Polarization holographic optical recording based on a new photochromic diarylethene compound

Article abstract of DOI:10.1007/s11458-010-0101-0

A new unsymmetrical photochromic diarylethene, namely1-[2-methyl-5-(p-N,N-dimethylaminophenyl)-3-thienyl]-2-[2-methyl-5-(3-methoxylphenyl)-3-thienyl] perfluorocyclopentene (1a), was synthesized. The compound showed good photochromism, high sensitivity and

Full text of DOI:10.1007/s11458-010-0101-0

Front. Chem. China 2010, 5(2): 234–240
DOI 10.1007/s11458-010-0101-0
RESEARCH ARTICLE
1 Introduction
With the increasing requirements of huge information storage,
the demand for high speed input and larger memory capacity
is becoming mandatory. Holographic recording is thought to
be one of the important forms of optical memories because of
its advantages of a large capacity and high speed of
information input and output [1]. However, materials are
always a challenging problem in holographic storage [2–6].
For a holographic recording system, the recording media must
satisfy stringent criteria, including high dynamic range,
dimensional stability, rapid response time, etc. [7].
Polarization holographic
optical recording based on a
new photochromic
diarylethene compound
Hui LI1, Shouzhi PU (✉)1, Gang LIU1,
Weijun LIU1 and Baoli YAO2
Generally, there are two main classes of materials used for
holographic recording, photorefractive materials and photo-
sensitive polymerizable acrylic materials [8–10]. Each kind of
material has its advantages and disadvantages. For instance,
photorefractive materials exhibit high dynamic range, and
photopolymer materials are sensitive to light throughout the
visible spectral region. However, slow response times for
photorefractive materials and special fixing information after
recording for photosensitive polymerizable acrylic materials
pose major disadvantages [1]. These disadvantages can be
overcome by using photochromic compounds as holographic
recording media. One of the most promising photochromic
candidates is diarylethene.
Upon alternating irradiation with UV and visible light,
photochromic diarylethenes reversibly change not only their
absorption spectra but also other physicochemical properties,
such as oxidation/reduction potentials, dielectric constants,
refractive index and so on, during the photochromic reaction
[11]. These molecular property changes are of great interest
because of their potential applications for designing multi-
stable switching materials [12–15]. One the other hand, the
advantages of photochromic diarylethenes (for instance, their
good thermal stability, remarkable fatigue resistance, high
sensitivity and rapid response to different wavelength lasers)
all satisfy the requirements of high-density photon-mode
optical recording [16]. To date, many researches have been
carried out on photochromic diarylethene compounds in
various optical recordings, such as multi-wavelength/level
optical recording [17–19], near-field optical recording
[20,21], and two-photon 3D optical [22,23], etc. However,
studies concerning polarization holographic optical recording
using diarylethenes as memory media are very rare except for
several publications [24–27]. Here in this work, a new
photochromic diarylethene, 1-[2-methyl-5-(p-N,N'-dimethy-
laminophenyl)-3-thienyl]-2-[2-methyl-5-(3-methoxylphe-
nyl)-3-thienyl]perfluorocyclopentene, was synthesized.
Mixing this compound into poly(methyl methacrylate)
(PMMA) thin film as recording medium, different types
of polarization holographic recordings and polarization
A new unsymmetrical photochromic diarylethene,
namely1-[2-methyl-5-(p-N,N-dimethylaminophenyl)-3-
thienyl]-2-[2-methyl-5-(3-methoxylphenyl)-3-thienyl]
perfluorocyclopentene (1a), was synthesized. The com-
pound showed good photochromism, high sensitivity
and remarkable fatigue-resistance both in solution and
in poly(methyl methacrylate) (PMMA) matrix with UV/
Vis light irradiation. The absorption maximum of its
closed-ring isomer was observed at 624 nm in PMMA
amorphous film. It is a nice match for the wavelength of
the recording laser (633 nm). Using this target com-
pound as recording medium, four types of polarization
holographic optical recordings were performed success-
fully using a He-Ne laser. The results showed that only
the orthogonal circular polarization recording could
obtain a hologram with high diffraction efficiency and
high signal-to noise-ratio. With multiplexing recording
technology, three types of polarization multiplexing
holographic optical recordings, including angular
multiplexing, polarization multiplexing, and angular
plus polarization multiplexing holographic recording,
were also carried out perfectly based on its photo-
induced anisotropic phenomenon accompanying the
photochromic reaction by photoirradiation. The results
demonstrate that the multiplexing recording technology
is an effective method to improve recording capacity
when using diarylethene 1 as recording medium.
Keywords diarylethene, photochromism, polarization
holographic recording, multiplexing recording technology
Received December 21, 2009; accepted January 5, 2010
1. Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and
Technology Normal University, Nanchang 330013, China
2. State Key Laboratory of Transient Optical and Photonics, Xi’an
Institute of Optics and Precision Mechanics, Chinese Academy of
Sciences, Xi’an 710068, China
E-mail: pushouzhi@tsinghua.org.cn
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2010

Polarization holographic optical recording based on a new photochromic diarylethene compound
235
multiplexing holographic recordings were performed success- 40.42, 55.34, 111.28, 112.49, 113.36, 118.202, 119.93,
fully by a He-Ne laser with a 633 nm wavelength. Its 121.74, 122.77, 125.59, 125.91, 126.56, 130.01, 134.73,
photochromic reaction is shown in Scheme 1.
139.13, 141.39, 141.86, 143.09, 150.24, 160.04; IR (KBr,ν,
cm–1): 777, 880, 811, 987, 1062, 1115, 1192, 1273, 1341,
1401, 1524, 1611. Anal. Calcd for C30H25F6NOS2 (%): C,
60.70; H, 4.24; N, 2.36. Found: C, 60.77; H, 4.31, N, 2.35.
2 Experimentals
2.1 Synthesis of recording material
2.2 Preparation of recording film
The synthetic route for diarylethene 1a is shown in Scheme 2.
Firstly, Suzuki coupling of 3-bromo-2-methyl-5-thienylboro-
nic acid (2) [28–30] with 1-bromo-3-methoxylbenzene
yielded 3-bromo-2-methyl-5-(3-methoxylphenyl)thiophene
(4). It was lithiated and then coupled with octafluorocyclo-
pentene to generate a mono-substituted diarylethene com-
pound (5). Finally, 3-bromo-2-methyl-5-[4-(N,N-
dimethylammino)phenyl]thiophene (3) [31] was lithiated
and then coupled with compound 5 to give the target
diarylethene compound 1a. The structure of 1a was confirmed
The target compound 1a (5 mg) was dissolved in PMMA–
chloroform solution (10%, w/w, 0.5 mL). The mixture
solution was then spin coated on a glass substrate (20 mm
 20 mm  1 mm) with a spin rotation speed of 1500 rpm.
The thickness of the film was about 10 μm. The film was dried
in air and kept in darkness at room temperature, and it was
colored homogeneously upon irradiation with 313 so that the
open-ring isomer 1a could convert completely to the closed-
ring isomer 1b before being recorded.
1
by NMR spectroscopy, IR, and elemental analysis. H NMR
2.3 Recording and readout
(400 MHz, CDCl3, ppm): δ 1.87 (s, 3H,–CH3), 2.06 (s, 3H,
–CH3), 2.92 (s, 3H, –CH3), 2.99 (s, 3H, –CH3), 3.78 (s, 3H,
–OCH3), 6.50 (s, 1H, benzene–H), 6.60 (t, 2H, J = 8.0 Hz,
benzene–H), 6.75 (d, 1H, J = 8.0 Hz, benzene–H), 6.98 (s, 1H,
thiophene–H), 6.99 (s, 1H, thiophene–H), 7.02-7.37 (m, 4H,
The experimental setup for polarization holographic recording
and readout is presented in Figure 1. A linearly polarized light
generated from a He-Ne laser passes through a half-wave plate
and a polarized beam splitter after filtering and collimating;
benzene–H); 13C NMR (100 MHz, CDCl3, ppm): δ 14.42, the light is divided into two parts as a reference beam and an
Scheme 1 Photochromism of diarylethene 1a.
Scheme 2 Synthes is of diarylethene 1a.
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

236
Hui LI, Shouzhi PU, Gang LIU, Weijun LIU and Baoli YAO
Figure 1 Schematic diagram of the holographic storage setup: BS = beam splitter; M = mirror.
object beam. The hologram was recorded by the intersecting absorption peak of the closed-ring isomer 1b shows a clear
of the object beam containing the information and a reference bathochromic shift (21 nm) in PMMA film, which may be
beam in the sample film, and the hologram images on the attributed to the polar effect of the polymer matrix and the
sample film were readout by the attenuated reference beam stabilization of molecular arrangement in solid state [33]. In
with different angles; the reconstructed hologram images were
recorded on a charge-coupled device (CCD) camera.
3 Results and discussion
3.1 Photochromism of diarylethene
The photochromic property of diarylethene 1a was investi-
gated both in hexane (c, 3Â10–5 mol$L–1) and in PMMA
amorphous film (10%, w/w). Its absorption spectral changes
induced by photoirradiation at room temperature are shown
in Figure 2. In hexane, the maximum absorption peak of
the open-ring isomer 1a was observed at 315 nm (e,
3Â104 L$mol–1$cm–1). Upon irradiation with a light at 254 nm,
the colorless solution turned blue due to the appearance of
a new visible absorption band at 603 nm attributable to
the formation of the closed-ring isomer 1b (ε,
1.9Â104 L$mol–1$cm–1). The solution reverted from blue to
colorless upon irradiation with the appropriate wavelength
visible light (l > 450 nm), indicating that 1b returned to the
initial state 1a, and a clear isosbestic point was observed at
350 nm (Figure 2(a)). The cyclization and cycloreversion
quantum yields of diarylethene 1 were 0.42 and 0.0017,
respectively, using 1,2-bis(2-methyl-5-phenyl-3-thienyl)per-
fluorocyclopentene as a reference [32]. Similarly, diarylethene
1a also underwent reversible photochromic reaction in
PMMA film, as shown in Figure 2(b). In PMMA amorphous
film, the open-ring isomer 1a has maximum absorption at
318 nm. Upon irradiation with 313 nm UV light, the colorless
1a turned blue with the appearance of a new broad absorption
band centered at 624 nm, which was assigned to the formation
Figure 2 Absorption spectral changes of diarylethene 1 (a) in
of the closed-ring isomer 1b. The colored 1b/PMMA film
hexane (3.0Â10–5 mol dm–3) and (b) in PMMA film (10%, w/w).
can revert to colorless upon irradiation with visible light
The insert graph shows the photoswitching repeated cycles of
(l > 500 nm). Compared to that in hexane, the maximum
diarylethene 1.
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

Polarization holographic optical recording based on a new photochromic diarylethene compound
237
addition, both in hexane and in PMMA film, the two isomers scattering noise. Therefore, a conclusion can be drawn that the
of diarylethene 1 were stable for more than 3 months at room orthogonal circular polarization holographic optical recording
temperature in darkness. The insert graphs in Figure 2 showed is the best method for diarylethene 1 among the four types of
that no degradation was detected by UV-Vis absorption after polarization recording methods. The reason is that a
12 cycles of photocoloration and photobleaching, indicating polarization modulation grating is mainly recorded there
that diarylethene 1 had relatively remarkable fatigue resis- rather than an absorption modulation grating, as is the case for
tance. Therefore, the thermal stability and fatigue-resistance parallel polarization recording [34].
of diarylethene 1 were satisfied with the requirement of optical
recording media.
3.2 Polarization holographic recording
With the experimental setup shown in Figure 1, we have
evaluated the potential application of photochromic diary-
lethene as an erasable polarization holographic optical
recording medium by recording, reading and erasing the
holograms in a real-time operation. For the sake of obtaining
high quality and large area solid films, we doped diarylethene
1 into a polymer matrix (PMMA) and prepared some solid
films on glass substrates by the spin-coating method. The
hologram was recorded by the intersecting of the object beam
containing the information and a reference beam in the
photochromic diarylethene sample [27]. Using diarylethene
1b/PMMA film as recording medium, we accomplished some
Figure 3 Comparison of retrieved diffracted images with
different types of polarization holographic recording in diary-
lethene 1b/PMMA film. (a) Parallel linear polarization recording;
different types of polarization holographic optical recordings
and polarization multiplexing holographic optical recordings.
(b) Parallel circular polarization recording; (c) Orthogonal linear
polarization recording; (d) Orthogonal circular polarization
recording.
3.2.1 Four types of polarization holographic optical
recording
In diarylethene 1b/PMMA film, four types of polarization
holographic optical recordings, including parallel linear
polarization recording, parallel circular polarization record-
ing, orthogonal linear polarization recording and orthogonal
circular polarization recording, were tested. The retrieved
diffracted images obtained by the four types of polarization
holographic recordings are shown in Figure 3. The experi-
mental conditions of each type of polarization holographic
recording were illustrated in detail in a previous paper [24].
As shown in Figure 3, we can see that both the parallel linear
polarization recording (Figure 3(a)) and the orthogonal linear
polarization recording (Figure 3(c)) obtain low diffraction
efficiency as well as low scattering noise when the
intensities of the recording wave and the readout wave are
fixed at 393 mW$cm–2; meanwhile, the orthogonal circular
polarization recording (Figure 3(d)) gets both high diffraction
efficiency and high signal-to noise-ratio (SNR), which are
both very important in holographic optical recording. The
orthogonal linear polarization recording (Figure 3(b)) has not
only the highest diffraction efficiency but also the highest
3.2.2 Polarization multiplexing holographic optical
recording
The polarization multiplexing scheme is based on the multiple
polarization hologram optical recording in polarization-
sensitive materials, done by alternating the object and
reference beams with parallel and orthogonal polarization,
either in linear or in circular polarization [25]. Photochromic
diarylethene materials can be potentially used as polarization
holographic recording candidates because there is a photo-
induced anisotropic phenomenon accompanying the photo-
chromic reaction by photoirradiation [26]. Based on
polarization modulation technology, we performed, perfectly,
three types of polarization multiplexing holographic optical
recordings such as angular multiplexing, polarization multi-
plexing, and angular multiplexing scheme combined with
polarization multiplexing scheme holographic optical record-
ing.
At first, two different images were recorded in the same
place with the dimension of 0.78 mm2 on diarylethene 1b/
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238
Hui LI, Shouzhi PU, Gang LIU, Weijun LIU and Baoli YAO
PMMA film with angular multiplexing technology, as shown region of the diarylethene 1b/PMMA medium. When the
in Figure 4. Each hologram was recorded by the intersecting angle between the polarization direction of P3 and the
of the object beam containing the information and a reference horizontal direction was 90 deg, namely, the reference beam
beam in diarylethene 1b/PMMA film. The intensities of was kept in left-hand circular polarization and the object beam
the object beam and the reference beam were fixed at was set to be right-hand circular polarization during recording,
393 mW$cm–2. The stored information was readout by the the first hologram was recorded as shown in Figure 5(a).
attenuated reference beam with an intensity of 80 mW$cm–2, When both the reference beam and the object beam were kept
and the reconstructed hologram image was recorded on CCD in left-hand circular polarization during recording (the angle is
camera. With 4 deg angular separation, two reference beam 0 deg), the second hologram was recorded in the same place of
angles were used in this experiment and two holograms were diarylethene 1b/PMMA sample, as shown in Figure 5(b).
recorded in the same region of the sample film. The exposure During the retrieval process, a single left-hand circularly
time for each hologram recording was 5 s. The results showed polarization beam is used to reconstruct the two orthogonally
that the two retrieved images had very good contrast and high circularly polarization images simultaneously. By rotating the
SNR. Above all, they could be separated completely from polarization angle of the polarizer P3, different images can be
each other without any cross talk, which is very important for selectively read out. When the readout linear polarization
successful realization of multiplexing holographic recording angle is 45°, an overlapping image appears as shown in
[25].
Figure 5(c). In this way, the two polarization holograms can
Furthermore, polarization multiplexing holographic optical be hidden in the pattern and recorded in the diarylethene 1b/
recordings were carried out successfully using diarylethene PMMA film based on its photochromic and photoinduced
1b/PMMA film. The retrieved holograms recorded by anisotropic properties, and they can be picked up separately
polarization multiplexing holographic recording method are only when the polarizer P3 is located on a certain polarization
shown in Figure 5. The hologram recording and reconstruc- angle. This phenomenon may be potentially applied in
tion were in the same optical setup, and the readout of the two camouflage technology [35].
object images was controlled by selectively setting the
Finally, we proposed to combine the angular multiplexing
polarization angle of the polarizer P3. In Figure 5, three scheme with the polarization multiplexing scheme based on
retrieved holograms were recorded by the circular polarization the multi-recording ability of diarylethene 1 described above.
multiplexing holographic recording method in the same The retrieved holograms of the combined multiplexing
Figure 4 Readout holograms for angular multiplexing holographic optical recording in diarylethene 1b/PMMA film.
Figure 5 Readout holograms for polarization multiplexing holographic optical recording in diarylethene 1b/PMMA film. (a) the
polarization of P is 90°; (b) the polarization of P is 0° ; (c) the polarization of P is 45°.
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

Polarization holographic optical recording based on a new photochromic diarylethene compound
239
Figure 6 Readout holograms for angular multiplexing scheme combined with polarization multiplexing scheme in diarylethene 1b/
PMMA film.
scheme are shown in Figure 6. In the combined multiplexing
scheme, the principal multiplexing scheme is angular multi-
4 Conclusion
plexing and the secondary scheme is polarization multi- In summary, a new unsymmetrical photochromic diarylethene
plexing. For each angle of the reference beam, two was synthesized and its photochromic properties were
polarization holograms, i.e. parallel circular polarization investigated both in hexane and in PMMA film. Using this
hologram and orthogonal circular polarization hologram, diarylethene compound as recording medium, different types
were recorded on diarylethene 1b/PMMA film. During the of polarization holographic optical recordings were performed
recording process, the intensities of the object beam and the successfully. The results demonstrated that high photosensi-
reference beam were both fixed at 393 mW$cm–2. The object tivity and easy information processing of photochromic
beam size on the sample plane was about 100 mm in diameter. diarylethene materials provide a substantial advantage for
With 4 deg angular separation, three angles of reference beam polarization holographic recording. The combined multi-
were used in this experiment. Thus, six holograms were plexing technique can effectively improve the recording
recorded in the same place of the diarylethene 1b/PMMA capacity. It will be helpful in accumulating evidence to realize
film, and the exposure time for each hologram recording was high-density optical recording using photochromic diary-
6, 5, 4, 4, 3, and 3 s, respectively. The recorded information lethene materials as recording media.
was readout by the attenuated reference beam with an
intensity of 80 mW$cm–2, and the retrieved holograms were
Acknowledgements This work was supported by the National
Natural Science Foundation of China (Grant No. 20962008),
recorded on CCD camera. As shown in Figure 6, it can be
Program for New Century Excellent Talents in University (Grant
seen that the six retrieved holograms showed good contrast
No. NCET-08-0702), the Foundation for Key Program of Ministry of
and no crosstalk with each other. Compared with the single
Education, China (Grant No. 208069), the Project of Jiangxi Youth
angular multiplexing scheme, the holographic recording
density can be increased doubly when using this combined
multiplexing technology, which will lead to a higher data
capacity of holographic recording than the single angular
multiplexing method. In addition, experimental tests showed
that the hologram recorded in the diarylethene 1b/PMMA film
is stable at room temperature in darkness and no significant
degradation is detected for retrieving of recorded holograms
after being stored for more than 2 months. The recorded
holograms can be erased entirely upon irradiation with UV
light, and no remarkable degradation in resolution of
reconstructed image after 12 write/erase cycles was found.
Scientist, and the Science Funds of the Education Office of Jiangxi,
China (Grant No. GJJ08365).
Shouzhi PU received his Ph.D. in 2003 from
Tsinghua University. He is currently working
at Jiangxi Science & Technology Normal
University. In 2008, he was appointed New
Century Excellent Talents by the Education
Ministry of China. His research interests
include synthesis of novel photochromic
compounds, and development of molecular
devices that apply in optical recording, photo-
switches and chemical sensor.
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Hui LI, Shouzhi PU, Gang LIU, Weijun LIU and Baoli YAO
18. Guo, H. B.; Zhang, F. S.; Wu, G. S.; Sun, F.; Pu, S. Z.; Mai, X.
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