Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems - Self Organized Nanostructures of Amphiphilic Block Copolymers - page 208

Fig. 6 Typical dependence
of ( a ) the fluorescence
lifetimes

2 ( 1 fast
process, 2 slow process) and
( b ) relative amplitude of the
fast process A 1 of the
dansyl-labeled PMA on pH

τ 1 and τ

F , F ,

and fluorescence lifetime do. It is also a good probe for anisotropy measurements.
The probes covalently attached to stretched parts of the chain (Fig. 5 ) are exposed to
polar water molecules and can rotate fast, but those entrapped in nonpolar collapsed
domains are considerably immobilized. It is evident that both the fluorescence and
anisotropy decays monitor changes in chain conformation.
The most important results can be summarized as follows: The time-resolved
fluorescence intensity decays measured in solutions of several PMA samples in a
wide range of pH and ionic strength were always double-exponential. Both the short
(units of nanosecond) and the long (tens of nanoseconds) fluorescence lifetimes
increase with decreasing pH. Typical data are shown in Fig. 6 a . The

τ 1 and τ 2

dependences vs. pH (curves 1 and 2, respectively) exhibit a pronounced sigmoid
shape, with the inflection point close to pH 5.5. The relative pre-exponential factor
of the short-living fluorescence component, A 1 , increases with increasing pH. The
A 1 dependence on pH is depicted in Fig. 6 b .
The obtained results can be explained as follows. In the pH region where the
necklace of pearls structure exists, a fraction of probes are entrapped in compact
globules and experience the nonpolar medium. The other probes attached to the
stretched parts of the chains are exposed to polar water molecules. The probes
are thus effectively distributed in two different microenvironments. The short life-
time corresponds to the water-exposed and the long one to the globule-embedded
probes. The pre-exponential factors, when corrected by corresponding quantum
yields, give the number fractions of both types of probes in the macroscopic ensem-
ble. In the first approximation, the factors reflect the average numbers of monomer
units in stretched and collapsed parts of the chain.
As the probe is more hydrophobic than the PMA monomer unit, we expect it
induces a preferential collapse of the PMA chain in its vicinity. Nevertheless, the

F , F ,