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a reduced conformational range of bay group orientation. Nevertheless as is evident
from Fig. 4.14 fluorescence spectra of individual PP molecules show that spectral
fluctuations still occur.
Notably, single molecule data provide more specific information on the structure
of the QD-PP nanoassemblies. According to Fig. 4.14 the spectral distribution of PP
fluorescence is blue shifted upon nanoassemblies formation. Such a shift indicates
that those conformations of PP are favored, for which the bay groups are most
extended as this situation is related to short fluorescence wavelengths [ 94 , 130 ,
134 ]. This corresponds to a situation where PP bay groups are as much within
the molecular plane as possible. Such a situation is reasonable since PP has to be
intercalated into the ligand shell which will be more easily accomplished for a nearly
flat molecule, since less ligands have to be excluded from the QD surface. Thus,
the distribution is shifted from a “free” PP molecule on SiO 2 surface to a “matrix-
isolated” type such as in a PMMA film (see Fig. 4.14 c). It is known also [ 94 ]that
spectral fluctuations are enhanced for assembled PP as compared to PP in a PMMA
film. From this it follows that the ligand shell imposes more flexibility than in a
PMMA film allowing for remaining conformational changes of PP molecules. An
alternative explanation is that the curved QD surface allows for more conformational
flexibility than on an SiO 2 substrate. The high flexibility of the PP phenoxy side
groups on the QD surface is only enabled in absence of steric hindrance. Therefore
(1) a nearly perpendicular geometry has to be established and (2) the ligand density
close to PP has to be low. In conclusion, the identification of PP conformations (1)
has been used to confirm the surface geometry already suggested by FRET results
and (2) serves as probe for the ligand density and dynamics on single quantum
dots.
4.3
Exciton Relaxation Pathways and QD Photoluminescence
Quenching in “QD-Dye” Nanoassemblies
The results presented in Sect. 4.2 clearly show the formation of “QD-Dye”
nanoassemblies accompanied by the PL quenching of the QD counterpart, both
from bulk experiments and single nanoassemblies detection. As it was mentioned
in the introduction, the QD PL quenching being observed in numerous “QD-
Dye” nanoassemblies needs a thorough analysis in order to evaluate the increased
non-radiative relaxation channels in the excited states of QD with attached dye
molecules. Such an approach has been carried out recently by us for “QD-
porphyrin” [ 62 - 65 , 114 , 123 ] and “QD-perylene diimide” nanoassemblies [ 64 , 65 ,
74 , 94 ] and we will summarize only the basic findings.
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