Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems - Self Organized Nanostructures of Amphiphilic Block Copolymers - page 198

The properties of the shell [density of polymer units, ionization degree (in weak
polyelectrolyte shells), ratio of free-to-bound (solvating) water molecules, effective
dielectric permittivity, etc.] change in the radial direction from the core-shell inter-
face. In dense parts of the shell, the fluorophore often competes for water molecules
with water-soluble polymer units. The relaxation of the solvate shell is very com-
plicated compared with isotropic systems of small molecules and contains several
contributions that differ significantly in relaxation rates. The complexity (when un-
derstood) offers an opportunity for detailed analysis, but a reliable knowledge of the
behavior obtained by an independent method is indispensable. The time-resolved
Stokes shift provides indirect information both on the solvation (hydration) of water-
soluble polymer chains and on the segmental dynamics, because water molecules
are engaged in the solvation of polymer segments, their motion is slowed down
and coupled with segmental dynamics of polymer chains. If the affinity of the fluo-
rophore to the nanoparticle is not high enough, the fluorescent part can move after
excitation in the radial direction with respect to the nanoparticle and can experience
higher microenvironment polarity during the lifetime of the excited state. This ap-
plies, e.g., to fluorescent surfactants with a short aliphatic tail that are not strongly
anchored to the hydrophobic core. It is thus obvious that it is impossible to offer
a universal scheme for the interpretation of solvent relaxation curves for systems
containing fluorophores bound to nanoparticles. In Sect. 3.3 , we will demonstrate
that a careful analysis of data obtained by a combination of fluorescence SRM mea-
surements with other experimental techniques provides details that are otherwise
inaccessible, and that such an analysis helps to formulate reliable conclusions on
the system behavior. As already mentioned, SRM has been very little used in poly-
mer research so far. The main goal of pertinent parts of the paper is to “advertise”
this technique and show its scientific potential.

2.4 Fluorescence Quenching and Nonradiative Excitation

Energy Transfer

2.4.1 Fluorescence Quenching

In the preceding sections, it was shown that all nonradiative processes that com-
pete with fluorescence shorten the fluorescence lifetime and weaken the emission
intensity. Some of them, such as vibrational relaxation, depend on the fluorophore,
solvent, and temperature. They predetermine the natural fluorescence lifetime,

τ F0 ,
which is defined as the lifetime in the absence of any additional factors that can
specifically quench the fluorescence. The molecules that strongly interact with the
excited fluorophore are therefore called quenchers.
Fluorescence quenching requires a close approach of the quencher to the
fluorophore and hence it can be used for studying various structural problems
and dynamic processes. When both the fluorophore and quencher are dissolved
in a solution, the time-resolved data report on the rate of diffusion. When the