Are Very Small Emission Quantum Yields Characteristic of Pure Metal-to-Ligand Charge-Transfer Excited States of Ruthenium(II)-(Acceptor Ligand) Chromophores?

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• 作者：Chia Nung Tsai ; Shivnath Mazumder ; Xiu Zhu Zhang ; H. Bernhard Schlegel ; Yuan Jang Chen ; John F. Endicott
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：August 1, 2016
• 年：2016
• 卷：55
• 期：15
• 页码：7341-7355
• 全文大小：778K
• 年卷期：0
• ISSN：1520-510X

文摘

Metal to ligand charge-transfer (MLCT) excited state emission quantum yields, ϕ_em, are reported in 77 K glasses for a series of pentaammine and tetraammine ruthenium(II) complexes with monodentate aromatic acceptor ligands (Ru-MDA) such as pyridine and pyrazine. These quantum yields are only about 0.2–1% of those found for their Ru-bpy (bpy = 2,2′-bipyridine) analogs in similar excited state energy ranges (hν_em). The excited state energy dependencies of the emission intensity are characterized by mean radiative decay rate constants, k_RAD, resolved from ϕ_em/τ_obs = k_RAD (τ_obs = the observed emission decay lifetime; τ_obs^–1 = k_RAD + k_NRD; k_NRD = nonradiative decay rate constant). Except for the Ru-pz chromophores in alcohol glasses, the values of k_NRD for the Ru-MDA chromophores are slightly smaller, and their dependences on excited state energies are very similar to those of related Ru-bpy chromophores. In principle, one expects k_RAD to be proportional to the product of (hν_em)³ and the square of the transition dipole moment (M_e,g).² However, from experimental studies of Ru-bpy chromophores, an additional hν_em dependence has been found that originates in an intensity stealing from a higher energy excited state with a much larger value of M_e,g. This additional hν_em dependence is not present in the k_RAD energy dependence for Ru-MDA chromophores in the same energy regime. Intensity stealing in the phosphorescence of these complexes is necessary since the triplet-to-singlet transition is only allowed through spin–orbit coupling and since the density functional theory modeling implicates configurational mixing between states in the triplet spin manifold; this is treated by setting M_e,g equal to the product of a mixing coefficient and the difference between the molecular dipole moments of the states involved, which implicates an experimental first order dependence of k_RAD on hν_em. The failure to observe intensity stealing for the Ru-MDA complexes suggests that their weak emissions are more typical of “pure” (or unmixed) ³MLCT excited states.