Intermolecular charge transfer in 1,4-dithioketo-3,6-diphenyl-pyrrolo- [3,4-c]-pyrrole

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Abstract
Certain organic vapors induce a morphological transformation in 1,4‐dithioketo‐3,6‐dipheyl‐pyrrolo‐[3,4‐c]‐pyrrole (DTPP) which is accompanied by enhanced absorption and photoconduction in the near infrared. In order to gain more insight into this phenomenon, intermolecular charge transfer caused by the vapor treatment has been investigated by Fourier transform‐infrared‐reflection absorption spectrometry (FT‐IR‐RAS) and x‐ray photoelectron spectroscopy (XPS). Spectral shifts have been observed in FT‐IR‐RAS spectra at wave numbers of about 3125 cm−1 (NH stretching), 1500 and 1600 cm−1 (CC stretching of the aromatic ring), 1180 cm−1 (C■S stretching), and 750 cm−1 (CH ‘‘out‐of‐plane’’ deformation of the aromatic ring). The N 1s and S 2p in XPS spectra have also been shifted towards lower binding energies by 0.1–0.2 eV, and this is fully consistent with the above results. These shifts are most likely attributed to the charge transfer interaction of C, N, and S atoms between the molecular planes along the b‐crystal axis. The electrostatic field induced by the charge transfer transition is responsible for the enhanced efficiency of the exciton dissociation into free‐charge carriers.