Mechanistic Studies of Photoinduced Electron Transport across Vesicle Bilayers

芘衍生物包埋在囊泡双层中，辐照可驱动从内水池中的抗坏血酸（Asc^-）到外水溶液中的甲基紫精（MV^<2+>）的电子传递。MV^<2+>还原的初始速率取决于芘环上的取代基;通过短亚甲基链与芘环连接的亲水性官能团充当电子传递的敏化剂。机理研究表明，电子传递主要是由Asc^-对芘单重激发态的还原猝灭引发的，并通过位于囊泡双层内外界面的芘之间的电子交换机制进行。设计并合成了一种新型的不对称取代芘类化合物，该化合物既含有由短亚甲基链连接的亲水基团，又含有疏水性长烷基，是一种优良的电子跨囊泡双层传递敏化剂。此外，为了提高电子传递效率，研究了脂质分子结构和内水池中电子供体对MV^<2+>还原率的影响。使用新合成的二聚芘桥由一个长的烷基链作为敏化剂导致电子传输效率的增强。虽然这些系统不幸地不能使用可见光工作，但它似乎是天然光合作用的最忠实的模型之一，因为电子在光能的帮助下在两个可逆氧化还原对之间以能量向上的方向通过脂质双层壁转移，防止电荷复合。

Endoergic electron transport across vesicle bilayers from ascorbate (Asc^-) in the inner waterpool to methylviologen (MV^<2+>) in the outer aqueous solution was driven by the irradiation of pyrene derivatives embedded in the vesicle bilayers. The initial rate of MV^<2+> reduction is dependent on the substituent group on the pyrenyl ring ; a hydrophilic functional group linked with the pyrenyl ring by a short methylene chain acts as a sensitizer for the electron transport. Mechanistic studies suggests that the electron transport is mainly initiated by the reductive quenching of the singlet excited state of pyrene by Asc^- and proceeds by a mechanism involving electron exchange between the pyrenes located at the inner and outer interface across the vesicle bilayer. Novel unsymmetrically substituted pyrenes having both a hydrophilic group linked by a short methylene chain and a hydrophobic long alkyl group was designed and synthesized, which acted as an excellent sensitizers for the electron transport across vesicle bilayers. Moreover, in order to enhance the electron transport efficiency, the effects of molecular structure of lipids and electron donors incorporated in the inner waterpool on the MV^<2+> reduction rate were investigated. The use of newly synthesized dimeric pyrenes bridged by a long alkyl chain as sensitizers resulted in the enhancement of electron transport efficiency. Although these systems unfortunately cannot work using visible light, it appears to be one of the most faithful model of natural photosynthesis in that an electron is transferred with the aid of light energy between two reversible redox couples in the energetically uphill direction through lipid bilayer walls preventing a charge recombination.