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)(3)Cl} and {Ir-(NC)(2)} metal fragment coordination. The obtained compounds were characterized by 1D H-1 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)(3)Cl} moiety in the imidazole-quinoline/pyridine position [Re (2) (NN2), Re- 2 (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)(3)Cl} moiety coordinated to the imidazole-quinoline/pyridine chelating function, confirmed the observed trend in the variation of the emission intensity.

Automated summary

Binuclear transition-metal complexes based on conjugated systems containing coordinating functions have potential applications in various fields. This study presents the synthesis, characterization, photophysical, and theoretical studies of rare rhenium-(I) and rhenium-(I)-iridium-(III) dyads.