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.