光激发四苯基卟啉钴氧化还原反应机理研究

Photo-Induced Redox Reaction Mechanism of Cobalt Tetraphenylporphyrin

金属卟啉在光激发条件下会呈现出重要催化特性和光学性质。四苯基卟啉亚钴(Co^(Ⅱ) TPP)性质稳定,难以通过强氧化剂氧化至四苯基卟啉钴(CoⅢTPP)。本文分别利用全波长氙灯与355nm激光脉冲诱导Co^(Ⅱ) TPP,探究其分子内的氧化还原过程。研究发现,Co^(Ⅱ) TPP的Soret带最大吸收峰随光照时长的递增出现红移,且吸收强度呈现先下降后上升的趋势,最大吸收峰位置由415nm逐渐红移至433nm,其Q带最大吸收峰位置由532nm逐渐红移至545nm。两种光源诱导Co^(Ⅱ) TPP均出现稳态吸光度的降低,分子内的活性中心钴离子由Co2+氧化到Co3+。咪唑使得Co^(Ⅱ) TPP受光诱导后更易出现稳态吸收峰红移。Co^(Ⅱ) TPP的瞬态吸收光谱有3个瞬态吸收峰,激光激发含咪唑的Co^(Ⅱ) TPP溶液仅在396nm出现单个瞬态吸收峰。数据表明光激发时,电子从卟啉的π成键分子轨道向具有配体特征的π*反键分子轨道迁移,发生金属对配体的电荷迁移,生成稳定的高价态CoⅢ。

Photo-excited metalloporphyrins exhibit important catalytical and optical properties. The center metal cobalt(Ⅱ) of Co^(Ⅱ) TPP is hard to be oxidized to cobalt(Ⅲ) by strong oxidant because of the low valence cobalt ion stability. A full-wavelength light from xenon lamp and 355 nm laser pulse were applied to irradiate Co^(Ⅱ) TPP for exploring the photo-induced redox process. Based on the UV-Vis spectra, the maximum Soret band absorption peaks of the Co^(Ⅱ) TPP red shifted with the irradiation time. The absorption peak at 415 nm of the Soret band gradually red-shifted to 433 nm, while the Q band peak gradually red shifted from 532 nm to 545 nm. The steady absorbance intensity of Co^(Ⅱ) TPP induced by the two light sources decreased obviously. The center metal cobalt ion in the molecule was oxidized to CoⅢ. As a ligand of the center metal, imidazole promoted the red-shifted rate of the steady absorption peaks of photo-induced Co^(Ⅱ) TPP. Three transient absorption peaks of laser excited Co^(Ⅱ) TPP could be observed in the transient absorption spectra, while only one single transient absorption peak at 396 nm appeared in the laser excited Co^(Ⅱ) TPP spectra with imidazole. The above data illustrated that electrons migrated from the π bonding molecular orbital to the ligand characteristic π* anti-bonding molecular orbital in the laser excited Co^(Ⅱ) TPP. The metal-ligand charge transfer resulted in a stable high-valence state CoⅢ coordinated complex.