[Cr(ddpd)2]3+: A Molecular, Water-Soluble, Highly NIR-Emissive Ruby Analogue

Bright, long-lived emission from first-row transition-metal complexes is very challenging to achieve. Herein, we present a new strategy relying on the rational tuning of energy levels. With the aid of the large N-Cr-N bite angle of the tridentate ligand ddpd (N, N'-dimethyl-N, N'-dipyridine-2-ylpyridine-2,6-diamine) and its strong sigma-donating capabilities, a very large ligand-field splitting could be introduced in the chromium(III) complex [Cr(ddpd)(2)](3+), that shifts the deactivating and photoreactive T-4(2) state well above the emitting E-2 state. Prevention of back-intersystem crossing from the E-2 to the T-4(2) state enables exceptionally high near-infrared phosphorescence quantum yields and lifetimes for this 3d metal complex. The complex [Cr(ddpd)(2)](BF4)(3) is highly water-soluble and very stable towards thermal and photo-induced substitution reactions and can be used for fluorescence intensity-and lifetime-based oxygen sensing in the NIR.