pH-Responsive Switching of the Near-Infrared Absorption of the Water-Soluble Bis(o-diiminobenzosemiquinonato)platinum(II) Complex

The electronic structure of a water-soluble near-infrared (NIR)-absorbing complex was determined to be singlet diradical [Pt(II)(L(ISQ))(2)](2-) [(L(ISQ))(2-) = o-diiminosulfobenzosemiquinonate radical] by X-ray diffraction and (1)H NMR spectroscopy. Cyclic voltammetry (CV) showed redox reactions of [Pt(II)(L(ISQ))(2)](2-) to give [Pt(II)(L(ISQ))(L(ISQ))(L(IBQ))](-) (E(1/2)(1) = -0.26 V vs. Fc(+)/Fc) and [Pt(II)(L(ISQ))(L(PDI))(13-) (E(1/2)(2) = -1.46 V) [(L(IBQ) = o-diiminosulfobenzoquinonate, (L(PDI))(3-) = o-phenylenediiminesulfonate]. The monoradical structure of the latter was confirmed by electron paramagnetic resonance (EPR) spectroscopy. Splitting of the reduction wave of [Pt(II)(L(ISQ))(L(IBQ))](-) in CV and the weak EPR signal suggested the formation of a dimer. Because the rest potential of the solution exceeds E(pa)(1) at pH 4.0, the decrease in the NIR absorption at pH < 4.0 can be attributed to the oxidation of [Pt(II)(L(ISQ))(2)](2-) followed by dimerization.