Theoretical Study On Singlet Fission Related Photophysical Processes Of BPEA Derivatives

Singlet fission process is a spin-allowed multi-exciton process.Two pairs of electron-hole pairs can be generated when a material absorbs a photon.Therefore,singlet fission has great potential in breaking the Shockley-Queisser limit and greatly improving the photovoltaic conversion efficiency of solar energy.The application of single exciton fission devices requires more stringent conditions,such as the matching conditions E(S₁)?2E(T₁),E(T₂)>2E(T₁),and the energy of T₁should not be too low(E(T₁)?1e V).Therefore,the materials with promising single fission are very limited.BPEA and some of its derivatives are new single exciton fission materials with high triplet energy in recent years.The yield of triplet in some material laboratories is more than 180%.BPEA molecule is highly modifiable in structure,which has great potential in further developing and optimizing BPEA derivatives as single exciton fission materials.Due to the lack of theoretical work on single exciton fission related to BPEA materials,there is a lack of effective guidance in further design and optimization.In this paper,four kinds of BPEA and their derivatives are selected as the research objects,which are systematically studied by density functional theory(DFT)and time-dependent density functional theory(TD-DFT).The main research contents and conclusions are as follows:1)by comparing the energy theory of singlet state and triplet state with the experiment,the appropriate functional and basis set are determined for subsequent calculation;2)The stacking mode of crystal materials has a great influence on the energy condition and efficiency of single fission.We selected dimers of several materials to scan the energy surface of singlet and triplet states in the longitudinal(X direction),lateral(Y direction)and vertical directions respectively.The scanning results show that the triplet energies of all BPEA and its derivatives are relatively stable(>1.1 EV),and the energy values change little(<0.1 EV)with the intermolecular slip,while the energy values of singlet states change greatly(about 0.4 e V)with the intermolecular slip;3)Through the analysis of electron transition density,the electron distribution of singlet state is delocalized,while the electron distribution of triplet state is relatively localized,and the energy of singlet state is greatly affected by the degree of intermolecular coupling;4)By studying the renormalization energy and radiation/non-radiation rate of single molecule and its dimer,it is shown that the non-radiation process and radiation process of derivative 1 are relatively small in several molecules.From monomer to dimer,the non-radiative process of BPEA was further inhibited,which was consistent with the high triplet yield of derivatives 1 and BPEA in the experiment,By mapping the transition density of the dimer,it was found that derivative 1 had low intermolecular charge transfer and delocalized density distribution,We speculate that the density distribution of delocalization and low intermolecular charge transfer may be conducive to single fission.The conclusions of this paper,especially the changing trend of the energy response of singlet and triplet states to the crystal stacking mode,provide important theoretical guidance for the design and optimization of new BPEA type single fission materials in the laboratory.