Phosphorescence versus thermally activated delayed fluorescence. Controlling singlet-triplet splitting in brightly emitting and sublimable Cu(I) compounds.

Photophysical properties of two highly emissive three-coordinate Cu(I) complexes, (IPr)Cu(py2-BMe2) (1) and (Bzl-3,5Me)Cu(py2-BMe2) (2), with two different N-heterocyclic (NHC) ligands were investigated in detail (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; Bzl-3,5Me = 1,3-bis(3,5-dimethylphenyl)-1H-benzo[d]imidazol-2-ylidene; py2-BMe2 = di(2-pyridyl)dimethylborate). The compounds exhibit remarkably high emission quantum yields of more than 70% in the powder phase. Despite similar chemical structures of both complexes, only compound 1 exhibits thermally activated delayed blue fluorescence (TADF), whereas compound 2 shows a pure, yellow phosphorescence. This behavior is related to the torsion angles between the two ligands. Changing this angle has a huge impact on the energy splitting between the first excited singlet state S1 and triplet state T1 and therefore on the TADF properties. In addition, it was found that, in both compounds, spin-orbit coupling (SOC) is particularly effective compared to other Cu(I) complexes. This is reflected in short emission decay times of the triplet states of only 34 µs (1) and 21 µs (2), respectively, as well as in the zero-field splittings of the triplet states amounting to 4 cm(-1) (0.5 meV) for 1 and 5 cm(-1) (0.6 meV) for 2. Accordingly, at ambient temperature, compound 1 exhibits two radiative decay paths which are thermally equilibrated: one via the S1 state as TADF path (62%) and one via the T1 state as phosphorescence path (38%). Thus, if this material is applied in an organic light-emitting diode, the generated excitons are harvested mainly in the singlet state, but to a significant portion also in the triplet state. This novel mechanism based on two separate radiative decay paths reduces the overall emission decay time distinctly.

Control of emission colour with N-heterocyclic carbene (NHC) ligands in phosphorescent three-coordinate Cu(I) complexes.

A series of three phosphorescent mononuclear (NHC)-Cu(I) complexes were prepared and characterized. Photophysical properties were found to be largely controlled by the NHC ligand chromophore. Variation of the NHC ligand leads to emission colour tuning over 200 nm range from blue to red, and emission efficiencies of 0.16-0.80 in the solid state.

Synthesis and characterization of phosphorescent three-coordinate Cu(I)-NHC complexes.

Cationic and neutral monomeric three-coordinate phosphorescent Cu(I) complexes were synthesized and characterized by XRD analysis, electrochemistry and photophysical studies in different environments. DFT calculations have aided the assignment of the electronic structure and excited state behavior of these complexes.