Resonance Raman scattering of rhodamine 6G as calculated by time-dependent density functional theory: Vibronic and solvent effects

The geometries, UV-vis absorption spectra, and resonance Raman (RR) intensities have been determined for the S₁ and S₃ excited states of rhodamine 6G (R6G) in vacuum and ethanol by means of DFT/TDDFT methodologies with the aim of better understanding the structures and properties of the excited states. The RR spectra have been simulated from the vibronic theory of RR scattering as well as within the short-time approximation, while the solvent effects have been modeled using the polarizable continuum model. The S₁ and S₃ states of R6G present UV-vis absorption bands with similar vibronic structure, i.e., a shoulder at smaller wavelengths, although this shoulder is relatively more intense and more sensitive to the solvent in the case of S₃. These differences are corroborated by the larger geometry relaxations upon excitation for S₃ and the fact that the charge transfer of S₃ is reduced in ethanol. Moreover, the differences between S₁ and S₃ are magnified when considering the RR spectra. On one hand, the RR spectrum of R6G in resonance with the S₀ → S₁ transition presents many transitions of which the relative intensities strongly vary when the excitation wavelength gets closer to the maximum of absorption. The RR spectrum of R6G in resonance with S₁ is however little influenced by the solvent. On the other hand, the RR spectrum of R6G in resonance with the S₀ → S ₃ transition displays only a few bands, strongly depends on the solvent, and is little affected when changing the excitation wavelength within the limits of the absorption band. As a consequence, the shorttime approximation is suitable to reproduce the RR spectrum of R6G in resonance with S₃ for a broad range of excitation wavelengths, whereas the vibronic theory approach is needed for describing the RR spectrum of R6G in resonance with S₁ close to resonance.