Luminescent P‑Chirogenic Copper Clusters

Abstract

P-chirogenic clusters of the cubanes [Cu₄I₄L₄] (L = chiral phosphine) were prepared from (+)- and (−)-ephedrine with L = (<i>S</i>)- or (<i>R</i>)-(R)­(Ph)­(<i>i</i>-Pr)P (with R = CH₃ (seven steps) or C₁₇H₃₅ (10 steps)) with e.e. up to 96%. The X-ray structure of [Cu₄I₄((<i>R</i>)-(CH₃)­(Ph)­(<i>i</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((<i>S</i>)-(CH₃)­(Ph)­(<i>i</i>-Pr)­P)₄] was established by the comparison of the X-ray powder diffraction patterns, and the opposite optical activity of the (<i>S</i>)- and (<i>R</i>)-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° (<i>d</i> ∼ 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