Photoresponsive dithienylethene-urea-based organogels with "reversed" behavior

Article abstract of DOI:10.1039/b802580k

Dithienylperhydrocyclopentene-bisurea-based low molecular weight gelators are described that function as photoresponsive organogels that show a remarkable gel-to-liquid transition upon irradiation. The two series of derivatives, with and without alkyl spacers between the urea hydrogen bonding groups and the photochromic unit, show different gelation behavior. Upon UV irradiation of the gels, a gel liquified at only 1.4% conversion of the photochromic unit. Transmission electron microscopy (TEM) shows that the gel fibres consist of thin ribbons. Semi-empirical (PM3) calculations indicate that the hydrogen bonding between the open-ring isomer (o) molecules is weak, and that formation of the closed-ring isomer (c) destabilises the hydrogen bonding further. The results indicate that a small amount of the closed-ring isomer will disrupt the intermolecular hydrogen-bonding, leading to disintegration of the gel fibre ribbons and hence reversible liquification. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b802580k

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Photoresponsive dithienylethene-urea-based organogels with “reversed”
behavior†
Masako Akazawa,a Kingo Uchida,*a Jaap J. D. de Jong,b Jetsuda Areephong,b Marc Stuart,b
Giuseppe Caroli,b Wesley R. Browneb and Ben L. Feringa*b
Received 14th February 2008, Accepted 19th March 2008
First published as an Advance Article on the web 1st April 2008
DOI: 10.1039/b802580k
Dithienylperhydrocyclopentene-bisurea-based low molecular
weight gelators are described that function as photore-
sponsive organogels that show a remarkable gel-to-liquid
transition upon irradiation. The two series of derivatives, with
and without alkyl spacers between the urea hydrogen bonding
groups and the photochromic unit, show different gelation
behavior. Upon UV irradiation of the gels, a gel liquified at
only 1.4% conversion of the photochromic unit. Transmission
electron microscopy (TEM) shows that the gel fibres consist
of thin ribbons. Semi-empirical (PM3) calculations indicate
that the hydrogen bonding between the open-ring isomer (o)
molecules is weak, and that formation of the closed-ring
isomer (c) destabilises the hydrogen bonding further. The
results indicate that a small amount of the closed-ring isomer
will disrupt the intermolecular hydrogen-bonding, leading to
disintegration of the gel fibre ribbons and hence reversible
liquification.
(Scheme 1a). Indeed, photochemical control of supramolecular
aggregation was used to achieve dynamic holographic pattern
formation.5
Scheme 1
We report here new dithienylethene-based bis-urea gelators that,
in sharp contrast to the light-switchable gelator systems described
earlier, show reversible gel-to-liquid phase transitions upon ring
closure of the photochromic unit (Scheme 2). Furthermore, it is
demonstrated that liquification takes place even with only 1.4%
conversion of the dithienylethene gelator molecules to the closed
state.
Urea moieties have proven to be versatile functional groups
in driving gelation of low molecular weight gelator systems. In
particular, compounds that contain two urea groups have been
demonstrated to be very potent and versatile gelating agents
for a wide range of organic solvents.6,7 Cyclohexane bis-urea
compounds are well known gelators not only of organic solvents
but for aqueous solutions also.8 The introduction of photochromic
responsiveness to urea-based gelators was reported previously in
the use of the azobenzene unit in conjunction with two urea groups
to produce thermoreversible gels.9
We report herein the synthesis and characterisation of a series
of dithienylcyclopentene-urea-based organogelators 1o–6o, and
explore the effect of the spacer between the urea group and
diarylethene moiety and the chain length of terminal alkyl
groups, together with the photo-responsiveness of the gela-
tors. Diarylethenes 1o–6o were prepared from 1,2-bis(5-chloro-
2-methylthien-3-yl)perfluorocyclopentene10 (see ESI†) and show
reversible photochromism in solution (Fig. 1 and Fig. 2).
Fig. 1 and Fig. 2 illustrate the changes in the absorption spectra
for compounds 1 and 4 in ethanol upon photo-irradiation. Upon
irradiation at kexc = 312 nm, the colourless solution turned red
and absorption bands assigned to 1c appeared at 523 nm (e: 1.3 ×
104 M−1 cm−1). Concomitantly the absorption of 1o at 283 nm (e:
2.8 × 104 M−1 cm−1) decreased in intensity. Upon irradiation at k >
500 nm, the red colour was bleached and 1o was regenerated, as
confirmed by UV/Vis absorption spectroscopy. Isosbestic points
were maintained at 206, 232, and 299 nm. The absorption maxima
Photochromic behaviour, the reversible unimolecular transfor-
mation between two states having different absorption spectra,
holds considerable potential towards application in molecular
switching, as control elements in molecular devices and in the
development of smart materials.1 Diarylethenes are amongst
the most promising of photochromic compounds,2 not only as
memory materials but also as switching unit components in
molecular devices and supramolecular systems.3 Recently, we
have shown that a dithienylcyclopentene disubstituted with chiral
(R)-N-phenylethylamide groups forms gels through self-assembly
due to the formation of multiple intermolecular hydrogen bonds
between the amide functionalities. The supramolecular chirality
of the gel governed the dynamic molecular chirality through the
hydrogen bonding networks formed, and upon photochemical ring
closure of the dithienylethene allows for locking of the molecular
chirality of the gel molecules stereoselectively.4 Furthermore, it was
shown that switching between gels of different aggregate (fibre)
stability could be achieved with the gel-to-liquid transition being
achieved by photochemical ring opening of gelator molecules
aDepartment of Materials Chemistry, Faculty of Science and Technology,
Ryukoku University, CREST-JST, Seta, Otsu, Shiga, 520-2194, Japan.
E-mail: uchida@chem.ryukoku.ac.jp; Fax: +81-77-543-7483; Tel: +81-77-
543-7462
bStratingh Institute for Chemistry, University of Groningen, Nijenborgh
4, 9747 AG, Groningen, The Netherlands. E-mail: b.l.feringa@rug.nl;
Fax: +31-50-363-4296
† Electronic supplementary information (ESI) available: Synthesis of
dithienylethene derivatives 1o–6o; DSC curves; spectral changes; TEM
images; 1H NMR spectra; IR spectra. See DOI: 10.1039/b802580k
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Scheme 2 Diarylethene derivatives having propylene spacers (1o–3o), and derivatives without spacers (4o–6o). The corresponding closed-ring isomers
1c–3c and 4c–6c are generated by UV irradiation. (o = open, c = closed).
Table 1 Gelation properties of compounds 1o–6o in organic solventsa
Solvent
1o
2o
3o
4o
5o
6o
Hexane
I
I
—
P
P
I
—
S
P
I
—
S
I
I
I
I
I
I
I
I
I
I
I
P
Cyclohexane
P-Xylene
Benzene
Toluene
Tetralin
P
D
D
I
P
P
P
I
I
—
S
P
P
P
P
P
P
S
P
P
P
I
I
—
S
P
P
P
P
I
P
S
I
I
P
I
I
I
P
P
I
S
P
P
P
P
I
I
S
I
P
P
P
P
P
S
S
S
S
P
P
I
1,4-Dioxane
N-Butyl acetate
Di-N-butyl ether
Cyclohexanone
Chloroform
Dichloromethane
Diphenyl ether
1,2-Dichloroethane
O-Dichlorobenzene
Chlorobenzene
THF
DMF
DMSO
1-Octanol
2-Propanol
1-Propanol
Ethanol
Acetonitrile
Water
G (19.2)
I
S
P
G (25.0)
S
P
I
—
G (25.0)
G (25.0)
G (25.0)
P
P
P
G (25.0)
S
P
—
—
P
P
P
I
G (12.5)
G (12.5)
G (12.5)
G (15.6)
Fig. 1 Absorption spectral changes of 1 in ethanol (0.33 mM), 1o: solid
line; photostationary state under 312 nm light: dashed line.
P
D
S
P
P
S
S
I
S
S
S
S
P
S
P
I
S
S
—
—
S
P
P
I
—
—
P
P
P
I
P
I
I
I
I
I
a G: gelation, P: precipitation, I: insoluble, S: soluble, D: decomposition.
The gelation concentrations (mg ml−1) are shown in parentheses.
For the gelators in which the urea moiety is attached directly
to the phenyl rings of the switching unit, only compound 5o did
not show gelation behaviour in any of the solvents tested, instead
forming precipitates. The minimum gelation concentrations for 4o
and 6o were almost half of that of 1o, indicating their superior
gelation abilities. The changes in gelation behaviour induced
by light were investigated in the THF gel of 1o and the o-
dichlorobenzene gels of 4o and 6o. The gel-to-sol transition
temperature was measure◦d by the tilt tube method in an air oven
with a heating rate of 0.5 C min−1.
Fig. 2 Absorption spectral changes of 4 in ethanol (0.31 mM), 4o: solid
line; photostationary state under 312 nm light: dashed line.
of 4o and 4c in ethanol are 315 nm (e: 3.10 × 104 M−1 cm−1) and
530 nm (e: 1.4 × 103 M−1 cm−1), respectively, as shown in Fig. 2.
The gelation properties were studied by first dissolving the
compounds by heating in the appropriate solvent, followed by
cooling to room temperature. For some of the solid–solvent
combinations, the formation of a gel was observed. The results
are shown in Table 1, in which the solvents are arranged in
order of increasing polarity according to their ET(30)-values. From
Table 1, in the compounds which incorporate propylene spacers,
only compound 1o showed gelation behaviour, while 2o and 3o do
not gelate any of the solvents tested but form precipitates instead.
The ◦melting temperature of the THF gel (25 mg ml−1) of 1o was
40–42 C, while that of the photostationary state (PSS) mixture
(1o:1c = 6 : 4) after UV irradiation showed a very modest increase
to 44–47 ◦C. The gel–sol temperature of o-dichlorobenzene gel
◦
(12.5 mg ml−1) of 4o was 113–116 C, while that for the mixture
(4o:4c = 98.6 : 1.4) formed after UV irradiation for 30 s was 96–
102 ◦C. The o-dichlorobenzene gel (12.5 mg ml−1) of derivative 6o,
which bears shorter terminal alkyl chains, undergoes the sol–gel
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transition at 116–119 ◦C, while that of the mixture (6o:6c = 98.2 :
1.8) formed after UV irradiation for 30 s was 108–115 ◦C. The
temperature differences obtained by DSC were not clear due to the
low content of closed-ring isomers (see ESI†). The most dramatic
sol–gel transition change was observed for 4. Upon UV irradiation
(312 nm, 30 s) of the o-dichlorobenzene gel of 4o, a photoinduced
gel–sol tr◦ansition was observed in the temperature range between
105–110 C, forming a transparent solution as a consequence of
photochemical ring closure of the gelator molecules. Visible light
irradiation to the sol at that temperature leads to re-formation of
the initial turbid gel. TEM image of the o-dichlorobenzene gel of
4o was obtained to further understand the origin of this dramatic
change. The gel fibres show a thin ribbon structure (Fig. 3).
Fig. 4 Dimer assemblies of 4o–4o (a, b), 4o–4c (c, d), and 4c–4c (e, f)
obtained by PM3 calculations (left: lateral view; right: longitudinal view).
moieties only. From the longitudinal views shown in Fig. 4, a
significant change in the relative position of the switch moieties is
indeed visible going from 4o–4o to 4c–4c.14
From the calculations, the interaction energies of 4o–4o, 4o–
4c, and 4c–4c are 30.86, 28.20, and 31.62 kcal mol−1, respectively.
From these numbers, it can be seen that the energy of interaction
of 4o–4c is 2.66 kcal mol−1, which is lower than that for 4o–
4o. The results of the calculations suggest that the hydrogen
bonding between 4o molecules is weak, and that photo-induced
formation of 4c slightly destabilizes the intermolecular interaction.
The experimental results indicate, in agreement with calculations,
that the small amount of 4c formed decreases the intermolecular
interactions, leading to break-up of the ribbons and eventual
liquification of the gel.12
In summary, we have described photo-switchable organogela-
tors consisting of a diarylethene and urea units. The gel-to-liquid
transition occurs even at very low conversion from the open
colourless state to the closed coloured state. This is attributed
to the weakness of the molecular level interactions and the thin
ribbon-like gel structure of the gel fibres. Visible light irradiation
recovers the gel. Compared with earlier switchable gelators, which
show gel-to-liquid phase changes upon ring opening, the present
system allows gel-to-liquid transitions triggered by photochemical
ring closure. This system is an excellent demonstration of the
delicate balance in supramolecular behaviour that can be achieved
in photo-reversible supramolecular systems and the possibility of
addressing aggregation with light.
Fig. 3 TEM images of the o-dichlorobenzene gel of 4o (12.5 mg ml−1
(scale bar: 1.0 lm).
)
Additionally, semi-empirical (PM3) calculations11 were carried
out12 to suggest a possible array of the molecules. Fig. 4 shows
the results for the optimized structures of 4o–4o assembly (a, b),
4o–4c assembly (c, d), and 4c–4c assembly (e, f), respectively. It
is remarkable that the hydrogen bond length in the complexes
˚
was found to be between 2.5 and 3 A; while the reported value
13
˚
for a hydrogen bond is about 2 A. There may be a significant
Acknowledgements
contribution from the interaction of the alkyl chains; however,
intermolecular H-bonding through the urea moieties is expected
to be the dominant interaction. The calculated distance of the
C12 chains is the same for all three complexes. This suggests that
the differences in energy of the intermolecular interactions of the
complexes 4o–4o, 4o–4c and 4c–4c are due to the switch and urea
This work was supported by a Grant-in-Aid for Science Research
in a Priority Area “New Frontiers in Photochromism (No. 471)”
and (B) (18350101) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan (MA and KU),
NWO-CW(BLF) and the University of Groningen (JA).
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molecules only might be not adequate for an accurate description of
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