The Nature of the Lowest Excited State in fac-Tricarbonylhalobis(4-phenylpyridine)rhenium(I) and fac-Tricarbonylhalobis(4, 4'-bipyridine)rhenium(I): Emissive Organometallic Complexes in Fluid Solution

Electronic absorption and emission spectral studies are reported for fac-XRe(CO)₃L₂ (X = Cl, Br, I; L = 4-phenylpyridine, 4, 4'-bipyridine) complexes in solution at 298 and 77 K. All complexes are emissive at 298 K from a state which is assigned as a Re ⟶ LCT state having considerable triplet character, and in every case the emission intensifies, blue shifts, and becomes longer lived at 77 K. For L = 4-phenylpyridine the complexes have a 77 K emissive state which has considerable intraligand (IL) (π-π*) triplet character, whereas the 4, 4'-bipyridine complexes exhibit emission which is dominated by Re ⟶ LCT character at any temperature between 77 and 298 K. The temperature dependence, over the range 30-300 K, of the emission lifetime and quantum yield in EPA, 2-propanol, and as a solid for fac-ClRe(CO)₃(4-phenylpyridine)₂ shows that both temperature and medium rigidity significantly influence the emission properties. The temperature dependence of the emission properties shows that both the nonradiative and radiative decay constants change with temperature, in accord with a variable admixture of IL and ReLCT character of the emissive state. Quenching of the excited fac-XRe(CO)₃(4-phenylpyridine)₂ complexes by trans-stilbene occurs at a diffusion-controlled rate and produces the spectroscopic trans-stilbene triplet which decays to the characteristic ratio of cis- and trans-stilbene. This result supports the conclusion that the emissive state has considerable triplet character.