From ruby to molecular ruby: Excited state understanding and design

The gemstone ruby, corundum (α-Al2O3) with Al3+ ions partially substituted by Cr3+ ions (Al2O3:Cr3+) possesses exceptional photophysical properties with an extremely long-lived (4270 μs) spin-flip phosphorescence at 694 nm with a very high photoluminescence (PL) 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 PL 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, and inductive-resonant energy transfer to high-energy oscillators) transitions. These challenges or disadvantages at first sight also include possibilities or levers to significantly tune specifically the photophysical properties and reactivity, such as emission energy, and to enable applications in photocatalysis and sensing.