A LARGE BATHOCHROMIC SHIFT FROM THE SOLUTION TO THE SOLID-STATE IN 1,4-DIKETO-3,6-DIPHENYL-PYRROLO-[3,4-C]-PYRROLE

A large bathochromic shift (about 1400 cm-1) from the solution to the solid state in the title compound has been investigated from the standpoint of the change in electron density on the nitrogen atom caused by deprotonation or by intermolecular hydrogen bonding. The electronic state of the diketopyrrolopyrrole chromophore is found to be most sensitively affected by an environment of proton acceptors which interact with the NH group. Mono and di-deprotonation in solution brings about large bathochromic displacements of 2650 and 3400 cm-1, respectively. The deprotonation leads to increased electron density on the nitrogen atom and, as a consequence, more overall electron density in the chromophore which contributes to the bathochromic shift. Similar increase in electron density is also operative in the solid state through the intermolecular hdyrogen bonding between the NH of one molecule and the oxygen of another. The contribution of the hydrogen bond to the large shift is estimated to be about 1000 cm-1 in the solid state.