Thermally assisted photonic inversion of supramolecular handedness

Full text of DOI:10.1002/anie.201205332

Angewandte
Chemie
DOI: 10.1002/anie.201205332
Photoswitchable Helicity
Thermally Assisted Photonic Inversion of Supramolecular
Handedness**
Anesh Gopal, Mohamed Hifsudheen, Seiichi Furumi, Masayuki Takeuchi, and
Ayyappanpillai Ajayaghosh*
Dedicated to Professor Samir K. Brahmachari on the occasion of his 60th birthday
Symmetry breaking, leading to a specific handedness (either
right or left) of biological structures is one of the most
fascinating phenomenon in nature.[1] Notably, nature is able to
translate molecular chirality into supramolecular handedness,
through genetic-information transfer, thereby creating func-
tionally incredible helical structures of nanoscopic and
macroscopic dimensions, including giant superstructures.
While several factors such as vortex motion,[2] stirring,[3]
magnetic field,[4] and redox forces[5] may be involved, natural
light may have a key role in controlling the chirality and
helical sense of biological helices.[6] Light is undoubtedly
a versatile external stimulus to control the chemical and
physical properties of molecules, both natural and synthetic.[7]
Chirality is one of the chemical properties that can be
manipulated using light. For example, photoisomerization of
azobenzene[8] has been used as a trigger to induce point
chirality on a molecular level,[9] whereas on a macromolecular
level, light is known to influence the helicity of polymers[10]
and self-assemblies.[11] While molecular chirality and single-
chain polymer helicity are easy to manipulate with circularly
polarized light, reversible control of supramolecular helicity
in a macroscopic self-assembly using unpolarized light is
challenging.
Chirality amplification and helicity induction are known
to be predominant in p-systems.[12] The thermodynamic and
kinetic complexities in supramolecular polymerization of p-
systems have recently been revealed by Meijer and co-
workers.[13] While co-assembly and guest binding have been
shown to influence supramolecular helicity,[14] the use of light
as a stimulus to control the macroscopic helical sense of self-
assembled structures has rarely been used. Herein, we report
that the helicity of supramolecular assemblies associated with
a specific chirality can be reversibly switched to the opposite
helical sense through a chiral-center-controlled photoisome-
rization of the attached azobenzene moieties. To understand
the role of light and heat on helicity at a supramolecular level,
we have synthesized the azobenzene linked phenyleneethy-
nylene (PE) derivatives 1–3 (Scheme 1) using a Pd-catalyzed
Sonogashira–Hagihara coupling method[15] and characterized
them by FTIR, NMR spectroscopy, MALDI-TOF MS, and
elemental analysis. The UV/Vis absorption spectra of 1–3 (1 ꢀ
10ꢀ5 m) in tetrahydrofuran (THF) showed two maxima at 324
and 419 nm.[16] In methyl cyclohexane (MCH), at a concen-
tration of 1 ꢀ 10ꢀ5 m, the absorption band of (S)-2 at 419 nm is
red-shifted to 440 nm with a shoulder at 480 nm owing to
aggregation, which was confirmed by temperature dependent
UV/Vis absorption spectral changes (Figure S2).[16]
Upon UV irradiation of an MCH solution (1 ꢀ 10ꢀ5 m) at
323 K with a band-pass filter l = 350 ꢁ 30 nm (intensity
0.1 Wcmꢀ2), the trans-(S)-2 (E,E) was isomerized to the
corresponding cis form (E,Z or Z,Z) as indicated by the
decrease in the absorption intensity at 330 nm, with a slight
increase in the absorption intensity at 440 nm through two
[*] A. Gopal, M. Hifsudheen, Prof. A. Ajayaghosh
Photosciences and Photonics Group, Chemical Sciences and
Technology Division
National Institute for Interdisciplinary Science and Technology
(NIIST), CSIR
Trivandrum-695019 (India)
E-mail: ajayaghosh@niist.res.in
Homepage: http://www.niist.res.in/english/scientists/
ajayaghosh-a-/personal.html
isosbestic points (298 and 396 nm) as shown in Figure S3.[16]
A
photostationary state (PSS) is attained within 15 minutes of
irradiation. The reverse transition was attained by visible light
irradiation using a band-pass filter l = 450 ꢁ 30 nm (light
intensity 0.3 Wcmꢀ2) for 30 minutes or by keeping the
solution at 343 K for 3–4 hours. The photoisomerization was
monitored by 1H NMR spectroscopy (Figure S4).[16] The
percent conversion was calculated from the change in the
integrated area of the signal at d = 7.68 ppm, with respect to
a reference signal d = 4.02 ppm (m, 12H), yielding nearly
53% cis-isomers at the PSS. In principle, 50% isomerization
of (S)-2 or (R)-3 could lead to 100% E,Z isomer. However,
experimentally, the irradiated solution at the PSS contained
a mixture of cis-isomers (E,Z and Z,Z) along with some
remaining E,E isomers.[16] We were unable to find the exact
percentage of each isomer in the mixture because these
isomers were difficult to separate by HPLC.
Prof. M. Takeuchi
Organic Materials Group, National Institute for Materials Science
(Japan)
Dr. S. Furumi
Applied Photonic Materials Group, National Institute for Materials
Science (Japan)
and
PRESTO Japan Science and Technology Agency (Japan)
[**] We are grateful to Prof. Bert Meijer for fruitful discussions and
suggestions. A.A. is grateful to the Department of Atomic Energy for
a DAE-SRC Outstanding Researcher award. A.G. and M.H. are
thankful to CSIR for fellowships. We thank Mr. C. K. Chandrakanth
(NIIST, Trivandrum) for SEM measurements. This is contribution
No. NIIST-PPG-330.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205332.
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intense negative signal at 464 nm with two positive
signals at 407 and 322 nm (Figure 1b). In this case
also, photoirradiation resulted in a reversal of the
CD signal. The quantitative CD response from the
chromophore excitonic coupling could be estimated
from the dissymmetry factor (gabs = De/e), which is
the ratio of the circularly polarized absorption to
the total absorption.[17] The values of j gabs j at
464 nm for (S)-2, before and after irradiation, are
0.0111 and 0.0092, respectively, with a difference of
0.0020. The corresponding values for (R)-3 are
0.0115 and 0.0093, respectively, with a difference of
0.0022.
The temperature-dependent CD spectral-inten-
sity variations (Fn) for (S)-2 (1 ꢀ 10ꢀ5 m, in MCH)
were monitored at 464 nm before and after photo-
isomerization and are shown in Figure 1c. In a slow
cooling experiment (2 Kminꢀ1), before photoiso-
merization, the molecules follow a nucleation elon-
gation pathway which is characteristic of a cooper-
ative self-assembly[17] with an elongation temperature (Te) of
310 K. Interestingly, after irradiation, the cooling curve of the
mixture of isomers showed a decreased Te of 303 K, with
reversal in the sign of the CD signal. Upon cooling, the CD
signal turned sharply negative, which upon further cooling
reverted back towards zero. However, with continued cool-
ing, the original trend towards the negative was regained, with
overall reversal of the original helicity (Figure 1c). This
unusual variation of the cooling curve indicates that after UV
irradiation the co-operative self-assembly involves more than
one type of aggregate. The initial nucleation may occur with
the partially isomerized species present in higher concentra-
tion, with an inverted helicity, followed by the elongation of
the helical chain with aggregates of the non-isomerized
molecules with the initial P-helicity. Upon further cooling,
aggregates of the fully isomerized molecules with lower
stability and inverted helicity may also join the growing
helices, further accelerating the overall opposite handedness.
The corresponding cooling curves obtained from the absorp-
tion spectra also support a multistep growth process after
irradiation (Figure 1d).
Scheme 1. Photoisomerization of the azobenzene-linked phenyleneethynylene
derivatives 1–3. A mixture of E,E; E,Z and Z,Z isomers are possible.
The molecule 1 was inherently achiral before and after
irradiation, whereas the CD spectra of (S)-2 and (R)-3 showed
they were chiral and had opposite Cotton effects. For
example, the CD spectrum of (S)-2 showed an intense
positive signal at 464 nm with two negative signals at
407 nm and 322 nm with a zero crossing at 421 nm corre-
sponding to the p–p* transition of the PE moiety (Figure 1a).
Surprisingly, the CD spectrum after UV irradiation at 323 K,
followed by cooling the solution to a lower temperature,
showed a reversal of the CD spectrum, however with a lower
intensity of the CD bands (Figure 1a). On the other hand, the
molecule (R)-3 with opposite stereocenters exhibited an
Circularly polarized luminescence (CPL) is characteristic
of helical assembly of luminophores, which exist in a dissym-
metric environment in the photoexcited state.[18] The degree
of CPL is given by the luminescence dissymmetry ratio, which
is defined as glum = 2(ILꢀIR)/(IL+IR), where IL and IR are the
luminescence intensities of left and right circularly polarized
light. As expected, (S)-2 before isomerization showed a pos-
itive CPL with a glum value of + 0.008 at 503 nm (Figure 2a).
Interestingly, after photoisomerization, the sign of CPL was
reversed with a decrease in the glum value (glum = ꢀ0.002 at
503 nm). In the case of (R)-3, a negative CPL emission with
g
lum = ꢀ0.01 was obtained before photoisomerization (Fig-
ure 2b).[19] After photoisomerization, the CPL sign was
reversed with a glum value of + 0.002 at 503 nm. The
corresponding emission spectra are shown in Figure 2c,d.
Even though these glum values are relatively low when
compared to other systems,[20] it is apparent that the CPL
seen here is associated with the respective helicity of the
Figure 1. a,b) CD spectra (top) and the corresponding UV absorption
spectra (bottom) for (S)-2 and (R)-3 before (c) and after (g) UV
irradiation at 273 K. c) Normalized CD intensity (Fn) at l=464 nm
*
*
upon cooling of (S)-2 before ( ) and after ( ) UV irradiation. d) The
corresponding normalized absorption spectral changes.
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*
*
Figure 2. a) CPL spectra for (S)-2 before ( ) and after ( ) UV irradi-
*
*
ation; b) CPL spectra for (R)-3 before ( ) and after ( ) UV irradiation.
The corresponding fluorescence emission spectra are shown in (c)
and (d), respectively. All spectra were obtained using 1ꢀ10ꢀ4 m (S)-2
or (R)-3 in MCH.
Figure 3. SEM images of (S)-2 a) before and b) after photoisomeriza-
tion. The SEM analysis was done on a silicon wafer substrate after
evaporating a drop of 5ꢀ10ꢀ5 m (S)-2 in MCH. AFM images for (S)-2
c) before and d) after photoisomerization. The AFM analysis was done
on a freshly cleaved mica surface drop cast with 1ꢀ10ꢀ5 m (S)-2 in
MCH. The arrows indicate the helical twist (P or M) of the fibers.
complex and not related to selective absorption of the
emission by the chiral compounds. This is clear from the
fact that the achiral derivative did not exhibit any CPL.
SEM analysis of (S)-2 before photoirradiation showed
entangled right-handed (P) helical ropes of diameters ranging
from 50 nm to 1 mm and lengths of several micrometers
(Figure 3a). After irradiation at 323 K followed by cooling,
the helicity of the fibers were found to be left-handed (M), as
shown in Figure 3b. AFM analysis of (S)-2 revealed reversal
of their native helicity to the induced opposite screw sense
after irradiation (Figure 3c,d). From CD, SEM, and AFM
analyses, it is apparent that the helical ropes are formed by the
coiling of the elementary fibrils, which are formed by the
helical packing of the chiral aggregates in a preferred helical
sense.
The relatively low CD intensity after photoirradiation
indicates the possibility for the coexistence of fibers with both
helicities. However, since the SEM and AFM images did not
show the presence of fibers with opposite helicity, it can be
inferred that the low intensity of the CD signal after
photoisomerization could be associated with the relatively
low stability of the cis isomer aggregates. This hypothesis is
clear from the temperature-dependent plots of the CD and
the absorption data of the aggregates before and after
photoisomerization (Figure 1c,d).
Interestingly, the inversion of helicity occurs through
a depolymerization pathway, which is clear from the plots of
the gabs obtained after irradiation at different temperatures
and the subsequent reassembly of the helix by cooling of the
solution (Figure 4a). For 1 ꢀ 10ꢀ5 m (S)-2 in MCH, the helicity
inversion occurs around 313 K or above. In the absence of
heating, the helicity did not change even after irradiation for
several hours, as shown in the plots of the gabs against the
irradiation time (Figure 4b). Even if there could be slow
isomerization of the molecules within the helical fibers, the
helical twist does not change significantly. However, rever-
Figure 4. Evidence for the depolymerization pathway of the helicity
inversion. a) Change in gabs for (S)-2 as a function of heating, UV
irradiation, and cooling. Each point on the plot was obtained by
irradiating at 350 nm at the specified temperature, followed by cooling.
The reversal of the CD signal occurs above 313 K, indicating depolyme-
rization of the aggregate. b) Plot of gabs versus time for the P- and M-
helices under UV and visible light irradiation, respectively, in the
absence of heating.
sible switching of helicity is possible for several cycles, as
shown by the sign inversion of the gabs values after heating the
initial helices above room temperature, followed by UV
irradiation and cooling (Figure S12).[16]
Based on the experimental data, a plausible mechanism
for the observed photoinduced helicity inversion is proposed
in Figure 5. After photoirradiation, a mixture of molecules
with partially isomerized (Y), fully isomerized (Z), and the
non-isomerized (X) azo groups may exist. The spectral and
microscopic changes after UV irradiation, suggest that the
major component at the PSS is the partially isomerized E,Z
isomer (Y), in which only one of the two azobenzene moieties
is isomerized. This argument is evident from the fact that the
inversion of helicity occurs only through the aggregate
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materials with switchable electronic properties, such as
conductivity and charge carrier mobility.
Received: July 6, 2012
Published online: && &&, &&&&
Keywords: azobenzenes · chirality · helicity ·
.
photoisomerization · self-assembly
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cis-isomer is required. Since the overall isomerization yield is
only 53% at the PSS, to have a high concentration of the E,Z
isomer (Y), the photoisomerization should restrict itself to the
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Figure 5.
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Photoswitchable Helicity
A. Gopal, M. Hifsudheen, S. Furumi,
M. Takeuchi,
A. Ajayaghosh*
&&&& — &&&&
Thermally Assisted Photonic Inversion of
Supramolecular Handedness
Spiraling into control: A photoresponsive
supramolecular assembly demonstrates
that light, along with heating (D) and
cooling ( ), can cause chiral communi-
cation between molecules. This effect
leads to bias in the helicity of the com-
plex, causing a reversible switching of
macroscopic handedness, as shown by
a reversal of sign of the circularly polar-
ized luminescence (CPL) that is emitted.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!