A Strongly Luminescent Chromium(III) Complex Acid

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.