Femtosecond excitation energy transport in triarylamine dendrimers

Article abstract of DOI:10.1021/ja025505z

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The

Full text of DOI:10.1021/ja025505z

Published on Web 05/16/2002
Femtosecond Excitation Energy Transport in Triarylamine Dendrimers
Mahinda I. Ranasinghe,† Oleg P. Varnavski,† Jan Pawlas,‡ Sheila I. Hauck,‡ Janis Louie,‡
John F. Hartwig,*,‡ and Theodore Goodson, III*,†
Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202, and Department of Chemistry,
Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107
Received January 2, 2002
Scheme 1. Synthesis of Triarylamine Dendrimer
An understanding of energy transfer in organic conjugated
dendrimers provides a conceptual basis for creating artificial light-
harvesting and -emitting nanostructured materials.1-3 The electronic
and structural properties of the branching point within the dendrimer
can influence this energy migration from branch to branch. At the
limit of weak electronic interactions, energy transfer from one
branch to another is incoherent, and a “hopping” mechanism
operates. In this case a Forster model applies. At the other extreme,
the junction between branches provides strong electronic interac-
tions, and the energy transfer is coherent.4,8c The presence of weak
or strong interactions in a system depends on the coupling parameter
(interaction strength) and the dephasing. In many cases, the junction
between branches disrupts conjugation, and the energy transport
in these dendrimers can be described by following an approach
that uses an exciton density matrix.5 Experimentally, the degree of
coherence in the energy-transfer process within branched molecular
systems containing multiple chromophores may be evaluated using
time-resolved fluorescence anisotropy.4b,6,7 The fluorescence de-
polarization measurement is very useful to investigate intramolecular
excitation transfer because the energy transfer is accompanied by
the reorientation of the transition dipole, resulting in depolarization
of the emission.
Triarylamine dendrimers containing p-phenylene diamine groups
comprise a unique family of hyperbranched materials that are highly
electron-rich and that can produce highly delocalized radical
cations.8 In contrast to unsaturated hydrocarbon dendrimers, the
amine materials contain exclusively p-phenylene units, and these
units are linked by sp2-hybridized nitrogens. The advent of
palladium-catalyzed aromatic C-N bond-formation allowed con-
struction of modestly sized materials of this topology.8 Here, we
use improved catalysts to prepare p-phenylene triarylamine den-
drimers of increased size. These materials, in combination with
small analogues, provide a platform for understanding energy
migration within a system with a strong interchromophore interac-
tion.
catalyst loadings and molecular weight of the ligand allowed for
simpler purification of the materials.6
The overall strategy for preparation of second- and third-
generation dendrimers is shown in Scheme 1. Reaction of ditolyl-
amine with diphenylmethyl-protected 4,4′-dichlorodiphenylamine
1 with a 1:1 combination of P(t-Bu)3 and Pd(dba)2 as catalyst
formed the double addition product 2 in 82% yield on a multigram
scale after a standard workup and precipitation of product.
Hydrogenolysis in cyclohexene solvent as the hydrogen source using
10% palladium on carbon as catalyst generated the free amine 3 in
24 h. This free amine was previously used to generate the second-
generation dendrimer TAA-G2 by reaction with 0.3 equiv of tris-
(4-bromophenyl)amine. In the presence of the recently developed
catalyst, the free amine reacted with diphenylmethyl-protected
dichlorodiphenylamine at room temperature to form after crystal-
lization the precursor to the third-generation dendrimer. Deprotec-
tion again gave the free amine smoothly, and crystallization gave
pure material in 73% yield. This free amine 6 was coupled with
tribromotriphenylamine to form the three-fold symmetric TAA-
G3 at room temperature in 89% isolated yield without difficulty
from steric crowding.
Two improvements in synthetic methods allowed for the prepara-
tion of substantial quantities of a third-generation triarylamine
dendrimer in analytically pure form. First, we employed diphenyl-
methyl protective groups on the amines to assist in deprotective
hydrogenolysis of the larger structures. Benzyl and even p-
methoxybenzyl protective groups underwent slow deprotection by
hydrogenolysis or acidic conditions. Second, highly active catalysts
for formation of both di- and triarylamines that are based on a 1:1
ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N
bond-formation and decreased reaction times.9 Moreover, the low
All materials showed the appropriate MALDI mass spectra for
the molecular ion and microanalytical data. With the exception of
the dendrimer TAA-G3 all materials showed 13C NMR signals that
1
were resolved. H and 13C{1H} NMR spectra of dendrimer TAA-
G3 showed overlapping resonances for the different layers of
aromatic units, but clean mass spectral data and a narrow peak in
the GPC trace corresponding to a PDI (Mw/Mn) of 1.02 were
obtained. Figure 1 shows the structures and absorption spectra of
the triphenylamine and three triarylamine dendrimers. Two absorp-
tion maxima are clearly seen for the dendrimers, one at 320 nm
and another at about 375 nm.
* Corresponding authors. E-mail: tgoodson@chem.wayne.edu, john.hartwig@
yale.edu.
† Wayne State University.
‡ Yale University.
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10.1021/ja025505z CCC: $22.00 © 2002 American Chemical Society

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
available free of charge via the Internet at http://pubs.acs.org.
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