"Quick-Silver" from a Systematic Study of Highly Luminescent, Two-Coordinate, d¹⁰ Coinage Metal Complexes

A systematic study is presented on the physical and photophysical properties of isoelectronic and isostructural Cu, Ag, and Au complexes with a common amide (N-carbazolyl) and two different carbene ligands (i.e., CAAC = (5 R,6 S)-2-(2,6-diisopropylphenyl)-6-isopropyl-3,3,9-trimethyl-2-azaspiro[4.5]decan-2-ylidene, MAC = 1,3-bis(2,6-diisopropylphenyl)-5,5-dimethyl-4-keto-tetrahydropyridylidene). The crystal structures of the (carbene)M^(I)(N-carbazolyl) (M^CAAC) and (MAC)M^(I)(N-carbazolyl) (M^MAC) complexes show coplanar carbene and carbzole ligands and C-M-N bond angles of ∼180°. The electrochemical properties and energies for charge transfer (CT) absorption and emission compounds are not significantly affected by the choice of metal ion. All six of the (carbene)M(Cz) complexes examined here display high photoluminescence quantum yields of 0.8-1.0. The compounds have short emission lifetimes (τ = 0.33-2.8 μs) that fall in the order Ag < Au < Cu, with the lifetimes of (carbene)Ag(Cz) roughly a factor of 10 shorter than for (carbene)Cu(Cz) complexes. Detailed temperature-dependent photophysical measurements (5-325 K) were carried out to determine the singlet and triplet emission lifetimes (τ_fl and τ_ph, respectively) and the energy difference between the singlet and triplet excited state, Δ E_S1-T1. The τ_fl values range between 20 and 85 ns, and the τ_ph values are in the 50-200 μs regime. The emission at room temperature is due exclusively to E-type delayed fluorescence or TADF (i.e., $$ ). The emission rate at room temperature is fully governed by Δ E_S1-T1, with the silver complexes giving Δ E_S1-T1 values of 150-180 cm^-1 (18-23 meV), whereas the gold and copper complexes give values of 570-590 cm^-1 (70-73 meV).