A simple orthogonal approach to poly(phenylenevinylene) dendrimers

Full text of DOI:10.1021/ja971790o

J. Am. Chem. Soc. 1997, 119, 9079-9080
Scheme 1. Syntheses of Dendritic Molecules
9079
A Simple Orthogonal Approach to
Poly(phenylenevinylene) Dendrimers
Shirshendu K. Deb, Todd M. Maddux, and Luping Yu*
Department of Chemistry and The James Frank Institute
The UniVersity of Chicago
5735 South Ellis AVenue, Chicago, Illinois 60637
ReceiVed June 2, 1997
Dendrimers possess a multibranched structure that radiates
out from a central core. The synthesis and characterization of
these unique molecules have been the focus of attention of
current polymer chemistry.1 These molecules are not only
aesthetically appealing but also possess some interesting physical
properties, such as unusual glass transition behavior and
viscosity, and an isolating effect with certain functional
molecules (e.g., porphyrin and rare earth ions).2,3 A wide range
of synthetic methodologies have been applied toward the
efficient syntheses of dendritic macromolecules.4 Two general
approaches have been developed to synthesize these mol-
ecules: a divergent growth approach and a convergent growth
approach.5,6 For the synthesis of well-defined dendritic mol-
ecules, the convergent approach is preferred, as exemplified by
the excellent works of Frechet, Moore, and others.7,8 A general
feature of these works is that the synthesis of each generation
of dendrons involves the protection and deprotection of certain
functional groups. Zimmerman disclosed an orthogonal ap-
proach in which aromatic moieties are alternatively linked by
flexible ester linkage and rigid acetylene linkages.9 However,
this approach leads to the formation of dendrimers that are not
homogeneous in chemical constituents due to the presence of
different linkages.
Scheme 2. Syntheses of G3.5 and Compound D
Recently, we described an orthogonal approach for the
synthesis of functionalized oligophenylenvinylenes in our
diblock copolymer project.10 In that approach two reactions,
the Heck and Horner-Wadsworth-Emmons reactions, were
alternatively utilized to construct a vinylene linkage. Due to
the fact that both reactions mutually tolerate the functional
groups involved, protection chemistry was not needed in each
step of growth. A logical extension of this approach is to the
synthesis of dendritic molecules bearing phenylenevinylene
linkages. We designed two AB2 type building molecules (A
and B, Scheme 1), one of which underwent the Horner-
Wadsworth-Emmons reaction and the other underwent the
Heck coupling reaction to produce higher generations of
dendrimer bearing vinyl linkages. This strategy allows us to
obtain homogeneous dendrons with a single functionality, which
is either a bromo or formyl group, at the focal point.
Benzylic bromination of 5-bromo-m-xylene followed by an
Arbuzov reaction yielded compound A. Compound B was
synthesized by monoformylation of 1,3,5-tribromobenzene
followed by exchange of the remaining two bromide groups by
vinyl groups via the Stille coupling reaction. Compound A
reacted with 3,5-di-tert-butylbenzaldehyde to obtain the first
generation dendrimer, which was highly soluble in many
common organic solvents such as CHCl3, CH2Cl2, THF, toluene,
etc. (see Scheme 1). The higher generations were obtained by
orthogonally using the Heck and Horner-Wadsworth-Emmons
coupling reactions between the preceding generation and the
appropriate building molecules (see Scheme 1). The yield using
the traditional Heck coupling conditions for the preparation of
G4 was poor.11 However, higher yields (45%) were obtained
using the protocol described by Meijere et al. which uses K2-
CO3 as a base and tetra-n-butylammonium bromide as a solid-
liquid phase transfer catalyst.12
Since each generation of the dendrimers is functionalized at
the central core, further chemical manipulation is possible. For
example, a generation 3 dendron can be used to react with
vinylbenzaldehyde to generate a molecule that has the aldehyde
functional group with less steric hindrance (G3.5, Scheme 2).
This molecule can then be used to couple with benzotriphosphate
(compound C) to obtain a molecule with a higher molecular
weight (compound D). Due to solubility problems, the yield
(1) (a) Issberner, J.; Moors, R.; Vogtle, F. Angew. Chem., Int. Ed. Engl.
1994, 33, 2413 (b) Ardoin, N; Astruc, D. Bull. Soc. Chim. Fr. 1995, 132,
875 (c) Tomalia, D. A. AdV. Mater. 1994, 6, 529.
(2) Frechet, J. M. J. Science 1994, 263, 1710.
(3) (a) Bhyrappa P.; Young J. K.; Moore J. S.; Suslick K. S. J. Am.
Chem. Soc. 1996, 118, 5708 (b) Dandliker P. J.; Diederich F.; Gross M.;
Knobler C. B.; Louati A.; Sanford E. M. Angew. Chem., Int. Ed. Engl.
1994, 33, 1739.
(4) (a) Vogtle F.; Weber E. Angew. Chem., Int. Ed. Engl. 1979, 18, 753.
(b) Buhleier E.; Wehner W.; Vogtle F. Synthesis, 1978, 155.
(5) (a) Tomalia D. A.; Naylor A. N.; Goddard, W. A., III. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 138. (b) Tomalia D. A.; Durst H. D. Top. Curr.
Chem. 1993, 165, 193. (c) Tomalia D. A. Aldrichimica Acta 1993, 26, 91.
(d) Moorefield C. N.; Newkome G. R.; Baker G. R. Aldrichimica Acta
1992, 25, 31.
(6) (a) Hawker C. J.; Frechet J. M. J. J. Chem. Soc., Chem. Commun.
1990, 1010. (b) Hawker C. J.; Frechet J. M. J. J. Am. Chem. Soc. 1990,
112, 7638. (c) Also see : Miller T. M.; Neenan T. X.; Zayas R.; Bair H.
E. J. Am. Chem. Soc. 1992, 114, 1018.
(7) Wooley K. L.; Hawker C. J.; Frechet J. M. J. J. Am. Chem. Soc.
1991, 113, 4252.
(8) (a) Zhang J.; Moore J. S.; Xu Z.; Aguirre R. A. J. Am Chem. Soc.
1992, 114, 2273. (b). Xu Z.; Moore J. S. Angew. Chem., Int. Ed. Engl.
1993, 32, 246.
(9) Zeng F.; Zimmerman S. C. J. Am .Chem. Soc. 1996, 118, 5326.
(10) Maddux, T. M.; Li, W. J.; Yu, L. P. J. Am. Chem. Soc. 1997, 119,
844.
(11) Heck, R. F. Org. React. 1982, 27, 345.
(12) Lansky, A.; Reiser, O.; De Meijere, A. Synlett 1990, 405.
S0002-7863(97)01790-3 CCC: $14.00 © 1997 American Chemical Society

9080 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Communications to the Editor
Figure 1. Mass spectrum (MALDI) of G4. The calculated molecular
weight is 4965.6. The molecular ion peak for compound D is shown
in inset. The theoretical value for that is 7673.8.
of this coupling process is low (9%), but molecule D can be
easily separated. It is a yellowish material with blue fluores-
cence. NMR and MALADI experiments have confirmed its
structure (see the inset of Figure 1).13
All of the dendrimers were characterized by using various
analytical techniques. 1H NMR spectra of the dentrimers
become complicated when the generations become higher but
the higher generations do possess the correct ratio of aliphatic
to aromatic protons. Monosubstitution was ruled out because
of the absence of a chemical shift caused by the terminal vinyl
proton or benzylic proton in the purified dendrimer. Dendrimers
G2, G3.5, and G4 all possess a proton chemical shift at about
10.15 ppm due to the formyl group. A chemical shift due to
one of the vinyl protons can be identified as a doublet at about
7.2 ppm.
Figure 2. UV/vis and fluorescence spectra of G1-G4. The fluores-
cence spectra were taken from solutions having UV absorbance close
to 0.2 AU and were normalized thereafter.
increases; the conjugation twists to different degrees from the
periphery to the core. This effect is reflected by the fact that
the extinction coefficient of G4 is almost identical with that of
G3. G3.5 has a longer conjugation system at the aldehyde
terminal and hence possesses a larger extinction coefficient.
These macromolecules fluoresce in the the near UV (G1) or
blue (D2-G4) region of the spectrum (see Figure 2). G1 showed
a low fluorescent quantum yield, probably due to effective
quenching by the bromine atom. G3 possesses a much higher
quantum yield than G1, although it also contains a bromine
group in the core position. G2, G3.5, and G4 all possess an
aldehyde group at the core and they exhibited similar fluores-
cence spectra. Their fluorescence quantum yields increase as
the generations increase.
G1 and G2 are crystalline materials with melting points at
150 and 350 °C, respectively. G3 and G4 did not show any
thermal transition between room temperature and 500 °C as
indicated by the DSC studies, both formed glassy materials.
In this work, there are several important points worth
emphasizing. First, the synthetic approach developed here
avoids the need for protection chemistry, which is needed almost
exclusively in all dendrimer syntheses, except for the work by
Zimmerman et al.9 The unique feature of this synthesis is that
the orthogonal approach leads to the formation of a homogenous
structure. This approach is also very versatile for the syntheses
of different dendritic structures, which can be achieved through
variation of the building molecules. The second point is that
the dendrimers obtained contain π-electron systems which can
be utilized in designing other functional materials, such as
photorefractive materials.
13C NMR spectra of these dendrimers were very crowded in
the 120-160 ppm region and it was difficult to assign every
peaks. The aliphatic primary carbons and the tertiary carbons
associated with the tert-butyl group appear around 32.5 and 35.9
ppm, respectively, for all of the dendrimers. The four different
carbons associated with the peripheral benzene unit with two
tert-butyl groups appeared around 152, 139.4, 123.3, and 121.9
ppm in each case. The aldehyde carbon for G2 and G4 had a
chemcal shift at 193 ppm.
Combustion results were in excellent agreement with the
theoretical composition within the experimental error.14 Firm
evidence for the molecular structure was obtained from mass
spectrometry results, using laser desorption ionization (MAL-
ADI). The mass spectra of all of the dendrimers showed the
molecular ion peak at the calculated position.13 Figure 1
exemplifies the mass spectrum of G4. The purity of these
dendrimers was confirmed by the SEC data where all showed
polydispersity close to 1.15 Since polystyrene was used as the
calibration standard, the molecular weights in all the cases were
more than doubled compared to the true value as obtained from
mass spectrometry (MALADI).
These molecules are almost colorless and the UV-vis
spectrum of the G1 dendrimer shows an absorption maxima at
306 nm (see Figure 2). A red shift was observed from G1 to
G2-G4 and was caused by substitution effect. Dendrimers
G2-G4 exhibit virtually identical absorption maxima at 320
nm because these molecules contain cross-conjugation, implying
that the major absorbing unit is stilbene in all the cases (the
λmax corresponding to the π-π* transition of stilbene is 294
nm). However, as the generations increase, the steric hindrance
In conclusion, the strategy outlined here enables us to
construct phenylenevinylene dendrimers with high molecular
weights and without carrying out any protection-deprotection
chemistries. The resulting dendrimers maintain the uniformity
of the structure in terms of linking units involved between the
generations. The reaction conditions are mild and simple and
can tolerate a large variety of functional groups. This approach
will be applicable to build up other structurally uniform and
well-defined conjugated functional dendrimers.
(13) MALADI results: G1, calcd for C38H49Br 585.7, found 584.7; G2,
calcd for C87H106O, 1167.8, found 1167.7; G3, calcd for C182H217Br, 2484.6,
found 2482.5; G3.5, calcd for C191H224O, 2535.9, found 2534.6; G4, calcd
for C375H442O, 4965.6, found 4962.6; compound D, calcd for C582H678
(trimer from G3.5) 7673.8, found 7666.8.
(14) G1: Calcd for C38H49Br, C, 77.93, H, 8.43, Br, 13.64. Found: C,
78.00, H, 8.42, Br, 13.58. G2: Calcd for C87H106O C, 89.48, H, 9.15.
Found: C, 89.49, H, 9.18. G3: Calcd for C182H217Br C, 87.98, H, 8.80,
Acknowledgment. This work was supported by AFOSR and the
National Science Foundation. Support from the National Science
Foundation Young Investigator programs is gratefully acknowledged.
This work was supported in part by the MRSEC Program of the
National Science Foundation under Award Number DMR-9400379.
Br, 3.22. Found: C, 87.97, H, 8.80, Br, 3.27. G3.5: Calcd for C191H224
O
Supporting Information Available: Experimental details and MS
spectra for all compounds (20 pages). See any current masthead page
for ordering and Internet access instructions.
C, 90.47, H, 8.90. Found: C, 90.22, H 8.95. G4: Calcd for C375H442O, C,
90.71, H, 8.97. Found: C, 90.88, H, 8.99.
(15) G1: Mn 1224, Mw 1227; PD 1.003. G2: Mn 2898, Mw 2907; PD
1.003. G3: Mn 5278, Mw 5293; PD 1.003. G4: Mn 13416, Mw 13718; PD
1.022.
JA971790O