Lewis Frederick D, Daublain Pierre, Zhang Ligang, Cohen Boiko, Vura-Weis Josh, Wasielewski Michael R, Shafirovich Vladimir, Wang Qiang, Raytchev Milen, Fiebig Torsten
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
J Phys Chem B. 2008 Mar 27;112(12):3838-43. doi: 10.1021/jp710718p. Epub 2008 Mar 5.
The excited-state behavior of synthetic DNA dumbbells possessing stilbenedicarboxamide (Sa) linkers separated by short A-tracts or alternating A-T base-pair sequences has been investigated by means of fluorescence and transient absorption spectroscopy. Electronic excitation of the Sa chromophores results in conversion of a locally excited state to a charge-separated state in which one Sa is reduced and the other is oxidized. This symmetry-breaking process occurs exclusively via a multistep mechanism-hole injection followed by hole transport and hole trapping-even at short distances. Rate constants for charge separation are strongly distance-dependent at short distances but become less so at longer distances. Disruption of the A-tract by inversion of a single A-T base pair results in a pronounced decrease in both the rate constant and efficiency of charge separation. Hole trapping by Sa is highly reversible, resulting in rapid charge recombination that occurs via the reverse of the charge separation process: hole detrapping, hole transport, and charge return to regenerate the locally excited Sa singlet state. These results differ in several significant respects from those previously reported for guanine or stilbenediether as hole traps. Neither charge separation nor charge recombination occur via a single-step superexchange mechanism, and hole trapping is slower and detrapping faster when Sa serves as the electron donor. Both the occurrence of symmetry breaking and reversible hole trapping by a shallow trap in a DNA-based system are without precedent.
通过荧光和瞬态吸收光谱法研究了具有由短A序列或交替A - T碱基对序列隔开的二苯乙烯二甲酰胺（Sa）连接体的合成DNA哑铃的激发态行为。Sa发色团的电子激发导致局部激发态转变为电荷分离态，其中一个Sa被还原，另一个被氧化。这种打破对称性的过程仅通过多步机制发生——空穴注入，随后是空穴传输和空穴捕获——即使在短距离也是如此。电荷分离的速率常数在短距离时强烈依赖于距离，但在较长距离时则不太明显。单个A - T碱基对的反向倒置破坏A序列会导致电荷分离的速率常数和效率显著降低。Sa对空穴的捕获是高度可逆的，导致快速的电荷复合，这通过电荷分离过程的逆过程发生：空穴解捕获、空穴传输和电荷返回以再生局部激发的Sa单重态。这些结果在几个重要方面与先前报道的以鸟嘌呤或二苯乙烯二醚作为空穴陷阱的结果不同。电荷分离和电荷复合都不是通过单步超交换机制发生的，并且当Sa作为电子供体时空穴捕获较慢而解捕获较快。在基于DNA的系统中，由浅陷阱导致的对称性破坏和可逆空穴捕获的发生都是前所未有的。
