Abstract
4, 4', 7, 7'-tetrachlorothioindigo (TCTI) and 4, 4', 5, 5', 7, 7'-hexachlorothioindigo (HCTI) are thionated derivatives of indigo that are known as reddish purple pigments as well as photoconductors. Although these molecules are quite similar in structure, TCTI is less soluble in organic solvents and more photoconductive than HCTI by about one order of magnitude. Intermolecular interactions are obviously responsible for these differences in physical properties. As a tool for evaluating intermolecular interactions, we have focused on the two-center energy which is involved in MO calculations and which relates to the bond energy within a molecule. We believed the two-center energy to be a powerful measure of intermolecular interactions when applied to non-bonded atom-pairs between two molecules. On the basis of this idea, we have extracted typical molecule pairs from the crystal lattice and evaluated their two-center energies. As a result, we found a two-dimensional CH…O hydrogen bond network both in TCTI and HCTI as well as other intermolecular forces. These results suggest that the intermolecular interactions are appreciably stronger in TCTI than in HCTI, indicating the insolubility of TCTI for organic solvents.
