Ligand Design Effects on Luminescence: A Case Study on [Cr(bimca^Me)₂]⁺

Luminescent and photoactive transition metal complexes play a key role in technologies like OLEDs or organic solar cells, sensing applications, photocatalysis, and biomedical applications. However, they typically rely on some of the most precious and expensive metals e.g. Ru or Ir, with high ligand field splitting and hence long-lived charge-transfer (CT) excited states. Recently, spin-flip (SF) emitters based on more abundant first and second row transition metals, like Cr^III, with d³ electron configuration are becoming increasingly significant in photochemistry and photophysics.^[2,3] They exhibit fundamentally different excited state properties compared to classical d⁶ charge-transfer (CT) emitters, resulting in advantageous characteristics, such as fast intersystem crossing, near infrared phosphorescence, sharp emission bands and long excited state lifetimes.^[3] However, clear design principles for efficient spin-flip emission are not yet well-established, and structural modifications can unexpectedly suppress luminescence. To better understand these effects, we combine spectroscopic methods with quantum chemical calculations to study the photophysical behavior of [Cr(bimca^Me)₂]⁺, where weak ligand-centered fluorescence can be observed. [1] C. Förster, K. Heinze, Chemical Society reviews 2020, 49, 1057–1070. [2] W. R. Kitzmann, J. Moll, K. Heinze, Photochemical & photobiological sciences : Official journal of the European Photochemistry Association and the European Society for Photobiology 2022, 21, 1309–1331. [3] W. R. Kitzmann, K. Heinze, Angewandte Chemie (International ed. in English) 2023, 62, e202213207.