Chiral alignment of OPV chromophores: Exploitation of the ureidophthalimide-based foldamer

Article abstract of DOI:10.1021/ja063405d

The ability of foldamers to adopt a secondary structure in solution has been exploited to organize peripheral functionality. Our previously reported poly(ureidophthalimide) foldamer proved to be an excellent scaffold for the chiral organization of periphe

Full text of DOI:10.1021/ja063405d

Published on Web 12/01/2006
Chiral Alignment of OPV Chromophores: Exploitation of the
Ureidophthalimide-Based Foldamer
†
†
†
Renatus W. Sinkeldam, Freek J. M. Hoeben, Maarten J. Pouderoijen,
‡
‡
‡
‡
Inge De Cat, Jian Zhang, Shuhei Furukawa, Steven De Feyter,
†
,†
Jef A. J. M. Vekemans, and E. W. Meijer*
Contribution from the Laboratory of Macromolecular and Organic Chemistry, EindhoVen
UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands and Laboratory
of Photochemistry and Spectroscopy, DiVision of Molecular and Nano Materials, Department of
Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200 F, B-3001 LeuVen, Belgium
Received June 2, 2006; Revised Manuscript Received October 6, 2006; E-mail: E.W.Meijer@tue.nl
Abstract: The ability of foldamers to adopt a secondary structure in solution has been exploited to organize
peripheral functionality. Our previously reported poly(ureidophthalimide) foldamer proved to be an excellent
scaffold for the chiral organization of peripherally positioned oligo(p-phenylenevinylene) (OPV) chro-
mophores. Facile high-yielding synthesis gave access to the required OPV-decorated building blocks. A
condensation polymerization provided polymers of sufficient length to allow construction of a helical
architecture comprising several turns. Short and long chains were separated by chromatography. Circular
dichroism studies in THF of the longer chains indicate the presence of helically arranged OPVs. However,
such an effect is not observed in CHCl
heptane. A bisignate Cotton effect is observed in heptane of a sample with a THF history. No Cotton effect
is observed in heptane of a sample with a CHCl history. In this example of supramolecular synthesis, the
3
. Remarkable are the measurements of the OPV foldamers in
3
solvent dictates the expression of supramolecular chirality in a secondary structure. The short-chain
oligomeric fractions that are unable to create a full turn revealed on scanning tunneling microscopy analysis
the presence of circular architectures at the graphite/1-phenyloctane interface. This is in full agreement
with the proposed conformation of the decorated foldamers.
4
,6
7
Introduction
recent literature. The folding of mPEs, oPE, and aromatic
8
electron donor-acceptor couples relies on solvophobic interac-
tions in combination with π-π stacking. Another class is
represented by the aromatic oligoamides reported by Huc et
al., Gong et al., and others, where π-π stacking is combined
1
Research on foldamers was pioneered by Gellman et al. and
2
Moore et al. for oligopeptides and abiotic, non-peptidomimetic
9
10
11
oligomers, respectively. The first abiotic oligomer capable of
folding was introduced by Hamilton et al. with his oligoanthra-
with hydrogen-bonding interactions to direct folding. The latter
3
nilamides, rapidly followed by the first oligo(m-phenylene-
4
ethynylene) (mPE) by Moore et al. The term “foldamer” is
used to categorize dynamic oligomeric systems that are capable
of folding into a conformationally ordered state in solution.
Through the years, the term was also used for structures that
are preferentially held in a helical ordered state while the
presence of a dynamic disordered state is less obvious. Since
the beginning, a large variety of synthetic foldamers have been
designed, each type utilizing a characteristic combination of
secondary interactions to direct the folding or well-defined
(5) (a) Block, M. A. B.; Kaiser, C.; Khan, A.; Hecht, S. Top. Curr. Chem.
2
005, 245, 89-150. (b) Sandford, A. R.; Yamato, K.; Yang, X.; Han, Y.;
Gong, B. Eur. J. Biochem. 2004, 271, 1416-1425. (c) Huc, I. Eur. J. Org.
Chem. 2004, 17-29. (d) Schmuck, C. Angew. Chem., Int. Ed. 2003, 2448-
2
2
452. (e) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173-180.
(6) (a) Matsuda, K.; Stone, M. T.; Moore, J. S. J. Am. Chem. Soc. 2002, 124,
1
1836-11837. (b) Brunsveld, L.; Meijer, E. W.; Prince, R. B.; Moore, J.
S. J. Am. Chem. Soc. 2001, 123, 7978-7984. (c) Prince, R. B.; Brunsveld,
L.; Meijer, E. W.; Moore, J. S. Angew. Chem., Int. Ed. 2000, 39, 228-
2
30. (d) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am.
Chem. Soc. 1999, 121, 3114-3121.
(
7) (a) Jones, T. V.; Blatchly, R. A.; Tew, G. N. Org. Lett. 2003, 5, 3297-
3
299. (b) Blatchly, R. A.; Tew, G. N. J. Org. Chem. 2003, 68, 8780-
8785.
5
(8) Nguyen, J. Q.; Iverson, B. L. J. Am. Chem. Soc. 1999, 121, 2639-2640.
conformation. A number of design strategies are prominent in
(
9) (a) Dolain, C.; Jiang, H.; L e´ ger, J. M.; Guionneau, P.; Huc, I. J. Am. Chem.
Soc. 2005, 127, 12943-12951. (b) Berl, V.; Huc, I.; Khoury, R. G.; Lehn,
J. M. Chem.-Eur. J. 2001, 7, 2798-2809.
†
Eindhoven University of Technology.
Katholieke Universiteit Leuven.
(
10) (a) Yuan, L.; Zeng, H.; Yamato, K.; Sanford, A. R.; Feng, W.; Atreya, H.
S.; Sukumaran, D. K.; Szyperski, T.; Gong, B. J. Am. Chem. Soc. 2004,
126, 16528-16537. (b) Yang, Z.; Yuan, L.; Yamato, K.; Brown, A. L.;
Feng, W.; Furukawa, M.; Zeng, X. C.; Gong, B. J. Am. Chem. Soc. 2004,
126, 3148-3162.
‡
(
(
(
(
1) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman,
S. H. J. Am. Chem. Soc. 1996, 118, 13071-13072.
2) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem.
ReV. 2001, 201, 3893-4011.
3) Hamuro, Y.; Geib, S. J.; Hamilton, D. A. J. Am. Chem. Soc. 1996, 118,
(11) (a) Hunter, C. A.; Spitaleri, A.; Tomas, S. Chem. Commun. 2005, 3691-
3693. (b) Tanatani, A.; Yokoyama, A.; Azumaya, I.; Takakura, Y.; Mitsui,
C.; Shiro, M.; Uchiyama, M.; Muranaka, A.; Kobayashi, N.; Yokozawa,
T. J. Am. Chem. Soc. 2005, 127, 8553-8561. (c) Hamuro, Y.; Geib, S. J.;
Hamilton, A. D. J. Am. Chem. Soc. 1997, 119, 10587-10593.
7
529-7541.
4) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997,
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2
10.1021/ja063405d CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 16113-16121
9
16113

A R T I C L E S
Sinkeldam et al.
thalimide)15 with OPVs since this type of chromophore has been
24
studied in great detail in supramolecular homo-assemblies and
2
5
hetero-assemblies. Furthermore, the large π-system of OPV
chromophores may help to stabilize the helical architecture.
Recently, we reported a general synthetic strategy to obtain an
2
6
array of 3,6-diaminophthalimides. These molecules form the
building blocks that give access to a collection of peripherally
functionalized foldamers. A high-yielding synthetic methodology
is described for the introduction of OPV3 and OPV4 chro-
mophores. Both chromophores possess chiral alkyl tails to allow
circular dichroism (CD) spectroscopy studies, and an additional
gallic acid derived moiety guarantees solubility in most common
organic solvents.
Figure 1. Poly(ureidophthalimide)s and a detailed hydrogen-bonding unit.
class is described in more detail in recent reviews.12 Less
common is the design based on oligo(aromatic urea)s: whereas
13
the ones reported by Gong et al. are present in a cisoid
conformation, those of Zimmerman et al.14 are forced in a
transoid conformation due to intramolecular hydrogen bonding.
Recently, we presented a helical foldamer based on poly-
Results and Discussion
Design and Synthesis. Target poly(ureidophthalimide)s
decorated with OPV3 (3a) and OPV4 (3b) were obtained by
26
(ureidophthalimide)s in which the urea linker adopts a cisoid
reaction of 3,6-diaminophthalimides 1a,b with the correspond-
ing diisocyanates 2a,b in refluxing toluene in the presence of
p-dimethylaminopyridine (DMAP) (Scheme 1).
conformation due to intramolecular hydrogen bonding (Figure
15
1
). Similar to the oligoamides, these poly(aromatic urea)s use
intramolecular hydrogen bonding to direct folding. Molecular
Monomers 1a,b are synthesized starting by reaction of 3,6-
bis(acetylamino)phthalic anhydride 4 with primary amines
16
modeling studies as well as an X-ray structure of a representa-
17
27 28
tive dimer strongly support the curved and coplanar conforma-
tion of the backbone. This curvature will lead to a helical
arrangement for longer oligomers with lengths exceeding 7-9
units, where it is proposed that π-π interactions further stabilize
the helical architecture once a turn is completed.
5a, b in refluxing dioxane for 17 h, which gives 3,6-bis-
2
6
(acetylamino)phthalimides 6 (Scheme 2). Subsequent amide
hydrolysis of 6 in the presence of aqueous HCl in refluxing
2
6
dioxane furnishes 3,6-diaminophthalimides 1a,b, which can
be quantitatively converted into the corresponding diisocyanates
2a,b by exposure to a 20 wt % solution of phosgene in toluene.
After polymerization, column chromatography on silica gel
with a gradient of chloroform to 10 v% ethyl acetate in
chloroform easily removes the nonmigrating DMAP and allows
separation of the longer from the shorter oligomers. The first
fraction 3aI contains oligomers of approximately 6-25 units
(66 wt %), whereas the second fraction 3aII consists of
oligomers with lengths between 2 and 7 units (15 wt %) based
A potentially interesting function for helical foldamers is the
exploitation of the inner void to host a guest. Depending on
the design, helical foldamers may possess an inner void of
18
19-21
sufficient size to accommodate ions or small molecules.
The efforts of this research may eventually lead to synthetic
trans-membrane ion channels.9b In addition to the exploitation
of the inner void, the helical architecture may also be a
promising candidate for the organization of peripheral func-
tionality. To the best of our knowledge, this opportunity has
not yet been addressed. Alignment of chromophores is of interest
in the field of molecular electronics such as organic photovol-
1
on GPC analysis (Figure 2). Although H NMR end-group
analysis has proven a most reliable method to estimate the
average oligomeric length, we found that GPC is more useful
2
2
23
1
taics and organic field effect transistors. In this article, we
report foldamers that utilize the periphery to align oligo(p-
phenylenevinylene) (OPV) chromophores in a chiral fashion.
We chose to decorate the previously reported poly(ureidoph-
in this case since the broad and overlapping signals in H NMR
2
9
1
hamper proper end-group analysis. Moreover, H NMR gives
the average length, whereas GPC gives more information on
the total distribution.
A similar separation on polymeric distribution 3b gave longer
OPV4-decorated ureidophthalimide oligomers 3bI with lengths
ranging from 5 to 21 units (61 wt %) and shorter oligomers
(
12) Huc, I. Eur. J. Org. Chem. 2004, 17-29.
(13) Zhang, A.; Han, Y.; Yamato, K.; Zeng, X. C.; Gong, B. Org. Lett. 2006,
8
, 803-806.
(
(
(
(
14) Corbin, P. S.; Zimmerman, S. C.; Thiessen, P. A.; Hawryluk, N. A.; Murray,
T. J. J. Am. Chem. Soc. 2001, 123, 10475-10488.
15) Van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer, E. W. Chem. Commun.
(24) A selection: (a) Hoeben, F. J. M.; Herz, L. M.; Daniel, C.; Jonkheijm, P.;
Schenning, A. P. H. J.; Silva, C.; Meskers, S. C. J.; Beljonne, D.; Phillips,
R. T.; Friend, R. H.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 1976-
1979. (b) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem.,
Int. Ed. 2003, 42, 332-335. (c) Schenning, A. P. H. J.; Jonkheijm, P.;
Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409-416.
(25) A selection: (a) Wolffs, M.; Hoeben, F. J. M.; Beckers, E. H. A.; Schenning,
A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 13484-13485.
(b) Schenning, A. P. H. J.; van Herrikhuyzen, J.; Jonkheijm, P.; Chen, Z.;
W u¨ rthner, F.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 10252-10253.
(c) Beckers, E. H. A.; van Hal, P. A.; Schenning, A. P. H. J.; El-Ghayoury,
A.; Peeters, E.; Rispens, M. T.; Hummelen, J. C.; Meijer, E. W.; Janssen,
R. A. J. J. Mater. Chem. 2002, 12, 2054-2060.
2
004, 60-61.
16) Sinkeldam, R. W.; van Houtem, M. H. C. J.; Pieterse, K.; Vekemans, J. A.
J. M.; Meijer, E. W. Chem.-Eur. J. 2006, 12, 6129-6137.
17) Katayama, H.; Kooijmans, H.; Vekemans, J. A. J. M.; Meijer, E. W.
Unpublished results.
(
(
18) Maurizot, V.; Linti, G.; Huc, I. Chem. Commun. 2004, 924-925.
19) (a) Goto, K.; Moore, J. S. Org. Lett. 2005, 7, 1683-1686. (b) Stone, M.
T.; Moore, S. J. Org. Lett. 2004, 6, 469-472. (c) Tanatani, A.; Hughes, T.
S.; Moore, J. S. Angew. Chem., Int. Ed. 2002, 41, 325-328. (d) Prince, R.
B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000, 122, 2758-2762.
20) Garric, J.; L e´ ger, J. M.; Huc, I. Angew. Chem., Int. Ed. 2005, 44, 1954-
(
(
(
1
958.
21) Hou, J. L.; Shao, X. B.; Chen, G. J.; Zhou, Y. X.; Jiang, X. K.; Li, Z. T.
J. Am. Chem. Soc. 2004, 126, 12386-12394.
(26) Sinkeldam, R. W.; van Houtem, M. H. C. J.; Koeckelberghs, G.; Vekemans,
J. A. J. M.; Meijer, E. W. Org. Lett. 2006, 8, 383-385.
22) Recent reviews: (a) Segura, J. L.; Martin, N.; Guldi, D. M. Chem. Soc.
ReV. 2005, 34, 31-47. (b) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004,
2
(27) The synthesis of 5a (OPV3-NH ) will be published elsewhere.
(28) (a) Peeters, E.; van Hal, P. A.; Meskers, S. C. J.; Janssen, R. A. J.; Meijer,
E. W. Chem.-Eur. J. 2002, 8, 4470-4474. (b) Syamakumari, A.;
Schenning, A. P. H. J.; Meijer, E. W. Chem.-Eur. J. 2002, 8, 3353-
3361.
1
9, 1924-1945.
(
23) Recent reviews: (a) Horowitz, G. AdV. Mater. 1998, 10, 365-377. (b)
Dodabalapur, A.; Katz, H. E.; Torsi, L. AdV. Mater. 1996, 8, 853-855. (c)
Sun, Y.; Liu, Y.; Zhu, D. J. Mater. Chem. 2005, 15, 53-65. (d) Kraft, A.
Chem. Phys. Chem. 2001, 2, 163-165.
(29) Dolain, C.; Grelard, A.; Laguerre, M.; Jiang, H.; Maurizot, V.; Huc, I.
Chem.-Eur. J. 2005, 11, 6135-6144.
16114 J. AM. CHEM. SOC.
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VOL. 128, NO. 50, 2006

Chiral Alignment of OPV Chromophores
A R T I C L E S
Scheme 1. Synthesis of Ureidophthalimide Foldamers Decorated with OPV3 (3a) and OPV4 (3b)a
a
(i) DMAP (1.0 equiv), PhCH3, reflux, 17 h.
Scheme 2. Synthesis of Polymer Precursors 1a,b26 and 2a,ba
a
(i) Dioxane, reflux, 17 h. 6a: 91%, 6b: 87%. (ii) HCl aqueous in dioxane, reflux, 4 h. 1a: 81%, 1b: 75%. (iii) COCl2 (20 equiv), PhCH3, rt to reflux,
∼
100%.
-1
3
bII with lengths of 1-5 units (7 wt %).30 This demonstrates
absorption to 401 nm (ꢀ ) 37 500 M-1 cm ). Both shifts are
3
2
that separation over silica gel is a facile method to remove the
shorter oligomers. Model studies and scanning tunneling
microscopy (STM) data published below have shown that
approximately 7-9 units may complete one turn.31 This means
that oligomeric lengths shorter than seven units will presumably
not contribute to the folding.
indicative for π-π stacking of the OPV units. When the
solution in CHCl3 was concentrated and redissolved in THF,
two CD effects were observed, of which the first was located
-4
at 328 nm (g ) -5.0 × 10 ) and the second displayed a
bisignate Cotton effect with a zero crossing at 399 nm and
-
4
maxima at 379 nm (g ) +4.4 × 10 ) and 416 nm (g ) -3.8
10 ). The zero crossing of this bisignate CD effect coincides
with the highest absorption maximum in the UV-vis spectrum
-4
×
UV-Vis and CD Spectroscopy of 3aI (OPV3). The nature
of the solubilizing π-conjugated peripherals enables UV-vis
and CD spectroscopic studies in various solvents. Thus, 3aI is
subjected to a study in CHCl3, THF, and heptane (Figure 3). A
solution in chloroform of 3aI displays two absorption maxima
-
1
-1
at 398 nm (ꢀ ) 43 700 M cm ). This absorption can be
attributed to the π-π* transition of the OPV3 unit, implying a
chiral stacking of the OPV3 moieties. It is important to mention
that 3,6-bis(acetylamino)-N-OPV3-phthalimide (6a),26 the pre-
cursor for diamino monomer 1a, shows no Cotton effect in THF
or CHCl3. Moreover, THF is known to be a good solvent for
isolated OPV3 chromophores, and thus aggregation is not
expected. Furthermore, the negative exciton coupling of the
bisignate Cotton effect suggests the presence of a left-handed
helical arrangement of the transition dipoles of the OPV
-
1
in the UV-vis spectrum located at 323 nm (ꢀ ) 38 700 M
-
1
-1
-1
cm ) and 405 nm (ꢀ ) 47 800 M cm ), which can be
attributed to the ureidophthalimide moiety and the OPV3 unit,
respectively. This solution proved to be CD silent, suggesting
an achiral conformation for the oligomers or a conformation
that lacks a preferred chiral orientation of the OPV side chains.
When the solution in CHCl3 was concentrated and the sample
was redissolved in heptane again, no Cotton effect was observed
although in the UV-vis spectrum a slight red shift of 6 nm
was observed for the first maximum to 329 nm (ꢀ ) 30 600
3
3,34
units.
In contrast to the measurement in heptane with a
chloroform history, the measurement in heptane with a THF
history does show a Cotton effect. This most remarkable
-1
-1
M
cm ) and a hypsochromic shift for the highest wavelength
(
32) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am.
Chem. Soc. 2001, 123, 409-416.
(30) GPC traces of OPV4-decorated oligo(ureidophthalimide) fractions 3bI and
(33) Jonkheijm, P.; Hoeben, F. J. M.; Kleppinger, R.; van Herrikhuyzen, J.;
Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2003, 125, 15941-
15949.
(34) Davydov, A. S. Zh. Eksp. Teor. Fiz. 1948, 18, 210-218.
3
bII are included in the Supporting Information.
(31) van Gorp, J. J. Helices by Hydrogen Bonding. Ph.D. Thesis, Eindhoven
University of Technology, Eindhoven, The Netherlands, 2004.
J. AM. CHEM. SOC.
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A R T I C L E S
Sinkeldam et al.
Figure 2. GPC analysis of (a) monomer 1a, (b) crude polymer 3a, (c) long oligomers fraction 3aI, and (d) short oligomers fraction 3aII. GPC is measured
in CHCl3 (UV detection at 420 nm) mixed-D column and polystyrene as internal standard.
Figure 3. UV-vis and CD spectra of longer oligomers fraction 3aI in (a) CHCl3, 2.4 × 10 M, (b) THF, 2.9 × 10-5 M, (c) heptane, 2.1 × 10-5 M with
-5
-
5
a CHCl3 history, and (d) heptane 2.3 × 10 M with a THF history.
observation implies the preservation of the secondary architec-
ture in the solid phase during concentration from THF and its
retention when subsequently redissolved in heptane. The
maximum of the first Cotton effect in heptane is located at 333
to heptane but is “erased” in chloroform. In addition to the
remarkable CD results, the UV-vis measurement clearly shows
a significantly lower (∼20%) molar absorption coefficient for
the long wavelength absorption of 3aI in heptane compared to
that in CHCl3 and THF. This hypochromic (decrease in
absorption intensity) effect is an indication for tight π-π
stacking that is also observed in DNA and π-stacked poly-
-
4
nm (g ) -6.6 × 10 ). The zero crossing of the bisignate
Cotton effect underwent a minor blue shift and is now positioned
-
4
at 395 nm with maxima at 373 nm (g ) +3.8 × 10 ) and 413
-
4
35,36
nm (g ) -4.2 × 10 ). The same blue shift is observed for the
absorption maximum of the OPV3 unit in the UV-vis spectrum
mers.
Above all, these experiments demonstrate that heptane is not
capable of inducing nor disrupting a chiral secondary architec-
ture but rather dissolves a preformed aggregate. This is in
agreement with temperature-dependent CD measurements in
THF and heptane (Figure 4). The gradually decreasing Cotton
effect in THF upon increasing the temperature, which is
reclaimed upon cooling, demonstrates the dynamics of the
secondary architecture in THF. A melting curve could be
constructed from the temperature-dependent Cotton effect at 415
-
1
-1
now located at 392 nm (ꢀ ) 37 200 M cm ). Furthermore,
-
1
-1
a small bathochromic shift to 333 nm (ꢀ ) 33 000 M cm )
is observed for the short wavelength absorption. Another
fascinating feature of this system is the fact that the CD effect
is lost upon concentration of the solution in heptane and
redissolution of the solid in chloroform. However, concentration
of the latter and redissolution in THF regenerate the Cotton
effect. This cycle can be repeated without observable changes
in the UV-vis and CD spectra. Seemingly THF is capable of
forming a chiral secondary architecture that can be transferred
(
35) Nakano, T.; Yade, T. J. Am. Chem. Soc. 2003, 125, 15474-15484.
(36) Schuster, G. B. Acc. Chem. Res. 2000, 33, 253-260.
16116 J. AM. CHEM. SOC.
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Chiral Alignment of OPV Chromophores
A R T I C L E S
-
5
Figure 4. Temperature-dependent CD measurements of solution of 3aI in (left) THF, 3.1 × 10 M from 20 °C (black line) in steps of 5 °C (gray lines)
-
5
to 55 °C (black line) and (right) heptane, 3.2 × 10 M with a THF history at 20 °C (black line) and 85 °C (dashed line).
Figure 5. Concentration-dependent UV-vis (left) and CD (right) spectra of 3aI in THF at 20 °C ranging from 5 × 10 M (solid black line) to 5 × 10-6
-4
M (dashed black line) and intermediate steps (gray lines).
Table 1. IR Absorptions of 3aI, NEAT from Different Solvents (ν
But the values differ from drop cast films of 3aI from CHCl3
and heptane with a CHCl3 history solution. Although the
differences are small, the results are in agreement with the CD
spectroscopy results in the sense that both solvents induce a
different secondary organization affecting the urea conformation.
However, for all solvents the IR spectra indicate strong hydrogen
-
1
in cm
)
urea
−H
imid C
dO
urea CdO
THF
THF f heptane
CHCl3
3342
3343
3347
3347
1727
1728
1724
1724
1699
1699
1696
1696
CHCl3 f heptane
3
8
bonding for the urea hydrogens and, hence, a cisoid urea
conformation. This is in full agreement with the molecular
1
6
17
nm in THF and is depicted in Figure 8 (vide infra). This is in
strong contrast to the temperature-dependent measurements in
heptane that clearly show stability of the secondary architecture.
Moreover, similar measurements in dodecane under prolonged
heating at 100 °C hardly altered the Cotton effect.
modeling and X-ray structure of a dimer. Therefore, the
differences in CHCl3, THF, and heptane are the result of subtle
organizational effects of both the backbone and the periphery.
These results imply that, despite the absence of a Cotton effect
in solution, a complete unfolding in CHCl3 is unlikely.
The UV-vis spectra of 3aI in THF at concentrations ranging
A final subjective but noteworthy remark is the observation
that solubilizing 3aI in heptane seemed to be easier from a
sample with a THF history than from one with a CHCl3 history.
This can be rationalized by considering the putative helical
organization in THF that shields the relatively polar ureidoph-
thalimide core from the apolar heptane. However, this is not
the case when 3aI has a chloroform history in which there is
no such shielding effect of the polar core due to folding. Solvents
with similar hydrogen bond accepting capabilities might harvest
the same effect. Although dioxane solution is structurally related
to THF, CD analysis of a dioxane solution of 3aI with a CHCl3
history reveals no Cotton effect. Measuring a dioxane solution
with a THF history also did not display a Cotton effect.
Although not explanatory, these results substantiate the unique
role of THF in the folding process.
-
4
-6
from 5 × 10 to 5 × 10 M display no differences in the
position nor in the intensity of the absorption maxima. Despite
the minor changes, the concentration-dependent CD spectra of
3
aI in THF at 20 °C reveal an almost concentration-independent
Cotton effect (Figure 5). This suggests that the Cotton effect is
the result of an intramolecular organization of the OPV3
chromophores and that interfoldamer aggregation is not respon-
sible for the chirality in the concentration range between 5 ×
-
4
-6
37
1
0
and 5 × 10 M.
To get more insight into the typical urea NsH, CdO, and
imide CdO vibrations, solid phase infrared (IR) spectra of 3aI
from different solvents have been collected (Table 1). The
measurements clearly reveal that films of 3aI drop cast from
THF and heptane with a THF history give the same values.
(
37) Matthews, J. R.; Goldoni, F.; Schenning, A. P. H. J.; Meijer, E. W. Chem.
(38) Versteegen, R. M.; Sijbesma, R. P.; Meijer, E. W. Macromolecules 2005,
38, 3176-3184.
Commun. 2005, 44, 5503-5505.
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A R T I C L E S
Sinkeldam et al.
Figure 6. UV-vis and CD spectra of 3bI in (a) CHCl3, 1.3 × 10 M, (b) THF, 1.5 × 10-5 M, (c) heptane, 1.4 × 10-5 M with a CHCl3 history, and (d)
-5
-
5
heptane 1.4 × 10 M with a THF history.
It is tempting to assume that THF induces or at least facilitates
the folding process. It has been previously reported that
especially diphenyl ureas substituted with electron-withdrawing
groups in the meta position are prone to cocrystallization with
hydrogen-bonding acceptors such as THF.39 Furthermore, therein
it is stated that hydrogen-bonding acceptor groups in ortho
position may form intramolecular hydrogen bonds with the urea
protons which themselves prefer an anti relationship to the urea
carbonyl. The combination of the above-described interactions
may induce a curvature that eventually nucleates folding in poly-
respectively (Figure 6). Furthermore, 3bI proved to be CD silent
in CHCl , suggesting a random coil conformation. The UV-
3
vis spectrum of 3bI in THF shows absorption maxima at 328
-1
-1
-1
-1
nm (ꢀ ) 39 600 M cm ) and 429 nm (ꢀ ) 78 500 M cm ).
The solution in THF reveals two bisignate Cotton effects:
-4
the first at 304 nm (g ) +1.6 × 10 ) and 328 nm (g ) -2.1
-4
×
10 ) with a zero crossing at 314 nm and a second bisignate
-4
Cotton effect at 385 nm (g ) +8.9 × 10 ) and 420 nm (g )
-
4
-
5.2 × 10 ) with a zero crossing at 401 nm. This is a strong
indication for the chiral organization of the OPV4 chromophores.
It is unclear why the zero crossing does not coincide with the
absorption maximum in UV-vis as was observed for the OPV3
foldamer (Figure 2). Moreover, in contrast to the CD measure-
ments on the OPV3 system, this system also shows a bisignate
Cotton effect in the short wavelength regime which can be
attributed to a chiral arrangement of the phthalimide units. A
solution in heptane with a CHCl3 history is CD silent with
(ureidophthalimide)s. Further evidence for this statement is given
below (in STM Analysis of Shorter Oligomers).
UV-Vis and CD Spectroscopy of 3bI (OPV4). Fraction
bI (longer oligomers decorated with OPV4 units) was subjected
3
to a similar photophysical study in CHCl3, THF, and heptane.
The remarkable “memory” effect reported for the OPV3-
decorated poly(ureidophthalimide) 3aI (Figure 3) was also
observed for the OPV4 analogue 3bI. Also in this system it
was possible to induce the secondary structure in THF, preserve
it in heptane, and “erase” it in chloroform. This cycle could be
repeated without noticeable changes in the UV-vis and CD
spectra. The UV-vis spectrum of a solution in CHCl3 shows
-
1
-1
somewhat shifted maxima at 330 nm (ꢀ ) 36 700 M cm )
-
1
-1
and 430 nm (ꢀ ) 66 900 M cm ), compared to the UV-vis
measurement in CHCl3. However, a solution in heptane with a
THF history reveals two bisignate Cotton effects. The short
wavelength bisignate Cotton effect has a zero crossing at 320
-
1
two absorption maxima located at 326 nm (ꢀ ) 39 500 M
-
4
-
1
-1
-1
nm and maxima at 305 nm (g ) +3.6 × 10 ) and 331 nm (g
cm ) and 436 nm (ꢀ ) 74 000 M cm ), which can be
-4
)
3
-2.2 × 10 ). The second bisignate effect has maxima at
-3 -4
attributed to the ureidophthalimide moiety and the OPV4 unit,
90 nm (g ) +1.0 × 10 ) and 425 nm (g ) -7.5 × 10 )
(
39) (a) Etter. M. C.; Urba n˜ czyk-Lipkowska, Z.; Zia-Ebrahimi, M.; Panunto,
T. W. J. Am. Chem. Soc. 1990, 112, 8415-8426. (b) Etter, M. C.; Panunto,
T. W. J. Am. Chem. Soc. 1988, 110, 5896-5897.
with a zero crossing at 407 nm. The accompanying UV-vis
spectrum shows a minor blue shift of the absorption maxima
16118 J. AM. CHEM. SOC.
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Chiral Alignment of OPV Chromophores
A R T I C L E S
-
5
Figure 7. Temperature-dependent UV-vis and CD spectroscopy of 3bI in (left) THF, 1.5 × 10 M, from 20 °C (black line) to 55 °C (black line) in steps
-
5
of 5 °C (gray lines) and (right) heptane, 1.4 × 10 M, from 20 °C (black line) to 80 °C (dashed line).
Figure 8. Melting curves of the Cotton effect in THF of 3aI (OPV3-decorated, 3.1 × 10 M) at 415 nm (left) and of 3bI (OPV4-decorated, 1.5 × 10-5
-5
M) at 439 nm (right). The curves represent a sigmoidal fit (dashed line) of the data points (b).
-
1
-1
to 332 nm (ꢀ ) 33 100 M cm ) and 420 nm (ꢀ ) 61 300
M
of both curves and the melting transition (∼312 K) are
comparable. It must be noted that the Cotton effect in both cases
does not reach zero, but the curve for OPV4-decorated foldamer
hints to a more cooperative unfolding than for the OPV3-
decorated foldamer. The stability of the folded chiral structures
in heptane is too high to see the transition from an ordered
helical state to a disordered state.
-1
-1
cm ). In addition, monomeric precursor 3,6-bis(acety-
lamino)-N-OPV4-phthalimide (6b) is CD silent in CHCl3, THF,
and heptane.
The stability of the putative helical architectures is again
investigated by subjecting solutions in THF and heptane of 3bI
to temperature-dependent UV-vis and CD experiments (Figure
7
). Raising the temperature of the solution in THF hardly affects
STM Analysis of Shorter Oligomers. To obtain experimen-
tal evidence for the curvature of the structures under investiga-
tion and hence to study their properties to arrive at a well-defined
folded state, we investigated the short-chain fractions 3aII
(OPV3-decorated) and 3bII (OPV4-decorated) containing chain
lengths between two and seven repeat units with STM. Their
organization on the solid-liquid interface of highly ordered
pyrolytic graphite (HOPG) and 1-phenyloctane could be studied
in detail with STM. Structural features appear in the STM
images that reflect individual molecules. On the basis of the
specific contrast of aromatic units (high tunneling efficiency)
in the STM images, in combination with the distance analysis,
we can correlate the bright rods with OPV units. Basically, two
types of images were recorded. At first glance, in the sample
containing OPV4-decorated ureidophthalimide oligomers (frac-
tion 3bII), the OPV4 rods appear mostly disordered (Figure
9a). Such images were also observed with a sample of 3aII,
containing OPV3-decorated ureidophthalimide oligomers (Fig-
ure 9c). The molecules are mobile as two images recorded
sequentially never show an identical arrangement of the
molecules (it takes ∼10 s to record an image). Given the
polydispersity of the sample, such a disordered arrangement is
not surprising. As the intermolecular lateral interactions between
the molecules are not optimized, the molecules are prone to
in-plane (diffusional) and out-of-plane (desorption-adsorption)
the intensity of the Cotton effect in combination with an almost
negligible hypsochromic shift of the absorption maxima. The
Cotton effect, however, does show a marked decrease upon
raising the temperature, but reclaims its original value upon
cooling and thereby demonstrates the dynamics of the system
in THF.
However, elevating the temperature of the solution in heptane
to 80 °C displays only a minor decrease of the Cotton effect,
indicating the remarkable stability of the supramolecular
architecture in heptane. Furthermore, it seems that the whole
spectrum undergoes a shift to shorter wavelength. A similar
effect is observed in the UV-vis spectrum. The nature of this
effect remains unclear. In addition, prolonged heating of both
solutions does not affect the Cotton effect any further.
Melting Curves of the Oligomers. Compared to the melting
curve of 3aI (decorated with OPV3) as recorded in THF, the
melting curve of 3bI (decorated with OPV4) in THF reveals a
steeper slope, indicating a difference in the change in enthalpy
involved in the unfolding process, albeit that the differences
are small (Figure 8). The almost linear relationship between
temperature and Cotton effect in 3aI indicates the absence of a
clear transition between folded and unfolded structures. The
difference can be attributed to the larger π-surface in the case
of the OPV4-decorated foldamer. The slope of the linear part
J. AM. CHEM. SOC.
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A R T I C L E S
Sinkeldam et al.
Figure 9. STM image of a random pattern of 3bII at the 1-phenyloctane/graphite interface. (a) Noncalibrated image with a size of approximately 22 nm
22 nm. It ) 0.80 nA. Vbias ) -0.57 V. The white arrows indicate dimers. Within the circle, a higher oligomer that could be an octamer is visible. (b) Zoom
×
of the central area in part a, showing the octamer with a superimposed molecular model. (c) 3aII, a circular structure of OPV3-decorated ureidophthalimide
is depicted in the circle. Image size is 21 nm × 21 nm. It ) 0.79 nA. Vbias ) -0.52 V.
Figure 10. STM image of 3bII at the 1-phenyloctane/graphite interface. (a) The unit cell is indicated in yellow. The lines in the inset at the lower right
corner of the image indicate the direction of the major symmetry axis of graphite. Image size is 16.2 nm × 19.6 nm. It ) 0.80 nA. Vbias ) -0.78 V. (b) Some
molecular models (dimers) superimposed on the STM image. (c) Zoom of STM image in part b with molecular model superimposed.
dynamic processes. In case of the OPV4-decorated ureidoph-
thalimide oligomers, the image shows mainly dimer molecules
indicated by white arrows (Figure 9a). The high abundance of
dimers is in agreement with the GPC trace. In spite of the
apparent random organization, both images clearly show higher
homologues in a circular orientation as depicted by the white
circle (Figure 9a,c). A superimposed model with all urea linkers
in a cisoid conformation fits very well (Figure 9b). Exactly this
shape is thought to be responsible for nucleation of folding in
solution. Previously, a rosette organization based on intermo-
lecular hydrogen bonding has been observed with OPV4
molecules equipped with a diamino triazine unit.40
In time, a second type of images appears for OPV4-decorated
ureidophtahlimide oligomers, revealing a more regular arrange-
ment of the molecules, locally leading to domains that are two-
dimensionally crystalline (Figure 10). The two-dimensional
arrangement can be described by a unit cell: a ) 5.2 ( 0.1
nm, b ) 4.3 ( 0.1 nm, γ ) 85 ( 2°, which contains four OPV
rods. The submolecular contrast variation along the rods is
attributed to the phenylene vinylene units. In addition to the
bright rods, also alkyl chains are visible which run perpendicular
to the OPV rods. Most likely, only two out of three dodecyloxy
chains per OPV unit are adsorbed with their long molecular
axis parallel to the substrate. Those adsorbed run (almost)
parallel to one of the main symmetry axes of graphite which is
frequently observed.
The formation of two-dimensional crystals is rather surprising
given the polydispersity of the sample. It suggests that, in time,
there is a selective adsorption of one given oligomer on the
surface. On the basis of the image contrast and the analysis, it
appears that tetramers with two cisoid and one transoid
conformation or dimers with one cisoid conformation can give
this regular packing. We cannot distinguish with absolute
certainty between them. However, the molecular model of the
tetramer underestimates the length of the “backbone” (2.4 nm),
while in the case of dimers, the experimental distance (2.7 nm)
fits the modeled one (2.6 nm) better. In addition, as the solution
contains dimers in excess, it is also plausible to conclude that
an excess of dimers will adsorb. The preferential adsorption of
dimers is most likely driven by the thermodynamics of the
adsorption-desorption process.
The study of the shorter oligomers on an HOPG surface
substantiates the tendency of the OPV-decorated ureidophtal-
imides to adopt a curved and hence folded structure that can be
attributed to the molecular design of the primary structure in
which the intramolecular hydrogen bonding between the urea
hydrogens and the adjacent imide carbonyls are highly direc-
tional in the folding process. This is in full agreement with the
IR data, the molecular modeling, and the X-ray structure of a
dimer. In contrast to the observation that short oligomers adopt
(
40) (a) Miura, A.; Jonkheijm, P.; De Feyter, S.; Schenning, A. P. H. J.; Meijer,
E. W.; De Schryver, F. C. Small 2005, 1, 131-137. (b) Jonkheijm, P.;
Miura, A.; Zdanowska, M.; Hoeben, F. J. M.; De Feyter, S.; Schenning,
A. P. H. J.; De Schryver, F. C.; Meijer, E. W. Angew. Chem., Int. Ed.
2
004, 43, 74-78.
16120 J. AM. CHEM. SOC.
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Chiral Alignment of OPV Chromophores
A R T I C L E S
ordered structures on a surface, they do not lead to a Cotton
effect in solution. This is due to their lack of length, being
insufficient to form at least one turn, and hence the lack of π-π
interactions to secure the formation of stable secondary struc-
tures. However, the STM data of the short length oligomers
support the proposal that the longer oligomers/polymers adopt
a folded helical conformation that is dynamic in THF and stable
in heptane and not present in CHCl3.
contrast to the longer oligomers, the shorter oligomers do not
seem to adopt a helical conformation in solution. However, STM
analysis revealed the presence of circular architectures for OPV3
as well as OPV4 oligomers. The high abundance of dimers in
the polydisperse sample resulted in the presence of a dense
packing of dimers to maximize surface coverage. The various
patterns observed on the graphite 1-phenyloctane interface are
a logical result of the polydisperse nature of the sample.
Conclusions
Acknowledgment. We thank Ralf Bovee for GPC analysis,
Dr. Albert Schenning for stimulating discussions, and Dr.
Theresa Chang for the artwork. The Council of Chemical
Science of the Dutch National Science Foundation (CW-NWO)
is acknowledged for the financial support of the Dutch team.
The authors in Leuven thank the Federal Science Policy through
IUAP-V-03 and the Institute for the Promotion of Innovation
by Science and Technology in Flanders (IWT). Financial support
from the Fund for Scientific Research-Flanders (FWO) and K.U.
Leuven is also acknowledged.
In summary, we have shown that a poly(ureidophthalimide)
backbone decorated with OPV chromophores assumes a random
coil conformation in CHCl3 but adopts a putative helical
conformation in THF. Measurements in heptane show that the
initial secondary organization is preserved depending on the
solvent used prior to heptane. Moreover, temperature-dependent
CD studies establish the dynamics of the secondary architectures
in THF and the remarkable stability in heptane. We are certain
that the intramolecular hydrogen bonding of the phthalimide
units is responsible for the initial chiral alignment of the OPV
chromophores. However, the rather small Cotton effects might
Supporting Information Available: GPC traces of OPV4-
decorated ureiodophthalimide oligomers 3bI and 3bII. Experi-
mental details concerning the synthesis of compounds 2a, 2b,
be an indication of the presence of frustrated stacks due to
nonchiral aggregation of OPV units.41 Although the role of THF
remains undefined, it is clear that solvent in general and THF
specifically play an important role in the folding process. In
1
13
3
a, and 3b and their characterization by H-, C NMR, and
IR. This material is available free of charge via the Internet at
http://pubs.acs.org.
(
41) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am.
Chem. Soc. 2001, 123, 409-416.
JA063405D
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