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

Abstract

Water-soluble alkynylplatinum(II) terpyridine complexes, [Pt{tpy(C 6 H 4 CH 2 NMe 3 -4)-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 p-p interactions and the emergence of a triplet metal-metal-to-ligand charge transfer ( 3 MMLCT) emission in the nearinfrared (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 3 MMLCT 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.

Molecular Design of Novel Classes of Luminescent Transition Metal Complexes and Their Use in Sensing, Biolabeling, and Cell Imaging

“…However, only a smaller extent of the MMLCT absorption changes was observed for 26 at pH 10 upon decreasing temperature, suggestive of a weaker extent of metal complex aggregation under such basic condition [60]. This could be rationalized by [60] with permission from The Royal Society of Chemistry the deprotonation of the phenolic functionalities that led to an enhanced hydrophilicity of the complex molecules in aqueous environment and hence their deaggregation. Upon photoexcitation, an NIR emission at ca.…”

“…In particular, a red-shifted MMLCT absorption was observed at pH 10 with respect to that at pH 4. This was attributed to an increase in electron-donating ability of the alkynyl ligands upon the deprotonation of the phenol functionalities and hence a rise in the dπ(Pt) orbital energy [60]. However, only a smaller extent of the MMLCT absorption changes was observed for 26 at pH 10 upon decreasing temperature, suggestive of a weaker extent of metal complex aggregation under such basic condition [60].…”

“…9 (left)) [60]. This complex was shown to undergo aggregation in aqueous solution at pH 4 at ambient temperature, as supported by the less broadening of terpyridine proton signals in the 1 H NMR spectra upon increasing temperature as well as the observation of large particles in μm range in the DLS experiment [60]. Other than the MLCT [dπ(Pt) !…”

“…465 nm which are typical of the alkynylplatinum(II) terpyridine complexes, 26 was found to exhibit a lower-energy absorption tail at ca. 600 nm in aqueous solution at pH 4, of which the absorbance was found to increase with decreasing temperature [60]. Together with the findings of 1 H NMR and DLS studies, the lower-energy absorption tail was suggested to be originated from the metal complex aggregates with the involvement of metal-metal and π-π stacking interactions and hence was assigned as the MMLCT transition.…”

“…This was attributed to an increase in electron-donating ability of the alkynyl ligands upon the deprotonation of the phenol functionalities and hence a rise in the dπ(Pt) orbital energy [60]. However, only a smaller extent of the MMLCT absorption changes was observed for 26 at pH 10 upon decreasing temperature, suggestive of a weaker extent of metal complex aggregation under such basic condition [60]. This could be rationalized by [60] with permission from The Royal Society of Chemistry the deprotonation of the phenolic functionalities that led to an enhanced hydrophilicity of the complex molecules in aqueous environment and hence their deaggregation.…”