A Strongly Luminescent Chromium(III) Complex Acid.

The synthesis, structure, reactivity, and photophysical properties of a novel acidic, luminescent chromium(III) complex [Cr(H2 tpda)2 ]3+ (23+ ) bearing the tridentate H2 tpda (2,6-bis(2-pyridylamino)pyridine) ligand are presented. Excitation of 23+ 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 23+ in the solid state. Deprotonation of the NH groups of 23+ by using a non-nucleophilic Schwesinger base in CH3 CN switches off the luminescence. Re-protonation by using HClO4 restores the emission. In water, the pKa value of 23+ amounts to 8.8, yet deprotonation is not reversible in the presence of hydroxide ions. Dioxygen quenches the emission of 23+ , but to a weaker extent than expected. This is possibly due to the strong ion-pairing properties of 23+ even in solution, reducing the energy transfer efficiency to O2 . Deuteration of the NH groups of 23+ approximately doubles the quantum yield and lifetime in water, demonstrating the importance of multiphoton relaxation in these NIR emitters.