Long-Lived Trimetallic Complexes of Fe(II), Ru(II), and Os(II) Based on a Heteroditopic Bipyridine–Terpyridine Bridge: Synthesis, Photophysics, and Electronic Energy Transfer

Abstract

Synthesis, characterization, and investigation of photophysical and redox behaviors of a new class of homoand heterotrimetallic complexes of composition [(bpy/ phen) 2 Ru(dipy-Hbzim-tpy)M(tpy-Hbzim-dipy)Ru(bpy/ phen) 2 ] 6+ (M = Fe II , Ru II , and Os II ) derived from a conjugated heteroditopic bipyridine−terpyridine bridge were carried out in this work. Trimetallic RuZnRu complexes of composition [(bpy/phen) 2 Ru(dipy-Hbzim-tpy)Zn(tpy-Hbzim-dipy)Ru(bpy/phen) 2 ] 6+ were also synthesized in situ as their photophysical properties are of particular interest in demonstrating the absorption and emission spectra of the complexes in the presence of a metal (Zn 2+ ) that has neither metal-toligand charge transfer (MLCT) nor metal-centered ( 3 MC) states. Complexes display intense absorption bands spanning almost the entire UV and visible region. The complexes also exhibit rich electrochemical behaviors with a number of metal-centered reversible oxidation and ligand-centered reduction waves. All complexes are luminescent at room temperature, and timeresolved emission spectral studies indicate that peripheral Ru II -centered emissive 3 MLCT states are quantitatively quenched, by intramolecular energy transfer to the low lying 3 MLCT (for central Ru and Os) or 3 MC states of the Fe II center (nonluminescent). Interestingly, Fe(II) does not adversely deteriorate the photophysics of the RuFeRu assembly. Thus, multicomponent complexes in the present work can serve as well-organized light-harvesting antennas as the light absorbed by multiple chromophoric subunits is efficiently channeled to the distinct component having the lowest-energy excited state.

Novel Ru(II)/Os(II)‐Exchange Homo‐ and Heterometallic Polypyridyl Complexes with Effective Energy Transfer

“…In comparison, the oxidation process of the Ru II /Ru III couple occurs at a slightly more positive potential than that of the Os II /Os III couple, as the occupied dπ orbitals of Os(II) have a higher energy than those of Ru(II) [2g,9] . The first two reduction processes of all acquired complexes are shifted to positive potentials compared with those of the reference compounds because the presence of the terpyridine units in the bridging ligands significantly stabilizes the ligand π* orbitals [10] .…”

“…Showing a weak emission band at 812 nm for S − Ru − Os , at 816 nm for D − Ru − Os , and at 833 nm for D − Os − Ru . The measured luminescence originated from the 3 MLCT state of the Os II ‐center, resulting in entirely quenched luminescence emission in the Ru−Os triads by the Os II ‐center [2g] . The Ru II ‐tpy or Os(II) excited state emission indicates that energy transfer occurs in the metal donor–acceptor complexes, which will be discussed in detail in the next section.…”

“…A similar absorption profile was observed for the Os‐based mononuclear complexes S − Os and D − Os . The Os‐based complexes S − Os , S − Ru − Os , S − Os − Ru , and D − Ru − Os exhibited not only a 1 MLCT band in the same wavelength range as the Ru(II) 1 MLCT transitions but also a weak 3 MLCT absorptions in the visible region at 645–670 nm, which stem from the spin‐forbidden 1 [Os II (dπ) 6 ] to 3 [Os II (dπ) 5 tpy‐ligand(π*) 1 ] transition [2g,3c,12] . The absorption of the multi‐metallic complexes in the range of 497–517 nm is ascribed to the dπ (Ru and Os)→tpy ligand of L 1 and L 2 1 MLCT transitions.…”

Controlling the Direction of Intercomponent Energy Transfer by Appropriate Placement of Metals in Long-Lived Trinuclear Complexes of Fe(II), Ru(II), and Os(II)

“…In order to utilize low-cost Fe(II) as an effective visible light sensitizer, we previously reported an array of trinuclear complexes based on the same bridging ligand where two pyridyl motifs are connected with a terpyridine unit via a phenyl-imidazole spacer. 53 The photophysics of the trinuclear complexes possessing two Ru 2+ units at the terminal and one Fe 2+ at the center are interesting with regard to achieving a long-lived excited state in mixed-metal RuFeRu complexes at RT. In addition, we observed photoinitiated intercomponent energy transfer from the periphery to the center in all the triads.…”

“…In addition, we observed photoinitiated intercomponent energy transfer from the periphery to the center in all the triads. 53 With the initial success, we are further interested to tune both the strength and direction of intramolecular electron and/ or energy transfers in multimetallic complexes which could be feasible either through variation of metal combinations or upon varying the electronic properties of bridging ligand. In this work, we synthesized and characterized a new array of trinuclear complexes of type, [(bpy) 2 Os(d-HIm-t)M(t-HImd)Os(bpy) 2 ] 6+ (M = Fe II , Ru II , and Os II ), by incorporating two Os 2+ metals in the periphery instead of Ru 2+ and comprehensively investigated their redox properties, absorption spectral behaviors, and both steady state and timeresolved luminescence spectral characteristics (Chart 1).…”