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 cocrysta llized (despite their structural incompatibility) by attaching a cationic " anchor" onto the arene which then clings to the POM anion by Coulombic forc es. As a result, novel charge-transfer (CT) salts are prepared from arene d onors and Lindqvist-type [M6O19](2-) and Keggin-type [SiM12O40](4-) accepte rs with overall 2:1 and 4:1 stoichiometry, respectively. The CT character o f the dark-colored (yellow to red) crystalline materials is confirmed by th e linear Mulliken correlation between the CT transition energies and the re duction potentials of the POM accepters, as well as by the transient (diffu se reflectance) absorption spectra (upon picosecond laser excitation) of an thracene or pyrene cation radicals (in monomeric and pi -dimeric forms). X- ray crystallographic studies reveal a unique "dimeric" arrangement of the c ofacially oriented arene couples which show contact points with the oxygen surface of the POMs that vary with distance, depending on the POM/arene com bination. Moreover, the combination of X-ray crystallographic and spectrosc opic 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 d ecay of the laser-excited charge-transfer state (due to back-electron trans fer).