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.