Thermally Activated Delayed Fluorescence from Ag(I) Complexes: A Route to 100% Quantum Yield at Unprecedentedly Short Decay Time

The four new Ag­(I) complexes Ag­(phen)­(P₂-nCB) (1), Ag­(idmp)­(P₂-nCB) (2), Ag­(dmp)­(P₂-nCB) (3), and Ag­(dbp)­(P₂-nCB) (4) with P₂-nCB = bis­(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were designed to demonstrate how to develop Ag­(I) complexes that exhibit highly efficient thermally activated delayed fluorescence (TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via influencing the radiative rate of the S₁ → S₀ transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S₁ and T₁ states. The oscillator strength of the S₁ ↔ S₀ transition f(S₁ ↔ S₀)an important parameter for the TADF efficiency being proportional to the radiative ratecan be increased from f(S₁ ↔ S₀) = 0.0258 for Ag­(phen)­(P₂-nCB) (1) to f(S₁ ↔ S₀) = 0.0536 for Ag­(dbp)­(P₂-nCB) (4), as calculated for the T₁ state optimized geometries. This parameter governs the radiative TADF decay time (τ^r) at ambient temperature, found to be τ^r = 5.6 μs for Ag­(phen)­(P₂-nCB) (1) but only τ^r = 1.4 μs for Ag­(dbp)­(P₂-nCB) (4)a record TADF value. In parallel, the photoluminescence quantum yield (Φ_PL) measured for powder samples at ambient temperature is boosted up from Φ_PL = 36% for Ag­(phen)­(P₂-nCB) (1) to Φ_PL = 100% for Ag­(dbp)­(P₂-nCB) (4). This is a consequence of a cooperative effect of both decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag­(phen)­(P₂-nCB) (1), Ag­(idmp)­(P₂-nCB) (2), and Ag­(dmp)­(P₂-nCB) (3) to Ag­(dbp)­(P₂-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S₁ state from the state T₁, ΔE(S₁–T₁). Experimentally it is determined for the complexes Ag­(dmp)­(P₂-nCB) (3) and Ag­(dbp)­(P₂-nCB) (4) to be of moderate size of ΔE(S₁–T₁) = 650 cm^–1.