Exceptional Electron-Rich Heteroaromatic Pentacycle for Ultralow Band Gap Conjugated Polymers and Photothermal Therapy

Stable redox-active conjugated molecules with exceptional electron-donating abilities are key components for the design and synthesis of ultralow band gap conjugated polymers. While hallmark electron-rich examples such as pentacene derivatives have been thoroughly explored, their poor air stability has hampered their broad incorporation into conjugated polymers for practical applications. Herein, we describe the synthesis of the electron-rich, fused pentacyclic pyrazino[2,3-b:5,6-b']diindolizine (PDIz) motif and detail its optical and redox behavior. The PDIz ring system exhibits a lower oxidation potential and a reduced optical band gap than the isoelectronic pentacene while retaining greater air stability in both solution and the solid state. The enhanced stability and electron density, together with readily installed solubilizing groups and polymerization handles, allow for the use of the PDIz motif in the synthesis of a series of conjugated polymers with band gaps as small as 0.71 eV. The tunable absorbance throughout the biologically relevant near-infrared I and II regions enables the use of these PDIz-based polymers as efficient photothermal therapeutic reagents for laser ablation of cancer cells.

Automated summary

In this study, the synthesis of electron-rich fused pentacyclic pyrazino[2,3-b:5,6-b']diindolizine (PDIz) motif is described, and its optical and redox properties are detailed. PDIz exhibits a lower oxidation potential and a reduced optical band gap compared to the isoelectronic pentacene, while maintaining better air stability in both solution and the solid state. A series of conjugated polymers with band gaps as small as 0.71 eV are synthesized using the PDIz motif, and their tunable absorbance throughout the biologically relevant near-infrared I and II regions enables efficient photothermal therapy for cancer cell laser ablation.