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

Abstract

The four new Ag(I) complexes Ag(phen)(P-nCB) (1), Ag(idmp)(P-nCB) (2), Ag(dmp)(P-nCB) (3), and Ag(dbp)(P-nCB) (4) with P-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 S → S transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S and T states. The oscillator strength of the S ↔ S transition f(S ↔ S)-an important parameter for the TADF efficiency being proportional to the radiative rate-can be increased from f(S ↔ S) = 0.0258 for Ag(phen)(P-nCB) (1) to f(S ↔ S) = 0.0536 for Ag(dbp)(P-nCB) (4), as calculated for the T state optimized geometries. This parameter governs the radiative TADF decay time (τ) at ambient temperature, found to be τ = 5.6 μs for Ag(phen)(P-nCB) (1) but only τ = 1.4 μs for Ag(dbp)(P-nCB) (4)-a record TADF value. In parallel, the photoluminescence quantum yield (Φ) measured for powder samples at ambient temperature is boosted up from Φ = 36% for Ag(phen)(P-nCB) (1) to Φ = 100% for Ag(dbp)(P-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)(P-nCB) (1), Ag(idmp)(P-nCB) (2), and Ag(dmp)(P-nCB) (3) to Ag(dbp)(P-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S state from the state T, ΔE(S-T). Experimentally it is determined for the complexes Ag(dmp)(P-nCB) (3) and Ag(dbp)(P-nCB) (4) to be of moderate size of ΔE(S-T) = 650 cm.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuIand AgIComplexes

“…However, these shortcomings may be suppressed to a large extent by rigidifying the molecular structure either by sterically demanding ligands or by a rigid environment. This behavior has already been discussed frequently in the literature . We will address these properties in Sections 3 and 6.…”

“…In contrast to Cu I complexes, TADF materials based on Ag I are rarely reported . This is related to the higher oxidation potential of Ag + compared to Cu + .…”

“…Suitable complexes can be built using the (P 2 ‐nCB) ligand in combination with 1,10‐phenanthroline (phen) or substituted phen ligands. Accordingly, a series of neutral Ag I complexes, referring to the numbers from 9 to 13 , is obtained (Table ) . In this section, we want to focus on these Ag(phen)(P 2 ‐nCB) type compounds and to demonstrate key steps for designing a material that shows TADF behavior.…”

“…Interestingly, design of a material, a silver complex, with Φ PL of 100 % becomes possible by following this strategy of rigidifying the molecular structure (see refs. and Section 6). Well designed TADF materials should exhibit relatively small energy separations Δ E (S 1 –T 1 ). For organo‐transition metal compounds this is related to the occurrence of metal‐to‐ligand charge transfer (MLCT) (and ligand‐to‐ligand charge transfer (LL′CT)) states having frontier orbitals, HOMO and LUMO, that are spatially largely separated.…”

“…Interestingly, design of a material, a silver complex, with Φ PL of 100 % becomes possible by following this strategy of rigidifying the molecular structure (see refs. and Section 6).…”