The time and frequency resolved optical response of wild-type green fluorescent protein (wt-GFP) has been measured at room temperature following 30 fs, 400 nm photo-excitation. In the wavelength range covering the stationary fluorescence spectrum of the protein, the stimulated emission rises on a time scale of roughly 20 ps due to excited-state proton-transfer (ESPT). The rise can be described phenomenologically by a sum of two exponentials. A long-time isosbestic behavior on the blue edge of the stationary emission implies a barrier for ESPT which is significantly larger than thermal excitations. In addition, an instantaneous component to the stimulated emission appears within the time resolution of our experiment. This observation is indicative of non-vertical cross-well transitions that prepare the proton-transferred configuration of the excited state directly from the equilibrium geometry of the ground-state neutral species during photo-excitation. Finally,
transient absorptions around 500 nm and 650 nm can be observed, which are attributed to transitions from different protonated forms of the excited-state of GFP to higher lying electronic configurations, Sn. The entire optical response of GFP is quantitatively simulated using a dynamic model that includes: (i) an energy-dependent rate coefficient for ESPT, (ii) intra- and intermolecular transfer of excess vibrational energy (IVR and VET), and (iii) an additional non-radiative decay pathway for the initially prepared Franck–Condon state leading to internal conversion via motion along a torsional coordinate. In particular, the nonexponential nature of the ESPT originates from overlapping time scales of reactive and non-reactive elementary processes following optical excitation.
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