Ultrafast, Non-Productive, Deprotonation of the Excited Flavylium Cation in Delphinidin-3-O-Rutinoside

Anthocyanins are widespread natural pigments responsible for the vibrant colors of fruits and flowers and play key roles in photoprotection. Despite their biological significance, the interplay between excited-state proton transfer (ESPT) and ultrafast relaxation remains poorly understood. Here, we investigate the ultrafast excited-state dynamics of delphinidin-3-O-rutinoside (D3R) in aqueous solution at pH 2—where the protonated flavylium cation dominates—using femtosecond broadband transient absorption spectroscopy. Excitation at 525 nm induces rapid spectral changes on the sub-picosecond timescale connected with sub 50 fs deprotonation in the excited state. Global analysis reveals two dominant processes: a 0.12 ps component assigned to ultrafast proton transfer from the excited flavylium cation to the solvent, forming the electronically excited quinoidal base (QB). This is followed by a 0.47 ps component characterized by strong bleach recovery, representing the primary relaxation pathway back to the ground state of at least 70%. Partial germinal reprotonation occurs upon electronic relaxation to the ground state. A mixture of vibrationally excited D3R and QB results. A characteristic time scale is around 3 ps. Reprotonation of QB occurs on the ps–ns timescale via diffusion-controlled processes. The overall process enables rapid dissipation of excitation energy, providing a mechanistic basis for the photoprotective function of anthocyanins in biological systems.