Oligo(p-phenyleneethynylene)s with hydrogen-bonded coplanar conformation

Article abstract of DOI:10.1021/ol800753z

(Chemical Equation Presented) A series of monodispersed oligo(p-phenyleneethynylene)s were synthesized bearing intramolecular hydrogen bonds between side chains of adjacent phenylene units in the backbone. Thus, all repeating units of the molecules are co

Full text of DOI:10.1021/ol800753z

ORGANIC
LETTERS
2008
Vol. 10, No. 13
2669-2672
Oligo(p-phenyleneethynylene)s with
Hydrogen-Bonded Coplanar
Conformation
Wei Hu, Ningbo Zhu, Wen Tang, 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 and Molecular Engineering, Peking UniVersity,
Beijing 100871, China
dhzhao@pku.edu.cn
Received April 2, 2008
ABSTRACT
A series of monodispersed oligo(p-phenyleneethynylene)s were synthesized bearing intramolecular hydrogen bonds between side chains of
adjacent phenylene units in the backbone. Thus, all repeating units of the molecules are constrained in a coplanar orientation. Such planarized
conformation is considered favorable for single-molecule conductance. Photophysical characterization results show narrowed bandgaps and
extended conjugation lengths, consistent with a rigid, planar backbone framework as a result of intramolecular hydrogen bonding.
Polymeric and oligomeric phenyleneethynylenes (PPEs
and OPEs)1 are a family of conjugated molecules that are
highly diversified in structure and have found a range of
applications in optoelectronics,2 sensors,3 supramolecular
architecture constructions,4 etc. More recently, with increased
attention devoted to molecular and nanoscale electronics,
OPEs are also intensively studied as promising molecular
wire candidates due to their optimal electrical conducting
properties.5 However, although OPEs possess desirable
backbone rigidity, the single bond joining the phenylene and
ethynylene units in the molecules displays a low rotational
barrier, rendering rapid torsional vibration of the phenylene
units at room temperature.6 Studies have shown that this
(1) (a) Tour, J. M. Chem. ReV. 1996, 96, 537. (b) Bunz, U. H. F. Chem.
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(5) For recent examples of molecular conductance studies on OPEs and
derivatives (including single-molecule studies) see: (a) Huber, R.; Gonza´lez,
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10.1021/ol800753z CCC: $40.75
Published on Web 05/29/2008
2008 American Chemical Society

conformational disorder plays a detrimental role in the
electrical conductance of OPEs, since the relative orientation
of the phenylene units determines the extent of orbital
overlapping and π-electron delocalization along the back-
bone.7 By affecting the conjugation length and bandgap, the
molecular conformation of the PE backbone inevitably affects
the spectroscopic properties and energy migration charac-
teristics along the molecules as well. Additionally, because
rigid, planar aromatic structures tend to promote strengthened
intermolecular interactions, the self-association behaviors are
also influenced by the backbone shape. Effectively, the
molecular dynamics of OPE/PPEs imposes critical influence
over all their aforementioned applications.
Despite the great importance of the planar conformation
of the PE structures, achieving precise control is challenging.
Previous strategies employed to constrain or maneuver the
relative orientation of the phenylene units in OPE/PPEs
involve covalently tethering,8a–c implementing surfactant
effect at the interface,8d increasing structural planarity in
nematic liquid crystal solutions,8e–g exploiting steric interac-
tions between backbone substituents,8h and introducing
charges into the molecules.8i,9 Our current study aimed to
sythesize a series of OPEs with their conformation confined
in the coplanar state by intramolecular H-bonds between the
side chains of adjacent phenylene units. A side-chain
H-bonding motif was previously introduced into meta-PE
oligomers, conferring a coplanar orientation for the diphe-
nylacetylene moieties.10 With newly developed synthetic
protocols, an analogous design is now implemented in the
para-PE system. The syntheses of such a series of OPEs
1-3 are presented here. Their monodispersed, systematically
extended chain lengths were designed to impart information
of intramolecular H-bonding effects and facilitate future
molecular-wire studies. Photophysical characterization of the
oligomers provided evidence for the rigid, planar structures
with extended effective conjugation lengths.
The synthetic routes of OPEs 1-3 are outlined in Scheme
1. The basic synthetic strategy was to join key intermediate
2,5-diethynyl-1,4-phenylenedihexanamide with (di)iodo-
substituted dihexyl terephthalate via repetitive Sonogashira
cross-coupling reactions and proper protection/deprotection
protocols.11 The NHs in the bisamide functionality and the
carbonyl groups of the terephthalate acted as the H-bonding
donor and acceptor, respectively, in the final OPEs. Such
H-bonding motif was previously proven to hinder the
rotational motion of the phenylene units in m-PE systems.10
As the chain length increased from 1 to 3, the solubility of
the molecules was noted to decrease drastically and OPE 3
was only sparsely soluble in chloroform, the best solvent
found.12 Thus, the purfication of 3 was difficult and strenu-
ous. This decrease in solubility suggested the presence of
rigid, planar backbones,13 which enhanced intermolecular
interactions and self-association.
The formation of intramolecular H-bonds in OPEs 1–3
1
was confirmed by H NMR spectroscopy. In intermediates
6 and 7 (as well as in compound 16), the amide protons (NH)
exhibited chemical shifts in the range of 7.8-8.0 ppm in
CDCl3, while all the amide protons in oligomers 1-3
displayed chemical shifts of ca. 9.0-9.2 ppm, indicating
intramolecular H-bond formation in the latter. It was hoped
that the formation and disruption of these H-bonds could be
reversibly controlled by varying the solution temperature or
by changing the solvent properties, thereby to study the
molecular properties under different conformations. Yet,
these intramolecular H-bonds proved to be robust. A minimal
chemical shift change was observed with the NH proton in
CDCl3 by varied-temperature NMR technique ranging from
20 to 60 °C (Figure S1), or by titrating DMSO-d6 into the
CDCl3 solution, indicating preservation of the H-bonds under
these conditions.14
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N. J. J. Am. Chem. Soc. 2005, 127, 9235.
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A. D.; Arcadi, A.; Cerichelli, G.; Aschi, M. Chem. Phys. Lett. 2007, 434,
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A. S.; Schiffrin, D. J.; Higgins, S. J.; Bryce, M. R.; Lambert, C. J.; Nichols,
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In order to delineate the effects of H-bonding, compounds
Cary, J. M.; Moore, J. S. Org. Lett. 2002, 4, 4663. (c) Yang, X.; Brown,
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Zeng, X. C.; Gong, B. J. Am. Chem. Soc. 2004, 126, 3148.
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(11) By carefully controlling the amount of potassium carbonate and
the reaction time, selectively cleaving one or both of the TMS groups in 5
can be achieved under respectively optimized conditions.
(12) OPEs 1-3 all have terephthalate as terminal units. Oligomers having
phenylenedihexanamide as terminal units had very low solubility in common
organic solvents or simply failed to be synthesized in a purified form due
to extremely low solubilities. Such low solubilities likely resulted from
intermolecular H-bonding formed by terminal amide groups not engaged
in intramolecular H-bonding.
(13) Similar solubility was observed among oligomers 13-15, confirm-
ing the solubility change in 1-3 is a result from planarization of the aromatic
backbone, rather than merely from the chain extension.
(14) The chemical shift of H-bonded NH only changed by ca. 0.09 ppm
as the temperature increased from 20 to 60 °C, compared to a 0.05 ppm
shift for non-hydrogen bonded NH proton and ca. 0.02 ppm shift for
aromatic protons. No significant chemical shift change was observed when
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Org. Lett., Vol. 10, No. 13, 2008

Scheme 1. Syntheses of Intramolecularly H-Bonded OPE 1-3 and Structures of Analogous Compounds without H-Bonds
13-17 were subsequently synthesized as analogues of 1-3
without H-bonds for comparison studies. Oligomers 13-15
have identical chain lengths with 1-3, respectively, but the
amide side chains were replaced with alkoxy groups. Thus,
intramolecular H-bonds were removed in 13-15, while the
electronic properties maintained comparable with those of
1-3. Compound 16 has a similar structure with OPE 1,
except that the terephthalates were replaced with isophtha-
lates, resulting in removal of the intramolecular H-bonds.
Molecule 17 is a control molecule with two isophthalate
terminal units and a dialkoxyphenylene in the middle. As
the stretching modes of N-H and CdO bonds are sensitive
to H-bonding, supportive evidence for intramolecular H-
bonds in OPE 1-3 was given by IR spectroscopy, although
it was complicated by the intermolecular H-bond formation
in 16 under conditions for recording IR spectra (see the
Supporting Information for details).
Figure 1. UV-vis absorption spectra of 1, 13, 16, and 17,
normalized based on the longest-wavelength absorption maxima.
by the UV-vis absorption spectra in Figure 1, compound
17 has an absorption peak at 372 nm and 16 at 364 nm.
Also, in this case by switching from dialkoxy to diamide
group, the onset of the absorption band was blue-shifted.
However, the opposite trend was observed in 1 and 13 upon
an analogous functionality change. The absorption band of
1 was noticeably red-shifted relative to that of 13. This
difference in spectroscopy change was therefore reasonably
attributed to another structural disparity between OPEs 1 and
13, i.e., the intramolecular H-bonds in 1. As the optical
Subsequently, the photophysical properties of OPEs 1-3
and 13-17 were systematically examined to investigate the
effect of H-bonds on the conformational charateristics
depicted by the molecular spectroscopic features. As shown
DMSO-d6 was titrated into the CDCl3 solution of 1 up to 15 vol%; high
volume percentage of DMSO or methanol in chloroform solution caused
the molecule to self-aggregate, as evidenced by the fluorescence quenching
of the system, which would have further stabilized the coplanar conformation
and the H-bonds.
Org. Lett., Vol. 10, No. 13, 2008
2671

spectra reflect the average property of a collective ensemble
of a distribution of conformations, this could serve as the
evidence supporting that the H-bonds effectively constrained
the phenylene rings to adopt more planarized orientations,
resulting in a more extended π-electron delocalization.
Next, the absorption and fluorescence emission spectra of
1-3 were compared to those of 13-15 (Figure 2). It was
disorded ground-state conformations. Thus, the red-shifting
is to a less extent in the fluorescence spectra. The magnitudes
of Stokes’ shift provided further evidence for the backbone
rigidity of 1-3. Flexible molecules tend to undergo energy-
dissipating internal conversions prior to fluorescent decay
and thus typically possess larger Stokes’ shifts. Here, OPEs
1-3 consistently exhibit smaller Stokes’ shifts than corre-
sponding 13-15, presumably due to H-bonding entailed
structural rigidity. For example, a rather small Stokes’ shift
of 1191 cm-1 was observed for 3, as compared to a shift of
2681 cm-1 for 15 (Table 1). All the above evidence supported
Table 1. Photophysical Characterizations of the OPEs
λab/nma
compounds (ε/104M-1cm-1
b
)
λem/nmc (|λem-λab|)/nm (/cm-1
)
1
2
3
13
14
15
16
17
402 (2.6)
456 (7.1)
471 (11)
387 (2.6)
419 (6.2)
437 (10)
364 (2.5)
372 (3.2)
468
499
499
461
486
495
420
416
66 (3508)
43 (1890)
28 (1191)
74 (4148)
67 (3290)
58 (2681)
56 (3663)
44 (2843)
a Absorption maximum of the longest-wavelength. b Extinction coef-
ficient of λab. c Emission maximum wavelengths.
the argument that the intramolecular H-bonds effectively
reduced the torsional motion of the phenylene rings in 1-3,
rendering more rigid and planar conformation.
In conclusion, a series of OPEs with repeating units
intramolecularly H-bonded into coplanar conformation are
synthesized. These OPEs will be further functionalized and
studied, e.g., regarding their molecular conducting properties.
As these planar conjugated structures demonstrated enhanced
self-association tendency, the system will also be modified
to explore their features and functions within ordered
supramolecular architectures.
Figure 2. UV-vis absorption and fluorescence spectra of OPEs 1
(upper, solid), 2 (middle, solid), 3 (bottom, solid), 13 (upper,
dashed), 14 (middle, dashed), and 15 (bottom, dashed), normalized
based on λab or λem in Table 1.
found that, at all different chain lengths oligomers having
intramolecular H-bonds (1-3) gave rise to absorption and
emission peaks of longer wavelengths than those of corre-
sponding oligomers without the H-bonds (13-15). Again,
such a red-shifting trend substantiated the hypothesis that
H-bonds may help planarize the backbone framework and
extend the effective conjugation length. It was also noted
that at given chain length the extent of red-shifting was more
pronounced in the absorption spectra than in the emission
spectra. It is reasonable that the H-bonds appear to be more
effective in planarizing the ground-state structures. For those
oligomers without H-bonds the equilibrium geometry of the
lowest excited state, from which fluorescence originates,
possibly has a more planarized structure than the average of
Acknowledgment. This work is supported by the National
Natural Science Foundation of China (No. 20774005 and
50603001), the Ministry of Science and Technology of China
(No. 2006CB921600), and Peking University.
Supporting Information Available: Synthetic procedures
and characterization data of the oligomers, including the
quantum yield information and discussion of IR spectros-
copy. This material is available free of charge via the Internet
at http://pubs.acs.org
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