Journal of Physical Chemistry B, Vol.117, No.25, 7675-7684, 2013
Excess Dynamic Stokes Shift of Molecular Probes in Solution
The solvation dynamics of molecular probes is studied by broad-band fluorescence upconversion. The time-dependent position of the S-1 -> S-0 emission band or of a vibronic line shape is measured with similar to 80 fs, 10 cm(-1) resolution. Polar solutes in acetonitrile and acetone, when excited into Si with excess vibrational energy, show a dynamic Stokes shift which extends to the red beyond the quasistationary, state. Equilibrium is then reached by a slower blue shift on a 10 ps time scale. In methanol, excess vibrational energy as large as similar to 14 000 cm(-1) shows no such effect. Nonpolar solutes exhibit an excess red shift of the emission band in both polar and nonpolar solvents even upon excitation near the vibronic origin. The observed dynamics are discussed in terms of transient heating of the excited chromophore, conformational change, and changes of the molecular cavity size. For solvation studies the optical excitation should be chosen close to the band origin.