An N,N′-diaryl urea based conjugated polymer model system

Article abstract of DOI:10.1016/j.tetlet.2004.02.126

An N,N′-diphenyl urea was designed as a model system for aggregation phenomenon in poly(phenyleneethynylenes) (PPEs). The unmethylated N,N′-diphenyl urea adopts an open, unfolded conformation in which the two diphenyl acetylene fluorophores are far enough away, mimicking the unaggregated state. Dimethylation forces the aromatic surfaces together into π-π contact, mimicking the aggregated state of PPEs. Analogous to bulk PPEs, this model system shows dramatic differences in quantum yield between the folded and unfolded states, with the unfolded urea having greater than 400-fold higher fluorescence quantum yield than its folded equivalent.

Full text of DOI:10.1016/j.tetlet.2004.02.126

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 3229–3232
An N,N0-diaryl urea based conjugated polymer model system
Holly L. Ricks, Linda S. Shimizu, Mark D. Smith, Uwe H. F. Bunz and Ken D. Shimizu*
Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208, USA
Received 19 February 2004; accepted 25 February 2004
Abstract—An N,N0-diphenyl urea was designed as a model system for aggregation phenomenon in poly(phenyleneethynylenes)
(PPEs). The unmethylated N,N0-diphenyl urea adopts an open, unfolded conformation in which the two diphenyl acetylene flu-
orophores are far enough away, mimicking the unaggregated state. Dimethylation forces the aromatic surfaces together into p–p
contact, mimicking the aggregated state of PPEs. Analogous to bulk PPEs, this model system shows dramatic differences in
quantum yield between the folded and unfolded states, with the unfolded urea having greater than 400-fold higher fluorescence
quantum yield than its folded equivalent.
ꢀ 2004 Published by Elsevier Ltd.
Polymer morphology and supramolecular ordering can
have a dramatic effect on optical properties and thus,
can either improve or diminish the utility of a polymer
for a particular application.1 For example, poly(phenyl-
eneethynylenes) (PPEs) have a quantum yield in solution
approaching unity. On the other hand, solid state de-
vices based on thin films of PPEs have dramatically
lower quantum yields.2 These differences presumably
arise from the differences in the properties between the
aggregated and unaggregated PPEs. A better under-
standing of the emission properties of polymer aggre-
gates could, therefore, lead to improvements in PPE
based devices. However, the study of the aggregated
state of conjugated polymers is difficult because the
observed emission behavior of the bulk material arises
from a combination of different aggregated and unag-
gregated structures. One solution has been to design
model systems that fold the conjugated polymer onto
itself, quantitatively mimicking intramolecular interac-
tions of the aggregated state. Typically, two conjugated
polymer chains are attached to a rigid U-shaped tem-
plate, which forces the two chains into a parallel p–p
stacked conformation (Fig. 1a). Examples of appropri-
ate U-shaped templates including: 1,8-anthracene, 1,8-
naphthalene, para-phenylcyclophanes, meso-porphyrins,
ethane, and ortho-benzene.3
(a)
(b)
n
n
O
Me
Me
R
R
O
N
R
N
R
N
H
N
H
(trans- / trans-)
(cis- / cis-)
Figure 1. Representation of a model system for conjugated polymer
aggregation based on a rigid U-shaped template.
a model of the aggregated and unaggregated states.
Comparison of the respective emission properties of the
cis,cis and trans,trans conformers (Fig. 1b) can then be
used to help gauge the effects of aggregation behavior on
the emission properties of PPEs. Control of the diphenyl
urea conformation was effectively exerted by alkylation.
Unmethylated N,N0-diphenyl urea exists in the trans,
trans conformations in which the phenyl groups are in a
divergent ÔunfoldedÕ conformation.4;5b Dimethylation
reverses the conformational preferences as the sterically
larger methyl groups occupy the trans-position adjacent
to the carbonyl oxygen. This forces the two phenyl rings
together into the folded cis,cis conformation. Azumaya
and co-workers have demonstrated that folded cis,cis
ureas are effective U-shaped templates for the study of
p-stacking interactions.5c Bradley Smith has utilized the
ability to control the cis,cis to trans,trans equilibrium as
a supramolecular switch. Lewis et al. has also used the
We report, herein, a new PPE model system based on
an N,N0-diphenyl urea template that can serve as both
Keywords: Urea; Conjugated polymers; Fluorescence.
* Corresponding author. Tel.: +1-803-777-6523; fax: +1-803-777-9521;
e-mail addresses: shimizu@mail.chem.sc.edu; shimizu@psc.sc.edu
0040-4039/$ - see front matter ꢀ 2004 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2004.02.126

3230
H. L. Ricks et al. / Tetrahedron Letters 45 (2004) 3229–3232
folded shape of N,N0-dimethylureas to study molecular
structure and luminescence.5a These studies have dem-
onstrated that in the folded cis,cis conformer, the two
phenyl rings are in p–p contact and the p-systems can
effectively communicate, while in the unfolded trans,
trans conformer the two phenyl rings are far apart.5
Another attractive characteristic of the chosen model
system was that it is easily assembled from common
precursors (Scheme 1). N,N0-Diiododiphenyl urea 1 was
prepared by treatment of p-nitrophenyl chloroformate
with an excess of p-iodoaniline in triethylamine and dry
dichloromethane. This urea was coupled to phenyl
acetylene via a Sonogashira6 coupling to give the
unfolded urea 2.
1
Figure 2. H NMR spectra of the folded 4 and unfolded 2 in DMSO-
d6.
folded cis,cis conformation. However, the arms are not
completely parallel and are skewed due to the short
Conversion of 1 to the N,N0-dimethyl derivative 3, is
obtained by alkylation with methyl iodide, sodium
hydride, and potassium iodide in anhydrous DMF.
Compound 3 was then coupled to phenyl acetylene
under Pd/Cu catalyzed conditions to give folded urea 4.
ꢀ
N–N distance (2.34 A) of the urea template and the 53.9ꢁ
Nurea–Caryl tortional angle between the two arms. The
first aromatic rings of the diphenyl acetylene arms are
within p–p stacking distances; whereas, the terminal
ꢀ
phenyl rings are far apart (9.1 A). An interesting thing to
note is that only three Caryl atoms of the first phenyl ring
The expected folded cis,cis and unfolded trans,trans
conformations of 4 and 2 were verified by 1H NMR and
are within the necessary distance for p–p stacking
1
X-ray spectroscopy. Comparison of the H NMR from
ꢀ
(<3.6 A).
both the methylated (4) and unmethylated (2) showed
dramatic differences consistent with the expected folded
and unfolded conformations (Fig. 2). The aryl protons
ortho to the urea are shifted approximately 0.5 ppm
upfield for the methylated urea 4, which is characteristic
of p–p stacking and formation of the folded cis,cis
conformation.5b The same protons in urea 2 are further
downfield, which is consistent with an unstacked
trans,trans conformation.
Crystals of the unfolded unmethylated urea 2 could not
be solved. However, many examples of N,N0-diphenyl
ureas exist in the literature and they all are in the
trans,trans conformation.4 Molecular modeling of urea
2 is consistent with the literature and predicts that it will
be in the unfolded trans,trans conformation in which the
two diphenyl acetylene arms are in a divergent confor-
mation and cannot interact by p–p stacking.
The respective conformations were further established
from X-ray crystal structures and molecular modeling
(Fig. 3). Crystals of dimethylated urea 4 were grown
from tetrahydrofuran. The crystal structure of 4 is in the
Once the folded and unfolded structures of 4 and 2 were
established, their spectroscopic properties were exam-
ined in acetonitrile (Table 1). The UV–vis spectra were
consistent with previous studies on methylated and
O
NH2
I
Cl
O
NO2
Et3N
CH2Cl2, 0oC
85%
O
Me
Me
CH3I, NaH,
KI, DMF, 35%
I
I
N
I
N
I
O
N
N
H
H
3
1
(PPh3)2PdCl2
CuI, piperidine
THF, 73%
(PPh3)2PdCl2
CuI, piperidine
THF, 37%
O
Me
Me
N
N
O
N
N
H
H
2
4
unfolded
folded
Figure 3. Top (a) and side (b) views of X-ray crystal structure of folded
ꢀ
urea 4. Close contacts (<3.6 A) between the two diphenyl acetylene
arms are shown as blue lines.
Scheme 1. Synthesis of folded and unfolded ureas 4 and 2.

H. L. Ricks et al. / Tetrahedron Letters 45 (2004) 3229–3232
3231
unmethylated diphenyl ureas. The kmax of folded dime-
thylated urea 4 were typically blue shifted in comparison
with unfolded urea 2. This blue shift of the aggregated
model system is consistent with the folded poly(m-
phenyleneethynylene)s (mPPEs) studied by Moore.7
However, aggregation studies on poly(p-phenylene-
ethynylene)s (pPPEs) show the reverse trend with the
formation of a sharp aggregation band at higher wave-
length. It is perhaps not surprising that the urea model
system is more consistent with the mPPEs as they share
similar diphenyl acetylene chromophores.
To confirm that the differences in fluorescence of 2 and
4 arose solely from their different conformations, two
control experiments were conducted to rule out the
possibility of intermolecular aggregation and the pres-
ence of small amounts of quenching impurities. First,
the absorbance spectra of both 2 and 4 showed a linear
dependence of absorbance with respect to concentration
in the concentration range of the spectroscopic exper-
iments. This suggests that there is no intermolecular
aggregation in the concentration range of the fluores-
cence measurements. Secondly, to rule out the possibil-
ity of small amounts of quenching impurity in 4
resulting in the much lower observed fluorescence, the
fluorescence of a solution containing a mixture of 2 and
4 was measured and found to be similar to the sum of
the two independent fluorescent spectra. This suggests
that there are no significant quenching impurities in 4 as
their presence would have also lead to a much lower
observed fluorescence for the mixture.
Differences in the fluorescence spectra between 2 and 4
were even more pronounced (Fig. 4). Unfolded urea 2
had an intense fluorescence with emission maxima at
348 nm. Folded urea 4, in contrast, showed significantly
lower fluorescence with a broad peak centered at
454 nm. The dramatically lower fluorescence of the
aggregated model system is again consistent with that
observed by Moore in mPPEs. The mPPEs in their
unfolded state have a strong fluorescence maximum at
340 nm. Whereas, folded mPPEs have a very weak red
shifted fluorescence at 425 nm.7 Similar trends are also
seen in pPPEs, where the unaggregated state displays a
strong fluorescence at 430 nm and the aggregated state
displays a weak fluorescent band at 500 nm.
The difference in fluorescence between 2 and 4 could
arise from a combination of two sources. The first is
from quenching due to interaction of the two chromo-
phores in the folded conformation. The second possible
source is due to the loss of conjugation of the urea
nitrogen with the diphenyl acetylene chromophore upon
methylation. In the crystal structure of folded urea 4, the
urea nitrogen is twisted out of plane with the aryl ring.
The Curea–Nurea–Caryl–Caryl torsional angle averages
40.5ꢁ. To investigate the contribution of each of these
sources, a Ôone-armÕ model system 7 was synthesized in
which the urea nitrogen is twisted out of plane with the
aromatic ring but does not have a second arm thus
eliminating the contribution of the chromophore–chro-
mophore interaction.8
The fluorescence efficiencies were quantified by mea-
surement of the respective quantum yields in solution
(Table 1). Unfolded and folded ureas 2 and 4 had
quantum yields of 0.85 and 0.002, respectively. The
differences are similar in trend but not in magnitude to
what is observed for the unaggregated ðUf ¼ 1:0Þ and
aggregated ðUf ¼ 0:13Þ pPPEs.1c The 425-fold difference
in fluorescence efficiencies of 2 and 4 are again more
similar to the mPPE foldamers.7
First, urea 5 was synthesized by reacting p-iodoaniline
with dimethyl carbamyl chloride and piperidine in dry
dichloromethane to give 5 quantitatively (Scheme 2).
Methylation of 5 with iodomethane, sodium hydride,
and potassium iodide in dry THF yielded 6. Pd/Cu cou-
pling of 6 to phenyl acetylene yielded the one-arm urea 7.
Table 1. Spectroscopic maxima and quantum yields ðUf Þ of ureas, all
measured at room temperature in acetonitrile
Urea
Fluorescence
UV–vis kmax/nm
kf /nm
Uf
The absorbance and fluorescence spectra of the one-
armed urea 7 fall directly between those measured for
folded and unfolded ureas 4 and 2 (Table 1). For
example, the fluorescence maximum of 378 nm is
between the values of 454 and 348 nm for 4 and 2. This
suggests that both the loss in conjugation of the urea
2
4
7
323
294
305
348
454
378
0.85
0.002
0.11
350
300
250
200
150
100
50
I
O
O
CH3I, NaH, KI
I
NH2
Me
10
5
Me
N
N
H
N
Cl
THF
99%
piperidine, CH2Cl2
99%
Me
Me
5
O
Me
Me
O
N
N
0
350
450
550
Me
Me
Me
Wavelength (nm)
N
N
I
Me
0
300
400
500
600
(PPh3)2PdCl2, CuI
Wavelength (nm)
piperidine, THF, 98%
6
7
Figure 4. Fluorescence of ureas 2 (blue) and 4 (red) at 2.5 · 10ꢀ6 M at
23 ꢁC in acetonitrile.
Scheme 2. Synthesis of one-armed urea 7.

3232
H. L. Ricks et al. / Tetrahedron Letters 45 (2004) 3229–3232
with the diphenyl acetylene chromophores and the thru-
space interactions of the two chromophores contribute
to the lower fluorescence efficiencies of folded urea 4.
Based on the comparisons of their quantum yields, the
primary contributor to the lower fluorescence of 4 was
due to the thru-space interactions of the chromophores
as the differences in quantum yield between 7and 4 were
approximately 50-fold. The smaller contributions of the
disruption in the conjugation of the urea nitrogen and
the diphenyl urea chromophore is seen by the smaller
7-fold difference in quantum yield between 7 and 2.
References and notes
1. (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605–1644; (b)
Tretiak, S.; Saxena, A.; Martin, R. L.; Bishop, A. R.
J. Phys. Chem. B 2000, 104, 7029–7037; (c) Deans, R.; Kim,
J.; Machacek, M. R.; Swager, T. M. J. Am. Chem. Soc.
2000, 122, 8565–8566; (d) Koren, A. B.; Curtis, M. D.;
Kampf, J. W. J. Chem. Mater. 2000, 12, 1519–1522.
2. (a) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.;
Studer-Martinez, S. L.; Bunz, U. H. F. Macromolecules
1998, 31, 8655–8659; (b) Fiesel, R.; Halkyard, C. E.;
Rampey, M. E.; Kloppenburg, L.; Studer-Martinez, S. L.;
Scherf, U.; Bunz, U. H. F. Macromol. Rapid Commun.
1999, 20, 107–111.
3. (a) Iyoda, M.; Kondo, T.; Makao, K.; Hara, K.; Kuwatani,
Y.; Yoshida, M.; Matsuyama, H. Org. Lett. 2000, 2(14),
2081–2083; (b) Iyoda, M.; Nakao, K.; Kondo, T.;
Kuwatani, Y.; Yoshida, M.; Matsuyama, H.; Fukami,
K.; Nagase, S. Tetrahedron Lett. 2001, 42, 6869–6872; (c)
Crisp, G. T.; Turner, P. D. Tetrahedron 2000, 56, 8335–
8344.
In conclusion, models based on a N,N0-diaryl urea
template were studied to better understand the difference
in emission properties of PPEs in the aggregated and
unaggregated states. Analogous to behavior seen for
PPEs, a dramatically higher emission intensity was
observed for the unfolded urea that mimics the unag-
gregated state in comparison to the folded urea that
mimics the aggregated state. Specifically, unfolded urea
2, exhibits a greater than 400-fold increase in fluores-
cence efficiency over the folded urea 4. The conjugation
length is shorter in the folded urea due to the twisting of
the Caryl–Nurea bond and it is feasible that thru-space
interactions between the folded arms of 4 could be the
major contributing factor as to the decrease of fluores-
cence. Even though there is minimal contact between the
two arms, via only three aryl-carbons, this contact is just
enough to drastically decrease fluorescence efficiency
and red shift the fluorescent spectra.
4. (a) Etter, M. C.; Panunto, T. W. J. Am. Chem. Soc. 1988,
~
110, 5896–5897; (b) Etter, M.; Urbanczyk-Lipkowska, Z.;
Zia-Ebrahimi, M.; Panunto, T. W. J. Am. Chem. Soc. 1990,
112, 8415–8426.
5. (a) Lewis, F. D.; Kurth, T. L.; Liu, W. Photochem.
Photobiol. Sci. 2002, 1, 30–37; (b) Kagechika, H.; Azu-
maya, I.; Yamaguchi, K.; Shudo, K. Chem. Pharm. Bull.
1996, 44, 460–462; (c) Kagechika, H.; Azumaya, I.;
Tanatani, A.; Yamaguchi, K.; Shudo, K. Tetrahedron Lett.
1999, 40, 3423–3426.
6. (a) Cassar, I. J. Organomet. Chem. 1975, 93, 253; (b)
Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
Lett. 1975, 16, 4467.
7. Moore, J. S. J. Am. Chem. Soc. 2000, 122, 11315–11319.
8. (a) Kurth, T. L.; Lewis, F. D.; Hattan, C. M.; Reiter, R. C.;
Stevenson, C. D. J. Am. Chem. Soc. 2003, 125, 1460–1461;
(b) Bazan, G. C.; Oldham, W. J.; Lachicotte, R. J.; Tretiak,
S.; Chernyak, V.; Mukamel, S. J. Am. Chem. Soc. 1998,
120, 9188–9204; (c) Morisaki, Y.; Chujo, Y. Macro-
molecules 2002, 35, 587–589.
Acknowledgements
This work was supported by the NSF (PI Bunz; DMR
0134156, PI Shimizu; CHE 0316395).