Near-Infrared Spin-Flip Luminophores with Earth-abundant Metal Ions 2.0 (NIR-SPINFLIP 2.0)

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Luminescent metal complexes are important building blocks in many applications such as organic light-emitting diodes. Currently, these technologies mainly rely on expensive metal precursors, while efficient luminophores based on cheap and earth-abundant metal ions are rare up to now. The development of materials that emit in the near-infrared (NIR) spectral region is particularly challenging due to the prevalence of many non-radiative decay pathways of electronically excited metal complexes, usually leading to low luminescence efficiencies. This project will develop and tune molecular spin-flip NIR emitters based on earth-abundant metal ions with d3 and d2 electron configurations (primarily vanadium and molybdenum ions). For the 3d2 vanadium(III) and 3d3 vanadium(II) metal ions, strong ligands will be coordinated in a pseudo-octahedral coordination geometry to increase the intrinsically weak ligand field splitting of vanadium and to push the energy of the interconfigurational ligand field excited states above the emissive spin-flip states. This will block the back-intersystem crossing (back-ISC) to these dark states. Installing bromine or iodine substituents at the ligands to exploit the heavy-atom effect will favor the ISC path from initially excited states to the spin-flip states. To increase the radiative rate for the spin-flip emission, heteroleptic, non-centrosymmetric complexes serve to mitigate Laporte’s rule. Systematic deuteration of the ligands and CH/CD overtone measurements serve to understand and decrease non-radiative decay via multiphonon relaxation. With these combined strategies, we will develop a redox-switchable luminescence (RSL) platform based on the vanadium(III/II) redox couple where both oxidation states exhibit luminescence in contrast to conventional redox-switchable on/off emitters.For the 4d metal ion molybdenum(III), which already possesses intrinsically large ligand field splittings and spin-orbit coupling constants, complementary strategies based on weak-field ligands and the installation of charge-transfer states slightly above the spin-flip states as antennas for efficient population of the spin-flip states will be developed.ISC rates will be determined by pump-probe spectroscopy. Step-scan FTIR spectroscopy will probe the structure of the spin-flip states, while NIR photoluminescene will be investigated by static and time-resolved spectroscopy in various environments. DFT calculations augmented by multireference calculations complement the spectroscopic studies.The results will deepen our understanding of excited state decay processes of transition metal complexes and further the systematic improvement for many key technologies involving metal complexes such as optical telecommunications or NIR biomedical imaging.