Poly(anthrylenebutadiynylene)s: Precursor-based synthesis and band-gap tuning

Article abstract of DOI:10.1002/anie.200703409

(Chemical Equation Presented) Skipping the monomer: A highly efficient reductive aromatization reaction transforms an unconjugated precursor polymer into a high-molecular-weight, butadiyne-linked anthracene homopolymer (see scheme). Photophysical and elec

Full text of DOI:10.1002/anie.200703409

Communications
DOI: 10.1002/anie.200703409
Conjugated Polymers
Poly(anthrylenebutadiynylene)s: Precursor-Based Synthesis and
Band-Gap Tuning**
Mark S. Taylor and Timothy M. Swager*
The utility of conjugated polymers in such diverse applica-
tions as sensors, photovoltaic devices, light-emitting diodes,
field-effect transistors, and electrochromic devices stems in
large part from the ability of chemists to systematically
modify the properties of these materials through structural
variation. A powerful strategy for varying the electronic
structure of organic polymers involves modulating the degree
of quinoid character of the conjugated backbone. Decreasing
the energy difference between aromatic and quinoid reso-
nance structures results in a decreased band gap, as demon-
strated by Wudl and co-workers in the synthesis of poly-
(isothianaphthene), the first low-band-gap polymer to be
prepared.[1] Since this report, poly(isothianaphthene) has
served as the prototype for several low-band-gap polymers
based upon fused heteroaromatic repeat units.[2] In contrast,
low-band-gap polymers based upon acene repeat units are
unknown, despite the documented proquinoid character of
anthracene and the higher acenes.[3] We report herein the
photophysical and electrochemical properties of high-molec-
ular-weight, butadiyne-linked anthracene homopolymers pre-
pared by a new synthetic route involving reductive aromati-
zation of polymeric precursors.
group to allow a coplanar arrangement of adjacent repeat
units or arising from the rotationally symmetric nature of its
electronic structure. These polymers also emerged as attrac-
tive targets in light of the utility of Eglinton-type oxidative
couplings for homopolymerizations.[9] However, the requisite
9,10-bis(ethynyl)anthracenes are unstable compounds, which
poses a stumbling block for the preparation of well-charac-
terized, high-molecular-weight materials.[10]
To address this challenge, we explored a precursor-
polymer-based approach using an established procedure for
acene synthesis: the reductive aromatization of dihydroace-
nediols (Scheme 1).[11] This venerable method continues to be
The synthesis of anthracene-based polymers has been the
subject of intensive research effort.[4] Significant advances
include the preparation of poly(9,10-anthrylene)s,[5]
poly(9,10-anthrylenevinylene)s,[6] and poly(9,10-anthrylene-
ethynylene)s.[7] Despite their interesting properties, these
materials display moderate band gaps (greater than 2.0 eV),
which are a consequence of the sterically hindered nature of
the 9- and 10-positions of the anthryl group. Müllen and co-
workers have demonstrated that poly(9,10-anthrylene)s and
oligo(9,10-anthrylenevinylene)s adopt twisted conformations,
and this same group has shown that a substantial lowering of
the band gap is effected by enforcing a coplanar conformation
in ladder poly(p-phenylene-alt-9,10-anthrylene)s.[8]
Scheme 1. Strategy for PAB synthesis.
employed extensively for the preparation of acene hydro-
carbons, including such challenging targets as hexacene and
heptacene.[12] The preparation of oligo(9,10-anthrylenes), in
which reductive aromatizations were employed as key steps,
constitutes the most direct precedent for the proposed
synthesis.[5] Apart from monomer stability, additional advan-
tages could result from this approach. The mild aromatization
conditions could minimize the decomposition of these
electron-rich macromolecules, and the solubility of protected
dihydroacenediol-based polymers might facilitate the prepa-
ration of high-molecular-weight materials.[13]
We became interested in the possibility that poly(anthry-
lenebutadiynylene)s (PABs) could exhibit substantial deloc-
alization owing to the ability of the butadiyne functional
[*]Dr. M. S. Taylor, Prof. Dr. T. M. Swager
Department of Chemistry and
Institute for Soldier Nanotechnologies
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Monomers 2a–2d, each bearing four substituents to
maximize polymer solubility, were prepared from anthraqui-
nones 1a–1d (Scheme 2; see the Supporting Information).
Alkoxy-substituted compounds 2c and 2d were prepared in
order to investigate the possibility of decreasing the band gap
by raising the energy of the highest occupied molecular
orbital (HOMO). This sort of substituent effect has precedent
Fax: (+1)617-253-7929
E-mail: tswager@mit.edu
[**]This work was supported by the U.S. Army through the Institute for
Soldier Nanotechnologies, under Contract DAAD-19-02-0002 and
W911NF-07-D-004 with the U.S. Army Research Office.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 2. Synthesis of substituted PABs. i) (Triethylsilyl)acetylene, nBuLi, THF, 0!238C. ii) CH3I, nBuLi, THF, 0!238C. iii) Tetra-n-butylammo-
nium fluoride (TBAF), THF, 0!238C. 2a 64%, three steps. 2b 56%. 2c 22%. 2d 22%. iv) p-benzoquinone, [Pd(PPh3)4], CuI, iPr2NH, toluene,
608C. v) SnCl2, 1m HCl, CH2Cl2, 238C (see Table 1).
for several classes of conjugated polymers. Homocoupling
cocatalyzed by palladium and copper with benzoquinone as
oxidant yielded colorless, high-molecular-weight polymers
3a–3d (Table 1).
The absorption and emission spectra of 4a, 4b, and 4d
(Table 2 and Figure 1) are red-shifted relative to conventional
Table 2: Photophysical properties of 4a, 4b, and 4d.
[a]
[b]
[c]
[a]
[d]
[e]
Polymer abs lmax
abs lmax
Eg,optical
em lmax
FF tF
Table 1: Yield and GPC data for polymers 3a–3d and 4a–4d.
[nm]
[nm]
[eV]
[nm]
[ns]
[a]
[a]
Polymer
Yield
Mn
Mw/Mn
4a
4b
4d
590
615
709
548
639
813
1.9
1.9
1.5
639
649
–
0.08 0.4
0.02 0.2
3a
3b
3c
3d
4a
4b
4c
4d
89%
96%
32%
8.7”104
1.2”105
3.3”104
1.6”105
8.7”104
6.6”104
–
3.0
2.7
2.6
2.9
5.3
4.6
–
–
–
[a]CHCl 3. [b]Film. [c]Band gap, estimated by onset of absorption
spectrum. [d]Quantum yield relative to cresyl violet perchlorate. [e]See
the Supporting Information.
87%
>95%
>95%
decomp
>95%
5.5”104
2.3
poly(aryleneethynylene)s or poly(arylenebutadiynylene)s.
The band gap of both 4a and 4b, as estimated by the onset
of the absorption spectra, is 1.9 eV. This value is in relatively
good agreement with the estimated 1.7-eV band gap of
unsubstituted PAB determined by DFT calculations on a
series of oligomers (B3LYP/6-31G(d), see the Supporting
Information). The alkoxy substituents of 4d have a dramatic
effect, resulting in a band gap of 1.5 eV. This is a remarkably
low band gap for a polymer with an all-hydrocarbon back-
bone; of such polymers, only the poly(indenofluorene)s
display comparable HOMO–LUMO gaps (LUMO = lowest
unoccupied MO).[14] Polymer 4d is nonfluorescent, whereas
4a and 4b are weakly emissive in chloroform solution and
non-emissive in the solid state. The red-shifted solid-state
absorption spectra of 4b and 4d are likely a result of
aggregation of the polymer backbones through relatively
strong p interactions between acene repeat units.
[a]Determined by gel permeation chromatography (GPC) relative to
polystyrene standards. Mn =number-average molecular weight; Mw =
weight-average molecular weight.
Aromatization of polymer 3a to PAB 4a was accom-
plished by treatment with tin(II) chloride and 1m hydrochloric
1
acid. The reaction was monitored by H NMR spectroscopy
(see the Supporting Information) and was accompanied by a
gradual change in color, resulting in a purple solution at full
conversion. While the synthesis of alkyl-substituted 4b
proceeded smoothly, alkoxy-substituted 3c decomposed
under the aromatization conditions, likely because of the
Brønsted acidic conditions employed. In contrast, the other
alkoxy-substituted polymer 3d underwent clean aromatiza-
tion to furnish polymer 4d as a dark blue solid. Analysis of the
aromatized polymers by GPC indicated a slight decrease in
Mn relative to the precursor polymers, although high-molec-
ular-weight products were obtained in each case (Table 1).
This result may reflect the moderate solubility of the
aromatized polymers or may stem from undefined side
reactions that lead to polymer-chain cleavage.
Alkoxy-substituted 4d also displays unique redox behav-
ior. Whereas 4a and 4b underwent irreversible oxidation at
0.7 and 0.8 V, respectively (referenced to ferrocene/ferroce-
nium (Fc/Fc+); see the Supporting Information), 4d was
oxidized reversibly (onset at 0.25 V, Figure 2). The in situ
conductivity of 4d was measured using interdigitated micro-
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low band gap of 4d does not arise from a planar arrangement
of backbone atoms, which is prevented by the peri substitu-
tion pattern (Figure 3). It remains to be determined whether
the dramatic influence of the alkoxy substituents is a simple
ꢁ
Figure 3. Energy-minimized geometry of dimer 5 ((MeO)4(C14H5)C
C(C14H5)(MeO)4, B3LYP/6-31g(d), see the Supporting Information).
substituent effect or if the through-space interaction between
the alkyne p system and the nonbonding electrons of the
oxygen atoms plays a role in the low band gap of 4d. The fact
that peri-substituted anthracenes have served as a scaffold for
the study of electronic interactions between adjacent p elec-
tron systems would appear to support the latter proposal.[16]
To the extent that the peri substituents may also play a role in
the steric stabilization of electron-rich 4d and its derived
radical cation (as demonstrated by cyclic voltammetry), the
enforced proximity of p systems to a conjugated backbone
could prove to be a general strategy for the design of stable,
low-band-gap materials.
Figure 1. Normalized absorbance (i CHCl3, ii thin film) and emission
spectra (iii) of 4a (A), 4b (B), and 4d (C).
Received: July 28, 2007
Published online: September 27, 2007
Keywords: alkynes · conjugation · polymers · reduction ·
.
semiconductors
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