From Ruby to Molecular Ruby: Excited State Understanding and Design

The gemstone ruby, corundum (α-Al 2 O 3 ) doped with Cr 3+ ions (Al 2 O 3 :Cr 3+ ) possesses exceptional photophysical properties with an extremely long-lived (4270 µs) spin-flip phosphorescence at 694 nm with very high photoluminescence quantum yield (90%). The phosphorescent soluble molecular congener [Cr(ddpd) 2 ] 3+ called "Molecular Ruby", emitting at 738 and 775 nm also shows record values in the field of chromium(III) complexes in photoluminescence lifetime (1122 µs) and quantum yield (13.7%). The comparably lower photophysical parameters stress the huge challenge of designing high-performance photoluminescent molecular systems. This tutorial review discusses general design concepts of Molecular Rubies, which are transferable to other photoactive molecular complexes. An introductory theoretical frame is given, including crystal field, ligand field and molecular orbital theory of transition metal complexes, also highlighting photophysical processes in vertical (absorption and emission) and horizontal (internal conversion, intersystem crossing, inductive resonant energy transfer to high energy oscillators) transitions. These challenges or disadvantages on first sight also include possibilities or levers to significantly tune specifically the photophysical properties and reactivity, such as emission energy and applications in photocatalysis.