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

Binuclear transition-metal complexes based on conjugated systems containing coordinating functions are potentially suitable for a wide range of applications, including light-emitting materials, sensors, light-harvesting systems, photocatalysts, etc., due to energy-transfer processes between chromophore centers. Herein we report on the synthesis, characterization, photophysical, and theoretical studies of relatively rare rhenium(I) and rhenium(I)-iridium(III) dyads prepared by using the nonsymmetrical polytopic ligands (NN2 and NN3) with the strongly conjugated phenanthroline and imidazole-quinoline/pyridine coordinating fragments. Availability of these different diimine chelating functions and targeted synthetic procedures allowed one to obtain a series of mononuclear (Re and Ir) and binuclear (Re-Re and Re-Ir) metal complexes with various modes of {Re(CO)3Cl} and {Ir(NC)2} metal fragment coordination. The obtained compounds were characterized by 1D 1H and 2D (COSY and NOESY) NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction crystallography. The photophysical study of the complexes (absorption, excitation and emission spectra, quantum yields, and excited-state lifetimes) showed that their emission parameters display strong dependence on the manner of metal center coordination to the diimine bidentate functions. The mononuclear complexes with an unoccupied imidazole-quinoline/pyridine fragment [Re(NN2), Re(NN3), and Ir(NC2)2(NN2)] or those containing a coordinated {Ir(NC)2} fragment in this position [Ir(NC2)2(NN1) and Re(NN2)Ir(NC1)2-Re(NN2)Ir(NC4)2] exhibit moderate-to-intense phosphorescence (quantum yields vary from 3% to 56% in a degassed solution), whereas the complexes containing a {Re(CO)3Cl} moiety in the imidazole-quinoline/pyridine position [Re2(NN2), Re2(NN3), and Ir(NC2)2(NN2)Re] demonstrate a strong reduction in the phosphorescence efficiency with a quantum yield of ≪0.1%. Quenching of the phosphorescence in the latter types of emitters is discussed in terms of a strong decrease in the radiative rate constants for these complexes compared to their analogues mentioned above, while the nonradiative constants remain nearly unchanged. Theoretical density functional theory (DFT) and time-dependent DFT (TD DFT) calculations, including evaluation of the radiative rate constants for the couple of structurally analogous complexes with and without a {Re(CO)3Cl} moiety coordinated to the imidazole-quinoline/pyridine chelating function, confirmed the observed trend in the variation of the emission intensity.

Luminescent organic dyes containing a phenanthro[9,10-D]imidazole core and [Ir(N

A family of diimine (N^N) and cyclometalating (N^C) ligands based on a phenanthro-imidazole aromatic system: 2-pyridyl-1H-phenanthro[9,10-d]imidazole (N^N); 2-R-1-phenyl-1H-phenanthro[9,10-d]imidazole, R = phenyl (N^C4), 3-iodophenyl (N^C5) and 4-nitrophenyl (N^C6) were prepared. It was found that N^C4 and N^C5 show π-π* fluorescence typical of aromatic systems of this sort, whereas the donor-acceptor architecture of N^C6 leads to strong emission solvatochromism and acidochromism, indicating the charge transfer character of the fluorescence observed. Six iridium(iii) complexes (1-6) [Ir(N^C#)2(N^N)]+, where # = 1-6 and N^C1 = 2-phenylpyridine, N^C2 = 2-(benzo[b]thiophen-2-yl)pyridine, and N^C3 = methyl 2-phenylquinoline-4-carboxylate, were also synthesized and characterized. The complexes obtained display moderate to bright phosphorescence with quantum yields up to 46% in degassed solution. The photophysical characteristics of 1-6 were studied in detail. DFT and TD DFT calculations were used for the assignment of electronic transitions responsible for the absorption and emission of these compounds. The variations in the cyclometalating ligand structure give rise to rich photophysics of the complexes obtained. It was found that the orbitals of both N^C and N^N ligands make a major contribution to the formation of emissive excited states and a delicate balance between the energy of the ligands' frontier orbitals determines the emission character.

Functionalized Pt(II) and Ir(III) NIR Emitters and Their Covalent Conjugates with Polymer-Based Nanocarriers.

Two NIR-emitting platinum [Pt(N^N^C)(phosphine)] and iridium [Ir(N^C)2(N^N)]+ complexes containing reactive succinimide groups were synthesized and characterized with spectroscopic methods (N^N^C, 1-phenyl-3-(pyridin-2-yl)benzo[4,5]imidazo[1,2-a]pyrazine, N^C, 6-(2-benzothienyl)phenanthridine, phosphine-3-(diphenylphosphaneyl)propanoic acid N-hydroxysuccinimide ether, and N^N, 4-oxo-4-((1-(pyridin-2-yl)-1H-1,2,3-triazol-4-yl)methoxy)butanoic acid N-hydroxysuccinimide ether). Their photophysics were carefully studied and analyzed using time-dependent density functional theory calculations. These complexes were used to prepare luminescent micro- and nanoparticles with the "core-shell" morphology, where the core consisted of biodegradable polymers of different hydrophobicity, namely, poly(d,l-lactic acid), poly(ε-caprolactone), and poly(ω-pentadecalactone), whereas the shell was formed by covalent conjugation with poly(l-lysine) covalently labeled with the platinum and iridium emitters. The surface of the species was further modified with heparin to reverse their charge from positive to negative values. The microparticles' size determined with dynamic laser scanning varies considerably from 720 to 1480 nm, but the nanoparticles' diameter falls in a rather narrow range, 210-230 nm. The species with a poly(l-lysine) shell display a high positive (>30 mV) zeta-potential that makes them essentially stable in aqueous media. Inversion of the surface charge to a negative value with the heparin cover did not deteriorate the species' stability. The iridium- and platinum-containing particles displayed emissions the spectral patterns of which were essentially similar to those of unconjugated complexes, which indicate retention of the chromophore nature upon binding to the polymer and further immobilization onto polyester micro- and nanoparticles for drug delivery. The obtained particles were tested to determine their ability to penetrate into different cells types: cancer cells, stem cells, and fibroblasts. It was found that all types of particles could effectively penetrate into all cells types under investigation. Nanoparticles were shown to penetrate into the cells more effectively than microparticles. However, positively charged nanoparticles covered with poly(l-lysine) seem to interact with negatively charged proteins in the medium and enter the inner part of the cells less effectively than nanoparticles covered with poly(l-lysine)/heparin. In the case of microparticles, the species with positive zeta-potentials were more readily up-taken by the cells than those with negative values.