A Strongly Luminescent Chromium(III) Complex Acid

Abstract

The synthesis, structure, reactivity, and photophysical properties of a novel acidic, luminescent chromium(III) complex [Cr(H2tpda)2]3+ (23+) bearing the tridentate H2tpda (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 CH3CN 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.