Chemistry Reference
In-Depth Information
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
400
500
600
λ
(nm)
700
800
FIGURE 12.21
Absorption (black line) and fluorescence (line
þ
symbol) spectra of com-
pound
in toluene. Fluorescence spectra were recorded upon excitation at 360 nm (blue),
490 nm (green), or 640 nm (red). Copyright fromRef. [123]. (See the color version of this figure
in Color Plates section.)
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chromophore as a core, 4 PMI chromophores in the scaffold and 8 NMI chromophores
at the periphery. This nanosized-emitter reveals a high degree of complexity since all
spatial locations, distances between the chromophores and their absorption and
emission spectra were adjusted carefully [132]. In this way, an efficient dendritic
triad was obtained absorbing light over the whole visible spectrum and showing
stepwise energy transfer over long distances from the periphery via the scaffold
toward the center of the dendrimer. The absorption spectrum and emission spectra
exciting in the NMI (360 nm), PMI (490 nm) and TDI (640 nm) region are given in
Figure 12.21. The low amount of fluorescence at 450 nm (NMI) and 550 nm (PMI)
indicate that very efficient energy transfer to the TDI is present.
According to the ensemble FRET data [123], highly efficient FRET is expected to
occur when probing single molecules of compound
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with 490 nm laser light. When
probing 100 molecules of compound
, 83% shows exclusive emission in the TDI
channel, before complete photobleaching occurs (see Figure 12.22a, upper and lower
panels). For such molecules, the fractional intensity
F
(TDI channel divided by the
sum of the TDI and PMI channel) shows a constant value around unity
(Figure 12.22a), therefore proving that FRET occurs efficiently from the PMI units
to the TDI core. Following the trajectory of the fluorescence decay time and of the
fluorescence spectrum of these molecules, both parameters stay constant during the
measurement, suggesting emission is related to a single excited species, here the TDI
chromophore [91,130]. The fluorescence intensity in the TDI channel decreases in
a stepwise fashion because of successive photobleaching of the PMI donors
(Figure 12.22a, upper panel) [91,130] Fluorescence spectra detected from such
molecules peak around 680 nm (Figure 12.22b) and resemble, both in shape and in
peak position, the ensemble solution spectrumof the bare TDI chromophore [91,129].
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