Two coordinate, monovalent copper complexes as chromophores and luminophores

Abstract

Two-coordinate Cu(I) complexes have recently emerged as promising luminescent materials for optoelectronic and photocatalytic applications. The complexes have a carbene-metal-amide (cMa) structure where an N-heterocyclic carbene ligand acts as an electron acceptor and the amide ligand serves as an electron donor. The donor-metal-acceptor motif leads to strong interligand charge transfer (ICT) absorption and emission transitions in the cMa complexes. The energy of the ICT transition is determined by the ionization potential of the amide and electron affinity of the carbene and can be varied over the entire visible spectrum by proper choice of the ligands. The highly efficient luminescence is characterized as thermally activated delayed fluorescence (TADF). Emission lifetimes are fast, typically in the 1-2 microsecond range, that are facilitated by strong spin-orbit coupling of the metal center which promotes rapid intersystem crossing between the lowest singlet and triplet states. The complexes have large permanent dipole moments in the ground state that reverse direction in the excited state. This change dipole orientation upon excitation leads to strong negative solvatochromism with increasing solvent polarity in absorption and emission. The radiative rate for the complexes is determined by the extent of overlap between the valence orbitals on the ligands mediated by the small contributions from the metal center. The radiative rate is also correlated to the distance separating the hole and electron in the excited state. The cMa complexes have been employed as luminescent dopants in organic light emitting diodes (OLEDs). OLEDs fabricated using the cMa complexes are highly efficient devices that rival the performance of devices that use precious metal phosphorescent dopants. The cMa complexes have also been used as photosensitizers to catalytically produce H2.