Induced self-assembly and disassembly of water-soluble alkynylplatinum(ii) terpyridyl complexes with "switchable" near-infrared (NIR) emission modulated by metal-metal interactions over physiological pH : Demonstration of pH-responsive NIR luminescent probes in cell-imaging studies

Water-soluble alkynylplatinum(ii) terpyridine complexes, [Pt{tpy(C ₆H₄CH₂NMe₃-4)-4′}(C≡C- Ar)](OTf)₂ [Ar = C₆H₃-(OH)₂-3,5 (1), C₆H₄OH-4 (2), C₆H₃-(OMe) ₂-3,5 (3)], have been synthesized and characterized. The photophysical and electrochemical properties of the complexes have been studied. Complex 1 has been found to undergo aggregation at low pHs, leading to metal-metal and/or π-π interactions and the emergence of a triplet metal-metal-to-ligand charge transfer (³MMLCT) emission in the near-infrared (NIR) region, the intensity of which has been enhanced 1350-fold over that at physiological pH. Such 'switchable' NIR emission of complex 1 was employed in cell-imaging experiments. The pH response of the ³MMLCT emission of complex 1 in cellular compartments has been studied using experiments with fixed Madin-Darby canine kidney (MDCK) cells, while live cell-imaging experiments revealed that complex 1 could function as a NIR luminescent probe for the tracking of the location of acidic organelles such as lysosomes. © 2013 The Royal Society of Chemistry.