Complementary helicity interchange of optically switchable supramolecular-enantiomeric helicenes with (-)-gel-sol-(+)-gel transition ternary logic

Article abstract of DOI:10.1021/ja401214t

A gallamide-containing pseudoenantiomeric helicene pair bearing a (10R,11R)-dimethoxymethyldibenzosuberane core can self-assemble by intermolecular amide H-bonding and π-π stacking into bundled helical fibers with helical tunnels of complementary helicity in CH₂Cl ₂. The helicenes undergo excellent complementary photoswitchings of ternary logic at 280, 318, and 343 nm through (-)-gel-sol-(+)-gel interconversion.

Full text of DOI:10.1021/ja401214t

Communication
pubs.acs.org/JACS
Complementary Helicity Interchange of Optically Switchable
Supramolecular-Enantiomeric Helicenes with (−)-Gel-Sol-(+)-Gel
Transition Ternary Logic
Chien-Tien Chen,*,† Chien-Hsiang Chen,‡ and Tiow-Gan Ong§
†Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan
‡National Taiwan Normal University, Taipei, Taiwan
§Institute of Chemistry, Academia Sinica, Taipei, Taiwan
S
* Supporting Information
Scheme 1. Photoswitchable Helicene Gelator by Self-
Assembly
ABSTRACT: A gallamide-containing pseudoenantio-
meric helicene pair bearing a (10R,11R)-dimethoxymethyl-
dibenzosuberane core can self-assemble by intermolecular
amide H-bonding and π-π stacking into bundled helical
fibers with helical tunnels of complementary helicity in
CH2Cl2. The helicenes undergo excellent complementary
photoswitchings of ternary logic at 280, 318, and 343 nm
through (−)-gel-sol-(+)-gel interconversion.
ransmission of chiral information from molecular to macro-
polymers bearing a photoswitchable fluorene-based helicene
head unit, allowing for efficient “sergeants and soldiers” and/or
“majority rules” induced control of polymer helicity.7g,8
As part of our ongoing research on the uses of C2-symmetric
dibenzosuberane (DBS)-based helicenes as conformation-
modulable, chirochromic optical switches having reversible,
complementary helicities (M or P′) in liquid-crystalline
materials,9 we sought to evaluate the feasibility of their use for
chiral organogel formation by the “sergeants and soldiers” or
“majority rules” effect,10c with complementary helicity inter-
changes that are switchable by light.
In contrast to our previous molecular designs of photo-
switchable helicenes, here we invoked a unique type of helicene,
1, incorporating aromatic amide (“aramide”) moieties at both the
C3 and C7 positions in the top C2-symmetric DBS template
(Scheme 1). In addition to H-bonding and π-π interactions, extra
vdW forces were introduced by appending long-chain alkyl
groups at the 3, 4, and 5 positions of the aramides to effect
interchain alignment, further facilitating organogel formation.10
Furthermore, a phenyl ring was introduced at the C8′ position of
the bottom tetralin template to enhance the π-π interactions
(edge-to-edge distance = 3.33 Å) and thus the helical nature of
the resulting organogels.11
1
Tor even supramolecular levels
(i.e., asymmetric tran-
scription) is a powerful mechanism adopted in nature for the
controlled self-assembly of chiral subunits into complex
superstructures with well-defined functions due to unique 3-D
molecular shapes and cavities. Helical domain(s) in macro-
molecules such as starch, protein, and DNA may thus be
generated.2 H-bonding, electrostatic, π-π, and van der Waals
(vdW) interactions are the major molecular forces involved in
triggering these self-assembly processes. As inspired by nature,
synthetic systems offer a diverse array of opportunities to control
the properties of materials in a dynamic, modulable, and even
reversible manner. By taking advantage of these molecular forces,
one can also trigger the self-assembly of custom-designed small
molecules into large polymer-like aggregates or gel networks.
Low-molecular-weight gelators (LMWGs) are organic com-
pounds with MW < 3000 Da3 that can form gels in organic
solvents and/or water at low concentrations (2−10 wt%).4 Weak
interactions like those mentioned above can lead to formation of
a gel network.5 The ordered arrangement in organogels made by
physical interplay can be perturbed by forces including heat, light,
chemical additives, and mechanical action. These factors facilitate
control over the sol-to-gel interchange. Despite advances in the
use of LMWGs in many applications,6 chiral helical variants of
LMWGs are relatively unexplored.7a−d To date, in situ direct
helicity interchanges in helical organogels by external stim-
ulators7e remain a challenge, even though helicity reversal in local
segment(s) of gel fibers has been observed by thermal control7f
or changes in gelator loading.7g,h Seminal work by Feringa et al.
successfully addressed this issue using 1,2-thienylcyclopentene
bearing chiral amide arms by temperature-modulated, light-
triggered reversible transcription of macroscopic into molecular
chirality.7b,i,j They also developed N-hexylurea-based helical
To identify two different irradiation wavelengths to achieve
photochemical switching between two pseudoenantiomeric
helicenes with complementary helicities, their individual UV/
vis and difference spectra with discernible extinction coefficient
changes (Δε) were measured, allowing their relative composi-
tion at the photostationary state (pss) of a given irradiation
wavelength to be assessed. The diastereomeric excess at the pss
Received: February 3, 2013
Published: March 22, 2013
© 2013 American Chemical Society
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Communication
Scheme 2. Photoisomerization Profiles of Helicenes (M)-1
and (P)-1′ in n-Hexane or CH2Cl2 (1×10−5 M)
Figure 2. (a) TEM images of the bundled superstructures of xerogels
from (top) (M)-1 and (bottom) (P)-1′. (b) Tapping-mode AFM image
of a sample obtained by spin-coating a solution of (M)-1 in CH2Cl2.
Inset: cross-section analysis along the green line. (c) AFM image of the
same sample taken after 1 week.
formed gels in CH2Cl2, CHCl3, and CH3CN at a minimum
concentration of 1×10−3 M (i.e., ≥2 mg/mL). The resulting gels
were stable at ambient temperature, and the gel-to-sol
interconversion was reversible upon repetitive heating (≥35
°C) and cooling cycles (Figure S3). Optimal concentrations for
gelation in CH2Cl2 ranged from 2 to 20 mg/mL.
Figure 1. (left) UV/vis and difference spectra of (M)-1 and (P)-1′ in
solution (n-hexane) and gel (CH2Cl2). (right) Stacked CD dynamic
plots before and after irradiation of (M)-1 (1.0×10−5 M) at 270 4 nm.
([de]pss) at a given wavelength can often be directly determined
by the difference in the extinction coefficients under the
condition that the photoisomerization quantum yields for the
two processes (ΦM→P and ΦP→M) stay similar. Thus, to attain
efficient photoswitching in a highly selective manner, it is better
to focus the photoirradiation on regions with the largest
complementary changes in the extinction coefficients.
UV/vis, UV/vis difference, and CD spectra of (M)-1- and (P)-
1′-doped CH2Cl2 gels were red-shifted by 10 nm relative to those
taken in n-hexane solution (Figure 1 left and Figure S4),
indicating their better π-π conjugation and/or stacking in the gel
states. Dried organogels (i.e., xerogels) formed from (M)-1- and
(P)-1′-doped CH2Cl2 gels showed left- and right-handed helical
fibril rope morphologies, respectively, as confirmed by trans-
mission electron microscopy (TEM) (Figures 2a and S5). For
both, the helical pitch was 1.5−1.6 μm. The tapping-mode
atomic force microscopy (AFM) image and cross-section profile
of a sample prepared by spin-coating a 1.0 mM solution of (M)-1
in CH2Cl2 onto highly ordered pyrolytic graphite shows four
repetitive 20 nm deep minor grooves for each 130 nm deep major
groove within a bundled fibril tube (Figures 2b and S7a). The
distance between major grooves is 800 nm, and that between
minor grooves is 160 nm. This indicates that a bundled fibril tube
consists of five bundled fibers (Figure S6), as evidenced by the
AFM image of the same sample taken after 1 week (Figures 2c
and S7b). Thus, hierarchical self-assembly of the helicene
monomers may be involved in formation of the bundled helical
fibril tubes.10e,k Furthermore, a 105 5 nm diameter helical
tunnel exists inside each bundled fibril tube.
Initial photoisomerization experiments were carried out by
irradiating (10R,11R,M)-1 [denoted as (M)-1] at 254 nm, as was
done previously for helicenes bearing (10R,11R)-diethyl-9a,b and
-dimethoxymethyl-substituted9c top templates. Notably, diaster-
eomeric switching of (M)-1 at 254 nm in n-hexane or CH2Cl2 led
to exclusive formation of (P)-1′ [(M)-1/(P)-1′ = <1/>99, as
determined by HPLC analysis of the reaction mixture (Scheme 2
and Figures S2 and S2′]. Subsequent photoisomerization
experiments were performed at 270, 308, and 335 nm based
on the UV/vis difference spectrum between (M)-1 and (P)-1′
(Figure 1 left). The detector wavelength for HPLC was set at 308
nm, the isosbestic point in the stacked UV/vis spectra of (M)-1
and (P)-1′ (Figure 1 left). Highly diastereoselective and
complementary switching selectivities could thus be achieved
upon photoirradiation of (M)-1 at 270 nm [(M)-1/(P)-1′ = <1/
>99] and (P)-1′ at 335 nm [(M)-1/(P)-1, 90/10] (Figures S2
and S2′). Notably, a pseudoracemic mixture [(M)-1/(P)-1′ =
50/50] was obtained upon irradiation at 308 nm in n-hexane or
CH2Cl2 (Scheme 2). In addition, split Cotton effects of
complementary exciton chirality were observed in the circular
dichroism (CD) spectra associated with the UV band at 280 nm
for (M)-1 and (P)-1′ (Figure 1 right), attributed to the change in
the electric transition dipole moment from the bottom 8′-phenyl
template to the top-right benzamide-substituted phenyl ring. In
contrast, only moderate diastereoselectivity (M/P′ = 25/75 at
280 nm) was observed for the helicene without the 8′-phenyl
appendage. Thus, introduction of the 8′-phenyl moiety was
essential to secure high switching profiles. Presumably, the
intramolecular π-π interaction between the 8′-phenyl group and
the DBS template can prevent undesired electrocyclization and
also increase the thermal stability of the helical configuration.
Of seven different solvents tested for possible gel formation,
(M)-1 and (P)-1′ remained dissolved in n-hexane, cyclohexane,
benzene, and toluene even at 0.01 M at ambient temperature but
Notably, (M)-1 and (P)-1′ helicenes led to the corresponding
M- and P-form helical fibers. Thus, the absolute chirality of the
helicene dictates the overall helical chirality of the self-assembled
helical fibers, presumably through intermolecular H-bonding and
π-π interactions.
FT-IR spectra of helicene (M)-1 both in solution (1.0×10−5
M) and in gel (1.7×10−3 M) were also examined (Figure S8). In
solution, the N−H and CO stretching modes appeared at
3439 and 1806 cm−1, respectively. After gel formation, these
bands were shifted to lower energy (3283 and 1733 cm−1,
respectively), presumably because of facile intermolecular N−
H···OC H-bonding in the gel. The downfield shift of the N−H
resonances of (M)-1 from 7.62 to 8.73 ppm in concentration-
dependent 1H NMR experiments (0.4−1.1 mM range in
CD2Cl2) confirmed the N−H···OC H-bonding (Figure 3).
To gain insight into the critical composition(s) of (M)-1/(P)-
1′ in CH2Cl2 to maintain the gel state at the minimal dopant
loading (i.e., 2−3 mg/mL or (1.2−1.7)×10−3 M), we performed
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Figure 3. Expanded stacked 1H NMR spectra of (M)-1 at 0.4−1.1 mM
in CD2Cl2 solution.
Figure 5. Expanded stacked CD/ORD plots of the photoisomerization
tracing experiments on the (M)-1 gel.
Figure 4. Photographs of the gel (3 mg/mL in CH2Cl2) at ambient
temperature after irradiation of the original (M)-1-containing gel at 318
nm for at least 60 min (middle) and then at 280 nm (right) or 343 nm
(left) for another 6−8 h.
the photoisomerization experiments using the gels at ambient
temperature. Upon irradiation of pure (M)-1-containing gel in a
sealed quartz cuvette (1 cm path length) at 318 4 nm with a
monochromator light source, the gel turned into a homogeneous
solution after 60 min, with the overall dopant composition
reaching (M)-1/(P)-1′ = 60/40 (Figures 4 and S9). Further
irradiation of the homogeneous solution at 280 4 nm for 60 min
led to an overall dopant composition of 25/75, where the
complementary helical gel state started to form, and the dopant
composition gradually shifted to pure (P)-1′ after another 6 h of
irradiation. Conversely, irradiation of the homogeneous solution
[(M)-1/(P)-1′ = 50/50] at 343 4 nm led to a dopant
composition of 69/31, in which the original gel state began to
be restored, and the dopant composition gradually returned to
the initial (M)-1 state [(M)-1/(P)-1′ = 90/10] after another 8 h
of irradiation. Thus, the helical superstructures of the bundled
helical fibril tubes can be controlled in a complementary and
reversible fashion by exposing the gel materials to either 280 or
343 nm irradiation.
Both UV/vis and CD/optical rotatory dispersion (ORD)
dynamic tracing experiments were carried out to gain insight into
the (−)-gel-sol-(+)-gel transition profiles. The UV λmax at 280
nm was completely shifted to 270 nm when the overall
composition of (M)-1/(P)-1′ in CH2Cl2 was changed from
70/30 to 60/40 (Figure S10), where the initial (−)-gel
completely turned into the sol. Conversely, the CD null point
at 291 nm, associated with the negative exciton chirality of the
bundled (−)-helical fibers, was completely shifted to 280 nm
(Figure 5).
Figure 6. TEM images of bundled xerogel superstructures with the
overall (M)-1/(P)-1′ composition (1.7×10−3 M in CH2Cl2) changed
from 100/0 to (a) 90/10, (b) 70/30, (c) 50/50 (inset: expanded image),
and (d) 20/80.
and CD/ORD throughout the photoswitching experiments and
found to remain intact for five consecutive cycles (Figure S11).
Two possible pathways can account for photoswitching in
gel−sol transitions. One involves local helical chirality switching
that subsequently induces the neighboring helicenes to switch
synergistically in the assembled single or bundled fibers. The
other involves initial detachment of the photoisomerized
helicenes or single fibers from the original homochiral bundled
fibers and subsequent reassembly of the freely dissociated
helicenes or single fibers of the same chirality to form the
supramolecular enantiomeric bundled fibers.
In a series of TEM images taken from these photo-
isomerization experiments, only either pure (M)- or (P)-form
bundled fibril tubes were observed when the overall composition
ranged from (M)-1/(P)-1′ = >99/<1 to 70/30 and 25/75 to <1/
>99, respectively (Figure 6; see Figure S12 for larger images).
Notably, increasing amounts of micelles or vesicles were
observed for the intermediate gel and sol states. In particular,
only micelles or vesicles (∼3.3 μm i.d., 0.52 μm thick) were
observed at 50/50 overall composition even though the helicenes
were still engaged in intermolecular H-bonding, as evidenced by
the IR spectrum in CH2Cl2 (Figure S8). Two different sets of N−
H and CO stretching modes were observed, indicating
formation of both homochiral and heterochiral aggregates.
The results suggest that photoisomerization in the gel may
proceed through the second mechanism, where initially detached
pseudoenantiomeric helicenes or single fibers from the original
homochiral bundled fibers get dissolved and confined in CH2Cl2.
They then gradually reassemble to form the complementary
bundled fibers, subsequently furnishing the complementary
In addition, the negative Cotton effect peak at 306 nm was
completely shifted to 296 nm. The sol remained unchanged until
the overall composition reached (M)-1/(P)-1′ ≈ 30/70, where
the UV λmax at 270 nm changed back to 280 nm and the sol
started turning into the complementary (+)-gel state. In the
meantime, an inverted CD/ORD peak at 300 nm started to
appear and then ultimately displayed a positive Cotton effect at
300 nm as well as a positive exciton chirality in the pure (P)-1′ gel
with the null point back to 291 nm. Photostabilities of both (M)-
1 and (P)-1′ in the warm gel states were monitored by UV/vis
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
ctchen@mx.nthu.edu.tw
■
Notes
The authors declare no competing financial interest.
Walther, P.; Moller, M. Adv. Mater. 2000, 12, 513. (e) Review of
dynamic helical structures: Maeda, K.; Yashima, E. Top. Curr. Chem.
̈
ACKNOWLEDGMENTS
■
2006, 265, 47. (f) Pratihar, P.; Ghosh, S.; Stepanenko, V.; Patwardhan,
We thank the National Science Council of Taiwan for financial
support of this research (98-2119-M-007-012-MY3). Dedicated
to Prof. Scott E. Denmark on the occasion of his 60th birthday.
S.; Grozema, F. C.; Siebbeles, L. D. A.; Wurthner, F. Beilstein J. Org.
̈
Chem. 2010, 6, 1070. (g) Kaiser, T. E.; Stepanenko, V.; Wurthner, F. J.
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Am. Chem. Soc. 2009, 131, 6719. (h) Wurthner, F.; Bauer, C.;
Stepanenko, V.; Yagai, S. Adv. Mater. 2008, 20, 1695. (i) Li, X.-Q.;
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