Fluorescence Excitation and Dispersed Fluorescence Spectra of the 1-Hydronaphthyl Radical (1-C₁₀H₉) in Solid...

Matrix isolation spectroscopy with para-hydrogen (p-H₂) has previously been employed to record IR absorption spectra of hydrogenated and protonated polycyclic aromatic hydrocarbons (PAHs), prospective carriers of unidentified infrared and diffuse interstellar bands. Despite the promising prospects of p-H₂ as matrix host, especially the rather weak interaction with the guest molecules and the resulting small matrix shifts, p-H₂ matrix isolation spectroscopy has rarely been applied to study electronic transitions of guest molecules. Here, we present the dispersed fluorescence and fluorescence excitation spectrum of the 1-hydronaphthyl radical (1-C₁₀H₉) isolated in solid p-H₂. We observed a strong 0₀⁰ band associated with the electronic transition to the first excited electronic state at 18881 cm^-1, red-shifted by ∼68 cm^-1 relative to a value reported for jet-cooled 1-C₁₀H₉. From a comparison of our experimental results to simulated vibrationally resolved electronic absorption and emission spectra computed on the basis of (TD-)DFT geometry optimizations and scaled harmonic vibration calculations using the FCclasses code, we derived assignments for observed vibronic transitions. The dispersed fluorescence spectrum of 1-C₁₀H₉ is new; it complements the infrared spectrum and identified many vibrational modes unidentifiable with infrared. The excitation spectrum covers a much wider spectral range than previous reports. We compare the excitation spectrum in solid p-H₂ to the reported electronic absorption spectrum of jet-cooled gaseous 1-C₁₀H₉ and that of 1-C₁₀H₉ isolated in solid Ne to assess the influence of p-H₂ as a matrix host on the electronic transition of 1-C₁₀H₉ and discuss a potential contribution of 1-C₁₀H₉ to the diffuse interstellar bands.