Cyclometalated Iridium(III) Complexes for Phosphorescence Sensing of Biological Metal Ions

Abstract

Phosphorescence signaling provides a valuable alternative to conventional bioimaging based on fluorescence. The benefits of using phosphorescent molecules include improved sensitivity and capabilities for effective elimination of background signals by time-gated acquisition. Cyclometalated Ir(III) complexes are promising candidates for facilitating phosphorescent bioimaging because they provide synthetic versatility and excellent phosphorescence properties. In this Forum Article, we present our recent studies on the development of phosphorescence sensors for the detection of metal ions based on cyclometalated iridium(III) complexes. The constructs contained cyclometalating (C^N) ligands with the electron densities and band-gap energies of the C^N ligand structures systematically varied. Receptors that chelated zinc, cupric, and chromium ions were tethered to the ligands to create phosphorescence sensors. The alterations in the C^N ligand structures had a profound influence on the phosphorescence responses to metal ions. Mechanistic studies suggested that the phosphorescence responses could be explained on the basis of the modulation of photoinduced electron transfer (PeT) from the receptor to the photoexcited iridium species. The PeT behaviors strictly adhered to the Rehm-Weller principle, and the occurrence of PeT was located in the Marcus-normal region. It is thus anticipated that improved responses will be obtainable by increasing the excited-state reduction potential of the iridium(III) complexes. Femtosecond transient absorption experiments provided evidence for the presence of an additional photophysical mechanism that involved metal-ion-induced alteration of the intraligand charge-transfer (ILCT) transition state. Utility of the mechanism by PeT and ILCT has been demonstrated for the phosphorescence sensing of biologically important transition-metal ions. In particular, the phosphorescence zinc sensor could report the presence of intracellular zinc pools by using confocal laser scanning microscopy and photoluminescence lifetime imaging microscopy techniques. We hope that the significant knowledge gained from our studies will be of great help in the design of new molecules as phosphorescence sensors.

Investigating Intracellular Localisation and Cytotoxicity Trends for Neutral and Cationic Iridium Tetrazolato Complexes in Live Cells

“…The emission detected from each of the cells following incubation at 4 °C was significantly reduced. This suggests that energy dependent pathways are predominantly involved in the cellular uptake of these complexes, a result that is consistent with previously studied cyclometalated iridium complexes incubated in various cell lines . Again, the lack of clear trends in Figure suggests variable mechanism of entrance even for subtle chemical changes.…”

Ir(iii) complexes designed for light-emitting devices: beyond the luminescence color array

“…[4][5][6]8,13 Research on OLEDs and LECs mainly focus on multi-color, highly-emissive complexes and new techniques for device fabrication. For this purpose, iridium(III) coordination compounds have played a prominent role in many application in photonics, as biological phosphorescent labels and sensors, [14][15][16][17][18] photodynamic therapy, 15,17 metallopharmaceuticals with antitumoral activities, 17 dye-sensitized solar cells, [19][20][21][22] and catalysis. [23][24][25][26][27] Most investigations have focused on photophysical properties, with promising applications in the active luminescent layers of OLEDs and LECs.…”

Blue-green emitting cationic iridium complexes with 1,3,4-oxadiazole cyclometallating ligands: synthesis, photophysical and electrochemical properties, theoretical investigation and electroluminescent devices

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] They have been successfully applied in photo-catalysis, 6, 7 biological imaging and sensing, 2, 6, 8 organic optoelectronics, [9][10][11][12] etc.. For instance, a variety of cationic iridium complexes have been developed as emitting materials in solid-state light emitting electrochemical cells (LECs); 9,10 moreover, some of the complexes display intriguing piezochromism, [14][15][16][17][18] which has been utilized for data recording or security protection. [1][2][3][4][5][6][7][8][9][10][11][12][13] They have been successfully applied in photo-catalysis, 6, 7 biological imaging and sensing, 2, 6, 8 organic optoelectronics, [9][10][11][12] etc.. For instance, a variety of cationic iridium complexes have been developed as emitting materials in solid-state light emitting electrochemical cells (LECs); 9,10 moreover, some of the complexes display intriguing piezochromism, [14]…”