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

Abstract

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 σ‐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 4T2 state well above the emitting 2E state. Prevention of back‐intersystem crossing from the 2E to the 4T2 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.

Strong, long‐lived, and sharp NIR phosphorescence (775 nm, Φ=12 %, τ=899 μs) is achieved in the soluble chromium(III) complex [Cr(ddpd)2]3+ (ddpd=N,N′‐dimethyl‐N,N′‐dipyridin‐2‐ylpyridine‐2,6‐diamine) by ligand‐field tuning through optimization of the ligand’s bite angle and σ‐donor strength. 3O2 quenches the emission allowing for optical oxygen sensing. The highly stable complex is easy to prepare in high yields from inexpensive starting materials.

Authors:   Sven Otto, Markus Grabolle, Christoph Förster, Christoph Kreitner, Ute Resch‐Genger, Katja Heinze
Journal:   Angewandte Chemie International Edition
Year:   2015
Pages:   n/a
DOI:   10.1002/anie.201504894
Publication date:   12-Aug-2015
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