Luminescent complexes of heavy metals such as iridium, platinum, and ruthenium play an important role in photocatalysis and energy conversion applications as well as organic light-emitting diodes (OLEDs). Achieving comparable performance from more–earth-abundant copper requires overcoming the weak spin-orbit coupling of the light metal as well as limiting the high reorganization energies typical in copper(I) [Cu(I)] complexes. Here we report that two-coordinate Cu(I) complexes with redox active ligands in coplanar conformation manifest suppressed nonradiative decay, reduced structural reorganization, and sufficient orbital overlap for efficient charge transfer. We achieve photoluminescence efficiencies >99% and microsecond lifetimes, which lead to an efficient blue-emitting OLED. Photophysical analysis and simulations reveal a temperature-dependent interplay between emissive singlet and triplet charge-transfer states and amide-localized triplet states.

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) are among the most nucleophilic (σ donating) and also electrophilic (π-accepting) stable carbenes known to date. These properties allow them to activate a variety of small molecules and enthalpically strong bonds, to stabilize highly reactive main-group and transition-metal diamagnetic and paramagnetic species, and to bind strongly to metal centers, which gives rise to very robust catalysts. The most important results published up to the end of 2013 are briefly summarized, while the majority of this Review focuses on findings reported within the last three years.

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 = (5R,6S)-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) (MCAAC) and (MAC)­M(I)(N-carbazolyl) (MMAC) 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 (τfl and τph, respectively) and the energy difference between the singlet and triplet excited state, ΔE S1–T1. The τfl values range between 20 and 85 ns, and the τph 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 ΔE S1–T1, with the silver complexes giving ΔE 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).

Transition-metal-catalyzed C-H activation has recently emerged as a powerful tool for the functionalization of organic molecules. While many efforts have focused on the functionalization of arenes and heteroarenes by this strategy in the past two decades, much less research has been devoted to the activation of non-acidic C-H bonds of alkyl groups. This Minireview highlights recent work in this area, with a particular emphasis on synthetically useful methods.