Liard Davina J, Kleverlaan Cornelis J, Vlcek Antonín
Department of Chemistry, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, United Kingdom.
Inorg Chem. 2003 Dec 1;42(24):7995-8002. doi: 10.1021/ic0346376.
The Re-->MQ(+) MLCT excited state of Re(MQ(+))(CO)(3)(dmb) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as Re(II)(MQ*)(CO)(3)(dmb). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) << ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) <<< CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the Re(MQ(+))(CO)(3)(dmb) ground state.
[Re(MQ⁺)(CO)₃(dmb)]²⁺（MQ⁺ = N-甲基-4,4'-联吡啶鎓，dmb = 4,4'-二甲基-2,2'-联吡啶）在400 nm光照下产生的Re→MQ(+) MLCT激发态，通过皮秒时间分辨红外光谱和共振拉曼光谱进行了表征，结果表明该激发态与基态相比存在很大的结构差异。Re→MQ(+) MLCT激发态可表示为[Re(II)(MQ*)(CO)₃(dmb)]²⁺。它通过MQ*→Re(II)的反向电子转移衰变回基态，其时间常数适度依赖于溶剂的分子性质，而非其整体参数：甲酰胺≈二甲基亚砜≈甲醇（1.2 - 2.2 ns）＜四氢呋喃、脂肪族腈（3.2 - 3.9 ns）＜＜乙二醇≈2 - 乙氧基乙醇（4.2 - 4.8 ns）＜吡啶（5.7 ns）＜甲氧基二甘醇（6.9 ns）＜苯甲腈（7.5 ns）＜硝基甲烷（8.6 ns）＜＜＜二氯甲烷、1,2 - 二氯乙烷（25.9 - 28.9 ns）。发现反向反应速率常数与古特曼给体数之间存在近似相关性。在二氯甲烷、甲醇和丁腈中测量的衰减速率的温度依赖性表明，反向的MQ*→Re(II)电子转移使基态的多个较高振动能级得以填充。电子转移速率的溶剂依赖性可通过溶剂对内部重组能和电子接受振动频率的影响、带正电的MQ⁺吡啶鎓环与电子转移产物即[Re(MQ⁺)(CO)₃(dmb)]²⁺基态中的溶剂分子之间的相互作用来解释。
