Photoresponsive rolling and bending of thin crystals of chiral diarylethenes.

Abstract

Dithienylhexafluorocyclopentene with (R)- or (S)-N-phenylethylamide substituents formed rod-like and 0.2-1.0 microm-thick platelike crystals by sublimation; upon UV irradiation, the crystals bent concavely to the incident light and finally rolled crystals were obtained; the bent crystals were reconverted to flat crystals by visible light irradiation.

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Photoresponsive rolling and bending of thin crystals of chiral
diarylethenes{
Kingo Uchida,*abShin-ichiro Sukata,aYuji Matsuzawa,aMasako Akazawa,aJaap J. D. de Jong,b
Nathalie Katsonis,bYuko Kojima,cShinichiro Nakamura,cJetsuda Areephong,bAuke Meetsmaband
Ben L. Feringa*b
Received (in Cambridge, UK) 4th October 2007, Accepted 21st November 2007
First published as an Advance Article on the web 4th December 2007
DOI: 10.1039/b715251e
Dithienylhexafluorocyclopentene with (R)- or (S)-N-phenyl-
ethylamide substituents formed rod-like and 0.2–1.0 mm-thick
plate-like crystals by sublimation; upon UV irradiation, the
crystals bent concavely to the incident light and finally rolled
crystals were obtained; the bent crystals were reconverted to flat
crystals by visible light irradiation.
Photochromism is defined as the reversible transformation of a
single molecule between two states having different absorption
spectra. Hence photochromic molecules often hold considerable
potential for application as molecular switches and as control
elements in molecular devices.1Diarylethenes are among the most
promising of photochromic compounds,2not only as memory
materials but also as switching units for molecular devices and in
supramolecular systems.3Recently changes in the surface mor-
phology of a diarylethene crystal were reported,4as well as
modification of crystal shape. In particular, Irie and co-workers5
reported the reversible bending of a rod-like crystal of 1,2-bis(5-
methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene upon UV and
visible light irradiation. The mechanism of bending was attributed
to differences in molecular stacking for the open- and the closed-
ring isomers, which resulted in macroscopic contraction of the
crystals along an axis. We have previously demonstrated that
dithienylperfluorocyclopentene derivatives like 1oRR (Fig. 1), with
chiral, (R)-N-phenylethylamide groups at both ends of the
molecule, undergo diastereoselective cyclisation in the crystalline
state under UV irradiation.6We also demonstrated that chiral
N-phenylethylamide-based diarylethenes form organogels and that
the properties of the fibres as well as the transmission of chiral
information from the molecular to the supramolecular level can be
controlled reversibly by light.7In the present communication we
disclose the photoresponse of thin crystals of 1oRR, and 2oRR,
and show for the first time that reversible light-induced bending of
crystals in both a convex and a concave manner can be achieved.
Furthermore, we discovered that plate-like crystals of 1oRR can be
rolled up upon UV irradiation.
Diarylethenes 1oRR, 1oRS, 1oSS and 2oRR have been
synthesised using procedures adapted from previous reports.6
Upon UV and visible light irradiation, crystalline diarylethenes
1oRR, 1oRS, 1oSS and 2oRR showed the same photochromic
behaviour as 1oRR in acetonitrile. The crystalline powders of
1oRR, 1oSS, 1oRS and 2oRR were inserted into a demountable
optical cell having a thickness of 1 mm. This cell was placed
horizontally, and a glass plate was placed on top of it.{ The cell
was subsequently heated at a temperature 10 uC below the
compounds’ melting points. Maintaining this temperature, thin
crystal plates grew from the original crystalline powder of 1oRR
and 1oSS.
Fig. 2a schematically shows a side view of the thin crystals
growing on top of the crystal powder material of 1oRR (shown at
the bottom). The growing rate was around 0.1 mm min21. The
thickness of the film was observed by SEM to be 0.2–1.0 mm as
shown in Fig. 2b. Compound 1oSS formed similar thin crystalline
plates. As expected, these thin crystals of 1oRR and 1oSS had the
same melting point of 269.9–271.2 uC. 1oRR and 1oSS also
formed thin plate-like crystalsfrom methanol, ethanol, 2-propanol,
and acetonitrile with mp 273.0–274.0 uC. The molecular packing of
1oRR in the crystal state has already been reported.6
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
cMitsubishi Chemical Group Science and Technology Research Center,
Inc. and CREST-JST, Japan Science and Technology Corporation, 1000
Kamoshida, Yokohama 227-8502, Japan
{ Electronic supplementary information (ESI) available: Absorption
spectra of the diarylethenes, X-ray analysis of 1oRR, description of
1oRR and 2oRR crystals and corresponding polar plots, IR spectra of
1oRR and 1cRR, ATR-IR spectra of the surface of the plate-like crystals
before and after irradiation, synthesis of the diarylethenes, and supporting
videos (growth of plate-like crystal of 1oRR, reversible bending of plate-
like crystal of 1oRR, bending of a needle-shaped crystal of 1oRR). See
DOI: 10.1039/b715251e
Fig. 1
closed form.
Dithienylethene derivatives used in the study. o = open form, c =
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By contrast, 1oRS did not form thin film crystals upon
sublimation – rather it melted in the cell, due to a change in
stereochemistry in the vicinity of the amide moieties. This indicates
that the intermolecular hydrogen-bonding plays an important role
in crystal growth from the gas phase. Under these conditions the
naphthyl derivative 2oRR formed both rod-like and plate-like
crystals. Both diarylethenes 1oRS and 2oRR formed needle-shaped
crystals from methanol, ethanol, 2-propanol, and acetonitrile with
melting points of 261.1–262.7 uC and 267.0–268.0 uC, respectively.
In single crystals of 1oRR, two conformers form a ladder-like
structure through multiple intermolecular hydrogen bonding (see
ESI{), and each molecule participates in four hydrogen bonds that
connect it to four (perpendicularly orientated) other molecules of
different conformation. The amide moieties, linked by hydrogen
bonds, form an infinite two-dimensional network along the base
vectors [100] and [010], i.e. in the (001) plane.
The polar plot of the vapour grown crystals of 1oRR is shown in
Fig. 3. The plot shape was different from that of the crystals
obtained from recrystallisation in solution,6which might be related
to the fact that melting points are also different from that obtained
from recrystallisation in solution. The order parameter (A//2 AH)/
(A//+ 2AH) obtained from the plot was 0.064, while the parameter
of the crystals from solution was 0.028. Because the plate-like
crystals are too thin, X-ray analysis of the thin crystal has so far
been unsuccessful in revealing a possible difference in structure.
The thin rod-like crystals of diarylethene 1oRR or 1oSS
(0.49 mm 6 0.07 mm) bent concave to the incident light upon
irradiation with UV light at 254 nm (Fig. 4a). More remarkably,
the plate-like crystals rolled up into a curled film crystal with
0.08 mm diameter as shown in Fig. 4b. Depending on the direction
of the incident light and the nature of the thin film crystals,
concave bending along the main axis of the plate-like crystals was
also observed (Fig. 4c). The bending of the rod-like crystals
(Fig. 4a) and concave bending along the main axis (Fig. 4c) are
fully reversible upon photochemical ring-opening of 1cRR to
1oRR.
Upon irradiation with UV light at 254 nm (254 nm Hg lamp
@12W), the bending of a plate-like crystal of 1oRR occurred
within 3 s (see video in ESI{). This plate-like crystal was 0.6 mm in
length and 0.2 mm in width. Its recovery from the rolling under
visible light irradiation took about 2 s (Xe lamp @500W). Both
response times depend on the light intensity which is used.
We assume that the bending mechanism which takes place in
our systems is similar to that reported by Irie and co-workers.5The
shrinkage would occur on the surface of the crystal in response to
the formation of the closed-ring isomer at the crystal surface. The
formation of the closed-ring isomer was ascertained by IR spectral
changes as observed in attenuated total reflection infrared (ATR-
IR) spectra of the crystal surface (see ESI{).
Thin crystals of 2oRR also rolled up under UV light. The degree
of bending was dependent on the thickness of the crystal.
Interestingly, both plate-like and rod-like crystals of the naphthyl
derivative 2oRR formed by sublimation, bent in a convex way to
Fig. 2
sublimation. b) SEM image of the side view of the thin crystal grown by
sublimation (magnification 15 000). The thickness of the plate-like crystal
is 0.2–1.0 mm. See also the growth of the plate-like crystals in the
supporting video (ESI).
a) Schematic representation of the set-up for crystal growth by
Fig. 3
Polar plot of the thin film crystal of 1oRR observed at l = 598 nm which is
the lmaxof the closed-ring isomer in the crystalline state.
a) Optical microscopy image of a plate-like crystal of 1oRR. b)
Fig. 4
irradiation at l = 254 nm. a) Bending of the needle-shaped crystal of 1oRR
occurs concavely upon UV light irradiation (see also supporting video in
the ESI{). b) Rolling of the plate-like crystal occurs concavely to the
incident UV light irradiation. c) Length-wise rolling of a plate-like crystal
is possible by using side irradiation. Length-wise rolling is reversible.
Rod-like and plate-like crystals of 1oRR before and after UV
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the incident UV light as shown in Fig. 5a–c. This opposite
behaviour as compared to the behaviour of plate-like crystals of
1oRR could be due to different crystal packings. Indeed, in crystals
of 2oRR, intermolecular hydrogen bonding expands along the long
axis of the crystal and the long axis of the diarylethene molecule is
perpendicular to the long axis of the crystal. The polar plot of the
crystalline rod of 2oRR, showing a dumbbell shape (Fig. 5d), is
consistent with this observation. The plot was perpendicular to the
long axis of the crystal, indicating a different alignment of
the molecules compared to the crystal of 1oRR. Although the
structure of the crystal formed by the closed isomer under UV
irradiation is still unclear, the differences in the molecular packing
are likely to affect the bending directions.
The bending of the rod-like crystal of 2oRR is fully reversible
upon irradiation with visible light (l . 500 nm) as seen in Fig. 5c.
The original shape of the rod-like crystal is recovered. Again we
attribute the reversible light-induced bending to a change in
packing (shrinkage or expansion) in the surface layer exposed to
the incident UV light beam. The photochemical ring closure results
in contraction of the crystal phase and bending of the rod-like
crystal. Ring-opening of 2cRR with visible light reverses this effect.
The response times of the thin crystals of 2oRR and 2cRR are
similar to those reported for 1oRR and 1cRR and similarly depend
on the light intensity which is used.
To fully understand why concave or convex changes in shape
occur, a detailed mechanistic study on the change in crystal
morphology upon UV and visible irradiation will be necessary.
The ability to tune bending (and curving) of crystals in a concave
or convex manner by rather small variations in molecular structure
offers fascinating opportunities to develop light-actuated mechan-
ical systems.
The present work was supported by 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.
Financial support from the Netherlands Organisation for
Scientific Research (NWO-CW) and the Zernike Institute for
Advanced Materials is gratefully acknowledged. We thank Zeon
Corp. Ltd. for supplying the octafluorocyclopentene.
Notes and references
{ A quartz cell AB20-UV-1.0 621016131 (GL Science Inc., Japan) and a
cover glass (Matsunami Neo Micro Cover Glass, 24 6 45 mm, thickness
0.12–0.17 mm) were used for making the thin plate crystals.
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Fig. 5
(0.2 mm 6 0.007 mm). (b) After UV (l = 254 nm) irradiation, the needle
crystal bent convex to incident UV light. (c) After visible (l . 500 nm)
light irradiation, the bent crystal recovered its original shape. (d) The polar
plot at l = 671 nm after UV irradiation of the crystal.
(a) A needle-shaped crystal of 2oRR before UV irradiation
328 | Chem. Commun., 2008, 326–328This journal is ? The Royal Society of Chemistry 2008