On-off-on Control of Molecular Inversion Symmetry via Multi-stage Protonation: Elucidating Vibronic Laporte Rule

The Laporte rule dictates that one- and two-photon absorption spectra of inversion-symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric- and non-inversion symmetric two-photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single- and double-protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two-photon absorption. By performing detailed one- and two-photon titration experiments, with supporting quantum-chemical model calculations, we explain how certain low-frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on-off-on inversion symmetry switch, opening new avenues for two-photon sensing applications.

Automated summary

The Laporte rule states that one- and two-photon absorption spectra of symmetric molecules should display alternatively forbidden electronic transitions. However, for organic fluorophores, the distinction between symmetric and non-inversion symmetric two-photon spectra is often unclear due to vibronic interactions. In this study, we use protonation to break and then reconstitute inversion symmetry in a symmetric diketopyrrolopyrrole, resulting in significant changes in two-photon absorption. Through experiments and calculations, we explain how low-frequency vibrational modes can lead to deviations from the Laporte rule and propose the system as an inversion symmetry switch for two-photon sensing applications.