Ultrafast Spectroscopic Study Of Intramolecular/Intermolecular Singlet Fission

Singlet Fission（SF）is a spin-allowed process that could convert a high-energy singlet exciton into two low-energy triplet excitons.It has the potential application into improving the photoelectric conversion efficiencies of organic solar cells by overcoming the Shockley-Queisser limitation.However,to date,the application of SF in organic solar cells has not been very successful.On one hand,the number of highly efficient SF materials with good stability,high triplet state energies and long triplet state lifetime is remain lacking.On the other hand,it is still a challenge to harvest efficiently the two triplet excitons generated from SF.Based on this,we designed and synthesized a series of SF materials with good stability,high triplet energy and long lifetime,and studied the electron transfer process between the triplet excitons and the electron acceptors.The detailed research content is as follows:1.Singlet fission in single-bond linked orthogonal anthracene dimersIn this chapter,we synthiszed a single-bond linked anthracene dimer and studied its photophysical property detailedly by time-resolved fluorescence spectra and transient absorption spectra.The study reveals that efficient SF（Φ_T=164%）can be conducted in this dimer film.This is benefited from the decreased triplet state energy that makes SF isothermic,which is caused by the orthogonal dimeric structure and perpendicular intermolecular packing.Furthermore,the triplet state energy of this dimer film can reach 1.51 e V,which is the highest one with efficient SF so far.The result provides valuable informations for designing new efficient SF materials based on anthracene.2.Singlet fission in a tetracene dimer with V-shaped structureIn this chapter,we synthesized a tetracene dimer with V-shaped structure.Steady-state spectra reveal that the ground-state interaction between the two tetracene units in this dimer is nearly neglible,but there is a centain interaction at the excited state.Fluorescence dynamics and transient absorption experiment demonstrate that SF can be conducted in this dimer and the generated triplet pair state can produce free triplet state via spatially separated triplet pair state（Φ_T=110%）.Furthermore,magnetic-field dependent fluorescence dynamics also confirms the occurrence of SF.Additionally,two-electron transfer can be proceeded from the free triplet to the electron acceptor（7,7,8,8-tetracyanoquinodimethane,TCNQ）with an efficiency of 200%.These results suggests that the relatively strong ground-state interaction is not the requirement for SF and only the presence of a centain excited-state interaction could ensure the occurrence of SF.This provides new insights for the design of efficient SF materials with long-lived triplet state.3.The study of the triplet electron transfer in linear tetracene oligomersIn this chapter,we studied the electron transfer process from the triplet state generated via SF to the electron acceptor（TCNQ）using transient absorption spectra.The electron transfer was not observed in dimer due to the low yield of the free triplet state formed from the separation of the triplet pair state.Instead,the electron transfer can be proceeded efficiently with the rate of 2.23×10⁹s^-1m^-1and 3.79×10⁹s^-1m^-1 in trimer and tetramer,respectively.We can see that the electron transfer rate of tetramer is faster than that of trimer.This is most likely due to the more negative gibbs free energy of the electron transfer in tetramer that is caused by the smaller coulomb repulsion brought by the longer distance between the two positive charge centers generated from triplet electron transfer.The result indicates that the electon transfer can be proceeded from the triplet state with long-lifetime instead of the triplet pair state with short-lifetime.In addition,the higher the separation degree of the triplet pair,the faster the triplet electron transfer.This provides some valuable informations for the utilization of the two triplet states via electron transfer.