An eumelanin-inspired core derived from the natural product, vanillin (vanilla bean extract) was utilized for the synthesis of eumelanin-inspired small molecules and polymer via Sonogashira cross coupling. The materials demonstrate that the methyl 4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate core can serve as a new building block for organic semiconductors.

In this work, five new eumelanin-inspired indole small molecules have been synthesized which differ by the substituents attached to the para position of the phenylene ring.

Four novel conjugated polymers containing the eumelanin-inspired indole core have been successfully synthesized using common cross coupling reactions. These polymers differed by the arylene and the carbon-carbon bond linkage. Optoelectronic experiments of these polymers suggest that the ethynylene linkage contributed to the red-shifted absorption spectra and blue-shifted emission spectra when compared to the vinylene linkage polymers. Furthermore, the optical bandgaps of the poly(indoylenearyleneethynylene)s (PIAEs) were smaller compared to the poly(indoylenearylenevinylene)s (PIAVs). Surprisingly, the HOMOs of these polymers were less affected by the nature of the carbon-carbon linkage. However, the LUMOs of the PIAEs were lower in comparison to the PIAVs. These eumelanin-inspired PIAEs and PIAVs are good fluorophores with fluorescence quantum yields ranging from 0.12 to 0.67 and have good thermal stability for applications such as in organic light-emitting diodes.