Enhancing spin-orbital coupling in deep-blue/blue TADF emitters by minimizing the distance from the heteroatoms in donors to acceptors

How to enhance the reverse intersystem crossing (RISC) rates of deep-blue/blue thermally activated delayed fluorescent (TADF) materials remains a difficult task to be solved. Incorporating heteroatoms can effectively enhance the spin-orbital coupling (SOC) between their singlet and triplet states, and boost their RISC processes thereby, though the relationship between the locations of heteroatoms and SOC remains rarely studied. Here, we have designed and synthesized six analogous TADF materials with different types and locations of heteroatoms. It is revealed that minimizing the distance from the heteroatoms in donors to acceptors can significantly enhance the contribution of heteroatoms to the natural transition orbitals of singlet and triplet states, promote their n-π* transition proportion, and enhance the SOC between them thereby. As a result, the maximum spin-orbital coupling matrix elements (SOCMEs) of deep-blue TADF material BFCZPZ1 containing heteroatom O and blue TADF material BTCZPZ1 containing heteroatom S are as high as 0.311 and 0.980 cm⁻¹, respectively, which accelerate their RISC processes. Fabricated deep-blue/blue TADF devices based on them exhibit high efficiencies with low efficiency roll-off. These findings provide a guideline in further designing high performance deep-blue/blue TADF materials containing heteroatoms.