Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I) Complexes Featuring Nonconventional N‑Heterocyclic Carbenes

A series of six luminescent two-coordinate
Cu­(I) complexes were
investigated bearing nonconventional N-heterocyclic carbene ligands,
monoamido-aminocarbene (MAC*) and diamidocarbene (DAC*), along with
carbazolyl (Cz) as well as mono- and dicyano-substituted Cz derivatives.
The emission color can be systematically varied over 270 nm, from
violet to red, through proper choice of the acceptor (carbene) and
donor (carbazolyl) groups. The compounds exhibit photoluminescent
quantum efficiencies up to 100% in fluid solution and polystyrene
films with short decay lifetimes (τ ≈ 1 μs). The
radiative rate constants for the Cu­(I) complexes (kr = 105–106 s–1) are comparable to state of the art phosphorescent emitters with
noble metals such as Ir and Pt. All complexes show strong solvatochromism
due to the large dipole moment of the ground states and the transition
dipole moment that is in the opposite direction. Temperature-dependent
studies of (MAC*)­Cu­(Cz) reveal a small energy separation between the
lowest singlet and triplet states (ΔES1–T1 = 500 cm–1)
and an exceptionally large zero-field splitting (ZFS = 85 cm–1). Organic light-emitting diodes (OLEDs) fabricated with (MAC*)­Cu­(Cz)
as a green emissive dopant have high external quantum efficiencies
(EQE = 19.4%) and brightness of 54 000 cd/m2 with
modest roll-off at high currents. The complex can also serve as a
neat emissive layer to make highly efficient OLEDs (EQE = 16.3%).