Thermally Activated and Nonactivated Excited State Decay of [Cr(dgpy)2]3.

Photoactive and luminescent chromium(III) complexes are typically coordinated by aromatic N-heterocyclic chelate ligands such as polypyridines. Photoluminescence quantum yields and lifetimes can achieve up to 30% and several milliseconds, respectively. The effect of aliphatic N-donor ligands (guanidines) on the nonradiative decay of emissive doublet states remained, however, underexplored. Here, we describe the preparation, structure, redox chemistry, gas phase stability, and photophysical properties of the complex [Cr(dgpy)2][PF6]3 with pyridine and guanidine donors (dgpy = 2,6-diguanidylpyridine). Variable-temperature luminescence spectroscopy, near-infrared overtone spectroscopy as well as quantum chemical calculations identified the major thermally activated and nonactivated decay pathways and inform on future design criteria for photoactive chromium(III) complexes.