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

• Polar N,N′-dinitrophenyl-substituted diketopyrrolopyrroles (DPPs) were prepared.
• The LUMO levels (3.65–3.69 eV) are almost independent of the polar substituents.
• DFT calculations reveal LUMO exclusively on 2,4-dinitro-phenyl substituents.
• The energy of LUMO is close to the optimal range for electron donors in BHJ SC.
• The HOMO is tunable by the character of the polar substituents.

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.

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