Bluish-Green Cu(I) Dimers Chelated with Thiophene Ring-Introduced Diphosphine Ligands for Both Singlet and Triplet Harvesting in OLEDs

Two new Cu(I) dimers chelated with thiophene ring-introduced diphosphine ligands Cu(μ2-I)dppt12 and Cu(μ2-I)dppt22 (dppt1 = 3,4-bis(diphenylphosphino)thiophene, dppt2 = 2,3-bis(diphenylphosphino)thiophene) have been prepared and studied in terms of photoluminescence and electroluminescence properties. Both dimers exhibited two independent radiative decay pathways, which are equilibrated thermally at room temperature: one is thermally activated delay fluorescence (TADF) via the first singlet excited state (S1) decay and the other is phosphorescence via the first triplet excited state (T1) decay. The dual emission mechanism for both singlet and triplet harvesting, as well as excellent photoluminescence properties such as bluish-green emission color (487 and 483 nm), short decay times (9.46 and 7.62 μs), and high photoluminescence quantum yields (69% and 86%) of the two Cu(I) dimers, implies their potential to be highly efficient emitter molecules for organic light emitting diode (OLED) applications. As a result, the optimized OLEDs with Cu(μ2-I)dppt22 showed the highest efficiency, exhibiting a current efficiency up to 32.2 cd A-1, a peak brightness of 3.67 × 103 cd m-2, as well as a maximum external quantum efficiency of 14.5%.