Biomedical Engineering Reference
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N
1
N
NH
N
1,0
N
N
HN
2
0,8
F
F
NH N
N
N
F
FF
3
N
0,6
N
1
2
3
F
F
F
F
0,4
F
NH N
N
N
0,2
HN
0,0
0123456789 0
Molar Ratio x = C Porph /C QD
Fig. 4.22
Relative PL intensity changes (quenching) I ( x )/ I (0) as function of the molar ratio
10 7 M) and various
porphyrin molecules. 1 :( m Pyr) 2 (Ph) 2 -H 2 P; 2 :( p Pyr) 2 (Ph) 2 -H 2 P; 3 :( m Pyr) 2 (5FPh) 2 -H 2 Pin
toluene at 295 K (
x
=
[ C Porph ]/[ C QD ] for CdSe/ZnS QD ( d CdSe =
2.5 nm, n ZnS
=
2, C QD
=
5
×
λ
=
465 nm,
λ
=
522 nm). Structures of porphyrins 1, 3, and 3 are also
exc
reg
presented on the right
QD clearly resembles the tunneling of an electron (through the ZnS barrier) to the
outer interface of the QD. Such tunneling is followed by the (self-)localization of the
electron-hole pair. This corresponds to the creation or modification of trap states in
the semiconductor band gap. Such traps might be subject to non-radiative channels,
e.g. via enhanced electron-phonon coupling. Thus, changes in local charge densities
result in changes in the quantum efficiency of the QD which have to be related
to PL lifetime changes and modification of blinking in case of single QDs [ 124 ,
131 , 151 ]. These results reveal also that single functionalized porphyrin molecules
can be considered as a probe for the complex interface physics and dynamics of
colloidal semiconductor quantum dots [ 63 , 64 , 74 , 75 , 94 , 150 ]. It is tempting to
conclude that the quenching is merely due to the influence of the pyridyl anchor
since the electronic properties of the porphyrins are obviously negligible [ 127 ](see
Fig. 4.22 ). Additionally, the PL quenching caused by PDI molecules is similar to
the one observed for porphyrins as long as assemblies are formed in toluene solvent.
However, obviously the anchor is not the main source for quenching, since titration
by orders of magnitude larger amounts of pyridine as compared to porhyrins does
not result in noticeable quenching as can be seen from Fig. 4.16 .
Concluding, we suggest that the non-FRET quenching mechanism is related
to the extension of the electronic wave function of the electron hole pair to the
outer interface of the (capped) QD influencing the formation of near band edge
states with new non-radiative features. However, though dye molecules heavily
induce PL quenching, this quenching is in many cases—as reported here—only to a
minor extent related to photoinduced charge and/or energy transfer. Nevertheless,
the identification of FRET is at least a proof that nanoassemblies are formed
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