ELECTRONIC-PROPERTIES OF 1,4-DITHIOKETO-3,6-DIPHENYL-PYRROLO-[3,4-C]-PYRROLE IN SOLUTION AND IN THE SOLID-STATE

The near IR optical absorption of the title compound (DTPP) is of practical importance for laser printers and information storage applications based on GaAs laser diodes. Electronic properties of DTPP have been investigated in solution and in the solid state from the standpoint of deprotonation and intermolecular hydrogen bonding. The electronic state of the dithioketopyrrolopyrrole chromophore is found to be most sensitively affected by an environment of proton acceptors which interact with the NH group. The deprotonation (=N-) in solution leads to increased electron density on the nitrogen atom. As a consequence, there is more overall electron density in the chromophore which contributes to a large bathochromic displacement. A similar increase in electron density is also operative in the solid state through the intermolecular hydrogen bonds between the NH group of one DTPP and the sulfur of the neighboring molecule. This is responsible for the large bathochromic shift of DTPP from the solution to the solid state. The visible absorption in the solid state is attributed to transitions along the molecular plane. On the other hand, the strong near IR absorption is interpreted as being due to interplanar pi-pi interactions along the stacking axis perpendicular to the molecular plane.