Theoretical Study On Singlet Fission Mechanisms Regulated By Solvated Structure Dynamics Of Chromophores

Singlet fission(SF)involves the spontaneous splitting of a photoexcited high-energy singlet state into a pair of low-energy triplets,absorbing one photon will generate two charges,and it has gained prominence as a method to improve the efficiency of organic photovoltaics beyond the Shockley-Queisser limit(～33%).For any light energy conversion system,the fluctuation of solvent structure,solvent polarity,dynamic interaction between solvent and chromophore,molecular vibration,relative position arrangement between monomers,temperature and other factors will cause the transient structural changes of chromophore before photoexcited.These structural deformations have a great impact on the excited energy,state mixing and superposition,orbital interaction between chromophores and so on,which further affect the SF channel,energetics,efficiency and mechanism.However,most of these transient structural deformations occur at the ultrafast level,which is extremely difficult to measure and characterize on the experiment,limiting the understanding and research on the realistic SF.Based on the above shortcomings,using the combination of ab initio molecular dynamics(AIMD)simulation sampling and time-dependent density functional theory(TD-DFT),this work studies the effects of chromophore dynamic structural deformation caused by solvent effect,molecular vibration,relative position arrangement between monomers and temperature on SF.The main research results and innovations are briefly described as follows:(1)General Dual-Switched Dynamic Singlet Fission Channels in Solvents Governed Jointly by Chromophore Structural Dynamics and Solvent Impact:The SF channels in many systems are controlled by the thermodynamic driving force(Switch 1)and kinetic barrier(Switch 2)both of which could be modulated by chromophore structure dynamics and solvent properties because they can cause chromophore different transient photoexcitation energetics.In this work,we simulate how the structural dynamics and solvent impact jointly govern the singlet pre-fission energetics,taking a T-shaped covalent BODIPY dimer(di-BODIPY,not meeting the SF criterion in the gas phase)in polar CHCl3 and apolar cyclohexane as examples,using AIMD simulation technique.We report a general dual-switched dynamic channel for intramolecular SF in solvents which can reasonably interpret the complicated SF phenomena and also suggest an effective AIMD-based configuration sampling method to characterize the joint effect of chromophore structure dynamics and solvent impact on SFs.Results,together with additional simulations on other systems,reveal that structural fluctuation and solvent impact not only cooperatively provide di-BODIPY much more opportunities for meeting the SF thermodynamic requirement in solvents(Switch 1 ON),but also jointly tune the charge transfer state towards removing the kinetic barrier(Switch 2 ON).Two factors jointly govern the dynamic dual-switched SF channel in di-BODIPY and anyone doesn’t open the channel alone.We suggest a general principle for dynamically dual-switching the SF channel in solvents by utilizing structural fluctuation dynamics of chromophore and impact of solvents to tune the pre-SF energetics of chromophore for photoexcitation and opening of the subsequent channel.AIMD sampling is used for the first time to discover explicit solvent-solute interaction and dynamics information and thus their effect on excitation energetics.High agreement of the findings here with experiments justifies our AIMD-based pre-SF energetic prediction to be a reliable way for exploring novel SF systems and their controllability.(2)Governing Energetics Population of Charge Transfer State and Its Coupling with Locally Excited State through Solvated Structure Dynamics of Chromophore in Intramolecular Singlet Fission:Understanding the mechanism of SF is essential for the development of new chromophores towards efficient fission-sensitized solar cells.Here,we propose the AIMD sampling to be applied to the SF mechanism,getting configuration ensemble of DP-Mes and DP-TIPS in ethanol solvent.We find configuration ensemble exhibits 5 different characteristics and behaviors in SF mechanism for DP-Mes and DP-TIPS based on energetics population of charge transfer(CT)state and its coupling with locally excited state,forming five subspaces,including R1,R2,V1,M1 channels and without SF.We uncover that coherent and incoherent SF can occur and the underlying SF multiple channels(R1,R2,V1 and M1)coexist in DP-Mes and DP-TIPS,but the main mechanisms and dominant SF channels are different in two chromophores due to the difference of energetics population of CT state and its coupling with locally excited state induced by solvated structure dynamics.SF multiple channels can convert which depends largely on proximity of the S1 state and CT state energies induced by solvated structure dynamics.Compared with DP-Mes,the dynamic structure fluctuation of side group-TIPS and the stretching of pentacene main bond length are more likely to break the symmetry of two monomers for DP-TIPS,which is the main reason for the difference of energetics population of CT state and its mixing with locally excited state.These results clearly indicate the role of the CT states and mixed electronic states in SF and emphasize the importance of the dynamic geometry of chromophore to the mechanism of the excited state photophysics and also provide detailed and complete insights into the SF mechanism.(3)Structural Dynamics Insight into Cooperativity of Chromophore Monomers in Promoting Singlet Fission in Perylenediimide Crystal:For SF in crystals,the current theoretical investigations are limited only to the static structures of chromophore monomers or dimers with tilted cofacial slip-stacked structures,ignoring the influence of adjacent monomers and dynamic structure fluctuation of monomers on SF at photoexcitation.This work explore the cooperativity of adjacent chromophore monomers in promoting SF on them under the influence of structure dynamics in perylenediimide(PDI)crystal.Simulations reveal that structure fluctuation can markedly adjust the structures of all monomers and make arbitrary two proximate monomers to find opportunity to transiently meet the SF energetic requirement,exhibiting high cooperativity in realizing SF.The high cooperativity of two arbitrary proximate chromophore monomers originates from the relative lowering of triplet state energy and HOMO-LUMO energy difference of the adjacent monomer due to the structure dynamics compared with that of the center chromophore monomer.Moreover,due to the anisotropic arrangement of the flake monomer in crystal which leads to different relative arranged dimer configurations,such structured dimer could exhibit different SF driving force and electronic coupling and also exhibit their different time evolution dynamics.The simulation results indicate that two monomers in the cofacial slip stacked arrangement plays a leading role in SF,while those in the head-to-head parallel contacted and tilted face-to-edge stacked configurations plays an auxiliary role in SF,which cooperate with each other to promote SF in the crystal.Not only do our findings reveal new light on the cooperative effect of correlated chromophore structure dynamics on the SF energetics but also provide a clue to construct the SF crystal materials for realizing highly efficient SF.(4)AIMD Sampling Reveals the Essence of Temperature-induced Conversion of Singlet Fission to Triplet-Triplet Annihilation in Rubrene:SF and triplet-triplet annihilation(TTA)upconversion are two important spin-coupled exciton interactions.TTA produces additional singlet excitons while SF consumes singlet exciton and generates a pair of triplet excitons.In this work,we use AIMD simulation and TD-DFT to prove that SF and TTA processes in rubrene are both dependent on temperature.It is surprised to find that a switch between TTA and SF in rubrene crystal when the temperature increases from 50 K to 300 K and present the coexistence of TTA and SF at 145 K by the simulation calculation of rubrene crystal at different temperatures.The coexistence and conversion of different kinetic processes in rubrene originates from the difference of chromophore dynamic structure induced by temperatures.The structural fluctuations induced by temperatures makes the HOMO-LUMO energy gap decrease with the temperature,resulting in the synchronous decrease of singlet excitation energy and triplet excitation energy.Therefore,rubrene satisfies the SF energy criterion at the higher temperature,whereas SF is almost suppressed gradually followed by an increasing TTA and TTA is prominent at the lower temperature.Therefore,rubrene has different potential properties at different temperatures,that is,as a SF material at higher temperatures,and as an annihilator at low temperatures.This study provides valuable information for a comprehensive understanding of the influence of temperature on SF and TTA,and provides theoretical support for the regulation of SF and other processes through external factors in the experiment.In conclusion,we for the first time applied AIMD simulation sampling technology to the SF,proposed AIMD sampling to get a series configuration ensemble from which the detailed comprehensive information can be extracted,including solvent effect,molecular vibration,structural dynamics caused by temperature,and their effect on the pre-SF energetics.Then dualswitched dynamic SF channels and a method to determine the SF mechanism were proposed,and the promotion effect of the coordination between the chromophores on intermolecular SF was revealed,as well as reasonably explains the essence of temperature-induced mutual conversion of SF and TTA.These findings highlight the importance of dynamic structural in SF,further promote the understanding of SF,and provide important theoretical support for the exploration of novel and complex SF systems.