Chemistry Reference
In-Depth Information
The close proximity of the PMI to the TDI chromophores as well as the overlap
between the emission and absorption spectra facilitates energy transfer from the PMI
to the TDI chromophores as was demonstrated by the steady state and single-molecule
data. In fact, the efficiency of this process is such that only TDI emission can be
observed in ensemble-level experiments, but no donor PMI emission, irrespective of
the excitation wavelength [130]. However, at high excitation intensities simultaneous
donor and acceptor emission can sometimes be observed [91,123]. An explanation of
this simultaneous emission could be exciton blockade. Exciton blockade occurs if two
donor chromophores are excited simultaneously then only one of these can transfer its
energy to the central TDI chromophore. The second excited PMI chromophore,
however, cannot transfer its energy to the excited TDI, and emits normally. The net
result is that both the donor and acceptor emit. This explanation however did not fully
explain the observed data at the single-molecule level [131]. It was found that
simultaneous donor and acceptor emission was more likely to occur after some of the
donor molecules in compound
had photobleached. Finally, it was also observed
that the donor and acceptor emission were uncorrelated, that is, the emission of a
donor or acceptor photon occurs in a stochastic fashion, with no apparent relation
between them. These findings showed that exciton blockade by itself was not a
satisfactory explanation for the simultaneous donor-acceptor emission. To fully
explain these observations, defocused wide-field imaging was performed, exciting
only the PMI chromophores, but separating the emission of the PMI and TDI
chromophores over two identical CCD cameras [124,131]. The defocused imaging
displayed that three different types of emission could be recovered: only acceptor,
only donor (after photobleaching of the acceptor), and simultaneous donor and
acceptor emission. After the acceptor had bleached, the donor emission would usually
be present as a single emission pattern only, indicating emission from a single donor
chromophore. However, there would be a progressive change to other emission
patterns until the last of the donor chromophores bleached. This can be explained by
the fact that some chromophores will have a lower energy than the others, and will act
as a preferential energy acceptor (see above). After the photobleaching of this lowest
energy chromophore another will take its place, until only a single donor chromo-
phore remains. Figure 12.19 shows images from two examples of single molecules of
compound
12
showing both emission from the PMI and TDI part, where the left
column shows the acceptor emission, and the right column shows the donor emission.
Details from these frames, showing emission patterns corresponding to simultaneous
donor and acceptor emission from the same molecule are shown in Figure 12.19b,
along with their fitted orientational patterns. In all the 15 measured molecules of
compound
12
that displayed this dual emissive behavior, the orientation of the donor
chromophore was perpendicular to that of the acceptor chromophore, which would
seriously hinder energy transfer between them.
Based on the confocal [91,131] and defocused imaging [124,131] the following
picture can be constructed: in intact dendrimers (no photobleached chromophores),
energy hopping among the donors ensures that the excitations end up at the acceptor,
even if unfavorably oriented donors are present in the dendritic molecule. Multiple
excitations within the dendrimer are quenched by very efficient singlet PMI-singlet
12
