Extending the conjugation of xanthene through N heteroatoms

Polymers with small molecule chromophores incorporated into the conjugated backbone have shown excellent electronic properties for use in organic semiconductor applications. The incorporation of xanthene moieties, the central motif in fluorescein and rhodamine dyes, into a fully conjugated polymer backbone has yet to be fully investigated due to the inherent break in conjugation within the structure. This study reports the syntheses of four different low-molecular weight polymers via Buchwald-Hartwig polyamination composed of xanthene units linked through various amines. The N heteroatom linkage promotes conversion to the quinoidal form, which extends the conjugation through a hypothesized zwitterionic backbone. Evidence of extended conjugation is seen in the reduced optical band gaps of the polymers with the lowest band gap observed at 1.58 eV. Furthermore, air stable unpaired electrons were detected via electron paramagnetic resonance (EPR), suggesting a stabilized equilibrium between ionic and radical forms in the backbone. The design and characteristics inherent in the materials synthesized should provide intrinsic electrical conductivity and magnetic properties of interest.