Macroscopic origin of circular dichroism effects by alignment of self-assembled fibers in solution

Article abstract of DOI:10.1002/anie.200703075

All lined up: Supramolecular assemblies are aligned by a convective flow in clear, nonviscous dilute solutions (see picture). The linear dichroism created by the alignment results in artifacts in circular dichroism measurements, the origins of which are explained. (Figure Presented).

Full text of DOI:10.1002/anie.200703075

Angewandte
Chemie
DOI: 10.1002/anie.200703075
Supramolecular Chemistry
Macroscopic Origin of Circular Dichroism Effects by Alignment of
Self-Assembled Fibers in Solution**
ˇ
´
Martin Wolffs, Subi J. George, Zeljko Tomovic, Stefan C. J. Meskers,*
Albertus P. H. J. Schenning,* and E. W. Meijer*
We report herein an unexpected circular dichroism
(CD) effect observed for a dilute solution of an achiral
oligo(p-phenylene vinylene) derivative, A-OPV3
(Scheme 1) in dodecane (c = 5 ” 10À5 m). This effect
likely originates from macroscopic phenomena during
the self-assembly process of A-OPV3 into fibers after
slow cooling from 363 K to 293 K at a rate of 60 KhÀ1
(Figure 1a). To our surprise, the CD response
increased in intensity after shaking the fully trans-
parent, nonviscous solution. To shed light on this
observation, we studied the self-assembly of A-OPV3
in more detail. Upon cooling the A-OPV3 solution
from 363 to 293 K, a typical blue shift of the absorption
maximum to l = 407 nm with a shoulder at l = 475 nm
is observed. This finding indicates that at high temper-
ature the molecules are molecularly dissolved, while at
Figure 1. a) CD spectra at 293 K of an A-OPV3 solution in dodecane after cooling
from 363 K at 60 KhÀ1 (black) and after shaking the solution (red).[2] b) Absorption
spectra at 363 K (black), at 293 K after cooling (red), and after shaking (green;
1 cm cuvette, c=5”10À5 m) c) LD spectra of an A-OPV3 solution after cooling
(black) and after shaking (red; c=5”10À5 m, T=293 K) d) AFM image of A-OPV3
on mica drop cast from a dodecane solution (c=1”10À5 m).
low temperature they are self-assembled into fibers (Fig-
ure 1b).[1] However, at low temperatures, a small response in
the linear dichroism (LD) spectrum is observed, which
increased slightly after shaking (Figure 1c). LD and CD are
absent at higher temperatures. The atomic force microscopy
(AFM) images of dried drop-cast solutions of these self-
assembled systems on mica showed large bundles of long
fibers (Figure 1d).
The three spectra (UV/Vis, CD, and LD) are almost
identical in shape (Figure 1a–c). However, when monitoring
the spectra during the temperature-induced self-assembly
process (absorption and CD intensity at l = 475 nm and the
LD intensity at l = 400 nm), the absorbance changes first,
Scheme 1. The molecular structures of A-OPV3 and S-OPV4.
[+]
ˇ
´
[*] M. Wolffs, Dr. S. J. George, Dr. Z. Tomovic, Dr. S. C. J. Meskers,
Dr. A. P. H. J. Schenning, Prof. Dr. E. W. Meijer
Laboratory for Macromolecular and Organic Chemistry
Eindhoven University of Technology
PO Box 513, 5600MB Eindhoven (The Netherlands)
Fax: (+31)40-245-1036
E-mail: s.c.j.meskers@tue.nl
a.p.h.j.schenning@tue.nl
e.w.meijer@tue.nl
[+] Current Address:
while the LD and CD signals arise together at lower
[2]
temperatures.
Because A-OPV3 molecules form fibrous
Elastogran, BASF group
Global PU Specialties Research
E-KUE/KFH-A10
aggregates with a length that increases upon lowering the
temperature, this behavior indicates that a certain fiber length
is needed to induce the LD and CD effects. Hence, the
increase in the LD is most likely associated with the (partial)
alignment of the A-OPV3 fibers in solution arising from
convective flow, which is caused by temperature differences in
the cuvette.[3] Additional measurements show that the align-
ment is less significant when a smaller cuvette (0.1 vs. 1 cm) is
used,[2] that is, when the flow pattern is altered. The flow
pattern induced by shaking might enhance the alignment
Elastogranstrasse 60, 49448 Lemförde (Germany)
[**] We would like to thank Prof. Dr. T. Aida and his group members for
stimulating discussions, Dr. P. E. L. G. Lecl›re for the AFM
measurements and the Council for Chemical Sciences of the
Netherlands Organization for Scientific Research (NWO-CW) for
funding.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
obtained by the convective flow. Furthermore, LD and CD
spectra of aligned A-OPV3 films show effects very similar to
those observed in solution.[2] Consistent with the LD of
aligned films and the convective nature of the flow in the
heated cell, the LD measurements show that the A-OPV3
fibers in solution orient preferentially in the vertical direc-
tion.[4] The apparent CD effect can thus be interpreted in
terms of an artifact resulting from LD in the partially aligned
solution together with the inherent optical imperfections of
the CD instrument, namely linear birefringence (LB) in the
photoelastic modulator and/or other optical elements.[5]
These imperfections cause a contamination of the pure left
(right) circular polarized light with a horizontal (vertical)
linear polarized component, which is then selectively absor-
bed by the partially aligned A-OPV3 fibers. Consistent with
this interpretation, the apparent CD effect arising from
alignment caused by convective flow (which is in the vertical
direction) always shows the same sign. The sign of the
apparent CD effect from the aligned film of A-OPV3 can be
inverted by rotating the film over 908 relative to the
propagation direction of the light beam. The maximal artifical
CD contribution can, in this first approximation, not be larger
than the flow-induced LD, so that only CD effects larger than
the LD should be considered for molecular structural
interpretations.
Figure 3. a) Schematic picture of the angle f between film 1 and
film 2. b) Results of the orientation study of aligned films of A-OPV3
for f=À458 (c), f=458 (a) and f=908 (g).
whose sign and intensity depend on the angle of rotation of
film 2 (Figure 3), are obtained; these effects are very similar
to those induced by stirring. For dye-doped cholesteric liquid
crystals, macroscopic chirality can induce artificial CD effects,
which can be explained by considering a two-layer sample.
With the first layer showing LB and the second layer showing
LD’ at Æ 458 with respect to the first layer, large apparent
circular dichroism is observed, which is essentially a cross
product of the LB and LD’ in the two layers.[8,9] Similarly, in
the case of A-OPV3, the CD signal originates on the
macroscopic length scale and is determined by the relative
orientation of the second layer (Æ 458).[8,9] The observed
bisignate CD effects for stirred solutions of A-OPV3 can now
be explained by the vortex flow aligning the fibers in the front
and back of the cell at different angles with respect to the
vertical direction and with the LB of the front and the LD’ of
back side of the cell combining to an apparent CD contribu-
tion. The LD spectrum of the fibers is monosignate (Fig-
ure 1c) so that the LB spectrum, which is the Kramers–
Kronig transform of the LD spectrum, is expected to be
bisignate with a zero-crossing near the maximum of the LD
intensity. For the combination of LB and LD into the artificial
CD contribution, a bisignate spectral signature is expected,
consistent with the experiment.[10] These experiments allow us
to asses three different contributions to the observed CD
effects of A-OPV3 in a stirred solution. The first results from
the macroscopic helical arrangement of the assemblies
induced by the helical flow pattern in the sample cell, leading
to a bisignate CD effect originating from the combined LB
and LD’ optical responses of the A-OPV3. Furthermore,
there can be a minor contribution arising from LD of the A-
OPV3 and birefringence in the optical system, similar to the
monosignate CD effect under convective flow (see above).
Lastly, there might be a contribution from stirring-induced
chirality on the molecular level, for which a bisignate spectral
shape is also expected, but we disregard this option, because
the observed CD effect appears and disappears very rapidly
when turning the stirring on and off.
As the result of discussions with Aida and co-workers,[6]
we were introduced to the large Cotton effects in CD that
occur in stirred solutions of aggregates of dendritic zinc
porphyrins. For these systems, the sign and intensity of the
signal is proportional to the direction and speed of the vortex
flow created in the cuvette. To discriminate this type of vortex
flow from the convective flow described above, we decided to
stir the aligned achiral A-OPV3 solution first clockwise and
then counterclockwise (Figure 2). Stirring of the slowly cooled
solution results in extraordinary, large bisignate CD effects,
with the zero-crossing at the maximum absorption and for
which the sign is affected by the stirring direction, in full
agreement with the results of Aida and co-workers.[2,7] After
removing the vortex flow, the CD effect reverts to that
observed for the aligned A-OPV3 solution.[2] The shape of the
CD spectrum induced by stirring is very different from that of
the spectrum induced by convective flow discussed above,
which suggests a different origin for the stirring-induced effect.
The CD effects caused by vortex flow in the cuvette can be
explained by considering two aligned A-OPV3 films[2] that
are rotated with respect to each other. Bisignate CD effects,
Since we have studied the self-assembly of the chiral S-
OPV4 in great detail before,[11] we also investigated the
influence of shaking, cooling, and stirring on its assembly.[12]
This oligomer has one phenylene vinylene unit more than A-
OPV3, and the stereocenters in the side chains bias the
helicity of the chiral stacked fibers. The CD spectrum taken at
ambient temperature without stirring (Figure 4a) is free from
Figure 2. CD measurements of a previously cooled (self-assembled) A-
OPV3 solution stirred clockwise (c) and counterclockwise (a;
c=5”10À5 m, T=293 K).
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stirring. This partial orientation induces an
artificial contribution in the circular dichroism
spectra of both chiral and achiral assemblies.
The shape, sign, and intensity of the CD
spectrum depend on the type of flow, and two
different CD effects are observed; one is due to
convective flow and is based on linear dichro-
ism, and the other is due to vortex flow patterns
and is based on linear dichroism and linear
birefringence. It is clear that the alignment of
supramolecular assemblies in dilute solutions
can create artifacts in the chiroptical properties
of the system, which should be clearly differ-
entiated from the effects originating from
chirality at the molecular length scale. To
reduce the flow, we are currently designing a cuvette with
improved heat transfer to decrease the temperature gradient.
The flow-induced CD, together with hydrodynamic modeling,
will be used to study size and shape of the assemblies.
Figure 4. S-OPV4 dodecane solution (1 cm cuvette, c=3”10À5 m). a) CD spectra at
293 K without stirring (c) and with clockwise (a) and counterclockwise (g)
stirring (solution obtained by cooling from 363 to 293 K with 60 KhÀ1 without stirring)
b) Temperature-dependent CD measurements at l=420 nm (ÀdT/dt=60 KhÀ1) with-
~
*
out stirring ( ) and with clockwise ( ) and counterclockwise (&) stirring.
the two types of artifacts discussed above and corresponds to
the spectra reported earlier.[1,2] Furthermore, shaking does
not affect the CD spectra.[2] Under stationary conditions, no
LD effects are present. A CD cooling curve, similar to that
reported previously, is shown in Figure 4b; it displays an
elongation temperature Te of 339 K (c = 2.4 ”10 À5 m).[1] Mon-
itoring the cooling of the S-OPV4 solution with linear
dichroism shows an increase in LD[2] that starts at 334 K,
that is, below the elongation temperature of the self-assembly.
Since the size of the S-OPV4 assemblies is much smaller than
that for A-OPV3, the S-OPV4 fibers are less susceptible to
the flow patterns created in the cuvette; hence, randomization
of the fibers is much faster.[1] Although LD is present during
cooling, the CD response at 293 K is only marginally affected
(typically by 4%). Previously, we suggested on the basis of
AFM data that S-OPV4 forms bundles of fibers at high
concentrations and ascribed the additional increase in CD
intensity during cooling to the lateral interactions of the
fibers.[1] When this S-OPV4 solution (c = 2.4 ”10 À4 m) is
monitored by LD, an increase in intensity occurs at the
same temperature as the additional increase in CD. This result
suggests that the deviation from the cooperative self-assembly
model is probably caused by the formation of bundled fibers
that are aligned by the convective flow, as visualized with
linear dichroism.[2] Hence, we conclude that for S-OPV4,
convective flows can also give rise to deviations of the optical
spectra; the magnitude is so low that all published data are
only marginally influenced by this phenomenon.
Stirring of the S-OPV4 solutions, on the other hand, again
has a significant influence on the CD effect, the sign of which
depends on the stirring direction (Figure 4a). The absorption
is not affected by the vortex flow.[2] The CD intensity upon
stirring is less than for A-OPV3, which is probably a result of
the different sizes of the assemblies. Furthermore, the CD
intensity was also monitored at l = 420 nm during cooling,
while the S-OPV4 solution was either not stirred or stirred
continuously in a clockwise or counterclockwise direction
(Figure 4b). The effect of stirring becomes visible 5 K below
the elongation temperature. This behavior again suggests that
a certain stack length is needed for the systems to be
influenced by stirring and that chirality at the molecular level
seems unaffected by the stirring.
Received: July 10, 2007
Published online: September 20, 2007
Keywords: alignment · circular dichroism · convective flow ·
.
self-assembly · vortex flow
[1] P. Jonkheijm, P. van der Schoot, A. P. H. J. Schenning, E. W.
Meijer, Science 2006, 313, 80 – 83.
[2] See the Supporting Information.
[3] J. R. Welty, C. E. Wicks, R. E. Wilson, G. Rorrer, in Fundamen-
tals of Momentum, Heat, and Mass Transfer, 4th ed., Wiley, New
York, 2001.
[4] C. R. L. P. N. Jeukens, P. Jonkheijm, F. J. P. Wijnen, J. C. Gielen,
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Aida, Angew. Chem. 2007, 119, 8346 – 8350; Angew. Chem. Int.
Ed. 2007, 46, 8198 – 8202.
[7] After quickly cooling the same solution by quenching in an ice
bath, stirring showed a less intense bisignate CD effect similar in
sign to the slowly cooled solution. See the Supporting Information.
[8] a) Y. Shindo, Y. Ohmi, J. Am. Chem. Soc. 1985, 107, 91 – 97; b) F.
Livolant, M. F. Maestre, Biochemistry 1988, 27, 3056 – 3068.
[9] a) J. Schellman, H. P. Jensen, Chem. Rev. 1987, 87, 1359 – 1399;
b) F. D. Saeva, P. E. Sharpe, G. R. Olin, J. Am. Chem. Soc. 1973,
95, 7656 – 7659; c) E. Sackmann, J. Voss, Chem. Phys. Lett. 1972,
14, 528 – 531.
[10] In dye-doped cholesteric LCs, usually monosignate CD effects
arise. In this case, the LB component originates from the host
liquid crystal, which usually does not show a pronounced
wavelength dependence in the region where the dye absorbed,
because the LC is transparent in this region. See: S. F. Mason,
R. D. Peacock, J. Chem. Soc. Chem. Commun. 1973, 712 – 713.
[11] The synthesis of the compound has been reported elsewhere: P.
Jonkheijm, F. J. M. Hoeben, R. Kleppinger, J. van Herrikhuyzen,
A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc. 2003, 125,
15941 – 15949.
[12] The LD contribution in the self-assembly of S-OPV3 (the exact
chiral analogue of A-OPV3 with S-methylbutyloxy side chains)
is negligible. See the Supporting Information.
In conclusion, we observed that supramolecular assem-
blies can be partially oriented by convective flow, shaking, or
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