Insights into the Lowest Excited State of Anthracene: Kerr-Gated UV Resonance Raman Spectroscopy (S₁ ← S₀) 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 S1 ← S0 transition had impeded the characterization of anthracene's S1 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 (Ag 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.