Dark-State-Mediated Efficient Energy Trapping in a Model GFP Chromophore

The functional properties of photoactive proteins are governed by the interplay between bright and dark excited states. While the bright states are well-studied, the dark states, which are fundamental to photostability and light harvesting, are notoriously difficult to characterize. Here, we report the direct observation and full characterization of an optically dark, low-lying singlet excited state in the isolated anion of the meta green fluorescent protein (GFP) chromophore. Using a combination of ultrafast time-resolved action absorption and photoelectron spectroscopy, we have captured the formation of this state in 100 fs and measured its remarkably long lifetime of 94 ps. We unambiguously assign its charge-transfer character and reveal the precise trapping mechanism through high-level ab initio calculations. Our findings uncover a photoprotective mechanism in biomolecular anions, where ultrafast internal conversion quenches electron emission, stabilizing long-lived electronic excitation even when the energy exceeds the electron detachment threshold.