Dipartimento di Chimica, Università di Ferrara, 44100 Ferrara, Italy.
J Phys Chem B. 2010 Nov 18;114(45):14273-82. doi: 10.1021/jp9107798. Epub 2010 Jan 12.
The photophysical behavior of a series of heterometallic arrays made of a central Sn(IV) porphyrin connected, respectively, to two (SnRu(2)), four (SnRu(4)), or six (SnRu(6)) ruthenium porphyrin units has been studied in dichloromethane. Two different motifs connect the ruthenium porphyrin units to central tin porphyrin core, axial coordination via ditopic bridging ligands and/or coordination to peripheral pyridyl groups of the central porphyrin ring. A remarkable number of electron transfer processes (photoinduced charge separation and recombination processes) have been time-resolved using a combination of emission spectroscopy and fast (nanosecond) and ultrafast (femtosecond) absorption techniques. In these systems both types of molecular components can be selectively populated by light absorption. In all the arrays, the local excited states of these units (the tin porphyrin singlet excited state and the ruthenium porphyrin triplet state) are quenched by electron transfer leading to a charge-separated state where the ruthenium porphyrin unit is oxidized and the tin porphyrin unit is reduced. For each array, the two forward electron transfer processes, as well as the charge recombination process leading back to the ground state, have been kinetically resolved. The rate constants obey standard free-energy correlations with the forward processes lying in the normal free-energy regime and the back reactions in the Marcus inverted region. The comparison between the trimeric (SnRu(2)) and pentameric (SnRu(4)) arrays shows that all the electron transfer processes are faster in the latter than in the former system. This can be rationalized in terms of differences in electronic factors (due to the different connecting motifs) and driving force. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted, leading to a switch (from electron transfer to triplet energy transfer) in the deactivation mechanism of the excited ruthenium triplet.
一系列杂化金属阵列的光物理行为已经在二氯甲烷中进行了研究,这些杂化金属阵列由一个中心锡(IV)卟啉连接,分别连接两个(SnRu(2))、四个(SnRu(4))或六个(SnRu(6))钌卟啉单元。两种不同的基序连接钌卟啉单元和中心锡卟啉核心,轴向配位通过双齿桥联配体和/或配位到中心卟啉环的外围吡啶基团。使用发射光谱和快速(纳秒)和超快(飞秒)吸收技术的组合,已经对许多电子转移过程(光诱导电荷分离和重组过程)进行了时间分辨。在这些系统中,两种类型的分子组件都可以通过光吸收选择性地激发。在所有的阵列中,这些单元的局部激发态(锡卟啉单重激发态和钌卟啉三重激发态)被电子转移猝灭,导致电荷分离态,其中钌卟啉单元被氧化,锡卟啉单元被还原。对于每个阵列,两个正向电子转移过程以及导致回到基态的电荷复合过程都已经被动力学解析。速率常数遵循标准自由能相关性,正向过程位于正常自由能区域,而反向反应位于马库斯反转区域。三聚物(SnRu(2))和五聚物(SnRu(4))阵列之间的比较表明,所有电子转移过程在后者中都比在前者中更快。这可以根据电子因素(由于不同的连接基序)和驱动力的差异来合理化。在极性较小的溶剂(如甲苯)中,电荷分离态的能量大大升高,导致激发态钌三重态的失活机制发生转变(从电子转移到三重态能量转移)。