Probing correlated spectral motion: two-color photon echo study of Nile blue

We performed two-color three-pulse photon echo peak shift experiments on Nile blue in ethylene glycol and acetonitrile to determine the role of solvent dynamics in correlated spectral motion. The system was pumped near the absorption maximum and the correlation between the initial state and the final state was probed at a number of wavelengths, from the absorption maximum to the fluorescence maximum. In addition to solvent dynamics, we found that strongly coupled intramolecular vibrations generated correlations between different spectral regions. The inertial solvent response was found for both solvents to have a time scale on the order of 100-145 fs. This response contributed half of the solvent interaction strength for acetonitrile, but less than a third for ethylene glycol. Several diffusive time scales were observed: 500 fs and 2.5 ps for acetonitrile, and 1, 15, and 100 ps for ethylene glycol. A single description of the solvation dynamics was insufficient to quantitatively describe the dynamics at all probe wavelengths, which could indicate different dynamics in the ground and excited states or the presence of an additional contribution to the signal from the excited-state absorption.