PH-responsive switching of near-infrared absorption of a diradical complex of Pt II and 3,4-diaminobenzoate formed in aqueous solutions

Abstract

In alkaline aqueous solutions, 3,4-diaminobenzoate (H 2( 2L PDA) −) reacts with Pt II to form a 1:2 (Pt:L) complex that intensely absorbs near-infrared (NIR) light at 713 nm ( ε = 8.0 × 10 4 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, [Pt II( 1L ISQ) 2] 2−, containing the analogous phenylenediamine ligand [( 1L ISQ) 2− = o-diiminobenzosemiquinonate radical], the complex can be formulated as [Pt II( 2L ISQ) 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 [Pt II( 2L ISQ) 2] 2− formed an ESR-silent complex assignable to the dimeric complex [{Pt II( 2L ISQ)( 2L IBQ)} 2] 2− [( 2L IBQ) = o-iminobenzoquinone form] in which the two radical centers at ( 2 L ISQ ) 2 - were antiferromagnetically coupled. Second, the one-electron reduced complex of [Pt II( 2L ISQ) 2] 2− exhibited an ESR signal attributed to [Pt II( 2L ISQ)( 2L PDA)] 3− ; 34% of the electronic spin was located at the Pt II 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 [Pt II( 2L ISQ) 2] 2− to [{Pt II( 2L ISQ)( 2L IBQ)} 2] 2− by the increase of the rest potential of the solution in the lower pH region.