Systematic study on the thermal cycloreversion reactivity of diarylethenes with alkoxy and alkyl groups at the reactive carbons

Article abstract of DOI:10.1007/s11164-012-0648-0

The relationship between the thermal cycloreversion reactivity of diarylethenes and the bulkiness of the substituents at the reactive carbons was systematically investigated. Two photochromic diarylethenes, 1,2-bis(2-isobutoxy-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-neopentoxy-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were newly synthesized and their optical properties and thermal cycloreversion reactivity were examined, because there is insufficient data for diarylethenes with alkoxy groups at the reactive carbons. The steric substituent constant was employed to correlate the relationship between the thermal cycloreversion reactivity of diarylethenes with alkyl and alkoxy groups at the reactive carbons and the bulkiness of the substituent. A good correlation was obtained for the substituent constant using CH₂ instead of oxygen in the alkoxy groups. The results indicate that this is a very useful strategy for the design of novel diarylethenes with desired thermal cycloreversion reactivity.

Full text of DOI:10.1007/s11164-012-0648-0

Res Chem Intermed (2013) 39:279–289
DOI 10.1007/s11164-012-0648-0
Systematic study on the thermal cycloreversion
reactivity of diarylethenes with alkoxy and alkyl
groups at the reactive carbons
•
Hiroaki Shoji Daichi Kitagawa Seiya Kobatake
•
Received: 13 October 2011 / Accepted: 7 November 2011 / Published online: 19 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract The relationship between the thermal cycloreversion reactivity of
diarylethenes and the bulkiness of the substituents at the reactive carbons was
systematically investigated. Two photochromic diarylethenes, 1,2-bis(2-isobutoxy-
5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-neopentoxy-5-phenyl-
3-thienyl)perfluorocyclopentene (2a), were newly synthesized and their optical
properties and thermal cycloreversion reactivity were examined, because there is
insufficient data for diarylethenes with alkoxy groups at the reactive carbons. The
steric substituent constant was employed to correlate the relationship between the
thermal cycloreversion reactivity of diarylethenes with alkyl and alkoxy groups at
the reactive carbons and the bulkiness of the substituent. A good correlation was
obtained for the substituent constant using CH2 instead of oxygen in the alkoxy
groups. The results indicate that this is a very useful strategy for the design of novel
diarylethenes with desired thermal cycloreversion reactivity.
Keywords Photochemistry ꢀ Photochromism ꢀ Diarylethene ꢀ Thermal bleaching
reaction
Introduction
Photochromism is the phenomenon of chromatic change due to the reversible
photoisomerization between two isomers having different absorption spectra [1, 2].
Compounds that have such a property are called photochromic compounds and can
reversibly change not only their molecular structures but also their refractive indices,
dielectric constants, oxidation–reduction potentials, and geometrical structures.
H. Shoji ꢀ D. Kitagawa ꢀ S. Kobatake (&)
Department of Applied Chemistry, Graduate School of Engineering, Osaka City University,
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
e-mail: kobatake@a-chem.eng.osaka-cu.ac.jp
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While various types of photochromic compounds have been reported to date,
they are classified into T- and P-types. Azobenzene, spiropyran, and spirooxazine
are well known as representative T-type photochromic compounds. Their photo-
generated isomers are thermally unstable and return to the original isomer at room
temperature in the dark. Therefore, they can be applied in photomodulated materials
such as photochromic ophthalmic lenses [3]. In contrast, both isomers in P-type
photochromic compounds such as furyl fulgide and diarylethene are thermally
stable at room temperature and can revert to the original isomer only by
photoirradiation. Therefore, they have potential for application to various
optoelectronic devices such as optical memory, photooptical switching, displays,
nonlinear optics, and photoresponsive actuators [4–9].
Although most of the diarylethene derivatives have thermal stability and fatigue
resistance, their properties are changed slightly according to the substituent [4].
Among these derivatives, it is quite important to control the thermal stability of the
colored closed-ring isomers by introduction of a substituent. The thermal stability of
the closed-ring isomer can be determined by the following three factors: (1)
aromatic stabilization energy of the aryl groups [10], (2) electron-withdrawing
substituents at the aryl groups [11, 12], and (3) the steric hindrance of the
substituents at the reactive carbons [13–15]. The theoretically calculated bond
length between the central carbons is related to the thermal cycloreversion reactivity
[16]. We have recently reported a correlation between the thermal cycloreversion
reactivity of the closed-ring isomers and steric substituent constants in relation to (3)
[17]. The values of three steric substituent constants, Es(R1) [18], Esc(R1) [19], and
v(R1) [20] were employed and a good correlation was obtained between the thermal
cycloreversion reactivity and the Esc(R1) value, while Es(R1) had no correlation with
the reactivity. Furthermore, the Es(CH2R1) value also has good correlation with the
reactivity. These can be applied only for alkyl groups (Scheme 1). It is difficult to
predict the thermal cycloreversion reactivity for substituent groups other than alkyl
groups, due to the absence of the substituent constants.
In the present work, two new diarylethenes (1a and 2a), which introduce
isobutoxy and neopentoxy groups at the reactive carbons, respectively, were
synthesized and their optical properties and thermal cycloreversion reactivity were
examined (Scheme 2). The correlation between the thermal cycloreversion reactiv-
ity and steric hindrance of the substituents was systematically investigated for
diarylethenes having alkyl and alkoxy groups at the reactive carbons.
F
F
F
F
F
F
F
F
UV
F
F
F
F
R1
R1
R1
R1
Vis.
S
S
S
S
Δ
Scheme 1 Photochromism of diarylethenes with alkyl groups
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Systematic study on the thermal cycloreversion reactivity
281
F
F
F
F
F
F
F
F
UV
F
F
F
F
OR2
R2O
OR2
R2O
Vis.
S
S
S
2
S
Δ
2
1a
1b
: R = CH2CH(CH3)2
2b: R2 = CH2C(CH3)3
: R = CH2CH(CH3)2
2a: R2 = CH2C(CH3)3
Scheme 2 Photochromism of diarylethenes with alkoxy groups
Experimental
General
1
Spectroscopic grade solvents were used and purified by distillation before use. H
NMR spectra were recorded at 300 MHz using a BRUKER AV-300 N
spectrometer. Tetramethylsilane (TMS) was used as an internal standard. Mass
spectra were obtained using a JEOL JMS-700/700S mass spectrometer. Absorp-
tion spectra were obtained using a JASCO V-560 absorption spectrophotometer.
High-performance liquid chromatography (HPLC) was carried out using a Hitachi
L-7150/L-2400 HPLC system equipped with a KANTO CHEMICAL Mightysil Si
60 column.
Materials
1,2-Bis(2-isobutoxy-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-
neopentoxy-5-phenyl-3-thienyl)perfluorocyclopentene (2a) were synthesized accord-
ing to methods similar to those in the literature [21]. The products were purified by
HPLC and recrystallization.
1a: mp. 145–146 °C; 1H NMR (300 MHz, CDCl3, TMS) d = 0.81 (d, J = 6.8 Hz,
12H, CH3), 1.75 (m, 2H, CH), 3.68 (d, J = 6.4 Hz, 4H, CH2), 7.15 (s, 2H, Aromatic),
7.15–7.48 (m, 10H, Aromatic). MS (FAB) m/z = 636.1592 (M?). Calcd for
C33H30F6O2S2 = 636.1591.
1
2a: mp. 175–176 °C; H NMR (300 MHz, CDCl3, TMS) d = 0.88 (s, 18H,
CH3), 3.65 (s, 4H, CH2), 7.07 (s, 2H, Aromatic), 7.21–7.44 (m, 10H, Aromatic). MS
(FAB) m/z = 664.1906 (M?). Calcd for C35H34F6O2S2 = 664.1904.
Photochromism
Photocyclization and cycloreversion quantum yields were determined in hexane as
relative quantum yields using diarylethene derivatives whose quantum yields have
already been determined [22]. The photocyclization and photocycloreversion
reactions were followed by absorption spectra. The samples were not degassed.
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Thermal cycloreversion reaction
The thermal cycloreversion reaction of the diarylethene closed-ring isomer was
carried out in toluene as follows. The diarylethene open-ring isomer was put in an
optical quartz cell degassed and sealed under vacuum. The solution in the cell was
irradiated with 313 nm light to give the closed-ring isomer. The photoirradiation
was carried out using a 200-W mercury-xenon lamp (MORITEX MUV-202) as the
light source. Monochromic light was obtained by passing the light through a
monochromator (Jobin–Yvon H10 UV). The cell was placed in a constant
temperature chamber (ESPEC ST-110) during the thermal cycloreversion reaction.
The reaction yields were periodically determined by absorption spectroscopic
measurement.
Results and discussion
Photochromism
Figure 1a shows the absorption spectral change of diarylethene 1a in hexane upon
irradiation with 313-nm light. The colorless solution of 1a has an absorption
maximum wavelength (kmax) at 311 nm. Upon irradiation with ultraviolet (UV)
light, the solution became blue with kmax at 627 nm. The blue color is due to the
closed-ring isomer 1b, and is thermally stable at room temperature. The blue-
colored solution gradually reverted to the initial colorless solution upon irradiation
with visible light (k [500 nm), which indicates that 1a exhibits photochromism in
hexane. Diarylethene 2a also exhibited a similar photochromic reaction in hexane
(Fig. 1b).
Table 1 shows the optical properties of diarylethenes with alkoxy groups at the
reactive carbons. The cyclization quantum yields of 1a and 2a were 0.41 and 0.22,
respectively; that for 1a was almost the same as those of other diarylethenes [21],
whereas that for 2a was slightly smaller than those of other diarylethenes. The steric
hindrance of the substituent on the b-carbon of the alkoxy group may affect the
photocyclization reaction. Diarylethene has two stable conformations in the open-ring
isomer, antiparallel and parallel, and the photocyclization reaction can proceed from
only the antiparallel conformation [23]. Therefore, the photocyclization quantum
yield depends on the content of each conformation. The content of the antiparallel
conformation in 2a may be lower than that in 1a due to the steric hindrance.
Both of the photocycloreversion quantum yields of 1b and 2b were estimated to
be 2.2 9 10-4. The cycloreversion quantum yields were lower than that of
diarylethene having methyl groups at the reactive carbons (R1=CH3) by a factor of
102. The decrease in the cycloreversion quantum yield is attributed to the activation
energy barrier for the cycloreversion process in the singlet excited state [24].
Diarylethenes with the alkoxy groups have a much higher energy barrier in the
excited state. When the bulkiness of the alkoxy groups increases, the cycloreversion
quantum yield is slightly increased, and this is considered to be ascribable to the
molecular strain of the closed-ring isomers by the bulky substituents.
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Systematic study on the thermal cycloreversion reactivity
283
Fig. 1 Absorption spectral change of 1 (1.30 9 10-5 M) (a) and 2 (1.25 9 10-5 M) (b) in hexane upon
irradiation with 313-nm light: Open-ring isomer (dashed line), the photostationary state under irradiation
with 313-nm light (dotted line), closed-ring isomer (solid line)
Thermal cycloreversion reaction
Diarylethenes having methyl groups at the reactive carbons exhibit thermally
irreversible photochromism, which means that diarylethenes have an excellent
advantage as P-type photochromic compounds [4]. However, diarylethenes with
bulky substituents introduced at the reactive carbons undergo the thermal
cycloreversion reaction [21].
The thermal cycloreversion reactivities of the closed-ring isomers 1b and 2b were
examined in toluene at 90–130 °C. Figure 2 shows that the decay curves for
absorbance at kmax in the closed-ring isomers at various temperatures; the decay
curves follow first-order kinetics. The rate constant (k) of the cycloreversion reaction
can be determined from the slope of the first-order kinetic plots. The half-lifetimes of
1b and 2b were estimated to be 33 and 23 h at 100 °C, respectively, and the
temperature dependence of k is plotted in Fig. 3. The slope and intercept of the linear
plots represent the activation energy (Ea) and frequency factor (A), respectively, which
were determined to be 130 and 128 kJ mol-1, and 7.7 9 1012 and 5.6 9 1012 s-1 for
1b and 2b, respectively. Table 2 shows the kinetic parameters for the thermal
cycloreversion reaction of diarylethenes with various alkoxy groups introduced at the
reactive carbons. The thermal cycloreversion reactivity was dependent on the
substituents and increased in the following order: OCH3 \ OCH2CH3 \ OCH2
CH(CH3)2 \ OCH2C(CH3)3 \ OCH(CH3)2 \ OC(CH3)3.
Correlation between thermal cycloreversion reactivity and steric substituent
constants
The Taft’s steric substituent constant [Es(R)] [18] was adopted to quantitatively
evaluate the size of the alkoxy groups at the reactive carbons. The constant
represents the rate of hydrolysis of RCOOCH2CH3, and in some cases the steric
effect can be interpreted using this constant. Hydrolysis is affected by hypercon-
jugation of the a-hydrogens of substituent R in the ester. The hyperconjugation
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Systematic study on the thermal cycloreversion reactivity
285
Fig. 2 Thermal bleaching curves of 1b (a) and 2b (b) in toluene
Fig. 3 Temperature dependence of the rate constants in thermal cycloreversion reaction of 1b (a) and 2b
(b) in toluene
Table 2 Rate constants, half-lifetimes, and Arrhenius parameters of thermal cycloreversion reactions of
diarylethenes having alkoxy groups
R2
k/s-1 at 100 °C
t
1/2/h at 100 °C Ea/kJ mol-1 A/s-1
kmax/nm Ref.
CH3
1.4 9 10-6
4.8 9 10-6
140
40
137
129
130
128
123
92
2.1 9 1012 625
5.5 9 1012 625
7.7 9 1012 627
5.6 9 1012 628
5.5 9 1012 635
5.7 9 1011 656
21
CH2CH3
CH2CH(CH3)2 5.8 9 10-6
21
33
This work
This work
21
CH2C(CH3)3
CH(CH3)2
C(CH3)3
8.4 9 10-6
3.3 9 10-5
8.2 9 10-2
23
5.8
0.0024
25
effect can be removed according to a method by Hancock et al. [19], as defined by
Ecs(R) in the following equation:
EscðRÞ ¼ EsðRÞ þ 0:306ðN ꢁ 3Þ
ð1Þ
where N is the number of hydrogen atoms of the a-carbon of substituent R. We
recently evaluated the thermal cycloreversion reactivity of diarylethenes with alkyl
groups (R1) at the reactive carbons using Es(R1) and Ecs(R1) [17]. Consequently, the
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H. Shoji et al.
steric effect of the thermal cycloreversion reaction could not be exactly evaluated
using the Es(R1) value, because it involves factors of both steric and hyperconju-
gation effects. Therefore, in order to except the hyperconjugation effect, the
Es(CH2R1) and Ecs(R1) values were applied to give a good correlation; however,
most of the steric substituent constants for alkoxy groups were absent.
Here, we applied the steric substituent constants for R2 in the alkoxy group (OR2)
to correlate the relationship between the steric substituent constant and the thermal
cycloreversion reactivity. For example, the steric substituent constant of methyl
group is employed when the substituent is a methoxy group. Table 3 shows the
steric substituent constants and Fig. 4 indicates the relationship between the steric
substituent constants and logarithm k at 100 °C or Ea in the thermal cycloreversion
reaction. When the Es(R2) value was used as the substituent constant, the Pearson
correlation coefficient (r factor) was estimated to be 0.557 and 0.571 for log(k) and
Ea, respectively. This indicates that Es(R2) was not correlated with the thermal
cycloreversion reactivity. However, when the Ecs(R2) value was used, the r factor
was increased to 0.745 and 0.760 for log(k) and Ea, respectively. Much better
correlation was given by the relationship between Es(CH2R2) and the thermal
cycloreversion reactivity.
Universal correlation for diarylethenes with alkyl and alkoxy groups
To generalize the correlation between the steric effect and thermal cycloreversion
reactivity of diarylethenes with alkyl and alkoxy groups, we discuss a novel method
for evaluation of diarylethenes with bulky substituents at the reactive carbons.
Ecs(CH2R2) was applied as the steric substituent constant for the alkoxy group (OR2),
due to the absence of Ecs(OR2), where CH2R2 corresponds to the substituent with
CH2 instead of oxygen in substituent OR2. For example, when the substituent is a
methoxy group, the steric substituent constant Ecs is applied for ethyl groups. In a
similar way, this can be evaluated as alkyl groups instead of alkoxy groups for
diarylethenes having alkyl and alkoxy substituents.
Figure 5 shows that there is a very good correlation between the thermal
cycloreversion reactivity and the steric substituent constant Ecs(CH2R2); the plots
for alkyl (R1) and alkoxy (OR2) groups are fitted in the same straight line. These
results show that oxygen atoms in alkoxy groups function as spacer without an
Table 3 Steric substituent constants, Es(R), Ecs(R) and Es(CH2R)
R
Es(R)
Ecs(R)
Es(CH2R)
CH3
0
0
-0.07
-0.36
-0.93
-0.51
-0.34
-1.74
CH2CH3
-0.07
-0.47
-0.93
-1.74
-1.54
-0.38
-1.08
-1.24
-2.05
-2.46
CH(CH3)2
CH2CH(CH3)2
CH2C(CH3)3
C(CH3)3
The Es(R) and Es(CH2R) values were cited from ref. [26]. The Ecs(R) values were calculated according
to Eq. (1)
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Systematic study on the thermal cycloreversion reactivity
287
Fig. 4 Relationship between substituent constants (Es(R2) (a, b), Esc(R2) (c, d), Es(CH2R2) (e, f) and
log(k) (a, c, e) or activation energy (Ea) (b, d, f)
electron-withdrawing inductive influence and an electron-donor influence with
conjugation.
Conclusion
Diarylethenes 1a and 2a, with isobutoxy and neopentoxy substituents introduced at
the reactive carbons, were synthesized and evaluated with respect to thermal
cycloreversion reactivity. These compounds exhibited thermal cycloreversion
reactions similar to diarylethenes with bulky substituents introduced at the reactive
carbons. The half-lifetimes of 1a and 2a at 100 °C were determined to be 33 and
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H. Shoji et al.
Fig. 5 Relationship between Ecs(R1) (open circle) or Esc(CH2R2) (filled circle) and log(k) (a) and
activation energy (Ea) (b)
23 h, respectively. The thermal cycloreversion reactivities of diarylethenes with
alkoxy substituents at the reactive carbons were quantitatively evaluated using the
Taft steric substituent constants, Es(R2), Esc(R2), and Es(CH2R2). There was a
good correlation between the thermal cycloreversion reactivity and Es(CH2R2).
The Ecs(CH2R2) steric substituent constant was used for diarylethenes with alkoxy
groups to generalize the thermal cycloreversion reactivity. There was a good
correlation between the thermal cycloreversion reactivity and Ecs(CH2R2) or Esc(R1).
The oxygen atoms in alkoxy groups were found to function like a spacer without
influences due to the electron-withdrawing inductive effect and the electron-donor
effect in conjugation.
Acknowledgments This work was partly supported by the Adaptable and Seamless Technology
transfer Program (A-STEP) through target-driven R&D, FS-Stage (Exploratory Research) from the Japan
Science and Technology Agency (JST). The authors also thank Nippon Zeon Co. Ltd. for providing
octafluorocyclopentene.
References
1. G.H. Brown, Photochromism (Wiley-Interscience, New York, 1971)
2. H. Du¨rr, H. Bouas-Laurent, Photochromism: Molecules and Systems (Elsevier, Amsterdam, 2003)
3. J.C. Crano, T. Flood, D. Knowles, A. Kumar, B.V. Gemert, Pure Appl. Chem. 68, 1395–1398 (1996)
4. M. Irie, Chem. Rev. 100, 1685–1716 (2000)
5. M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, Nature 420, 759–760 (2002)
6. B.L. Feringa, Molecular Switches (Wiley-VCH, Weinheim, 2001)
7. C. Bechinger, S. Ferrer, A. Zaban, J. Sprague, B.A. Gregg, Nature 383, 608–610 (1996)
8. J.A. Delaire, K. Nakatani, Chem. Rev. 100, 1817–1846 (2000)
9. S. Kobatake, S. Takami, H. Muto, T. Ishikawa, M. Irie, Nature 446, 778–781 (2007)
10. S. Nakamura, M. Irie, J. Org. Chem. 53, 6136–6138 (1988)
11. S.L. Gilat, S.H. Kawai, J.M. Lehn, Chem. Eur. J. 1, 275–284 (1995)
12. S. Nakamura, S. Yokojima, K. Uchida, T. Tsujioka, A. Goldberg, A. Murakami, K. Shinoda, M.
Mikami, T. Kobayashi, S. Kobatake, K. Matsuda, M. Irie, J. Photochem. Photobiol. A 200, 10–18
(2008)
13. M. Irie, T. Lifka, S. Kobatake, N. Kato, J. Am. Chem. Soc. 122, 4871–4876 (2000)
14. S. Kobatake, K. Shibata, K. Uchida, M. Irie, J. Am. Chem. Soc. 122, 12135–12141 (2000)
15. S. Kobatake, K. Uchida, E. Tsuchida, M. Irie, Chem. Lett. 11, 1340–1341 (2000)
123

Systematic study on the thermal cycloreversion reactivity
289
16. D. Chen, Z. Wang, H. Zhang, J. Mol. Struct. 859, 11–17 (2008)
17. D. Kitagawa, K. Sasaki, S. Kobatake, Bull. Chem. Soc. Jpn. 84, 141–147 (2011)
18. R.W. Taft Jr, J. Am. Chem. Soc. 74, 3120–3128 (1952)
19. C.K. Hancock, E.A. Meyers, B.J. Yager, J. Am. Chem. Soc. 83, 4211–4213 (1961)
20. M. Charton, J. Am. Chem. Soc. 97, 1552–1556 (1975)
21. K. Morimitsu, K. Shibata, S. Kobatake, M. Irie, J. Org. Chem. 67, 4574–4578 (2002)
22. Y. Yokoyama, Y. Kurita, J. Synth. Org. Chem. Jpn. 49, 364–372 (1991)
23. M. Irie, O. Miyatake, K. Uchida, J. Am. Chem. Soc. 114, 8715–8716 (1992)
24. D. Guillaumont, T. Kobayashi, K. Kanda, H. Miyasaka, K. Uchida, S. Kobatake, K. Shibata, S.
Nakamura, M. Irie, J. Phys. Chem. A 106, 7222–7227 (2002)
25. K. Morimitsu, K. Shibata, S. Kobatake, M. Irie, Chem. Lett. 31, 572–573 (2002)
26. C. Hansch, A. Leo, Substituent Constants for Correlation Analysis in Chemistry and Biology (Wiley,
New York, 1979)
123