Non-alternant-hydrocarbon-based conjugated phosphole oxides

The synthesis and characterization of two species of a new family of ring-fused phosphole oxides that feature azulene as a key component is reported. Azulene is known for inducing distinct photophysics to conjugated materials due to its inherently polar nature, and this structure-property study reveals its impact on fused phosphole-containing species for the first time. Both new species feature four fused rings in their π-conjugated scaffold that only differ in one terminal ring (i.e., pyridine vs. thiophene). The pyridine-terminated species can be further modified via N-methylation leading to an overall cationic scaffold. Supporting computational studies provide details on the frontier molecular orbitals, polarities, and the ring currents within the scaffolds. All compounds are deeply blue or purple colored, because of the polar nature of the azulene building block with absorptions between ∼300 and 700 nm in their UV–vis spectra. Moreover, they show largely (quasi)reversible reduction features between −0.5 and −2.0 V (vs. Ag/AgCl), as determined via cyclic voltammetry, a hallmark of phosphole-oxide containing conjugated materials. This study thus supports the successful marriage of key functional properties of azulene and phosphole-based conjugated materials.