First-principles Modeling of the Absorption spectrum of Crystal Violet in Solution - The Importance of Environmentally-driven Symmetry Breaking

Theoretical spectroscopy plays a crucial role in understanding the properties of materials and molecules. One of the most promising methods for computing optical spectra of chromophores embedded in complex environments from the first principles is the cumulant approach, where both (generally anharmonic) vibrational degrees of freedom and environmental interactions are explicitly accounted for. In this work, we verify the capabilities of the cumulant approach in describing the effect of complex environmental interactions on linear absorption spectra by studying Crystal Violet (CV) in different solvents. The experimental absorption spectrum of CV strongly depends on the nature of the solvent, indicating strong coupling to the condensed-phase environment. We demonstrate that these changes in absorption lineshape are driven by an increased splitting between absorption bands of two low-lying excited states that is caused by a breaking of the D3 symmetry of the molecule, and that in polar solvents this symmetry-breaking is mainly driven by electrostatic interactions with the condensed phase environment, rather than distortion of the structure of the molecule, in contrast with conclusions reached in a number of previous studies. Our results reveal the importance explicitly including a counterion in the calculations in nonpolar solvent due to electrostatic interactions between CV and the ion. In polar solvent, these interactions are strongly reduced due to solvent screening effects, thus minimizing the symmetry breaking. Computed spectra in methanol are found to be in reasonable agreement with experiment, demonstrating the strengths of the outlined approach in modeling strong environmental interactions.