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)dppt1]₂ and [Cu(μ₂-I)dppt2]₂ (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)dppt2]₂ 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%.