Luminescent Dinuclear Ruthenium Terpyridine Complexes with a Bis-Phenylbenzimidazole Spacer

Abstract

A conjugated bis-terpyridine bridging ligand, 2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-6-(2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-1H-benzo[d]imidazol-6-yl)-1H-benzo[d] imidazole (tpy-BPhBzimH-tpy), was designed in this work by covalent coupling of 3,3'-diaminobenzidine and two 4'-(p-formylphenyl)-2,2':6',2″-terpyridine units to synthesize a new series of bimetallic Ru(II)-terpyridine light-harvesting complexes. Photophysical and electrochemical properties were modulated by the variation of the terminal ligands in the complexes. The new compounds were thoroughly characterized by H NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Absorption spectra of the complexes consist of very strong ligand-centered π-π* and n-π* transitions in the UV, metal-to-ligand, and intraligand charge transfer bands in the visible regions. Steady-state and time-resolved emission spectral measurements indicate that the complexes exhibit moderately intense luminescence at room temperature within the spectral domain of 653-687 nm having luminescence lifetimes in the range between 6.3 and 55.2 ns, depending upon terminal tridentate ligand and solvent. Variable-temperature luminescence measurements suggest substantial increase of the energy gap between luminescentmetal-to-ligand charge transfer state and nonluminescent metal centered in the complexes compared to the parent [Ru(tpy)]. Each of the three bimetallic complexes exhibits only one reversible couple in the positive potential window with almost no detectable splitting corresponding to simultaneous oxidation of the two remote Ru centers. All the complexes possess a number of imidazole NH protons, which became sufficiently acidic upon metal ion coordination. By utilizing these NH protons, we thoroughly studied anion recognition properties of the complexes in pure organic as well as predominantly aqueous media through multiple optical channels and spectroscopic methods. Finally computation investigations employing density functional theory (DFT) and time-dependent DFT were done to examine the electronic structures of the complexes and accurate assignment of experimentally observed optical spectral bands.

Synthesis and characterization of a new ditopic bipyridine-terpyridine bridging ligand using a Suzuki cross-coupling reaction

“…The key materials of bromo-and boric acid derivatives 4, 5, 8 and 9 were prepared as shown in Scheme 1. The 2,2′-bipyridine-4-boronic acid (5) ligand was prepared in a five-steps reaction starting with 2,2′-bipyridine and the 4′-(4-boronatophenyl)-2,2′:6′,2′′-terpyridine (9) ligand was synthesized in a four-steps reaction involving a Kröhnke reaction to prepare 1-[2-oxo-2-(2-pyridyl)ethyl]pyridinium iodide (6), 4-bromo-2′azachalcone (7), and 4′-(4-bromophenyl)-2,2′:6′,2′′-terpyridine (8). All compounds (1), 27 (2), 27 (3), 28 (4), 28 (5), 29 (6), 30 7, 31 (8), 31 and (9) 22,29, were characterized using conventional methods such as NMR and mass spectrometry.…”

“…Overall, since the two synthetic methods of the target ligand (I) are different in their last steps of the reactions, considering the yields of the bromo and boronic acid derivatives (i.e. 61%, 73%, 79%, and 73% for compounds (4), (5), (8), and (9), respectively, see Experimental Section), compound (4) is the one which can form the smallest amount of the product considered and can be counted as a limiting compound for the synthesis in both routes and using compound (4) directly can influence the overall yield of the product. In this prospect, based on the yields obtained, the route two (i.e.…”

“…31 4-bromobenzaldehyde (3.7 g) was used as starting material according to the already published method. 31 The light yellow solid which was collected was used to prepare compound (8) without further purification, therefore the yield and melting point were not determined.…”

“…Das et al 5 developed a polypyridylimidazole system where the number of the donor sites on the ligand as well as the types of the binding sites are differentiated for the purpose of multichannel anion and cation sensing studies. Although there are many reports on the synthesis of polypyridyl bridging ligands in the literature, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] but polypyridyl bridging ligands with two binding sites where the number of donor atoms are different have been less studied. 3,4,[21][22][23] In our series of researches development of bridging ligands with bipyridine types at one end and terpyridine types of binding sites at the other end appealed.…”

Synthesis, Photophysics, and Switchable Luminescence Properties of a New Class of Ruthenium(II)–Terpyridine Complexes Containing Photoisomerizable Styrylbenzene Units

“…The Ru–N bond lengths varying between 1.971(2) and 2.080(2) Å are comparable to those of the reported structurally related compounds. 79−81…”