Brightly Blue and Green Emitting Cu(I) Dimers for Singlet Harvesting in OLEDs

Abstract

With the chelating aminophosphane ligands Ph2P-(o-C6H4)-N(CH3)2 (PNMe2) and Ph2P-(o-C6H4)-NC4H8 (PNpy), the four halide (Cl, Br, I)-bridged copper coordination compounds [Cu(μ-Cl)(PNMe2)]2 (1), [Cu(μ-Br)(PNMe2)]2 (2), [Cu(μ-I)(PNMe2)]2 (3), and [Cu(μ-I)(PNpy)]2 (4) were synthesized and structurally characterized. Their photophysical properties were studied in detail. The complexes exhibit strong blue (λmax = 464 (3) and 465 nm (4)) and green (λmax = 506 (1) and 490 nm (2)) luminescence as powders with quantum yields of up to 65% at decay times as short as 4.1 μs. An investigation of the emission decay behavior between 1.3 and 300 K gives insight into the nature of the emitting states. At temperatures below T ≈ 60 K, the decay times of the studied compounds are several hundred microseconds long, which indicates that the emission originates from a triplet state (T1 state). DFT calculations show that this state is of (metal+halide)-to-ligand charge transfer (3)(M+X)LCT character. Investigations at 1.3 K allow us to gain insight into the three triplet substates, in particular, to determine the individual substate decay times being as long as a few milliseconds. The zero-field splittings are smaller than 1 or 2 cm(-1). With an analysis of these data, conclusions about the effectiveness of spin-orbit coupling (SOC) can be drawn. Interestingly, the large differences of SOC constants of the halides are not obviously displayed in the triplet state properties. With a temperature increase from T ≈ 60 to 300 K, a significant decrease of the emission decay time by almost 2 orders of magnitude is observed, and at ambient temperature, the decay times amount only to ∼4-7 μs without a significant reduction of the emission quantum yields. This drastic decrease of the (radiative) decay time is a result of the thermal population of a short-lived singlet state (S1 state) that lies energetically only a few hundred wavenumbers (460-630 cm(-1)) higher than the T1 state. Such an emission mechanism corresponds to a thermally activated delayed fluorescence (TADF). At ambient temperature, almost only a delayed fluorescence (∼98%) is observed. Compounds showing this mechanism are highly attractive for applications in OLEDs or LEECs as, in principle, it is possible to harvest all singlet and triplet excitons for the generation of light in the lowest excited singlet state. This effect represents the singlet harvesting mechanism.