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

作者：Tsai Chia Nung; Mazumder Shivnath; Zhang Xiu Zhu; Schlegel H Bernhard; Chen Yuan Jang; Endicott John F

来源：Inorganic Chemistry, 2016, 55(15): 7341-7355.

DOI：10.1021/acs.inorgchem.6b00374

Metal to land charge-transfer (MLCT) excited state emission quantum yields, phi(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 p-yrazine. 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 (h nu(em)). The excited state energy dependencies of the emission; intensity are characterized by mean radiative decay rate constants, k(RAD), resolved from phi(em)/tau(obs) = k(RAD) (tau(obs) = the observed emission decay lifetime; tau(-1)(obs) = 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 (h nu(em))(3) and the square of the transition dipole moment (M-e,M-g)(2). However, from experimental studies of Ru-hpy chromophores, an additional h nu(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,M-g. This additional h nu(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 singe 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; tins is treated by setting M-e,M-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 h nu(em). The failure to observe intensity stealing for the Ru-MDA complexes suggests that their weak emissions are more typical of "pure" (or unmixed) (MLCT)-M-3 excited states.

• 出版日期2016-8-1