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A heterometallic octanuclear coordination cage [Os 4 Zn 4 (L nap ) 12 ]X 16 (denoted Os•Zn; X = perchlorate or chloride) has been prepared (L nap 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 3 MLCT 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 10 3 -10 4 M -1 , resulting in quenching of the phosphorescence arising from the Os(II) units. A crystal structure of an isostructural Co 8 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.

inorganic chemistry

Photophysics of Cage/Guest Assemblies: Photoinduced Electron Transfer between a Coordination Cage Containing Osmium(II) Luminophores, and Electron-Deficient Bound Guests in the Central Cavity