Low-Energy and Long-Lived Emission from Polypyridyl Ruthenium(II) Complexes Having A Stable-Radical Substituent

Novel polypyridyl
ruthenium­(II) complexes having a 2,2′-bipyridine (bpy) derivative
which possesses a 1,5-dimethyl-6-oxoverdazyl radical (OV) group as
a stable-radical substituent were designed and synthesized. The radical–ruthenium­(II)
complexes showed low-energy/intense MLCT absorption and low-energy/long-lived
MLCT emission, and these characteristics of the complexes were explained
by the electron-withdrawing nature of the OV group. Furthermore, the
radical-substituent effects were enhanced by the presence of the electron-donating
methyl groups at the 4- and 4′-positions of bpy in the ancillary
ligands. The detailed electrochemical, spectroscopic, and photophysical
properties of the complexes were discussed in terms of the systematic
modification of the second coordination sphere in the main and ancillary
ligands.