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Mechanism of direct UV photolysis of pyridine herbicide triclopyr (TRI) was revealed by the combination of nanosecond laser flash photolysis, steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. Both the detection of short-lived intermediates and the detailed identification of final products were done for the first time. The quantum yield of TRI photodegradation is about 4% at both UVC (254 nm) and UVB (308 nm) excitation. The primary stage is the heterolytic cleavage of C-Cl bond in dissociative triplet state of TRI with the formation of phenyl cation followed by a fast nucleophilic attack by a solvent molecule. The minor channel is the photohydrolysis leading to the formation of 3,5,6-trichloropyridin-2-ol. Primary photoproducts undergo secondary photolysis by the mechanism similar to initial TRI with the formation of products of acetic group elimination, sequential substitution of chlorine atoms to hydroxyl groups and, finally, oxidation and opening of the pyridine ring. Obtained results can be important for understanding the fate of pyridine herbicides in the processes of UVC disinfection and in natural waters under action of the sunlight.

Quantum chemical calculations (CASSCF and XMCQDPT level of theory, IMCP-SR1 and SBKJC basis sets) of the structures and electronic absorption spectra of the OsIVCl5(H2O)− and OsIVCl5(OH)2− complexes, which are the products of OsIVCl62− photoaquation, were performed. The satisfactory agreement between the experimental and calculated spectra was achieved using both triplet and quintet manifolds. The dissociation of the aquacomplex with the formation of the hydroxocomplex was explained by the thermochemical data.

The photochemistry of the OsIVCl62- complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump-probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d-d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(iv) to Os(iii), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(iii) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(iv) and Os(iii) intermediates determines the synthetic potential of OsIVCl62- photochemistry.

A general analytical expression for the transient fluorescence spectrum is derived. The formation of a wave packet in the excited state of a fluorophore is described, assuming that the pump pulse has a Gaussian time-profile. The expression explicitly connects the relaxation characteristics of the medium with the spectral dynamics of a fluorophore. Fitting the expression to experimental spectral dynamics allows obtaining the solvent relaxation function. So far this approach was applicable for the analysis of experimental data when the pump pulse does not populate excited sublevels of intramolecular high-frequency vibrational modes. Here, the approach is generalized to include vibrational relaxation in the excited electronic state. In this case, fitting to the experimental spectral dynamics provides reliable information not only on the solvent relaxation, but also on the relaxation time constants of intramolecular high-frequency vibrational modes. This approach is applied to the excited state dynamics of coumarin 153 in multiple solvents, obtained from broadband fluorescence upconversion spectroscopy.

Introduction of push-pull system between carbonyl and dimethylamino groups in the ethene bridge of diarylethene provides access to multifunctional fluorescent switches, responsible for various stimuli (UV and visible light, solvent polarity, and pH change). In this work a thorough study of luminescent and photochromic properties of three cyclopentenone-based derivatives in various solvents was performed. Quantum yields of luminescence as well as forward and backward photochemical reactions were measured, and the kinetic properties of the luminescence decay were determined. The ICT (intramolecular charge transfer) excited state is responsible for the emissive properties, which determines the strong dependence of the emissive properties vs. solvent polarity. Kinetic behavior of the luminescence and its dependence of solvent polarity were explained by the model based on the equilibrium between two isomers of the DAEs open form (parallel, P, and anti-parallel, AP). Dialkylaminobenzylidene moiety, conjugated with cyclopentenone bridge, provides unique combination of bidirectional manipulation by visible light only and complex switching behavior, presumably associated with competitive processes of 6π-electrocyclization and E-/Z-isomerization.

Photodegradation of imipramine (IMI), a widely used tricyclic antidepressant, in the presence of Fe(III) oxalate and potassium persulfate (PS) was studied for the first time by combination of steady state and laser flash photolysis. It was demonstrated that Fe(III) oxalate system exhibits an effective photooxidation of IMI due to the formation of reactive oxygen species (ROS) with a very high quantum yield, φROS(308 nm) ∼0.3. The quantum yield of the most important ROS, hydroxyl radical, was also measured for the first time (φOH(308 nm) = 0.25 ± 0.02) as well as the rate constant of the reaction between •OH radical and IMI (kOH = (1.5 ± 0.1) ×1010 M−1s−1). Nature and spectral properties of short lived organic radicals formed in this reaction were also determined experimentally and proved by quantum-chemical calculations. At a IMI concentration of less than 10 µM it is possible to achieve full disappearance of the initial compound in Fe(III)-oxalate system. Main aromatic by-products of the IMI oxidation are connected with its consequent hydroxylation and elimination of aliphatic substituent. Complete degradation of both IMI and all aromatic byproducts needs joint application of Fe(III) oxalate and PS, as additional oxidizing agent. Combined Fe(III) oxalate – PS system demonstrates high photoactivity under UV irradiation and could be used for the effective degradation of IMI and its close analogues in aqueous media.

Using methods of stationary and time-resolved femtosecond laser spectroscopy, as well as time-dependent density functional theory, the spectral-kinetic properties of excited electronic states of synthesized 2,7-diaminoxanthone (DAX) have been studied in polar protic and aprotic solvents. The intramolecular charge transfer from amino-groups to the central oxygen-containing moiety manifests itself in appearance of a new intensive long-wavelength absorption band in DAX. Considerable bathochromic Stokes shift observed in DAX fluorescence spectra with increasing aprotic solvents polarity indicates growth of charge transfer degree and, respectively, of transition dipole moment between the ground to the first singlet-excited states. In alcohols, hydrogen bond formation between DAX and solvent results in an increase of Stokes shift and fluorescence quenching. The observed spectral dynamics reflects the solvent relaxations and is well reproduced with the dynamic parameters of the solvents measured and reported elsewhere.

Two mechanisms of OsIVCl62− photolysis were studied by means of quantum chemical calculations in gas and aqueous phases. The difference between these mechanisms is in the nature of the possible Os(IV) key intermediates (KI). According to calculations, the intermediate is an OsIVCl5− complex of square pyramidal coordination geometry. The calculations do not give an opportunity to make an unambiguous choice between the triplet and quintet multiplicities of OsIVCl5−. The calculated CASSCF/IMCP-SR1 transition energies for 5OsIVCl5− are lower than for 3OsIVCl5−, while the calculated XMC-QDPT2/SBKJC spectra for the triplet state are in better agreement with the experimental absorption spectrum of the KI than for the quintet state.

Ribulose 1,5-bisphosphate (RuBP) undergoes enolization to initiate fixation of atmospheric carbon dioxide in the plant carbon cycle. The known model assumes the binding of RuBP to the Rubisco active site with the subsequent formation of 2,3-enediol (2,3,4-trihydroxypent-2-ene-1,5-diyl diphosphate). In the present study, it is assumed that 1,2-enol (2,3,4-trihydroxypent-1-ene-1,5-diyl diphosphate) can be formed in the enolization step to initiate the carboxylation reaction. We have used Kohn–Sham density functional theory on WB97X-D3/Def2-TZVP levels to compare the reaction barriers in the two ways. We considered the pathways of carboxylation of 1/2-ene (mono/di)ol via the C1 and C2 carbons without taking into account the binding of RuBP to the magnesium ion. Calculations of Gibbs free energies confirm the equal probability of the formation of 2,3-enediol and 1,2-enol. Quantum–chemical modeling of enolization and carboxylation reactions supports the important role of the bridging water molecule and diphosphate groups, which provide proton transfer and lower reaction barriers. The results show that carbon dioxide fixation can occur without a magnesium ion, and binding with C1 can have a lower barrier (~12 kcal/mol) than with C2 (~23 kcal/mol).

Cerium ammonium nitrate (NH4)2CeIV(NO3)6 (CAN) is widely used as a photolytic source of NO3 radicals in acetonitrile. In spite of that, the mechanistic aspects of CAN photochemistry were poorly discussed in the literature. In this work CAN photochemistry in CH3CN was studied using the combination of stationary methods and laser flash photolysis. In accordance with the literature, the inner-sphere electron transfer was found to be the primary photochemical process. The (NH4)2CeIII(NO3)5(CH3CN) complex was found to be the only reaction product formed with the rather high quantum yield (0.6 and 0.4 upon 308 and 355 nm excitation correspondingly). The NO3 radicals decay is mainly caused by the second-order reactions; their rate constants are determined. In addition to the direct release of the NO3 radicals, a part of the light-excited CAN molecules were found to form the radical complex [(NH4)2CeIII(NO3)5…NO3 ], which lifetime is ca. 2 μs.

A new compound, 1,3-dimethoxy xanthione (DXT), has been synthesized and its absorption (stationary and transient) and luminescence spectra have been measured in n-hexane and compared with xanthione (XT) spectra. The pronounced broadening of xanthione vibronic absorption band related to the electronic transition to the second singlet excited state has been observed. Distinctions between the spectra of xanthione and its methoxy derivatives are discussed. Quantum chemical calculations of these compounds in the ground and excited electronic states have been accomplished to clarify the nature of electronic spectra changes due to modification of xanthione by methoxy groups. Appearance of a new absorption band of DXT caused by symmetry changes has been discussed. Calculations of the second excited state structure of xanthione and its methoxy derivatives confirm noticeable charge transfer (about 0.1 of the charge of an electron) from the methoxy group to thiocarbonyl group. Fitting of the transient spectra of XT and DXT has been fulfilled and the time constants of internal conversion S 2 → S 1 and intersystem crossing S 1 → T 1 have been determined. A considerable difference between the time constants of internal conversion S 2 → S 1 in XT and DXT is uncovered.

We introduce bsmKinetic , an implementation of the stochastic multichannel point-transition approach to simulation of the charge transfer kinetics in molecular systems with reorganization of many intramolecular high-frequency vibrational mode in solvents with several relaxation timescales. The software provides simulation of the charge transfer kinetics in the molecular systems with many electronic states involved in photochemical transformations. It also allows simulating the charge transfer occurring in both equilibrium and nonequilibrium regimes. bsmKinetic is open-source software distributed under the terms of the GPL without additional components. Software is implemented on multiple computing platforms. It exploits Matsumoto and Nishimura source code for pseudorandom number generator and a hierarchy of custom parallelization of the stochastic trajectories built on MPI . Program summary Program Title: bsmKinetic Program Files doi: http://dx.doi.org/10.17632/ywd9cwzzx9.1 Licensing provisions: GPLv3 Programming language: C++ Supplementary material: MPI Nature of problem: Simulation of dynamics and kinetics of charge transfer in solution. Solution method: Brownian simulation method for diffusional dynamics, stochastic point-transition approach for electronic and vibronic transitions. Additional comments including Restrictions and Unusual features: Binary search of transition-points for each diffusion step on crosses of vibrational manifolds, initial electron-vibrational distribution includes pump pules characteristics, classical description of the solvent fluctuations, description of the vibronic states in terms of populations ignoring the quantum coherence, usage of the diabatic basis, parabolic terms. Reference: [1] M. Matsumoto, T. Nishimura Dynamic Creation of Pseudorandom Number Generators, in book Monte Carlo and Quasi-Monte Carlo Methods, 1998, Springer, 2000, 56–69.

Quantum chemical calculations (CASSCF and XMCQDPT level of theory, IMCP-SR1 and SBKJC basis sets) of the structures and electronic absorption spectra of the OsIVCl5(H2O)- and OsIVCl5(OH)2- complexes, which are the products of OsIVCl62- photoaquation, were performed. The satisfactory agreement between the experimental and calculated spectra was achieved using both triplet and quintet manifolds. The dissociation of the aquacomplex with the formation of the hydroxocomplex was explained by the thermochemical data.

Nitro derivatives of xanthione, 2,7-dinitro-9 H-xanthene-9-thione and 2,4,7-trinitro-9 H-xanthene-9-thione, have been first synthesized and their stationary and transient spectra have been measured. The stationary spectra show that the attachment of the nitro groups to the xanthione scaffold leads to strong quenching of S

An analysis of theoretical modeling results of ultrafast kinetics of photoinduced intramolecular charge separation from the second excited singlet state in the dyad Zn-tetraphenylporphyrin-aminonaphthalenediimide (Zn-TPP-ANDI) in a solution of toluene is presented. The calculations are performed within the framework of the stochastic multi-channel model, which includes four electron states (the ground, first and second excited singlet states, the state with charge separation), as well as their vibration sublevels corresponding to the excitation of highfrequency intramolecular vibration modes. A bimodal kinetic curve of population of the state with charge separation observed in experiments is quantitatively reproduced. The absolute yield values of the state with charge separation are determined. The results of the modeling show that intramolecular modes make a significant contribution to the reorganization of low-frequency modes. Quantum chemical calculations were performed, determining the degrees of freedom related to the intramolecular slow motion of nuclei of high amplitude in the dyad Zn-TPPANDI on going from the ground state to the state with charge separation.

Two mechanisms of Os

A model for simulating the transient electronic absorption spectra of donor-acceptor dyads undergoing ultrafast intramolecular charge transfer in solution has been developed. It is based on the stochastic multichannel point-transition approach and includes the reorganization of high-frequency intramolecular modes (treated quantum mechanically) and of low frequency intramolecular and solvent modes (described classically). The relaxation of the slow modes is assumed to be exponential with time constants taken from experiments. The excited-state dynamics is obtained by simulating the population distribution of each quantum state after optical excitation and upon electronic and vibrational transitions. This model was used to simulate the transient electronic absorption spectra measured previously with a pyrylium phenolate in acetonitrile. A very good agreement between the simulated and measured spectra was obtained assuming a three-level model including the ground state, the optically excited state, and a dark state with large charge-transfer character and a substantially different geometry relative to that of the optically excited state. The merit of this approach to disentangle the contributions of both population changes and relaxation processes to the ultrafast spectral dynamics will be discussed.

We derive an analytical expression for calculating the transient absorption signal measured in the pump–probe experiment. The expression explicitly accounts for dynamic properties of the medium, the population decay of the photoexcited state, and the angle θ between the directions of the reaction coordinates corresponding to the electron transitions at the pump and probe stages. We investigate numerically the influence of the carrier frequency of the pump pulse and θ on the transient absorption signal. We study the signal dynamics and its deviation from that of the excited state population under variation of these parameters. We show that these effects are manifested in complexes including methyl-substituted benzene and tetracyanoethylene in polar solvents.

The yield orotic aldehyde at the oxidation of 6-methyl-2,4-dihydroxypyrimidine was found to increase when selenious acid is used as an oxidizer. A comparative analysis of 6-methyl-2,4-dioxypyrimidine oxidation with selenium dioxide and selenious acid was performed using the ab initio quantum chemical method in the 6-31G** basis.

Analysis of the oxidation mechanism of 6-methyl-2,4-dioxopyrimidine with selenium oxide and selenous acid with the formation of orotic aldehyde has been carried out by the ab initio quantum-chemical method on the 6-31G** basis. The mixed anhydride of acetic and selenous acids is formed with a gain of energy. It possesses high activity and steric accessibility in electrophilic attack on position 5 of the pyrimidine ring. The three-stage mechanism of the oxidation of the methyl group in 6-methyluracil by the mixed anhydride of selenous and acetic acids has been analyzed.

Reactions of 5-benzylidene-2-thioxo-2,3-dihydro-(1H,5H)-pyrimidine-4,6-diones with a singlet difluorocarbene afford difluoromethoxy derivatives in 20–34% yield. The quantum-chemical analysis of the reaction mechanism showed that N,N-dimethylformamide is involved into the formation of difluoromethoxy derivatives.

A theoretical approach to simulation of the transient spectra in molecular systems with ultrafast photoinduced nonradiative electronic transitions is developed. The evolution of the excited and ground state populations as well as the nonradiative transitions between them are calculated in the framework of the stochastic multichannel point-transition model involving the reorganization of the medium and the intramolecular high frequency vibrational modes. Simulations of transient spectra of donor-acceptor pairs excited in the charge-transfer band that are accompanied by ultrafast charge recombination into the ground state demonstrate a possibility of positive band appearance in the transient absorption spectrum caused by those systems in the ground state, which returned there from the excited state. The region of the parameters of the donor-acceptor systems where a positive ground state absorption signal can be observed is discussed. A qualitative comparison of the simulated transient spectra with the experimental data on betaine-30 is presented.

The excitation of a charge transfer band by a laser pulse of finite duration and the ensuing charge recombination are calculated in the framework of the perturbation theory. The influence of the spectral characteristics of the laser pulse on the charge recombination dynamics is investigated for models including several nuclear modes that differ greatly in their timescales. It is shown that, in the area of applicability of the perturbation theory, the variation of the pulse carrier frequency inside the absorption band can significantly change the effective charge recombination rate constant.

The dependence of the ultrafast photoinduced electron transfer dynamics in donor-acceptor complexes on the excitation pulse carrier frequency (spectral effect) has been investigated in the framework of a model involving three electronic state. The spectral effect has been shown to strongly depend on the angle theta between the reaction coordinate directions corresponding to optical and charge transfer transitions. Describing the solvent as a linear homogenous polar medium and accounting for Coulombic interaction of the transferred charge with the medium polarization fluctuations, the angle theta has been found out to be typically in the area 40 degrees -85 degrees. Exactly in this area of theta the spectral effect is predicted to be most pronounced.

The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.

The electronic structure of the starting, intermediate, and final compounds was studied by MNDO calcualtions with the Dewar and Thiel parameters [2]. As follows from the calculations, the most probable reaction pathway involves initial formation of (dimethoxyphosphinoyl)methanol (VI). Energetically, the most advantageous participant of the acid-catalyzed reaction is a five-coordinate phosphorus phosphite structure that forms with II an energetically favorable transition state VII. DOI: 10.1134/S1070363208070335