Excited-State Symmetry Breaking in a Quadrupolar Molecule Visualized in Time and Space.

The influence of the length of the push-pull branches of quadrupolar molecules on their excited-state symmetry breaking was investigated using ultrafast time-resolved IR spectroscopy. For this, the excited-state dynamics of an A-π-D-π-A molecule was compared with those of an ADA analogue, where the same electron donor (D) and acceptor (A) subunits are directly linked without a phenylethynyl π-spacer. The spatial distribution of the excitation was visualized in real time by monitoring C≡C and C≡N vibrational modes localized in the spacer and acceptor units, respectively. In nonpolar solvents, the excited state is quadrupolar and the excitation is localized on the π-D-π center. In medium polarity solvents, the excitation spreads over the entire molecule but is no longer symmetric. Finally, in the most polar solvents, the excitation localizes on a single D-π-A branch, contrary to the ADA analogue where symmetry breaking is only partial.