P∩N Bridged Cu(I) Dimers Featuring Both TADF and Phosphorescence. From Overview towards Detailed Case Study of the Excited Singlet and Triplet States

We present an overview over eight brightly luminescent Cu(I) dimers of the type Cu 2X 2(P∩N) 3 with X = Cl, Br, I and P∩N = 2-diphenylphosphino-pyridine (Ph 2Ppy), 2-diphenylphosphino-pyrimidine (Ph 2Ppym), 1-diphenylphosphino-isoquinoline (Ph 2Piqn) including three new crystal structures (Cu 2Br 2(Ph 2Ppy) 3  1-Br, Cu 2I 2(Ph 2Ppym) 3  2-I and Cu 2I 2(Ph 2Piqn) 3  3-I). However, we mainly focus on their photo-luminescence properties. All compounds exhibit combined thermally activated delayed fluorescence (TADF) and phosphorescence at ambient temperature. Emission color, decay time and quantum yield vary over large ranges. For deeper characterization, we select Cu 2I 2(Ph 2Ppy) 3, 1-I, showing a quantum yield of 81%. DFT and SOC-TDDFT calculations provide insight into the electronic structures of the singlet S 1 and triplet T 1 states. Both stem from metal+iodide-to-ligand charge transfer transitions. Evaluation of the emission decay dynamics, measured from 1.2 ≤ T ≤ 300 K, gives ∆E(S 1-T 1) = 380 cm -1 (47 meV), a transition rate of k(S 1→S 0) = 2.25 × 10 6 s -1 (445 ns), T 1 zero-field splittings, transition rates from the triplet substates and spin-lattice relaxation times. We also discuss the interplay of S 1-TADF and T 1-phosphorescence. The combined emission paths shorten the overall decay time. For OLED applications, utilization of both singlet and triplet harvesting can be highly favorable for improvement of the device performance.