Relaxation Dynamics of Tryptophan in Water: A UV Fluorescence Up-Conversion and Molecular Dynamics Study

We report on an ultrafast experimental and simulations study of the early relaxation events of photoexcited tryptophan in water. Experimentally, we used fluorescence up-conversion in both polychromatic and single wavelength detection modes in the 300-480 nm range with polarization dependence. We report on the time evolution of the Stokes shift, bandwidth, and anisotropy from tens of femtoseconds to picoseconds. These observables contain signatures of the simultaneous occurrence of intramolecular and solvent-molecule interactions, which we disentangle with the help of nonequilibrium molecular dynamics simulations. We also observe a breakdown of the linear response approximation to describe our results.