Nanocrystallization of diarylethene and photochromic properties

Article abstract of DOI:10.1021/cg100441h

We have succeeded, for the first time, in fabrication of diarylethene (DAE), 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-perfluorocyclopentene, nanocrystals using two nanocrystallization processes based on the reprecipitation method. Process I was the microwave irradiation after reprecipitation step. Process II uses a nucleating agent of a closed-DAE molecule without microwave irradiation. The crystal structure of the obtainedDAEnanocrystals was elucidated as the lattice structure of an open-DAE bulk crystal. Moreover, the DAE nanocrystals clearly exhibited photochromism by alternate irradiation of UV and visible light.

Full text of DOI:10.1021/cg100441h

DOI: 10.1021/cg100441h
Nanocrystallization of Diarylethene and Photochromic Properties
2010, Vol. 10
2857–2859
Norio Tagawa,*,† Akito Masuhara,*,‡ Hitoshi Kasai,†,§ Hachiro Nakanishi,† and
Hidetoshi Oikawa†
†Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira,
Aoba-ku Sendai, Miyagi 980-8577, Japan, ‡Graduate School of Science and Engineering,
Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan, and §PRESTO,
Japan Science and Technology Agency (JST ), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Received April 2, 2010; Revised Manuscript Received June 3, 2010
ABSTRACT: We have succeeded, for the first time, in fabrication of diarylethene (DAE), 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-
perfluorocyclopentene, nanocrystals using two nanocrystallization processes based on the reprecipitation method. Process I was the
microwave irradiation after reprecipitation step. Process II uses a nucleating agent of a closed-DAE molecule without microwave
irradiation. The crystal structure of the obtained DAE nanocrystals was elucidated as the lattice structure of an open-DAE bulk crystal.
Moreover, the DAE nanocrystals clearly exhibited photochromism by alternate irradiation of UV and visible light.
Tetrahydrofuran (THF) was purchased from Wako Pure Chemical
Industries, Ltd. Water was purified up to 18.2 MΩ cm using Arium
611UV (Sartorius Mechtronics Japan K.K.).
Introduction
Photochromism is a reversible transformation induced by
photoirradiation between two forms with different chemical
structures.1,2 Thedifferentchemical structures maycause changes
of various physicochemical properties, such as absorption, emis-
sion, refractive index, dielectric constant, and redox potential.
Photochromic compounds attract much attention because of their
potential applications in photochromic lenses, optical memories
and switches, holographic displays, and light-driven actuators.3,4
Diarylethene (DAE) has excellent characteristics among photo-
chromic compounds, e.g., thermal stability of both isomers, fati-
gue resistance, and high photosensitivity. Moreover, DAE exhi-
bits photochromism not only in a solution state but also at a bulk
crystal state.5 The crystal state color change is observed. Its pro-
perty can be exploited for applications in dichroism and three-
dimensional memory.
Using two processes developed based on the reprecipitation
method,12-14 the DAE nanocrystals were fabricated. In process I,
DAE was first dissolved in THF to produce a concentration of DAE
solution 0.20-10 mM. Then 20 mL of this solution was injected into
water (500 mL) used as a poor solvent with a pulseless syringe pump
(260D; TeledyneIscoInc.) atroomtemperature ina darkroom, while
it was stirred vigorously using a ramond stirrer (1400 rpm). The
resulting aqueous dispersion liquid of DAE nanocrystals was irra-
diated immediately (within 30 s) with microwaves (2.45 GHz, 500 W,
7 min). The other process (II) was almost identical, but the UV-
irradiated DAE solution, which means the open-DAE solution
containing closed-DAE by irradiation of UV light, was injected.
The UV irradiation was conducted using a UV lamp (SUV-16; AS
ONE corporation).
The DAE nanocrystal size and shape were evaluated using a dyna-
mic light scattering instrument (DLS: Zetasizer Nano series Nano-
ZS; Sysmex Co.) and atomic force microscopy (AFM: Innova SPM;
Nihon Veeco K.K.). Powder X-ray diffraction (XRD) patterns were
measured (D8 Advance; Bruker AXS K.K.). The irradiation wave-
Fabrication of DAE nanocrystals is now anticipated because
their chemical and physical properties are expected to exhibit
distinctive behavior associated with size-dependence.6 In addi-
tion, organic crystals can be dispersed in an aqueous colloidal
liquid or thin-layer matrix and can avoid problems of light
scattering loss or opacity in bulk crystals.
˚
length was 1.5 A (Cu KR). The UV-vis absorption spectrum was
measured with a UV-visible spectrometer (V-570DS; Jasco Corp.).
Some research groups have reported fabrication of DAE
nanoparticles.7-10 Unfortunately, it was extremely difficult to
fabricate monodispersed and size-controlled DAE nanocrystals
as well as DAE nanoparticles. A previous report described our
preparation of monodispersed and size-controlled DAE nano-
particles using a reprecipitation method and revealed a red-shift
of absorption peak positions with increasing size.11 Nanocrys-
tallization of DAE, however, remains a challenging topic.
Herein we report, for the first time, the fabrication of DAE
nanocrystals by development of two processes based on a repre-
cipitation method. Nanocrystallization, crystal structure, and the
photochromism of these DAE nanocrystals are discussed in detail.
Results and Discussion
Two forms of DAE isomers, designated as open-DAE and
closed-DAE (see Figure 1a), exhibit attractive physicochemical
properties because of their different conjugation. However, open-
DAE is a photochemical equilibrium with closed-DAE, which
means that open-DAE is not completely converted into closed-
DAE, even in the solution state, when UV light irradiated.
Therefore, we attempted to fabricate a DAE nanocrystal, which
consists only of the colorless open-DAE molecules.
As described in the Experimental Section, we fabricated DAE
nanocrystals using two processes based on the reprecipitation
method. Specifically, the use of a pulseless syringe pump and
ramond stirrer made it possible to realize mass-production
(50 times compared with the conventional reprecipitation meth-
od), high-reproductivity, and monodispersibility because of high
injection and a stirring rate effect. The resulting DAE nanocrys-
tals were dispersed into a stable dispersion without using a
surfactant.
Experimental Section
1,2-Bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene
(Figure 1a) was used as the DAE. It changes from colorless to
blue upon UV irradiation and reverts back to colorless again by
irradiation of visible light. DAE was purchased from Tokyo Chemi-
cal Industry Co. Ltd. It was used without further purification.
Figure 1b presents an AFM image of DAE nanocrystals
fabricated using process I. The DAE nanocrystals were spherical;
their size was approximately uniform and almost identical to that
from DLS measurement. Additionally, it was possible to control
*To whom correspondence should be addressed. E-mail: tagawa@mail.
tagen.tohoku.ac.jp and masuhara@yz.yamagata-u.ac.jp. Telephone and
Fax: þ81-22-217-5645 and þ81-238-26-3891.
r
2010 American Chemical Society
Published on Web 06/15/2010
pubs.acs.org/crystal

2858 Crystal Growth & Design, Vol. 10, No. 7, 2010
Tagawa et al.
Figure 1. (a) Chemical structures of open-diarylethene and closed-
diarylethene (DAE); (b) AFM image of DAE nanocrystals prepared
by process I.
Figure 3. Powder XRD patterns: DAE nanocrystals prepared
using a DAE solution containing (a) 1.5% and (b) 37% of closed-
DAE in process II, or (c) closed-DAE bulk crystal.
were in an almost amorphous state before microwave irradiation
(see Supporting Information, Figure S3). In all probability, the
microwave irradiation would have promoted nanocrystallization
of DAE nanoparticles because of a rapid and homogeneous
heating effect. As a consequence, the residual good solvent,
THF, was evaporated from DAE nanoparticles. Simultaneously,
DAE molecules were reoriented into the nanocrystal state during
heating by microwave irradiation.6
We have also fabricated DAE nanocrystals using a small
amount of closed-DAE molecules as a nucleating agent in process
II because a closed-DAE molecule shows lower solubility in THF
than open-DAE. Figure 3a shows the powder XRD pattern of
DAE nanocrystals fabricated from a THF solution of open-DAE
containing 1.5% of closed-DAE molecules, the value of which
was estimated from ref 16. Compared with parts a and b of
Figure 2, these DAE nanocrystalswereof the same crystal state as
that of the open-DAE bulk crystal. For these nanocrystals too,
the size was controllable by changing the concentration of the
injected solution in process II.
However, when the ratio of closed-DAE molecule was in-
creased to 37%, the DAE nanocrystals became a mixed crystal of
open-DAE and closed-DAE crystal, as shown in Figure 3b and c.
Perhaps, the domains of closed-DAE crystals were formed
initially before open-DAE molecules were crystallized. This fact
suggests that one should optimize the concentration of closed-
DAE as a nucleating agent.
It seemed difficult to nanocrystallize DAE having bulky sub-
stituents such as fluorine and phenyl groups. In addition, the
three rings of the open-DAE molecule remain rotatably in the
solution state; it is difficult to obtain a stable crystal form.
Therefore, it is important to optimize the nanocrystallization
process. In one experiment, open-DAE nanocrystals could not
be stabilized at a stirring rate of less than 700 rpm in process II.
Probably, the degree of supersaturation was insufficient in
process II.
Figure 4a exhibits the UV-vis absorption spectral changes of
the aqueous dispersion liquid of DAE nanocrystals during alter-
nate UV and visible light irradiation. During UV light irradiation
(λ = 254 nm), the colorless aqueous dispersion liquid turned to
bluish-purple; that is, new absorption peaks appeared at 373 and
572 nm. The bluish-purple color of the aqueous dispersion liquid
disappeared when irradiated with visible light (λ > 420 nm);
that is, the absorption peaks were restored approximately 100%.
Furthermore, these color changes were repeated several times, as
Figure 2. Powder XRD patterns: (a) DAE nanocrystals prepared
by process I; (b) open-DAE bulk crystal; (c and d) a-b plane (100)
and a-c plane (010) in the lattice structure of the open-DAE bulk
crystal, respectively.
the DAE nanocrystal size to ca. 60-120 nm, by changing the
concentration of the injected solution. The size increased with
concentration (see Supporting Information, Figures S1 and S2).
Figure 2a portrays the powder XRD pattern of DAE nano-
crystals identified with Figure 2b.15 Comparison of parts a and b
of Figure 2 shows that the DAE nanocrystals take the same
crystal lattice form as that of the open-DAE bulk crystal shown in
parts c and d of Figure 2.
In process I, the microwave irradiation plays an important
role in nanocrystallization because only DAE nanoparticles

Communication
Crystal Growth & Design, Vol. 10, No. 7, 2010 2859
DAE bulk crystal. Moreover, the DAE nanocrystals clearly
exhibited photochromism by alternate irradiation of UV and
visible light. For use in optically functional devices and other
applications, DAE nanocrystals are anticipated as promising
nanomaterials.
Supporting Information Available: Size-control of DAE nano-
crystals details; powder XRD pattern of DAE nanoparticles before
microwave irradiation in the process (I). This material is available
free of charge via the Internet at http://pubs.acs.org.
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