Synthetic Control Over Photoinduced Electron Transfer in Phosphorescence Zinc Sensors

Abstract

Despite the promising photofunctionalities, phosphorescent probes have been examined only to a limited extent, and the molecular features that provide convenient handles for controlling the phosphorescence response have yet to be identified. We synthesized a series of phosphorescence zinc sensors based on a cyclometalated heteroleptic Ir(III) complex. The sensor construct includes two anionic cyclometalating ligands and a neutral diimine ligand that tethers a di(2-picolyl)amine (DPA) zinc receptor. A series of cyclometalating ligands with a range of electron densities and band gap energies were used to create phosphorescence sensors. The sensor series was characterized by variable-temperature steady-state and transient photoluminescence spectroscopy studies, electrochemical measurements, and quantum chemical calculations based on time-dependent density functional theory. The studies demonstrated that the suppression of nonradiative photoinduced electron transfer (PeT) from DPA to the photoexcited Ir(IV) species provided the underlying mechanism that governed the phosphorescent response to zinc ions. Importantly, the Coulombic barrier, which was located on either the cyclometalating ligand or the diimine ligand, negligibly influenced the PeT process. Phosphorescence modulation by PeT strictly obeyed the Rehm-Weller principle, and the process occurred in the Marcus-normal region. These findings provide important guidelines for improving sensing performance; an efficient phosphorescence sensor should include a cyclometalating ligand with a wide band gap energy and a deep oxidation potential. Finally, the actions of the sensor were demonstrated by visualizing the intracellular zinc ion distribution in HeLa cells using a confocal laser scanning microscope and a photoluminescence lifetime imaging microscope.

Phosphorescent Iridium(III) Complexes for Bioimaging

“…Only the most long-lived component that corresponds to complex 82 is elongated from 140 to 160 ns upon addition of Zn 2+ . Similar intracellular lifetime response has also been observed when using structurally related iridium(III) DPA complex 81 [63].…”

“…Binding of Zn 2+ suppresses the nonradiative photoinduced electron-transfer (PeT) process from DPA to the excited cyclometalated iridium(III) polypyridine cores, leading to phosphorescence enhancement of these complexes. Similar Zn 2+ -induced phosphorescence response has also been observed in an intracellular environment using complexes 81 and 82 via confocal laser-scanning microscopy [62,63]. Another DPA-containing iridium(III) complex [Ir(ppy) 2 (btp-DPA)] (88) displays green and red dual phosphorescence because of limited internal conversion between two MLCT emissive states [64].…”

“…[60]. A series of iridium(III) complexes 77-87 equipped with a 2,2 0 -dipicolylamine (DPA) unit has been designed as Zn 2+ sensors [61][62][63]. Binding of Zn 2+ suppresses the nonradiative photoinduced electron-transfer (PeT) process from DPA to the excited cyclometalated iridium(III) polypyridine cores, leading to phosphorescence enhancement of these complexes.…”

Zinc(II) and pyrophosphate selective fluorescence probe and its application to living cell imaging

“…1 H NMR spectra were recorded on a Bruker DRX-300 NMR spectrometer. 13 NMR spectrometer. Fluorescent images were taken on a Leica TCS SP5 X AOBS confocal fluorescence microscope.…”

“…Disorders in Zn 2+ metabolism have been connected to several neurological diseases, including Alzheimer's disease (AD) [4,5], cerebral ischemia [6], and epilepsy [7]. Therefore, the development of techniques for Zn 2+ detection in biological processes has been an important research topic [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”