Molecular Orbital Calculations of Diketo‐dipyridyl‐pyrrolopyrrole, Diketo and Dithioketo‐diphenyl‐pyrrolopyrrole on Protonation and Deprotonation

Abstract PPP, CNDO/S and INDO/S calculations have been carried out on the title compounds (abbreviated to DPPP, DPP and DTPP, respectively; industrially important pigments) in order to study spectroscopically the diketopyrrolopyrrole chromophore on protonation and deprotonation. Protonation which takes place at the pyridyl nitrogen (DPPP only) and deprotonation (in all three molecules) which occurs at the NH group induce large spectral shifts in solution towards longer wavelengths (protonation: 35 nm (ca. 1236 cm−1); deprotonation: 77–150 nm (ca. 1820–4380 cm−1)). Geometry optimization by the AM1 method showed that the pyridyl rings in DPPP and the phenyl rings in DPP and DTPP are twisted, in the same direction, out of the plane of the planar heterocyclic system by 15–21°. The deviation is asymmetrically reduced due to mono‐deprotonation: the ring on the deprotonated side is less twisted than the other. Di‐deprotonation makes the molecule completely planar. On mono‐protonation of DPPP, the protonated pyridyl ring becomes less twisted while the unprotonated remains almost unchanged. In contrast, on di‐protonation, the two pyridyl rings become even more twisted than in the initial state. The electronic spectra calculated on the basis of the optimized structures reproduce well qualitatively the trends of the optical absorption, but generally underestimate the experimental values.