Ando Hideo, Nakao Yoshihide, Sato Hirofumi, Sakaki Shigeyoshi
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.
J Phys Chem A. 2007 Jun 28;111(25):5515-22. doi: 10.1021/jp0714163. Epub 2007 Jun 5.
The mechanism of light-induced excited spin state trapping (LIESST) of [FeIII(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) was discussed on the basis of potential energy surfaces (PESs) of several important spin states, where the PESs were evaluated with the DFT(B3LYP) method. The PES of the quartet spin state crosses those of the doublet and sextet spin states around its minimum. This means that the spin transition occurs from the quartet spin state to either the doublet spin state or the sextet spin state around the PES minimum of the quartet spin state. The PES minimum of the sextet spin state is slightly less stable than that of the doublet spin state by 0.18 eV (4.2 kcal/mol). This small energy difference is favorable for the LIESST. The doublet-sextet spin crossover point is 0.41 eV (9.6 kcal/mol) above the PES minimum of the sextet spin state. Because of this considerably large activation barrier, the thermal spin transition and the tunneling process do not occur easily. In the doublet spin state, the ligand to ligand charge transfer (LLCT) transition is calculated to be 2.16 eV with the TD-DFT(B3LYP) method, in which the pi orbital of the phenoxy moiety and the pi* orbital of the imine moiety in the pap ligand participate. This transition energy is moderately smaller than the visible light of 550 nm used experimentally. In the sextet spin state, the ligand to metal charge transfer (LMCT) transition is calculated to be at 2.36 eV, which is moderately higher than the visible light (550 nm). These results indicate that the irradiation of the visible light induces the LIESST to generate the sextet spin state but the reverse-LIESST is also somewhat induced by the visible light, indicating that the complete spin conversion from the doublet spin state to the sextet one does not occur, as reported experimentally.
基于几种重要自旋态的势能面（PESs），讨论了[FeIII(pap)2]+（pap = N - 2 - 吡啶基亚甲基 - 2 - 羟基苯基氨基）的光诱导激发自旋态捕获（LIESST）机制，其中PESs采用DFT(B3LYP)方法进行评估。四重态自旋态的PES在其最小值附近与二重态和六重态自旋态的PES相交。这意味着在四重态自旋态的PES最小值附近会发生从四重态自旋态到二重态自旋态或六重态自旋态的自旋转变。六重态自旋态的PES最小值比二重态自旋态的稍微不稳定0.18 eV（4.2 kcal/mol）。这个小的能量差有利于LIESST。二重态 - 六重态自旋交叉点比六重态自旋态的PES最小值高0.41 eV（9.6 kcal/mol）。由于这个相当大的活化能垒，热自旋转变和隧穿过程不容易发生。在二重态自旋态中，用TD - DFT(B3LYP)方法计算的配体到配体电荷转移（LLCT）跃迁为2.16 eV，其中pap配体中苯氧基部分的π轨道和亚胺部分的π*轨道参与。这个跃迁能量略小于实验中使用的550 nm可见光。在六重态自旋态中，配体到金属电荷转移（LMCT）跃迁计算为2.36 eV，略高于可见光（550 nm）。这些结果表明，可见光照射诱导LIESST产生六重态自旋态，但可见光也在一定程度上诱导了反向LIESST，这表明如实验报道的那样，并没有发生从二重态自旋态到六重态的完全自旋转换。
