Indole-5,6-quinones display hallmark properties of eumelanin

Melanins are ubiquitous biopolymers produced from phenols and catechols by oxidation. They provide photoprotection, pigmentation and redox activity to most life forms, and inspire synthetic materials with desirable optical, electronic and mechanical properties. The chemical structures of melanins remain elusive, however, creating uncertainty about their roles, and preventing the design of synthetic mimics with tailored properties. Indole-5,6-quinone (IQ) has been implicated as a biosynthetic intermediate and structural subunit of mammalian eumelanin pigments, but its instability has prevented its isolation and unambiguous characterization. Here we use steric shielding to stabilize IQ and show that 'blocked' derivatives exhibit eumelanin's characteristic ultrafast nonradiative decay and its ability to absorb light from the ultraviolet to the near-infrared. These new compounds are also redox-active and a source of paramagnetism, emulating eumelanin's unique electronic properties, which include persistent radicals. Blocked IQs are atomistically precise and tailorable molecules that can offer a bottom-up understanding of emergent properties in eumelanin and have the potential to advance the rational design of melanin-inspired materials.

Automated summary

Melanins are biopolymers produced from phenols and catechols that provide various functions such as photoprotection, pigmentation, and redox activity. The chemical structures of melanins remain unknown, hindering the design of synthetic materials with specific properties. However, the stabilization of indole-5,6-quinone (IQ) derivatives has allowed the exploration of eumelanin's unique electronic and optical properties, which can contribute to the development of melanin-inspired materials.