Novel charge-transfer materials via cocrystallization of planar aromatic donors and spherical polyoxometalate acceptors

Spherical polyoxometalates (POMs) such as M6O192-- and SiM12O404- (with M = Mo or W) and planar arene donors (anthracenes and pyrenes) can be cocrystallized (despite their structural incompatibility) by attaching a cationic "anchor" onto the arene which then clings to the POM anion by Coulombic forces. As a result, novel charge-transfer (CT) salts are prepared from arene donors and Lindqvist-type [M6O19](2-) and Keggin-type [SiM12O40](4-) accepters with overall 2:1 and 4:1 stoichiometry, respectively. The CT character of the dark-colored (yellow to red) crystalline materials is confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM accepters, as well as by the transient (diffuse reflectance) absorption spectra (upon picosecond laser excitation) of anthracene or pyrene cation radicals (in monomeric and pi -dimeric forms). X-ray crystallographic studies reveal a unique "dimeric" arrangement of the cofacially oriented arene couples which show contact points with the oxygen surface of the POMs that vary with distance, depending on the POM/arene combination. Moreover, the combination of X-ray crystallographic and spectroscopic techniques results in the observation of a logical structure/property relationship-the shorter the distance between the POM surface and the arene nucleus, the darker is the color of the CT crystal and the faster is the decay of the laser-excited charge-transfer state (due to back-electron transfer).