Fluorescence Excitation and Dispersed Fluorescence Spectra of the First Electronic Excited (S 1 ) State of peri -Hexabenzocoronene (C 42 H 18 ) Isolated in Solid para -Hydrogen.

Large polycyclic aromatic hydrocarbons (PAH) and their cationic, hydrogenated, and protonated derivatives have long been considered as promising candidates for the carriers of the diffuse interstellar bands. peri -Hexabenzocoronene ( peri -HBC, C 42 H 18 ) is a large, compact PAH, and, to the best of our knowledge, the largest centrosymmetric all-benzenoid PAH for which electronic spectroscopy data has been published. In this work, we present the dispersed fluorescence and fluorescence excitation spectra of the first electronic excited (S 1 ) state of peri -HBC isolated in solid para -H 2 and provide the first detailed vibronic analysis of observed features. The observed spectra agree with the emission and absorption spectra simulated according to optimized geometries and scaled harmonic vibrational frequencies calculated at the density functional theory (DFT) level using a Franck-Condon Herzberg-Teller approach; the spectral bands are associated solely with vibrational normal modes of approximate e 2g symmetry and their combinations with vibrational modes of approximately a 1g symmetry. We clearly observed the position of the S 1 -S 0 electronic transition origin of peri -HBC at 22,088 cm -1 (452.7 nm), which was unreported previously. The matrix shift of ∼110 cm -1 to the red relative to the gas-phase value was estimated by comparison of two reported gas-phase bands with our work. Because of the significant deviation from the reported wavelengths of DIB, the weakness of the S 1 -S 0 electronic transitions, and the lack of reported DIB at <400 nm where the intense S 4 ← S 0 band of peri -HBC is located, peri -HBC is unlikely to contribute to DIB.