Hyperbranched: A universal conjugated polymer platform

Full text of DOI:10.1002/anie.200900980

Communications
DOI: 10.1002/anie.200900980
Polymer Synthesis
Hyperbranched: A Universal Conjugated Polymer Platform**
Juan Tolosa, Chris Kub, and Uwe H. F. Bunz*
Post-functionalization strategies allow functional tuning late
in a reaction sequence, but classic polymer chemistry operates
on a different principle, as each polymer starts from its own
monomer.[1] Weckꢀs universal polymer backbone, by which a
simple polynorbornene carrying a supramolecular attachment
site is reversibly occupied by complementary species, is an
important step towards post-functionalization strategies.[2]
Another is the 1,3-dipolar cycloaddition of alkynes and
azides that allows functionalities to be clicked onto a polymer
backbone.[3]
In the case of conjugated polymers, post-functionalization
schemes that allow the manipulation of the electronic
structure by interposition of a suitable reagent are rare. End
functionalization is possible, but problematic, owing to the
low concentration of end groups. The synthesis of a hyper-
branched conjugated polymer by polycondensation of an AB2
monomer would circumvent this issue. Once formed, such a
hyperbranched polymer would have one functional reactive B
group per monomer. Such a polymer should—as long as the
polycondensation reaction that forms the polymer is irrever-
sible—allow the facile post-functionalization with any species
that carries a complementary functional group.
therefore soluble and processible, and 9 attains variable
functionalities.
Starting from 1, Horner reaction with 2a furnishes 3,
which, after a second Horner reaction with 2b and subsequent
deprotection, gives the monomer 4, which carries two iodine
groups. Classic Sonogashira polymerization of 4 in a mixture
of THF and piperidine with CuI as co-catalyst furnishes the
hyperbranched polymer 5 in 87% yield, with a molecular
weight of 2.4 ꢁ 104 and a polydispersity index Mw/Mn of 2.0
(Scheme 1, Figure 1). In a similar fashion, the model com-
pound 7 and the linear conjugated polymer 6 (Mn = 2.5 ꢁ 104,
Mw/Mn = 2.5, Scheme 2) were prepared (see the Supporting
Information). By coincidence, both polymers had a similar
molecular weight, which allowed a comparison of their
intrinsic viscosity in chloroform, namely [h] = 0.19 dLgÀ1 for
5 and [h] = 0.32 dLgÀ1 for 6.
Linear and hyperbranched[4] poly(phenyleneethynylene)s
and dendrimeric[5] species based upon 1,2,4-trisubstituted
benzenes have been made, but their potential with respect to
postfunctionalization schemes that manipulate electronic
structure and potential binding affinities to metal ions or
other potential analytes of interest has been surprisingly
rarely exploited.[6] The most thorough investigation of hyper-
branched PPEs was performed by Moore et al.,[7] but their
system, which is based on 1,3,5-triethynylbenzene units, was
not designed nor intended to show enhanced electronic
interactions, and they only reported post-functionalization of
an insoluble hyperbranched polymer with 3,5-bis(tert-butyl)-
phenylacetylene to obtain a soluble material. Weder et al.[8]
prepared truly conjugated branched PPEs with interesting
optical properties. Herein, we introduce a dodecyloxy group
into the monomer 4; the resulting polymers 5 and 9 are
[*] Dr. J. Tolosa, C. Kub, Prof. U. H. F. Bunz
School of Chemistry and Biochemistry, Georgia Institute of
Technology
901 Atlantic Drive, Atlanta, GA 30332 (USA)
Fax: (+1)404-510-2443
E-mail: uwe.bunz@chemistry.gatech.edu
[**] Published on the occasion of the 25th anniversary of the Max Planck
Institute for Polymer Research, Mainz.
We thank the Department of Energy-Basic Energy Sciences (DE-
FG02-04ER46141)for generous financial support. J.T. thanks Junta
de Comunidades de Castilla La Mancha (Fondo Social Europeo FSE
2007-2013) for his postdoctoral grant.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900980.
Scheme 1. Synthesis of the hyperbranched polymer 5. TMS=trimethyl-
silyl.
4610
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 4610 –4612

Angewandte
Chemie
Figure 2. Solutions of 5–7 and 9a–9d in dichloromethane under a
black light (lmax =365 nm) at a concentration of 10À5 molLÀ1
.
Table 1: Optical properties of 5–7 and 9a–9d.
lmax abs[a]
lmax em[a]
F [%]
e[b]
Yield [%]
7
6
5
9a
9b
9c
9d
337, 389
380
392
397
397
472
500
510
513
519
528
553
87
30
3
25
19
13
6
39800
66450
84000
43600
32600
60700
65600
na
na
na
97
84
89
85
386
392
[a] Maximum wavelength for absorption and emission in nm; all
measurements carried out in dichloromethane. [b] e in LmolÀ1 cmÀ1
per repeat unit for the polymers.
Figure 1. Synthesis and palladium-catalyzed post-functionalization of
5.
tion analysis, the polymers that formed had an iodine content
between 0.2 and 0.5 wt%, indicating efficient substitution. For
9a–9d, the quantum yields of emission were increased by a
factor of 3–10 (Table 1) as a consequence of the removal of
the iodine groups. The emission wavelength changed from
510 nm in 5 to 553 nm in 9d, documenting a significant red
shift of emission upon functionalization. The addition of a
donor group, as represented by the dibutylaniline unit in 9d,
has the strongest bathochromic effect upon the emission. The
position of the absorption maxima of 5 and 9a–9d are almost
unaffected and vary from 386 to 397 nm. Figure 2 shows
representative solutions of the model compound 7, the linear
polymer 6, 5, and 9a–9d (corresponding spectra are given in
the Supporting Information). It is important to note that all of
the hyperbranched polymers show a red-shifted absorption
and emission relative to 6 and to the model 7.
It was of interest to establish whether 9 could also support
blue-shifted emission. Upon protonation of 9d, a material
with green emission (lmax (em) = 487 nm) and an absorption
maximum (lmax (abs) = 346 nm) results, suggesting that the
optical properties of hyperbranched PAEs 9 can be engi-
neered not only to be more red-emissive, but also into blue- or
green-emissive materials. Protonation is not only an excellent
way to shift the electronic properties of the prepared hyper-
branched polymers, but it also indicates that the introduction
of electronegative residues into the hyperbranched polymers
should lead to hypsochromically shifted derivatives of 9.
An attractive aspect of hyperbranched conjugated poly-
mers is their branched character, which should increase the
rate of energy transfer, as a general matter of structural
principle.[9] A useful tool to test these polymers for increased
energy transfer through branching is the quenching of their
fluorescence by paraquat. Swager et al.[10] demonstrated that
quenching of conjugated polymers of the PPE type by
paraquat displays an approximately 80-fold increased effi-
ciency in comparison to that of small molecules, which is a
Scheme 2. Linear polymer 6 and model compound 7 along with
alkynes 8a–8d used for post-functionalization.
As expected, the hyperbranched polymer 5 is less viscous
than a linear polymer with the same molecular weight. As 5
contains one iodine atom per repeat unit (17.5% iodine by
combustion analysis), its fluorescence is quenched by the
heavy atom effect (Figure 2), but 5 is freely soluble in
dichloromethane and chloroform. We performed a Sonoga-
shira coupling of 5 with 8a–8d (Figure 1) to obtain 9a–9d
smoothly as yellow or orange powders after standard workup,
in yields that ranged from 84–97% (Table 1). From combus-
Angew. Chem. Int. Ed. 2009, 48, 4610 –4612
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4611

Communications
result of the molecular-wire effect. The factor of 80 is the
9 are amorphous and form free-standing films of excellent
maximum number of monomer units that is sampled by a
single exciton in a linear chain. Branching should increase the
efficiency of an exciton to sample polymer chains. Figure 3
quality.
Received: February 19, 2009
Revised: March 25, 2009
Published online: April 16, 2009
À
Keywords: alkynes · C C coupling · hyperbranched polymers ·
palladium · post-functionalization
.
[1] For poly(para-phenyleneethynylene)s and related compounds,
see: a) U. H. F. Bunz, Chem. Rev. 2000, 100, 1605 – 1644;
b) U. H. F. Bunz, Adv. Polym. Sci. 2005, 177, 1 – 52; c) see also
Poly(arylene ethynylene)s: From Synthesis to Application, Adv.
Polym. Sci. 2005, 177.
[2] a) L. P. Stubbs, M. Weck, Chem. Eur. J. 2003, 9, 992 – 999;
b) J. M. Pollino, M. Weck, Chem. Soc. Rev. 2005, 34, 193 – 207.
[3] a) R. Huisgen, Angew. Chem. 1963, 75, 604 – 637; Angew. Chem.
Int. Ed. Engl. 1963, 2, 565 – 598; b) R. Huisgen, G. Szeimies, L.
Mꢂbius, Chem. Ber. 1967, 100, 2494 – 2501; c) B. Helms, J. L.
Mynar, C. J. Hawker, J. M. J. Frechet, J. Am. Chem. Soc. 2004,
126, 15020 – 15021; d) B. C. Englert, S. Bakbak, U. H. F. Bunz,
Macromolecules 2005, 38, 5868 – 5877; e) R. J. Thibault, K.
Takizawa, P. Lowenheilm, B. Helms, J. L. Mynar, J. M. J. Frechet,
C. J. Hawker, J. Am. Chem. Soc. 2006, 128, 12084 – 12085;
f) V. D. Bock, H. Hiemstra, J. H. van Maarseveen, Eur. J. Org.
Chem. 2006, 51 – 68; g) W. H. Binder, R. Sachsenhofer, Macro-
mol. Rapid Commun. 2007, 28, 15 – 54; h) J. F. Lutz, Angew.
Chem. 2007, 119, 1036 – 1043; Angew. Chem. Int. Ed. 2007, 46,
1018 – 1025.
[4] a) Y. H. Kim, O. W. Webster, Macromolecules 1992, 25, 5561 –
5572; b) J. M. J. Frꢃchet, C. J. Hawker, I. Gitsov, J. W. Leon, J.
Macromol. Sci. Part A 1996, 33, 1399 – 1425; c) D. Hꢂlter, A.
Burgath, H. Frey, Acta Polym. 1997, 48, 30 – 35; d) B. Voit, J.
Polym. Sci. Part A 2000, 38, 2505 – 2525; e) A. Sunder, J.
Heinemann, H. Frey, Chem. Eur. J. 2000, 6, 2499 – 2506.
[5] a) Z. H. Peng, Y. C. Pan, B. B. Xu, J. H. Zhang, J. Am. Chem.
Soc. 2000, 122, 6619 – 6623; b) J. S. Melinger, Y. C. Pan, V. D.
Kleiman, Z. H. Peng, B. L. Davis, D. McMorrow, M. Lu, J. Am.
Chem. Soc. 2002, 124, 12002 – 12012; c) for thiophene-containing
dendrimers, see: G. Ramakrishna, A. Bhaskar, P. Bauerle, T.
Goodson, J. Phys. Chem. A 2008, 112, 2018 – 2026.
[6] Y. N. Li, G. Vamvounis, S. Holdcroft, Macromolecules 2002, 35,
6900–6906.
[7] a) P. Bharathi, J. S. Moore, Macromolecules 2000, 33, 3212 –
3218; b) P. Bharathi, J. S. Moore, J. Am. Chem. Soc. 1997, 119,
3391 – 3392.
[8] a) J. D. Mendez, M. Schroeter, C. Weder, Macromol. Chem.
Phys. 2007, 208, 1625 – 1636; b) E. Hittinger, A. Kokil, C. Weder,
Angew. Chem. 2004, 116, 1844 – 1847; Angew. Chem. Int. Ed.
2004, 43, 1808 – 1811.
[9] a) A. Bejan, Shape and Structure, from Engineering to Nature,
Cambridge University Press, Cambridge, 2000; b) D. Bray,
Science 2003, 301, 1864 – 1865.
Figure 3. Quenching of 9b (b) and 6 (c) by paraquat. See text
for details.
shows the quenching of 9b and linear polymer 6 by paraquat.
The Stern–Volmer equation is used, with F0/F[Q] = Ksv[Q] + 1,
where F0 denotes the fluorescence intensity without added
quencher Q, F[Q] is the fluorescence intensity of the solution in
the presence of the concentration [Q] of the quencher Q, and
Ksv is the Stern–Volmer constant.[11] In this equation, the
easily measured total concentration of quencher [Q] is used as
an approximation for its free concentration; for the condition
[fluorophore][Ksv] < 1, this approximation holds well, and Ksv
values of 2.0 ꢁ 102 and 4.1 ꢁ 102 result. These Ksv values are
low, as there is no binding element engineered into either 9b
nor 6. However, the hyperbranched polymer 9b is more
efficiently quenched by paraquat than 6 by a factor of about
two. We can compare the two values, as the molecular weights
of the two polymers are approximately equal and their degree
of polymerization is considerably below Pn < 80. In hyper-
branched polymers of higher molecular weight, this effect
should be even more distinctive. The immediate application
for hyperbranched polymers would be in sensors, where
efficient quenching is desired but the simple molecular-wire
effect, even if combined with multivalent interactions, is not
sufficient to obtain enough sensitivity.[12] Hyperbranching
should give a sensitivity boost to such systems.
In conclusion, 5 is easily functionalized into 9a–d in a
proof-of-concept study, making 5 and its derivatives 9 an
attractive proposition and competition for the more tradi-
tional conjugated polymers in sensory applications, but also
potentially in organic electronics, as the fluorescent polymers
[10] Q. Zhou, T. M. Swager, J. Am. Chem. Soc. 1995, 117, 12593 –
12602.
[11] R. L. Phillips, O. R. Miranda, D. E. Mortenson, C. Subramani,
V. M. Rotello, U. H. F. Bunz, Soft Matter 2009, 5, 607 – 612.
[12] a) I. B. Kim, B. Erdogan, J. N. Wilson, U. H. F. Bunz, Chem. Eur.
J. 2004, 10, 6247 – 6254; b) I. B. Kim, A. Dunkhorst, J. Gilbert,
U. H. F. Bunz, Macromolecules 2005, 38, 4560 – 4562.
4612
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 4610 –4612