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

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.

One Guest or Two? A Crystallographic and Solution Study of Guest Binding in a Cubic Coordination Cage

“…Thus the carbonyl group of a range of bulky aliphatic ketone guests is always anchored in this way, as is the carboxylate terminus of adamantane‐1‐carboxylate . When a single guest contains two H‐bond accepting functional groups with the correct separation it can span the cavity diagonally and interact with both H‐bond donor sites simultaneously, as we saw recently using 1,2,4,5‐tetracyanobenzene as a guest …”

“…In some of our earlier studies of guest binding, NMR titrations of the host cage with guests in solution sometimes showed that free and bound guest are in fast exchange on the NMR timescale: this results in a steady shift for the signal being observed as the titration proceeds, and the resulting binding curves could normally be satisfactorily fit to a 1:1 model . The sole exception to this was with the small guest DMMP (mentioned earlier), for which clear evidence of a H⋅G 2 complex was obtained from the NMR titration.…”

“…The sole exception to this was with the small guest DMMP (mentioned earlier), for which clear evidence of a H⋅G 2 complex was obtained from the NMR titration. In many other cases luminescence titrations have been used, and again—with a range of guests—binding curves that fit to a 1:1 binding model were obtained . Thus, neither NMR nor luminescence titrations have provided—in any of our extensive previous work—any indication that guests such as those reported in this paper could form 1:2 H⋅G 2 complexes in solution.…”

“…In addition, three molecules of 2 are located outside the cavity in the spaces between host cages, with these fractional site occupancies amounting to a total of 2.25 additional molecules per cage. Along with the guest 1,2,4,5‐tetracyanobenzene, this is only the second example of a bifunctional guest which spans the cavity and is anchored to the cage interior surface using both H‐bonding pockets and illustrates how the arrangement of two H‐bond donor pockets can organise a bifunctional guest of appropriate dimensions inside the cavity.…”

“…The similarity of all of the H⋅G 2 structures in this series—over a range of guest sizes and degrees of saturation—suggests that this structural behaviour is general such that other bicyclic guests of this general size and shape should give the same type of structure. This proved to be the case using the guests indan‐2‐one ( 10 ), isoquinoline‐ N ‐oxide ( 11 ; a particularly good H‐bond acceptor due to the high partial negative charge on the exocyclic oxygen atom), naphthoquinone ( 12 ; a quencher of the excited state in two isostructural cages which incorporate either metal‐based or ligand‐based luminophores in the superstructure), and 1‐naphthaldehyde ( 13 ). The disposition of the pair of guests in the cage cavity, and the main H‐bonding interactions with the H‐bond donor pockets on the cage interior surface, are summarised in Figure and Figure .…”