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 (τr) at ambient temperature, found to be τr = 5.6 μs for Ag(phen)(P₂-nCB) (1) but only τr = 1.4 μs for Ag(dbp)(P₂-nCB) (4)-a record TADF value. In parallel, the photoluminescence quantum yield (ΦPL) measured for powder samples at ambient temperature is boosted up from ΦPL = 36% for Ag(phen)(P₂-nCB) (1) to ΦPL = 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.