Selective Hypochlorous Acid Detection by Electronic Tuning of Platinum-4,5-bis(diphenylphosphino)acridine-Thiolate Complexes

Four [Pt(PNP)(L)]⁺ pincer complexes (PNP = 4,5-bis(diphenylphosphino)acridine; L = Cl^- (1), CN^- (2), benzenethiolate ^-SPh (3), and 4-thiolatobenzoic acid ^-SPhCO₂H (4)) were synthesized and structurally characterized. The oxidation of 3 and 4 by HOCl gave the corresponding sulfinate complexes 3″ (L = ^-SO₂Ph) and 4″ (L = ^-SO₂PhCO₂H), which were characterized by X-ray crystallography. All of the complexes show intense acridine-based π → π* absorption at λ_max = 398-430 nm, but the spectra of 3 and 4 display low-energy absorptions at 525 and 482 nm, respectively, which are predominantly ligand (3p of S)-to-ligand (π* of acridine) charge transfer (CT) in nature. 1, 2, 3″, and 4″ exhibit ¹ππ* fluorescence, which is quenched by internal conversion to the dark CT excited state in 3 and 4. This photophysical property of 3 and 4 was harnessed for the detection of hypochlorous acid HOCl and peroxynitrite ion (ONOO^-). The cyclic voltammetry (CV) of the complexes is consistent with the notion that 3 is more prone to oxidation than 4, as the CV of the former exhibits an irreversible S oxidation at +0.68 V vs Fc⁺/Fc, which is anodically shifted to +0.79 V in the CV of 4. The addition of HOCl or ONOO^- to a solution of 3 promptly switches on its emission as the complex is oxidized to 3″. The emission intensity reaches its maximum at a [HOCl]/[3] ratio of 2. On the contrary, 4, because its thiolate S atom is less electron-rich, responds only to HOCl, indicating that electronic tuning of the auxiliary ligand can lead to different selectivities. Our findings show that 3 and 4 are fast responding probes for HOCl or ONOO^- with reasonably good detection limits.