Distinguishing between Dexter and rapid sequential electron transfer in covalently linked donor-acceptor assemblies.
Distinguishing between Dexter and rapid sequential electron transfer in covalently linked donor-acceptor assemblies.
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DOI：
10.1021/ja077096i
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发表时间：
2008-03
影响因子：
15
通讯作者：
M. Soler;J. McCusker
中科院分区：
文献类型：
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作者：
M. Soler;J. McCusker
具有通式 [M2(L)(mcb)(Ru(4,4'-(X)2-bpy)2)](PF6)3 的络合物的合成、物理和光物理性质（其中 M = Mn(II) 或 Zn(II)，X = CH3 或 CF3，mcb 是 4'-甲基-4-羧基-2,2'-联吡啶，L 是衍生自 的席夫碱大环化合物描述了2,6-二甲酰基-4-甲基苯酚和双(2-氨基乙基)-N-甲胺)。同构分子均由与 Ru(II) 聚吡啶络合物共价连接的双核金属核组成。 [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (4) 在脱氧 CH2Cl2 溶液中的光激发导致 Ru(II) 发色团的 3MLCT 激发态的发射特性，但具有寿命 (tau(obs) = 5.0 +/- 0.1 ns) 和辐射量子产率 (Phi(r) 约为 7 x 10(-4)) 相对于 Zn(II) 模型复合物 [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) （分别为 tau(obs) = 730 +/- 30 ns 和 Phi(r) = 0.024）显着减弱。在 [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3) 的情况下，3MLCT 激发态的淬灭更加广泛，其测量的寿命 (tau(obs) = 45 +/- 5 ps) 比相应的模型复合体短 >10(4) [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (tau(obs) = 1.31 +/- 0.05 微米）。室温下两种含锰配合物的时间分辨吸收测量揭示了与探针波长无关的动力学；没有观察到电子转移光产物的光谱特征。在 CH2Cl2 溶液中在 200-300 K 范围内获取的配合物 4 的时间分辨发射数据可以拟合 k(nr) = k0 + A x exp{-DeltaE/kB T} 形式的表达式，其中 k0 = 1.065 +/- 0.05 x 10(7) s(-1), A = 3.7 +/- 0.5 x 10(10) s(-1)，DeltaE = 1230 +/- 30 cm(-1)。假设电子转移机制，复合物 4 上的可变温度数据将需要大约 0.4-0.5 eV 的 lambda 重组能，该能量太小而无法与该系统中的电荷分离相关。这一结果加上在低于溶剂的玻璃-流体转变温度下缺乏增强的发射以及缺乏与 Mn(II)2 核相关的可见吸收特征，使得 Dexter 转移明确指定为主要的激发态反应途径。对于配合物 3 也得出了类似的结论，部分原因是电子转移驱动力较小 (DeltaG0(ET) = -0.1 eV）、由于供体激发态与二锰受体更接近而导致 Dexter 转移概率增加，以及在 80 K 形成光学玻璃时化合物缺乏发射。Dexter 转移的电子耦合常数确定为约 10 cm(-1) 和约 0.15 cm(-1)分别在配合物3和4中，表明激发态从桥（配合物3）到发色团外围（配合物4）的空间定位的变化导致与二锰核心的电子耦合减少了近2个数量级。除了深入了解供体/受体接近度对交换能量转移的影响外，这项研究还强调了在 Dexter 和电子转移机制可能导致难以区分的光谱可观测值的情况下变温测量的效用。
The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M2(L)(mcb)(Ru(4,4'-(X)2-bpy)2)](PF6)3 (where M = Mn(II) or Zn(II), X = CH3 or CF3, mcb is 4'-methyl-4-carboxy-2,2'-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a Ru(II) polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the 3MLCT excited state of the Ru(II) chromophore but with a lifetime (tau(obs) = 5.0 +/- 0.1 ns) and radiative quantum yield (Phi(r) approximately 7 x 10(-4)) that are significantly attenuated relative to the Zn(II) model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) (tau(obs) = 730 +/- 30 ns and Phi(r) = 0.024, respectively). Quenching of the 3MLCT excited state is even more extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (tau(obs) = 45 +/- 5 ps) is >10(4) shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (tau(obs) = 1.31 +/- 0.05 micros). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form k(nr) = k0 + A x exp{-DeltaE/kB T} with k0 = 1.065 +/- 0.05 x 10(7) s(-1), A = 3.7 +/- 0.5 x 10(10) s(-1), and DeltaE = 1230 +/- 30 cm(-1). Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of lambda approximately 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn(II)2 core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (DeltaG0(ET) = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be approximately 10 cm(-1) and approximately 0.15 cm(-1) in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) results in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude. In addition to providing insight into the influence of donor/acceptor proximity on exchange energy transfer, this study underscores the utility of variable-temperature measurements in cases where Dexter and electron-transfer mechanisms can lead to indistinguishable spectroscopic observables.