Strategies for highly luminescent d10 coinage metal compounds challenging PtII- and IrIII-based emitters

• Applicant: Professor Dr. Andreas Steffen
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2011 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 195201394

Our preliminary results have shown that non-traditional carbene ligands, such as CAACs, can act as exceptionally potent pi-chromophore ligands, leading to very high radiative rate constants, which is a prerequisite for high quantum yields and application of transition metal complexes in OLEDs or as NIR single photon sources. Furthermore, we have shown that dispersive interactions between d10 metal centers as well as between ligands can also have a beneficial influence on the excited state properties. However, a detailed structure-property relationship is still missing, although we have formulated first design principles, which is mandatory prior to applying d10 coinage metal complexes bearing such types of ligands or such dispersive interactions. We thus aim at a) exploiting exceptional pi-acceptor carbenes for enhancing spin-orbit coupling (SOC), and b) a deep understanding of the influence of metal-metal interactions on the photophysical properties of the excited states, both of which are necessary in order to fully control the properties and to be able to design materials for a given application.In most of the T1 and TADF emitters based on d10 coinage metal complexes both absorption S0-S1 and emission T1-S0 are dominated by an overlap and symmetry forbidden HOMO-LUMO transition, with the HOMO being of Cu(d-sigma) and the LUMO of ligand(pi*) character. As SOC can only occur between triplet and singlet states which differ in one and only one M(d) orbital, phosphorescence is ensured only by coupling of the triplet excited state T1 with 1MLCT states of reasonable oscillator strength Sn-S0, which are usually energetically very high lying, limiting the radiative rate constant k(r). By either raising the energy of the M(d) orbitals of pi symmetry with respect to the carbene centered LUMO, or by decreasing the energy of the M(d-sigma) orbital below the M(d-pi) we will be able to decrease the energy gap between T1 and those 1MLCT states with a high transition dipole moment coupling them with the ground state S0. We will work out the design criteria for pure T1 emitters in the blue, and TADF emission in the red to near-IR. Suitable candidates will be exploited in devices as well as time-correlated single photon sources.Metallophilic interactions pre-designed in the ground state allow both metals to participate in the excited state and have a great influence on the radiative rate constants for T1 emission as well as for TADF. Also, intra- and intermolecular pi-stacking of the chromophore ligands can drastically alter the photophysical properties of a given system. We aim to explore these concepts further to I) be able to design emitter materials for a given application and II) generate stimuli-responsive photoactive materials.

• DFG Programme: Research Grants