Meylemans Heather A, Lei Chi-Fong, Damrauer Niels H
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA.
Inorg Chem. 2008 May 19;47(10):4060-76. doi: 10.1021/ic701776k. Epub 2008 Apr 12.
Synthesis, ground-, and excited-state properties are reported for two new electron donor-bridge-acceptor (D-B-A) molecules and two new photophysical model complexes. The D-B-A molecules are Ru(bpy)2(bpy-phi-MV)4 (3) and Ru(tmb)2(bpy-phi-MV)4 (4), where bpy is 2,2'-bipyridine, tmb is 4,4',5,5'-tetramethyl-2,2'-bipyridine, MV is methyl viologen, and phi is a phenylene spacer. Their model complexes are Ru(bpy)2(p-tol-bpy)2 (1) and Ru(tmb)2(p-tol-bpy)2 (2), where p-tolyl-bpy is 4-(p-tolyl)-2,2'-bipyridine. Photophysical characterization of 1 and 2 indicates that 2.17 eV and 2.12 eV are stored in their respective (3)MLCT (metal-to-ligand charge transfer) excited state. These values along with electrochemical measurements show that photoinduced electron transfer (D*-B-A-->D (+)-B-A(-)) is favorable in 3 and 4 with DeltaG degrees(ET)=-0.52 eV and -0.62 eV, respectively. The driving force for the reverse process (D(+)-B-A(-) --> D-B-A) is also reported: DeltaG degrees(BET)=-1.7 eV for 3 and -1.5 eV for 4. Transient absorption (TA) spectra for 3 and 4 in 298 K acetonitrile provide evidence that reduced methyl viologen is observable at 50 ps following excitation. Detailed TA kinetics confirm this, and the data are fit to a model to determine both forward (k(ET)) and back (k(BET)) electron transfer rate constants: k(ET)=2.6 x 10(10) s(-1) for 3 and 2.8 x 10(10) s(-1) for 4; k(BET)=0.62 x 10(10) s(-1) for 3 and 1.37 x 10(10) s(-1) for 4. The similar rate constants k ET for 3 and 4 despite a 100 meV driving force (DeltaG degrees(ET)) increase suggests that forward electron transfer in these molecules in room temperature acetonitrile is nearly barrierless as predicted by the Marcus theory. The reduction in electron transfer reorganization energy necessary for this barrierless reactivity is attributed to excited-state electron delocalization in the (3)MLCT excited states of 3 and 4, an effect that is made possible by excited-state conformational changes in the aryl-substituted ligands of these complexes.
报道了两种新型电子供体-桥-受体（D-B-A）分子和两种新型光物理模型配合物的合成、基态和激发态性质。D-B-A分子分别为Ru(bpy)2(bpy-phi-MV)4（3）和Ru(tmb)2(bpy-phi-MV)4（4），其中bpy为2,2'-联吡啶，tmb为4,4',5,5'-四甲基-2,2'-联吡啶，MV为甲基紫精，phi为亚苯基间隔基。它们的模型配合物分别为Ru(bpy)2(p-tol-bpy)2（1）和Ru(tmb)2(p-tol-bpy)2（2），其中对甲苯基联吡啶为4-(对甲苯基)-2,2'-联吡啶。对1和2的光物理表征表明，在它们各自的（3）MLCT（金属到配体电荷转移）激发态中储存了2.17 eV和2.12 eV的能量。这些值以及电化学测量结果表明，光诱导电子转移（D*-B-A→D(+)-B-A(-)）在3和4中是有利的，ΔG°(ET)分别为-0.52 eV和-0.62 eV。还报道了逆过程（D(+)-B-A(-)→D-B-A）的驱动力：3的ΔG°(BET)=-1.7 eV，4的ΔG°(BET)=-1.5 eV。在298 K乙腈中对3和4进行的瞬态吸收（TA）光谱提供了证据，表明在激发后50 ps时可观察到还原的甲基紫精。详细的TA动力学证实了这一点，并且数据拟合到一个模型以确定正向（k(ET)）和反向（k(BET)）电子转移速率常数：3的k(ET)=2.6×10(10) s(-1)，4的k(ET)=2.8×10(10) s(-1)；3的k(BET)=0.62×10(10) s(-1)，4的k(BET)=1.37×10(10) s(-1)。尽管驱动力（ΔG°(ET)）增加了100 meV，但3和4的k ET速率常数相似，这表明在室温乙腈中这些分子中的正向电子转移几乎没有势垒，正如Marcus理论所预测的那样。这种无势垒反应性所需的电子转移重组能的降低归因于3和4的（3）MLCT激发态中的激发态电子离域，这种效应是由这些配合物的芳基取代配体中的激发态构象变化所实现的。
