Yuasa Junpei, Fukuzumi Shunichi
Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan.
J Am Chem Soc. 2008 Jan 16;130(2):566-75. doi: 10.1021/ja0748480. Epub 2007 Dec 20.
Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and a series of metal ions (M(n+) = Sc3+, Y3+, Ho3+, Eu3+, Lu3+, Nd3+, Zn2+, Mg2+, Ca2+, Ba2+, Sr2+, Li+), where TPPZ forms a 2:1 complex [(TPPZ)2-M(n+)] and a 1:1 complex [TPPZ-M(n+)] with Mn+ at low and high concentrations of metal ions, respectively. The fluorescence intensity of TPPZ begins to increase at high concentrations of metal ions, when the 2:1 (TPPZ)2-M(n+) complex is converted to the fluorescent 1:1 TPPZ-M(n+) complex. This is regarded as an "OFF-OFF-ON" fluorescence sensor for metal ions depending on the stepwise complex formation between TPPZ and metal ions. The fluorescence quantum yields of the TPPZ-M(n+) complex vary depending on the metal valence state, in which the fluorescence quantum yields of the divalent metal complexes (TPPZ-M2+) are much larger than those of the trivalent metal complexes (TPPZ-M3+). On the other hand, the binding constants of (TPPZ)2-M(n+) (K1) and TPPZ-M(n+) (K2) vary depending on the Lewis acidity of metal ions (i.e., both K1 and K2 values increase with increasing Lewis acidity of metal ions). Sc3+, which acts as the strongest Lewis acid, forms the (TPPZ)2-Sc3+ and TPPZ-Sc3+ complexes stoichiometrically with TPPZ. In such a case, "OFF-OFF-ON" switching of electron transfer from cobalt(II) tetraphenylporphyrin (CoTPP) to O2 is observed in the presence of Sc3+ and TPPZ depending on the ratio of Sc3+ to TPPZ. Electron transfer from CoTPP to O2 occurs at Sc3+ concentrations above the 1:2 ratio ([Sc3+]/[TPPZ]0 > 0.5), when the (TPPZ)2-Sc3+ complex is converted to the TPPZ-Sc3+ complex and TPPZ-(Sc3+)2, which act as promoters of electron transfer (ON) by the strong binding of O2*- with Sc3+. In sharp contrast, no electron transfer occurs without metal ion (OFF) or in the presence at Sc3+ concentrations below the 1:2 ratio (OFF), when the (TPPZ)2-Sc3+ complex has no binding site available for O2*-.
观察到2,3,5,6-四（2-吡啶基）吡嗪（TPPZ）与一系列金属离子（M(n+) = Sc3+、Y3+、Ho3+、Eu3+、Lu3+、Nd3+、Zn2+、Mg2+、Ca2+、Ba2+、Sr2+、Li+）之间逐步形成配合物，其中TPPZ在低金属离子浓度和高金属离子浓度下分别与Mn+形成2:1配合物[(TPPZ)2-M(n+)]和1:1配合物[TPPZ-M(n+)]。当2:1的(TPPZ)2-M(n+)配合物转化为荧光性的1:1 TPPZ-M(n+)配合物时，TPPZ的荧光强度在高金属离子浓度下开始增加。这被视为一种基于TPPZ与金属离子之间逐步形成配合物的金属离子“关-关-开”荧光传感器。TPPZ-M(n+)配合物的荧光量子产率因金属价态而异，其中二价金属配合物（TPPZ-M2+）的荧光量子产率远大于三价金属配合物（TPPZ-M3+）。另一方面，(TPPZ)2-M(n+)（K1）和TPPZ-M(n+)（K2）的结合常数因金属离子的路易斯酸度而异（即K1和K2值均随金属离子路易斯酸度的增加而增加）。作为最强路易斯酸的Sc3+与TPPZ化学计量地形成(TPPZ)2-Sc3+和TPPZ-Sc3+配合物。在这种情况下，在Sc3+和TPPZ存在下，根据Sc3+与TPPZ的比例，观察到从四苯基卟啉钴（II）（CoTPP）到O2的电子转移的“关-关-开”切换。当(TPPZ)2-Sc3+配合物转化为TPPZ-Sc3+配合物和TPPZ-(Sc3+)2时，电子从CoTPP转移到O2发生在Sc3+浓度高于1:2比例（[Sc3+]/[TPPZ]0 > 0.5）时，TPPZ-(Sc3+)2通过O2*-与Sc3+的强结合充当电子转移促进剂（开）。与之形成鲜明对比的是，在没有金属离子时（关）或在Sc3+浓度低于1:2比例时（关）不发生电子转移，此时(TPPZ)2-Sc3+配合物没有可供O2*-结合的位点。
