Superior Photostability of the Unnatural Base 6-Amino-5-nitropyridin-2-ol: A Case Study Using Ultrafast Broadband Fluorescence, Transient Absorption, and Theoretical Computation

6-Amino-5-nitropyridin-2-ol (Z), a nitroaromatic compound and a base for Hachimoji nucleic acids, holds significant potential in expanding the genetic alphabet, as well as in synthetic biology and biotechnology. Despite its promising applications, the spectral characterization and photoinduced properties of Z have remained largely unexplored until now. This study presents a comprehensive investigation into its excited state dynamics in various solvents, utilizing state-of-the-art ultrafast broadband time-resolved fluorescence and transient absorption spectroscopy, complemented by computational methods. The acquired results provide direct experimental evidence that, upon photoexcitation, Z emits prompt fluorescence from a nearly planar structure in its excited state, independent of solvent properties. This state deactivates nonradiatively within sub-picoseconds through internal conversion with a unitary yield, primarily mediated by the rotation of the nitro group. This unusually rapid deactivation pathway entirely excludes the involvement of long-lived nπ* states, triplet states, and photoproducts, which are commonly observed in most nitroaromatic compounds and natural DNA and RNA bases. Our findings underscore that Z, as an unnatural base, exhibits superior photostability compared to canonical natural bases. This provides valuable insights into the photodynamics of nitroaromatic compounds, which is beneficial for strategic substitution design in environmental and biological applications.