Fluorescence excitation and dispersed fluorescence spectra of iso-quinolinyl radicals 4-, 5-, and 8-iso-HC9H7N isolated in solid para-hydrogen

Nitrogen-substituted polycyclic aromatic hydrocarbons and their derivatives have been proposed as possible carriers of the unidentified infrared emission feature near 6.2 µm in astronomical observations and might also contribute to the diffuse interstellar bands (DIB). We present the previously unobserved dispersed fluorescence and fluorescence excitation spectra of three isomers of hydrogenated iso-quinoline, iso-quinolinyl radicals (iso-HC₉H₇N), isolated in solid para-hydrogen (para-H₂). The three observed progressions, originating at 18492, 19430, and 18688 cm⁻¹ and exhibiting distinct fluorescence lifetimes, are assigned to the D₁−D₀ electronic transitions of the 4-, 8, and 5-iso-HC₉H₇N isomers, respectively. Assignments are supported by Franck-Condon Herzberg-Teller simulations based on (TD-)uB3PW91/6-311++G(2d,2p) optimized geometries and scaled harmonic vibrational frequencies. The dispersed fluorescence and fluorescence excitation spectra predominantly involve vibrational normal modes of a' symmetry. We compare our spectra to previously reported IR spectra and discuss how the position of hydrogenation influences vibronic structure. The potential contribution of 4-, 8, and 5-iso-HC₉H₇N to the DIB spectrum is discussed.