Chemistry Reference
In-Depth Information
dendrimer was obtained for multichromophores based on this design concept.
Figure 12.17b shows the absorption and emission spectra of compound
. Absorp-
tion features around 500 and 675 nm can be attributed to the PMI and TDI parts,
respectively. When exciting the PMI part at 500 nm, the emission is almost uniquely
coming from the TDI, indicating a very efficient energy transfer efficiency (see
Figure 12.17b). At the single-molecule level, the use of APDs with their limited time
resolution makes the detection of fast kinetic components of directional FRET from
the PMI to the TDI, as suggested by the ensemble solution time-resolved data,
impossible [129]. However, due to the excellent photostability and brightness of
compound
12
23 ps response time) could be used,
which is more suited to detect a fast rise term if one uses appropriate reconvolution
procedures [91]. For each single molecule, all the detected photons from the acceptor
were used to build a decay histogram that was fitted with a biexponential model
function with positively and negatively contributing time constants by MLE fitting.
The residual graphs in Figure 12.18a show that a monoexponential fit does not fit
the datawell, whereas a biexponential fit with a negative pre-exponential factor for the
short time constant gives a good result. In all cases, the ratio of the pre-exponential
factors accounting for the contributions of the recovered time constants was around
12
, a multichannel plate detector (
12
is built up only from FRET from the PMI chromophores. A typical analyzed single-
molecule decay from an individual molecule of compound
1.0
0.2. This demonstrates that the excited state of the TDI part in compound
is given in
Figure 12.18a. The histogram of the rise components detected from 100 different
single molecules of compound
12
shows a broad distribution with a mean value of
48 ps (Figure 12.18b). This finding points to a broad range of FRET time constant
values at the single-molecule level that implies that there is a range of
interchromophoric distances and orientations between the donor and acceptor
chromophores in the dendrimers when immobilized in Zeonex [91].
12
FIGURE 12.18
(a) Top panel shows fluorescence decay histogram recorded with an
MCP-PMT detector of the acceptor emission from a single molecule of compound
12
in
Zeonex upon selective 488 nm excitation of the donors. The instrumental response function of
the system and the biexponential fit curve are also shown. The middle and bottom panels show
residual graphs for mono- (middle panel) and biexponential (bottom panel) fits. (b) Histogram
of the rise times of 100 single molecules of compound
12
. Copyright from Ref. [91].
