Department of Chemistry and Bioengineering, Tampere University of Technology, P. O. Box 541, 33101 Tampere, Finland.
Phys Chem Chem Phys. 2011 Jan 14;13(2):397-412. doi: 10.1039/c0cp01106a. Epub 2010 Oct 29.
The present paper highlights results of a systematic study of photoinduced electron transfer, where the fundamental aspects of the photochemistry occurring in solutions and in artificially or self-assembled molecular systems are combined and compared. In photochemical electron transfer (ET) reactions in solutions the electron donor, D, and acceptor, A, have to be or to diffuse to a short distance, which requires a high concentration of quencher molecules and/or long lifetimes of the excited donor or acceptor, which cannot always be arranged. The problem can partly be avoided by linking the donor and acceptor moieties covalently by a single bond, molecular chain or chains, or rigid bridge, forming D-A dyads. The covalent combination of porphyrin or phthalocyanine donors with an efficient electron acceptor, e.g. fullerene, has a two-fold effect on the electron transfer properties. Firstly, the electronic systems of the D-A pair result in a formation of an exciplex intermediate upon excitation both in solutions and in solid phases. The formation of the exciplex accelerates the ET rate, which was found to be as fast as >10(12) s(-1). Secondly, the total reorganization energy can be as small as 0.3 eV, even in polar solvents, which allows nanosecond lifetimes for the charge separated (CS) state. Molecular assemblies can form solid heterogeneous, but organized systems, e.g. molecular layers. This results in more complex charge separation and recombination dynamics. A distinct feature of the ET in organized assemblies is intermolecular interactions, which open a possibility for a charge migration both in the acceptor and in the donor layers, after the primary intramolecular exciplex formation and charge separation in the D-A dyad. The intramolecular ET is fast (35 ps) and efficient, but the formed interlayer CS states have lifetimes in microsecond or even second time domain. This is an important result considering possible applications.
本文重点介绍了光诱导电子转移的系统研究结果，其中结合并比较了溶液中和人工或自组装分子体系中发生的光化学的基本方面。在溶液中的光化学电子转移（ET）反应中，电子给体 D 和受体 A 必须扩散到短距离，这需要高浓度的猝灭剂分子和/或激发供体或受体的长寿命，而这并不总是可以安排的。通过用单键、分子链或链或刚性桥将供体和受体部分共价键合，形成 D-A 偶联物，可以部分避免该问题。卟啉或酞菁供体与高效电子受体（例如富勒烯）的共价结合对电子转移性质具有双重影响。首先，在溶液和固态中激发时，D-A 对的电子体系导致形成激基复合物中间体。激基复合物的形成加速了 ET 速率，发现其速率快于>10(12) s(-1)。其次，总重组能可以小至 0.3 eV，即使在极性溶剂中也是如此，这允许电荷分离（CS）态的纳秒寿命。分子组装可以形成固态不均匀但有组织的体系，例如分子层。这导致更复杂的电荷分离和重组动力学。有组织的组装中 ET 的一个显著特点是分子间相互作用，这为在主分子内激基复合物形成和 D-A 偶联物中的电荷分离之后，在受体和供体层中进行电荷迁移提供了可能性。分子内 ET 是快速（35 ps）且有效的，但形成的层间 CS 态的寿命在微秒甚至秒时间域内。考虑到可能的应用，这是一个重要的结果。
