pH-responsive switching of near-infrared absorption of a diradical complex of PtII and 3,4-diaminobenzoate formed in aqueous solutions

In alkaline aqueous solutions, 3,4-diaminobenzoate (H2(2LPDA)−) reacts with PtII to form a 1:2 (Pt:L) complex that intensely absorbs near-infrared (NIR) light at 713 nm (ε = 8.0 × 104 M−1 cm−1). The absorption disappeared at pH < 3 (in DMSO), showing pH-responsive switching of the NIR absorption. By comparing the NIR-absorbing behavior of this complex to that of a complex, [PtII(1LISQ)2]2−, containing the analogous phenylenediamine ligand [(1LISQ)2− = o-diiminobenzosemiquinonate radical], the complex can be formulated as [PtII(2LISQ)2]2−. The assignment of the entity was consistent with the redox and spectroelectrochemical behaviors and electronic spin resonance (ESR) spectroscopy. First, one-electron oxidation of [PtII(2LISQ)2]2− formed an ESR-silent complex assignable to the dimeric complex [{PtII(2LISQ)(2LIBQ)}2]2− [(2LIBQ) = o-iminobenzoquinone form] in which the two radical centers at ( 2 L ISQ ) 2 - were antiferromagnetically coupled. Second, the one-electron reduced complex of [PtII(2LISQ)2]2− exhibited an ESR signal attributed to [PtII(2LISQ)(2LPDA)]3−; 34% of the electronic spin was located at the PtII center rather than on the ( 2 L ISQ ) 2 - moiety. The pH-responsive switching-off of the NIR absorption was thus rationally explained by oxidation of [PtII(2LISQ)2]2− to [{PtII(2LISQ)(2LIBQ)}2]2− by the increase of the rest potential of the solution in the lower pH region.