PRESSURE-TUNING SPECTROSCOPY OF CHARGE-TRANSFER SALTS - X-RAY CRYSTALLOGRAPHY AND COMPARATIVE-STUDIES IN SOLUTION AND IN THE SOLID-STATE

The highly colored pyridinium (P+) and cobaltocenium (C+) iodides are charge-transfer salts by virtue of the new electronic absorption bands that follow Mulliken theory. X-ray crystallography establishes the relevant interionic separation and steric orientation of the cation/anion pairs P+I- and C+I- constrained for optimum charge-transfer interaction in the crystal lattice. Spectral comparisons of the charge-transfer (CT) transitions by absorption (solution) and by diffuse reflectance (solid-state) measurements reveals the commonality of contact ion pairs (CIP) in aprotic nonpolar solvents (dichloromethane) with those extant in crystalline charge-transfer salts. As such, the compression of the charge-transfer salts P+I- dissolved in dichloromethane by the application of pressures up to 10 kbar leads to the expected blue shift of the CT bands. Such a compressional effect can be attributed to increased ground-state stabilization of CIP arising from the enhanced solute-solvent interactions by the pressure-induced change in the dielectric constant (of dichloromethane), since it quantitatively follows the solvatochromic trend (Figure 13). By comparison, the compression of the charge-transfer salts P+I- in the solid state by the application of pressures up to 140 kbar leads to unusual red shifts of the CT bands indicative of the dominance of destabilizing charge-transfer interactions. X-ray crystallographic analysis of the cation/anion packing in the unit cell suggests that compressional effects on cation/cation and anion/anion interactions in the crystal lattice are superimposed on the relevant CT interactions of cation/anion pairs. The effects of pressure on the cryptic CT bands of crystalline cobaltocenium iodide are also compared with those observed in solution. © 1990 American Chemical Society.