Highly Efficient Thermally Activated Delayed Fluorescence in Dinuclear Ag(I) Complexes with a Bis-Bidentate Tetraphosphane Bridging Ligand.

A series of highly emissive neutral dinuclear silver complexes [Ag(PPh3)(X)]2(tpbz) (tpbz = 1,2,4,5-tetrakis(diphenylphosphanyl)benzene; X = Cl (1), Br (2), I (3)) was synthesized and structurally characterized. In the complexes, the silver atoms with tetradedral geometry are bridged by the tpbz ligand, and the ends of the molecules are coordinated by a halogen anion and a terminal triphenylphosphine ligand for each silver atom. These complexes exhibit intense white-blue (λmax = 475 nm (1) and 471 nm (2)) and green (λmax = 495 nm (3)) photoluminescence in the solid state with quantum yields of up to 98% (1) and emissive decay rates of up to 3.3 × 105 s-1 (1) at 298 K. With temperature decreasing from 298 to 77 K, a red shift of the emission maximum by 9 nm for all these complexes is observed. The temperature dependence of the luminescence for complex 1 in solid state indicates that the emission originates from two thermally equilibrated charge transfer (CT) excited states and exhibits highly efficient thermally activated delayed fluorescence (TADF) at ambient temperature. At 77 K, the decay time is 638 µs, indicating that the emission is mainly from a triplet state (T1 state). With temperature increasing from 77 to 298 K, a significant decrease of the emissive decay time by a factor of almost 210 is observed, and at 298 K, the decay time is 3.0 µs. The remarkable decrease of the decay time indicates that thermal population of a short-lived singlet state (S1 state) increases as the temperature increases. The charge transfer character of the excited states and TADF behavior of the complexes are interrogated by DFT and TDDFT calculations. The computational results demonstrate that the origin of TADF can be ascribed to 1,3(ILCT + XLCT+ MLCT) states in complexes 1 and 2 and 1,3(XLCT) states mixed with minor contributions of MLCT and ILCT in complex 3.

Experimental and theoretical studies of highly emissive dinuclear Cu(I) halide complexes with delayed fluorescence.

A series of luminescent homo-dinuclear Cu(I) halide complexes, [PPh2PAr2Cu(µ-X)2CuPPh2PAr2] (X = I (1), Br (2), Cl (3)) (PPh2PAr2 = (1-bis(2-methylphenyl)phosphino-2-diphenylphosphino)benzene) were synthesized from the reaction of the corresponding cuprous halide and the chelating bisphosphine ligand PPh2PAr2 in CH3CN. The complexes were structurally characterized by X-ray single crystal analysis. Their photophysical properties were studied in detail. The Cu(I) atoms in these complexes are four-coordinated and adopt a tetrahedral coordination geometry. In each complex, the copper centers are bridged by two halide anions and each Cu(I) is chelated further terminally by a PPh2PAr2 ligand. The[Cu(µ-X)2Cu] cores have similar butterfly-type configurations. The distances between the Cu(I) atoms in each complex are over 2.94 Å. In the solid state, these complexes are highly emissive and exhibit bluish-green photoluminescence (emission peaks, λmax = 488 nm (1), 482 nm (2), 490 nm (3)) with short lifetimes (4.9-5.9 µs) and high quantum yields (ϕ = 0.42-0.95) at room temperature. In this series of complexes, the ligand-field strengths of the ions (I(-) < Br(-) < Cl(-)) do not have obvious effect on the emission maxima. The studies on varied temperature emission spectra and decay behaviours of these complexes indicate that the mechanism of their emissions involves two thermal-equilibrium excited states. At room temperature, the complexes display thermally activated delayed fluorescences with short decay lifetimes. With a decrease of the temperature, a significant increase of emission decay times by almost 2 orders of magnitude is observed. At temperatures below T ≈ 100 K, the decay times of the studied complexes are over one hundred microseconds long, which indicates that the emission originates mainly from the triplet state (T1 state). To interpret the varied temperature photophysics of these complexes, an equilibrated 2 excited states model S0 ← T1 ↔ S1 → S0 was used. The results of the experimental and DFT calculations suggest that the emission in the solid state originates from the (1,3)(MLCT + XLCT + ILCT) excited states, in which emissive excited states, (1)S and (3)T, are in equilibrium with an energy difference of about 0.055 eV. The process of the reverse intersystem crossing was estimated to be in the order of 2 ns.