Photochemical control of dielectric properties based on intermolecular proton transfer in a hydrogen-bonded diarylethene crystal

Article abstract of DOI:10.1039/c0cc05729k

The photochromic reaction of a diarylethene derivative having imidazoline rings reversibly modulates the dielectric properties of the crystal, which is based on the intermolecular proton transfer in one-dimensional (1D) hydrogen-bonded chains.

Full text of DOI:10.1039/c0cc05729k

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COMMUNICATION
Photochemical control of dielectric properties based on intermolecular
proton transfer in a hydrogen-bonded diarylethene crystalw
Masakazu Morimoto*ab and Masahiro Iriea
Received 22nd December 2010, Accepted 10th February 2011
DOI: 10.1039/c0cc05729k
The photochromic reaction of a diarylethene derivative having
imidazoline rings reversibly modulates the dielectric properties
of the crystal, which is based on the intermolecular proton
transfer in one-dimensional (1D) hydrogen-bonded chains.
Hydrogen-bonding interactions in molecular crystals play an
important role as essential tools for crystal engineering.1 The
robustness and direction-specific character of hydrogen bonds
have been utilized for the control of crystal structure, chemical
reactivity, and solid-state physical properties.2 In addition,
intra- or intermolecular proton transfer through hydrogen
bonds in molecular crystals induces characteristic electrical
functions, such as dielectric response3 and ferroelectricity.4
In these systems, proton transfer in networks containing
O–Hꢀ ꢀ ꢀO, N–Hꢀ ꢀ ꢀN, and O–Hꢀ ꢀ ꢀN hydrogen bonds is induced
by application of external electric fields and its dynamic
behaviour is detected as dielectric properties. Several molecular
crystals containing 1D or 2D hydrogen-bonded networks, in
which protons are ordered in polar crystal lattices, have been
reported to exhibit spontaneous polarization and ferroelectric
properties.5–8
Scheme 1 Proton transfer in a 1D hydrogen-bonded chain of
imidazoline.
light.9 In order to achieve the photo-modulation of the proton
transfer dynamics, we designed and synthesized a photochromic
diarylethene derivative 1 having imidazoline rings (Scheme 2).
Electronic and geometrical structure changes of the photo-
chromic molecule upon photoisomerization are expected to
affect the proton transfer behaviour in the hydrogen-bonded
network of the imidazoline rings. Crystal structure, dielectric
properties, and photochromic reactions of the single crystal
have been examined.
Plate-like single crystals were obtained by slow evaporation
of a chloroform solution of the open-ring isomer 1o.z X-Ray
crystallographic analysis for 1o was carried out at 93 K. The
details of the crystallographic data are shown in Table S1 in
ESI.w The crystal has an orthorhombic crystal system with a
space group of Pnna and Z = 4. The 1o molecule has a
C2 symmetry and half of the molecule is crystallographically
independent. The molecules are fixed in an anti-parallel confor-
mation and the distance between the reacting carbon atoms of
the thiophene rings is 3.470 A (Fig. S2 in ESIw), which is short
enough for the photocyclization reaction to take place in the
crystal.10 Fig. 1a shows a molecular packing diagram viewed
along the a-axis. It is noted that the imidazoline rings of 1o are
hydrogen-bonded in the N–Hꢀ ꢀ ꢀN manner with those of the
adjacent molecules to form 1D hydrogen-bonded chains along
the c-axis. The motif of the crystal structure can be regarded as
Herein, we report on reversible photo-modulation of
dielectric properties based on proton transfer in a hydrogen-
bonded system. As a hydrogen-bonded structure, in which
cooperative intermolecular proton transfer potentially takes
place, we focused on a 1D hydrogen-bonded chain of imidazoline
formed by intermolecular N–Hꢀ ꢀ ꢀN interactions (Scheme 1).
Upon application of an external electric field, the inter-
molecular proton transfer through the hydrogen bonds takes
place, resulting in the reversal of electric dipole moments,
which can be detected as a dielectric response of the crystal. As
a photo-switching unit, we chose a photochromic diarylethene
molecule which reversibly isomerizes between open- and closed-
ring isomers upon irradiation with ultraviolet (UV) and visible
a Department of Chemistry and Research Center for Smart Molecules,
Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku,
Tokyo 171-8501, Japan. E-mail: m-morimoto@rikkyo.ac.jp;
Fax: +81 3-3985-2397; Tel: +81 3-3985-2397
b Precursory Research for Embryonic Science and Technology
(PRESTO), Japan Science and Technology Agency (JST),
Honcho, , 4-1-8, Kawaguchi-shi, Saitama 332-0012, Japan
w Electronic supplementary information (ESI) available: Experimental
section and supplementary data. CCDC 805211–805213. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc05729k
Scheme 2 Photochromism of diarylethene 1 having imidazoline
rings.
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Fig. 2 (a) Frequency- and (b) temperature-dependence of relative
dielectric permittivity (real part, e0) of single crystal of 1o measured
along the c-axis. (c) Photoinduced change of e0 measured along the
c-axis at 300 K.
Fig. 1 (a) Crystal structure of 1o viewed along the a-axis. Blue dotted
lines indicate intermolecular hydrogen bonds. Disordered N–H hydrogen
atoms in the hydrogen bonds are shown as red spheres. (b) Schematic
illustrations of two equivalent orientations of the N–H hydrogen
atoms in the disordered structure.
permittivity (real part, e0) measured along the c-axis parallel
to the hydrogen-bonded chain. The e0 value remains constant
at about 73 from cryogenic temperature to around 240 K.
Above 240 K, a distinct dielectric response was observed in the
low-frequency region (0.1–100 Hz) and the e0 value reaches
308 at 300 K and 0.1 Hz. The e0 values measured along the
a- and b-axes, which are perpendicular to the hydrogen-
bonded chains, are lower than those observed along the c-axis
(Fig. S3 in ESIw). The dielectric anisotropy suggests that
the dielectric response arises from the proton transfer in the
hydrogen-bonded chains of the imidazoline rings. The observed
Nꢀ ꢀ ꢀN distances in the hydrogen-bonded chain (2.915 A and
2.842 A) suggest a double-well potential energy curve of the
proton coordinate between the nitrogen atoms.11–14 Taking
into consideration the symmetry of the crystal structure along
the c-axis, a potential energy curve for the two equivalent
orientations of the N–H protons in the hydrogen-bonded
chain (Fig. 1b) is symmetric. At low temperature, the energy
barrier cannot be overcome even under application of the
electric field, and the proton transfer hardly takes place. As
the temperature increases, the protons start to transfer to the
adjacent nitrogen atoms with assistance of thermal activation.
The transfer leads to the fluctuation of the short-range ordered
domains and the resultant dynamic reversal of the dipole
moments of the imidazoline rings is detected as the dielectric
response. This situation is similar to the dielectric responses
observed in 1D hydrogen-bonded chains of dabcoH+ and
squaric acid derivatives.11–16 The relatively large dielectric loss
(e00) suggests the contribution of the migration of charged kink
solitons in the imidazoline chains (Fig. S3 in ESIw).11,15,16
The diarylethene 1 showed reversible photochromism in the
single crystal (Fig. S4 in ESIw).17 Upon irradiation with UV
light (l = 400 nm), the pale-yellow single crystal of 1o turned
blue due to the formation of the closed-ring isomer 1c. Sub-
sequent irradiation with visible light (l > 520 nm) returned
a 2D sheet in the bc plane, in which the chains of the imidazoline
rings are bridged by the diarylethene linkers. The longitudinal
and transverse axes of the diarylethene molecule are directed
along the c- and b-axes, respectively. The hydrogen-bonded chain
includes two different intermolecular NꢀꢀꢀN distances of 2.915 A
and 2.842 A, which are within the typical range for the N–HꢀꢀꢀN
hydrogen bond, but longer than the distances in salts of mono-
protonated diazabicyclo[2.2.2]octane (dabcoH+), which exhibit
large dielectric responses.11–14 A noteworthy point is the positions
of the hydrogen-bonding protons between the nitrogen atoms
in the chains. The X-ray analysis revealed that there exist two
sites of the N–H protons between adjacent nitrogen atoms and
the sites are occupied in the ratio of 50 : 50. This reflects the
fact that each hydrogen-bonded chain includes disordered
structures of two equivalent orientations (up and down) of
the N–H protons in the possibility of 50 : 50, as shown in the
schematic illustrations (Fig. 1b). The disordered structure is
also reflected by the geometry of the imidazoline ring (Fig. S2
in ESIw). Both of the two nitrogen atoms in the imidazoline
ring are equally protonated in the occupancy ratio of 50 : 50.
The lengths of the two C–N bonds (1.307 A and 1.329 A) are
almost the same and the bond alternation is not clear as a
result of the average of the single (C–N) and double (CQN)
bond lengths. Weak correlations between the proton sites
result in almost random coexistence of short-range ordered
domains of the two proton orientations rather than long-range
ferroelectric or antiferroelectric orderings.11–13
Dielectric properties were measured for the single-crystalline
sample of 1o by using an impedance analyzer. Fig. 2a and b
show frequency- and temperature-dependence of dielectric
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increase of the component diarylethene molecules upon photo-
cyclization, resulting in the expansion of the c-axis. Similar
lattice deformation induced by photochromic reactions also
has been observed in a diarylethene–perfluoronaphthalene
co-crystal.19 Such geometrical structure changes of the
diarylethene molecules upon photoisomerization modify the
hydrogen bonds in the imidazoline chains and are considered
to affect the dielectric response due to the proton transfer.
In conclusion, the diarylethene derivative having imidazoline
rings shows the dielectric response due to the proton transfer
in the hydrogen-bonded chains and the photochromic reactivity
in the crystal. The photoreversible changes of the dielectric
property suggest the correlation between the electronic and
geometrical structure changes of the diarylethene molecules
and the proton transfer behaviour.
Fig. 3 Optimized structures of (a) 1o and (b) 1c calculated by
RHF/6-31G*.
the blue crystal to the original pale-yellow one. The photo-
chromic reaction of the diarylethene molecules in the crystal
affected the dielectric response of the hydrogen-bonded chains.
Fig. 2c shows the photoinduced change of the dielectric
constants measured along the c-axis at 300 K. Upon irradia-
tion with UV light (l = 400 nm), the crystal turned blue and
the e0 values in the low-frequency region decreased. The
UV-irradiated state was thermally stable at this temperature.
Upon irradiation with visible light (l > 520 nm), the crystal
returned to the pale-yellow one and the e0 values also recovered.
Such reversible change along with the photochromic reaction
suggests that the isomerizations of the diarylethene molecules
affect the dielectric property originated from the hydrogen-
bonded chains of the imidazoline rings.
The present work was supported by PRESTO, JST and
Grant-in-Aid for Scientific Research on Priority Areas
‘‘Photochromism (471)’’ (No. 19050008) from MEXT, Japan.
Notes and references
z Synthesis and characterization of 1o are described in ESI.w
Photochromic diarylethene derivatives have been widely
utilized to control various properties of materials.9 In most
cases, electronic structure changes of the diarylethene units are
considered to affect the properties. Molecular orbital calcula-
tions on the present diarylethene molecule suggest that
electron densities in HOMOs and electrostatic potentials
around the imidazoline moieties are slightly altered by the
photocyclization reaction (Fig. S5 in ESIw). It is probable that
such electronic structure changes upon isomerization modulate
the proton transfer behaviour in the hydrogen-bonded chains of
the imidazoline rings.
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Another possible mechanism is that the geometrical struc-
ture change of the diarylethene upon photoisomerization
modulates the geometry of the hydrogen bonds. In situ
X-ray crystallographic analysis was carried out for the photo-
irradiated crystal of 1o. Upon UV irradiation (l = 400 nm),
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observed with the conversion ratio of 9%. (Fig. S6 in ESIw).18
Accompanying the photocyclization reaction of the diarylethene
molecule, the change of the cell parameters was observed
(Table S1 in ESIw). Upon UV irradiation, the length of the
c-axis, which is parallel to the hydrogen-bonded chain, was
expanded from 9.2876 A to 9.3271 A (+0.43%) (1o-UV in
Table S1, ESIw). Upon visible irradiation (l > 520 nm), the
structure of the closed-ring isomer disappeared and the
cell parameters returned to the original values (1o-Vis in
Table S1, ESIw).
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the geometrical structure change of the diarylethene molecule
by the photoreaction. As shown by the optimized structures of
the open- and closed-ring isomers in Fig. 3, the longitudinal
height (H) of the molecule increases from 6.586 A to 6.850 A
upon photocyclization. In the crystal, the longitudinal axes of
the molecules are parallel to the hydrogen-bonded chains of
the imidazoline rings along the c-axis (Fig. 1a). The hydrogen-
bonded chains are forced to be elongated by the height
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This journal is The Royal Society of Chemistry 2011