Song Hee-eun, Kirmaier Christine, Taniguchi Masahiko, Diers James R, Bocian David F, Lindsey Jonathan S, Holten Dewey
Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, USA.
J Am Chem Soc. 2008 Nov 19;130(46):15636-48. doi: 10.1021/ja805673m.
Excited-state charge separation in molecular architectures has been widely explored, yet ground-state hole (or electron) transfer, particularly involving equivalent pigments, has been far less studied, and direct quantitation of the rate of transfer often has proved difficult. Prior studies of ground-state hole transfer between equivalent zinc porphyrins using electron paramagnetic resonance techniques give a lower limit of approximately (50 ns)(-1) on the rates. Related transient optical studies of hole transfer between inequivalent sites [zinc porphyrin (Zn) and free base porphyrin (Fb)] give an upper limit of approximately (20 ps)(-1). Thus, a substantial window remains for the unknown rates of ground-state hole transfer between equivalent sites. Herein, the ground-state hole-transfer processes are probed in a series of oxidized porphyrin triads (ZnZnFb) with the focus being on determination of the rates between the nominally equivalent sites (Zn/Zn). The strategy builds upon recent time-resolved optical studies of the photodynamics of dyads wherein a zinc porphyrin is electrochemically oxidized and the attached free base porphyrin is photoexcited. The resulting energy- and hole-transfer processes in the oxidized ZnFb dyads are typically complete within 100 ps of excitation. Such processes are also present in the triads and serve as a starting point for determining the rates of ground-state hole transfer between equivalent sites in the triads. The rate constant of the Zn/Zn hole transfer is found to be (0.8 ns)(-1) for diphenylethyne-linked zinc porphyrins and increases only slightly to (0.6 ns)(-1) when a shorter phenylene linker is utilized. The rate decreases slightly to (1.1 ns)(-1) when steric constraints are introduced in the diarylethyne linker. In general, the rate constants for ground-state Zn/Zn hole transfer in oxidized arrays are a factor of 40 slower than those for Zn/Fb transfer. Collectively, the findings should aid the design of next-generation molecular architectures for applications in solar-energy conversion.
分子结构中的激发态电荷分离已得到广泛研究，然而基态空穴（或电子）转移，尤其是涉及等效色素的转移，却鲜有研究，并且直接定量转移速率往往很困难。先前使用电子顺磁共振技术对等效锌卟啉之间的基态空穴转移进行的研究给出了速率下限约为(50纳秒)(-1)。对不等价位点[锌卟啉(Zn)和游离碱卟啉(Fb)]之间空穴转移的相关瞬态光学研究给出了上限约为(20皮秒)(-1)。因此，等效位点之间基态空穴转移的未知速率仍有很大的区间。在此，对一系列氧化卟啉三联体(ZnZnFb)中的基态空穴转移过程进行了探测，重点是确定名义上等效位点(Zn/Zn)之间的速率。该策略基于最近对二元体系光动力学的时间分辨光学研究，其中锌卟啉被电化学氧化，附着的游离碱卟啉被光激发。氧化的ZnFb二元体系中产生的能量和空穴转移过程通常在激发后100皮秒内完成。这些过程在三联体中也存在，并作为确定三联体中等效位点之间基态空穴转移速率的起点。发现对于二苯乙炔连接的锌卟啉，Zn/Zn空穴转移的速率常数为(0.8纳秒)(-1)，当使用较短的亚苯基连接体时，仅略有增加至(0.6纳秒)(-1)。当在二芳基乙炔连接体中引入空间位阻时，速率略有下降至(1.1纳秒)(-1)。一般来说，氧化阵列中基态Zn/Zn空穴转移的速率常数比Zn/Fb转移的速率常数慢40倍。总的来说，这些发现应有助于设计用于太阳能转换应用的下一代分子结构。
