Insights into the Lowest Excited State of Anthracene: Kerr-Gated UV Resonance Raman Spectroscopy (S ₁ ← S ₀ ) with Tunable Excitation and DFT Analysis

In this work, we present the first, well-resolved Raman spectrum of anthracene, a model system of polycyclic aromatic hydrocarbons, in resonance with the first electronic state. Until now, the autofluorescence associated with the <i>S</i>₁ ← <i>S</i>₀ transition had impeded the characterization of anthracene's <i>S</i>₁ state through resonance Raman. To overcome this limitation, we employ state-of-the-art Kerr-gated UVRR spectroscopy with tunable excitation for effectively suppressing the UV-excited autofluorescence. Experimental RR excitation profiles with laser excitation wavelengths in the 366-383 nm range, covering the first fundamental vibronic transitions, were recorded. With the exception of the first overtone of normal mode 59 (<i>A</i>_<i>g</i> symmetry) observed at 785 cm^-1, the experimental Raman excitation profiles (REPs) broadly follow the experimental UV absorption spectrum. Notably this overtone gains maximum intensity when excited with an excitation blue-shifted to the absorption maximum (376 nm), in resonance with the |59¹⟩ ← |0⟩ transition. These REP excitation profiles highlight the central influence of vibronic coupling on resonance Raman intensities and emphasize that, under the Franck-Condon assumption, truly predictive quantum-chemical simulations require sum-over-states treatments with explicit inclusion of the vibrational manifold of the intermediate electronic excited state.