Synthesis and optical properties of simple amine-containing conjugated polymers

Article abstract of DOI:10.1021/ma0478815

Conjugated polymers (CPs) containing amino groups have been synthesized, and their optical properties in both solution and thin film have been studied. New monomers required for the synthesis of these polymers have been readily prepared via efficient synt

Full text of DOI:10.1021/ma0478815

2716
Macromolecules 2005, 38, 2716-2721
Synthesis and Optical Properties of Simple Amine-Containing
Conjugated Polymers
Samuel W. Thomas, III, and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139
Received October 13, 2004; Revised Manuscript Received January 21, 2005
ABSTRACT: Conjugated polymers (CPs) containing amino groups have been synthesized, and their optical
properties in both solution and thin film have been studied. New monomers required for the synthesis of
these polymers have been readily prepared via efficient synthetic routes. These monomers have been
successfully polymerized with a variety of comonomers. The spectral positions of the absorption and
emission spectra correlate with the degree of electron density on the polymer chain. Polymers containing
N-alkylcarbazole units display similar optical properties in solution to most CPs. Polymers with
dialkylamino groups, however, display very different optical properties, including broadened absorbance
and emission spectra, larger Stokes shifts, and longer excited state lifetimes. These results are consistent
with a significant difference between the molecular geometries of the absorbing and emitting states. The
solid state emission of most of the polymers is sufficient to warrant them for consideration as fluorescent
sensing materials.
Chart 1. Amine-Containing Building Blocks Used
Introduction
Organic semiconductors have uniquely tunable optical
and electronic properties that can be tailored for par-
ticular applications. Important design principles natu-
rally depend on the application for which the conjugated
polymer is intended. Sensor systems based on conju-
gated polymers developed in our research group and
elsewhere typically rely on amplified signal transduction
by electron-transfer quenching from the conjugated
polymer to a bound analyte1 or by fluorescence reso-
nanance energy transfer from the polymer to an energy
acceptor.2 Amplification arises from the ability of delo-
calized photogenerated excitons to sample many poten-
tial binding sites within one excited state lifetime. Some
recent important design principles in this field have
included the incorporation of shape-persistent three-
dimensional structures to prevent aggregation of con-
jugated polymer chains in the solid state,1c,3 the intro-
duction of electron-poor substituents to reverse the
direction of electron transfer with electron-rich ana-
lytes,4 and the taking advantage of biological binding
events to give highly specific sensing.5
One of our recent goals has been to expand the
versatility of our conjugated polymer sensors to include
the ability to sense, via an amplified photoinduced
electron transfer quenching scheme, analytes that have
less favorable reduction potentials than nitroaromatics
and quinones. This requires the development of conju-
gated polymers with stronger electron-donating char-
acter and smaller work functions in order to induce
analyte binding and electron transfer. As part of our
efforts to realize this goal, we observed that there is
relatively little available literature concerning simple
amine-containing conjugated polymers. Notable excep-
tions are carbazole-containing conjugated polymers
(principally of the poly(phenylene) class), which have
recently received a large degree of interest in the organic
light emitting device field,6 as well as poly(bis(dialky-
lamino)phenylenevinylene)s.7 Herein we report the
synthesis and photophysical properties of a series of
simple amine-containing conjugated polymers, in which
the amine groups are part of the conjugated system.
Results and Discussion
Monomer Synthesis. The three basic amine-con-
taining synthetic building blocks used in the design and
synthesis of these polymers are illustrated in Chart 1.
These include the 2,7-disubstituted carbazole unit, the
2,5-disubstituted aniline unit, and the 2,5-disubstituted
p-phenylenediamine unit. Carbazole-containing conju-
gated systems polymerized through the 2- and 7-posi-
tions are preferable to the more easily prepared 3,6-
disubstituted systems because of a greater degree of
delocalization along the polymer backbone.6c,e There
have been efficient published syntheses of N-alkyl-2,7-
dichloro- or dibromocarbazoles for the purpose of mak-
ing polymers via metal-mediated cross-coupling chem-
istry.6c,8 Scheme 1 illustrates how the 2,7-diiodide 1 can
be obtained in 45% yield after recrystallization by
lithiation of the corresponding dibromide8 followed by
quenching with molecular iodine.
N,N-Dialkylanilines containing iodides or terminal
acetylenes for the synthesis of poly(phenylyeneethy-
nylene)s, heretofore unreported in the literature, can
be obtained in a straightforward and efficient manner
as shown in Scheme 1. Dialkylation of 2,5-dibromoa-
niline using sodium hydride and the appropriate alkyl
bromide gave the fully alkylated dibromoaniline in 84%
yield. Transhalogenation as described above gave the
diiodide 2 in 72% yield. The bis(TMS-acetylene) was
isolated in 64% yield by Sonogashira cross-coupling with
a small excess of (trimethylsilyl)acetylene. As a byprod-
uct of this reaction, the monoiodide illustrated in
* Corresponding author. E-mail tswager@mit.edu.
10.1021/ma0478815 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/05/2005

Macromolecules, Vol. 38, No. 7, 2005
Amine-Containing Conjugated Polymers 2717
Scheme 1. Synthesis of Amine-Containing Monomers for Cross-Coupling Polymerizations
Table 1. Polymer Solution Optical Properties (THF)
polymer
Mn (DP)a
abs λ(onset) (nm)
abs λ(max) (nm)
emis λ(max) (nm)
Φf
τf (ns)d
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
6700 (7)
6300 (8)b
11K (14)
16K (23)
32K (46)c
20K (19)
31K (33)
14K (18)
6600 (9)
4000 (4)
490
450
485
495
495
550
550
415
425
445
420
386
350
426
435
353
453
368
366
360
458
423
476
471
480
580
594
413
453
512
0.28
0.51
0.64
0.30
0.58
0.50
0.39
0.86
0.57
0.22
0.6
0.5
1.8
1.0
0.9
3.9
3.2
0.6
1.9
4.2
a Molecular weights were determined by gel permeation chromatography (GPC) and are reported relative to polystyrene standards.
b Reflects the molecular weight of only the soluble portion (∼50%). c Reflects the molecular weight of only the soluble portion (∼30%).
d Reported fluorescence lifetimes are fit to a single-exponential function.
Chart 2. Comonomers Used in Cross-Coupling
Polymerizations
Scheme 1 was also isolated in 16% yield. The regio-
chemistry of this byproduct was confirmed by analysis
of coupling constants and chemical shifts in the 1H NMR
spectrum. This is the expected byproduct, since a
combination of electron-donating ability and steric
influence of the amine retards cross-coupling reactions
at the ortho position. Each of these monomer precursors
could be readily deprotected to reveal the terminal
acetylenes 3 and 4.
Tetraalkylated diiiodophenylenediamines have been
reported previously.9 Scheme 1 shows that by carrying
out a Curtius rearrangement with the diacyl azide
(prepared from the corresponding diacyl chloride1b) the
diiodophenylenediamine was efficiently prepared. Tet-
raalkylation with a primary bromide in the presence of
sodium hydride and 15-crown-5 gave the desired diio-
dide monomer 5. This was also transformed into the
diacetylene monomer via a similar Sonogashira coupling/
deprotection sequence as described above to give 6.
Polymer Synthesis. These amine-containing mono-
mers were effectively polymerized using standard cross-
coupling polymerization methods. Table 1 summarizes
the molecular weights and degrees of polymerization.
Poly(phenyleneethynylene)s (PPEs) were prepared us-
ing tetrakis(triphenylphosphine)palladium and copper
iodide catalysts in the presence of an amine solvent. It
was found that a mixture of toluene and diisopropy-
lamine was a better solvent system than morpholine,
giving higher molecular weights and yields of the PPEs.
Several different comonomers with different steric and
electronic properties were used, as illustrated in Chart
2. Polymerization reactions with amine-containing aryl
iodides were less efficient that those with amine-
containing aryl alkynes, giving lower degrees of polym-
erization with longer reaction times and higher tem-
peratures. Whereas polymers prepared with amine-
containing dialkynes (P5 and P7) could be precipitated
after 6 h at 65 °C, P3 and P6 were polymerized for 24-
48 h at 90 °C. The presence of very strong electron-
donating groups on aryl halides typically has a detri-
mental effect on cross-coupling reactions.10
Similar reactivity trends were observed in the syn-
thesis of poly(phenylene)s. These polymers P8, P9, and
P10 were prepared via biphasic Suzuki coupling using
tetrakis(triphenylphosphine)palladium and potassium
carbonate in the presence of a phase transfer catalyst.
A diboronic ester of 9,9-dioctylfluorene was used as the
comonomer in these polymerizations. Whereas using a
standard p-dialkoxy diiodide under these conditions
gave a molecular weight of 14 000 (GPC, DP ) 18),
using the diiodoaniline 2 gave a more modest molecular
weight of 6600. The diamino diiodide 5 proved to be the

2718 Thomas and Swager
Macromolecules, Vol. 38, No. 7, 2005
Chart 3. Polymers Synthesized in This Study
least efficient coupling partner, giving P10 as a mixture
of oligomers with a number-average molecular weight
of 4000. In the case of poly(phenylene)s, optical proper-
ties were found to be very similar for fractionated (via
prep-GPC) high molecular weight polymer as for bulk
precipitated polymer.6d The structures of the polymers
studied in this work are displayed in Chart 3.
with a PPE containing only dialkylamino substituents.12
This effect can be attributed to a more varied collection
of polymer conformations in solution.
Also, in contrast to the carbazole-containing polymers,
several of the dialkylamino polymers presented here
also display solvatochromism in their absorption spec-
tra. Table 2 displays this effect in toluene and THF. This
is most easily visualized in Figure 3 with P3, which is
a copolymer PPE with alternating aniline and pentip-
tycene units. The maximum absorbance shifts from 338
nm in toluene to 350 nm in tetrahydrofuran (THF) to
358 nm in N,N-dimethylformamide (DMF). Comparison
of GPC traces of P3 in THF and DMF shows the same
degree of polymerization and no apparent aggregation.
The observation of solvatochromic behavior typically
indicates the presence of charge-transfer character. In
these polymers, there are no traditional electron-
withdrawing groups along the polymer chain and there-
fore no clear donor-acceptor CT states. This is indica-
tive of the strong donor character of the amine in the
PPE framework.
Solution State Fluorescence. Figures 1 and 2
display fluorescence spectra of several of the amino-
containing polymers in THF, while Table 1 summarizes
important fluorescence data of these polymers. As
expected, when more electron density is injected into
the polymer main chain, there is a corresponding red
shift of the emission spectrum. On an absolute scale the
poly(phenylene)s give blue-shifted emission spectra
relative to PPEs made up of electronically similar
monomers. For instance, comparing the emission spec-
tra of P3 and P9 shows a 23 nm blue shift for the poly-
(phenylene). PPPs are known to be a more blue-emitting
class of conjugated polymers relative to PPEs due to a
Solution Electronic Spectra. UV/vis spectra of all
the polymers synthesized in this study were acquired
in solution. As can be observed in Figure 1 and Table
1, the absorption stretches further to red as a function
of electron density in the polymer chain. This is expected
since increasing electron density in a conjugated poly-
mer reduces the oxidation potential, thereby raising the
energy of the valence band. This effect is well docu-
mented for moderately electron-donating alkoxy groups.11
Amine groups show this trend to an even greater extent.
PPEs containing carbazole in this study behave very
similarly to well-known dialkoxy-PPEs. They have a
relatively narrow absorption band in the near visible
or ultraviolet region of the spectrum and display little
solvatochromism. In contrast, the polymers that contain
dialkylamino groups in general do not follow these
trends. Most show very broad absorption bands with
local maxima in the ultraviolet and long tails that decay
well into the visible. P3, P4, P6, and P7 even display
what appear to be two distinct transitions in this
spectral region, with one occurring at about 350 nm and
the other in the visible. Initial speculation that this was
due to the presence of lower molecular weight oligomers
was overturned because samples (obtained via prep-
GPC) of high molecular weight (DP > 15) and narrow
polydispersity (PDI ∼ 1.05) showed the same spectra.
Similarly broad electronic spectra have been observed

Macromolecules, Vol. 38, No. 7, 2005
Amine-Containing Conjugated Polymers 2719
Figure 2. Emission spectra of fluorene-based poly(p-phe-
nylene)s P8, P9, and P10 in THF.
Figure 1. Absorption and emission spectra of the structurally
analogous PPEs P2 (carbazole), P3 (monoamine), and P6
(diamine) in THF.
Table 2. Polymer Solvatochromism
absorbance λmax (nm)
emission λmax (nm)
polymer
toluene
THF
toluene
THF
P3
P4
P5
P6
P7
P9
P10
338
425
434
350
453
366
350
350
426
435
353
453
366
360
462
467
478
561
582
445
500
475
471
480
580
594
453
512
decrease in effective conjugation along the polymer main
chain and higher aromatic character stemming from the
twisted biphenyl-type linkages.13
As was observed in the absorption spectra, there is
significant solvatochromism in the emission spectra of
some of the dialkylamino conjugated polymers studied.
The solvent-dependent emission of P3, illustrated in
Figure 3, shifts from 462 nm in toluene to 499 nm in
DMF. This corresponds to a stabilization of ∼0.2 eV,
similar to that observed in the absorption spectra. The
degree of solvatochromism is largest when the difference
between the electron-donating ability of the comonomers
(i.e., P3, P6, and P10) is maximized.
There are several other interesting and unique trends
in the emission of conjugated polymers containing
dialkyl amino groups. Whereas most conjugated poly-
mers display at least some vibronic structure in their
room temperature emission spectra (including the car-
bazole PPEs presented here), these conjugated polymers
with dialkylamino groups have spectra that are com-
paratively broad and featureless. This is accompanied
by a large Stokes shift, which is greater in magnitude
Figure 3. Solvent-dependent absorption and emission of P3.
for diamino polymers relative to monoamine polymers.
Finally, there is a dramatic lengthening of excited state
lifetime upon introduction of dialkylamino groups. This
is easily observed in structurally similar polymers P2,
P3, and P6 (pentiptycene-containing PPE class, from
0.5 to 3.9 ns), P1, P5, and P7 (dialkoxyphenylene-
containing PPE class, from 0.6 to 3.2 ns), and P8
through P10 (PPP class, from 0.6 to 4.2 ns).
This information taken together suggests a significant
conformational difference between a relatively twisted
absorbing ground state and the emitting excited state,
which possibly adopts a more planar excited state
conformation, in the dialkylamino polymers. Large
Stokes shifts accompanied by broadened emission spec-
tra are consistent with this model.1c,14 This is further
supported by a strong correlation between the Stokes

2720 Thomas and Swager
Table 3. Thin Film Optical Properties
Macromolecules, Vol. 38, No. 7, 2005
tion and emission spectra, in that there is minimal
bathochromic shifting in the thin film relative to solu-
tion,15 as well as retention of strong emission in the solid
state. These qualities are among those that have made
them so popular in PLED research.16 The PPE series
also shows interesting solid state behavior. Carbazole-
based polymers P1 and P2 are highly quenched in thin
film and also exhibit a new band that is highly red-
shifted relative to the solution emission, without a
corresponding change in the absorbance spectra. These
results are consistent with formation of an exciplex,
which carbazole is known to form with the emissive
layers of OLEDs.17
abs λmax emis λmax
abs λmax emis λmax
(nm)
polymer
(nm)
(nm)
polymer
(nm)
P1
P2
P3
P4
P5
420
385
352
451
450
570a
P6
P7
P8
P9
P10
356
480
376
380
362
557
581
415
462
499
460a
472
518
486
a New exciplex band in solid state.
Most of the dialkylamino PPEs, on the other hand,
still retain significant solid state emission intensity
upon spin-casting thin films. Excluding P4, the emission
spectra in the solid state are very similar in spectral
position and shape (shifts in spun-cast films were less
than 10 nm) relative to their solution spectra. Distinct
aggregation bands were not observed in the emission
spectra. This is true even for polymers P5 and P7 that
do not contain rigid pentiptycene groups. Nevertheless,
those polymers with pentiptycene groups, P3 and P6,
show a smaller red shift in the maxima of their
absorption spectra between solution and film (3-5 nm)
than do P5 and P7, which show a shift of 15-20 nm.
This highlights the ability of the pentiptycene moiety
to prevent interchain interactions in the solid state.1c
Figure 4 illustrates these properties for several of the
polymers studied. The well-behaved solid state photo-
physics of most of these polymers, even some of those
without pentiptycene moieties, make them candidates
for fluorescent chemosensing materials.
Conclusion. We have designed, synthesized, and
photophysically characterized a series of new simple
amine-containing conjugated polymers in which the
amines are part of the conjugated system. The color of
emission is easily tunable over a broad range as a
function of electron density on the polymer chain, with
more electron-rich polymers giving red-shifted emission.
The solution optical properties of carbazole-containing
PPEs are similar to most related polymers, while their
high degree of self-quenching in the solid state makes
them poor candidates for sensory applications. On the
other hand, PPEs and PPPs containing dialkylamino
groups display unique photophysical properties, includ-
ing larger Stokes shifts and longer excited state life-
times. Most are not strongly self-quenched in the solid
and therefore retain a significant degree of solid state
luminescence. The potential of these polymers in sens-
ing applications, particularly for analytes that have a
less favorable reduction potential than nitroaromatics
or quinones, is currently being evaluated.
Figure 4. Toluene solution (solid line) and thin film (dotted
line) absorption and emission spectra of P7 (top) and P6
(bottom).
shift and excited state liftetime. Longer excited state
lifetimes indicate slower rates of radiative decay, also
consistent with the smaller overlap of the vibronic wave
functions of structurally different ground and excited
states. Evidence for this significant of a structural
difference between ground and excited states is not
observed in more thoroughly studied dialkoxy conju-
gated polymers or in the carbazole-based polymers
studied here.
Thin Film Behavior. Absorption and emission spec-
tra of spun-cast thin films of these polymers from either
chloroform or THF were acquired. Many of the polymers
show adequate solid state brightness for use in sensory
devices. Table 3 summarizes some of the optical proper-
ties of these polymers in thin film. These polymers
follow some of the same trends in the solid state as in
solution. For instance, polymers with more electron-
donating groups are further red-shifted than those with
less. Dialkylamino polymers give broad and featureless
spectra, while polymers containing only alkoxy groups
retain more vibronic structure in their thin film emis-
sion.
Acknowledgment. The authors acknowledge The
Transportation Security Administration, The Technical
Support Working Group, Sandia National Laboratories,
and The Institute for Soldier Nanotechnologies for
support as well as Mr. Jean Bouffard for the donation
of 2,5-diiodo-1,4-dibenzoyl chloride.
Supporting Information Available: Experimental de-
tails for monomer and polymer synthesis as well as additional
absorbance and fluorescence spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
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