Benchmarking the Calculated Resonance Raman Spectrum of a Diarylethene-Based Molecular Switch Using the Gradient Approximation

Resonance Raman spectroscopy provides important insight into the initial nuclear motions of a molecule following optical excitation. The vibrational frequencies in the spectrum represent the ground-state structure, but the intensities of the Raman bands reflect the dynamics after excitation to an electronically excited state. Calculations are often necessary to assign vibrations in the experimental spectrum in order to correctly identify the excited-state dynamics, but there is a risk of misassignment when using calculations that do not include resonance-enhancement effects. Off-resonance calculations typically give frequencies that are accurate within 10–20 cm^–1 but may not correctly represent the relative Raman scattering intensities of an experimental resonance Raman spectrum. Two approaches for including resonance-enhancement effects in the Raman spectrum are the Franck–Condon method and the gradient approximation method. This contribution examines both methods for obtaining resonance Raman intensities for a diarylethene-based molecular switch with 129 normal modes. Comparing experimental spectra measured both on- and off-resonance with simulated spectra using off-resonance, Franck–Condon, and gradient approximation methods highlights the need to include resonance-enhancement effects in the calculations in order to make accurate mode assignments. The gradient approximation gives good agreement with the experimental resonance Raman spectrum while avoiding potential complications and the computational cost of finding the optimized geometry and normal modes of the excited state. The vibrational assignments reveal key stretching motions involved in the initial excited-state dynamics of the molecular switch.