Abstract

The synthesis, structural and optoelectronic properties of a series of electron deficient chromophores based on benzaldehyde- and benzoate-functionalised diketopyrrolopyrroles (DPPs) is described. All compounds have high molar absorption coefficients and fluorescence quantum yields. The benzoate derivative, with a protonated lactam nitrogen that can engage in hydrogen bonding, shows unusually high solubility in THF. N-Alkylation of the lactam unit disrupts the hydrogen bonding of the chromophores, rendering derivatives soluble in solvents ranging in polarity from toluene to acetonitrile, in which they display negative solvatochromism. In the solid state, single crystal structures show that for the benzoate family, N-alkylation modulates the π-π stacking interactions from a sandwich interaction between phenyl and lactam moieties to CH-π interactions between adjacent phenyl rings distorted from ideal orthogonality between lactam and phenyl units. In contrast, the benzaldehyde derivative displays a similar sandwich interaction to the ester with the unsubstituted lactam. Theoretical mobility measurements based on the crystal packing indicate values approaching 0.05 cm² V^-1s^-1 for holes and 0.08 cm² V^-1s^-1 for electrons^, suggesting promise as potential semiconductors for organic electronic applications. All compounds display multiple redox processes, with each having a single irreversible oxidation process and multiple accessible reversible reduction processes, showing their electron accepting ability. Electron paramagnetic resonance spectroscopy of the singly reduced species shows the delocalisation of the unpaired electron over the whole aromatic part of the molecule. Spectroelectrochemical visible absorption spectra tracking the first and second reduction processes give orders of magnitude increase in their molar absorption coefficient in most cases, attributed to the diradical taking the quinoidal form and suggest suitability for electrochromic applications.

Keywords: Dyes, Reduction, Radicals, Chromophores, Redox

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Humphreys, J.; Malagreca, F.; Hume, P. A.; Pop, F.; Davies, E. S.; Argent, S. P.; Newton, G. N.; Amabilino, D. B. Beilstein Arch. 2026, 202629. doi:10.3762/bxiv.2026.29.v1

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