Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and a series of metal ions (Mn+ = Sc3+, Y3+, Ho3+, Eu3+, Lu3+, Nd3+, Zn2+, Mg2+, Ca2+, Ba2+, Sr2+, Li+), where TPPZ forms a 2:1 complex [(TPPZ)(2)-Mn+] and a 1:1 complex [TPPZ-Mn+] 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)-Mn+ complex is converted to the fluorescent 1:1 TPPZ-Mn+ 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-Mn+ 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)-Mn+ (K-1) and TPPZ-Mn+ (K-2) vary depending on the,Lewis acidity of metal ions (i.e., both K-1 and K-2 values increase with increasing Lewis acidity of metal ions). Sc3+, which acts as the strongest Lewis acid, forms the (TPPZ)(2)-Sc3+ and TPPZSc(3+) complexes stoichiometrically with TPPZ. In such a case, "OFF-OFF-ON" switching of electron transfer from cobalt(II) tetraphenylporphyrin (CoTPP) to O-2 is observed in the presence of Sc3+ and TPPZ depending on the ratio of Sc3+ to TPPZ. Electron transfer from CoTPP to O-2 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 O-2(center dot-) 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 O-2(center dot-).