Influence of symmetry breaking on the absorption spectrum of crystal violet: from isolated cations at 5 K to room temperature solutions

We report the resolution of a long-standing puzzle in molecular spectroscopy: the origin of the shoulder in the room temperature solution absorption spectrum of crystal violet (CV) - an archetypal cationic triphenylmethane dye. This was achieved by comparing experimental and theoretical results for CV in solution at room temperature and as an isolated cation in gas-phase at 5 K. The two lowest energy electronically excited states involved in the visible region absorption are degenerate and coupled via a Jahn–Teller (JT) mechanism involving phenyl torsions, making CV particularly sensitive to environmental perturbations. The shoulder is absent in the low-temperature isolated cation spectrum, and vibronic simulations based on time dependent density functional theory (TD-DFT) indicate negligible JT effects under these conditions. Combining vibronic simulations with molecular dynamics, demonstrates that in water and toluene solution at room temperature the shoulder arises mainly from an intermolecular, Jahn–Teller-like symmetry-breaking effect induced by the fluctuating electrostatic potential of the disordered solvent environment, rather than from molecular distortions.