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 IBQ )] – ( E ${1 \over 2}$ 1 = –0.26 V vs. Fc + /Fc) and [Pt II (L ISQ )(L PDI )] 3– ( E ${1 \over 2}$ 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.