Selective Luminescent Chemosensing of Chloride Based on a Cyclometalated Platinum(II) Complex in Water: Crystal Structures, Spectroscopic Studies, Extraction, and Bioimaging

Abstract

Selective anion sensing by luminescent chemosensors capable of operating in aqueous conditions is a central field of modern supramolecular chemistry that impacts analytical and biological chemistry. A cationic cyclometalated [Pt(N^C^N)NCCH 3 ]OTf complex, 1 [N^C^N = 1,3-bis(1-( p -tolyl)-benzimidazol-2′-yl)benzene, OTf = triflate], was prepared, structurally described by single-crystal X-ray diffraction and studied in-depth as a luminescent chemosensor for anions in aqueous phase and solid state. A series of related neutral [Pt(N^C^N)X] complexes (X = Cl, 2; CN, 3 and I, 4 ) were formed readily upon treatment of 1 with the respective NaX salt in aqueous media and were described structurally by X-ray diffraction. Complex 1 is hydrostable with phosphorescent green emission originated by intraligand transitions, and [d yz (Pt) → π*(N^C^N)] charge transfer transitions, as evidenced by TD-DFT calculations and lifetime. Additions of halides, pseudohalides, oxyanions, and dicarboxylates to a neutral aqueous solution of 1 modified its green emission intensity with a pronounced affinity ( K = 1.5 × 10 5 M –1 ) and turn-on signal toward Cl – within the micromolar concentration range. Pt complex 1 is two orders of magnitude more selective for Cl – than the other halides, CN – and basic oxyanions. Such Cl – affinity for a metal-based chemosensor in aqueous media is still rare. On the basis of X-ray crystallographic analysis and multiple spectroscopic tools (NMR, UV–vis, luminescence, MS, lifetimes) the origin of this selectivity hinges on the cooperative three-point recognition involving one coordination bond (Pt–Cl) and two convergent short C–H···Cl – contacts. This strong affinity and efficient optical response can be utilized in quantitative Cl – sensing in real samples and solid–liquid extractions. Additionally, chloro-Pt complex, 2 may be relevant to bioimaging as a marker for cell nuclei, as revealed by its emission within living cells and intracellular distribution by confocal microscopic studies. These results demonstrate the usefulness of the new water-stable luminescent Pt-N^C^N complexes as effective analytical tools in anion sensing and extraction agents.

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