Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 24, Issue 48, Pages 12555-12563Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201802797
Keywords
chromium; deuteration; luminescence; polypyridine; tridentate ligands
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Funding
- Deutsche Forschungsgemeinschaft [HE 2778/10-1, RE 1203/23-1]
- Johannes Gutenberg University, Mainz
- Deutsche Forschungsgemeinschaft (Materials Science in Mainz) [GSC 266]
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The synthesis, structure, reactivity, and photophysical properties of a novel acidic, luminescent chromium(III) complex [Cr(H(2)tpda)(2)](3+) (2(3+)) bearing the tridentate H(2)tpda (2,6-bis(2-pyridylamino)pyridine) ligand are presented. Excitation of 2(3+) at 442 nm results in strong, long-lived NIR luminescence at 782 nm in water and in acetonitrile. X-ray diffraction analysis and IR spectroscopy reveal hydrogen-bonding interactions of the counter ions to the NH groups of 2(3+) in the solid state. Deprotonation of the NH groups of 2(3+) by using a non-nucleophilic Schwesinger base in CH3CN switches off the luminescence. Re-protonation by using HCIO4 restores the emission. In water, the pK(a) value of 2(3+) amounts to 8.8, yet deprotonation is not reversible in the presence of hydroxide ions. Dioxygen quenches the emission of 2(3+), but to a weaker extent than expected. This is possibly due to the strong ion-pairing properties of 2(3+) even in solution, reducing the energy transfer efficiency to O-2. Deuteration of the NH groups of 2(3+) approximately doubles the quantum yield and lifetime in water, demonstrating the importance of multiphoton relaxation in these NIR emitters.
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