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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Transient Absorption and Resonance Raman Investigations on the Axial Ligand Photodissociation of Halochromium(III) Tetraphenylporphyrin
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Transient Absorption and Resonance Raman Investigations on the Axial Ligand Photodissociation of Halochromium(III) Tetraphenylporphyrin

机译：卤代铬（III）四苯基卟啉的轴向配体光解离的瞬态吸收和共振拉曼研究

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摘要

The axial ligand photodissociation processes of halochromium(III) tetraphenylporphyrin (XCr~(III)TPP, X = Cl, Br) have been investigated in noncoordinating and coordinating solvents by transient Raman and absorption spectroscopic techniques. In noncoordinating solvents such as benzene, the upshift of the v_2 and v_4 bands and the disappearance of Cr-X stretching mode in the transient Raman spectra demonstrate the core size reduction of the porphyrin macrocycle accompanied by the photodissociation of axial halogen ligand atoms in the excited state. In coordinating solvents such as tetrahydrofuran (THF), where the solvent molecule is already attached to XCr~(III)TPP as an axial ligand to form XCr~(III)TPP(THF), the transient spectroscopic data indicate that the axial halogen ligand atoms photodissociate to form the five-coordinate Cr~(III)TPP(THF) on photoexcitation. The temporal evolutions of photoinduced absorption and bleaching signals of XCr~(III)TPP in benzene exhibit biphasic decay profiles with time constants of 1 and 20 ms. The shorter decay is likely due to the four-coordinate photoexcited Cr~(III)TPP~* species, and the relatively slow decay component seems to be the recombination process returning to the original five-coordinate XCr~(III)TPP species. On the other hand, a significant reduction in the lifetime of photoexcited ClCr~(III)TPP in THF was observed as compared with that in benzene. This behavior seems to be caused by the excited five-coordinate Cr~(III)TPP(THF)~* species, which decays rapidly with a time constant of 632 ps due to the participation of low-energy states in the deactivation process below the normally emissive tripmultiplet (π, π~*) states. The electronic nature of the lowest excited state of the five-coordinate Cr~(III)TPP(THF)~* species is suggested to possess (π, d_π) charge transfer character based on the comparison of transient Raman and absorption spectral features with those of other paramagnetic metalloporphyrins. We explain the axial ligand photodissociation processes in terms of the electron density change in metal d orbitals, which is particularly sensitive to the interaction with σ-donor axial ligands.
机译：通过瞬态拉曼和吸收光谱技术研究了非配位和配位溶剂中卤化铬（III）四苯基卟啉（XCr〜（III）TPP，X = Cl，Br）的轴向配体光解过程。在非配位溶剂（例如苯）中，瞬态拉曼光谱中v_2和v_4谱带的上移以及Cr-X拉伸模式的消失证明了卟啉大环的核尺寸减小，同时在激发态中轴向卤素配体原子发生光解离州。在配位溶剂（例如四氢呋喃（THF））中，溶剂分子已经作为轴向配体与XCr〜（III）TPP结合形成XCr〜（III）TPP（THF），瞬态光谱数据表明轴向卤素配体原子在光激发下光解离形成五配位的Cr〜（III）TPP（THF）。 XCr〜（III）TPP在苯中的光诱导吸收和漂白信号的时间演化表现出双相衰减曲线，其时间常数为1和20 ms。较短的衰变很可能是由于四坐标的光激发的Cr〜（III）TPP〜*物种，而相对较慢的衰变成分似乎是重新回到原始的五坐标的XCr〜（III）TPP物种的重组过程。另一方面，与苯相比，在THF中光激发的ClCr〜（III）TPP的寿命显着降低。这种现象似乎是由激发的五配位Cr〜（III）TPP（THF）〜*物质引起的，由于低能态参与了低于Pb的失活过程，其以632 ps的时间常数迅速衰减。通常为三重发射（π，π〜*）状态。根据瞬态拉曼光谱和吸收光谱特征与之比较，认为五配位Cr〜（III）TPP（THF）〜*物种的最低激发态的电子性质具有（π，d_π）电荷转移特性。其他顺磁性金属卟啉。我们根据金属d轨道中的电子密度变化来解释轴向配体光解离过程，该过程对与σ供体轴向配体的相互作用特别敏感。

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