The correct assignment of vibrationally-resolved absorption spectra of protonated anthracene isomers

The assignment of experimental optical absorption spectra of protonated anthracene has been under debate for years. It is complicated by the presence of rich vibronic spectral features and the possible co-occurrence of two isomers, 9H-An⁺ and 1H-An⁺. In this study, the vibrationally resolved absorption spectra of 9H-An⁺ and 1H-An⁺ have been calculated using time-dependent density functional theory. The calculated vibronic spectra profiles of 9H-An⁺ and 1H-An⁺ are in excellent agreement with the corresponding experimental results and provide unambiguously spectra assignments. It shows that the previously reported assignments based on vertical excitation energy are largely wrong. The onset located at 493.8 nm of the experimental spectrum can be assigned to the S₀ → S₁ transition of 9H-An⁺, while the origin band located at 453.5 nm corresponds to the S₀ → S₂ transition of 1H-An⁺.