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 S1 → S0 emission band or of a vibronic line shape is measured with ∼80 fs, 10 cm–1 resolution. Polar solutes in acetonitrile and acetone, when excited into S1 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 ∼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.