Fig. 4: Ultrafast proton transfer in Chili-DMHBO⁺.

a Global deconvolution of time resolved fluorescence spectra from the fluorescence up-conversion measurement of Chili–DMHBO⁺. The peak at 470 nm is a Raman signal of water. The black emission spectrum (peaking at 510 nm) refers to the protonated form of DMHBO⁺, the red to magenta spectra to the deprotonated form. The time constants τ₂-τ₄ (0.8–38 ps) refer to a continuous dynamic Stokes shift of the emission spectrum caused by solvent and molecular relaxation. The last time constant (1.4 ns) refers to ground state recovery. b Schematic model of excitation, proton transfer, relaxation, and emission processes observed in (a). For the ESPT a barrier is assumed because of the distinct differences of the emission spectra of both states. c Scheme of the proposed photo cycle in Chili-DMHBO⁺. The neutral chromophore (P) is excited from the ground state. The transition from the excited phenol form (P*) to the anionic form (A*) occurs through ESPT in 130 fs. After fluorescence emission, the ground state P is regenerated.