Design of BPEA-based derivatives with high singlet fission performance: a theoretical perspective

The development of singlet fission (SF) is greatly hindered by the severe shortage of the types and numbers of SF materials. Here, essential energy conditions and SF-related competitive processes of a series of BPEA derivatives, which are a kind of new promising SF material, are investigated theoretically. Encouraging advantages and interesting laws of key energy conditions of those derivatives were found and potential BPEA derivatives were predicted. Those derivatives present mild exothermic SF processes with 0.3-0.4 eV free energies (ΔE(S₁-2T₁)) consistently. Their lowest triplet states (T₁) are stable and totally enter into the ideal energy window (≥1.0 eV), which is beneficial for achieving the maximum efficiency of PCE. Their large ΔE(T₂-2T₁) can suppress the higher-state annihilation of T₁ well. The E(S₁) and ΔE(S₁-2T₁) of the derivatives are sensitive to both the slip patterns of the dimer and ending substituents. Terminal substituents with both strong electron-withdrawing and electron-donating abilities can lower E(S₁), and decreases in the former are more obvious due to the larger intramolecular charge transfer. Interestingly, it is found firstly that the terminal substituent modulation effect on E(S₁) and ΔE(S₁-2T₁) is more effective when large longitudinal slips are included in their stacking modes. The reason is that the direction of the transition dipole moments (μ_s) is along X, and large longitudinal slips will bring about the approach of positive and negative charge centers of monomers, and lead to large Davydov splitting. By further evaluation of important radiation and non-radiation processes, it is predicted that the BPEA-based derivatives, which have rigid -Cl, -Br, or -CN terminals and include large longitudinal slips in their crystal packing, are expected to achieve excellent SF performances. Our work provides useful ideas for developing or optimizing acene-derivative SF materials with high efficiency.