Interstellar Formation\nof Nitrogen Heteroaromatics\n[Indole, C<sub>8</sub>H<sub>7</sub>N; Pyrrole, C<sub>4</sub>H<sub>5</sub>N; Aniline, C<sub>6</sub>H<sub>5</sub>NH<sub>2

Nitrogen-substituted polycyclic aromatic hydrocarbons\n(NPAHs) are\nnot only fundamental building blocks in the prebiotic synthesis of\nvital biomolecules such as amino acids and nucleobases of DNA and\nRNA but also a potential source of the prominent unidentified 6.2\nμm interstellar absorption band. Although NPAHs have been detected\nin meteorites and are believed to be ubiquitous in the universe, their\nformation mechanisms in deep space have remained largely elusive.\nHere, we report the first bottom-up formation pathways to the simplest\nprototype of NPAHs, indole (C<sub>8</sub>H<sub>7</sub>N), along with\nits building blocks pyrrole (C<sub>4</sub>H<sub>5</sub>N) and aniline\n(C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub>) in low-temperature model\ninterstellar ices composed of acetylene (C<sub>2</sub>H<sub>2</sub>) and ammonia (NH<sub>3</sub>). Utilizing the isomer-selective techniques\nof resonance-enhanced multiphoton ionization and tunable vacuum ultraviolet\nphotoionization reflectron time-of-flight mass spectrometry, indole,\npyrrole, and aniline were identified in the gas phase, suggesting\nthat they are promising candidates for future astronomical searches\nin star-forming regions. Our laboratory experiments utilizing infrared\nspectroscopy and mass spectrometry in tandem with electronic structure\ncalculations reveal critical insights into the reaction pathways toward\nNPAHs and their precursors, thus advancing our fundamental understanding\nof the interstellar formation of aromatic proteinogenic amino acids\nand nucleobases, key classes of molecules central to the <i>Origins\nof Life</i>.