Investigation of the clear structure−property relationship and microscopic mechanism of thermally activated delayed fluorescence (TADF) emitters with high emission quantum yield is a direction worthy of continuous efforts. The instructive theoretical principle of TADF material design is critical and challenging. Here, we carried out theoretical calculation on two experimental Cu(I) complexes with the same 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate (dppnc) but different N^N ligands [dmbpy = 6,6′-dimethyl-2,2′-bipyridine (1) or dmp = 2,9dimethyl-1,10-phenanthroline (2)] to briefly elaborate the structure−TADF performance relationship and luminescence mechanism. It was found that enhanced rigidity by the fused benzene ring between two pyridyl units in complex 2 leads to (i) higher allowedness of S 1 → S 0 , (ii) more effective reverse intersystem crossing (RISC), and (iii) better relative stability of the T 1 state, which could be responsible for its excellent TADF behavior. Thus, a strategy of extending π conjugation in the N^N ligand could be deduced to further enhance the quantum yield. We validated it and have succeeded in designing analogue complex 4 by extending π conjugation with an electron-withdrawing pyrazinyl. Benefiting from the smaller energy gap (ΔE ST ) and plunged reorganization energy between the S 1 and T 1 states, the rate of RISC in complex 4 (1.05 × 10 8 s −1 ) increased 2 orders of magnitude relative to that of 2 (5.80 × 10 6 s −1 ), showing more superiority of the TADF behavior through a better balance of RISC, fluorescence, and phosphorescence decay. Meanwhile, the thermally activated temperature of 4 is only 165 K, implying that there is a low-energy barrier. All of these indicate that the designed complex 4 may be a potential TADF candidate.

Profound understanding of the luminescence mechanism and structure− property relationship is vital for Cu(I) thermally activated delayed fluorescence (TADF) emitters. Herein, we theoretically simulated luminescent behavior in both solution and solid phases for two Cu(I) complexes and found the following: (i) The strengthened spin−orbit coupling (SOC) effect by more d x 2 −y 2 orbital contributions and well-restricted structural distortion via remarkable intramolecular interaction in [Cu(dmp)(POP)] + enable the emission at room temperature to be a mixture of direct phosphorescence (10%) and TADF (90%). (ii) Benefiting from enhanced steric hindrance and the electron-donating ability of the paracyclophane group, the narrowed S 1 −T 1 energy separation (ΔE ST ) in [Cu(dmp)(phanephos)] + accelerates the reverse intersystem crossing, promoting the TADF rate (1.88 × 10 5 s −1 ) and intensity ratio (98.3%). These results indicate that the small ΔE ST is superior for reducing the lifetime and that the strong SOC stimulates the phosphorescence to compete with TADF, which are both conducive to avoiding collision-induced exciton quenching and reducing the roll-off in devices.

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