tionary state. The backward ring-opening reaction of 2c to
regenerate 2o took place on irradiation with visible light
(w580 nm) from a 500 W xenon lamp. A similar spectral
change was also observed for the amorphous ®lm of 1o. The
photocyclised compound 2c was isolated from a benzene
solution irradiated with 334 and 365 nm light using HPLC
(eluent: methanol±acetone (3 : 1)), and puri®ed by recrystallisa-
tion from THF±ethanol. Compound 2c was also found to
readily form an amorphous glass with a Tg of 104 ³C. Both 1c
and 2c were characterised by electronic absorption and 1H
NMR spectroscopy.17
Table 1 summarises quantum yields for the photocyclisation
(WoAc) and the backward photochemical ring-opening reaction
(WcAo), together with the molar ratio of the photocyclised
molecule to the total amount of the starting and photocyclised
molecules at the photostationary state (Ypss). Compounds 1o
and 2o exhibited relatively high WoAc in solution (0.79 for 1o
and 0.61 for 2o) compared with the values reported for other
dithienylethene derivatives in solution (mostly in the range
from 0.3 to 0.5),9 although the highest value (0.86) has been
reported for a dithienylethene-backbone photochromic poly-
mer.18
Fig. 1 Electronic absorption spectral change of 2o amorphous ®lm. a)
before photoirradiation. b) photostationary state upon irradiation with
334 and 365 nm light.
1,2-bis[2,4-dimethyl-5-(4-tert-butylphenyl)thiophen-3-yl]-3,3,4,-
4,5,5-hexa¯uorocyclopentene has been shown to form an
amorphous glass.15
Compound 1o was synthesised by lithiation of 5-{4-[bis(4-
methylphenyl)amino]phenyl}-3-bromo-2-methylthiophene (3)
(3.5 g, 7.8 mmol) followed by reaction with 1-(2-methylbenzo-
[b]thiophen-3-yl)-2,3,3,4,4,5,5-hepta¯uorocyclopentene (3.0 g,
8.8 mmol), prepared by the reaction of 3-lithio-2-methylben-
zo[b]thiophene with 1,2,3,3,4,4,5,5-octa¯uorocyclopentene.
Compound 2o was synthesised by lithiation of 3 (5.4 g,
12 mmol) followed by reaction with 1,2,3,3,4,4,5,5-octa¯uor-
ocyclopentene (1.2 ml, 8.9 mmol). Compounds 1o and 2o were
puri®ed by silica-gel column chromatography, followed by
recrystallisation from THF±ethanol and toluene±hexane to
give 1.8 g (yield 33%) and 2.5 g (yield 45%), respectively. They
were identi®ed by various spectroscopic methods, mass
spectrometry, and elemental analysis.16
Both 1o and 2o were found to readily form stable amorphous
glasses when the melt samples were left to stand in air, as
evidenced by differential scanning calorimetry (DSC), X-ray
diffraction, and polarising microscopy. The glass-transition
temperatures (Tgs) and melting points are 66 and 158 ³C for 1o
and 94 and 207 ³C for 2o, as determined by DSC. The
amorphous glasses of these two materials are very stable, no
crystallisation phenomenon being observed upon heating
above their Tgs. Compounds 1o and 2o form uniform
amorphous ®lms by spin coating.
Compounds 1o and 2o were found to exhibit photochromism
in their amorphous ®lms as well as in solution. Fig. 1 shows the
electronic absorption spectral change of an amorphous ®lm of
2o prepared by spin coating from benzene solution (ca.
6.661023 mol dm23). Upon irradiation with 334 and 365 nm
light (1.6 mW cm22) from a 500 W super-high-pressure mer-
cury lamp in air, the band with a maximum at around 360 nm
gradually decreased and new absorption bands with maxima at
around 442 and 635 nm together with a shoulder at ca. 675 nm
appeared due to the transformation of 2o into 2c by
photocyclisation, and the system ®nally reached a photosta-
It is noteworthy that the quantum yield for the photo-
cyclisation (WoAc) in the amorphous ®lm is approximately a
half of that in solution for both 1o and 2o. It is known that
dithienylethene derivatives have two conformers, ``antipar-
allel'' (ap) conformer having two thienyl moieties with C2
symmetry and ``parallel'' (p) conformer containing two
thienyl moieties with mirror symmetry, and that only the
former undergoes photocyclisation reaction.9,10 It is thought
that no transformation takes place between the two
conformers in the solid state and hence, the photocyclisation
quantum yield in the solid state is decreased due to the
presence of the p-conformer which does not undergo
photocyclisation. In addition, intermolecular interactions
in the solid state may cause enhanced radiationless
deactivation, leading to lower quantum yields in the
amorphous ®lm than in solution. Also noteworthy is that
the Ypss value of the amorphous ®lm of 2o is double that of
1o. This result suggests that the population of the ap-
conformer in the amorphous ®lm of 2o is larger than that of
1o. It is suggested that the steric effect due to the two bulky
triphenylamine moieties is responsible for this difference.
The CPK models of these two molecules show that while
both the ap- and p-conformers of 1o and the ap-conformer of
2o permit free rotation around the C±C bond connecting the
thienyl and triphenylamine moieties, this rotation is
restricted for the p-conformer of 2o because of steric
crowding. Therefore, the ap-conformer is suggested to be
more populated for 2o. Irradiation of an amorphous ®lm of
the photocyclised compound 2c with light of the wavelength
longer than 580 nm produced 2o with the photoactive
antiparallel conformer due to the photochemical ring-open-
ing reaction with a WcAo of 0.001. The 2o compound thus
produced underwent the photocyclisation to give 2c with a
WoAc of 0.41.
In summary, a novel class of photochromic amorphous
molecular materials based on dithienylethene, 1o and 2o, has
been created. These novel dithienylethene derivatives were
found to readily form stable amorphous glasses and uniform
amorphous ®lms by spin coating, exhibiting photochromism in
solution and in the amorphous ®lm. Comparison of photo-
chromic behavior between 1o and 2o in their amorphous ®lms
suggests that the introduction of two bulky triphenylamine
moieties increases the population of the ap-conformer of the
molecule, leading to higher conversion of the photocyclisation
reaction. The present study presents a new guideline for the
molecular design of photochromic amorphous molecular
materials based on dithienylethene.
Table 1 Quantum yields for the photocyclisation reaction by irradia-
tion with 365 nm monochromatic light (WoAc) and the backward
photochemical ring-opening reaction by irradiation with 600 nm
monochromatic light (WcAo), and the molar ratio of the photocyclised
molecules to the total molecules at the photostationary state upon
irradiation with 334 and 365 nm light (Ypss
)
System
WoAc
WcAo
Ypss
1, benzene solution
1, amorphous ®lm
2, benzene solution
2, amorphous ®lm
0.79
0.33
0.61
0.33
0.015
0.010
0.001
0.001
1.00
0.36
0.98
0.77
J. Mater. Chem., 2000, 10, 2436±2438
2437
Utsumi
