[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 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 (4)T2 state well above the emitting (2)E state. Prevention of back-intersystem crossing from the (2)E to the (4)T2 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.
Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128 Mainz (Germany).
Sven
S
Otto
Division 1.10, Federal Institute for Materials Research and Testing (BAM), Richard-Willstätter-Strasse 11, 12489 Berlin (Germany).
Markus
M
Grabolle
Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128 Mainz (Germany).
Christoph
C
Förster
Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128 Mainz (Germany).|Graduate School Materials Science in Mainz, Staudingerweg 9, 55128 Mainz (Germany).
Christoph
C
Kreitner
Division 1.10, Federal Institute for Materials Research and Testing (BAM), Richard-Willstätter-Strasse 11, 12489 Berlin (Germany). ute.resch@bam.de.
Ute
U
Resch-Genger
Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128 Mainz (Germany). katja.heinze@uni-mainz.de.
Katja
K
Heinze
D002857:Chromium / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D012995:Solubility; D019265:Spectroscopy, Near-Infrared / Q000379:methods*; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
D056831:Coordination Complexes; D014867:Water; D002857:Chromium
NIR luminescence; chromium complexes; intersystem crossing; ligand-field splitting; photophysics