Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems - Self Organized Nanostructures of Amphiphilic Block Copolymers - page 196

Fig. 4 Solvent relaxation: energies of the electronic states of a solvated fluorophore are depicted
by bold lines , vibrational states by thin lines ; the long arrows and wavy lines show individual
processes; the changes of the dipole moment and geometry of the fluorophore upon excitation and
emission are depicted by different orientations of the short arrow and the ellipsoidal prolongation.
The changes in the arrangement and orientation of polar solvent molecules are also indicated
that occurs independently of the solvent polarity, the solvent relaxation proceeds
only in polar solvents. Its extent is given by the strength of the dipole-dipole inter-
action between the fluorophore and the molecules in the solvate shell (local polarity
of the nanoenvironment), while its rate reflects the mobility of the solvate shell (local
viscosity of the nanoenvironment). In non-viscous solvents, the solvent relaxation
can be quite fast and may partially overlap with the vibrational relaxation.
The next process is the emission of a photon. As already mentioned, the emission
of photons in a macroscopic system of fluorophores proceeds on the nanosecond
timescale. This means that most photons are usually emitted and detected from
excited states with fully relaxed solvate shell. Because the excited state population
decays exponentially, fast measurements enable detection of “hot” photons from
non-relaxed states at early times. When discussing the relaxation at the level of a
single molecule, we have to consider different timescales. An isolated single emis-
sion event is as fast as the absorption, and the electronic transition takes less than
10 − 15 s. It is evident that the fluorophore does not reach the relaxed ground state
immediately after the emission of a photon. What follows, is a cascade of processes
that resemble the mirror image of the above-described relaxations. Firstly, the vi-
brational relaxation occurs and, finally, the solvent equilibrates corresponding to