The synthesis, absorption and fluorescence of polar diketo-pyrrolo-pyrroles

The effect of electron-donor (piperidino) and electron-acceptor (cyano) groups on the absorption maxima of some diketo-pyrrolo-pyrroles in DMSO solution was investigated both experimentally and theoretically. The syntheses of push–pull substituted 3-(4-piperidinophenyl)-6-(4-cyanophenyl)-2,5-dihydropyrrolo[3,4-c]pyrrol-1,4-dione are reported for the first time; 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrol-1,4-diones substituted with either one or two piperidino or cyano groups in the para-positions of the phenyl rings were also synthesized. Whilst both types of substituent produce a bathochromic and hyperchromic shifts with respect to the parent unsubstituted compound, the influence of the piperidino group is strongest. Symmetrically substituted derivatives show well-resolved vibronic structures, while the spectra of polar, unsymmetrical compounds were poorly resolved because of strong dipole–dipole interaction with the polar solvent. The fluorescence of all compounds was intense and a significant increase in the Stokes shift of polar derivatives was another consequence of dipole–dipole interactions with the solvent in the excited state.