Cu(I) and Ag(I) Complexes with a New Type of Rigid Tridentate N,P,P-Ligand for Thermally Activated Delayed Fluorescence and OLEDs with High External Quantum Efficiency

Abstract

Neutral Cu(I) and Ag(I) complexes with a new rigid tridentate N,P,P ligand (dmpzpp, 3,5-dimethyl-1-(2-((2-(di-o-tolyl)phosphanyl) (o-tolyl)-phosphanyl)phenyl)-1H-pyrazole), giving Cu(dmpzpp)Cl 6, Cu(dmpzpp)Br 7, Cu(dmpzpp)I 8, Cu(dmpzpp)SPh 9, and Ag(dmpzpp)I 10 with SPh = thiophenylato, were prepared and their crystal structures, TD-DFT electronic structures, and phosphorescence and thermally activated delayed fluorescence (TADF) properties were studied in detail. The photoluminescence quantum yields ΦPL of neat powder materials lie between 70 and 90% with emission colors from blue to yellow. Compound 9, with bulky ligands showing ΦPL = 90%, was used for detailed emission studies from T = 1.7 to 300 K. Up to T ≈ 70 K, 9 shows only long-lived phosphorescence with a radiative decay time of T1 of τr(phos) = 1 ms because of weak spin–orbit coupling. Accordingly, the zero-field splittings of T1 in three substates are < 1 cm–1 (0.1 meV). Individual decay times of 2400, 2250, and 292 μs are estimated. Presumably, the phosphorescence is essentially induced by spin–vibronic mechanisms. Up to T = 300 K, the radiative decay time decreases by more than two orders of magnitude to τr(TADF) = 5.6 μs because of the TADF effect. This short decay time is determined by the small gap of ΔE(S1–T1) = 600 cm–1 (74 meV) and the fast radiative S1 → S0 rate of 1.1 × 107 s–1 (91 ns). For fabrication of OLED devices, we applied sublimable 8 and 9 using cohost device structures. For example, with a concentration of 2 wt % of 8, a green-emitting OLED showing CIE coordinates of (0.33; 0.52), a high external quantum efficiency of up to EQE = 16.4%, and a high luminance of almost 10,000 cd m–2 could be fabricated. Strategies for designing compounds giving higher EQE are presented.