Sequential photochemical processes in biological and artificial molecular systems

这项研究项目旨在展示高度组织化的分子体系中光化学过程的一个新方面，在这些体系中，反应分子处于接近和最佳取向的位置。用飞秒时间分辨荧光光谱监测了电子转移和激发转移中的超快和非平衡过程的反应动力学。本研究的结果总结如下。(1)在Langmuir-Blodgett多层膜中，发现在室温下(约10ps)发生层间和层内激发转移的速度比在10K(约50ps)时要快得多。理论分析表明，分子的空间涨落可能在激发转移中起重要作用。(2)在人工卟啉阵列和光合作用类胡萝卜素-叶绿素系统中，在热平衡前的80-140fs内，激发能直接从施主S2态转移。这一发现违反了众所周知的卡夏定律，即光化学反应从热平衡的最低激发态开始。(3)外电场效应研究发现，LB膜中的电子转移受外加电场的作用，其反应速率仅与初始激发态有关。该项目的成果现在指向量子计算选材和光开关分子器件的开发。

This research project aims at presenting a new aspect of photochemical processes in highly organized molecular systems in which reacting molecules are located with close proximity and optimal orientation. The reaction dynamics of ultrafast and non-equilibrium processes in electron transfer and excitation transfer were monitored with femtosecond time-resolved fluorescence spectroscopy. The result of this study is summarized in the following. (1) In Langmuir-Blodgett multilayers, it was found that the inter- and intra-layer excitation transfer took place much faster at room temperature (ca. 10 ps) than in 10K (ca. 50 ps). The theoretical analyses suggest that the spatical fluctuation of molecules may play an important role in excitation transfer. (2) In artificial porphyrin arrays and photosynthetic carotenoid-chlorophyll systems, the excitation energy transfer occurs directrly from the donor S2 state during 80-140 fs before thermal equilibration. This finding violates the well-known Kasha's rule that the photochemical reaction proceeds from the thermally equilibrated lowest excited state. (3) External electric field effect It was found that the electron transfer in LB films undergoes the external electrid field and its reaction rate depend stronly on the initially excited state. The results of the project are now directed toward development of quantum computing elecments and photo-switching molecular devices.