Photochromism of a dithienylethene having diphenylamino side groups in the bulk amorphous phase

Article abstract of DOI:10.1246/cl.2001.70

1,2-Bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene was found to form an amorphous state in bulk by introducing diphenylamino substituents at both ends of the molecule. The glass transition temperature was as high as 103 °C. The amorphous film e

Full text of DOI:10.1246/cl.2001.70

70
Chemistry Letters 2001
Photochromism of a Dithienylethene Having Diphenylamino Side Groups
in the Bulk Amorphous Phase1
Masato Fukudome, Kazushi Kamiyama, Tsuyoshi Kawai† and Masahiro Irie†*
Research and Development Center, Kyocera Corporation Ltd., 1-4, Yamashitacho, Kokubu-shi, Kagoshima 899-4312
†Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, and
CREST, Japan Science and Technology Corporation (JST), 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
(Received October 2, 2000; CL-000894)
1,2-Bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclo-
pentene was found to form an amorphous state in bulk by intro-
ducing diphenylamino substituents at both ends of the mole-
o
cule. The glass transition temperature was as high as 103 C.
The amorphous film exhibited reversible color and refractive
index changes by alternate irradiation with ultraviolet and visi-
ble light.
Various types photochromic compounds, such as spiroben-
zopyrans, azobenzenes, furylfulgides, and diarylethenes, have
been so far developed for applications to optoelectronic
devices. Among the compounds diarylethenes are the most
promising for the applications because of fatigue resistant and
thermally irreversible photochromic performance.2,3 Diaryl-
ethenes exhibit photo-reversible changes not only in the optical
absorption and fluorescence spectra but also in other optical
properties such as refractive index and non-linear optical
effect.4–10 The refractive index change is of special interest
because wave-guided optical switches, holographic grating and
3D memories are based on the refractive index change.
For the practical applications photochromic reactions
should take place in solid matrices. So far polymers have been
used for the solid matrices, but polymer matrices have several
defects. In general it is not easy to increase the dye concentra-
tion and segregation takes place to some extent after long term
storage. The ideal solid matrices are bulk amorphous or crys-
talline states. Bulk amorphous materials have advantageous
characteristics, such as high dye density and low optical scatter-
ing, and thin films can be easily prepared from the materials by
spin-coating or vacuum deposition methods.6,11 Optical isotropy
of the amorphous state is also favorable for device fabrication.
Although several amorphous diarylethenes have been so far
reported, the glass transition temperatures are below 80 °C.6,12–16
For practical use it is preferred to increase Tg above 100 °C. In
the present study we report on a new diarylethene which forms
an amorphous state in bulk with Tg higher than 100 °C. We also
studied the photostimulated reversible change in refractive index
in the bulk amorphous state.
room temperature. Thermal property of the sample was studied
with a differential scanning calorimeter (Seiko Instruments,
DCS6200). The temperature scan rate was typically 5 °C/min.
Refractive index of the amorphous sample was measured with a
prism-coupler system (Metricon, 2010) utilizing IR-diode laser
(817 nm) as a light source.
Hexane solution of 1a showed absorption bands at 306 nm
(εmax = 48000 M–1cm–1) and 333 nm (εmax = 51700 M–1cm–1).
Upon irradiation with UV light (λ = 366 nm), the colorless
hexane solution turned blue, in which absorption peaks were
observed at 595 nm and 420 nm. The visible absorption bands
disappeared by irradiation with visible light (λ > 480 nm). An
isosbestic point was observed at 360 nm. The photochemical
colored product was isolated from the UV-irradiated hexane
solution by HPLC (silica-gel/hexane). NMR and mass spectra
assigned the product to the closed-ring isomer, 1b.17 The
absorption coefficients are εmax = 20700 M–1cm–1 at 595 nm and
εmax = 24100 M–1cm–1 at 420 nm. The conversion ratio at the
photostationary state under irradiation with 313 nm light was
estimated to be 99%.
The cyclization and cycloreversion quantum yields in
hexane solution were measured using furylfulgide as the refer-
ence.18 The cyclization quantum yield was as large as 0.44,
while the cycloreversion quantum yield was very low (0.005).
The substitution of electron donating diphenylamino groups is
considered to suppress the cycloreversion quantum yield from
0.015 to 0.005.18 The high conversion in the photostationary
state can be explained by the large quantum yield difference
between cyclization and cycloreversion reactions.
The amorphous films of 1a and 1b showed similar pho-
tochromic behavior. The 1a film was colorless and showed the
absorption peaks at 304 nm and 334 nm. Upon irradiation with
UV light (λ = 313 nm) the film turned blue and the absorption
peaks were observed at 432 nm and 606 nm, which are same as
the peaks of the 1b film. The absorption peak wavelengths of
the bulk films were slightly longer than those in solution. The
A new dithienylethene derivative having diphenylamino
groups, 1,2-bis[2,4-dimethyl-5-[4-N,N-bis(4-methylphenyl)
amino]phenyl-3-thienyl]-3,3,4,4,5,5-hexafluorocyclopentene,
1a, was prepared as shown in Scheme 1. The product was puri-
fied by GPC and HPLC (silica-gel/hexane) and characterized
by the conventional methods.17 After evaporation of the sol-
vent, 1a was obtained as colorless amorphous glass. The thin
film of 1a was prepared on a quartz substrate by a spin-coating
method from its toluene solution. Optical absorption spectra
were measured with a spectrophotometer (Hitachi, U-3500) at
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
71
conversion ratio at the photostationary states of 1a and bleached
1b films under irradiation with 313 nm light were 35% and
48%, respectively. The bleached amorphous film prepared
from 1b exhibited higher conversion ratio in the photostation-
ary state than that of the film prepared from 1a. The open-ring
isomers of diarylethenes have two conformations, parallel and
anti-parallel conformations, and the photochemical cyclization
reaction takes place only from the anti-parallel conformation.
Although interconversion between the two conformers is
allowed in solution, the interconversion is strongly suppressed
in the amorphous film. When 1a is produced by visible light
irradiation from 1b in the amorphous film, the photogenerated
1a is considered to be in the photoactive anti-parallel conforma-
tion and hardly interconverts to the photo-inactive parallel con-
formation. This is the reason why the conversion in the photo-
stationary state of the film prepared from 1b is higher than that
of the film from 1a. The low conversion in the film prepared
from 1a is that a part of molecules are in the photo-inactive par-
allel conformation.
indices of 1a and 1b was 0.040, which is similar to the value of
our previous result on amorphous film.13 It should also be
noted that the refractive index of the bleached 1b film was
slightly higher than that of 1a film. This difference suggests
that the structure of 1a molecule in the 1a film is different from
that in the bleached 1b film and, therefore, density is slightly
different. The refractive index in the photostationary state of
the 1b film was found to be about 0.015 larger than the
bleached 1b film, which is 43 % of the refraction index change
of 1b film. This ratio is roughly in accordance with the conver-
sion ratio at the photostationary state, 48%, suggesting a linear
relationship between composition and refractive index.
Thermal property was studied by DSC as shown in Figure
1. Upon heating, 1a showed a clear endothermal peak at 113.7
°C , and clear shift in the base line was observed at around this
peak, which is typical for the glass-to-liquid transition. The
endothermal peak can be assigned to the intermolecular interac-
tion in the glass state. Based on the threshold temperature, the
glass transition temperature, Tg, was determined to be 103 °C.
Thermal behavior of 1b was also investigated and clear glass
transition behavior was observed above 120 °C and Tg was
determined to be 124 °C, as shown in Figure 1(b). Formation
References and Notes
1
M. Fukudome, K. Kamiya, T. Kawai, and M. Irie, Abstract of Papers,
78th Spring Meeting of The Chemical Society of Japan, Tokyo, March
2000, 3A4 29.
o
of 1a indicates that the exothermal peak at about 178 C corre-
sponds to the thermal cycloreversion reaction. The higher Tg of
1b is attributed to the planer molecular structure of 1b.
2
3
4
5
6
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The change in the refractive index of 1 was also studied by
a prism coupling method.14 At the monitoring wavelength of
817 nm both 1a and 1b have no optical absorption. The refrac-
tive index changes upon UV (λ = 313 nm) and visible (λ > 480
nm) light irradiations are shown in Figure 2. The refractive
index of 1a film was 1.619 and increased to 1.635 upon irradia-
tion with UV light. The refractive index of film of 1b
decreased from 1.659 to 1.624 upon irradiation with visible
light. These changes in the refractive index clearly indicate that
the closed-ring isomer 1b has higher polarizability than that of
the open-ring isomer 1a. The difference between the refractive
16 M. -S. Kim, T. Kawai, and M. Irie, Chem. Lett., 1188 (2000).
1
17 1a: H NMR(CDCl3): δ = 2.07(s, 3H), 2.10(s, 3H), 2.32(m, 18H),
6.9–7.3(m, 24H). MS m/z = 938(M+). 1b: 1HNMR (CDCl3); δ =
2.21(s, 6H), 2.32(m, 18H), 6.9–7.3(m, 24H). Anal. Found: C,73.30;H,
5.53; N, 2.89%. Calcd. for C57H48F6N2S2: C, 72.90; H, 5.15; F, 12.14;
N, 2.98; S, 6.83%. MS m/z (M+) = 938.
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19 During the review process a paper dealing with a similar compound
appeared; H. Utsumi, D. Nagahama, H. Nakano, and Y. Shirota, J.
Mater. Chem., 10, 2436 (2000).