generation dendrimer 6, the higher dendrimer generations
represent shape-persistent nanoparticles.
Here we investigate the influence of the branching unit on the
shape of the dendrimer employing 1 as core and a four-
position temperatures well above 450 °C for the TIPS
substituted and 580 °C for the unsubstituted dendrimers under
air.
The special features of the results presented above can be
summarized as follows: (i) in contrast to the established concept
based on metallo-organic coupling reactions for the construc-
4 2
directional A B building block 3 rather than the A B building
block 2. Thereby, we introduce 2,3,4,5-tetrakis[4-triisopro-
pylsilylethynylphenyl)cyclopenta-2,4-dienone 3 as a powerful
multifunctional reagent that allows the rapid construction of a
spherical polyphenylene architecture.
12
tion of polyphenylene dendrimers (as well as the closely
related hyperbranched polyphenylenes),13 the growth of the
dendrimers presented here is achieved via [2+4]cycloaddition
Cyclopentadienone 3 contains four dienophile units and one
4
followed by a desilylation; (ii) using the A B building block 3
diene function and can thus be regarded as an A
4
B building
and the core 1 the second dendrimer generation exhibits
properties such as spherical shape in spite of the core’s D2h
symmetry and the densest packing of benzene rings which one
would generally expect only for higher generation numbers; (iii)
using a defined dendrimer core, the shape of the dendrimer
formed depends on the cyclopentadienone used.
Our current investigations involve light scattering experi-
ments on higher dendrimer generations as well as microscopic
visualization after adsorption on substrate surfaces.
block which represents, to the best of our knowledge, the first
example of a dendrimer branching element possessing a
5
multiplicity higher than 3. Compound 3 was prepared by the
base-catalyzed condensation of 4,4’-bis(triisopropylsilyl)benzil
and 1,3-bis(4-triisopropylsilylethynyl)acetone (83%, red crys-
6
7
tals). The acetone was obtained via the Sonogashira coupling
of triisopropylsilylacetylene and 1,3-di(4-bromophenyl)acetone
82%). The latter was synthesized by converting 4-bromobenzyl
bromide with Fe(CO) in the presence of NaOH under phase
transfer conditions (CH Cl –H O; 43%).
The four-fold Diels–Alder reaction of the A
with the tetraethynylbiphenyl 1 led to the first dendrimer
(
5
2
2
2
Notes and References
4
B building block
†
‡
E-mail: muellen@mpip-mainz.mpg.de
The amount of higher Diels–Alder cycloaddition products can be
3
generation 5a (77%, white solid). After cleaving the TIPS
groups (5b), further reaction with 3 provided a mixture of
products, determined by mass spectrometry, consisting of the
desired sixteen-fold Diels–Alder product, and mainly of the
fourteen-fold [2+4]cycloaddition product.† In contrast, addition
of tetraphenylcyclopentadienone 8 to 5b led smoothly to the
desired sixteen-fold Diels–Alder product 7 exclusively, which
was isolated as a white solid in 62% yield (see Scheme 1).
The different results obtained by the reaction of 5b with 2 or
increased by repeated treatment with cyclopentadienone 2b.
Selected data for 3: FD-MS: m/z 1105, calc. for C73 100OSi
§
H
4
: 1106; d
, 303 K) 7.37–7.28 (m, 8H), 7.17 (d, J 8.0, 4H, HAryl),
.89 (d, J 8.0, 4H, HAryl), 1.13 (s, 84H, HTIPS); d (75 MHz, CD Cl , 303
H
3
(
6
2 2
200 MHz, CD Cl
3
C
2
2
K) 199.7 (CNO), 154.8, 133.5, 132.5, 132.4, 131.4, 130.8, 130.1, 126.2,
i
i
124.8, 123.5, 107.7 [ArC°CSi(Pr ) ], 107.3 [ArC°CSi(Pr ) ], 93.4
3
3
i
i
[ArC°CSi(Pr )
[CH(CH ]; mp 296 °C (decomp.). For 7: MALDI-TOF-MS: m/z 7762,
calc. for C612 410: 7764; d (500 MHz, THF, 303 K) 7.70–7.34 (br, 24H),
.25–6.03 (br, 386H); d (125 MHz, THF, 303 K) 145.55, 145.45, 145.36,
3
], 92.7 [ArC°CSi(Pr )
3 3 2
], 19.2 [CH(CH ) ], 12.1
3 2
)
H
H
7
1
1
1
1
1
C
3
can be easily rationalized by looking at ball-and-stick models,
44.60, 144.54, 144.43, 144.22, 144.18, 144.05, 144.03, 143.99, 143.91,
43.87, 143.80, 143.73, 143.69, 143.62, 143.58, 143.45, 142.99, 142.86,
42.80, 142.69, 142.64, 142.56, 135.60, 135.50, 135.45, 135.38, 135.33,
35.29, 135.16, 135.09, 135.02, 134.95, 134.87, 134.76, 134.70, 134.65,
34.57, 133.49, 133.38, 133.34, 130.92, 130.29, 130.24, 130.18, 129.95,
which were generated using the MM2 (85) force field with the
CERIUS 2 program package and applying the Conjugate
8
Gradient 200 algorithm (compare Scheme 1): As expected, the
appearances of the corresponding first generations 4 and 5 are
very similar, as both dendrimers contain 22 benzene rings and
differ only in the number of substituents, while having the same
spatial extension. The corresponding second generations al-
ready exhibit clearly dissimilar shapes. The dendrimer 6, with
129.86, 129.55, 129.49, 129.04, 128.87, 128.72, 128.60, 109.67; GPC
analysis (polystyrene as standard): M /M = 1.04; mp > 300 °C.
w
n
1
G. R. Newkome, C. N. Moorefield and F. Vögtle, Dendritic Molecules,
VCH Verlag, Weinheim, 1996
6
2 benzene rings, synthesized using the A
possesses a dumb-bell like structure, whereas the use of the A
2
B building block 2,
4
B
2 J. M. J. Fréchet and C. J. Hawker, in Comprehensive Polymer Science,
2. Supl., ed. G. Allen, S. L. Aggarwal and S. Russo, Elsevier, Oxford,
1996, S. 70–129.
building block 3 leads to ‘nanoball’ architecture 7, which
possesses 102 benzene rings. As the second generations of both
dendrimers have a maximum extension of about 4 nm, the
density of benzene rings is dramatically increased in 7. In the
case of the more strongly branched dendrimer 7 the space
available for a new dendrimer shell is sufficient to accom-
modate the incoming eighty unsubstituted benzene rings
spherically around the dendrimer core. The fact that the 64
3
4
5
F. Morgenroth, E. Reuther and K. Müllen, Angew. Chem., 1997, 109,
47; Angew. Chem., Int. Ed. Engl., 1997, 36, 631.
F. Morgenroth, C. Kübel and K. Müllen, J. Mater. Chem., 1997, 7,
207.
It ought to be mentioned in this context that Fréchet and Otha have
recently described the synthesis of an end-reactive AB dendron (made
from two different AB building blocks) for the accelerated synthesis of
6
1
4
2
sterically demanding TIPS groups of the A
4
B building blocks
dendritic polymers: M. Ohta and J. M. J. Fréchet, J. Macromol. Sci.,
Pure Appl. Chem., 1997, A34, 2025.
6 F. Morgenroth and K. Müllen, Tetrahedron, 1997, 53, 15349 and
references cited therein.
cannot be accommodated, proves the concept of densest
packing.9
,10
The preferred formation of the fourteen-fold
cycloaddition product can be understood according to Tomalia
11
7 S. Takahashi, Y. Kuroyama, K. Sonogashira and N. Hagihara, Synthesis,
as a sterically induced stoichiometry.
The obtained dendrimers exhibit unexpectedly good sol-
2
ubility in common organic solvents such as toluene and CH Cl .
1
980, 627.
2
8
9
CERIUS , Molecular Simulations Inc., Waltham, MA, USA. For more
details see ref. 4. Additional data are to be published.
M. Maciejewski, J. Macromol. Sci. Chem., 1982, A17, 689.
2
Therefore, they can be fully characterized by matrix-assisted
laser desorption ionization time of flight mass spectrometry
1
0 P.-G. de Gennes and H. Hervet, J. Phys. Lett., 1983, 44, L-351.
1
13
(
MALDI-TOF-MS) as well as H and C NMR spectroscopy.
11 D. A. Tomalia and H. D. Durst, Top. Curr. Chem. 1993, 165, 193.
12 T. M. Miller, T. X. Neenan, R. Zayas and H. E. Bair, J. Am. Chem. Soc.,
1992, 114, 1018.
The perfect agreement between calculated and experimentally
determined m/z ratios for the dendrimers as well as GPC
analysis confirm their monodispersity.§ With respect to the
physical properties of the dendrimers their thermal stability is
noteworthy. The thermogravimetric analysis yielded decom-
1
3 Y. H. Kim and O. W. Webster, J. Am. Chem. Soc. 1990, 112, 4592.
Received in Cambridge, UK, 18th February 1998; 8/01395K
1140
Chem. Commun., 1998
Morgenroth, Frank
