Photophysics of cage/guest assemblies: photoinduced electron transfer between a coordination cage containing Os(II) luminophores, and electron-deficient bound guests in the central cavity.

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.

of the main text; residuals from the fits are shown in red at the bottom. . Some of the broad signals associated with the paramagnetic cage are split into separate signals for free host and host/guest species, but in addition much sharper new signals associated with bound guest are apparent in each case (labelled *).

Fig. S6.
Top: UV/Vis spectra of the Os 4 Cd 4 cage in the fully reduced Os(II) form (black), and the fully oxidised Os(III) form (red), taken from previous work (ref. 10a, main text). Bottom; difference between the two UV/Vis spectra showing spectroscopic changes associated with oxidation of Os(II) to Os(III) (compare with the TA spectra in Fig. 5, main text).

Fig. S7.
Decay-associated spectra for the free cage Os•Zn in aqueous solution fitted to a 3-component sequential model. The top figure shows the normalised spectra, whereas the bottom figure is prenormalisation and therefore reflects the relative intensities of the three components.

Fig. S8
. Decay-associated spectra derived from TA spectroscopy of Os•Zn/NQ (top) and Os•Zn/NN (bottom) relating to the short time constant (ca. 200 ps, red trace, from the cage/guest complex) and the longer time constant (ca. 25 ns, black trace, from the free unquenched cage in the equilibrium mixture); the difference between these was used to calculate the absorption spectra of the transient charge-separated pairs shown in Fig. 7b of the main text (green and red spectra, respectively).   Data were collected at the University of Sheffield on a Bruker Apex II diffractometer with a CCD detector. One sixth of the complete cage is found in the asymmetric unit, with the Zn and Os atoms occupying the same sites such that these were refined as a 50/50 mixture of the two atom types. Fourteen of the required sixteen anions could be located from the data; the other two are presumably too badly disordered to locate but were included in the calculated formula. All anions were refined with isotropic displacement parameters; all anions, and the nitromethane molecule, needed weak DFIX restraints to keep their geometry sensible during the refinement. Weak global restraints were applied to all C, N, O atoms to achieve a chemically reasonable model (RIGU for the cage ligands, and SIMU/DELU commands for the solvents and anions). All hydrogen atoms were added in calculated positions, apart from those on the water molecules which were not located from the data but were included in the reported formula. After the cage and anions were located, a large solvent-accessible void containing diffuse electron density remained. This region of diffuse electron density was removed with the SQUEEZE command on PLATON; full details are given in the CIF.
Substantial regions of diffuse electron density that could not be meaningfully modelled were removed from the final refinement using the SQUEEZE function in PLATON; S1 full details are in the CIF. We emphasise that the cage complex cations and the surface-bound perchlorate ions are well defined with acceptable thermal displacement parameters. CCDC deposition number: 1871131 Data were collected at beamline i-19 at the Diamond Light Source. The complex molecule lies astride an inversion centre such that half of it lies in the asymmetric unit. Rigid bond and similarity restraints were applied to the anisotropic and isotropic displacement parameters of all atoms in the structure (RIGU and SIMU). Methanol residues O21S, O41S, O51S and water residue O2W were refined with isotropic displacement parameters fixed at values of 0.1. All other non-hydrogen atoms in the structure were refined with isotropic displacement parameters. The anisotropic displacement parameters of tetrafluoroborate residue B31 were restrained to have more isotropic character (ISOR); a measure likely necessary as a result of un-modelled disorder in the partial occupancy residue. The occupancies of cage encapsulated 1,2,4,5-tetracyanobenzene residue and methanol residue O11S were refined to values of 0.66(1) and 0.55(1) respectively. The occupancy of tetrafluoroborate residue B31 was allowed to refine to a value of 0.68(1). The occupancies of tetrafluoroborate residues B41 and B51 were constrained to sum to unity and refined to values of 0.33(1) and 0.67(1) respectively. The occupancies of methanol residues O21S, O41S, O51S and water residue O2W were refined to values of 0.71(1), 0.49(1), 0.43(1) and 0.42(2) respectively.
The geometries of methylene-pyrazolyl-pyridine and naphthyl moieities were restrained to have similar geometries (SAME). The geometries of all tetrafluoroborate residues were restrained to have similar geometries reflecting their tetrahedral shape (SADI). The C-O bond lengths of methanol residues O21S, O41S and O51S were restrained to a target value of 1.42 Å (DFIX). The pairs of chemically equivalent cyano C-N and C-C distances were restrained to have similar bond lengths (SADI).
A short intermolecular distance 2.90(1) A is observed between cage encapsulated methanol oxygen atom O11S and cage encaspulated 1,2,4,5-tetracyanobenzene cyano carbon atom C14G. This distance and angle (O11s...C14G-N15G, 101.6(3)) are consistent with the Burgi-Dunitz angle describing the approach of a nucleophile to an unsaturated carbon atom. S2 The methyl H atoms of methanol residues O21S, O41S, O51S, hydroxy hydrogen atoms of all methanol residues and hydrogen atoms of water residue O2W were not observed in the electron density map and are not included in the model, however, all hydrogen atoms were included in the unit cell contents. All other hydrogen atoms in the structure were geometrically placed and refined with a riding model.
Disordered solvent molecules could not be sensibly modelled, so the structure was treated the SQUEEZE function in PLATON; S1 full details are in the CIF. A total of 2178 electrons were accounted from the P1 cell in this way, equating to 30 methanol molecules per asymmetric unit, which have been included in the unit cell contents and calculation of derived parameters. CCDC deposition number: 1884249. S1 (a) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7;(b) van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect.