Vibrational and electronic absorption spectroscopy of 2,3-benzofluorene and its cation

Benzofluorene (C ₁₇H ₁₂) has been studied in argon matrices via Fourier transform infrared and UV-visible absorption spectroscopy. The analysis of the infrared absorption spectra of neutral and cationic 2,3-benzofluorene was supported by density functional theory (DFT) B3LYP/6-311+G** calculations of the harmonic-mode frequencies. Extensive time-dependent DFT calculations of the electronic vertical excitation energies with BLYP/6-31++G** and B3LYP/6-31++G** functionals/basis sets and the Casida-Salahub asymptotic correction were performed to assign the observed electronic absorption bands of the neutral species. Although the observed low-energy absorption bands are predicted well by theory, the higher-energy bands (S _n ← S ₀ transitions, n ≥ 4) have been assigned only tentatively. However, the observed electronic absorption bands for the parent, singly dehydrogenated cationic and neutral species are in accord with TDDFT (BLYP/6-31G**) results. The possibility that the 2,3-benzofluorene cation contributes to the unidentified infrared (UIR) bands observed from interstellar space is discussed briefly.