Chirally organized oligothiophenes: Towards modeling interchain interactions within π-conjugated systems

Article abstract of DOI:10.1002/chem.201001079

(Figure Presented) Interchain modeling: A model is proposed to compare the strength of the interaction between two conjugated oligomers (see graphic). It consists of probing changes in the angle between the parts of a binaphthalene hinge, invoked by the interaction between the oligomers. This allows the strength of the interaction between any set of oligomers to be compared or changes in the interactions upon oxidation or deprotonation to be monitored.

Full text of DOI:10.1002/chem.201001079

COMMUNICATION
DOI: 10.1002/chem.201001079
Chirally Organized Oligothiophenes: Towards Modeling Interchain
Interactions Within p-Conjugated Systems
David Cornelis, Thierry Verbiest, and Guy Koeckelberghs*[a]
Research in the field of conjugated polymers and oligo-
mers is driven on the one hand by the optimization of the
intrinsic molecular features of the conjugated system itself,
and on the other hand by the fine-tuning of the intermolecu-
lar interactions between the conjugated segments.[1] These
intermolecular interactions play a crucial role in the proper-
ties of conjugated materials, not only in the neutral but also
in the oxidized state. Several oligomeric model compounds,
composed of stacked oligothiophenes, have recently been
prepared to model the intermolecular p dimerization upon
oxidation.[2] This p-dimer formation has even been used as
the driving force for molecular actuators, which are com-
posed of tweezer-like oligothiophenes with flexible molecu-
lar hinges.[3] Finally, chirality has also been implemented in
conjugated models, which has created the possibility of visu-
alizing the induced conformational changes through chirop-
tical techniques.[4]
The present approaches to monitor the strength of the in-
termolecular interactions and their dependence on external
stimuli, such as oxidation, typically focus on the spectral
changes of the oligomers induced by the interaction. How-
ever, such studies are usually severely impeded by the fact
that both the interaction and the external stimulus affect the
electronic properties of the oligomer. Indeed, if oxidation is
taken as an example, oxidation results in both the appear-
ance of new, (bi)polaronic bands attributable to the altered
electronic structure and (blue)shifts, originating from p di-
merization, that is, the interaction.[2,3,5] Moreover, the mag-
nitude and nature of the spectral changes do not only
depend on the strength of the particular interaction (such as
p interaction or p dimerization), but also on the oligomer
itself and the external stimulus (such as oxidation or depro-
tonation). Therefore, the present studies remain typically fo-
cused on a comparison of the spectral changes induced by
the same external stimulus (e.g., oxidation) in an analogous
series of oligomers.
In contrast, a more general protocol can be obtained if
two oligomers are attached to a flexible hinge molecule and
if the relative orientation of the two parts of this hinge is
monitored, because in that case the spectral changes do not
depend on the particular oligomer or external stimulus.
Indeed, external stimuli such as oxidation can alter the inter-
action and, consequently, the distance between the two
oligoACTHNURGTNEmNUG ers held together by the molecular tweezer. The
change in this distance is translated to an altered relative
orientation and the distance of the two parts of the flexible
hinge is also altered (Figure 1). Finally, these changes can
conveniently be monitored by, for instance, circular dichro-
ism (CD) spectroscopy. As a consequence, this protocol is
not limited to studying differences in interactions within the
same class of molecules, but it can, in principle, generally be
used for comparing totally different molecules and interac-
tions, provided that the same linking group (e.g., ethenylene
group) and oligomers of similar bulkiness and rigidity are
employed.
A survey of the molecules studied is shown in Figure 1.
As a flexible, chiral linker, a binaphthalene moiety (BN)
was used. An overall X-type geometry of the oligomers was
preferred because this results in the strongest intermolecular
interactions.[6] All model compounds were prepared by a
Wittig–Horner reaction of (S)-1,1ꢀ-binaphthalene-2,2ꢀ-di-
AHCTUNGTREGaNNNU ldehyde with different phosphonated oligothiophenes in a
1
trans-selective configuration, as evidenced by H NMR spec-
troscopy.[7] To verify whether the conformation as shown in
Figure 1 is indeed possible, some Hyperchem simulations
were performed on MDC 2. These calculations revealed that
the proposed sandwich-like conformation is energetically fa-
vored.[7]
[a] D. Cornelis, Prof. Dr. T. Verbiest, Dr. G. Koeckelberghs
Department of Chemistry, Katholieke Universiteit Leuven
Celestijnenlaan 200F, 3001 Heverlee (Belgium)
Fax : (+32)16-32-7990
The CD spectrum of a chiral BN is composed of a bisig-
nate couplet(s), the intensity (and sign) of which depend on
the dihedral angle (q), the strength of the transition dipole
moment, and the orientation of these transition dipole mo-
E-mail: guy.koeckelberghs@chem.kuleuven.be
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001079.
Chem. Eur. J. 2010, 16, 10963 – 10967
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10963

2) The CD signal must not be affected by variations in the
strength or orientation of the monitored transition dipole
moment. Although substituents can indeed influence the
orientation, this effect is only important for quantitative
studies and can therefore be neglected.[8]
Concerning the influence of changes in e, it might be
speculated that oxidation could affect e and, hence, the CD
response as it changes the electronic properties of the oligo-
thiophene and introduces additives. Therefore, MDC 3 was
oxidized by gradual addition of Meerweinꢀs salt
(Et3OSbCl6). This oxidant was chosen because it has been
shown to result in a one-electron oxidation of oligothio-
phenes without any other side reaction.[3,11] The UV/Vis
spectra (Figure 2c) show that oxidation of the oligothio-
phene indeed occurs, but, importantly, the Cotton effect
near 320 nm is not affected. Therefore, possible changes of
DeBN of MDC 2 and MDC 4 upon oxidation cannot be due
to an altered orientation or strength of the transition dipole
moment of the monitored BN transition, but must originate
from a different dihedral angle.
Next, MDC 4, in which an electron-poor oligothiazole and
the pentathiophene are stacked, was subjected to oxidation.
Owing to the electron poverty of oligothiazole, only oxida-
tion of the oligothiophene is possible, which is again visual-
ized in the UV/Vis spectra. Importantly, DeBN decreases
upon oxidation, revealing a decline in the dihedral angle
and, hence, a change in the interaction between the conju-
gated pentamers.[12] Note that MDC 4 shows a clear bisig-
nate Cotton effect near 400 nm, which originates from chiral
exciton coupling between the two oligomers and is therefore
absent in MDC 3, which is equipped with only one oligomer.
In this way, the BN moiety transfers its chirality to the oligo-
thiophenes. Finally, MDC 2, which is composed of two oxi-
dizable pentathiophenes, is oxidized. From the UV/Vis spec-
tra, it can be concluded that oxidation indeed occurs and
that at higher degrees of oxidation, additional, blue-shifted
peaks (near 650 and 950 nm) arise, which can be attributed
to p dimerization.[2,3,5] A second indication of interactions
between the oxidized oligothiophenes is provided by cyclic
and differential-pulse voltammetry (Figure 3).[13] The cyclic
voltammogram of MDC 2 shows two quasireversible oxida-
tions. In the case of MDC 4 one quasireversible oxidation is
present together with an irreversible second oxidation at
higher potential, which can be attributed to the instability of
an oxidized oligothiazole moiety. The first oxidation of
MDC 2 clearly occurs at a lower potential than the first oxi-
dation of MDC 4, showing that the oxidized pentathiophene
is more stabilized by a pentathiophene (MDC 2) than by an
oligothiazole (MDC 4). This observation clearly demon-
strates the interaction between the (oxidized) oligothio-
phenes.
Figure 1. General protocol and the structure of MDC 1–4.
ments with respect to the naphthalene axis.[8] In particular,
the high-energy lobe of the CD signal in a typical (S)-BN
geometry evolves from a small positive to a strong negative
effect, with a zero-crossing at approximately 1108 and a
moderate signal at approximately 908, the typical angle of
most BN molecules.[8] By further decreasing q, the intensity
again drops (Figure 6). Note that the dihedral angle can
vary over a broad range and even adopt small values, pro-
vided that an additional driving force, such as incorporation
into a ring, is present.[9] Consequently, probing this CD
signal can be used to visualize changes in the dihedral angle
of the BN core and, hence, interactions between the oligo-
mers and even qualitatively compare them,[10] provided that
following requirements are fulfilled:
1) The tracked signal must solely originate from the BN
couplet, that is, it may not interfere with signals from the
oligomer. A comparison of the UV/Vis and CD spectra
of MDC 1 and the oligothiophene reveals that indeed
overlapping occurs.[7] However, when l>380 nm, MDC 1
does not give any signal and the UV/Vis and CD spectra
are exclusively governed by the oligomer. On the other
hand, for 280<l<320 nm, in which the shape of the CD
spectra of MDC 1 and MDC 2 coincide and the chromo-
phores hardly absorb, the Cotton effects solely arise
from the BN hinge molecule. Therefore, De at the mini-
mum of the lobe near 310 nm, denoted as DeBN, can be
monitored as the indicative signal.
Oxidation also affects the CD spectra. Moreover, the
clear absence of an isosbestic point is in agreement with two
subsequent one-electron oxidations (one on each pentathio-
phene), the latter being accompanied by p dimerization.
This is in contrast with the single-step oxidation of MDC 4,
10964
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 10963 – 10967

Chirally Organized Oligothiophenes
COMMUNICATION
Figure 2. UV/Vis and CD spectra of MDC 2 (c=1.47.10À5 m) (a, b), MDC 3 (c=3.94.10À5 m) (c, d), and MDC 4 (c=3.13.10À5 m) (e, f) in dichloromethane
during gradual oxidation with Et3OSbCl6. In the region 280<l<320 nm, indicated by the grey areas, the CD signals originate from the BN hinge; in the
region l>380 nm, indicated by the checked areas, the UV/Vis and CD spectra arise from the oligomer. See the Supporting Information for more de-
tailed spectra.
in which such an isosbestic point can be observed. Finally,
the comparison of the magnitude of the change of DeBN of
MDC 2 and MDC 4 reveals that the interaction between
oxidized pentathiophenes (although both are electron poor
and charged) is much stronger[12] than the interaction be-
tween oxidized pentathiophene and electron-poor but neu-
tral oligothiazole, which clearly reflects the strength of the p
dimerization.
As already mentioned, the magnitude of DeBN directly de-
pends on the interaction between the two moieties (if pres-
ent) that are connected to the naphthalenes. Therefore,
probing DeBN cannot only be used to monitor changes in in-
teractions upon oxidation, but it can already reveal differ-
AHCTUNGTREGeNNUN nces in p interactions in different neutral oligothiophenes.
Indeed, DeBN of MDC 1, in which no interactions between
oligomers are present, is approximately 60mÀ1 cmÀ1,[7] where-
as in contrast, DeBN of MDC 2 and MDC 4 are approximate-
ly 100mÀ1 cmÀ1, indicating a smaller q and thus reflecting the
p-interactions between the oligomers in the latter com-
pounds. Second, because the model does not rely on the
chirACTHNUGRTENUNGoptical properties of the oligomers, it is not restricted to
studying p interactions and their differences induced by oxi-
dation, but it can more generally be used to probe any inter-
action between moieties connected to the binaphthalene
core through the same ethenylene linker. To test this hy-
pothesis, MDC 5–6 (Figure 4) were prepared, in which de-
protonation instead of oxidation allows for the manipulation
of the interactions between the oligomers.
To confirm that the deprotonation does not wrongly
affect DeBN, KOtBu is first gradually added to MDC 5. The
red shift (from 400 to 440 nm, Figure 5) confirms the depro-
Figure 3. Calibrated cyclic voltammograms (top) of MDC 2 (a) and
MDC 4 (c) in dichloromethane with ferrocene as the internal standard
versus a standard calomel electrode (SCE; scan rate=100 mVsÀ1). Cali-
brated and normalized differential pulse voltammograms (bottom) under
the same conditions, but with a pulse amplitude of 75 mV (50 ms) and a
scan rate of 20 mVsÀ1
.
Chem. Eur. J. 2010, 16, 10963 – 10967
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10965

G. Koeckelberghs et al.
spectra are shown in grey, Figure 5). The necessity to use
large quantities of the base for the second deprotonation
suggests that the deprotonation of the first oligomer signifi-
cantly decreases the acidity of the second one. This is con-
firmed by the fact that if the less basic KOH is employed,
the second deprotonation does not occur, even at a very
large excess. If the CD spectra are considered, it is striking
that DeBN again amounts to approximately 100mÀ1 cmÀ1
(MDC 6). The first deprotonation results in a relatively
small drop in DeBN; the second deprotonation further in-
creases the interactions between the oligomers, which is re-
flected by the smaller DeBN and, hence, q.[12]
As already mentioned, the universality of the approach
allows the qualitative comparison of interactions between a
set of two oligomers, because this is reflected by the magni-
tude of DeBN. Such a comparison (Figure 6) shows that in
the absence of any interactions (MDC 1) DeBN is approxi-
mately 60mÀ1 cmÀ1, corresponding to a typical qꢀ908,[8] and
p interactions (MDC 2, MDC 4, and MDC 6) decrease q.
The fact that similar values are found (DeBN ꢀ100mÀ1 cmÀ1)
can be attributed to the fact that the interaction is in all
cases the same and/or the DeBN to q relationship reaches a
plateau. Both oxidation and deprotonation increase the in-
teractions between the oligomers (q further decreases), but
the (multiple) oxidation increase the interactions to a larger
extent than the (multiple) deprotonation.
Figure 4. Structures of MDC 5 and 6.
tonation of the oligomer, but DeBN remains unchanged. This
again demonstrates that neither changing the nature of the
chromophore nor the introduction of ions influence DeBN
.
Aggregation can also be ruled out because MDC 6 dissolves
in tBuOH.[7] Moreover, the fact that the CD spectra of
MDC 3 and MDC 5 are very similar in both shape and mag-
nitude again supports the fact that they are essentially due
to the chiral BN, irrespective of the presence and nature of
the oligomer. Next, MDC 6 was subjected to deprotonation,
which can be visualized in both the UV/Vis and CD spectra.
Interestingly, after deprotonation the original, neutral situa-
tion can be restored by the addition of HCl.[7] Furthermore,
a more detailed inspection of the spectra reveals that the de-
protonation is a two-step process, with the second deproto-
nation only occurring at large quantities of KOtBu (these
In conclusion, a protocol that allows (changes in) interac-
tions between two oligomers to be monitored is reported. It
consists of the attachment of two oligomers to a chiral, flexi-
ble binaphthalene core and monitoring DeBN. Because this
Figure 5. UV/Vis and CD spectra of MDC 5 (c=2.41.10À5 m) (a, b) and MDC 6 (c=2.41.10À5 m) (c, d) in THF/tBuOH (1:1) during gradual deprotonation
with KOtBu and MDC 6 (c=2.41.10À5 m) (e, f) in THF/MeOH (8:2) during gradual deprotonation with KOH. In the region 280<l<320 nm, indicated
by the grey areas, the UV/Vis and CD signals originate from the BN hinge; in the region l>380 nm, indicated by the checked areas, the UV/Vis and
CD spectra arise from the oligomer. See the Supporting Information for more detailed spectra.
10966
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 10963 – 10967

Chirally Organized Oligothiophenes
COMMUNICATION
[3] a) C. Song, T. M. Swager, Org. Lett. 2008, 10, 3575; b) R. Takita, C.
Song, T. M. Swager, Org. Lett. 2008, 10, 5003.
[4] a) R. Rathore, V. J. Chebny, E. J. Kopatz, I. A. Guzei, Angew. Chem.
2005, 117, 2831; Angew. Chem. Int. Ed. 2005, 44, 2771; b) M. Fukui,
T. Mori, Y. Inoue, R. Rathore, Org. Lett. 2007, 9, 3977.
[5] a) P. Bꢂuerle, U. Segelbacher, K.-U. Gaudl, D. Huttenlocher, M.
Mehring, Angew. Chem. 1993, 105, 125; Angew. Chem. Int. Ed.
Engl. 1993, 32, 76; b) P. Bꢂuerle, U. Segelbacher, A. Maier, M.
Mehring, J. Am. Chem. Soc. 1993, 115, 10217; c) L. L. Miller, K. R.
Mann, Acc. Chem. Res. 1996, 29, 417; d) J. J. Apperloo, R. A. J. Jans-
sen, P. R. L. Malenfant, L. Groenendaal, J. M. J. Frꢃchet, J. Am.
Chem. Soc. 2000, 122, 7042.
[6] D. Cornelis, H. Peeters, S. Zrig, B. Andrioletti, E. Rose, T. Verbiest,
G. Koeckelberghs, Chem. Mater. 2008, 20, 2133.
[7] See the Supporting Information for more details.
[8] a) N. Harada, K. Nakanishi, Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Stereochemistry, Oxford University Press,
Oxford, 1983; b) A. Rodger, B. Nordꢃn, Circular Dichroism and
Linear Dichroism, Oxford University Press, Oxford, 1997; c) L.
Di Bari, G. Pescitelli, P. Salvadori, J. Am. Chem. Soc. 1999, 121,
7998; d) L. Di Bari, G. Pescitelli, F. Marchetti, P. Salvadori, J. Am.
Chem. Soc. 2000, 122, 6395.
Figure 6. Comparison of the dihedral angle (q) in the model compounds
before and after the external stimulus. The evolution of DeBN as a func-
tion of q was calculated by using classical physics, assuming standard bi-
naphthalene geometries, Gaussian absorption bands (s=1.5ꢄ1014 sÀ1),
and a transition dipole strength of 50 D2 located along the naphthalene
long axis.[14]
[9] E. Hendrickx, C. Boutton, K. Clays, A. Persoons, S. van Es, T. Bie-
mans, B. Meijer, Chem. Phys. Lett. 1997, 270, 241.
[10] Because De does not scale (linearly) with the dihedral angle and the
(change of the) dihedral angle does not (linearly) depend on the
(change in the) strength of the interaction, and also owing to some
simplifications, the proposed model cannot be used to quantitatively
measure the strength of the interactions between the oligomers, but
only allows the qualitative scaling of interactions between oligo-
mers.
property is independent of the nature of the oligothiophene
or external stimulus performed, it can be generally used to
monitor and compare interactions between two p-conju-
ACHTUNGTRENNUNGgated oligomers.
[11] R. Rathore, A. S. Kumar, S. V. Lindeman, J. K. Kochi, J. Org. Chem.
1998, 63, 5847.
Acknowledgements
[12] If q is such that a maximal CD intensity is obtained, any variation of
q, and, hence, of the interaction between the oligomers (decrease or
increase), results in a decline of the CD intensity. Applied to the ex-
periments performed, the drop of the CD intensity upon applying
the external stimulus can, in principle, both indicate an increase or
decrease of q, provided that it was started from a maximal CD in-
tensity. However, the nature of the interaction (such as delocaliza-
tion of charges or p dimerization) and the vast amount of literature
on these subjects rules out the possibility that the interaction weak-
ens.
This work was supported by the Katholieke Universiteit Leuven (GOA),
the Fund for Scientific Research (FWO-Vlaanderen). D.C. is a doctoral
fellow of the IWT and G.K. is a postdoctoral fellow of FWO-Vlaanderen.
Keywords: binaphthalenes
·
chirality
·
conjugation
·
interchain interactions · oligothiophenes
[13] The first and second oxidation, resulting in MDC 2+C and MDC 2++
,
[1] a) A. Mishra, C.-Q. Ma, P. Bꢂuerle, Chem. Rev. 2009, 109, 1141, and
references therein; b) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer,
A. P. H. J. Schenning, Chem. Rev. 2005, 105, 14911, and references
therein.
respectively, occur at a lower potential than oxidation of the parent,
uncoupled pentathiophene, which can be attributed to charge deloc-
alization of the radical cation by the second pentathiophene and p
dimerization. These results will be published elsewhere.
[2] a) T. Sakai, T. Satou, T. Kaikawa, K. Takimiya, T. Otsubo, Y. Aso, J.
Am. Chem. Soc. 2005, 127, 8082; b) K. M. Knoblock, C. J. Silvestri,
D. M. Collard, J. Am. Chem. Soc. 2006, 128, 13680; c) C. Edder,
J. M. J. Frꢃchet, Org. Lett. 2003, 5, 1879.
[14] S. F. Mason, R. H. Seal, D. R. Roberts, Tetrahedron 1974, 30, 1671.
Received: April 23, 2010
Published online: August 16, 2010
Chem. Eur. J. 2010, 16, 10963 – 10967
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10967