Polar solvation dynamics of coumarin 153 by ultrafast time-resolved fluorescence.

Polar solvation dynamics of coumarin 153 in acetonitrile, methanol, and butanol are investigated by dynamic Stokes shift function, S(t). In small protic solvents, it is known that an initial ultrafast component below 50 fs constitutes more than half of the total solvation process. We use fluorescence up-conversion technique via two-photon absorption process, which can provide 40 fs time resolution for the whole emission wavelength range. Moreover, time-resolved fluorescence spectra are recorded directly without the spectral reconstruction. We observe a temporal oscillation in frequency of whole emission spectrum in the solvation curve. In the obtained S(t), initial solvation time scale is 120 fs, invariant to solvents used in this experiment, although its amplitude varies in different solvents.