Abstract
The stabilization of light-induced charge separation in two axially bound triads based on aluminum(III) porphyrin (AlPor) are investigated using the electron spin polarization patterns of the final radical pair state. In the triads, TTF-(Ph)n-py-AlPor-AQ, (n=0, 1) anthraquinone (AQ) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) on the opposite face of the porphyrin ring via the appended pyridine (py). The dyad AlPor-AQ has been studied previously (M. Kanematsu, P. Naumov, T. Kojima, S. Fukuzumi, Chem. Eur. J. 17 (2011) 12372.) and shown to undergo fast light-induced charge separation and triplet recombination. Here, it is shown that by coordinating pyridine-appended TTF to the porphyrin, the charge separation can be stabilized. The spin polarized transient EPR spectra of the state TTF·+AQ·− can be observed in both the glass phase and in liquid solution and show that the state is formed from a singlet precursor on a timescale of less than ~0.5 ns. Using structural models to fix the geometry of the radical pair and the strength of the dipolar coupling, it is possible to determine the sign and approximate magnitude of the exchange coupling between TTF·+ and AQ·−. In contrast, other similar triads, which display relatively large ferromagnetic coupling, the exchange coupling is found to be small and antiferromagnetic. This difference can be rationalized as a result of differences in the structure of the bridge between the porphyrin and the acceptor.
摘要
利用最终自由基对状态的电子自旋极化模式,研究了基于铝(III)卟啉(AlPor)的两个轴向结合三元组中光诱导电荷分离的稳定性。在三元组中,TTF-(Ph)n-py-AlPor-AQ(n = 0,1)蒽醌(AQ)共价连接到Al(III)中心,而给体四硫富瓦烯(TTF)通过附加的吡啶(py)与Al(III)在卟啉环的相反面上配位。之前已经研究过二元体系AlPor-AQ(M. Kanematsu,P. Naumov,T. Kojima,S. Fukuzumi,Chem. Eur. J. 17(2011)12372),并显示其经历了快速的光诱导电荷分离和三重态复合。在这里,展示了通过将附加吡啶的TTF配位到卟啉上,可以稳定电荷分离。在玻璃相和液体溶液中,可以观察到状态TTF·+AQ·−的自旋极化瞬态EPR光谱,显示该状态从一个单重态前体形成的时间尺度小于~0.5 ns。使用结构模型来确定自由基对的几何形状和偶极耦合的强度,可以确定TTF·+和AQ·−之间的交换耦合的符号和近似大小。相比之下,其他类似的三元组显示出相对较大的铁磁耦合,交换耦合被发现很小且反铁磁。这种差异可以理解为由于卟啉和受体之间桥接的结构差异导致的结果。