Intrinsic 77 K Phosphorescence Characteristics and\nComputational Modeling of Ru(II)-(Bidentate Cyclometalated-Aromatic\nLigand) Chromophores: Their Relatively Low Nonradiative Rate Constants\nOriginating from Low Spin−Orbit Coupling Driven Vibronic

We\ninvestigated the photoinduced relaxation of Kasha-type emitting\nruthenium-(bidentate cyclometalated aromatic ligand), Ru-CM, chromophores\nof [Ru(pzpy)<sub>2</sub>(CM)]<sup>+</sup> ions (CM = 1-phenylisoquinoline,\n2,3-diphenylpyrazine, and 1,4-diazatriphenylene and pzpy = 2-pyrazol-1-yl-pyridine).\nThis is the first report of the phosphorescence behavior of pure Ru-(bidentate\nCM) chromophores. The 77 K photoinduced relaxation characteristics\nof phosphorescence chromophores showed emission quantum yields higher\nthan those of reference Ru-bpy (bpy = 2,2′-bipyridine) chromophores\nin the emission region of 670–900 nm. This phenomenon of the\nRu-CM chromophores could be attributed to their unusually low magnitudes\nfor 77 K nonradiative rate constants (<i>k</i><sub>NRD</sub>), although their radiative rate-constants (<i>k</i><sub>RAD</sub>) are not remarkable. In order to examine the 77 K photoinduced\nbehavioral relaxation difference between Ru-CM and Ru-bpy chromophores,\nwe used computational simulation, applying the fundamental formalism\nof <i>k</i><sub>RAD</sub> and temperature-independent <i>k</i><sub>NRD</sub> equations, which included calculated spin–orbit\ncoupling values.