-planar structure of p-cresol and bisphenol A promotes an increase in the efficiency of singlet-triplet conversion and a decrease in the fluorescence quantum yield. Results of calculations have established the mechanisms of photolysis of the considered molecular systems. It has been shown that the O–H bond in phenol and p-cresol is broken by the pre-dissociation mechanism. Photolysis of bisphenol A upon exposure to solar radiation proceeds by the mechanism of direct dissociation, but with a noticeable potential barrier (~12000 cm–1).

Spectral-luminescent properties and photolysis of charged forms (cation and anion) of bisphenol A are studied experimentally and by methods of quantum chemistry. The calculations and the experiment demonstrate that no new absorption bands appear in the absorption spectra of charged forms in the region of 200–600 nm as compared to a neutral molecule. The polar solvent (water) shifts the absorption spectrum bands of the ionic forms to the region of low energies. In this case, the shift of cation absorption spectrum is insignificant, while the shift of anion absorption spectrum is significant as the intensity of absorption bands increases. The low quantum yield of fluorescence of ionic forms is explained by the predominance of singlet-triplet conversion efficiency over the efficiency of the radiation channel of fluorescent state decay. The low quantum yield of fluorescence of the anion form is due not only to the effective singlet-triplet conversion, but also to the low efficiency of the radiation decay of the fluorescent state of the anion caused by a change in its orbital nature. Calculations demonstrate that the potential curves of the excited states of bisphenol A and its ionic forms have a significant potential barrier for photolysis. An increase in the potential reaction barrier in the cation decreases the efficiency of its photolysis. The increase in the photodissociation efficiency of the bisphenol A anion is caused by a noticeable decrease in the potential barrier and an increase in the overlap between the absorption spectra of the bisphenol A anion and the solar radiation.

The spectral luminescent properties and photolysis of the ionic forms (neutral, cationic and anionic) of bisphenol A have been studied experimentally and by methods of quantum chemistry. Calculations and experiment have shown that, in comparison with a neutral form, no new absorption bands appear in the absorption spectra of ionic form in the range 200–600 nm. The polar solvent (water) shifts the absorption spectrum bands of ionic forms towards low energies. In this case, the shift of the absorption spectrum of the cation is insignificant, and the shift of the anion is significant with an increase in the intensity of the absorption bands. The low quantum yield of fluorescence of ionic form is explained by the prevalence of the efficiency of singlet-triplet conversion over the efficiency of the radiation channel of decay of the fluorescent state. The low quantum yield of fluorescence of the anionic form is due not only to the effective singlet-triplet conversion, but also to the low efficiency of the radiative decay of the fluorescent state of the anion caused by a change in its orbital nature. Calculations have shown that the potential curves of the excited states of bisphenol A and its ionic forms have a significant potential barrier to photolysis. The increase in the efficiency of the process of photodissociation of the bisphenol A anion is caused by a noticeable decrease in the potential barrier and an increase in the overlap of the absorption spectra of the bisphenol A anion and solar radiation.

Методами квантовой химии исследованы спектрально-люминесцентные свойства заряженных (анионной и катионной) форм трех замещенных бензальдегидов, проявляющих биологическую активность: o-анисового (2-метоксибензальдегид) и сиреневого (3,5-диметокси-4-гидроксибензальдегид) бензальдегидов и ванилина (3-метокси-4-гидроксибензальдегид). Расчеты показали, что в случае заряженных форм исследуемых молекул состояние S1 в отличие от нейтральных форм является состоянием ππ*-типа и по своей локализации аналогично S2 (ππ*)-состоянию нейтральных молекул (анисового альдегида и ванилина) или S3 (ππ*)-состоянию сиреневого бензальдегида. По результатам расчета показано, что в области спектра 240-420 nm нет новых электронных переходов, формирующих полосы поглощения в спектре, отличающиеся по природе и локализации от электронных переходов нейтральных молекул. Рассчитанные характеристики флуоресценции заряженных форм исследованных молекул показали, что в отличие от нейтральных форм эффективность радиационного распада заряженных форм много выше, что связано с изменением орбитальной природы состояния S1 при переходе от нейтральной к заряженной форме. Согласно анализу расчетных и экспериментальных данных флуоресценции исследуемых замещенных бензальдегидов в спиртовых растворах, флуоресценция на 410 nm принадлежит катионным формам. В ванилине и сиреневом альдегиде также имеется возможность слабой флуоресценции анионных форм этих соединений. Ключевые слова: замещенные бензальдегиды, спектрально-люминесцентные свойства, методы квантовой химии.

A quantum chemical study of the spectral and luminescent properties of the BPA + 2H2O complex was carried out. Calculations were performed by the semi-empirical method of intermediate neglect of differential overlap using a program complex and a special parameterization. The spectral behavior of BPA in water was modeled by a complex with water molecules in the ratio 1: 2 forming the hydrogen bond. The calculated data were compared with the results of investigation of the isolated BPA molecule. The nonplanar BPA structure leads to the strong mixing of the π- and σ-type atomic wave functions. The main reason for the low quantum yield of the BPA fluorescence is the efficient process of singlet-triplet conversion in the S1(ππ*) ≳ Tn(πσ*) channel of the BPA molecule and its complex with water. A study of the photolysis of the isolated BPA molecule upon exposure to solar radiation, the short-wavelength boundary of which on the Earth’s surface is located at ~290 nm (~34480 cm–1), showed that the energy of the photodissociative state localized on the O–H bond is much higher than this value for BPA. The binding curve is characteristic of the S1(ππ*) state, while the singlet and triplet states of the πσ* type, localized on the single C–C bonds of the central fragment of the molecule, are repulsion curves with a barrier. From our point of view, the low efficiency of the BPA degradation under the influence of solar radiation is due to the presence of a significant potential barrier to the photolysis in the singlet or triplet state. The mechanisms of bond breaking in the BPA + 2H2O complex are different for the singlet and triplet states, namely, for the S3(πσ*) state, the break occurs by the predissociation mechanism, and for the Tn(πσ*) state, due to its population through the singlet-triplet conversion in the S1(ππ*) → Тn(πσ*) channel.

The role of Professor V. G. Plotnikov in the development of the theoretical molecular photonics is discussed. A review is given of the principal results and the list of his publications covering the creation of the Nurmukhametrov–Plotnikov–Shigorin spectral-luminescent systematics of molecules. The main results of the theory for the radiationless conversion in polyatomic molecules, constructed by V. G. Plotnikov, are presented.