Photodynamics simulation insights into excited-state relaxation mechanisms via Z/E isomerization in para-amino substituted GFP chromophores

The ability of green fluorescent protein (GFP) chromophore and its derivatives to undergo cis-trans photoswitching behavior has been widely acknowledged as of great interest because of its emerging applications in optogenetics and optoelectronics. However, key aspects of the internal conversion process in the GFP chromophore derivatives remain largely unclear. Based on quantum chemical methods and on-the-fly nonadiabatic dynamics simulations, we investigated the ultrafast photoinduced cis-trans isomerization phenomenon in the para-amino (NH2-HBDI) derivative of the GFP chromophore, which is intrinsically non-fluorescent. We demonstrate that upon excitation to the excited (S1) state, there occurs nonselective progression along the distortion of exocyclic methine-bridged imidazolinone (I) and phenylamine (P) bonds, leading to ultrafast nonradiative relaxation via S1(1ππ*)/S0 internal conversion. This internal conversion process is facilitated by multiple S1/S0 conical intersections with dominant imidazolinone (ΦI) dihedral rotations. Our results provide insight into the effect of the amino group on the dynamics of fluorescent probes. Therefore, these observations contribute to a valuable understanding of photoinduced switching phenomena and find a relation between structure and dynamics.