Synthesis of stair-stepped polymers containing dibenz[a, h]anthracene subunits

Article abstract of DOI:10.1021/ma902486y

Polycyclic aromatic monomers based on substituted dibenz[a,h]anthracene frameworks have been prepared in high yields. From these, several fluorescent stair-stepped conjugated polymers containing fused aromatic subunits have been synthesized via Sonogashira polycondensations and Glaser-type oxidative couplings. The new polymers were characterized by NMR spectroscopy, gel-permeation chromatography (GPC), UV-vis, and fluorescence spectroscopy.

Full text of DOI:10.1021/ma902486y

Macromolecules 2010, 43, 2789–2793 2789
DOI: 10.1021/ma902486y
Synthesis of Stair-Stepped Polymers Containing
Dibenz[a,h]anthracene Subunits
Julian M. W. Chan, Steven E. Kooi, and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,
Massachusetts 02139
Received November 9, 2009; Revised Manuscript Received February 5, 2010
ABSTRACT: Polycyclic aromatic monomers based on substituted dibenz[a,h]anthracene frameworks have
been prepared in high yields. From these, several fluorescent stair-stepped conjugated polymers containing
fused aromatic subunits have been synthesized via Sonogashira polycondensations and Glaser-type oxidative
couplings. The new polymers were characterized by NMR spectroscopy, gel-permeation chromatography
(GPC), UV-vis, and fluorescence spectroscopy.
Introduction
Scheme 1. Synthesis of Monomers 3 and 5
Poly(aryleneethynylene)s (PAEs)1 represent a class of conju-
gated polymers that has received much attention over the past
two decades due to their interesting electronic and photophysical
properties. In general, PAEs are highly fluorescent and relatively
photostable. These properties, along with their ease of synthesis,
make PAEs a popular class of conjugated polymers that has
found numerous technological applications, e.g., in the field of
chemical sensors.2 Thegeneral structureof a PAE, i.e., alternating
aromatic blocks and carbon-carbon triple bonds, allows for
great flexibility in the design and tunability of the polymer’s
physical properties. Harnessing the power and versatility of
organic synthesis, it is possible to prepare diverse monomers
containing virtually any degree of structural complexity, which
could then be used to synthesize a range of polymers with varying
properties. As a result of our recent work on dibenz[a,j]-
anthracene-based J-aggregating macrocycles,3 we became inter-
ested in incorporating the isomeric dibenz[a,h]anthracene motif
into conjugated polymers. Over the years, the aryl component of
PAEs has seen a wide variety of different aromatic groups, with
the simple benzene ring being the most common, i.e., poly-
(phenyleneethynylene)s (PPEs). Polycyclic aromatic moieties
such as anthracene have also been used frequently,4 and previous
work in our group has also employed more complex building
blocks such as dibenzochrysene5 and triphenylene.6 Inmost of the
aforementioned examples, the resulting polymers can be approxi-
mated to linear rigid rods, as a consequence of the para-substitu-
tion pattern between repeat units of the polymer backbone. With
6,13- and 5,12-linkage patterns on our dibenz[a,h]anthracene
building block, the resulting polymers would adopt a “stair-
stepped” structure instead, and we were curious as to how this
aromatic moiety, along with the unusual substitution patterns,
would affect the physical properties of the resulting polymers.
Herein, we report the synthesis of new monomers containing
dibenz[a,h]anthracene subunits and their successful incorpora-
tion into conjugated polymers.
double Suzuki coupling with 2 equiv of 4-hexylphenylboronic
acid under generic conditions led to the terphenyl derivative 2,
which was then converted to monomer 3 via a 2-fold electrophilic
cyclization brought about by treatment with iodine monochlor-
ide at -78 °C. Diiodide 3 can be converted to monomer 5 over
several steps, starting with the formation of dialdehyde 4 via a
lithiation/formylation procedure, followed by a Corey-Fuchs
homologation7 to establish two terminal alkynes. Straightfor-
ward conversion of diiodide 3 to dialkyne 5 via a Sonogashira
coupling/deprotection protocol tends to be unsuccessful, as we
have recently described elsewhere.3
Results and Discussion
The dibenz[a,h]anthracene-based monomers 3 and 5 were
synthesized in good yields from the same dibromide 1 over two
and four steps, respectively (Scheme 1). Subjecting dibromide 1 to
Diiodide monomer 3 was used in the preparation of polymers 8
and 9 via palladium-catalyzed Sonogashira polycondensation
with comonomers 6 and 7, respectively (Scheme 2). We reacted
*Corresponding author. E-mail: tswager@mit.edu.
r 2010 American Chemical Society
Published on Web 02/26/2010
pubs.acs.org/Macromolecules

2790 Macromolecules, Vol. 43, No. 6, 2010
Chan et al.
Scheme 2. Synthesis of Polymers 8 and 9
the simple dialkyne 6 with the sterically hindered diiodide 3 in
order to test the latter’s viability in polymer formation. This
turned out to be facile, giving the desired polymer in 89% yield. In
the second reaction, the sterically bulky monomer 7 (6,13-
diethynylpentiptycene), which had been previously used in the
production of nonaggregating poly(arylene ethynylene)s,8 was
successfully copolymerized with 3. The successful reaction of
these two hindered molecules attested to the reactivity and
viability of diiodide 3 as a monomer. Gratifyingly, polymers
were obtained in high yield in both cases, with 8 being isolated as
an orange solid and 9 as a lemon-yellow solid. We also attempted
a homopolymerization of monomer 5 using modified Glaser-type
coupling conditions with p-benzoquinone as the oxidant. Un-
fortunately, the yield in this case was much lower (ca. 60%), and
the purification process also proved challenging. As a result, only
a very minute quantity of the butadiyne-type polymer could be
isolated, precluding the possibility of rigorous characterization.
Furthermore, the polymer was found to have low solubility in
various organic solvents. In conclusion, using dialkyne 5 to form
a stair-stepped homopolymer appears to be unsuccessful. Based
on the above findings, the adequately substituted dibenz[a,h]-
anthracene motif can easily be incorporated into poly(aryle-
neethynylene)s via Sonogashira polycondensation, but accessing
butadiyne-type polymers using Glaser coupling proved to be
more problematic.
[a,j]anthracene-based macrocycles. Beginning with dibromide 1,
double Suzuki coupling with 4-methoxyphenylboronic acid af-
fords terphenyl 10 in good yield. The terphenyl is subjected to
iodine monochloride to produce the bis-spirocyclic compound
11, which is partially purified and subsequently converted to the
desired diiodide monomer via an acid-catalyzed rearrangement.
Using Sonogashira conditions identical to those used in synthe-
sizing polymer 8, monomer 12 was copolymerized with dialkyne 6
over 3 days to give poly(aryleneethynylene) 13. The conversion
occurred in high yield, and polymer 13 was isolated as a red solid.
Following the successful application of the dibenz[a,h]-
anthracene-based monomers 3 and 12 in Sonogashira polymer-
ization, we then explored other dibenz[a,h]anthracenes bearing
different substitution patterns. This led to the synthesis of
monomer 18, in which iodine atoms are situated at carbons 5
and 12 of the polycyclic arene instead of carbons 6 and 13, which
would produce a structurally different polymer backbone featur-
ing “stair steps” of gentler gradient. The synthesis of polymer 19
(Scheme 4) begins with the commercially available 1-bromo-2-
iodobenzene, which is subjected to an iodine-selective Sonoga-
shira coupling to give compound 15. For unknown reasons,
attempts to prepare terphenyl 17 by Suzuki coupling 2 equiv of 15
with benzene-1,4-diboronic acid were unsuccessful. However,
when 15 was first converted to boronic acid 16, followed by
Suzuki coupling with 1/2 equiv of 1,4-diiodobenzene, the terphe-
nyl derivative 17 could be obtained in reasonable yield. Subse-
quent double electrophilic cyclizations with iodine monochloride
were carried out on 17 to give the target monomer 18. The
diiodide was then copolymerized with dialkyne 6 to afford
polymer 19 (deep red solid) in high yield.
To investigate the effects of alkoxy vs alkyl substitution on the
photophysical properties of the resulting polymers, the analogous
monomer 12 was synthesized. This diiodide 12 can once again be
prepared using a sequence of transformations (Scheme 3) similar
to those used in the synthesis of our J-aggregating dibenz-

Article
Macromolecules, Vol. 43, No. 6, 2010 2791
Scheme 3. Synthesis of Polymer 13
Scheme 4. Synthesis of Polymer 19
Polymer 8 shows an absorption maximum (λmax) of 436 nm in
solution and is a blue-green emitter (Table 1, Figure 1). Its
solution photoluminescence spectrum shows peak emission at
450 nm, with a secondary peak at 483 nm. Polymer 9, which
incorporates the bulky pentiptycene unit, displays blue-shifted
absorption and emission relative to polymer 8. The steric bulk of

2792 Macromolecules, Vol. 43, No. 6, 2010
Table 1. Characterization and Spectroscopic Data for Polymers 8, 9, 13, and 19
Chan et al.
λ
max (nm)
lifetimes (bimodal) (ns)
λ
em (nm)
Φf
solutionc
(thin film)d
a
Mn/Mw
(103 g mol-1
solution
(ε/L mol-1 cm-1 b
solution
(thin film)
polymer
)
)
thin film
447
solution
thin film
8
21.5/50.2
436 (4200)
392 (21900)
448 (26600)
454 (35300)
450 (509)
430 (505)
457 (550)
507 (579)
0.29 (0.050)
0.34 (0.049)
0.32 (0.070)
0.23 (0.018)
1.0 (94%)
2.7 (6%)
2.0 (93%)
6.0 (7%)
1.4 (97%)
3.5 (3%)
1.1 (92%)
2.6 (8%)
1.4 (98%)
5.1 (2%)
1.7 (97%)
6.7 (3%)
2.0 (95%)
7.4 (5%)
1.2 (98%)
4.2 (2%)
9
10.6/25.7
19.0/72.0
10.4/17.4
336
457
450
13
19
a Determined by GPC using polystyrene standards. b Molar absorptivity based on the molecular weight of a repeating unit. c Against quinine sulfate in
0.1 M H2SO4 (Φf = 0.546, λex = 366 nm).9 d Against perylene in PMMA (Φf = 0.87, λex = 412 nm).10
Figure 1. UV-vis (solid lines) and fluorescence (dashed lines) spectra of polymers 8, 9, 13, and 19 in chloroform solution (black) and in thin films (red;
normalized).
the pentiptycenes probably reduces overall conjugation along the
polymer backbone by twisting the monomer subunits relative to
one another. These factors lead to the observation of pure blue
emission instead of blue-green. Another interesting feature of
polymer 9 is that unlike the pentiptycene-containing PPEs pre-
viously synthesized in our group,11 the presence of that bulky
comonomer does not seem to prevent aggregation, as evidenced
by the red shift and reduced fluorescence quantum yield of
polymer 9 films compared to the corresponding solutions. We
believe there are two reasons for this. First, unlike the pentipty-
cene-containing PPEs, polymer 9 incorporates rather large poly-
cyclic aromatic dibenz[a,h]anthracene units. By contrast, the
much smaller comonomers (single benzene ring) of our pentipty-
cene-containing PPEs are dwarfed by the larger pentiptycene
units, resulting in polymers whose backbones are exceedingly
well-shielded sterically, thus preventing aggregation and self-
quenching in concentrated solutions and thin films. With such
large π-surface areas present in polymer 9, the three-dimensional

Article
Macromolecules, Vol. 43, No. 6, 2010 2793
bulk of the pentiptycene units are unable to effectively shield the
polymer backbone, and as a result, interchain π-π stacking
between polycyclic aromatic units still occurs, producing bath-
ochromic shifts and lowered quantum yields. Second, it must be
noted the polymer 9 is structurally a stair-stepped polymer rather
than a PPE, whereby the two flanking pentiptycene groups are
connected to the dibenz[a,h]anthracene unit at different “levels”
of the “staircase” (vs para substitution in PPEs), resulting in
further reduction of the pentiptycene shielding efficiency. As for
polymer 13, the replacement of alkyl chains with alkoxy groups
(compare with polymer 8) in the dibenz[a,h]anthracene subunits
brought about a small red shift. Apart from this, the spectral
profiles of the structurally similar 8 and 13 are nearly identical.
Polymer 19, which features 5,12-substituted dibenz[a,h]anthra-
cene repeat units, experiences less intrachain steric repulsions
between the monomer repeat units compared to polymer 8. Thus,
increased conjugation and a corresponding bathochromic shift in
the absorption and fluorescence spectra were expected. This was
indeed observed, making polymer 19 the most spectroscopically
red-shifted polymer of the series, showing maximum absorp-
tion at 454 nm and green emission at 507 nm (with a shoulder at
541 nm).
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0001088.
Supporting Information Available: Complete experimental
procedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) (a) Bunz, U. H. F. Macromol. Rapid Commun. 2009, 30, 772–805.
(b) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605–1644.
(2) (a) Burnworth, M.; Rowan, S. J.; Weder, C. Chem.;Eur. J. 2007,
13, 7828. (b) Knapton, D.; Burnworth, M.; Rowan, S. J.; Weder, C.
Angew. Chem. 2006, 45, 5825. (c) Zhang, S. W.; Swager, T. M. J. Am.
Chem. Soc. 2003, 125, 3420.
ꢀ
(3) Chan, J. M. W.; Tischler, J. R.; Kooi, S. E.; Bulovic, V.; Swager,
T. M. J. Am. Chem. Soc. 2009, 131, 5659–5666.
(4) (a) Taylor, M. S.; Swager, T. M. Angew. Chem., Int. Ed. 2007, 46,
8480–8483. (b) Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048. (c)
Baumgarten, M.; M€uller, U.; Bohnen, A.; M€ullen, K. Angew. Chem.
1992, 104, 482–485. Angew. Chem., Int. Ed. Engl. 1992, 31,
448-451. (d) Weitzel, H.-P.; Bohnen, A.; M€ullen, K. Makromol.
Chem. 1990, 191, 2815–2835. (e) Weitzel, H.-P.; M€ullen, K. Makro-
€
mol. Chem. 1990, 191, 2837–2851. (f) Heun, S.; Bassler, H.; M€uller,
U.; M€ullen, K. J. Phys. Chem. 1994, 98, 7355–7358. (g) Ishow, E.;
Bouffard, J.; Kim, Y.; Swager, T. M. Macromolecules 2006, 39, 7854–
7858. (h) Egbe, D. A. M.; Cornelia, B.; Nowotny, J.; G€unther, W.;
Klemm, E. Macromolecules 2003, 36, 5459–5469. (i) Kaneko, T.;
Matsubara, T.; Aoki, T. Chem. Mater. 2002, 14, 3898–3906.
(j) Moroni, M.; Le Moigne, J.; Pham, T. A.; Bigot, J.-Y. Macromole-
cules 1997, 30, 1964–1972. (k) Swager, T. M.; Gil, C. J.; Wrighton,
M. S. J. Phys. Chem. 1995, 99, 4886–4893. (l) Yang, C.; Jacob, J.;
M€ullen, K. Macromolecules 2006, 39, 5696–5704.
Conclusions
Several new polycyclic aromatic monomers based on the
dibenz[a,h]anthracene motif have been synthesized from readily
available starting materials. These monomers are easily co-
polymerized with a variety of comonomers in cross-coupling
polymerizations to give fluorescent stair-stepped poly(aryl-
eneethynylene)s. These conjugated polymers exhibit high solu-
bilities as well as relatively blue-shifted absorption and emis-
sion maxima. We have found that the degree of blue-shifting
can be tuned by employing comonomers of varying steric bulk
and also by changing the substitution pattern on the poly-
cyclic aromatic component. These properties suggest potential
applications as blue-to-green emitters in various optoelectronic
devices.
(5) Yamaguchi, S.; Swager, T. M. J. Am. Chem. Soc. 2001, 123, 12087–
12088.
(6) Rose, A.; Lugmair, C. G.; Swager, T. M. J. Am. Chem. Soc. 2001,
123, 11298–11299.
(7) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 3769–3772.
(8) (a) Zhu, Z.; Swager, T. M. Org. Lett. 2001, 3, 3471–3474. (b) Zhao,
D.; Swager, T. M. Macromolecules 2005, 38, 9377–9384.
(9) (a) Olmsted, J., III J. Phys. Chem. 1979, 83, 2581–2584. (b) Demas,
J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024. (c) Melhuish,
W. H. J. Phys. Chem. 1961, 65, 229–235. (d) Dawson, W. R.; Windsor,
M. W. J. Phys. Chem. 1968, 72, 3251–3260.
(10) Melhuish, W. H. J. Opt. Soc. Am. 1964, 52, 183–186.
(11) (a) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321–
5322. (b) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120,
11864–11873. (c) Bouffard, J.; Swager, T. M. Macromolecules 2008,
41, 5559–5562.
Acknowledgment. This work was supported by the National
Science Foundation, Division of Materials Research. This
material is based upon work supported as part of the Center
for Excitonics, an EnergyFrontierResearchCenterfunded bythe