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- non-inversion symmetric is often obscured due to prevalent vibronic interactions. We take advantage consecutive single- double-protonation break then reconstitute inversion symmetry in a nominally diketopyrrolopyrrole, causing large changes absorption. By performing detailed titration experiments, with supporting quantum-chemical model calculations, we explain how certain low-frequency vibrational modes may lead apparent deviations from strict rule. As result, system be indeed considered as an on-off-on switch, opening new avenues sensing applications.