Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA.
J Org Chem. 2010 May 21;75(10):3193-202. doi: 10.1021/jo100527h.
Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the pi-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein (1)H and (14)N serve as the hyperfine "clocks", and in arrays containing site-specific (13)C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a (13)C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with (13)C-nitromethane. The resulting singly (13)C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and (13)C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a(1u)-like for the chlorins versus a(2u)-like for the porphyrins; the a(1u)-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a(2u)-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.
理解多组分分子体系中相互作用成分之间的电子通信对于包括人工光合作用和分子电子学在内的多个领域的合理设计非常重要。一种研究氧化四吡咯阵列中基态空穴/电子转移的策略依赖于分析 EPR 光谱中π-阳离子自由基观察到的超精细相互作用。该策略已被用于研究正常同位素组成的氧化多卟啉阵列中的空穴/电子转移过程，其中（1）H 和（14）N 用作超精细“时钟”，并且在含有特定位置（13）C 标记的阵列中，用作附加超精细时钟。在此，超精细时钟策略应用于二氢卟啉（叶绿素）的偶联物。叶绿素在结构上与叶绿素的关系比卟啉更密切。采用从头合成策略在叶绿素大环的 19 位引入（13）C 标记，该位置的电子/空穴密度大，并且可以从（13）C-硝基甲烷开始进行合成。所得的单（13）C 标记的叶绿素与未标记的叶绿素偶联，得到一个偶联物，其中二苯乙炔连接物跨越两个锌叶绿素的 10 位。天然丰度和（13）C 标记锌叶绿素偶联物以及基准锌叶绿素单体的单阳离子的 EPR 研究表明，空穴/电子转移的时间尺度在 4-7 ns 范围内，比类似的卟啉阵列长 5-10 倍。与卟啉偶联物相比，叶绿素与卟啉偶联物的空穴/电子转移速率较慢归因于这样一个事实，即 HOMO 对于叶绿素是 a(1u)-样的，而对于卟啉是 a(2u)-样的；a(1u)-样轨道在连接体附着处几乎没有（或没有）电子/空穴密度，而 a(2u)-样轨道在该位置具有显著的电子/空穴密度。总体而言，叶绿素和卟啉偶联物的研究提供了对影响空穴/电子转移过程的结构特征的深入了解。
