Photoswitchable exciton coupling in merocyanine-diarylethene multi-chromophore hydrogen-bonded complexes

Article abstract of DOI:10.1002/anie.201205504

Light on: Photocontrol of J-type exciton interactions by using chromophores is reported (see picture). Hydrogen-bonded merocyanine dyes could be switched reversibly through photoinduced ring-closure/ring-opening reactions of diarylethene receptors. Addition of H-aggregation-inducing bismelamine receptors enabled the partial interconversion between J- and H-type exciton coupling. Copyright

Full text of DOI:10.1002/anie.201205504

Angewandte
Chemie
DOI: 10.1002/anie.201205504
Photochemistry
Photoswitchable Exciton Coupling in Merocyanine–Diarylethene
Multi-Chromophore Hydrogen-Bonded Complexes
Shiki Yagai,* Kazunori Iwai, Takashi Karatsu, and Akihide Kitamura
The use of phototriggered conformational change of
photochromic molecules to tune the physical proper-
ties of functional molecules through noncovalent
interactions is a challenging research topic[1] inspired
by the sophisticated function in vision.[2] Aida and co-
workers reported an elegant example in which the
photoinduced conformational change of a scissorlike
azobenzene–ferrocene–zinc porphyrin conjugated host
is transmitted successfully to mechanical twisting of the
guest molecule through two-fold axial ligation, thus
enabling a remote photocontrol of the atropisomerism
of the guest.[3] Andrꢀasson and co-workers succeeded
to transduce a photoinduced conformational change of
diarylethene to the mechanical bending of diacetylene-
tethered zinc porphyrin dimers through two-fold axial
ligation, demonstrating a remote photocontrol of the
absorption of the porphyrin dimer.[4] In these examples,
conformations of chromogenic molecules are elabo-
rately modulated by the structural change of the
photochromic molecules through noncovalent interac-
tions. On the other hand, exciton interactions of
pigments play important roles in biological systems,
through which they show distinct photochemical prop-
erties compared to the monomeric state.[5] However,
none of the artificial photoresponsive supramolecular
systems succeeded to control exciton interactions
Figure 1. Structures of a) merocyanine 1, b) bismelamine receptors BM3 and
BM12, and c) diarylethene receptor 2 (2o: open form and 2c: closed form). In
b) and c), the complexation events are summarized.
between pigments by using a conformational change of
exciton interactions. In the present study melamine-equipped
diarylethene 2[10] was used as a photoresponsive receptor for
1. Chiral substituents[11] were introduced to melamine moi-
eties to induce chiral exciton coupling, which is conveniently
studied by circular dichroism (CD) spectroscopy. The photo-
triggered morphological change between the open-form 2o
and closed-form 2c enabled “on/off” switching of chiral J-type
exciton coupling between 1 in oligomeric species. Further-
more, a sufficiently large difference between the stabilities of
the “on” and the “off” assemblies allowed a partial inter-
conversion between J- and H-type exciton interactions by
noncovalently bound photochromic molecules.[6]
We herein report a supramolecular system where exciton
interactions of chromogenic components can be controlled
through the conformational change of photochromic compo-
nents. Previous our study on supramolecular complexation
between merocyanine dye 1[7] (Figure 1a) and bismelamine
(BM) receptors[8] through multiple hydrogen bonds demon-
strated that the oligomethylene linker length of the receptors
can control the J- (BM3) and H-type (BM12) exciton
interactions of 1 (Figure 1b).[9] From the spectroscopic and
the NMR studies, it was suggested that the formation of
discrete oligomers (1 + BM3) and folded supramolecular
polymers (1 + BM12) are responsible for such contrasting
a
ternary mixture including BM12 as “H-aggregation-
inducer”.[12]
Merocyanine 1 (c = 1.0 ꢁ 10ꢀ5 m) exists as monomeric state
in a nonpolar solvent such as cyclohexane, exhibiting
a charge-transfer (CT) absorption band (M-band) at lmax
=
[*] Prof. Dr. S. Yagai, K. Iwai, Prof. Dr. T. Karatsu, Prof. Dr. A. Kitamura
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
E-mail: yagai@faculty.chiba-u.jp
456 nm (the full-width at half-maximum, FWHM, is about
1300 cmꢀ1). Upon adding aliquots of open-form DAE (diaryl-
ethene) receptor 2o, a new band (J-band) grew at 495 nm at
the expense of the M-band (Figure 2a). The pronounced red-
shift (about 1730 cmꢀ1) of the maximum wavelength indicates
the J-type exciton coupling of the dye in the hydrogen-bonded
complexes (1·2o)n. The intensity of the J-band increased
almost linearly with the amount of 2o and leveled off at
Prof. Dr. S. Yagai
CREST, JST, Chiyoda-ku 102-0075 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205504.
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free 2o is CD inactive. Because the absorption band of
2o could be attributed to the p–p* transition of the
phenylenethienylene moieties, its induced CD signal
strongly suggests that this molecule adopts the parallel
conformation[13] upon complexation with 1. The com-
plexation-induced CD signal of 2o was not observed
when nonchromophoric N-dodecylcyanurate[10] was
used as a guest, indicating that the J-type aggregation
of 1 is responsible for the formation of the specific
assemblies.
Monomeric 1 is almost nonemissive due to non-
radiative deactivation through a bond twist in the
excited states. However, a clear fluorescence was
observed at 532 nm upon exciting the J-band of (1·2o)n
(Figure 2e). The fluorescence intensity maximized at
a molar ratio of 1:1 (Figure 2 f). A relatively small
Stokes shift of 37 nm (about 1400 cmꢀ1), and a mirror-
image relationship between the absorption and the
fluorescence bands are both characteristic of J-type
aggregates.[14]
With the above evidence for the J-type exciton
coupling of 1 upon complexation with 2o in hand, we
performed light-irradiation experiments of the 1:1
complex. When a cyclohexane solution was irradiated
at around 313 nm, the progress of the ring-closure
reaction was indicated by the growth of the absorption
bands at 395 and 606 nm with the bleaching of the
band at 316 nm (Figure 3a). The photostationary state
(PSS) was achieved with 6 minutes of irradiation
under the applied conditions (see the Supporting
Information), resulting in the quantitative conversion
of 2o to 2c. The ring-closure reaction was accompanied
by a decrease of the J-band of 1 at 495 nm, which is
compensated by a new band at 475 nm (M2c-band).
The M2c-band is red-shifted from the M-band
(456 nm) by 19 nm, which could be ascribed to the
Figure 2. a,c,e) Changes of a) UV/Vis, c) CD, and e) fluorescence spectra of
merocyanine 1 (c=1ꢀ10ꢀ5 m) upon addition of 0 (blue) to 1 equiv (red) of 2o in
cyclohexane. The dashed curve in (e) is the J-band normalized to the
fluorescence spectrum of the 1:1 mixture. b,d,f) Plots of the respective spectral
changes versus equivalent of [2o] (0 to 2 equiv).
a molar ratio of 1:1 (Figure 2b). This result suggests that the
stoichiometric complexation is achieved at low concentration
by the formation of stable assemblies. Although the FWHM
of the J-band (about 2000 cmꢀ1) at 208C is greater than that of
the M-band, the value is considerably smaller than that of the
previously reported complex (1·BM3)n featuring a flexible
trimethylene linker (FWHM of about 3000 cmꢀ1). This
finding suggests that more uniform J-type exciton interaction
is achieved upon complexation with 2o owing to a decrease in
the conformational freedom of the linker moiety.
Circular dichroism spectroscopy provided further support
for the exciton coupling in (1·2o)n. Upon mixing 2o to the
monomeric solution of 1, a bisignate CD signal was induced in
the CT absorption region (Figure 2c), the intensity of which
once levelled off at a molar ratio of 1:1 (Figure 2d). The zero-
crossing point locates at 482 nm, which is blue-shifted by
13 nm from the maximum wavelength of the J-band. This
might be attributed to the presence of other small CD signals
in the same region. The positive and negative CD signs from
longer wavelength indicate the P-type (clockwise) chiral
exciton coupling occurring in excess between transition
dipoles of 1. Notably, another bisignate CD signal was
observed in the absorption region of 2o despite the fact that
electronic effect of hydrogen-bonding interaction on the
barbituric acid moiety.[9b] The tunable exciton interaction of
1 upon ring-closure reaction was supported by CD spectros-
copy. At the PSS, the bisignate CD signal of 1 was significantly
weakened, and the maximum molar circular dichroism (De)
value of the positive CD sign decreased from 13 to
3.0mꢀ1 cmꢀ1. Notably, the bisignate CD signal arising from
the diarylethene scaffold completely disappeared (Fig-
ure 3b). This observation strongly supports the hypothesis
that 2o indeed adopts the parallel conformation in (1·2o)n
(Figure 3e). Under a dynamic equilibrium between aggre-
gated and monomeric state, however, 2o can adopt the
antiparallel conformation even in the presence of 1, and
thereby the ring-closure photoreaction can take place. The
resulting rigid receptor 2c spatially separates the bound guests,
allowing only weak exciton coupling.
As expected, the fluorescence of 1 was quenched upon
ring-closure of 2o probably due to the loss of J-type exciton
coupling (Figure 3c). In the present system, however, the
possibility of energy transfer from 1 to 2c was indicated by the
following experiment. When the complex (1·2c)n was irradi-
ated at the absorption maximum of 1 (475 nm) at which 2c
alone did not show ring opening, an efficient ring-opening
2
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Figure 4. a) Plots of mole fraction of aggregates (a) versus temper-
*
ature for variable temperature UV/Vis spectra of (1·2o)n ( ) and (1·2c)n
( ) in methylcyclohexane (c=1ꢀ10ꢀ5 m). b) Comparison of 1H NMR
*
spectra of (1·2o)n in [D12]cyclohexane (c=5ꢀ10ꢀ4 m) before (upper
panel) and after UV irradiation (lower panel). c) Plots of the aggrega-
tion number N (=cmon/ccollig) determined by VPO versus stoichiometric
Figure 3. a–c) Changes of a) UV/Vis, b) CD, and c) fluorescence spec-
tra of the 1:1 complex of 1 (c=1ꢀ10ꢀ5 m) and 2o (c=1ꢀ10ꢀ5 m) upon
UV irradiation at around 313 nm for 6 minutes at 208C. The inset in c)
shows photographs of the solution before and after UV irradiation.
d) Change of the molar extinction coefficient (e) at 495 nm (J-band of
1) upon UV and visible light irradiation cycle. e) Proposed photo-
triggered structural change of the complexes. Only the local binding
structures were shown.
*
*
concentrations (cmon) of the components for (1·2o)n ( ) and (1·2c)n ( ).
plexes) of 458C and 278C were determined for (1·2o)n and
(1·2c)n, respectively, by fitting with the Boltzmann equation
½a ¼ 1=ð1 þ exp½ðT ꢀ TmÞDTꢁÞꢁ. A pronouncedly higher sta-
bility of (1·2o)n relative to that of (1·2c)n despite its higher
entropic penalty associated with the flexible open-form
diarylethene core implies the formation of cyclic supramolec-
ular assemblies.[17]
reaction was observed at the same reaction rate as that rate
obtained for direct irradiation of 2c at 600 nm. The ring-
opening reaction caused the recovery of the J-band of
1 together with the green fluorescence and the bisignate CD
signal. A plausible explanation for this result is that fluores-
cence resonance energy transfer (FRET) takes place from
weakly exciton-coupled 1 to 2c in (1·2c)n.[15] As another
possibility, we must consider energy transfer from nonfluor-
escent 1 to 2c.[16] Although further studies are needed to
disclose the mechanism, it is worth noting that “non-photo-
chromic” merocyanine 1 show “photochromic fluorescence”
through the supramolecular complexation with 2. The switch-
ing of the J-type exciton coupling of 1 could be repeated at
least 10 times (Figure 3d).
The 1H NMR spectrum of (1·2o)n (c = 5 ꢁ 10ꢀ4 m) in
[D12]cyclohexane showed two major resonances between
12–14 ppm, ascribable to the hydrogen-bonded NH proton
atoms of 1 (upper, Figure 4b). The relatively sharp resonan-
ces suggest the formation of well-defined supramolecular
assemblies. Upon converting 2o to 2c with UV irradiation,
these resonances displayed significant broadening (Figure 4b,
bottom). This observation suggests the formation of complex
mixtures composed of various hydrogen-bonded assemblies
with different aggregation numbers and supramolecular
structures.[18]
To gain insight into the present photoresponsive assem-
blies, the stabilities of (1·2o)n and (1·2c)n were compared by
variable-temperature (VT) UV/Vis measurements in methyl-
cyclohexane. Both assemblies (c = 1.0 ꢁ 10ꢀ5 m) showed rever-
sible changes in the absorption band of 1 between aggregated
(495 nm with 2o; 475 nm with 2c) and monomeric states
(456 nm) upon varying the solution temperatures between 10
and 908C (see Figure S1 in the Supporting Information). Plots
of the fraction of aggregated molecules (a) versus temper-
ature gave dissociation curves (Figure 4a), from which
melting temperatures (Tm, 50% of molecules form com-
Vapor pressure osmometry (VPO) was used to further
analyze the properties of (1·2o)n and (1·2c)n. For this purpose,
solutions of (1·2o)n and (1·2c)n were prepared separately at
four total concentrations of monomers (cmon = [1] + [2]):
c
mon = 5.0 ꢁ 10ꢀ3, 1.0 ꢁ 10ꢀ2, 1.5 ꢁ 10ꢀ2, and 2.0 ꢁ 10ꢀ2 m. In the
case of (1·2o)n, the colligative concentrations measured by
VPO (ccollig) were 1.2 ꢁ 10ꢀ3, 2.7 ꢁ 10ꢀ3, 4.5 ꢁ 10ꢀ3, and 5.8 ꢁ
10ꢀ3 m for the above stoichiometric concentrations, respec-
tively. From these results, aggregation numbers N (= cmon
/
ccollig) were calculated to be 4.2, 3.7, 3.4, and 3.5 for the above
four concentrations, respectively (Figure 4c). A relatively
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Communications
concentration-insensitive number N with an average value of
3.7 indicates the formation of closed [2+2] assemblies
[(1·2o)2]. In striking contrast, the ccollig values of (1·2c)n were
measured as 2.4 ꢁ 10ꢀ3, 3.6 ꢁ 10ꢀ3, 4.3 ꢁ 10ꢀ3, and 4.7 ꢁ 10ꢀ3 m,
for the four stoichiometric concentrations, providing N values
of 2.1, 2.8, 3.5, and 4.3, respectively (Figure 4c). An increase
of N upon increasing of concentration illustrates the forma-
tion of open-ended oligomeric species.[19]
All the above spectroscopic and VPO studies demonstrate
that the present multi-chromophore assemblies can change
the supramolecular structures between closed [(1·2o)2] and
open-ended oligomeric state [(1·2c)n] in response to UV and
visible light. This structural change enables the “on/off”
switching of the J-type exciton interaction of 1. To expand this
system to a JQH interconvertible system, we incorporated
BM12 to the binary system of 1 and 2. This attempt is based
on the fact that the Tm value of (1·BM12)n at 1.0 ꢁ 10ꢀ4 m is
528C, which is in between those of (1·2o)2 (638C) and (1·2c)n
(488C). The low stability of (1·2c)n compared to (1·2o)2 could
enable selective conversion of the former assemblies to
(1·BM12)n that organizes into folded supramolecular poly-
mers by intrachain H-type aggregation of 1.[9b,c] We thus
prepared a 1:1:1 ternary mixture 1 + 2o + BM12 with the
concentrations of all the components at 1.0 ꢁ 10ꢀ4 m. As shown
by the red curve in Figure 5, the addition of BM12 resulted in
J-type exciton coupling between the merocyanine compo-
nents could be switched reversibly “on/off” by irradiation
with UVand visible light. The “on” state of the J-type exciton
coupling was confirmed by the red-shifted absorption band,
the bisignate CD signals, and the enhanced fluorescence.
1H NMR and VPO studies revealed that distinct types of
supramolecular assemblies are interconverted by the ring-
closure/ring-opening photoreactions of the diarylethene units.
Furthermore, a relatively large change in the stabilities of
these assemblies allowed the extension of the “on/off”
switching property to the J/H switchable functionality by
using a third component, BM12, that can convert the “off”
state to the H-aggregated state. The present study thus
demonstrates that dynamic molecular assemblies based on
rational designs of building blocks could realize complicated
stimuli-responsive systems similar to those found in natural
systems.
Received: July 12, 2012
Published online: && &&, &&&&
Keywords: exciton coupling · hydrogen bonds ·
.
photochemistry · photochromism · self-assembly
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Communications
Photochemistry
Light on: Photocontrol of J-type exciton
interactions by using chromophores is
reported (see picture). Hydrogen-bonded
merocyanine dyes could be switched
reversibly through photoinduced ring-
closure/ring-opening reactions of diaryl-
ethene receptors. Addition of H-aggrega-
tion-inducing bismelamine receptors
enabled the partial interconversion
S. Yagai,* K. Iwai, T. Karatsu,
A. Kitamura
&&&& — &&&&
Photoswitchable Exciton Coupling in
Merocyanine–Diarylethene Multi-
Chromophore Hydrogen-Bonded
Complexes
between J- and H-type exciton coupling.
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!