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Project titleUltrafast photochemical processes in donor-acceptor molecular systems and their manifestations in transient spectra

Research area 03 - CHEMISTRY AND MATERIAL SCIENCES, 03-504 - Electronically excited systems, photochemistry, radiation chemistry, spin chemistry

Keywordselectron transfer, transient spectra, charge transfer, solvent polarization, intramolecular reorganization, solvent relaxation, ultrafast photochemical processes.

Annotation
Developing new theoretical approaches and software systems that allow quantitatively calculating the dynamics of ultrafast photoinduced charge transfer in solutions, as well as nonstationary spectra registered in the visible / UV and IR regions, is the project objective. The project is aimed at developing new methods of photochemical processes controlling, new approaches to the design of highly efficient molecular devices. The increase of photoinduced charge separation in donor-acceptor systems efficiency is the central aim here. A distinctive feature of this research is that the description will not be limited by considering the only stage of charge separation, but will include a joint description of all the stages, including the process of photoexcitation, primary charge separation, hot and thermal charge recombination, and charge transfer to a secondary acceptor, leading to a long-living state with separated charges. The need for such an integrated approach is determined by the fact that ultrafast processes proceed in non-equilibrium conditions and separate stages influence each other. Thus, they significantly differ from the relatively slow chemical processes proceeding in the thermal regime, for which the Ostwald principle postulating the independence of simple reactions is applicable. Using the multichannel stochastic model developed by our group, a theory of ultrafast charge transfer describing both the chemical dynamics and the dynamics of IR and optical spectra will be developed. The implementation of this approach will open up the possibility of quantitative interpretation of experimental data and obtaining previously unavailable detailed information on ultrafast photochemical processes. This will allow reaching a new level of control over the rates of product formation and the product yields. Complex organic compounds are widely used as donors and acceptors of electrons, including those with various kinds of symmetry (symmetric dyads, centrosymmetric (quadrupole) triads, and octupole molecules). It is often observed symmetry breaking by charge transfer in the excited state of such molecules with charge localization on one of the branches of a molecule, which manifests itself in nonstationary optical and IR spectra. Such localization of charge can significantly affect the efficiency of a given molecule as an electron donor or acceptor. The study of the effect of symmetry breaking on donor-acceptor properties will be carried out within the theory of symmetry breaking in excited states of quadrupole molecules developed by our group. In addition, the influence of symmetry breaking degree on IR spectra will be studied. This theory has been shown to provide a quantitative description of a number of experimental data. This project is planned to develop the theory of charge transfer symmetry breaking and its manifestations in nonstationary optical and IR spectra in symmetric dyads and octupole molecules. Experiments show that symmetry breaking in octupole molecules is more pronounced than in quadrupole molecules. This makes such molecules promising for a variety of applications. Implementation of the proposed approach will expand the possibilities of quantitative interpretation of nonstationary spectra and extraction of information on chemical dynamics from the spectra. Information on the pathways of electronic energy conversion to vibrational degrees of freedom and its subsequent migration reveals the details of the mechanism of ultrafast photochemical processes. Knowledge of the mechanism is critical for controlling the rates of product formation and their yields in multistage ultrafast processes. Ultrafast energy and charge transfer processes are attracting more and more interest, as they determine the functionality of a wide range of molecular systems. The development of ultrafast laser technologies, as well as a general orientation towards solar energy, have led to a surge in research in this direction. Research is already going beyond simple molecular systems and is increasingly focusing on the underlying molecular mechanisms that prevail in nanomaterials, native proteins, and hybrid systems. The scientific significance of the project's planned results is caused by the creation of new methods for interpreting experimental data and revealing a detailed microscopic mechanism of ultrafast chemical processes in molecular and supramolecular systems. This will reveal new possibilities for solving problems of controlling specific photochemical processes and designing molecular electronic devices. The expected results will solve a number of modern scientific problems associated with the interpretation of experimental data and the identification of a detailed microscopic mechanism of ultrafast chemical processes. They can be widely used in Russia and abroad in the design of molecular devices, in particular, organic photocells. The achievability of the project results is ensured by the fact that the proposed approaches are the extension of the models of ultrafast photoreactions developed by our group, as well as the experience of our team in computer modeling of processes of such type.

Expected results
It is proposed to obtain new, more detailed information on the microscopic mechanisms of ultrafast intra- and intermolecular photoinduced charge separation in organic donor-acceptor systems within the project. The following will be created: (1) a theory of ultrafast multistage photochemical processes capable of describing both chemical dynamics and non-stationary fluorescence spectra and difference spectra obtained by the pump-probing scheme; (2) a software system for computer modeling of the chemical and spectral dynamics of nonequilibrium molecular systems in solutions; (3) a technique for obtaining quantitative information on the dynamics of ultrafast processes by the simultaneous fitting of spectral dynamics and stationary absorption and emission spectra; (4) a technique for determining the detailed pathways of energy dissipation in excited molecules from nonstationary fluorescence spectra; (5) theory of symmetry breaking by asymmetric charge transfer and manifestations of this breaking in the spectral dynamics of symmetric dyads and octupole molecules. The obtained results will allow formulating new methods for controlling the dynamics of photoinduced charge separation in donor-acceptor molecular systems. The scientific significance of the results planned is due to the creation of new methods for interpreting experimental data and revealing a detailed microscopic mechanism of ultrafast chemical processes. This will provide new opportunities for solving problems of controlling specific photochemical processes and designing molecular electronic devices. The implementation of this project will provide our group with the leading positions in the world in the field of the theory of ultrafast photochemical processes in the liquid phase. The results will be made public in the form of publishing at least 18 articles in leading world scientific journals, reports at leading international conferences, and in the creation of intellectual property.

Annotation of the results obtained in 2022
1. A four-level model of symmetry breaking charge transfer in symmetric excited dyads and dimers is developed. A connection is established between the degree of dissymmetry and the parameters of the model, which has a clear physical meaning. The model predicts that for a given value of the charge separation free energy gap, symmetry breaking is possible only if the reorganization energy λ exceeds the threshold value. The threshold value depends on the magnitude of the electronic couplings and the energy gap. On the other hand, for a given reorganization energy λ, symmetry breaking is possible only if the gap is smaller than the threshold value. In this case, the threshold is determined by λ and off-diagonal electronic matrix elements. Dyads have four closely spaced electronic levels. Changing the parameters of the dyad can lead to the degeneracy of these levels, which is accompanied by a resonant increase in the degree of charge separation. The ranges of parameters in which the convergence and crossing of energy levels occurs are established. It has been established that the model predicts the resonance dependence of the degree of dissymmetry on the parameters of the dyad. It should be emphasized that in the vicinity of the resonant region, the degree of dissymmetry is strictly equal to zero. The height and width of the resonance depend on the proximity of the resonance parameters to those at which symmetry breaking is possible without degeneracy of the energy levels: the closer, the wider and higher the resonance. This behavior of the resonance is very unusual, since in the typical case, when the parameters are varied, the height of the resonance decreases with increasing width. A change in the electronic matrix elements and the energy gap between the locally excited and ionic states, which leads to broadening of the resonance, simultaneously leads to an increase in the resonant height. This opens up wide possibilities for controlling the degree of charge separation. The predictions of the theory agree with the available experimental data. 2. For the first time, a model has been created that describes the effect of the formation of hydrogen bonds (H-bond) of excited quadrupole molecules with a solvent on symmetry breaking charge transfer. It is assumed that the molecule consists of an electron donor located in its central part and two acceptors located on the periphery so that the molecule has central symmetry. Acceptor groups include atoms capable of forming H-bonds with a solvent. Oxygen or nitrogen atoms usually act as such atoms, and alcohols act as solvents. The H-bond effect is described by two quantities: (1) the free energy of H-bond formation between an excited symmetric dye with a protic solvent and (2) a parameter that determines the growth of the H-bond energy with increasing charge on the H-bond acceptor. The model takes into account that an increase in dye asymmetry results in a charge shift from one acceptor to another, making one acceptor more negative and the other less negative. As a result, the strength of the H-bond increases on one branch and decreases on the other. A mathematical model of the phenomenon is constructed, and its analytical solution is obtained. It is shown that, regardless of the strength of the H-bond, its effect on the degree of symmetry breaking is small, as long as the free energy of formation of a weaker H-bond is negative. This result is explained by the fact that when a charge is transferred from one acceptor to another, the binding energy increases at one acceptor and decreases at the other, so that the total free energy of the molecule changes only slightly. A strong effect is expected only if the free energy of formation of the weaker H-bond becomes positive, i.e., when the H-bond is formed on only one arm. An unexpected result of the weakening of the degree of dissymmetry due to the strengthening of the H-bond is predicted and discussed. An approach is proposed for quantitative determination of the sensitivity of the H-bond strength to the charge of the H-bond acceptor. 3. A theory has been developed for ultrafast charge transfer reactions in macromolecular systems with multiple redox centers, taking into account the complex multicomponent dynamics of medium polarization. The theory serves as a basis for mathematical and numerical modeling of photoreactions in real molecular devices, as well as for calculating the response of a nonequilibrium system to probe optical pulses within the "excitation-probe" scheme. Within the framework of the general theory, a method is proposed for constructing the space of nonequilibrium states of a medium in media with several relaxation components. As an illustration of the general approach, the problem of charge transfer in a three-center molecular system in a medium with a two-component relaxation function is considered, an algorithm for constructing diabatic free energy surfaces is described, and a system of equations is written for the evolution of particle distribution functions in these states. The results obtained within the framework of the general model are shown to reproduce the known solutions in special cases.

Publications

1. Antipov I.F., Ivanov A.I. Minimal model of excited-state symmetry breaking in symmetric dimers and covalently linked dyads The Journal of Chemical Physics, 22, 157, 224104 (year - 2022) https://doi.org/10.1063/5.0129697

2. Ivanov A.I. Modeling the Effect of H-Bonding of Excited Quadrupolar Molecules with a Solvent on Charge Transfer Symmetry Breaking The Journal of Physical Chemistry B, 44, 126, 9038–9046 (year - 2022) https://doi.org/10.1021/acs.jpcb.2c05984

3. Yermolenko I.P., Mikhailova V.A., Ivanov A.I. Определение релаксационных характеристик растворителей по нестационарным спектрам флуоресценции: роль длительности стробирующего импульса Оптика и спектроскопия, 130, 10,1499-1505 (year - 2022) https://doi.org/10.21883/OS.2022.10.53619.3924-22

Annotation of the results obtained in 2023
1. A theory describing the statistics of H-bond clusters in mixtures of alcohols and co-solvents accepting H-bonds has been developed. The dependences of the concentration of clusters of various lengths and composition on the concentration of methanol have been calculated. Using the example of a methanol-DMF mixture, it is shown that the concentration of clusters capable of acting as H-bond donors becomes significant only at methanol concentrations exceeding 9 M. 2. A theory of the fluorescence quenching of a photoexcited centrosymmetric acridinedione derivative due to the formation of H-bonds with methanol clusters in binary mixtures has been developed. The theory is shown to provide a quantitative description of the concentration dependence of the fluorescence quantum yield in mixtures of methanol and DMF. In particular, the unusual threshold Stern-Volmer dependence observed earlier in the experiment is explained. The theory, along with the Stern-Volmer dependence, well reproduces the non-exponential kinetics of fluorescence decay. 3. A program code for generation of training and testing data sets has been developed. A trained neural network has been created that determines the values of physical parameters. The final version of the neural network was tested on a series of already published experimental data. The operating time of a neural network is about one second on a personal computer. A machine learning model has been developed for computationally efficient calculation of the medium reorganizational energy and the change in free energy from stationary fluorescence spectra. An approach to generating separate data sets for training and validating a machine learning model is proposed. Studies have shown that the medium reorganization energy and the change in free energy can be predicted with an average absolute error of ~0.024 eV and ~0.009 eV, respectively. 4. A theory of symmetry breaking in excited octupolar molecules in polar media has been developed and manifestations of symmetry breaking in optical spectra have been identified. An equation for the evolution of the particle distribution function in the space of two coordinates of the reaction function is derived. A strict definition of reaction coordinates is given and their dynamic characteristics are determined. The free energy surface in the space of these coordinates is calculated. A relationship has been established between the degree of symmetry breaking (a parameter proportional to the dipole moment of the molecule) with the parameters of the molecule (the amount of splitting of the one- and two-quantum absorption bands) and the parameter that determines the intensity of its interaction with the polarization of the medium (the energy of reorganization of the medium for the transition of the molecule from a state with zero dipole moment to a state with maximum possible dipole moment). It is shown that the obtained dependences of the spectral characteristics on the polarity of the medium are consistent with the available experimental data. It has been established that the degeneracy of excited states inherent in octupole dyes in the excited state leads to a sharp increase in the degree of symmetry breaking. The effect of symmetry breaking on the dependence of the Stokes shift on solvent polarity is calculated and compared with the available experimental data. 5. Multidimensional spaces of nonequilibrium nuclear configurations were constructed, as well as diabatic free energy surfaces for states with electron transfer in non-Debye polar media with a multicomponent relaxation function. The influence of the interaction of the system with the environment on the kinetics of electron transfer was revealed, and the mechanisms for increasing the efficiency of charge separation in macromolecular compounds were analyzed. An approach has been developed to optimize the efficiency of photoinduced charge separation in multicenter donor-acceptor organic structures. A study was carried out to identify the conditions for maximizing the quantum yield of states with separated charges and the energy yield. The influence of geometric and energy factors on quantum yields is revealed. 6. The nature of the second excited state in quadrupolar molecules of the type A-D-A, where A and D are an electron acceptor and donor, respectively, has been studied. The effect of a flip of the dipole moment of a molecule during a phototransition between the first and second excited states is predicted. It has been shown that dipole flip leads to high energy reorganization of the solvent. The dependence of the phototransition energy between the first and second excited states on the polarity of the solvent has been established. The manifestations of the dipole moment flip on the absorption spectrum have been established. A quantitative description of the effect of symmetry breaking on the magnitude of the transition dipole moment between the first and second excited states is given. Available experimental data confirm these theoretical predictions.

Publications

1. Anatoly I. Ivanov Electric dipole flip in a quadrupolar molecule with broken symmetry upon excited state absorption The Journal of Chemical Physics, V.159, P. 054302 (year - 2023) https://doi.org/10.1063/5.0161041

2. Igor P. Yermolenko, Alexey E. Nazarov How good can a simple artificial neural network predict the medium reorganization energy and the free energy gap from a steady-state fluorescence spectrum? Journal of Molecular Liquids, V. 390, P.123130 (year - 2023) https://doi.org/10.1016/j.molliq.2023.123130

3. Nikolay B. Siplivy and Anatoly I. Ivanov The effect of energy level degeneracy on symmetry-breaking charge transfer: Excited octupolar dyes The Journal of Chemical Physics, V. 158, P. 164301 (year - 2023) https://doi.org/10.1063/5.0147328

4. Serguei V. Feskov Semiclassical Theory of Multistage Nonequilibrium Electron Transfer in Macromolecular Compounds in Polar Media with Several Relaxation Timescales International Journal of Molecular Sciences, V. 23(24), P. 15793 (year - 2023) https://doi.org/10.3390/ijms232415793

5. Serguei V. Feskov, Stanislav L. Bondarev, Tamara F. Raichenok, Sergei A. Tikhomirov, Anatoly I. Ivanov Symmetry-breaking charge transfer in an excited acridine-dione derivative: Effects of hydrogen bonding, clustering, and cooperativity in mixtures of methanol and dimethylformamide Journal of Molecular Liquids, V. 385, P. 122386 (year - 2023) https://doi.org/10.1016/j.molliq.2023.122386