HOMO and LUMO energy levels of N,N′-dinitrophenyl-substituted polar diketopyrrolopyrroles (DPPs)

Abstract

A series of four [3,4-c]pyrrole-1,4-diones (diketopyrrolopyrroles, DPPs) with (substituted) phenyl rings in 3,6-positions was prepared by direct N,N′-arylation of corresponding diketopyrrolopyrrole pigments with 1-fluoro-2,4-dinitro-benzene. While the energies of the HOMO levels depend strongly on the nature of the p-substituents on the 3,6-phenyl rings, the LUMO levels obtained by cyclic and rotating disc voltammetry were found to be almost independent of the substituent. The absorption spectra show either a hypsochromic or bathochromic shift with respect to parent pigments, depending on the electron-donating and -accepting character of the p-substituents. This behaviour was rationalized by density functional theory calculations, showing that highest occupied molecular orbital is delocalized over the whole 3,6-diphenyl-diketopyrrolopyrrole conjugated system as in the parent pigments, while low-lying LUMO is completely different from the precursors, as it is localized exclusively on the 2,4-dinitrophenyl substituents, i.e. its shape and energy are not affected by a substitution on the 3,6-phenyl rings.