Exploration of Zn(II) complexes for efficient phosphorescence and TADF

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The investigation of transition metal complexes as emitters is of fundamental im¬portance for the development of highly innovative applications in lighting and display technology. Because the electron-hole-recombination process produces ca. 75% triplet states, it is highly desirable to exploit these states for luminescence. In the last decade, luminescent Cu(I) complexes have seen a phoenix-like rise, with many proof-of-concept applications in OLEDs. Their attractive¬ness is mainly due to the absence of low-energy d-d* states which could give rise to very fast non-radiative decay and to the fact that many of these complexes show thermally activated delayed fluorescence (TADF), providing an emission mecha¬nism bypassing the spin-forbidden T1→S0 transition by thermal re-population of the singlet excited state S1 with subse¬quent fluorescence S1→S0. The beauty of this mech¬anism is that it re¬moves the necessity of strong spin–orbit coupling (SOC) and greatly increases the overall radiative rate constant kr. Interestingly, Zn(II) complexes, which also exhibit a d10 electron configuration, have received very little attention as emitters to exploit triplet excitons by phosphorescence or TADF. Based on our previous achievements with regard to photoactive d10 transition metal carbene complexes and our preliminary results, we aim to close this gap. This collaborative project proposal combines the synthetic and spectroscopic expertise of the Steffen group and the theoretical expertise of the Marian group to develop Zn(II) complexes for efficient luminescence exploiting triplet excitons, either via phosphorescence or via TADF, by employing carbene ligands of varying π-acceptor strength in various coordination geometries to enable LLCT and LMCT states with strong SOC. We will study environmental influences on the properties of the long-lived excited states, establish theoretical models for their photokinetic excited state behavior, and demonstrate the use of selected candidates in first proof-of-concept applications.