C O M M U N I C A T I O N S
value within ∼100 fs. The value of the residual anisotropy decreases
with an increase in generation number. The systematic decrease of
the residual value can be associated with the increase of the number
of accessible dipole orientations in a nonplanar geometry.11 This
fast decay of anisotropy and the systematic decrease in the residual
value of anisotropy indicates a fast energy delocalization via a
coherent excitonic mechanism between dipoles that are oriented in
different directions.
The fast energy transfer may be considered in terms of the
formation of coherent domains (intramolecular chromophore clus-
tering).6 Equilibration within a cluster leads to an initial fast decay
in the value of the anisotropy, while the remainder of the
depolarization decay is due to incoherent intercluster energy
transfer.6
An important feature of this chromophore clustering is that
anisotropy dynamics occurs on two time scales, with the slower
decay component reflecting incoherent hopping. The anisotropy
decay for all three generations of the dendrimer showed no evidence
of such a second slow component in the time range g150 fs. Thus,
the presence of the fast component of the anisotropy decay for each
of the dendrimers could be explained by the coherent domains
extending over large portions of the each dendrimer, including the
largest TAA-G3. The most striking features of these results are
the decrease in the residual anisotropy and the persistence of a fast
component with increasing generation. These features may be a
consequence of strong interactions over relatively large molecular
distances in the branched system.
Figure 1. Absorption spectra of the TAA molecule and the three
triarylamine dendrimer systems in THF. The peak at 320 nm (which was
consistent for all dendrimer systems) was normalized to unity to illustrate
the red-shift and increase in amplitude of the longer-wavelength peak.
In summary, we provide the first systematic investigation of
energy migration due to a coherent excitonic mechanism in a family
of homogeneous dendrimers. The fast anisotropy decay to smaller
residual value with the increase in generation is an indication of
the exciton delocalization in multichromophore-branched systems.
Figure 2. The fluorescence anisotropy decay for the three dendrimer
systems in THF. The excitation wavelength is 390 nm, and emission
wavelength is 480 nm. The scale extends to 1.5 ps so that the ultrafast
decay can be well illustrated. The residual anisotropies for TAA-G1, TAA-
G2, and TAA-G3 are 0.22,0.105, and 0.045, respectively.
The absorption spectra of the dendrimers are shifted to the red
with respect to the absorption line of the triarylamine (TAA) NPh3,
which is the basic building block of the dendrimers. Comparison
of the spectra of the three dendrimers also shows that the longer-
wavelength component increases in intensity and wavelength
maximum as the generation number increases (Figure 1). Such a
trend was not observed for analogous spectra of dendrimers with
weak intersegment interactions.
Acknowledgment. T.G. acknowledges the NSF (DMR-0088044)
and AFOSR for support, and J.F.H. thanks the DOE for support.
Supporting Information Available: Synthetic procedures, NMR
data and steady-state fluorescence spectra (PDF). This material is
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