The cellular uptake and localization of non-emissive iridium(III) complexes as cellular reaction-based luminescence probes

Improvement of cellular uptake and subcellular resolution remains a major obstacle in the successful and broad application of cellular optical probes. In this context, we design and synthesize seven non-emissive cyclometalated iridium(III) solvent complexes [Ir(C-N)(2)(solv)(2)]L-+(-) (LIr2-LIr8, in which C-N = 2-phenylpyridine (ppy) or its derivative; solv = DMSO, H2O or CH3CN: L- = PF6- or OTf-) applicable in live cell imaging to facilitate selective visualization of cellular structures. Based on the above variations (including different counter ions, solvent ligands, and C-N ligands), structure-activity relationship analyses reveal a number of clear correlations: (1) variations in counter anions and solvent ligands of iridium(III) complexes do not affect cellular imaging behavior, and (2) length of the side carbon chain in C-N ligands has significant effects on cellular uptake and localization/accumulation of iridium complexes in living cells. Moreover, investigation of the uptake mechanism via low-temperature and metabolism inhibitor assays reveal that [Ir(4-Meppy)(2)(CH3CN)(2)]+OTf- (LIr5) with 2-phenylpyridine derivative with side-chain of methyl group at the 4-position as C-N ligand permeates the outer and nuclear membranes of living cells through an energy-dependent, non-endocytic entry pathway, and translocation of the complex from the cell periphery towards the perinuclear region possibly occurs through a microtubule-dependent transport pathway. Nuclear pore complexes (NPCs) appear to selectively control the transport of iridium(III) complexes between the cytoplasm and nucleus. A generalization of trends in behavior and structure-activity relationships is presented, which should provide further insights into the design and optimization of future probes. (C) 2012 Elsevier Ltd. All rights reserved.