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FSDS is Fluorescence Spectral Dynamics Simulator, designed to investigate photoinduced charge transfer and its manifestations in time-resolved fluorescence spectra of a dye in a solvent. The project is implemented in C, using the MPI library. The computer model simulates stationary absorption and fluorescence spectra, as well as the time evolution of population distributions along the solvent reaction coordinate, to calculate spectral dynamics. Excitation and relaxation of intramolecular high-frequency vibrations are described at the quantum level. FSDS can fit the physical parameters of the dyes to experimental data. The dye excitation is consistently described accounting for the finite duration of the pump pulse. The program code implements two evolutionary models: a spin-boson model within the harmonic potential and a stochastic model based on the Smoluchowski diffusion operator for an arbitrary free energy surface. Computer simulation shows the numerical coincidence of the calculations performed in the framework of these models for the harmonic free energy surface. The detail description of the code and numerical schemes are presented. Program summary Program Title: Fluorescence Spectral Dynamics Simulator (FSDS) CPC Library link to program files: https://doi.org/10.17632/d39rrrkvhr.1 Code Ocean capsule: https://codeocean.com/capsule/6544976 Licensing provisions: GPLv3 Programming language: C Supplementary material: Sample configuration files, User Manual Nature of problem: The time-resolved spectra contain reach information about photochemical processes. However, the chemically important information about separate elementary processes is entangled due to they proceed in parallel. Time-resolved spectra modeling allows extracting information about these processes. The presented program code makes it possible to determine the relaxation characteristics of the solvent and the times of intramolecular relaxation. Solution method: Finite-difference time-domain and Brownian dynamics simulation methods.

Regularities of ultrafast charge recombination kinetics in photoinduced intramolecular electron transfer in polar solvent are investigated. The multichannel stochastic model is used to simulate kinetics of charge separation and ensuing ultrafast charge recombination. The rate constant of ultrafast charge recombination occurring in non-equilibrium mode is shown to depend on the free energy gap of the preceding charge separation stage. This is a clear demonstration of violation of one of the fundamental principles of formal chemical kinetics, namely, the postulate of independence of elementary chemical reactions. The simulations revealed two main trends: (i) the charge recombination rate constant typically increases with decreasing the charge separation free energy gap, (ii) the maximum of the charge recombination rate constant, as a function of the charge recombination free energy gap, |ΔGCR|, shifts toward smaller values of |ΔGCR| with decreasing the free energy gap at the previous, charge separation, stage of the reaction. The prediction of ultrafast charge recombination in donor–acceptor dyads with small charge recombination free energy gap is shown to be supported by the experimental data on the kinetics of charge recombination into the first excited state followed the charge separation from the second excited state.

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.

We study the fluorescence spectral dynamics of a molecule with two excited fluorescent states (called the precursor and product) within the stochastic point transition model, which includes a consistent description of the dynamics of the particles distribution in excited states and transitions between them. Crucial changes in spectral dynamics with increasing the transition rate between the precursor and the product are discovered. Analysis of the results of spectral dynamics modeling allows us to determine how the transition rate between the precursor and the product affects the isoemissive point in the spectra collected with an increasing time delay. The following main regularities were revealed: fluorescence spectra with a short delay not exceeding several solvent relaxation times ( τ L ) may not pass through the isoemissive point; the isoemissive point shifts to the red with a decrease in reaction time constant ( τ eff ). The shifted isoemissive point exists in the range 10 τ L τ eff 40 τ L . For faster reactions with 5 τ L τ eff 10 τ L , the isoemissive point blurs. When the reaction proceeds in the ultrafast regime τ eff 5 τ L , the fluorescence spectrum shifts from a vicinity of the precursor spectrum to the product spectrum as a whole, changing its shape.