4,4′,7,7′-tetrachlorothioindigo (TCTI) is an industrially important red pigment which also is used as a photoconductor for electrophotographic photoreceptors. Although TCTI is quite similar in molecular and crystal structures to 4,4′,5,5′,7,7′-hexachlorothioindigo (HCTI), the electrophotographic gain of TCTI has been reported to be about four times higher than that of HCTI. The high photosensitivity was interpreted in terms of the formation of weakly bound intermolecular charge transfer (CT) excitons and their subsequent electron hole pair dissociation; whereas tightly bound Frenkel excitons prevail in HCTI. In order to directly prove the formation of CT excitons in TCTI, an attempt has been made in the present investigation to observe the CT transition by means of polarized light using single crystals. No CT transition was, however, observed either in TCTI or HCTI, and both electronic structures are quite similar. Furthermore, the photoconductivity of TCTI and HCTI is found to be equally high, but depends significantly on the electrode material. This indicates that the exciton is extrinsically dissociated at the Schottky barrier between TCTI (or HCTI) and the electrode. These results lead us to conclude that the difference in electrophotographic gain between TCTI and HCTI-based photoreceptors might presumably be attributed to the carrier formation or injection efficiency at the interface between the charge generation layer and charge transport layer.
Jin Mizuguchi, Misato Mochizuki, "Electronic Structure of Thioindigo Derivatives as Studied by Polarized Reflection Spectra on Single Crystals" in Journal of Imaging Science and Technology, 2003, pp 18 - 24, https://doi.org/10.2352/J.ImagingSci.Technol.2003.47.1.art00005