Metal-to-Ligand Charge-Transfer Emissions of Ruthenium(II) Pentaammine Complexes with Monodentate Aromatic Acceptor Ligands and Distortion Patterns of their Lowest Energy Triplet Excited States

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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：2015
• 出版时间：September 8, 2015
• 年：2015
• 卷：54
• 期：17
• 页码：8495-8508
• 全文大小：728K
• ISSN：1520-510X

文摘

This is the first report of the 77 K triplet metal-to-ligand charge-transfer (³MLCT) emission spectra of pentaammine鈥揗DA鈥搑uthenium(II) ([Ru(NH₃)₅(MDA)]²⁺) complexes, where MDA is a monodentate aromatic ligand. The emission spectra of these complexes and of the related trans-[Ru(NH₃)₄(MDA) (MDA鈥?]²⁺ complexes are closely related, and their emission intensities are very weak. Density functional theory (DFT) calculations indicate that the energies of the lowest ³MLCT excited states of Ru鈥揗DA complexes are either similar to or lower than those of the lowest energy metal-centered excited states (³MC_X(Y)), that the barrier to internal conversion at 77 K is large compared to k_BT, and that the ³MC_X(Y) excited states are weakly bound. The [Ru(NH₃)₅py]²⁺ complex is an exception to the general pattern: emission has been observed for the [Ru(ND₃)₅(d₅-py)]²⁺ complex, but its lifetime is apparently very short. DFT modeling indicates that the excited state distortions of the different ³MC excited states are very large and are in both Ru鈥搇igand bonds along a single Cartesian axis for each different ³MC excited state, nominally resulting in ³MC_X(Y), ³MC_(X)Y, and ³MC_Z lowest energy metal-centered states. The ³MC_X(Y) and ³MC_(X)Y states appear to be the pseudo-Jahn鈥揟eller distorted components of a ³MC_(XY) state. The ³MC_X(Y) states are distorted up to 0.5 脜 in each H₃N鈥揜u鈥揘H₃ bond along a single Cartesian axis in the pentaammine and trans-tetraammine complexes, whereas the ³MC_Z states are found to be dissociative. DFT modeling of the ³MLCT excited state of [Ru(NH₃)₅(py)]²⁺ indicates that the Ru center has a spin density of 1.24 at the ³MLCT energy minimum and that the ³MLCT 鈫?³MC_Z crossing is smooth with a very small barrier (<0.5 kcal/mol) along the D₃N鈥揜u鈥損y distortion coordinate, implying strong ³MLCT/³MC excited state configurational mixing. Furthermore, the DFT modeling indicates that the long-lived intermediate observed in earlier flash photolysis studies of [Ru(NH₃)₅py]²⁺ is a Ru^II-(畏²(C鈺怌)-py) species.