Controlling Solubility and Modulating Peripheral Function in Dendrimer Encapsulated Dyes

Article abstract of DOI:10.1021/ja037133f

The synthesis of large dendrons and dendrimers with site-isolated dyes at their core has been explored. The dyes selected for this work were coumarin 343 and pentathiophene, as energy transfer processes prevail when the two dyes are intimately mixed but each should behave independently of the other if site-isolation is achieved. Because the two dyes have very different functional characteristics, a protocol involving orthogonal protecting groups and allowing the use of a single family of electroactive dendrons for their encapsulation had to be developed. The synthetic protocol must balance the need to incorporate electroactive groups at the periphery of the dendrons with the requirement for high solubility and a size sufficient to fully encapsulate the central dye. Because of their poor solubility and tendency to crystallize, dendrons with uniform triarylamine substitution proved unsatisfactory leading to the development of new unsymmetrical dendrons with alternating branched alkyl groups and triarylamine moieties at their periphery. These dendrons, which show excellent solubility and no tendency to crystallize, were assembled into large dendrimers using a modular protocol with the light emitting dye at their core. It is expected that the large size of the dendritic shell will provide effective site-isolation for the encapsulated central dyes enabling them to exhibit their intrinsic emission properties with minimal energy transfer between neighboring core fluorophores when processed in bulk thin films.

Full text of DOI:10.1021/ja037133f

Published on Web 10/07/2003
Controlling Solubility and Modulating Peripheral Function in
Dendrimer Encapsulated Dyes
Paul Furuta and Jean M. J. Fre´chet*
Contribution from the Department of Chemistry, UniVersity of California,
Berkeley, California 94720-1460 and Material Sciences DiVision,
Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received July 8, 2003; E-mail: frechet@cchem.berkeley.edu
Abstract: The synthesis of large dendrons and dendrimers with site-isolated dyes at their core has been
explored. The dyes selected for this work were coumarin 343 and pentathiophene, as energy transfer
processes prevail when the two dyes are intimately mixed but each should behave independently of the
other if site-isolation is achieved. Because the two dyes have very different functional characteristics, a
protocol involving orthogonal protecting groups and allowing the use of a single family of electroactive
dendrons for their encapsulation had to be developed. The synthetic protocol must balance the need to
incorporate electroactive groups at the periphery of the dendrons with the requirement for high solubility
and a size sufficient to fully encapsulate the central dye. Because of their poor solubility and tendency to
crystallize, dendrons with uniform triarylamine substitution proved unsatisfactory leading to the development
of new unsymmetrical dendrons with alternating branched alkyl groups and triarylamine moieties at their
periphery. These dendrons, which show excellent solubility and no tendency to crystallize, were assembled
into large dendrimers using a modular protocol with the light emitting dye at their core. It is expected that
the large size of the dendritic shell will provide effective site-isolation for the encapsulated central dyes
enabling them to exhibit their intrinsic emission properties with minimal energy transfer between neighboring
core fluorophores when processed in bulk thin films.
Introduction
An attractive target in dendrimer chemistry is one that makes
use of the size and isolated internal environment of dendrimers
The concept of site isolation is widespread in nature with
systems in which proteins or other naturally occurring macro-
molecular assemblies encapsulate an active center to endow it
with unique properties that would not prevail in the absence of
encapsulation. Dendrimers can be thought of as ideal synthetic
structures that can be prepared in an attempt to mimic nature’s
encapsulation of functional molecules such as hemes in cyto-
chromes or hemoglobin. The structures of dendrimers1 can be
tailored to spatially arrange different functionalities at the
periphery, while placing a single functionality at the core, or
focal point. Both divergent2 and convergent3 synthetic strategies
have been used successfully to create macromolecules possess-
ing “isolated” microenvironments within the dendrimer.4 Their
flexible synthetic methodology allows for the design of well-
defined structures, where both the number of peripheral groups
as well as their distance from the core can be precisely
controlled.
to make catalysts with enhanced reactivity.5 Examples include
the manganese porphyrin dendrimer of Bhyrappa et al.,5a,b in
which epoxidation of olefins is more selective to less hindered
subtrates,5c and the amphiphilic dendrimer of Piotti et al.6 The
latter behaves as a reverse unimolecular micelle where the polar
interior acts as a transition state catalyst for E1 elimination
reactions, while also performing as a thermodynamic pump,
bringing in the polar reactants and expelling the less polar
products.
Besides providing reactive environments, dendrimers have
also been used to enhance the photophysical behavior of
chromophores they encapsulate.7 Kawa and Fre´chet have shown
that self-assembled luminescent dendrimers holding a light
emitting rare earth cation at their core8 were free of the self-
quenching behavior characteristic of the unshielded cations and
had a luminescence efficiency that increased with the size of
the dendrons surrounding the cation. Such site-isolation is an
important feature for application in optical amplifiers. In earlier
work,9 the concept of dendritic antennae was demonstrated with
systems in which an energy transfer (ET) interaction or similar
(1) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and
Dendrons: Concepts, Syntheses, Applications; Wiley-VCH: Weinheim,
New York, 2000. (b) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and
Other Dendritic Polymers; Wiley: Chichester, New York, 2001.
(2) (a) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, C.; Martin, S.;
Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (b) Newkome, G.
R.; Yao, Z.;. Baker, G. R.;. Gupta, V. K. J. Org. Chem. 1985, 50, 2003.
(3) (a) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638. (b)
Hawker, C. J.; Fre´chet, J. M. J. Chem Commun. 1990, 1010.
(4) (a) Hecht, S. and Fre´chet, J. M. J. Angew. Chem. Int. Ed. 2001, 40, 74. (b)
Gorman, C. B.; Smith, J. C. Acc. Chem. Res. 2001, 34, 60.
(5) (a) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem.
Soc. 1996, 118, 5708. (b) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick,
K. S. J. Mol. Catal. A 1996, 113, 109. (c) Bhyrappa, P.; Vaijayanthimala,
G.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708. (d) Uyemura, M.;
Aida, T. J. Am. Chem. Soc. 2002, 124, 11 392
(6) Piotti, M. E.; Rivera, F.; Bond, R.; Hawker, C. J.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 1999, 121, 9471.
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J. AM. CHEM. SOC. 2003, 125, 13173-13181
13173

A R T I C L E S
Furuta and Fre´chet
In a preliminary report,11 we have demonstrated that some
degree of encapsulation of light emitting dyes within dendritic
shells might enable two dyes to coexist as a mixture while still
preserving some of their ability to emit individually as a result
of partial site-isolation. Following early work by Devadoss et
al.,9c with a single color emitter that was subject to self-
quenching caused by solid-state aggregation, Freeman et al.11
successfully utilized triarylamine (TAA) HT-labeled poly(benzyl
ether) dendrimers possessing Coumarin 343 (C343) and pen-
tathiophene (T5) core to give blue and green OLEDs, respec-
tively. Initial attempts to achieve synergistic emission from the
two types of dendrimers by blending them in a single layer were
only slightly successful due to detrimental energy transfer from
the C343 laser dye to the smaller band-gap T5. Due to the large
Fo¨rster radius of these dyes, energy transfer was an unfortuitous
consequence of the small size and flexibility of the surrounding
dendrons that were attached at a single point of the lumophores.
Attempts to synthesize larger, more highly shielding dendrimers
were unsuccessful due to the poor solubility and increased
crystallinity of the triarylamine dendrons beyond the second
generation. In general, such crystallinity is also highly detri-
mental to device performance due to the inhomogeneity of the
resulting solid films.
We now describe the use of a new synthetic protocol and
the design of novel building blocks that enable the preparation
of dual functionssite isolating and electroactivesdendrimers
with vastly improved solubilities and molecular dimensions that
may be adjusted readily to test more fully the fundamental
concept of site-isolation through simultaneous emission from
two vicinal dyes in a dendritic light emitting diode configuration.
Several recent reports have also described the use of dendrimers
as components of monochromatic light emitting diodes.12
In an accompanying manuscript the photoluminescence and
electroluminescence properties of the dendrimer-encapsulated
dyes and their mixtures are described.
Figure 1. Schematic representation of energy transfer between dyes (a)
and steric encapsulation preventing energy transfer between dyes (b)
electronic link was introduced between the periphery and the
core. In a dendritic antenna, an array of terminal donor
chromophores collects many photons and transfers their energy
through space (Fo¨rster energy transfer) to the core or focal point
acceptor unit, which can also be excited independently of the
periphery. Because emission is observed from the core only,
the system serves as a spatial and spectral energy concentrator,
in other words it acts as a “molecular lens”.
In contrast, it is normally difficult to prepare a light-emitting
layer in which two or more dyes emit simultaneously as energy
transfer between the dyes leads to light emission by the dye
with the lowest band gap10 as shown schematically in Figure
1a. Best results for simultaneous emission are generally obtained
by using highly diluted systems in which the dye molecules
are kept at mutual distances that exceed their Fo¨rster radius,
thereby reducing these nonluminescent interactions. The chal-
lenge posed by the creation of a single layer light emitting diode
capable of emitting two or more colors simultaneouslys
ultimately perhaps the three primary colors of the visible
spectrum to afford white lightsis an excellent vehicle to test a
dual function dendrimer design affording not only site-isolation
at its core but also contributing key electronic transport through
its periphery. The dendritic encapsulation shown schematically
in Figure 1b would enable multicolor emission from a single
layer containing a mixture of encapsulated dyes.
Results and Discussion
A. Design of the Light Emitting Dendrimers. To maximize
site-isolation, our dendrimer design features the light emitting
chromophore at the center of the dendrimer with a multi-
functional periphery held by large Fre´chet-type poly(aryl ether)
dendrons.3 With a two chromophore system involving for
example coumarin 343 (C343) and pentathiophene (T5) as the
central dyes, this arrangement keeps the core moieties separated
from each other to limit both self-quenching and the transfer
of energy from the dye with the higher band gap (C343) to the
lower band gap emitter (T5). The periphery of the dendrimers
is fitted with triarylamine (TAA) moieties that serve a charge
transport functionshole transport (HT)sfor the device and also
act as energy donors for the core acceptor chromophores. The
dendrimer generation can then be used to control the average
distance between the TAA units and the acceptor chromophores.
The hole transport moieties are expected to work in conjunction
with an oxadiazole (PBD) electron transporter added to the
(7) (a) Wang, P.-W.; Liu, Y.-J.; Devadoss, C.; Bharathi, P.; Moore, J. S. AdV.
Mater. 1996, 3, 237. (b) Shortreed, M. R.; Swallen, S. F.; Shi, Z.-Y.; Tan,
W.; Xu, Z.; Devadoss, C.; Moore, J. S.; Kopelman, R. J. Phys. Chem. B.
1997, 101, 6318. (c) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454. (d)
Aida, T.; Jiang, D.-L. J. Am. Chem. Soc. 1998, 120, 10 895. (e) Sato, T.;
Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10 658. (f) Balzani,
V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem.
Res. 1998, 31, 26. (g) Maruo, N.; Uchiyama, M.; Kato, T.; Arai, T.; Akisada,
H.; Nishino, N. Chem. Commun. 1999, 2057. (h) Plevoets, M.; Vo¨gtle, F.;
Cola, L. D.; Balzani, V. New J. Chem. 1999, 23, 63. (i) Schenning, P. H.
J.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 4489. (j) Peng,
Z.; Pan, Y.; Xu, B.; Zhang, J. J. Am. Chem. Soc. 2000, 122, 6619. (k)
Weil, T.; Wiesler, U. M.; Herrmann, A.; Bauer, R.; Hofkens, J.; De
Schryver, F. C.; Mu¨llen, K. J. Am. Chem. Soc. 2001, 123, 8101
(8) Kawa, M. and Fre´chet, J. M. J. Chem. Mater. 1998, 10, 286.
(9) (a) Gilat, S. L.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem. Int. Ed. 1999,
38, 1422. Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 18, 1701.
(c) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118,
9635. (d) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354.
(10) Shoustikov, A.; You, Y.; Thompson, M. E. IEEE J. Sel. Top. Quantum
1998, 4, 3.
(11) Freeman, A. W.; Koene, S. C.; Malenfant, P. R. L.; Thompson, M. E.;
Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122, 12 385.
(12) Representative publications include (a) Pogantsch, A.; Wenzl, F. P.; List,
E. J. W.; Leising, G.; Grimsdale, A. C.; Mu¨llen, K. AdV. Mater. 2002, 14,
1061. (b) Anthopoulos, T. D.; Markham, J. P. J.; Namdas, E. B.; Samuel,
I. D. W.; Lo, S.; Burn, P. L. Appl. Phys. Lett. 2003, 82, 4824. (c) Satoh,
N.; Cho, J.; Higuchi, M.; Yamamoto, K. J. Am. Chem. Soc. 2003, 125,
8104.
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Dendrimer Encapsulated Dyes
A R T I C L E S
Scheme 1
Figure 2. Schematic representation of asymmetric encapsulation of
pentathiophene (T5)1.
dendrimer layer of the device. The photoemission band of the
TAAs overlaps well with both the C343 and the T5 core
absorption bands ensuring efficient energy transfer. Due to both
the favorable band overlap and the limited distance between
the donor TAAs and acceptor cores, the probability of photon
emission via energy transfer is favored over nonemissive or
core-core energy transfer, provided that the overall dendrimer
framework is large enough. Energy transfer in these dendrimers
may occur via photoinduced excitation as observed in other light
harvesting dendrimer systems,7-9 but it also occurs when a
voltage potential is applied to a bulk film of the dendrimer, as
in a device where electrons recombine with hole transporting
TAAs in their excited state.
B. Dendronizing both Extremities of an Unsymmetrical
T5 for Site-Isolation. Given the poor solubility of TAA
dendrons above the second generation, larger TAA dendrons
are difficult to purify and cannot be used to help solubilize T5.
In addition, the sheer length of the T5 chromophore used in
our previous work makes it impossible to achieve a high degree
of encapsulation through dendronization of one of its extremities
only. Therefore, a new strategy involving a bifunctional T5
moiety of the type shown in Figure 2 was explored. In this
strategy, one terminus of an unsymmetrically substituted T5
chromophore, 1,13 is capped with a small end-protected dendron
followed by attachment of a solubilizing poly(benzyl ether)
dendron at the other extremity. Following deprotection of the
Conditions: (i) Ph3P, DEAD, CH2Cl2, 2 eq dimethyl-5-hydroxyisoph-
thalate; (ii) LAH, THF, 55 °C; (iii) DHP, p-TSA, DMF; (iv) n-BuLi, THF,
-78 °C, (Bu)3SnCl; (v) 1, Pd(II), DMF, 90 °C; (vi) KOH, pH5 buffer;
(vii) [G-4]-CH2OH, DCC, DPTS; (viii) p-TSA, MeOH, THF.
first extemity, final assembly is achieved by attachment of small,
second generation TAA dendrons affording the desired encap-
sulated, yet still soluble structure with a fourth generation
electroactive [G-4] TAA terminus.
1. Preparing the Pentathiophene for Dendron Attachment.
Given the intrinsic low solubility of the pentathiophene core,
our synthetic blueprint requires that the poorly soluble TAA
functionalities be added last. We have shown earlier that
attaching appropriately chosen dendrons to oligo- and even poly-
thiophenes can greatly increase their solubility.13a,13b Therefore
the unsymmetrical pentathiophene 7, with one end protected
by a benzyl ester group and the other by a small dendron con-
taining four latent tetrahydropyranyl-protected alcohol coupling
sites, was prepared (Scheme 1). The benzyl ester of 7 will later
(13) (a) Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J. M. J. J. Am. Chem.
Soc. 1998, 120, 10 990. (b) Malenfant, P. R. L.; Jayaraman, M.; Fre´chet,
J. M. J. Chem. Mater. 1999, 11, 3420. (c) Malenfant, P. R. L.; Fre´chet, J.
M. J. Macromolecules 2000, 33, 3634. (d) Briehn, C. A.; Schiedel, M.;
Bonsen, E. M.; Schuhmann, W.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2001,
40, 4680. (e) Caras-Quintero, D.; Ba¨uerle, P. Chem. Commun. 2002, 2690.
9
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A R T I C L E S
Furuta and Fre´chet
Scheme 2
Conditions: (i) K2CO3, 18-Crown-6, Acetone, 60 °C; (ii) LAH, THF, 0 °C; (iii) CBr4, Ph3P, DIPEA, THF; (iv) KOH, MeOH, H+.
be replaced by a solubilizing fourth generation Fre´chet-type
poly(benzyl ether) dendron, while the four coupling sites at the
other extremity will be used to introduce low generation
electroactive dendrons containing TAA termini.
lithiation followed by stannylation to afford the corresponding
tributyltin derivative 6, which could be used directly without
further purification. Finally, attachment of 6 to the pen-
tathiophene core 1 to form 7, was achieved in 55% yield using
Stille coupling conditions.17
Pentathiophene 1 was selected as the starting material as it
has orthogonal bromide and carboxylic ester functionalities to
facilitate the construction of the type of asymmetric “barbell”13a
structure shown in Figure 2. The protected tetrafunctional [G-2]
dendron 5 was chosen to provide the desired latent tetraol
functionality of 7. Compound 5 was obtained using the
“reversed” convergent benzyl ether approach of Tyler and
Hanson.14 Starting with diol 2,15 treatment with two equivalents
of dimethyl-5-hydroxyisophthalate under Mitsunobu etherifi-
cation conditions16 afforded the [G-2] tetraester dendron 3 in
82% yield after purification. Reduction of 3 into 4 had to be
carried out at 55 °C using LiAlH4 in THF due to the poor
solubility of the starting tetraester. After recrystallization of 4,
THP protection of its four hydroxyl groups led to hypermonomer
5, which is freely soluble in many aprotic solvents, including
hexane. The focal point of 5 was subsequently activated by
2. Attachment of the Solubilizing Dendron: Preparation
of Unsymmetrical Dendrimer 10. Although the solubility of
pentathiophene carboxylic acids is generally too low to enable
their efficient use in the elaboration of dendronized structures,
the excellent solubilizing properties of the THP-protected
dendron located at one extremity of 7 allow removal of the
benzyl ester protecting group from the other extremity. Because
solubility is preserved, the subsequent coupling of a large
dendron can be achieved without the adverse effects that would
arise from undue dilution or the use of solvents inappropriate
for the coupling step. Therefore, highly soluble pentathiophene
benzyl ester 7 was saponified and washed with pH 5 buffer to
liberate the free acid 8, without affecting the acid-labile THP
acetals. A fourth generation Fre´chet-type poly(benzyl ether)
dendron3 [G-4]-OH was then coupled to 8 using DCC and
DPTS18 in CH2Cl2 to afford 9 in 54% yield. The terminal poly-
(14) (a) Tyler, T. L.; Hanson, J. E. Polym. Mater. Sci. Eng. 1995, 73, 356. (b)
Gilat, S. L.; Andronov, A.; Fre´chet, J. M. J. J. Org. Chem. 1999, 64, 7474.
Tyler, T. L.; Hanson, J. E. Chem. Mater. 1999, 11, 3452-3459.
(15) Gumbley, S. J.; Stewart, R. J. Chem. Soc., Perkin Trans. 2 1984, 3, 529.
(16) Mitsunobu, O. Synthesis 1981, 1, 1.
(17) (a) Milstein, D.; Stille, J. K.. J. Am. Chem. Soc. 1979, 101, 4992. (b) Stille,
J. K.; Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(18) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65.
9
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Dendrimer Encapsulated Dyes
A R T I C L E S
Scheme 3
Figure 3. Schematic representation of the utility of methyl ester focal point
transformations.
(benzyl ether) dendron now contributes to the solubility of the
whole and allows removal of the THP acetals to give soluble
tetraol 10.
Conditions: (i) DEAD, Ph3P, THF; (ii) LAH, THF, 0 °C; (iii) K2CO3,
18-Crown-6, Acetone, 60 °C; (iv) CBr4, Ph3P, DIPEA, THF.
3. Preparation of the TAA Dendrons and their Attach-
ment to Unsymmetrical Dendrimer 10. The strategy chosen
(Figure 2) requires the preparation of small, soluble [G-2] TAA
dendrons, and their single-step attachment to the four coupling
sites of dendrimer 10 to form the desired pentathiophene 20 in
which the chromophore is flanked by two encapsulating
generation four dendrons (Scheme 2). The hole transporting
TAA dendrons, which serve as emissive donors while also
providing steric bulk for encapsulation of the core T5 lumo-
phore, were synthesized using a convergent approach. THP
acetal 11,19 obtained from commercially available 3-bromoben-
zyl alcohol was used in a palladium cross coupling reaction20
with N-phenyl-2-naphthylamine to afford 12. After bromination
using Ph3P and Br2 in CH2Cl2 at 0 °C, the first generation hole
transporting dendron 15 (TAA2-[G-1]-COOCH3) was prepared
from 13 by Williamson etherification17 of methyl 3,5-dihy-
droxybenzoate 14. The methyl ester focal point was then reduced
quantitatively to the corresponding alcohol 16 using LAH in
THF at 0 °C followed by Baeckstro¨m’s21 workup. Although
the use of methyl-3,5-dihydroxybenzoate instead of the com-
monly employed 3,5-dihydroxybenzyl alcohol for the prepara-
tion of benzyl ether dendrons requires an additional reduction
step, the electron withdrawing ester group of methyl-3,5-
dihydroxybenzoate suppresses C-alkylation side reactions and
increases the overall yield as the additional reduction step is
quantitative. The methyl ester focal point is also a useful
precursor for a variety of coupling reactions used in the synthesis
of our dendrimers (Figure 3). It can be reduced and transformed
to the corresponding electrophilic bromide for reaction with a
nucleophilic reactant or core. It can also be saponified, to
produce the corresponding nucleophilic carboxylates useful in
subsequent assembly with appropriate electrophiles under mildly
basic conditions, or to form an activated ester capable of
coupling with suitable nucleophiles. The routes ultimately
chosen for construction of the dendrimers must take into account
solubility issues, the compatibility of functional groups, and the
type of bond desired. This functional versatility is also useful
for our purposes because one of the fluorescent dyes we plan
to use as a core is nucleophilic while the other is electrophilic.
Alcohol 16 was transformed into bromide 17 using N,N-
diisopropylethylamine (DIPEA), CBr4, and Ph3P in THF at 0
°C. DIPEA was used to neutralize any trace of potentially
harmful acid that may be formed in the reaction. The [G-2]
dendron 18 was constructed from 17 and diol 14 under the same
alkylation conditions used previously for 15. The ester focal
point was saponified and subsequently acidified to give TAA4-
[G-2]-COOH 19, which was coupled to tetraol 10 using DCC
as a coupling agent to give the target encapsulated lumophore
20 in 40% yield.
(19) Delorme, D.; Ducharme, Y.; Brideau, C.; Chan, C.; Chaulet, N. J. Med.
Chem. 1996, 39, 3951.
With dendrimer 20, the solubility issue appeared to have been
solved as 20 was soluble in common organic solvents (acetone,
chloroform, THF, etc.), and preliminary studies of mixtures of
(20) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7217.
(21) Baeckstro¨m, P.; Li, L.; Mahinda, W.; Norin, T. Synth. Commun. 1990, 20,
423. This procedure greatly simplifies the workup and purification of the
product via a single filtration.
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A R T I C L E S
Furuta and Fre´chet
Scheme 4
Conditions: (i) K2CO3, 18-Crown-6, Acetone, 60 °C; (ii) LAH, THF, 0 °C; (iii) CBr4, Ph3P, THF, DIPEA; (iv) KOH, MeOH, H+.
20 with coumarin dyes suggested that the encapsulating den-
drons were large enough to show some improved isolation.
However, the large [G-4] TAA terminated dendrons show a
strong tendency to crystallize causing the rapid appearance of
haziness in solution cast films of 20, thus negating their use in
organic light emitting diodes. Although other examples of
triarylamine dendrimers22 with meta substituents on their
aromatic rings have been shown to form amorphous films from
vacuum deposition, the poor quality of solution cast films of
20 could not be changed by the use of other solvents. As a
result, the functional testing of the light emission and encapsula-
tion characteristics of 20 required the solution casting of films
containing 50 wt % of poly(vinyl carbazole). Therefore, while
20 achieved the initial goals of solubility and increased size,
the presence of the large dendron with TAA end groups confers
undesirable properties that interfere with the measurement of
site isolation.
C. Designing a New, More Soluble and Less Crystalline
Dendrimer Periphery. The problem of crystallinity uncovered
with dendrimer 20 is likely to re-occur with any other en-
capsulated chromophoric system based on a dense TAA
periphery since both the TAA dendrons and the chromophores
selected for encapsulation, coumarin 343 and T5 have limited
intrinsic solubilities.
A new strategy was therefore developed both to solve the
problems of solubility and crystallinity, and to allow the
construction of larger dendrimers thereby enabling a more
thorough exploration of the concept of dye encapsulation. To
reduce the stacking of the TAA groups, which is likely to be
the major contributor to any issue of crystallinity, a new dendron
in which electroactive peripheral TAA groups alternate with
(22) (a) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.;
Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807-
809. (b) Okumoto, K.; Shirota, Y. Chem. Lett. 2000, 29, 1034-1035.
9
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Dendrimer Encapsulated Dyes
A R T I C L E S
Scheme 5
Conditions: (i) K2CO3, 18-Crown-6, Acetone, 56 °C; (ii) Pd(II), DMF, 90 °C.
solubilizing23 branched alkyl ether groups was designed (Scheme
3). The synthesis of the unsymmetrical TAA-alkyl ether [G-1]-
COOMe dendron 24 was only slightly more complicated than
that of symmetrical dendron 15, requiring an additional step to
monofunctionalize the basic building block 14. Once the
terminal moieties have been introduced, the subsequent growth
steps used to construct higher generation dendrons involve a
conventional convergent protocol with both growth and workup
facilitated by the high solubility of the new structures.
D. Synthesis of the Unsymmetrical Dendrons. A combina-
tion of divergent and convergent steps, also known as the
double-stage convergent approach,24 enabled the construction
of dendrons that remained soluble and amorphous through the
largest fifth generation molecules we studied. The solubilizing
terminal moiety with its twin alkyl ether groups was obtained
by coupling 2-methyl-1-pentanol with 14 under Mitsunobu
conditions, generating methyl ester 21 (Scheme 3). Following
reduction of the ester, the resulting alcohol 22 was attached to
only one of the two hydroxyls of 14, affording phenol ester 23.
Williamson etherification of TAA 13 with 23 produced the first
generation dendron 24, having both TAA and alkyl ether
terminal functionalities. LAH reduction of 24 generated alcohol
25, which was subsequently brominated to obtain benzylic
bromide 26. The standard dendron growth procedure involving
stepwise alkylation of 14 followed by reduction and activation
of 28 to the bromide 29 was then applied. Both [G-3] and [G-4]
dendrons with methyl ester focal points are available in one
(23) (a) Halim, M.; Samuel, I. D. W.; Pillow, J. N. G.; Monkman, A. P.; Burn,
P. L. Synthetic Met. 1999, 102, 1571. (b) Robinson, M. R.; O′Regan, M.
B.; Bazan, G, C. Chem. Commun. 2000, 1645.
(24) Wooley, K. L.; Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1991,
113, 4252.
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A R T I C L E S
Furuta and Fre´chet
step from 29 (Scheme 4), depending on whether 14, methyl
4,4-di(4-hydroxyphenyl)pentanoate, or the tetrafunctional hy-
permonomer25 32 is used. Standard etherification methodologies
were used for all of these coupling reactions leading to third
generation (30 and 33), and fourth generation (36) methyl ester
dendrons. A similar synthetic strategy was used to build the
fifth generation methyl ester 40.
E. Synthesis of the Encapsulated Chromophores. The
electrophilic C343 core moiety11 44 was prepared by reaction
of coumarin 343 with a large excess of tetra-bromide26 43 under
standard phase transfer conditions to maximize monosubstitu-
tion. The resulting core moiety still possesses three electrophilic
benzylic bromide groups that can each be used to attach en-
capsulating electroactive dendrons. In contrast to our preliminary
work,11 in which only small coumarin dendrimers could be used
due to their poor solubility, much larger third and fourth
generation dendrons such as 31 and 39 with enhanced potential
for encapsulation can be used as a result of the introduction of
the terminal solubilizing groups. An initial attempt at encapsula-
tion of the somewhat congested functionalized core dye 44 was
made using the soluble electroactive dendron 31 as it was
expected that its extended focal point would facilitate the
coupling step via nucleophilic substitution with its carboxylate
anion. This reaction was effected using 3.15 mol of carboxylic
acid dendrons 31 per mole of 44 under standard phase transfer
conditions affording dendrimer 45 with its coumarin core
encapsulated in generation three dendrons. Although the use of
the extended focal point in dendron 31 was successful,
subsequent studies with a shorter focal point were also carried
out in an attempt to provide added rigidity to the final product.
Therefore, the coupling of 44 with the large fourth generation
dendron 39 with its shorter and less flexible focal point led to
a 47% isolated yield of encapsulated C343 dye 46.
Figure 4. MALDI-TOF of (a) bithiophene [G-5] dendron 53 and (b)
pentathiophene bis [G-5] dendrimer 54.
F. Characterization of the Dendrons and Dendrimers. The
dendrons and dendrimers were characterized by a combination
of IR, NMR, and MALDI mass spectroscopy. The main utility
of IR was the monitoring of transformations involving ester focal
points. As the ester focal point is transformed into a carboxylic
acid, the CdO stretch shifts from 1745 to 1692 cm-1. Similarly,
the 1H NMR chemical shifts of methyl ester protons, as well as
the benzyl protons of the alcohol and bromide focal points are
easily followed confirming focal point transformations.
MALDI-TOF mass spectroscopy is most valuable for higher
molecular weight compounds (MW > 1500) where other meth-
ods become less applicable because diagnostic NMR signals
become more difficult to discern due to the growing intensities
of the signals corresponding to the dendrimer interior and
periphery. For example, few differences are observable in the
NMR signals for 54 and its precursor 53 as the peak corre-
sponding to the additional thiophene ring is hidden within the
large aromatic resonances of the dendrons, whereas the masses
of the two compounds differ widely (27 203 vs 13 640 Daltons)
(Figure 4a,b).
Given the success achieved in the preparation of larger
encapsulating moieties for the coumarin chromophores, similar
building blocks were used in the preparation of the encapsulated
T5 dyes. In this case however, the great solubility of the
dendritic arms enabled the use of a vastly simplified synthetic
blueprint. Scheme 5 details the construction of generation 3, 4,
and 5 dendrons with a bromo-bithiophene focal point by
coupling the respective benzylic bromide dendrons with bro-
mobithiophene carboxylic acid 47.27 In all three cases these
dendrons 49, 51, and 53 were obtained in excellent yields of
85, 83, and 88%, respectively.
Assembly of the final dendrimers with two generation 3, 4,
or 5 dendrons surrounding a pentatiophene core was achieved
using the double Stille coupling17 of the electroactive bithiophene
dendrons (49, 51, and 53) onto bis(trimethylstannyl)thiophene
48. Surprisingly high yields are obtained in this coupling reaction
despite the rather large size of the dendrons. For example, the
large bis-G-3 (50), bis-G-4 (52), and bis-G-5 (54) encapsulated
pentathiophenes are obtained in yields of 85%, 86%, and 69%,
respectively by reaction of only a very slight excess (less than
3%) of 49, 51, or 53 with the small bifunctional thiophene 48.
It is worth noting that the largest encapsulated dye obtained by
this process has a molecular weight of more than 27 000
Daltons.
Conclusion
Since chromophores such as C343 and T5 are capable of
energy transfer over a range of several nanometers, site isolation
requires that fairly large structures be built, and the measurement
(25) Freeman, A. W.; Chrisstoffels, L. A. J.; Fre´chet, J. M. J. J. Org. Chem.
2000, 65, 7612.
(26) Vinod, T.; Hart, H. J. Org. Chem. 1990, 55, 881.
(27) Kilbinger, A. F. M.; Schenning, A. P. H. J.; Goldoni, F.; Feast, W. J.;
Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 1820.
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Dendrimer Encapsulated Dyes
A R T I C L E S
of the relative degree of encapsulation they afford mandates
experiments involving light emission that require the fabrication
of thin film devices. As a result, the dendrimers must not only
possess the required radial arrangement of a central chromophore
surrounded by peripheral electroactive groups, but also ap-
propriate physical properties allowing the preparation of good
amorphous films. Although early dendrimers based on TAA
dendrons were capable of providing a sterically demanding
environment, their tendency to crystallize led to synthetic
difficulties with lowered yields and film-forming properties too
poor to allow the measurement of ultimate properties derived
from site isolation. The synthetic versatility of the convergent
growth approach has now enabled the introduction of groups
that contribute both physical (solubility) and electronic proper-
ties, while also allowing the high yield preparation of structures
sufficiently large to provide a significant level of steric shielding
and site isolation. Not surprisingly, site isolation required the
development of very large structures with a size comparable to
that of many enzymes. The demonstration of site-isolation and
the functional properties of these light-emitting dendrimers are
described in the accompanying paper.
Acknowledgment. Financial support for this research was
provided by the Air Force Office of Scientific Research. The
authors also acknowledge partial support by the U.S. Department
of Energy under Contract No. DE-AC03-76SF00098. Pen-
tathiophene 1 was prepared at UC Berkeley by Patrick Malen-
fant.
Supporting Information Available: Detailed experimental
procedures and characterization data for compounds 2-13,
15-31, 33-42, 44-46, and 48-54 are available in the complete
Experimental Section included in Supporting Information (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA037133F
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