Folding a conjugated chain: Oligo(o-phenyleneethynylene-alt-p- phenyleneethynylene)

Article abstract of DOI:10.1021/ol801677s

(Chemical Equation Presented) An oligo(o-phenyleneethynylene-alf-p- phenyleneethynylene) was synthesized to create a conjugated molecule capable of adopting a helical secondary structure. A special feature of such a folded molecule is that the effective directions of energy/charge transport via covalent conjugation and through π-π stacking are converged to be along the helix axis. The transition from random conformations to the helix, driven by solvophobic and aromatic stacking interactions, was controlled by solvent properties. UV-vis and fluorescence spectroscopies gave supportive evidence for the folding process.

Full text of DOI:10.1021/ol801677s

ORGANIC
LETTERS
2008
Vol. 10, No. 19
4283-4286
Folding a Conjugated Chain: Oligo(o-
phenyleneethynylene-alt-p-phenyleneethynylene)
Ningbo Zhu, Wei Hu, Shuliang Han, Qi Wang, and Dahui Zhao*
Beijing National Laboratory for Molecular Sciences (BNLMS), Department of Applied
Chemistry and the Key Laboratory of Polymer Chemistry and Physics of the Ministry
of Education, College of Chemistry, Peking UniVersity, Beijing 100871, China
dhzhao@pku.edu.cn
Received July 23, 2008
ABSTRACT
An oligo(o-phenyleneethynylene-alt-p-phenyleneethynylene) was synthesized to create a conjugated molecule capable of adopting a helical
secondary structure. A special feature of such a folded molecule is that the effective directions of energy/charge transport via covalent
conjugation and through π-π stacking are converged to be along the helix axis. The transition from random conformations to the helix,
driven by solvophobic and aromatic stacking interactions, was controlled by solvent properties. UV-vis and fluorescence spectroscopies
gave supportive evidence for the folding process.
Due to a superior combination of (semi)conducting,
illuminating, optoelectronic, mechanical properties, and
solution processability, conjugated polymers1 have found a
wide range of applications, such as field-effect transistors,2
light-emitting diodes,3 solar cells,4 chemo- and biosensors,5
lasing devices,6 etc. In spite of the vast merits of solution
processing, the device performance is sometimes limited by
the amorphous nature of the resulting materials. Self-
assembly of conjugated (macro)molecules into ordered nano-
or microstructures is envisioned to help improve certain
properties while maintaining the solution processability.7
Also, self-assembly of conjugated systems offers a potent
bottom-up approach to fabricating (supra)molecular devices.
Currently, studies on conjugated-molecule self-assembly
are mainly focused on intermolecular organization behaviors
and architectures. Manipulations of single molecules with
optical or electronic functions into well-defined, higher-order
structures are less common,8,9 although natural and artificial
systems of nonconjugated structures exhibiting single-
molecule supramolecular architectures are ubiquitous and
(1) Kiebooms, R.; Lee, K.; Menon, R. Synthesis, Electrical, and Optical
Properties of Conjugated Polymers. In Handbook of AdVanced Electronic
and Photonic Materials and DeVices; Nalwa, H. S., Ed.; Academic Press:
2000; Vol. 8, pp 1-102.
(2) Reese, C.; Roberts, M.; Ling, M.; Bao, Z. Mater. Today 2004, 7
(9), 20.
(3) (a) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.;
Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas,
J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (b) Braun, D.
Mater. Today 2002, 5 (6), 32.
(6) Samuel, I. D. W.; Turnbull, G. A. Mater. Today 2004, 7 (9), 28.
(7) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J.
Chem. ReV. 2005, 105, 1491.
(4) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107,
1324.
(5) (a) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. ReV. 2007,
107, 1339. (b) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV.
2000, 100, 2537.
(8) Han, J. J.; Shaller, A. D.; Wang, W.; Li, A. D. Q. J. Am. Chem.
Soc. 2008, 130, 6974.
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10.1021/ol801677s CCC: $40.75
Published on Web 09/09/2008
2008 American Chemical Society

widely investigated for their structure-enabled functions. For
example, foldamers of varied structures and functions have
been widely explored,10,11 among which m-phenyleneethy-
nylene (PE) oligomers are intensively studied.12 Analogous
to m-PE helices with six repeating units per turn, oligo-
(o-PE) chains may fold into helices of 3 PE units per turn.
Among a few other studies,13 Tew and co-workers demon-
strated with NMR spectroscopy that o-PE oligomers folded
into helices.14
Here, we introduce a design that folds a conjugated chain
into a helical conformation. Such a helical structure imparts
a novel characteristic to the system. In the folded state, π-π
stacking takes place intramolecularly. The effective directions
of energy/charge transport via covalent conjugation and
through π-π stacking are thus converged to be along the
helix axis in a single molecule. The two electronic coupling
mechanisms may thus work jointly and cooperatively. This
unique feature may bring about new properties and functions
inaccessible with traditional, linear conjugated polymers.
The idea of folding a conjugated molecule is first tested
with a PE oligomer. Besides capable of folding, the molecular
structure was specifically devised to maintain conjugation
along the backbone, such that the molecule preserves
desirable optical and electronic properties. ortho-Phenylene
was selected as the repeating unit to confer nonlinear bond
angle and constitute “turns” within helices. This choice was
also made because the linkage displays a certain degree of
conjugation.15 As depicted in Figure 1, the oligomer back-
bone is constructed with alternating o-PE and p-PE units.16
The reasons for incorporating p-PE units are as follows. In
order to accommodate a helical pitch corresponding to the
typical distance between stacked aromatic rings, a folded
oligomer comprising exclusively o-PE units has to assume
a significant dihedral angle at the o-phenylene linkage.13,14
With a p-PE segment inserted between every other o-PE
units, the torsional strain at the o-phenylene linkage can be
Figure 1. Chemical structures of oligomers 1 and 2, and top- and
side-views of a space-filling model of 1 in a helical conformation
(dodecyl side chains were truncated as methyl groups for clarity).
greatly relieved in the folded state, and the coplanarity of
adjacent phenyl rings is also improved. This would favor
π-electron delocalization and thus energy/charge transport
along the covalent chain. The p-PE units also help extend
the effective conjugation length, making the molecule more
suitable for photonic or opto-electronic applications. As a
modular design, either or both of the p- and o-PE units can
be substituted with alternative aromatic moieties of proper
bond angles. Thereby, functionalization and fine-tuning of
the optical or electronic properties of the system can be
realized.
Based on the above considerations, oligomer 1 was
synthesized. This molecule contains 15 phenyl(ene) units and
has dodecyloxy side chains appended to the o-phenylenes.
The helical architecture was to be attained by virtue of
solvophilicity difference between the aromatic backbone and
aliphatic side chains. Specifically, a nonpolar, aliphatic
solvent will be used to create a solvophobic environment
for the aromatic backbone, while keeping the alkyl side
chains soluble.7 Imaginably, under such conditions a helical
conformation will be favored, as it offers a solvophilicity
balance by having most backbone units buried and all side
chains exposed at the helix periphery.12 Additionally, the
helix can be stabilized by intramolecular π-π stacking
interactions.12a Another prerequisite for stabilizing the helical
architecture is that the molecule should be long enough to
gain sufficient energy from intramolecular π-π stackings
to counteract the entropy loss for adopting an ordered
conformation.12b Thus, a shorter homologue of 1, oligomer
2, consisting of five phenyl rings and therefore incapable of
folding, was employed as a reference molecule for compari-
son studies.
(10) (a) Foldamers: Structure, Properties and Applications; Hecht, S.,
Huc, I., Eds.; Wiley-VCH: Weinheim, 2007. (b) Smaldone, R. A.; Moore,
J. S. Chem.sEur. J. 2008, 14, 2650. (c) Cheng, P. Curr. Opin. Struct. Biol.
2004, 14, 512. (d) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.;
Moore, J. S. Chem. ReV. 2001, 101, 3893. (e) Cheng, R. P.; Gellman, S. H.;
DeGrado, W. F. Chem. ReV. 2001, 101, 3219, and references therein
.
(11) For most recent studies on foldamers and their functions, see: (a)
Cai, W.; Wang, G.-T.; Xu, Y.-X.; Jiang, X.-K.; Li, Z.-T. J. Am. Chem.
Soc. 2008, 130, 6936. (b) Meudtner, R. M.; Hecht, S. Angew. Chem., Int.
Ed. 2008, 47, 4926. (c) Bao, C.; Kauffmann, B.; Gan, Q.; Srinivas, K.;
Jiang, H.; Huc, I. Angew. Chem., Int. Ed. 2008, 47, 4153. (d) Horne, W. S.;
Boersma, M. D.; Windsor, M. A.; Gellman, S. H. Angew. Chem., Int. Ed.
2008, 47, 2853, and ref 9
.
(12) (a) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science
1997, 277, 1793. (b) Stone, M. T.; Heemstra, J. M.; Moore, J. S. Acc. Chem.
Res. 2006, 39, 11. (c) Smaldone, R. A.; Moore, J. S. J. Am. Chem. Soc.
2007, 129, 5444. (d) Tan, C.; Pinto, M. R.; Schanze, K. S. AdV. Mater.
2004, 16, 1208. (e) Ishitsuka, Y.; Arnt, L.; Majewski, J.; Frey, S.; Ratajczek,
M.; Kjaer, K.; Tew, G. N.; Lee, K. Y. C. J. Am. Chem. Soc. 2006, 128,
13123.
The synthesis of pentadecamer 1 is illustrated in Scheme
1. The final step was accomplished by joining three segments
together, a bisfunctional compound 3 with two equivalent
4, via a Sonogashira-Hagihara reaction. The advantages of
such a three-component coupling over linking two longer
pieces into one (e.g., a heptamer with an octamer) are not
only the less tedious synthetic routes to the shorter precursors
but also that the byproduct from oxidative coupling of
terminal acetylenes has a distinct chain length from that of
the target molecule and may be conveniently separated with
column chromatography. As shown in Scheme 1, precursors
(13) (a) Grubbs, R. H.; Kratz, D. Chem. Ber. Recl. 1993, 126, 149. (b)
Shotwell, S.; Windschief, P. M.; Smith, M. D.; Bunz, U. H. F. Org. Lett.
2004, 6, 4151. (c) Khan, A.; Hecht, S. J. Polym. Sci., Part A 2006, 44,
1619.
(14) (a) Jones, T. V.; Blatchly, R. A.; Tew, G. N. Org. Lett. 2003, 5,
3297. (b) Jones, T. V.; Slutsky, M. M.; Laos, R.; de Greef, T. F. A.; Tew,
G. N. J. Am. Chem. Soc. 2005, 127, 17235. (c) Slutsky, M. M.; Jones,
T. V.; Tew, G. N. J. Org. Chem. 2007, 72, 342. (d) Jones, T. V.; Slutsky,
M. M.; Tew, G. N. New J. Chem. 2008, 32, 676.
(15) Anand, S.; Varnavski, O.; Marsden, J. A.; Haley, M. M.; Schlegel,
H. B.; Goodson, T., III. J. Phys. Chem. A. 2006, 110, 1305.
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Org. Lett., Vol. 10, No. 19, 2008

Scheme 1. Synthetic Route of Oligomer 1
3 and 4 were prepared through multistep Sonogashira cross-
couplings, along with orthogonal protecting and masking
protocols for the terminal acetylene and aryl halide.
Complementary to the NMR studies conducted by Tew
et al.,14b-d for the current system UV-vis absorption and
fluorescence emission spectra handily provided evidence for
folding. In chloroform, which is a good solvent for both the
backbone and side chains, the oligomer should adopt random
conformations, whereas cyclohexane was selected as the
aliphatic solvent that might induce helical folding of the
oligomer. Hence, solvent titration experiments were per-
formed with the two solvents to probe the folding transition
of 1. But first, oligomer 2 was examined on how the optical
spectra responded to such solvent changes when folding was
not involved. It was found that when the volume ratio of
cyclohexane to chloroform was increased in the binary
solvent, oligomer 2 showed a slight blue-shift and better
resolved vibronic structures in the emission band, whereas
the absorption spectra exhibited no prominent change (parts
a and b of Figure 2). The occurrence of such blue-shifting
and fine structures in the emission is fairly common for
chromophores experiencing solvent polarity decrease, due
Figure 2. UV-vis absorption and fluorescence emission spectra
of 2 (a,b) and 1 (c,d) in chloroform-cyclohexane cosolvent (the
legend shows the vol % of cyclohexane; arrows indicate the
spectrum shifting trend from chloroform to cyclohexane).
to greater solvent relaxations in more polar solvents. In stark
contrast, longer oligomer 1 displayed rather distinct responses
when subject to the same solvent changes. With an increased
cyclohexane content in the mixed solvent, the original
absorbance maximum at 315 nm and a side-peak around 400
nm attenuated while a new peak emerged at about 300 nm
and the original shoulder peak at ca. 345 nm escalated and
(16) For examples of previously reported molecules containing the p-PE-
o-PE unit, see: (a) Ohkita, M.; Ando, K.; Suzuki, T.; Tsuji, T. J. Org. Chem.
2000, 65, 4385. (b) Anderson, S. Chem.sEur. J. 2001, 7, 4706. (c)
Nierengarten, J.-F.; Zhang, S.; Gegout, A.; Urbani, M.; Armaroli, N.;
Marconi, G.; Rio, Y. J. Org. Chem. 2005, 70, 7550. (d) Shirai, Y.; Osgood,
A. J.; Zhao, Y.; Yao, Y.; Saudan, L.; Yang, H.; Yu-Hung, C.; Alemany,
L. B.; Sasaki, T.; Morin, J.-F.; Guerrero, J. M.; Kelly, K. F.; Tour, J. M.
J. Am. Chem. Soc. 2006, 128, 4854.
Org. Lett., Vol. 10, No. 19, 2008
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became a major maximum in pure cyclohexane (Figure 2c).
During this process multiple isosbestic points were observed,
indicating that the molecule was undergoing a gradual
transition between two defined states, likely the transoid and
cisoid conformational states.12a Moreover, the fluorescence
intensity of oligomer 1 drastically decreased as the solvent
changed from chloroform to cyclohexane, accompanied with
a slight red-shift in wavelength (Figure 2d). Since fluores-
cence quenching is a typical signature of π-π stacking, it
suggested that longer oligomer 1 adopted a more compact
conformation in cyclohexane, consistent with the helical
structure. This solvent titration experiment was then con-
Figure 3. Chemical structure and UV-vis absorption spectrum of
MC in cyclohexane (blue) in comparison with those of 1 in
ducted at a concentration as low as approximately 10-8
M
cyclohexane (green) and chloroform (red).
of 1, giving nearly identical spectral changes as shown in
parts c and d of Figure 2. This result suggested that the
observed spectral changes could originate from a single-
molecule behavior, i.e., chain folding, rather than intermo-
lecular interactions. This assumption was further supported
by dynamic light scattering (DLS) studies. No particles of
measurable sizes were detected by DLS in cyclohexane
solutions of 1 from 10-8 to 10-6 M, the same concentration
range for recording the optical spectra.17
It can be imagined that under alternative, less optimal
conditions, e.g., in poor solvents for both the backbone and
side chains, such an ordered helical conformation is unlikely
to exist. Rather, the molecules may collapse into compact
yet random conformations and/or form ill-defined intermo-
lecular aggregates. Indeed, when adding acetonitrile or
methanol into the chloroform solution of 1, less well-defined
changes were recorded with the absorption, although fluo-
rescence quenching took place to a greater extent (Figures
S1 and S2, Supporting Information). These results further
corroborated the assertion that the spectral changes exhibited
in parts c and d of Figure 2 corresponded to a transition to
a specific, helical structure.
macrocycle and a turn in the helix. The particularly large
A350nm/A300nm value of MC might result from its rigid, cyclic
geometry or the unique closed-ring electronic characteristics.
Likely due to the more dynamic and crescent backbone
conformation, the helix gave a similar yet not identical band
shape. Alternatively, this difference might indicate that the
helical structure of 1 is not completely stabilized in cyclo-
hexane. Investigations at quantifying the folding stability and
possibly further stabilizing the helical structure with other
solvent conditions are underway.
The current system is a proof-of-principle design, illustrat-
ing that higher-order structures such as helices may be
realized in conjugated chain molecules. Photophysical char-
acterizations of a monodispersed oligo(o-PE-alt-p-PE) gave
supportive evidence for a helical structure under suitable
conditions. Alternative aromatic units will be incorporated
into the backbone in future studies, making the architecture
a versatile folding scaffold to achieve structural and function
diversity.
Additional evidence for the helical conformation came
from a macrocyclic analogue, MC (Figure 3). This molecule
shares the same backbone repeating units with oligomer 1.
The absorption spectra in Figure 3 clearly show that the band
shape of 1 in cyclohexane resembles that of MC, with two
major peaks at ca. 300 and 350 nm, more than that in
chloroform. This further supported the helical structure of 1
in cyclohexane, considering the similarity between the
Acknowledgment. This work is supported by the National
Natural Science Foundation of China (No. 50603001 and
20774005), the Ministry of Science and Technology of China
(No. 2006CB921600), and Fok Ying-Tung Educational
Foundation (No. 114008).
Supporting Information Available: Synthesis procedures
and characterizations of 1, 2, and MC. This material is
available free of charge via the Internet at http://pubs.acs.org.
(17) The emission quantum yield of 1 in cyclohexane did not decrease
with increasing concentration, which also suggested that no significant
intermolecular aggregation occurred.
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