pH-responsive switching of near-infrared absorption of a diradical complex of Pt II and 3,4-diaminobenzoate formed in aqueous solutions Kousaku Tamura a , Atsuko Masuya a , Nobuhiko Iki a,⇑ , Yasunori Ohba b , Seigo Yamauchi b , Hitoshi Hoshino a a Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan article info Article history: Received 31 May 2011 Received in revised form 2 August 2011 Accepted 10 August 2011 Available online 25 August 2011 Keywords: Radical complex o-Phenylenediamine Pt II Near infrared light Redox abstract In alkaline aqueous solutions, 3,4-diaminobenzoate (H 2 ( 2 L PDA )  ) reacts with Pt II to form a 1:2 (Pt:L) complex that intensely absorbs near-infrared (NIR) light at 713 nm (e = 8.0  10 4 M 1 cm 1 ). The absorp- tion disappeared at pH < 3 (in DMSO), showing pH-responsive switching of the NIR absorption. By compar- ing the NIR-absorbing behavior of this complex to that of a complex, [Pt II ( 1 L ISQ ) 2 ] 2 , containing the analogous phenylenediamine ligand [( 1 L ISQ ) 2 = o-diiminobenzosemiquinonate radical], the complex can be formulated as [Pt II ( 2 L ISQ ) 2 ] 2 . The assignment of the entity was consistent with the redox and spectro- electrochemical behaviors and electronic spin resonance (ESR) spectroscopy. First, one-electron oxidation of [Pt II ( 2 L ISQ ) 2 ] 2 formed an ESR-silent complex assignable to the dimeric complex [{Pt II ( 2 L ISQ )( 2 L IBQ )} 2 ] 2 [( 2 L IBQ )= o-iminobenzoquinone form] in which the two radical centers at ð 2 L ISQ Þ  2 were antiferromagneti- cally coupled. Second, the one-electron reduced complex of [Pt II ( 2 L ISQ ) 2 ] 2 exhibited an ESR signal attributed to [Pt II ( 2 L ISQ )( 2 L PDA )] 3Å ; 34% of the electronic spin was located at the Pt II center rather than on the ð 2 L ISQ Þ  2 moiety. The pH-responsive switching-off of the NIR absorption was thus rationally explained by oxidation of [Pt II ( 2 L ISQ ) 2 ] 2 to [{Pt II ( 2 L ISQ )( 2 L IBQ )} 2 ] 2 by the increase of the rest potential of the solution in the lower pH region. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction In the field of bio-molecular imaging, fluorescent probes have been widely utilized since they are able to bind to a target mole- cule or tissue and respond to the microenvironment [1]. Recently, to achieve a high signal-to-noise (S/N) ratio, emission in the near- infrared (NIR) region (7001200 nm) has drawn much attention because biological tissue is essentially transparent in this wave- length region allowing imaging of deep tissue [2]. In contrast, less attention has been paid to the application of NIR absorption to bio-imaging. However, in the field of applied spectroscopy, novel techniques based on NIR absorption such as optical coherence tomography (OCT) [3] and photoacoustic tomography (PAT) [4,5] are emerging, and are able to provide images with greater resolu- tion (10 lm for OCT and <100 lm for PAT) at a depth of 510 mm (for PAT) to complement existing imaging modalities. In such techniques, contrasting agents play a pivotal role in enhancing the S/N ratio of the acquired images. So far, NIR-absorbing organic dyes [6] and nano-materials [7,8] have been utilized as contrast- ing agents, but there are no examples of metal complexes. In general, metal complexes have many advantages over dyes and nano-materials including ease of preparation by simple reaction of the metal ion and ligand, resistance to photobleaching, and feasibility in bio-conjugation (as compared to the case of nano- materials); in some cases they also provide very large molar absorptivity (e). These features prompted us to design a metal complex suitable for NIR-absorption contrasting agent applica- tions, which require that the complex is bio-compatible, i.e., water-soluble and able to undergo bio-conjugation. Very recently, we clarified that Pt II ion and 3,4-diaminobenzenesulfonate (DBS, Scheme 1) formed a water-soluble complex [Pt II ( 1 L ISQ ) 2 ] 2 with a large e value of 1.1  10 5 M 1 cm 1 at 705 nm, which is attributed to the ligand-to-ligand charge-transfer (LLCT) transition (Scheme 2) [9]. Moreover, the NIR absorption can be switched on and off depending upon the pH of the solution. In order to add conjugat- ing ability to the biomolecule without sacrificing water-solubility, we herein study the ability of 3,4-diaminobenzoic acid (DBA), which has a carboxy group as an anchor to bind to the biomole- cule, to form an NIR-absorbing complex with Pt II in aqueous solu- tions and report the pH-responsive switching of the NIR absorption and the mechanism using (spectro)electrochemical and ESR measurements. 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.08.018 ⇑ Corresponding author. E-mail address: iki@m.tohoku.ac.jp (N. Iki). Inorganica Chimica Acta 378 (2011) 81–86 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica