Abstract
Exciton transformation, a non-radiative process in changing the spin multiplicity of an exciton usually between singlet and triplet forms, has received much attention recently due to its crucial effects in manipulating optoelectronic properties for various applications. However, current understanding of exciton transformation mechanism does not extend far beyond a thermal equilibrium of two states with different multiplicity and it is a significant challenge to probe what exactly control the transformation between the highly active excited states. Here, based on the recent developments of three types of purely organic molecules capable of efficient spin-flipping, we perform ab initio structure/energy optimization and similarity/overlap extent analysis to theoretically explore the critical factors in controlling the transformation process of the excited states. The results suggest that the states having close energy levels and similar exciton characteristics with same transition configurations and high heteroatom participation are prone to facilitating exciton transformation. A basic guideline towards the molecular design of purely organic materials with facile exciton transformation ability is also proposed. Our discovery highlights systematically the critical importance of vertical transition configuration of excited states in promoting the singlet/triplet exciton transformation, making a key step forward in excited state tuning of purely organic optoelectronic materials.
Highlights
When an organic molecule is excited to form a singlet or triplet exciton, the spin multiplicity of the exciton can transform either from singlet to triplet via intersystem crossing (ISC) or from triplet to singlet via reverse intersystem crossing (RISC)[1,2,3]
By comparing to the experimental results, the most applicable functionals for thermally activated delayed fluorescence (TADF), hybridized local charge-transfer (HLCT), and organic ultralong RTP (OURTP) molecules are PBE0, M062X, and PBE0, respectively (Supplementary Table S1); they were adopted in the following TD-Density functional theory (DFT) studies of the corresponding materials
At the ground states (Figs 3 and S1), the donor (D) moieties of dihydroacridine and diphenyl amine in respective TADF molecules of DMAC-DPS and Spiro-CN are almost orthogonally connected to acceptor (A) moieties with large twisting angles (88.3° and 89.9° respectively), which is important to support the strong charge transfer (CT) feature with small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) overlap extent (IH/L) and single-triplet splitting between S1 and T1 (ΔEST11) for efficient ISC and RISC processes[31]
Summary
When an organic molecule is excited to form a singlet or triplet exciton, the spin multiplicity of the exciton can transform either from singlet to triplet via intersystem crossing (ISC) or from triplet to singlet via reverse intersystem crossing (RISC)[1,2,3]. Organic ultralong RTP (OURTP) molecules (Fig. 1c) has been found to exhibit ultralong phosphorescence with lifetime up to 1.35s upon photoexcitation under ambient conditions at room temperature[17] In all these new-emerged purely organic materials of TADF, HLCT, and OURTP molecules, efficient exciton transformation has been experimentally confirmed and theoretically explained to be the origin of their extraordinary properties. Photophysical studies of TADF molecules with the small bandgap between the lowest singlet (S1) and triplet (T1) excited states offer an important path to understand the transformation mechanism Both efficient ISC and RISC between S1 and T1 with rate constants up to ~106 and 104 s−1 respectively for exciton transformation have been experimentally identified in TADF materials and proved to be the key factor for their high OLED performance with external quantum efficiency (EQE) about 30%18. Where λ denotes the Marcus reorganization energy, ∆E is the energy difference between the initial and final states, and HS0
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