Indolonaphthyridine: A Versatile Chromophore for Organic Electronics Inspired by Natural Indigo Dye

Organic electronics is an exciting field of research offering innovative technologies from roll-to-roll inkjet-printed solar cells to foldable displays for cellphones and televisions. These functional devices exploit the flexible nature of conjugated organic materials, both polymeric and molecular, to absorb and emit light and to facilitate transport of charge carriers. A major driving force of development within the field is the creation of novel high-performance building blocks, providing a fruitful and ever-growing library of materials for tailored applications. Most of these building blocks contain chromophores that are entirely synthetic, yet there exist many naturally occurring building blocks, which have been relatively overlooked, despite their innate high stability and inexpensive nature. Indigo is the most produced dye worldwide and has one of the richest histories of all known textile dyes, dating before 4000 BC. Indigo's superior photostability has been linked to fast, favorable deactivation pathways following light absorption. But through one straightforward reaction, the chromophore of indigo can be transformed to a new chromophore with remarkable optoelectronic properties.In this Account, we discuss this chromophore, indolonaphthyridine, and give an overview of our research into the synthesis and optoelectronics properties of functional organic electronic materials derived from it. The unit's strong, fused planar construction contains bis-imide functional groups in similarity to the field-favorite diketopyrrolopyrrole, and similarly requires solubilizing with long alkyl chains, the installation of which is nontrivial and achieved using a protecting group strategy. Our solubilized indolonaphthyridine monomer allows us to copolymerize it with simple archetypal comonomers (thiophene, benzothiadiazole, etc.), in contrast to the other research groups working on the chromophore, who employ complex alkylated comonomer units. We discovered materials with extraordinary performance in organic photovoltaics, affording power conversion efficiencies up to 4.1% in the near-IR region of the spectrum. In organic field-effect transistors, the copolymers exhibited ambipolar transport and notable n-type mobilities up to 3.1 cm²/(V s), well above the benchmark set by silicon (1 cm²/(V s)). The strong absorption in the near-IR allowed us to explore the use of the polymers as contrast agents in photoacoustic imaging, an emerging technique capable of achieving deep tissue penetration without the need for ionizing radiation, while maintaining high contrast and high accuracy responses. Finally, we discuss an exciting aspect of the photophysics of molecular indolonaphthyridine: its ability to undergo singlet fission. Moreover, most singlet fission materials exhibit poor ambient stability; however our molecular indolonaphthyridines exhibit superior stability. It is our hope that this Account showcases the remarkable potential of this relatively unexplored, versatile chromophore and leads to wider adoption in the future.