Design, synthesis, and self-assembly of oligothiophene derivatives with a diketopyrrolopyrrole core

Oligothiophenes containing diketopyrrolopyrrole (DPP) cores were synthesized, and their electrochemistry, photophysics, and film morphology were studied. Density functional theory calculations indicate that the highest occupied molecular orbital and the lowest unoccupied molecular orbital can be tuned by changing the number of thiophene units. The compounds synthesized in this study had electrochemical gaps ranging from 1.80 to 2.25 V and intense absorption bands in the UV, assigned to the thiophene units, as well as bands in the visible part of the spectrum, attributed to charge-transfer transitions between the electron-rich thiophene moieties and the electron-withdrawing DPP core. All of the compounds were found to be luminescent in solution with emission wavelengths ranging from 562 to 682 nm. Atomic force microscopy studies show that the length and type of alkyl substitution on the DPP core affect the film morphology, and hence, the solid state photophysical and electronic properties. Long and straight alkyl chains lead to rod-like or needle-like structures, whereas compounds with branched alkyl chains do not show ordering in the solid state. The self-assembling process can also be manipulated by changing the number of thiophene units. Nanoscale fibrous structures were obtained when either two or four thiophene units were added to the initial bis-thiophenyl-DPP unit.