Electronic Characterization of Reaction Intermediates: The Fluorenylium, Phenalenylium, and Benz[f]indenylium Cations and Their Radicals

Three vibrationally resolved absorption systems commencing at 538, 518, and 392 nm were detected in a 6 K neon matrix after mass-selected deposition of C₁₃H₉⁺ ions (m/z=165) produced from fluorene in a hot-cathode discharge ion source. The benz[f]indenylium (BfI⁺: 538 nm), fluorenylium (FL9⁺: 518 nm), and phenalenylium (PHL⁺: 392 nm) cations are the absorbing molecules. Two electronic systems corresponding to neutral species are apparent at 490 and 546 nm after irradiation of the matrix with λ<260 nm photons and were assigned to the FL9 and BfI radicals, respectively. The strongest peak at 518 nm is the origin of the 2 ¹B₂←X̃ ¹A₁ absorption of FL9⁺, and the 490 nm band is the 2 ²A₂←X̃ ²B₁ origin of FL9. The electronic systems commencing at 538 nm and 546 nm were assigned to the 1 ¹A₁←X̃ ¹A₁ and 1 ²A₂←X̃ ²A₂ transitions of BfI⁺ and BfI. The 392 nm band is the 1 ¹E′←X̃ ¹A₁′ transition of PHL⁺. The electronic spectra of C₁₃H₉⁺/C₁₃H₉ were assigned on the basis of the vertical excitation energies calculated with SAC-CI and MS-CASPT2 methods.