Hyperbranched conjugated Polymers: Postfunctionalization

Article abstract of DOI:10.1021/ma902642a

In this contribution, the synthesis of two different hyperbranched poly(phenylene vinylenephenylene ethynylene) (HB-PPV-PPE, 3a-b) scaffolds are reported. The necessary AB₂ monomer 2 is obtained by an asymmetric Horner reaction and contains two

Full text of DOI:10.1021/ma902642a

2124 Macromolecules 2010, 43, 2124–2129
DOI: 10.1021/ma902642a
Hyperbranched Conjugated Polymers: Postfunctionalization
Chris Kub,† Juan Tolosa,† Anthony J. Zucchero,† Psaras L. McGrier,†
Chandramouleeswaran Subramani,‡ Abraham Khorasani,‡ Vincent M. Rotello,‡ and
Uwe H. F. Bunz*,†
†School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia
‡
30332, and Department of Chemistry, University of Massachusetts, 710 North Pleasant Street, Massachusetts
01003
Received December 2, 2009; Revised Manuscript Received February 1, 2010
ABSTRACT: In this contribution, the synthesis of two different hyperbranched poly(phenylene vinylene-
phenylene ethynylene) (HB-PPV-PPE, 3a-b) scaffolds are reported. The necessary AB2 monomer 2 is
obtained by an asymmetric Horner reaction and contains two iodine substituents and one ethynyl group. This
AB2 monomer provides, after Sonogashira polymerization, a hyperbranched conjugated polymer decorated
with iodine groups, suitable to be further functionalized with terminal alkynes 4a-t by a Pd-catalyzed
coupling. Elemental analysis of the postfunctionalized polymers 5a-t and 6a-q reveals nearly complete
substitution of the iodine groups. As a consequence of the replacement, substantial increases in fluorescence
quantum yields as well as variation in the optical response of the new polymers are observed. The
spectroscopic properties of the postfunctionalized polymers were investigated in solution and in the solid
state. The emission of the formed hyperbranched polymers shows a strong dependence on the functional
groups attached to the conjugated structure and ranges from 510 nm for 5a to 602 nm for 6h. In all cases, solid-
state emission is surprisingly strong and red-shifted compared with emission observed in solution.
Introduction
Herein12 we present polyfunctional and well-soluble conju-
gated HBPs prepared from the AB2 monomers 2a and 2b. The
Froma general perspective of polymer chemistry, there are two
ways to introduce functionalities into a polymeric structure:
functionalization of the monomeric unit or postfunctionalization
of an already formed polymer. The first method requires sig-
nificantly more synthetic effort if libraries or “diversets” of
polymers are desired because each polymer starts from its own
monomer. Therefore, efficient postfunctionalization pathways
are of great interest in terms of economy and versatility. Weck
and coworkers1 have used ring-opening metathesis polymeriza-
tion (ROMP) of functionalized norbornenes and cyclooctenes to
create a universal polymeric backbone containing a supramole-
cular connector. The addition of a molecule with a complemen-
tary functionality creates a host of new polymers in a mix-and-
match approach.
In conjugated polymers, such as poly(para-phenylene
ethynylene)s (PPEs),2 an efficient postfunctionalization should
also allow for manipulation of their electronic structure. Leclerc3
and Holdcroft4 have successfully postfunctionalized polythio-
phenes. Similar routes of postfunctionalization have been inves-
tigated in PPEs through 1,3-dipolar cylcoaddition of azides to
alkynes.5 However, in most cases, the modifications do not
directly affect the conjugated backbone because the reaction
occurs on nonconjugated side chains. Inherently, hyperbranched
polymers6 (HBPs) offer a possibility for postfunctionalization, as
in a polymer formed from n AB2 units, n reactive B groups will
remain. Described from a theoretical point of view as an inter-
molecular condensation of ABx monomers by Flory in 1952,7
HBPs have been exploited over the last several years in live-cell
imaging,8 solar cells,9 drug delivery systems,10 and surface
chemistry.11
properties of the formed HBPs (3a and 3b in Scheme 1), accessible
on a multigram scale, are tuned by a subsequent Pd-catalyzed
alkynylation reaction. We have prepared 24 derivatives of these
novel conjugated HBPs.
Results and Discussion
Scheme 1 displays the synthesis of polymers 3a and 3b.
Following the same approach used for the preparation of
asymmetrical cruciforms,13 monomers 2a and 2b were con-
structed by monoalkenylation of the diphosphonate to furnish
1a and 1b in acceptable yields. After a second Horner reaction
with 4-(trimethylsilylethynyl)benzaldehyde and subsequent de-
protection with potassium carbonate, the desired monomers 2a
and 2b were obtained. Sonogashira polymerization of 2a and 2b
leads to the hyperbranched polymers 3a and 3b in 87 and 81%
yield, respectively. These hyperbranched polymers are powdery
orange solids soluble in common organic solvents. Both show
expectedly low quantum yields (0.014 and 0.004, respectively) in
solution because of the heavy atom effect of the iodine groups
present in the polymeric skeleton. The molecular weight of 3a is
2.4 Â 104 with a polydispersity index Mw/Mn of 2.0 according to
gel permeation gel (GPC). GPC measurements of 3b show only
peaksassigned to aggregated species. We attempted to disrupt the
aggregation of the polymer 3b by catalytic hydrogenation of the
conjugated structure,14 but again, only signals due to aggregated
polymers were recorded. The incorporation of dialkylanilines
into the structure seems to induce the aggregation of the polymer
inside the column, hampering an accurate measurement of their
molecular weight. Despite the lack of a measurement of the
molecular weight of 3b, the 1H NMR spectra of 3a and 3b
show broad signals compared with those of the corresponding
*Corresponding author. E-mail: uwe.bunz@chemistry.gatech.edu.
Fax: 404-385-1795. Tel: 404-510-2443.
pubs.acs.org/Macromolecules
Published on Web 02/10/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 5, 2010 2125
Figure 1. 1H NMR spectra of monomers 2a and 2b (bottom) and polymers 3a and 3b prepared from them (top). Broad signals in 3a and 3b indicate a
comparable degree of polymerization.
Scheme 1. Synthesis of Hyperbranched Iodinated Polymers by Pd-Catalyzed Coupling
The yields are generally >85%, with all products isolated as
yellow-to-red powders (5a-t from 3a and 6a-q from 3b). In most
cases, NMR spectroscopy was not very useful to elucidate the
functionalization of the polymer because of the broad signals in
the aromatic region of the spectrum. Therefore, combustion
analyses were performed to assess the remaining amount of
iodine present in the polymer after the reaction and to have a
handle on the degree of the postfunctionalization (Table 1). In all
cases but 5n and 5f, the iodine content is < 1%. These values
indicate that >95% of iodine groups have been replaced by the
corresponding alkyne. A graphic of the number-average mole-
cular weight of the new polymers, as measured by gel permeation
chromatography (GPC), compared with their theoretical values,
Figure 2. Normalized absorption and emission spectra of solutions of
3a and 3b in dichloromethane.
if complete substitution of the iodines in 3a is assumed, also
confirms the high degree of functionalization (Figure 3). A linear
regression of these values shows a gradient (Mn /Mn ) near the
exp
th
monomers 2a and 2b (Figure 1), suggesting a similar degree of
polymerization for both polymers. Polymers 3a and 3b contain
one iodine substituent per repeat unit, however, different from
linear or truly dendritic polymers, these iodine groups are
statistically distributed over terminal and linear modules in the
formed hyperbranched polymers. The donor character of the
amino groups in 3b destabilizes its HOMO. As a result, a strong
bathochromic effect compared with 3a is observed in its absorp-
tion and fluorescent response (Figure 2).
We obtained a diverset of hyperbranched polymers 5 and 6 by
coupling 3a and 3b to the alkynes 4a-t (Scheme 2) using
Sonogashira conditions. The set of alkynes 4a-t includes donor
and acceptor species with different heteroatoms, aliphatic and
aromatic substituents, heterocycles, and complex structures such
as 4r, a T-shaped alkyne featuring two dibutylamino groups,
or 4t, a flavin derivative potentially useful in supramolecular
chemistry.15 All reactions were followed by a simple workup
consisting of the precipitation of the polymers into methanol.
expected 1 (y=1.1x).
To elucidate the effect of the substituents on the electronic
properties of polymers, we recorded their UV-vis and emission
spectra in solution and in the solid state. Figure 4 displays
photographs of solutions of the polymers 5 and 6 in dichloro-
methane as well as a selection of them in their solid state. The
pictures were taken under black light illumination at λ = 366 nm.
Qualitatively, we observe that emission of the polymers varies
from blue-green for 5a to orange-red for 6h. The strong solid-
state emission of the polymers shows bathocromically shifted
features compared with their spectra in solution, ranging from
yellow (5l) to red (6h), depending on the substitution. The
principal spectroscopic properties of 3a-b and the postfunctio-
nalized polymers 5a-t and 6a-q are shown in Table 1. Without
much changes in the absorption spectra, the incorporation of
different functional groups into the polymeric structure leads to a
general increase in the quantum yields up to a factor of 20, as a
consequence of the removal of the iodine groups. The derivatives

2126 Macromolecules, Vol. 43, No. 5, 2010
Scheme 2. Postfunctionalization of the Hyperbranched Polymer 3 by Pd-Catalyzed Couplings to Different Alkynes 4
Kub et al.
Table 1. Chemical and Spectroscopic Data of Polymers 5 and 6
λ
max absorption (nm)a
λmax emission (nm)
Φ (%)c
ε (L cm-1 mol-1
)
yield (%)
% iodo
3a
5a
5b
5c
5d
5e
5f
5g
5h
5i
392
397
397
397
397
375
396
395
396
395
395
389
396
386
386
392
407
378
394
437
441
442
354, 442
357, 438
362, 440
382, 441
510 (553)b
513 (558)
513 (557)
522 (565)
523 (560)
533 (570)
505
1.4
24
25
11
13
15
2.1
7.2
10
16
14
5.9
24
4.3
13
2.1
0.7
2.6
1.7
0.4
9.2
15
6.0
5.1
8.9
13
84 000
43 600
58 400
32 000
42 600
59 000
51 600
45 000
54 200
44 500
32 300
32 300
51 900
25 400
60 700
65 600
N/A
17.5
0.8
0.1
0.2
0.4
0.1
1.8
0.2
0.5
0.3
0.1
1.1
0.1
3.8
0.2
0.3
0.1
0.7
N/A
21.8
0.3
0.2
0.8
0.4
0.3
0.3
97
59
84
95
95
62
90
52
77
51
89
86
90
89
85
83
88
70
519 (578)
527 (582)
528 (568)
523 (571)
524 (559)
527 (562)
519 (560)
528 (565)
553 (569)
517, 595d (612)
502 (581)
504 (559)
586 (613)
593 (605)
587 (597)
593 (618)
602 (629)
593 (616)
592 (622)
5j
5k
5l
5n
5o
5p
5r
5s
5t
3b
6a
6b
6c
6h
6m
6q
8400
57 200
66 200
58 700
60 200
64 200
57 900
66 500
70 100
88
93
91
98
94
71
a Fluorescence measurements were performed in dichloromethane solutions of the polymers. b Wavelength of maximum emission of the polymers in
the solid state in parentheses. c Quinine sulfate was used as standard in quantum yield measurements. d 5r presents a significant shoulder around 595 nm.
5f, 5r, and 5t are exceptions to this behavior. The presence of
pyridinic nitrogen leads to an almost quenched emission in 5f and
5t. The fluorescence of 5r in dichloromethane shows a dual
emission with a main band centered at 515 nm and a shoulder
around 595 nm corresponding to 4r (Figure 5). The emissive
lifetimes of the two bands are significantly different (τ515 = 1.2 ns
and τ595 = 2.0 ns).16 According to the longer lifetime of the red-
shifted band, we postulate an inefficient energy transfer from the
polymeric backbone to the bis(dibutylaminostyryl) moiety in
solution to explain the low quantum yields of 5r. The energy
transfer is more efficient in the solid state, and this secondary
in the solid state but almost nonfluorescent in solution, reinfor-
cing our assumption of the efficient energy transfer.
From 5a to 5e, an extension of the conjugation causes a red
shift of the fluorescence. Naphthyl-substituted 5e is slightly
stronger bathochromically shifted than the indole-substituted
polymer 5k. The presence of electron-withdrawing groups also
induces a red shift, as is observed in 5g-i and especially in 6h, a
donor-acceptor system, which shows an emission maximum at
602 nm. Electron-donating substituents in the polymers prepared
from 3a cause a red shift of the fluorescence. On the other hand,
no significant effect is observed in those prepared from 3b,
because a strong donor is already present in the structure.
Polymer 5p shows the strongest bathochromic effect upon the
band appears to be the only one observed in solid state (λmax
=
612 nm). Wealso notefromFigure 4 that 5r isbrightly fluorescent

Article
Macromolecules, Vol. 43, No. 5, 2010 2127
emission in solution, whereas 5r is the most red-shifted polymer
of its family in the solid state. Sterics also have a significant
influence on the fluorescence of the polymers; 6a and 6b,
TMS- and TIPS-substituted polymers, respectively, show a
different fluorescence response both in solution and in solid
state. In the solid state, the bulkier substituent in 6b increases
the quantum yield and induces a blue shift in fluorescence
compared with 6a. This behavior is surprisingly not observed
for 5b and 5a.
In 5p and 6m, the pre- and postfunctional groups are similar
but switched. The amino groups in 6m seem to have a stronger
influence on the electronic structure of the polymer than in 5p,
with a significant red shift in both absorption (48 nm) and
emission (40 nm) of 6m compared with 5p. The difference in the
spectroscopic properties of 6m and 5p might be due to the
difference in connection of the dialkylamino functionalities to
the conjugated backbone. A recent study of the electronic
characteristics of substituted distyrylbenzenes and bisaryl-
ethynylbenzenes reveals a general red shift of the fluorescence
of distyrylbenzene derivatives compared with their corre-
sponding bisarylethynylbenzene pairs because of the vinylic
connection.17 However, the significant change observed in the
fluorescence of 6m and 5p has not been recorded in small
molecules; as a consequence, we assume that other factors
might play a role. An examination of the polymer structure
shows that in 6m, the dialkylamino groups are present in each
monomeric unit of the polymer, whereas in 5p, the amines are
located only where there were iodine groups in 3a, that is, not
necessarily on each monomer because the position of the
iodine groups depends on the degree of branching of the
hyperbranched polymers (Figure 6). Only in an ideal linear
polymer will every monomeric unit contain an amine, whereas
in a purely dendritic system, only the peripheral units will carry
two iodine substituents each. If we assume the theoretical 50%
degree of branching calculated for this type of monomer by
Frechet et al.,18 then the number of amino groups situated in
the inner part of the backbone will be much lower in 5p than in
6m. This difference in the distribution of amino groups might
play a critical role in the different optical gaps of these two
pseudoisomeric polymers.
Figure 3. Number-average molecular weight by GPC and theoretical
molecular weight-average number in postfunctionalization reactions
for derivatives of 5.
We attempted to confer water solubility to 5 by hydrolysis of 5i
to the corresponding carboxylate and by the deprotection of the
phenol group from 5l. Although the results are promising and
both 5i and 5l dissolve in mixtures of methanol/water and THF/
water, they are not completely soluble in water. In the future we
will use alkynes that carry more than one carboxylate group to
imbue water solubility to these polymers.
Because the frontier molecular orbitals of the polymer are
influenced by electron pairs such as those in dialkylamino groups,
their optical properties should vary upon metal complexation or
protonation. To explore this response, the effect of protonation
upon the photophysics of 5f, 5n, 5o, 5p, 5r, and 5t as well as upon
Figure 4. Photographs of fluorescence emission of polymers 5 and 6 in
dichloromethane solutions (top) and in the solid state (bottom) under
illumination using a hand-held blacklight (λem = 366 nm).
Figure 5. Emission spectra of 5r in dichloromethane solution (blue) and in solid state (red). Fluorescence lifetimes of the two bands observed in the
emission of 5r in dichloromethane are presented.

2128 Macromolecules, Vol. 43, No. 5, 2010
Kub et al.
Figure 6. Simplified representation of the pseudoisomers 5p and 6m
showing the different connectivities of the functional groups to the
polymer (top). Schematic view of the distribution of the alkynes after
postfunctionalization (purple spheres) depending on the degree of
branching (DB). Branching units (blue) do not bear postfunctional
groups (bottom).
Table 2. Absorption and Emission of the polymers upon addition of
TFA in dichloromethane
λ
max absorption (nm)a
λmax emission (nm)
Φ (%)
5f
396/396
386/355
386/395
392/358
407/368
394/389
441/388
442/384
442/388
438/388
440/386
441/378
505/quench
519/498
528/494
553/494
595/499
504/501
593/498
587/485
593/496
602/495
593/498
592/510
2.1/quench
4.3/0.9
13/0.4
2.1/5.8
0.7/10.8
1.7/0.5
9.2/62
15/66
5n
5o
5p
5r
Figure 7. Selected fluorescence emission of some of the dialkylamino
containing polymers (5o, 5p, 5r, and 6m) in dichloromethane (black
traces) and upon addition of TFA (gray traces).
5t
6a
6b
6c
6h
6m
6q
Conclusions
6.0/32
5.1/28
8.9/66
13/19
In summary, we have prepared the iodinated hyperbranched
polymers 3a and 3b. The facile modification of these polymers
was achieved by Sonogashira coupling; a wide set of alkynes was
incorporated into the structure. We have demonstrated that the
functionalization of the polymers occurs to near completion. We
have also shown that the position and distribution of the electron-
donating dibutylamino groups change the optical properties of
these hyperbranched polymers. The styryl groups introduced in
the first step of the synthesis have a strong influence on the
electronic properties of the polymers. In this regard, it would be
of great interest to introduce this functionality likewise in a latter
synthetic step to gain control of the styryl functionality once the
polymerization reaction is completed.
The demonstrated simplicity and versatility of the synthetic
strategy involved in the preparation of these hyperbranched
conjugated polymers is attractive. The use of novel methods of
functionalization as well as a better understanding of their
behavior will allow us to pursue further uses for these structures
in organic electronics, sensory applications, supramolecular
chemistry and for the synthesis of organic and inorganic hyper-
branched structures. Particularly attractive is the generation of
diversity in the last step; that is, starting from a common
precursor, a diverset of new and different polymers can be
obtained.
a Values on the right correspond to the protonated species.
6a-q was examined. The results are shown in Table 2. (All
spectra are presented in the Supporting Information.) Upon the
addition of an excess of trifluoreacetic acid (TFA) to solutions of
the polymers in dichloromethane, a strong blue shift in the
absorption is observed for the dialkylamino-substituted polymers
as well as for the triphenylphosphine-substituted derivative 5n.
These shifts are mirrored in their emission spectra, where hypso-
chromic shifts are also measured upon protonation. These shifts
are significant in 6a-q (the polymers prepared from 3b) and are
accompanied by a large increase in their quantum yields. Poly-
mers 5p and 5r also experience a modestincrease in their quantum
yields upon protonation of their amino groups. It is important to
note that all of the emission maxima of the protonated species of
the dialkylamino-containing polymers converge at around
495-500 nm regardless of the relative position of the amino
group in the polymer (Figure 7). Because the electron pairs of the
amino groups are no longer available, the fluorescence originates
from the “naked” conjugated backbone. In the case of 5f and 5t,
pyridine- and flavin-substituted derivatives, respectively, the
effects of protonation are almost imperceptible in absorption.
Regarding the fluorescence, quenching of 5f is observed, whereas
5t does not experience any change in either emission wavelength
or quantum yields.
Acknowledgment. We thank the Department of Energy,
Office of Basic Science (DE DE-FG02-04ER46141) for generous
financial support. J.T. thanks the Junta de Comunidades de

Article
Macromolecules, Vol. 43, No. 5, 2010 2129
Castilla La Mancha (Fondo Social Europeo FSE 2007-2013) for
his postdoctoral grant.
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Supporting Information Available: Experimental details are
provided for the synthesis of the polymers and alkynes reported
in this work as well as details for the photophysical experiments.
This material is available free of charge via the Internet at http://
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