Direct Visualization of Excited-State Symmetry Breaking Using Ultrafast Time-Resolved Infrared Spectroscopy

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Abstract
Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S1 electronic excited state. This is usually explained by a break-up of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D--A--D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S1 state was found to strongly depend on the solvent polarity: 1) in non-polar solvents, it is symmetric and quadrupolar; 2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry-broken S1 state with one arm bearing more excitation than the other; and 3) in highly polar solvents, the excited state evolves further to a purely dipolar S1 state with the excitation localized entirely on one arm. The timescales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations.
Funding Information
  • Université de Genève
  • National Natural Science Foundation of China (21404048)
  • Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (200020-147098)

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