TADF for singlet harvesting: next generation OLED materials based on brightly green and blue emitting Cu(I) and Ag(I) compounds

Detailed photophysical studies are presented for Cu₂Cl₂(dppb)₂ and Ag₂Cl₂(dppb)₂. Both compounds show very effective thermally activated delayed fluorescence (TADF) at ambient temperature with an emission quantum yield of the Ag(I) complex of Φ_PL(300 K) = 93 %. This emission is blue shifted by 65 nm (2500 cm^-1) with respect to the emission of the Cu(I) complex, demonstrating a valuable strategy for engineering blue light emitters. Potentially, these materials are well suited for taking advantage of the singlet harvesting effect in an OLED device. Moreover, both compounds do not show effects of concentration quenching at high emitter concentration, a property which might be attractive for reducing the efficiency roll-off at higher current densities. Investigations down to T = 1.6 K show that spin-orbit coupling (SOC) is particularly weak. This is displayed in the very long emission decay times of the triplet states (T₁ states) of metal-toligand charge transfer (³MLCT) character, amounting to τ(Ag₂Cl₂(dppb)₂) = 1.1 ms and τ(Cu₂Cl₂(dppb)₂) = 2.2 ms. According to the TADF mechanism, which leads to the additional decay channel at ambient temperature via the S₁ state (of ¹MLCT character), an increase of the radiative rate by a factor of 70 and almost 500 for Ag₂Cl₂(dppb)₂ and Cu₂Cl₂(dppb)₂, respectively, is induced. This results in radiative rates at ambient temperature of k_r = 6.2 ∙ 10⁴ s^-1 (τ_r = 16 μs, Ag(I) complex) and 11.7 ∙ 10⁴ s^-1 (τ_r = 8.5 μs, Cu(I) complex). Simple approaches are presented that allow us to understand the weakness of SOC on the basis of results from DFT and TD-DFT calculations. Investigations of the emission decay properties down to T = 1.6 K further support the conclusions with respect to the SOC strength.