Two-photon spectroscopy reveals protonation-induced symmetry-breaking in the ground electronic state of nominally centrosymmetric organic fluorophores*

One-photon absorption (1PA) and two-photon absorption (2PA) of inversion-symmetric systems display alternative parity rule, i.e. transition to 1PA-allowed excited electronic states are 2PA-forbidden. Therefore, comparison of 1PA and 2PA spectra of nominally centrosymmetric organic chromophores offers valuable insights into symmetry-breaking due to conformational changes, vibronic interactions, solvent-chromophore interactions. etc.

Here, we use femtosecond 2PA spectroscopy to show, for the first time, that a nominally centrosymmetric organic fluorophore can be switched from a centrosymmetric- to non-centrosymmetric state and back by  variation of pH of the solution.  We synthesize novel fluorophores comprising pyrrollopyrrole core with symmetrically-attached pair of moieties with affinity to protonation.  In pH-neutral solvent such as neat methanol, there are no protons attached, and the 1PA and 2PA spectra show hallmark alternative behavior. Adding a small amount of diluted triflic acid causes protonation of one of the two moieties, which breaks the ground state inversion symmetry as evidenced by simultaneously 1PA- and 2PA-allowed lowest-energy electronic transition. Upon further decrease of the pH, both moieties become protonated, thus restoring nominal inversion symmetry.