Luminescent P‑Chirogenic Copper Clusters

P-chirogenic
clusters of the cubanes [Cu4I4L4]
(L = chiral phosphine) were prepared from (+)- and (−)-ephedrine
with L = (S)- or (R)-(R)­(Ph)­(i-Pr)P (with R = CH3 (seven steps) or C17H35 (10 steps)) with e.e. up to 96%. The X-ray structure
of [Cu4I4((R)-(CH3)­(Ph)­(i-Pr)­P)4] confirmed the cubane
structure with average Cu···Cu and Cu···I
distances of 2.954 and 2.696 Å, respectively. The cubane structure
of the corresponding [Cu4I4((S)-(CH3)­(Ph)­(i-Pr)­P)4] was
established by the comparison of the X-ray powder diffraction patterns,
and the opposite optical activity of the (S)- and
(R)-ligand-containing clusters was confirmed by circular
dichroism spectroscopy. Small-angle X-ray scattering patterns of one
cluster bearing a C17H35 chain exhibit a weak
signal at 2θ ∼ 2.8° (d ∼
31.6 Å), indicating some molecular ordering in the liquid state.
The emission spectra exhibit two emission bands, both associated with
triplet excited states. These two bands are assigned as follows: the
high energy emission is due to a halide-to-ligand charge transfer,
XLCT, state mixed with LXCT (ligand-to-halide-charge-transfer). The
low energy band is assigned to a cluster-centered excited state. Both
emissions are found to be thermochromic with the relative intensity
changing between 77 and 298 K for the clusters in methylcyclohexane
solution. Several differences are observed in the photophysical parameters,
emission quantum yields and lifetimes for R = CH3 and C17H35. The measurements of the polarization along
the emission indicate that the emission is depolarized, consistent
with an approximate tetrahedral geometry of the chromophores.