Abstract
A heterometallic octanuclear coordination cage [Os4Zn4(Lnap)12]X16 (denoted Os•Zn; X = perchlorate or chloride) has been prepared (Lnap is a bis-bidentate bridging ligand containing two pyrazolyl-pyridine chelating units separated by a 1,5-naphthalenediyl spacer group). The {Os(NN)3}2+ units located at four of the eight vertices of the cube have a long-lived, phosphorescent 3MLCT excited state which is a stronger electron donor than [Ru(bipy)3]2+. The chloride form of Os•Zn is water-soluble and binds in its central cavity the hydrophobic electron-accepting organic guests 1,2,4,5-tetracyanobenzene, 1,4-naphthoquinone and 1-nitronaphthalene, with binding constants in the range 103-104 M-1, resulting in quenching of the phosphorescence arising from the Os(II) units. A crystal structure of an isostructural Co8 cage containing one molecule of 1,2,4,5-tetracyanobenzene as a guest inside the cavity has been determined. Ultrafast transient absorption measurements show formation of a charge-separated Os(III)/guest•- state arising from cage-to-guest photoinduced electron transfer; this state is formed within 13-21 ps, and decays on a time scale of ca. 200 ps. In the presence of a competing guest with a large binding constant (cycloundecanone) which displaces each electron-accepting quencher from the cage cavity, the charge-separated state is no longer observed. Further, a combination of mononuclear {Os(NN)3}2+ model complexes with the same electron-accepting species showed no evidence for formation of charge-separated Os(III)/guest•- states. These two control experiments indicate that the {Os(NN)3}2+ chromophores need to be assembled into the cage structure to bind the electron-accepting guests, and for PET to occur. These results help to pave the way for use of photoactive coordination cages as hosts for photoredox catalysis reactions on bound guests.
Highlights
Coordination cages, in which a combination of metal ions and bridging ligands self-assemble into a hollow three-dimensional array with a well-defined central cavity, have been of particular interest for functions associated with binding of small molecule guests in the cavity.1 In many cases, this function relies on the structure of the host container molecule with the precise chemical nature of the component parts of the cage being of secondary importance: the cage is an inert box of given dimensions, solubility, and charge
We reported recently how the octanuclear, approximately cubic coordination cage [Cd8(LnapW)12]16+, which is luminescent by virtue of the array of 12 naphthyl chromophores in the ligands around the cage periphery, could effect photoinduced electron transfer from a naphthyl group in its excited state to an electron-deficient guest such as 1,4-naphthoquinone (NQ) or 1,2,4,5-tetracyanobenzene (TCNB)
We have prepared a water-soluble octanuclear cubic host cage OsZn, which contains [Os(pyrazolylpyridine)3]2+ units at four of the eight vertices, by a stepwise “complexes as ligands” strategy. These Os(II) complex units have a long-lived 3MLCT excited state which is a good photoelectron donor and is capable of effecting photoinduced electron-transfer to any of three different electron-deficient aromatic guests which bind in the cage cavity, giving in each case a cage+/guest− chargeseparated state with a lifetime of ca. 200 ps
Summary
Coordination cages, in which a combination of metal ions and bridging ligands self-assemble into a hollow three-dimensional array with a well-defined central cavity, have been of particular interest for functions associated with binding of small molecule guests in the cavity. In many cases, this function relies on the structure of the host container molecule with the precise chemical nature of the component parts of the cage being of secondary importance: the cage is an inert box of given dimensions, solubility, and charge. Further analysis of the TA data was performed on each of the OsZn/guest assemblies through spectral subtractions, to reveal the transient spectral features associated only with these short-lived charge-separated states in the cage/guest structures; this was performed by subtraction of the longest time delay spectrum (5 ns after excitation) from a series of spectra obtained after shorter time delays from 5 ps to 1 ns This subtraction was necessary to account for the transient absorption signals arising from ca. Information, Figure S13), there is no detectable absorption feature associated with formation of TCNB−, confirming that it is not just the presence of the Os(II) chromophores that is essential for the PET to occur and their assembly into a cage that can bind the guests in its central cavity
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