pH-Responsive N^C-Cyclometalated Iridium(III) Complexes: Synthesis, Photophysical Properties, Computational Results, and Bioimaging Application

Abstract

Herein we report four [Ir(N^C)2(L^L)]n+, n = 0,1 complexes (1–4) containing cyclometallated N^C ligand (N^CH = 1-phenyl-2-(4-(pyridin-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole) and various bidentate L^L ligands (picolinic acid (1), 2,2′-bipyridine (2), [2,2′-bipyridine]-4,4′-dicarboxylic acid (3), and sodium 4,4′,4″,4‴-(1,2-phenylenebis(phosphanetriyl))tetrabenzenesulfonate (4). The N^CH ligand precursor and iridium complexes 1–4 were synthesized in good yield and characterized using chemical analysis, ESI mass spectrometry, and NMR spectroscopy. The solid-state structure of 2 was also determined by XRD analysis. The complexes display moderate to strong phosphorescence in the 550–670 nm range with the quantum yields up to 30% and lifetimes of the excited state up to 60 µs in deoxygenated solution. Emission properties of 1–4 and N^CH are strongly pH-dependent to give considerable variations in excitation and emission profiles accompanied by changes in emission efficiency and dynamics of the excited state. Density functional theory (DFT) and time-dependent density functional theory (TD DFT) calculations made it possible to assign the nature of emissive excited states in both deprotonated and protonated forms of these molecules. The complexes 3 and 4 internalize into living CHO-K1 cells, localize in cytoplasmic vesicles, primarily in lysosomes and acidified endosomes, and demonstrate relatively low toxicity, showing more than 80% cells viability up to the concentration of 10 µM after 24 h incubation. Phosphorescence lifetime imaging microscopy (PLIM) experiments in these cells display lifetime distribution, the conversion of which into pH values using calibration curves gives the magnitudes of this parameter compatible with the physiologically relevant interval of the cell compartments pH.

Unusual Effects of the Metal Center Coordination Mode on the Photophysical Behavior of the Rhenium(I) and Rhenium(I)–Iridium(III) Complexes

“…The electronic absorption spectra of mononuclear species Re(NN1)−Re(NN3), Ir(NC2) 2 (NN1), and Ir(NC2) 2 (NN2) (Figure S26) are reminiscent of those found for the previously studied tricarbonylrhenium(I) diimine emitters 47,52−54 and bisorthometalated iridium(III) diimine complexes, 49,50,55,56 with the metal centers bound to closely analogous chelating S26) expectedly display a superposition of the spectra of the corresponding mononuclear complexes. According to the results of DFT calculations, the long-wavelength region of the spectrum of the binuclear homometallic rhenium Re 2 (NN2) complex is similar to the corresponding spectrum of mononuclear analogues and displays mainly the rhenium(I)-to-phenanthroline (S 0 → S 2 ) and rhenium(I)-to-imidazole-quinoline (S 0 → S 1 ) 1 MLCT character (Tables S11−S14 and Figures S32 and S33) with a noticeable admixture of 1 LLCT transitions from Cl − and CO to the aromatic system of the bridging ligand.…”

“…The mononuclear iridium complexes Ir(NC2) 2 (NN1) and Ir(NC2) 2 (NN2) containing the {Ir(NC2) 2 } moiety coordinated to different diimine chelating functions display rather strong orange/red and yellow phosphorescence (Table 1). The nature of emissive excited states in both cases is typical for these types of iridium [(NC) 2 Ir(NN)] + complexes 49,50,55 and may be assigned to a mixture of 3 MLCT/ 3 LLCT/ 3 LC emissive transitions localized at the corresponding diimine fragments of the bridging ligands (Tables S39 and S43 and Figure S48).…”

Investigation of the N^C Ligand Effects on Emission Characteristics in a Series of Bis-Metalated [Ir(N^C)2(N^N)]+ Complexes

“…Ligands in the coordination sphere of these complexes form a pseudo-octahedral environment at the iridium ion, with the nitrogen and carbon atoms of the N^C ligands disposed in trans - and cis -positions, respectively. This structural pattern is typical for complexes of this sort; bond angles and lengths in the coordination octahedron are not exceptional and fall in the range characteristic for the cationic [Ir(N^C) 2 (N^N)] + complexes [ 26 , 27 , 28 ]. The observed deviations from the ideal octahedral geometry around the iridium center are due to short bite angles of the N^C ligands, which are about 80° in these structures.…”

Photophysical Study on the Rigid Pt(II) Complex [Pt(naphen)(Cl)] (Hnaphen = Naphtho[1,2-b][1,10]Phenanthroline and Derivatives

“…The study of luminescent materials based on transition metal complexes is motivated by a number of importantapplications in fields such as photocatalysis [ 1 , 2 , 3 ], sensing and bioimaging [ 4 , 5 , 6 , 7 ], and optoelectronic devices [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. In the field of OLEDs (organic light emitting diodes), phosphorescent metal complexes are of particular interest due to their ability to harvest all generated excitons through the availability of excited triplet states.…”

“…The necessary intersystem crossing (ISC) is facilitated through efficient spin-orbit coupling (SOC) associated with the heavy metal centres, which also boosts the phosphorescent relaxation [ 8 , 10 , 17 ]. Typical phosphorescent (triplet) emitters are based on metal cations such as Ir(III), Ru(II), and Pt(II) [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. While the d 6 metal centres of Ir(III) and Ru(II) adopt an octahedral coordination environment, the d 8 configured Pt(II) complexes and the less frequently used Pd(II) and Au(III) coordination compounds adopt square planar geometries with open coordination sites in the axial positions [ 16 , 18 ].…”