The X-ray crystal structures of thioindigo derivatives, 4,4＇7,7＇-tetrachlorothioindigo and 4,4＇,5,5＇,7,7＇-hexachlorothioindigo, have been determined to clarify the photocarrier generation process in these compounds. The crystals belong to the same space group (P2(1)/a), but the stacking manner of the molecules differs with respect to the slip angles of the molecular stacking columns. In tetrachlorothioindigo, the stacking molecules are arranged in such a way that all of the atoms of the indigoid part effectively overlap, so that the superior overlap between the HOMOs and between the LUMOs of the nearest stacking neighbors are realized to those in hexachlorothioindigo and thioindigo. These overlaps and the resultant intermolecular interactions in the tetrachlorothioindigo crystal are also revealed by differences between the bond lengths in the indigoid part of the isolated molecule and those of the solid-state molecule. In hexachlorothioindigo, there are no such differences, indicating that neither significant overlap of molecular orbitals nor strong intermolecular interactions exists. As a result, in the tetrachlorothioindigo crystal, intermolecular charge transfer and subsequent electron-hole pair dissociation occur more efficiently than in the others. This leads to a higher photocarrier generation efficiency of tetrachlorothioindigo expressed experimentally by a large thermalization length of 6 nm.