Ru–Os dyads based on a mixed bipyridine–terpyridine bridging ligand: modulation of the rate of energy transfer and pH-induced luminescence switching in the infrared domain

Abstract

A series of heterobimetallic complexes of compositions [(bpy/phen)Ru(dipy-Hbzim-tpy)Os (tpy-PhCH/Hpbbzim)] (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, tpy-PhCH = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine and Hpbbzim = 2,6-bis(benzimidazole-2-yl)pyridine)), derived from a heteroditopic bpy-tpy bridging ligand, were synthesized and thoroughly characterized in this work. The heterometallic complexes exhibit two successive one-electron reversible metal-centered oxidations corresponding to Os/Os at lower potential and Ru/Ru at higher potential. All the four dyads exhibit very intense, ligand centered absorption bands in the UV region and moderately intense MLCT bands in the visible region. The dyads also show intense infrared emission with the emission maximum spanning between 734 nm and 775 nm with reasonably long room temperature lifetimes varying between 30 ns and 104 ns. Both steady state and time resolved luminescence spectroscopic investigations indicate that efficient and fast intramolecular energy transfer from the MLCT state of the Ru(ii) center to the Os-center takes place in all the four dyads. In addition, the rate of energy transfer was found to depend on the terminal ligand on the Os-site. Due to the presence of a number of imidazole NH protons in the dyads, significant modulation of both the ground and excited state properties of the complexes was made possible by varying the pH of the solution. By varying the terminal ligand, pH-induced "on-off", "off-off-on" and "on-off-on" emission switching of the complexes was nicely demonstrated in the infrared region.

Stimuli-Responsive Near-Infrared Emissive Os(II)–Terpyridine Complexes with a Sense of Logic

“…44,46 tpy-Hbzim-dipy = 4′-[4-(4,5-dipyridin-2-yl-1H-imidazol-2-yl)-phenyl]-[2,2′:6′,2″]terpyridine, bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline, tpy-PhCH 3 = 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine, H 2 pbbzim = 2,6-bis(benzimidazol-2-yl)- pyridine. 38,49,76 bpy = 2,2′-bipyridine and HL = 2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-1H-imidazo[4,5-f][1,10] phenanthroline). 77 bpy = 2,2′-bipyridine and Htppip = 2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-1Himidazo[4,5-f] [1,10] phenanthroline.…”

“…It is of interest to compare the pH-sensing behaviors of closely related complexes. In Table , we provided a comparison of p K a and p K a * values and luminescence switching behaviors of some structurally related Ru­(II) and Os­(II) complexes mainly derived from heteroditopic bpy-tpy and phen-tpy type of ligands. ,,,,− …”

“…[(tpy-PhCH 3 )OsCl 3 ] and [(H 2 pbbzim)-OsCl 3 ] were synthesized following literature procedure. 49 Dipy-Hbzim-tpy and Os(II) complexes were synthesized by some modified procedures reported by us. 49,74 Synthesis of [(tpy-PhCH 3 )Os(tpy-Hbzim-dipy)](ClO 4 ) 2 • 2H 2 O (1).…”

“…49 Dipy-Hbzim-tpy and Os(II) complexes were synthesized by some modified procedures reported by us. 49,74 Synthesis of [(tpy-PhCH 3 )Os(tpy-Hbzim-dipy)](ClO 4 ) 2 • 2H 2 O (1). [(tpy-PhCH 3 )Os Cl 3 ] (0.10 g, 0.16 mmol) and dipy-Hbzim-tpy (0.12 g, 0.23 mmol) were mixed together in 25 mL ethylene glycol and heated at ∼180 °C with continuous stirring under argon atmosphere for 5 h. After cooling, the reaction mixture was filtered and poured into a saturated aqueous solution of NaClO 4 •H 2 O when the complex got precipitated.…”

“…In order to accomplish our goal, we have designed two Os­(II)–terpyridine complexes based on a bifunctional terpyridine–bipyridine ligand, dipy-Hbzim-tpy, and thoroughly studied their stimuli-responsive photophysical properties (Chart ). Although a sizeable number of reports on polypyridyl Os­(II) complexes exhibiting NIR luminescence are available in the literature from our laboratory − as well as from other research groups, − but to our knowledge, there is no report wherein the optical as well as NIR luminescence properties of bis-terpyridine Os­(II) complexes were tuned and/or switched simultaneously upon interaction with anions, cations, pH, protons, and solvents.…”