Excess Dynamic Stokes Shift of Molecular Probes in
Solution
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Abstract
The
solvation dynamics of molecular probes is studied by broad-band
fluorescence upconversion. The time-dependent position of the S<sub>1</sub> → S<sub>0</sub> emission band or of a vibronic line
shape is measured with ∼80 fs, 10 cm<sup>–1</sup> resolution.
Polar solutes in acetonitrile and acetone, when excited into S<sub>1</sub> 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<sup>–1</sup> 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