Pushing the synthetic limit: Polyphenylene dendrimers with "exploded" branching units - 22-nm-diameter, monodisperse, stiff macromolecules

Article abstract of DOI:10.1002/anie.200501368

(Figure Presented) Structural perfection up to the fifth generation of a polyphenylene dendrimer has been achieved by inserting p-terphenyl spacers into the arms of each branching unit (see picture). The stiff, monodisperse macromolecules with diameters u

Full text of DOI:10.1002/anie.200501368

Communications
the molecule, such as the nature of the molecular density and
the location and orientation of the chain ends.[1] Rigidification
of the dendrimer, and in some cases, control over the chain-
end locations, has come from a variety of approaches,[2]
including the use of stiff building blocks such as that of
poly(1-3,5-phenylene ethynylene),[3] poly(1-3,5-phenylene),[4]
and polyphenylene dendrimers based on pentaphenylben-
zene[5] (Scheme 1a, b, and c, respectively). Beyond a critical
Dendrimers
DOI: 10.1002/anie.200501368
Pushing the Synthetic Limit: Polyphenylene
Dendrimers with “Exploded” Branching Units—
22-nm-Diameter, Monodisperse, Stiff
Macromolecules**
Scheme 1. Stiff, dendritic AB2 repeat units: a) poly(1-3,5-phenylene
ethynylene), b) poly(1-3,5-phenylene), c) poly(4’-4,4’’-(3’,6’-diphenyl o-
terphenylene)), and d) “exploded” polyphenylene by the extension of
each arm by insertion of a linear terphenyl between each set of dashed
lines of the general polyphenylene repeat unit shown in (c).
Ekaterina V. Andreitchenko, Christopher G. Clark, Jr.,
Roland E. Bauer, Günter Lieser, and Klaus Müllen*
generation, the convergent synthesis of dendrimers necessi-
tates the use of flexible building blocks or smaller dendrons.[6]
To circumvent this limitation, a gradient of branching unit
lengths has been employed[7] in the case of a dendrimer with
1-3,5-phenylene ethynylene branching points, thus installing
greater volume at the interior of the molecule. A similar size-
modulation approach to the branching unit is herein proposed
for the enlargement of the parent, polyphenylene dendrimers.
Linear enlargement of the arms of the general dendritic
repeat unit (Scheme 1d) should directly translate into
increased dendrimer radii without requiring a larger core
moiety (Figure 1). The synthesis of polyphenylene dendrim-
ers around a tetraphenylmethane core has thus far been
limited to the fourth generation[8] because of incomplete
conversion at higher generations, presumably the prohibitive
result of steric crowding at the chain ends.[9] In light of this
observation, a terphenyl spacer was introduced into each arm
of the branching unit to decrease the congestion of the chain
ends at higher generations (Figure 1). Since the connectivity
of the branching points in the dendrimers with extended arms
The majority of effort in dendrimer chemistry has been
devoted to constructions with flexible branching units,
thereby introducing a variety of conformational aspects into
[*] E. V. Andreitchenko, Dr. C. G. Clark, Jr., Dr. R. E. Bauer, Dr. G. Lieser,
Prof. Dr. K. Müllen
Max-Planck-Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-100
E-mail: muellen@mpip-mainz.mpg.de
[**] The research was supported by the Deutsche Forschungsgemein-
schaft (DFG) in the frame of the Sonderforschungsbereich (SFB)
625. C.G.C. is greatfulfor financialsupport from a US National
Science Foundation MPS Distinguished InternationalPostdoctoral
Research Fellowship (MPS-DRF; Award: DMR-0207086). We would
like to thank M. Wagner for NMR results and A. Rouhanipour and
H.-J. Räder for mass spectrometric results and helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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than 20 nm, which well exceeds the
dimension of the narrowly disperse but
not monodisperse particles that have
been synthesized by self-assembly or
multiphase approaches.[10]
The key component in the
dendrimer synthesis is the masked
AB2 building block 3,4-bis-(2’,5’-
dimethyl-4’’’-triisopropylsilylethynyl-
1,1’:4’,1’’:4’’,1’’’-quaterphenyl-4-yl)-2,5-
diphenylcyclopentadienone (6; Sche-
me 2) which bears an active tetraphe-
nylcyclopentadienone and two terminal
aryl acetylenes, or Diels–Alder dieno-
philes, protected with bulky triisopro-
pylsilyl (TiPS) groups. While the solu-
bilities of the final dendrimers were not
anticipated to be poor, the solubilities
of the p-terphenyl intermediates were
expected to complicate the synthesis.
As such, two methyl groups were intro-
duced as side chains from the beginning
by using 1-bromo-4-iodo-2,5-dimethyl-
benzene. An efficient, iterative, selec-
tive Suzuki coupling[11] (Scheme 2)
using the more-reactive aryl iodide,
compared to the aryl bromide, with
commercially available 4-trimethylsilyl-
phenylboronic acid was performed
to
give
4-bromo-2,5-dimethyl-4’-
trimethylsilylbiphenyl (1). Removal of
the trimethylsilyl group with ICl[12]
provided
4-bromo-4’-iodo-2,5-dime-
thylbiphenyl (2). The cycle was
repeated to obtain 4-bromo-4’’-iodo-
2,5-dimethyl-1,1’:4’,1’’-terphenyl
(4).
The aryl iodide was treated with trime-
thylsilylacetylene under Sonogashira/
Hagihara conditions[13] to afford 5.
The final step was the twofold Suzuki
coupling of the remaining aryl bromide
with
3,4-bis(4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenyl)-2,5-
diphenylcyclopentadienone to furnish
6.[14] Chromatographic purification was
performed for all steps.
The strategy for the synthesis of the
new dendrimers with extended arms
(Scheme 3) was directly analogous to
the synthesis of the parent polypheny-
lene dendrimers. Fourfold Diels–Alder
reactions of the masked AB2 building
block 6 with tetra(4-ethynylphenyl)me-
thane[8] produced the first-generation
dendrimer 7 bearing eight TiPS chain
Figure 1. Scaling of the dendrimer radii for general polyphenylene dendrimers and new
“exploded” polyphenylene dendrimers with identical branching-point connectivity but increased
branching unit arm length through the use of a 2’,5’-dimethyl-p-terphenylspacer. Second-genera-
tion dendrimers for each type are illustrated in greater detail. The iterative illustrations were gen-
erated using POV-Ray and are qualitatively similar to the results of MMFF94 molecular mechan-
ics calculations for generations 1–4. The maximum radius was determined by the length of the
longest linear oligophenylene arm for each generation.
is the same as for the parent polyphenylene dendrimers, the
radii should be 150% greater than the comparable radii of the
parent cases. This situation would enable the stepwise syn-
thesis of a structurally perfect particle with a diameter greater
end groups. Removal of excess 6 was accomplished by
chromatographic purification. Removal of the TiPS groups
with tetrabutylammonium fluoride trihydrate gave the first-
generation dendrimer 8 with eight terminal ethynyl units
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Scheme 2. Synthesis of the masked, AB2 building block 6: a) K2CO3, [Pd(PPh3)4], toluene, water, reflux, 62%; b) ICl, CCl4, CH2Cl2, 08C, 96%;
c) K2CO3, [Pd(PPh3)4], toluene, water, ethanol, reflux, 83%; d) ICl, CH2Cl2, 08C, 87%; e) [PdCl2(PPh3)2], CuI, PPh3, NEt3, CH2Cl2, 80%; f) K2CO3,
[Pd(PPh3)4], toluene, water, ethanol, reflux, 48%.
quantitatively. After quenching the reaction with water and
precipitation in methanol, 8 was ready for subsequent
generational growth. The cycle was repeated through to the
fourth generation, and a final coupling yielded the fifth-
generation dendrimer with 128 TiPS chain ends. Removal of
excess 6 from the reaction mixture was achieved, beginning
with the synthesis of the second-generation dendrimer 9, by
first adsorbing the dendrimer onto silica gel. The adsorbate
was washed with acetone until the purple color of the eluent
disappeared, and then the dendrimer was extracted with THF
and reprecipitated in acetone. Synthetic details for all
compounds presented here are included in the Supporting
Information.
results (THF; Figure 2) showed a progressive decrease in the
elution volume for each subsequent generation (7, 9, 11, 13,
and 15), while the decrease in the elution volume was smaller
going to the fifth-generation dendrimer 15;[15] the peak shape
confirmed that 15 is smaller than the exclusion limit of the
columns. A high-molecular-weight shoulder was present,
which gave rise to a broader polydispersity than the earlier
generation dendrimers. The shoulder, presumed to be the
result of aggregation, has been observed for large generation
dendrimers[2c] and was further broadened at higher concen-
1
tration. Neither H NMR spectroscopy nor SEC, however,
has the inherent dynamic range to determine whether the
final coupling reactions go exclusively to completion (for
example, whether the pure dendrimer or a mixture of the
dendrimer and nearly complete conversions is produced).
MALDI-TOFanalysis (using dithranol as matrix, and in some
cases, a suitable cationization agent was added) was relied
upon to detect the presence of the final dendrimers. The
Diels–Alder reactions were stopped when the product
molecular ion was the only large molecular ion observed
(Table 1). The molecular ions of the desired dendrimeric
products were detected for the first three generations, 7, 9,
and 11. At higher generation (13, Figure 3) an ion corre-
sponding to MALDI fragmentation at the core could be
observed (calcd: m/z 16370, found: m/z 16339) in addition to
the molecular ion (calcd: m/z 65494, found: m/z 65430). This
fragmentation mechanism, removal of one dendron from the
core, has been typically observed with polyphenylene den-
1H NMR spectroscopic analysis (see Supporting Informa-
tion) is consistent with the growth of each new generation of
dendrimer (7, 9, 11, 13, and 15). The aromatic resonances for
the third-, fourth-, and fifth-generation dendrimers (11, 13,
and 15, respectively) were virtually identical, as the reso-
nances from the tetraphenylmethane core were diluted.
Resonances for the four inequivalent methyl groups (two
closely spaced near d = 2.3 ppm and two others further
separated at d = 2.13 and 2.10 ppm for 7) of the repeat unit
were nearly unchanged upon deprotection and growth to the
next generations, as they are too far from sites of reactivity.
Integration of the TiPS chain end resonances, relative to both
the aromatic backbone resonances and the methyl side-chain
resonances, is in agreement with complete conversion into the
dendrimer product. Size-exclusion chromatography (SEC)
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Scheme 3. Synthesis of the first- through to fifth-generation “exploded” polyphenylene dendrimers with a tetraphenylmethane core: a,c,e,g,i) o-
xylene, reflux; b,d,f,h) tetrabutylammonium fluoride trihydrate, THF, RT; a) 85%, b) 76%, c) 96%, d) 75%, e) 87%, f) 75%, g) 64%, h) 58%,
i) 65%.
drimers bearing a tetraphenylmethane core.[16] No ions could
be observed in the MALDI-TOF spectrum of the fifth-
generation dendrimer 15 using dithranol as matrix; however,
the MALDI-TOF spectrum obtained using DCTB[17] as
matrix (Figure 4) and sufficient laser power to observe any
ions showed a weak signal for the molecular ion (calcd:
m/z 134162, found: m/z 135290). Further proof of the exis-
tence of 15 came from the detected ions, which resulted from
a large degree of fragmentation and coalescence of the highly
reactive fragments with the molecular species. In addition to
fragmentation at the core (m/z 33948, 67980, 101273), an
additional fragmentation pathway was identified at the first
branching point away from the core (m/z 16975, 50971,
84626, 118281, and higher coalescence ions). Increasing the
relative amount of matrix or the introduction of cationization
agents did not improve the intensity of the molecular ion. The
fragmentation is possibly a result of the spectral overlap
between the nitrogen laser (337 nm) used for excitation of the
matrix and the UV absorbance for the higher generation
dendrimers (lmax = 300 nm). The fourth-generation den-
drimer 13 appears to be the limit for efficient generation
and detection of the molecular ion without substantial
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Table 1: Summary and comparison of the size characteristics for new polyphenylene dendrimers 7, 9, 11, 13, and 15 compared with hydrogen end-capped
general polyphenylene dendrimers A, B, and C.[8,16] [a]
Compound
Molecular
formula
Aryl
rings ends mass
[gmolÀ1
Chain Molecular MALDI-TOF SEC Mp(PS)[b] SEC Theo.
DLS
TEM
Dendrimer
MS [gmolÀ1
]
[gmolÀ1
]
PDI radius[c] radius[d] radius[e] density[f]
]
[nm]
[nm]
[nm]
[gmLÀ1
]
“exploded” dendrimers
C(Ph([G1](CꢀC-TiPS)2)4 (7)
C393H388Si8
C1041H964Si16
48
136
312
664
8
16
32
64
5336
13924
31101
65454
134162
5443 (Ag)
13935 (K)
31168
65430 (K)
135290
8000
22000
38900
83200
114200
1.04
1.05
1.03
1.05
2.5
4.2
6.9
9.0
2.0
3.6
5.2
9.0
11.8
–
–
–
–
11
0.26
0.12
0.09
0.04
0.03
C(Ph([G2](CꢀC-TiPS)4)4 (9)
C(Ph([G3](CꢀC-TiPS)8)4 (11) C2337H2116Si32
C(Ph([G4](CꢀC-TiPS)16)4 (13) C4929H4420Si64
C(Ph([G5](CꢀC-TiPS)32)4 (15) C10113H9028Si128 1368 128
1.10 11.0
general dendrimers
C(Ph([G2](H)4)4 (A)
C(Ph([G3](H)8)4 (B)
C(Ph([G4](H)16)4 (C)
C385H260
C865H580
C1825H1220
64
144
304
16
32
64
4886
10974
23150
4916 (K)
11113 (Ag)
23194 (K)
3900
7900
13800
1.04
1.03
1.05
1.8
2.5
3.2
1.5
2.2
2.9
–
2.6
3.0
0.57
0.41
0.38
[a] All MALDI-TOF mass spectra were acquired by irradiating the 1:250 analyte/matrix mixture with a nitrogen laser (337 nm) using dithranol as matrix and
optional cationization agent, a trifluoroacetate salt, if specified, except for that of 15 which was acquired using DCTB.[17] [b] SEC was performed at room
temperature in THF using SEC columns with 500, 104, and 106 Š porosities and calibrated against narrowly disperse, linear polystyrene (PS) standards.
[c] Theoretical radii were determined using MMFF94 molecular mechanics calculations, by evaluating the distance between the central carbon atom and the
chain end of the longest linear oligophenylene arm. [d] DLS experiments were performed using a krypton ion laser (647.1 nm), a toluene filled index matching
bath, and detected at 60, 90, 120, and 1508 with respect to the incident beam and were measured in THF for 7, 9, 11, 13, and 15 and in toluene for A, B, and
C.[8,16] The hydrodynamic radii were calculated using the Stokes–Einstein equation. [e] TEM samples were prepared by spin-coating 10À8–10À9 m CH2Cl2
solutions onto mica substrates and subsequently shadowed with a tungsten/tantalum alloy at high vacuum, followed by deposition of a thin carbon support.
The films were floated off the mica substrates onto a water surface and transferred to 600-mesh hexagonal copper grids for imaging. [f] The dendrimer densities
were calculating by dividing the molecular mass by the hydrodynamic volume determined from DLS studies, and are substantially lower for the new
polyphenylene dendrimer series, which suggests much higher porosities for the same series than those of the general polyphenylene dendrimers.
Figure 2. SEC traces for TiPS-ethynyl-functionalized polyphenylene den-
drimers 7, 9, 11, 13, and 15 in THF with 500, 104, and 106 Š column
porosities.
Figure 3. MALDI-TOF mass spectrum (dithranol, potassium trifluoro-
acetate) of C(Ph([G4](ethynyl-TiPS)16)4 (13); G4=fourth generation.
fragmentation for this family of dendrimers using these
experimental conditions.[18] While fragmentation from
MALDI is generally regarded as undesired, the iterative
nature of the dendrimer, and a clear fragmentation point
(such as at the core) enables conclusive evidence to be drawn
from such a “digestion.” All the detected ions from this
MALDI “digestion,” combined with the H NMR spectrum
of the intact dendrimer 15 unequivocally prove the complete
conversion into the fifth generation.
Transmission electron microscopy (TEM; Figure 5,
Table 1) and dynamic light scattering (DLS) studies
(Table 1) in THF were performed to gauge the sizes of the
dendrimers. The TEM samples were obtained by spin-coating
dilute CH2Cl2 solutions onto freshly cleaved mica substrates
and subsequent shadowing with a tungsten/tantalum alloy by
electron-beam evaporation at high vacuum, followed by
deposition of a thin carbon support.[8,19] The fourth-gener-
ation dendrimer 13 showed only filmlike aggregates, but the
molecules of fifth-generation dendrimer 15 were well-sepa-
rated with an average radius of 11.0 nm.[20] There is agreement
between the diffusional radius (11.8 nm) and the radius
calculated by MMFF94 molecular mechanics (11.0 nm), thus
suggesting that the chain ends are not appreciably backfolded
and that the shape is preserved in solution. Such agreement,
combined with surface accessibility of the chain ends, suggests
that the rigid, pentaphenylbenzene branching units provide a
conformational stiffness and directionality to the chain ends,
similar to that of the parent case.[21] The SEC elution behavior
(Table 1) clearly indicates how much more compact the
parent polyphenylene dendrimers (A, B, C)[8,16] are than are
the new dendrimers 7, 9, 11, 13, and 15. The polystyrene (PS)-
1
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evaluation of growth to a higher generation with complete
certainty, but the results are highly encouraging using
MALDI “digestion.” Perhaps the most compelling synthetic
observation is that, unlike the syntheses of the parent
polyphenylene dendrimers which necessitated several weeks
for the final growth reaction to go exclusively to completion,
the reaction conversion for the fifth generation required less
than 50 h. This result suggests that the steric demands at the
chain ends required by the Diels–Alder cycloaddition are well
below any critical threshold, completely unlike that of the
parent polyphenylene dendrimers. The control over the
topology of the branching units and the high porosity result
in efficient growth cycles that lead to structural perfection,
even for constructions containing 664 and 1368 phenyl rings
(for the fourth- and fifth-generation dendrimers 13 and 15,
respectively). To the best of our knowledge, this is the first
report of the stepwise chemical synthesis and proof of a truly
monodisperse molecule with a diameter greater than 20 nm.
Furthermore, no evidence was observed for the physical
encapsulation of the branching unit within the dendrimers
during the synthesis, thus suggesting that the cavities within
the structures are large and may be exploited as novel hosts
for large guests.[22] Efforts are underway to evaluate the
stiffness and to clarify the aggregate formation for the fifth-
generation dendrimer 15.
Figure 4. MALDI-TOF mass spectrum (DCTB) of C(Ph([G5](ethynyl-
TiPS)32)4 (15). G5=fifth generation.
Received: April 20, 2005
Published online: September 7, 2005
Keywords: cycloaddition · dendrimers · mass spectrometry ·
.
nanostructures · structure elucidation
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molecular ions with MALDI techniques had precluded
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Angew. Chem. Int. Ed. 2005, 44, 6348 –6354