Molecular dynamics of Auramine O in low-viscosity solutions as investigated by an ultrafast lensing effect

Ultrafast change in refractive index after excitation by a femtosecond lase r pulse was observed for dilute Auramine O solutions in low-viscosity solve nts by detecting the change as an ultrafast lensing effect (ULE). The decay of the ULE signals was found to consist of two components, and both their relaxation times depended on solvents. This is the first reported observati on of the faster component by this technique. The slower component had a si milar relaxation time to that of the fluorescence lifetime. Molecular orbit al calculation results attributed the faster to relaxation in the lowest ex cited singlet state S-1 by way of internal rotation and the slower to the s ubsequent relaxation from S-1 to the ground state. Solvent dependence of th e experimentally determined relaxation time was strongly correlated with th e molecular weight of the solvents rather than their bulk viscosity. A simp lified model explaining these results was proposed in which solute and solv ent molecules interact only via hydrogen bonding, the strength of which is solvent independent.