Exploration on dual emission mechanism of CPzP and CPzPO with thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) molecules with dual emission have attracted extensive attention recently. However, the dual emission mechanism is complex and difficult to be studied especially in solid state. In this paper, asymmetrical diphenylketone derivatives CPzP and CPzPO are investigated in both benzene and in aggregation state with polarisable continuum model (PCM) and combined quantum mechanics and molecular mechanics (QM/MM) method respectively. It is found that there are both quasi-axial (ax) and quasi-equatorial (eq) conformations for CPzP and CPzPO in benzene. The ax conformation is responsible for the high energy band emission, while the low energy band emission is contributed by eq conformations. The dimers with intermolecular hydrogen bond have similar emission wavelengths with monomers. The small energy gap between the first singlet excited state and the first triplet excited state as well as large reverse intersystem crossing rate in eq conformations confirms the generation of TADF in eq conformation, which agree with experimental results that the low energy band emission is TADF. Our calculation results revealed the dual emission mechanism of two TADF molecules, which would favor the understanding of light emitting properties and the design of new type dual emission TADF emitters.