Unraveling the electronic structure of 2-cyanonaphthalene: A cosmic PAH

The recent astronomical detection of 2-cyanonaphthalene in Taurus Molecular Cloud-1 has highlighted the importance of substituted polycyclic aromatic hydrocarbons in astrochemistry. Here, we present a combined experimental and theoretical study of the electronic absorption spectrum of 2-cyanonaphthalene in the ultraviolet (UV) and vacuum ultraviolet (VUV) regions (3.7-10.8 eV). Photoabsorption spectra in the UV region 3.7-4.9 eV are recorded in an n-hexane solvent at room temperature. Gas-phase photoabsorption spectra and absolute cross sections are measured at 343 K using synchrotron radiation in the VUV region (5.1-10.8 eV). Analysis supported by TDDFT calculations reveals that the UV region is dominated by valence π* ← π transitions, while the VUV region contains extensive Rydberg series converging to the first three ionization potentials. The strongest absorption band, arising from the HOMO-LUMO transition, peaks at 224 nm, close to the 217.5 nm interstellar extinction bump, suggesting that 2-CNN may contribute to this astrophysical feature. A well-resolved vibronic structure is observed throughout the spectrum. Franck-Condon simulations including the Herzberg-Teller effect for the S1, S2, and S5 states reproduce the experimental spectra and identify the dominant progressions as arising from various C-H bending modes. Temperature-dependent simulations (15-500 K) of vibronic bands are used to study changes in spectral profile across various temperatures. Potential energy curves of singlet and triplet excited states with respect to C-H and C-CN bond lengths provide insights into the photostability of 2-cyanonaphthalene. The spectroscopic data and theoretical simulations presented here provide valuable inputs for understanding the photophysics and astrochemical evolution of 2-cyanonaphthalene.