Crystal chemistry of mixed-stack π–π* molecular compounds

Abstract

The room-temperature crystal structures of many mixed stack charge transfer molecular compounds can be grouped into a relatively small number of crystallochemical families. Although the mixed stack arrangement predominates, there are structures which deviate to a greater or lesser extent from such an arrangement, for reasons which are often not clear. These maverick structures are reviewed and then the more common mixed stack arrangements are classified into a number of structural groups. Compounds with quinonoid acceptors sometimes have special structural features because of the mode of interaction of the carbonyl groups with aromatic rings. In the quinhydrone family, the (aromatic ring)...carbonyl interaction is supplemented by hydrogen bonding between carbonyl and hydroxyl oxygens, and this leads to considerable structural homogeneity. Many CT compounds where the π-π* interaction is supplemented by hydrogen bonding show special structural features and physical properties different from those with only π-π* interaction. The mixed stack compounds with ionic ground states generally resemble those with neutral ground states in structural terms, but have different physical properties. In the isomeric compounds, the possibility of having both electron and proton transfer leads to variations on the mixed stack theme.