Mono- versus Bicyclic Carbene Metal Amide Photoemitters: Which Design Leads to the Best Performance?

Abstract

New luminescent "carbene-metal-amide" (CMA) Cu, Ag and Au complexes based on monocyclic (C6) or bicyclic six-ring (BIC6) cyclic (alkyl)(amino)carbene ligands illustrates the effects of LUMO energy stabilization, conformational flexibility, excited state energy and geometry on the photoluminescent properties, leading to 100% luminescence quantum yields, short excited state lifetimes Cu > Au > Ag down to 0.5 µs and high radiative rates of 10 6 s -1 . Gold complexes with the BIC6 ligand exhibit exceptional photostability under hard and soft UV-light compared with analogous complexes with C5 and C6 carbenes. Steady-state and time-resolved photoluminescence spectroscopy at 298 K and 77 K enabled an estimate of the energy levels of the charge transfer (CT) and locally excited (LE) states with singlet and triplet character. A four-state model is applied to describe thermally activated delayed fluorescence (TADF) properties in CMA materials and correlates excited state lifetimes with the energy difference between LE and CT states.

Synthesis and Thermochromic Luminescence of Ag(I) Complexes Based on 4,6-Bis(diphenylphosphino)-Pyrimidine

“…The DFT computations performed on complex 1 support this suggestion revealing that HOMO and near-HOMOs are contributed by silver d-orbitals and p-orbitals of nitrate oxygen atoms, while LUMO and near-LUMOs are pure pyrimidine π-orbitals (Figure S10). Note the previously reported Ag(I) complexes feature a similar HOMO/LUMO distribution pattern [22][23][24][25][26][27][36][37][38][39][40][41][42]. Thus, it can be assumed that the emissive excited state of 1 has a 1,3 (M + L')LCT character (L' = NO 3 , and L = pyrimidyldiphosphine).…”

“…As a result, the emission energy shifts in the blue region upon warming from 77 to 300 K, resulting in thermochromic luminescence of 1 and 2. Considering the literature data on the related Ag(I) complexes exhibiting TADF [22][23][24][25][26][27][36][37][38][39][40][41][42], we believe that the S1 and T1 excited states As seen from Figure 5, the luminescence of both compounds at ambient temperature represents TADF because the τ obs (T) curves attain the high-temperature plateau. The pure phosphorescence begins to appear when the τ obs (T) curve reaches the low-temperature plateau.…”

“…As a result, the emission energy shifts in the blue region upon warming from 77 to 300 K, resulting in thermochromic luminescence of 1 and 2. Considering the literature data on the related Ag(I) complexes exhibiting TADF [22][23][24][25][26][27][36][37][38][39][40][41][42], we believe that the S 1 and T 1 excited states of 1 and 2 are of MLCT or (M + L')LCT nature. The DFT computations performed on complex 1 support this suggestion revealing that HOMO and near-HOMOs are contributed by silver d-orbitals and p-orbitals of nitrate oxygen atoms, while LUMO and near-LUMOs are pure pyrimidine π-orbitals (Figure S10).…”

“…On account of highly electron-donating ligands, the silver d 10 orbitals begin to contribute to HOMO and near-HOMO, while the π-acceptors facilitate charge transfer from the metal. As a result, (M + L')LCT excited states can be generated, thereby inducing thermally activated delayed fluorescence (TADF) in Ag(I) compounds [22][23][24][25][26][27][36][37][38][39][40][41][42]. In the context of OLED application, it is relevant to note that the (M + L')LCT emission of Ag(I) benefits over that of Cu(I) analogues since the former (i) is shorter in the lifetimes [36,41,42], and (ii) generally appears in the higher energy domain [36,37,39,[43][44][45].…”

“…As a result, (M + L')LCT excited states can be generated, thereby inducing thermally activated delayed fluorescence (TADF) in Ag(I) compounds [22][23][24][25][26][27][36][37][38][39][40][41][42]. In the context of OLED application, it is relevant to note that the (M + L')LCT emission of Ag(I) benefits over that of Cu(I) analogues since the former (i) is shorter in the lifetimes [36,41,42], and (ii) generally appears in the higher energy domain [36,37,39,[43][44][45]. On the whole, Ag(I) compounds that emit apparent luminescence at ambient temperature, especially, that of TADF nature, are still rare.…”