Interpretation of the Near-IR Absorption of 1,4-Dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole in Terms of Exciton Coupling Effects

Abstract

The title compound (abbreviated to DTPP) is known to possess three crystal modifications, among which only modification III exhibits an intense near-IR absorption. Because of this, DTPP has attracted attention as a material suitable for laser printers as well as optical information storage systems based on GaAsAl laser diodes. Modifications I and II are quite similar and crystallize in a “herringbone” fashion; whereas modification III is characterized by the “bricks in a brick wall” structure composed of homochiral hydrogen-bond pairs and heterochiral convex and concave pairs. The mechanism of the near-IR absorption has been therefore investigated from the standpoint of exciton coupling effects. The exciton displacement energies due to the nearest molecule-pairs within a sphere of about 30 A (12 pairs for modifications I&II and 16 pairs for modification III) have been calculated on the basis of the crystal structure. The hydrogen-bond pairs are found to displace the optical absorption towards longer wavelengths by about 1500 cm^-1 in all modifications. In modification III, the convex and concave pairs additionally contribute to the bathochromic shift. These results indicate that the “bricks in a brick wall” structure plays the decisive role for the appearance of the near-IR absorption.