Chemistry Reference
In-Depth Information
200
(a)
150
100
50
1.0
(b)
0.5
0.0
-0.5
-1.0
0
50
100
Time (s)
150
200
100
(c)
80
60
40
20
0
-0.8
-0.4
0.0
0.4
0.8
p
-value
FIGURE 12.12
(a) Example of a fluorescence intensity trajectory of a single molecule of
compound
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where the parallel and perpendicular component were detected separately. (b)
Polarization value of the trace in panel a. (c) Histogram of the four
p
-value levels of the trace in
panel b. Copyright from Ref. [51]. (See the color version of this figure in Color Plates section.)
polarization trajectory and corresponding histogram (Figure 12.12c) displaying four
distinct
p
-values of this spatially well-defined system indicate that emission occurs
always from the chromophore that is lowest in energy at each point in time [80,95].
This energetically lowest PMI chromophore acts as a fluorescent trap while the other
chromophores are communicating via energy hopping to it, leading to discrete
p
-
values in the polarization trajectory. Based on the overlap between the ensemble
absorption and emission spectra (see Figure 12.11b), a F
orster radius of 4.78 nm can
be calculated [51,84,87]. The interchromophoric distances in compound
€
are well
within this radius, indicating that indeed energy hopping is very efficient. An
alternative way of demonstrating energy hopping can be done by rotating the
excitation polarization during the experiment (at a rate of a few Hz) and looking
whether the parallel and perpendicular components of the emission are in phase or
not [84,85,89]. If they are not in phase that means the chromophores emit indepen-
dently and there is no hopping to a specific emissive trap. If they are in phase and the
modulation of the parallel and perpendicular component of the emission does not go
down to zero, this indicates that there are multiple absorbers and one chromophore
acts as an emissive trap. If the modulation goes down to zero, this indicates that there
are either multiple parallel oriented chromophores that absorb and a single fluorescent
trap or that there is only one chromophore present. For example, when performing this
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