The determination of single donor EET rate constants from the line widths of
low temperature excitation spectra as described above was first accomplished for
the multichromophoric poly(phenylene) dendrimer 31, in which the TDI acceptor is
residing in the dendrimer core and the donors are located at the rim of the structure
[26]. In this study it was shown that frequency-selective high resolution spectroscopy at low temperature allows the elucidation in great detail of complex energy
transfer processes in individual multichromophoric assemblies by controllably
interrogating the donors as well as the acceptors. In particular, the EET time
constants for individual donors within the dendrimer could be determined. In a
room temperature single-molecule study of the same dendrimer it had previously
been shown that the acceptor rise time, which corresponds to an average over the
energy transfer times of the four donors, could be accessed by time-resolved
fluorescence spectroscopy [27]. A similar type of time-resolved experiment has
been performed for dyad 2. In particular, the EET time constants were accessed by
analyzing differences in the fluorescence rise/decay time profiles of the acceptor,
recorded successively for a dyad prior to and after bleaching of the donor [8]. These
investigations had already indicated that the distribution of EET times obtained for
single dyads cannot be reproduced by a purely Fo ¨rster-type description of the EET
process.
The study of 1 allowed addressing coupling mechanisms and more principal
photophysical aspects of the EET process. Metivier et al. have determined the
distribution of EET time constants for single molecules of 1 embedded in PMMA
[2]. The EET times were in the range of several picoseconds, which translated into
an EET efficiency of almost 100%. Accordingly, in emission spectra of 1 only
acceptor emission was observed. These results were similar to those obtained for
the dendrimer 31, which actually was not too surprising because the chromophores
and mutual distances were comparable in both systems. Again, a large discrepancy
was found between the experimentally determined distribution of EET rates of
1 and a distribution that was calculated within the Fo ¨rster model (see Fig. 21a).
The details for the calculation can be found in the literature [2, 9]. Considering the
average values obtained from the experimental (hk
exp
EET i ¼ 3.2 Â 10
11 s
À 1 ) and the
calculated distributions (hk
Förster
EET i ¼ 3.9 Â 10
10 s
À 1 ), it is seen that the discrepancy
amounts to a factor of ~8. Figure 21a also displays the expected distribution of EET
times due to the conformational flexibility of 1 (see Sect. 3.1). This distribution is
appreciably smaller than the distribution of EET times caused by the variations in
spectral overlaps, which originates in the inhomogeneous broadening of the optical
transitions.
Besides the obvious discrepancy between the measured distributions of EET
rates and those calculated according to Fo ¨rster theory (illustrated in Fig. 21a), the
validity of the Fo ¨rster model was further checked by plotting the experimentally
determined EET rates k
exp
EET against the spectral overlaps of individual molecules
of 1, which according to the standard Fo ¨rster expression of the EET rate should
yield a linear dependence (see Eq. (1), [12]). The dashed line in Fig. 21b represents
a linear fit to the data. Although the slope of this line showed good agreement with
Optical Properties of Assemblies of Molecules and Nanoparticles
99
low temperature excitation spectra as described above was first accomplished for
the multichromophoric poly(phenylene) dendrimer 31, in which the TDI acceptor is
residing in the dendrimer core and the donors are located at the rim of the structure
[26]. In this study it was shown that frequency-selective high resolution spectroscopy at low temperature allows the elucidation in great detail of complex energy
transfer processes in individual multichromophoric assemblies by controllably
interrogating the donors as well as the acceptors. In particular, the EET time
constants for individual donors within the dendrimer could be determined. In a
room temperature single-molecule study of the same dendrimer it had previously
been shown that the acceptor rise time, which corresponds to an average over the
energy transfer times of the four donors, could be accessed by time-resolved
fluorescence spectroscopy [27]. A similar type of time-resolved experiment has
been performed for dyad 2. In particular, the EET time constants were accessed by
analyzing differences in the fluorescence rise/decay time profiles of the acceptor,
recorded successively for a dyad prior to and after bleaching of the donor [8]. These
investigations had already indicated that the distribution of EET times obtained for
single dyads cannot be reproduced by a purely Fo ¨rster-type description of the EET
process.
The study of 1 allowed addressing coupling mechanisms and more principal
photophysical aspects of the EET process. Metivier et al. have determined the
distribution of EET time constants for single molecules of 1 embedded in PMMA
[2]. The EET times were in the range of several picoseconds, which translated into
an EET efficiency of almost 100%. Accordingly, in emission spectra of 1 only
acceptor emission was observed. These results were similar to those obtained for
the dendrimer 31, which actually was not too surprising because the chromophores
and mutual distances were comparable in both systems. Again, a large discrepancy
was found between the experimentally determined distribution of EET rates of
1 and a distribution that was calculated within the Fo ¨rster model (see Fig. 21a).
The details for the calculation can be found in the literature [2, 9]. Considering the
average values obtained from the experimental (hk
exp
EET i ¼ 3.2 Â 10
11 s
À 1 ) and the
calculated distributions (hk
Förster
EET i ¼ 3.9 Â 10
10 s
À 1 ), it is seen that the discrepancy
amounts to a factor of ~8. Figure 21a also displays the expected distribution of EET
times due to the conformational flexibility of 1 (see Sect. 3.1). This distribution is
appreciably smaller than the distribution of EET times caused by the variations in
spectral overlaps, which originates in the inhomogeneous broadening of the optical
transitions.
Besides the obvious discrepancy between the measured distributions of EET
rates and those calculated according to Fo ¨rster theory (illustrated in Fig. 21a), the
validity of the Fo ¨rster model was further checked by plotting the experimentally
determined EET rates k
exp
EET against the spectral overlaps of individual molecules
of 1, which according to the standard Fo ¨rster expression of the EET rate should
yield a linear dependence (see Eq. (1), [12]). The dashed line in Fig. 21b represents
a linear fit to the data. Although the slope of this line showed good agreement with
Optical Properties of Assemblies of Molecules and Nanoparticles
99