Conditions for forming excited multiplet states: magnetic interactions between excited triplet (phthalocyaninato)zinc and doublet nitroxide radical
作者:Kazuyuki Ishii、Takeo Ishizaki、Nagao Kobayashi
DOI:10.1039/b104656j
日期:2001.10.31
Excited state phthalocyanine complexes comprising (tetra-tert-butylphthalocyaninato)zinc (ZnPc) coordinated by nitroxide radicals have been studied by time-resolved electron paramagnetic resonance (TREPR). Five ZnPc complexes coordinated by an axial ligand containing a nitroxide radical NRX (where X (= 4, 5, 6, 8 or 10) denotes the number of bonds from the zinc to the nitroxide nitrogen) were selected, and the magnetic interactions between the excited triplet ZnPc and respective NRX have been investigated in terms of the conditions for forming the excited quartet (Q1) state. Optimum structures calculated using a PM3 Hamiltonian show that the bond number X is well correlated with the distance between zinc and nitroxide nitrogen atoms (Δr). TREPR spectra of ZnPc complexes, which are coordinated by NR8 or NR10, are almost the same as that of ZnPc coordinated by pyridine (ZnPc–py), indicating that the electron
exchange interaction, J, between the excited triplet ZnPc and doublet nitroxide is much smaller than the zero field splitting parameter D value (D(T1) = 0.720 GHz) of the excited triplet ZnPc–py. On the other hand, TREPR spectra of the NR4, NR5 and NR6 complexes are assigned to the Q1 state constituted by the excited triplet ZnPc and doublet nitroxide radical. The D value of the Q1 state (D(Q1)) decreases in the order ZnPc–NR6 (0.205 GHz) > ZnPc–NR5 (0.190 GHz) > ZnPc–NR4 (0.165 GHz). This decrease is interpreted in terms of a magnetic dipole–dipole interaction between the triplet ZnPc and doublet nitroxide, which is opposite in sign to D(Q1), and increases in the order ZnPc–NR6 < ZnPc–NR5 < ZnPc–NR4. Calculations of resonance magnetic fields indicate that the |J| values of the NR4, NR5 and NR6
complexes are larger at least than the D(T1) value. It is found that the D and |J| values are well correlated with the bond number X and distance Δr. This EPR study is useful for understanding the photophysical and photochemical properties of chromophores.
通过时间分辨电子顺磁共振(TREPR)研究了由硝氧自由基配位的(四叔丁基酞菁)锌(ZnPc)组成的激发态酞菁配合物。研究人员选择了五种由含有亚硝基自由基 NRX 的轴向配体配位的 ZnPc 复合物(其中 X(= 4、5、6、8 或 10)表示锌与亚硝基氮之间的键数),并根据形成激发四元态(Q1)的条件研究了激发三重 ZnPc 与相应 NRX 之间的磁相互作用。使用 PM3 哈密顿计算得出的最佳结构表明,键数 X 与锌和亚硝基氮原子间的距离 (Δr)密切相关。由 NR8 或 NR10 配位的 ZnPc 复合物的 TREPR 光谱与由吡啶配位的 ZnPc 复合物(ZnPc-py)的 TREPR 光谱几乎相同,这表明激发的三重态 ZnPc 与双重态亚硝基之间的电子交换相互作用 J 远远小于激发的三重态 ZnPc-py 的零场分裂参数 D 值(D(T1) = 0.720 GHz)。另一方面,NR4、NR5 和 NR6 复合物的 TREPR 光谱被归入由激发的三重 ZnPc 和双亚硝基构成的 Q1 态。Q1 态的 D 值(D(Q1))按照 ZnPc-NR6 (0.205 GHz) > ZnPc-NR5 (0.190 GHz) > ZnPc-NR4 (0.165 GHz) 的顺序递减。这一下降可以用三重 ZnPc 和双重硝基之间的磁偶极-偶极相互作用来解释,它与 D(Q1) 的符号相反,并按照 ZnPc-NR6 < ZnPc-NR5 < ZnPc-NR4 的顺序增加。共振磁场的计算表明,NR4、NR5 和 NR6 复合物的|J|值至少大于 D(T1)值。研究发现,D 和 |J| 值与键数 X 和距离 Δr 有很好的相关性。这项 EPR 研究有助于了解发色团的光物理和光化学性质。