Acid-induced photochromic system switching of diarylethene derivatives between P- and T-types

Article abstract of DOI:10.1039/b700177k

P-type photochromic diarylethenes having a diethylamino group can switch to T-type photochromic system by the addition of trifluoromethanesulfonic acid as external stimuli. The Royal Society of Chemistry.

Full text of DOI:10.1039/b700177k

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COMMUNICATION
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Acid-induced photochromic system switching of diarylethene derivatives
between P- and T-types{
Seiya Kobatake* and Yuko Terakawa
Received (in Cambridge, UK) 5th January 2007, Accepted 23rd January 2007
First published as an Advance Article on the web 9th February 2007
DOI: 10.1039/b700177k
P-type photochromic diarylethenes having a diethylamino
group can switch to T-type photochromic system by the
addition of trifluoromethanesulfonic acid as external stimuli.
Photochromism has attracted particular attention because the
chemical and physical properties can be changed by the external
stimuli of light. Photochromic compounds are classified into two
types, thermally unstable (T-type) and thermally stable (P-type) of
the photogenerated isomers.1 Azobenzene and spiropyran belong
to T-type photochromic compounds. Diarylethene and furylful-
gide can undergo thermally irreversible P-type photochromic
reactions. Therefore, P-type photochromic compounds potentially
can be used for sustainable application such as optical memory
media, switching devices, and display materials.2–7 If materials
switch between P-type and T-type photochromism by other
external stimuli, the photochromic materials can be developed as
novel material applications such as rewritable papers of write-by-
light/erase-by-heat systems.
Diarylethenes 1a and 2a showed thermally irreversible photo-
chromism in acetonitrile as well as in hexane, toluene, and other
organic solvents. Fig. 1 shows the absorption spectral changes of
1a in acetonitrile. Upon irradiation with ultraviolet light (UV), the
acetonitrile solution of 1a changed from colorless to violet. The
absorption maximum of 1b appeared at 540 nm. The colored
solution was thermally stable and returned to the colorless one by
irradiation with visible light. The closed-ring isomer 1b was
isolated by HPLC. The photocyclization conversion was deter-
mined by comparing the absorption spectra of 1b with the
photostationary solution. The conversion in acetonitrile was 100%
Most of the diarylethenes having heterocyclic aryl groups
belong to P-type photochromic compounds.8 Even at 100 uC, the
colored closed-ring isomers have a long-term stability.2 However,
some diarylethenes having electron-withdrawing substituents on
the aryl group undergo a thermal cycloreversion reaction.9 For
example, the closed-ring isomer having 2,2-dicyanovinyl substi-
tuents at the end of diarylethene completely returned to the open-
ring isomer within ten minutes at 60 uC.9 If aryl groups of
diarylethenes change from electron-donating to electron-with-
drawing substituents by external stimulus, they have a possibility
to switch between P-type and T-type photochromism. An acid–
base-induced system as the external stimulus can easily exchange
donor and acceptor of the substituents. The effect of the addition
of an acid to a diarylethene having diethylamino groups has been
reported so far.10,11 However, the thermal stability has not been
reported in the literature. We synthesized novel diethylamino-
substituted diarylethenes (1a and 2a),{ and found that the
protonated closed-ring isomers underwent thermal cycloreversion
reaction even at room temperature. In this communication, we
focus on photochromic system switching between P-type and
T-type of 1a and 2a by the addition of an acid.
Department of Applied Chemistry, Graduate School of Engineering,
Osaka City University and JST-PRESTO, 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka, Japan. E-mail: kobatake@a-chem.eng.osaka-
cu.ac.jp; Fax: +81-6-6605-2797; Tel: +81-6-6605-2797
{ Electronic supplementary information (ESI) available: synthesis of 1a
and 2a, and photochromic properties of 1a, 2a, and the related
compounds. See DOI: 10.1039/b700177k
Fig. 1 Absorption spectra of 1a (—), 1b (-?-), 1a-H+ (– –), and 1b-H+
…
formation of 1a-H+ and 1b-H+.
(
) in acetonitrile. 3 eq. of trifluoromethanesulfonic acid was used for
1698 | Chem. Commun., 2007, 1698–1700
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upon irradiation with 313 nm light. When trifluoromethanesulfo-
nic acid (TFMSA) was added to 1b, the absorption spectrum
immediately showed hypochromic change to give 1b-H+, as shown
in Fig. 1. The e value decreased to 81% of 1b. The absorption
coefficient of 1b-H+ is almost the same as that of the unsubstituted
compound,
1,2-bis(2-methyl-5-phenylthien-3-yl)cyclopentene.12
When the diethylamino groups are protonated, the electron-
donating property of the diethylamino group is lost. The
absorption intensity was also recovered by the addition of
triethylamine. The protonation and deprotonation can be
reversibly carried out by the addition of an acid and a base,
respectively.
The acetonitrile solution of 2a also showed similar photo-
chromic behavior (see ESI{). The photocyclization conversion
from 2a to 2b was 66% in acetonitrile upon irradiation with 313 nm
light. As summarized in Table 1, the absorption maximum of 2b
was shifted to short wavelength as long as 40 nm by protonation.
When the solution of the protonated compound 1b-H+ was kept
in the dark at room temperature, the color of the solution was
gradually bleached with a slow rate. Fig. 2 shows color changes in
the absence and presence of 10 eq. of TFMSA in acetonitrile by
UV irradiation and heating at 80 uC. Upon irradiation with UV
light the color of the solutions turned to violet (1b and 1b-H+),
green (2b), or blue (2b-H+). After heating, the solutions in the
presence of TFMSA returned to colorless, whereas non-
protonated compounds were thermally stable. When the bleached
solution was irradiated with UV light again, the solution turned to
violet (1b-H+) or blue (2b-H+). These results indicate that 1b-H+
and 2b-H+ are thermally unstable and return to the colorless
solutions. The thermal bleaching of 2b-H+ proceeded at a much
slower rate than that of 1b-H+. The difference between
diarylcyclopentene and diarylperfluorocyclopentene must be
directly related to the different electron densities at the central
single bond of the closed-ring forms because the perfluorocyclo-
pentene ring has an electron-withdrawing character. This indicates
that there is a significant intramolecular electronic interaction
between the phenylthienyl moiety and perfluorocyclopentene ring
in the closed-ring isomers.13
Fig. 2 Color changes of diarylethenes 1a (top) and 2a (bottom) in the
absence (left) and presence (right) of 10 eq. of TFMSA in acetonitrile. (a)
and (d): before photoirradiation. (b) and (e): after UV irradiation. (c): after
heating for 30 min at 80 uC, (f) after heating for 5 h at 80 uC.
1b), 6.7 (2a), and 5.0 (2b) (see ESI{). This means that the pKa of
the protonated species of 2a/2b in acetonitrile can change by
photoisomerization. Some pKa-switching systems of diarylethenes
in water/methanol have been reported.14,15 The pKa value of a
phenol group by the switching of a p-conjugated chain led to a
large change (a 16-fold change in pKa). In our systems, the
electron-withdrawing perfluorocyclopentene ring plays an impor-
tant role in decreasing the p-conjugated electron density of the
diethylaminophenyl group in the closed-ring isomer, whereas in
the open-ring isomer it does not. As a result, diarylcyclopentene
did not change the pKa between the open- and closed-ring isomers,
whereas diarylperfluorocyclopentene led to a 50-fold change in
pKa between the open- and closed-ring isomers in acetonitrile.
The thermal bleaching reactions of 1b-H+ and 2b-H+ in the
degassed acetonitrile solution were followed by the decrease of the
absorbance at 534 and 590 nm for 1b-H+ and 2b-H+, respectively,
at different temperatures. The protonated compounds were
prepared by the addition of 10 eq. of TFMSA. The thermal
bleaching reaction proceeded to follow a first-order kinetics. The
rate constants of the thermal cycloreversion reactions were
determined from the slope of the first-order plots. Fig. 3 shows
The effect of the protonation of 1a and 2a on the addition of
TFMSA in acetonitrile was evaluated from changes in the
absorption spectra. The absorption spectra changed on the
addition of TFMSA, but no further changes were observed when
more than 3 eq. of the acid were added to 1a, 1b, and 2a. On the
other hand, for 2b changes in the absorption spectrum continued
to be observed even with more than 3 eq. of acid added until
finally the addition of a large amount of acid (.10 eq.) to the
solution caused no further spectral change. The protonated species
are in the fully protonated state in the presence of 10 eq. of
TFMSA. From the titration by TFMSA, pKa values of these
protonated species in acetonitrile were determined to be 6.4 (1a,
Table 1 Absorption maxima and coefficients of the diarylethenes in
the absence and presence of TFMSA in acetonitrile
l
max/nm e/M21 cm21
l
max/nm e/M21 cm21
1a
1a-H+ 280
2a 335
2a-H+ 293
329
26600
29800
32500
37400
1b
1b-H+ 534
2b 630
2b-H+ 590
540
22400
16700
28300
15800
Fig. 3 Temperature dependence of rate constants of thermal cyclorever-
sion reaction of 1b (#), 1b-H+ ($), 2b (%), and 2b-H+ (&) in acetonitrile.
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such as 160 uC. The protonation and deprotonation of the open-
Table 2 Activation energy and frequency factor of thermal cyclo-
reversion reaction of the closed-ring isomers
and closed-ring isomers were reversible. Such photochromic
materials could be useful for application as reversible switching
materials in write-by-light/erase-by-heat systems.
Ea/kJ mol21 A/s21
t1/2 at 160 uC/s t1/2 at 25 uC/days
1b
1b-H+ 96
2b 109
2b-H+ 104
121
1.3 6 1013 21
980
This work was partly supported by PRESTO, JST and Grants-
in-Aid for Scientific Research (No. 16685014) from MEXT, Japan.
5.5 6 1011
0.48
0.96
2900
64
3.5 6 1010 280
2.1 6 1011 11
Notes and references
{ 1a: 1H-NMR (CDCl3, 400 MHz, TMS): d = 1.16 (t, J = 7.2 Hz, 6H), 1.95
(s, 3H), 1.98 (s, 3H), 2.05 (quintet, J = 7.6 Hz, 2H), 2.83 (t, J = 7.6 Hz, 4H),
3.35 (q, J = 7.2 Hz, 4H), 6.62 (d, J = 8.8 Hz, 2H), 6.85 (s, 1H), 7.05 (s, 1H),
7.2–7.6 (m, 7H); FAB-MS m/z = 483.2058 (M+) (calc. 483.2054).
a temperature dependence of the rate constants for the thermal
cycloreversion reaction. The slope corresponds to the activation
energy of the reaction. The kinetics parameters determined are
summarized in Table 2. The thermal cycloreversion reactions of 1b
and 2b were very slow. The thermal half-life times of the non-
protonated closed-ring isomers were 2.7 and 7.9 years for 1b and
2b, respectively, at 25 uC. The colored states are thermally stable in
the absence of acid. They return at a slow rate to the open-ring
isomers at a high temperature such as 160 uC. On the other hand,
the enhancement of the thermal cycloreversion reaction by
protonation was observed. The thermal half-life times of 1b-H+
and 2b-H+ were 35 s and 5.5 min at 100 uC and 0.48 s and 11 s at
160 uC, respectively. These results indicate that 1b is stable for a
few years at room temperature, and the protonation led to the very
rapid thermal cycloreversion reaction at 160 uC. 1b-H+ was more
effective than 2b-H+ because of the smaller activation energy. As a
result, the non-protonated diarylethenes underwent P-type photo-
chromic reactions and protonated diarylethenes underwent T-type
photochromic reactions.
1
2a: H-NMR (CDCl3, 400 MHz, TMS): d = 1.18 (t, J = 7.2 Hz, 6H),
1.92 (s, 3H), 1.95 (s, 3H), 3.37 (q, J = 7.2 Hz, 4H), 6.65 (d, J = 8.8 Hz, 2H),
7.08 (s, 1H), 7.29–7.40 (m, 6H), 7.54 (d, J = 7.2 Hz, 2H); FAB-MS m/z =
591.1483 (M+) (calc. 591.1489).
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In conclusion, we have demonstrated that diethylamino-
substituted diarylethenes can switch photochromic systems
between P-type and T-type photochromism. Non-protonated
diarylethenes exhibit a thermally stable photochromic reaction,
and the diarylethenes can change to a T-type system by the
addition of TFMSA. Therefore, the protonated colored isomers
return to open-ring isomers at a fast rate at elevated temperature
1700 | Chem. Commun., 2007, 1698–1700
This journal is ß The Royal Society of Chemistry 2007