pKa switching induced by the change in the π-conjugated system based on photochromism

Article abstract of DOI:10.1002/chem.200501292

Two diarylethene derivatives 1a and 2a containing a 2, 5-diaryl-3-thienyl group have been designed and synthesized. The pK^a values of these compounds change upon photoirradiation. They have a phenol group as a proton source and a pyridinium gro

Full text of DOI:10.1002/chem.200501292

FULL PAPER
DOI: 10.1002/chem.200501292
pKa Switching Induced by the Change in the p-Conjugated System Based on
Photochromism
Yuka Odo,[a] Kenji Matsuda,[a, b] and Masahiro Irie*[a]
Abstract: Two diarylethene derivatives 1a and 2a containing a 2,5-diaryl-3-thienyl
group have been designed and synthesized. The pKa values of these compounds
change upon photoirradiation. They have a phenol group as a proton source and a
pyridinium group as an acceptor unit at each end of the p-conjugated chain. The
cyclization/cycloreversion reactions can be used to control the length of the p-con-
jugated chain between the proton source and the acceptor. The change in the p-
conjugated chain length caused the pKa-switching.
Keywords: conjugation
ethenes molecular devices
photochromism · pKa switches
·
diaryl-
A
·
·
Introduction
was accelerated in the closed-ring isomer by the enhanced
acceptor effect of the pyridinium ion.
Various molecular switching systems have been extensively
explored to apply them to molecular devices.[1] Among
them, photoswitching systems are advantageous from the
viewpoint of fast response and high sensitivity. Most photo-
switching systems are composed of a photochromic unit and
a functional group. Photochemical as well as photophysical
property changes of the photochromic unit control the per-
formance of the functional group. Diarylethene derivatives
are widely used as the photochromic unit to control photo-
switching systems.[2] Changes in the p-conjugated chain
length of diarylethene derivatives upon photoirradiation can
be successfully used to control electronic conduction,
donor–acceptor interactions, and magnetic interactions.[3–6]
Lehn and co-workers reported pKa-switching systems that
use a diarylethene with a phenol group as a proton source
and a pyridinium group as an acceptor at each end of the di-
arylethene p-conjugated chain.[7] The proton dissociation
The switching of diarylethenes so far reported has been
based on the fact that p conjugation between two aryl
groups is disconnected in the open-ring isomer and connect-
ed in the closed-ring isomer. The switching mode can be re-
versed by placing two interaction units in the same aryl unit
(Scheme 1). The interconversion of the orbital hybridization
of the reactive carbon from sp2 to sp3 can be used to control
the p-conjugated chain length.
Scheme 1. Photoswitching by placing two interaction units (A and B) in
the same aryl unit.
[a] Y. Odo, Dr. K. Matsuda, Prof. Dr. M. Irie
Department of Chemistry and Biochemistry
Graduate School of Engineering
Kyushu University
In this work, we report on pKa-switching systems based
on the interconversion of the orbital hybridization.
Moto-oka 744, Nishi-ku, Fukuoka 819-0395 (Japan)
Fax : (+81)92-642-3568
E-mail: irie@cstf.kyushu-u.ac.jp
Results and Discussion
[b] Dr. K. Matsuda
Design and synthesis: To control the pKa, diarylethene de-
rivatives 1a and 2a were designed (Scheme 2). These com-
pounds have a phenol group as a proton source and a pyridi-
nium group as an acceptor unit.[8] In compound 1, both the
PRESTO, Japan Science and Technology Agency (JST)
Kawaguchi 332-0012 (Japan)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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4283

radiation with l=365 and
>600 nm light (see the Sup-
porting Information). Upon ir-
radiation with UV light, a solu-
tion of 3 in methanol changed
from colorless to blue and a
new absorption maximum was
observed at l=600 nm. On the
other hand, the solution of N-
methylated
compound
1
changed from light yellow to
green. The absorption maxi-
mum of the photogenerated
isomer showed a redshift to l=
662 nm.
Upon addition of base, the
absorption spectra of 1a and
the photostationary-state solu-
tion changed as shown in
Figure 1. The absorption band
of 1a at l=380 nm decreased
and a new band appeared at
l=465 nm. The absorption
maximum showed a redshift of
as much as l=80 nm, which
suggests the presence of
a
strong intramolecular donor–ac-
ceptor interaction between the
phenoxide
and
pyridinium
units.[13,14] In the photostation-
ary state, the bands at l=380
and 660 nm decreased and the
band at l=470 nm increased.
Scheme 2. Design of pKa switching systems.
pyridinium and phenol units are located on the same thio-
phene ring. The donor and acceptor groups of the open-ring
isomer 1a can interact through the p-conjugated chain, how-
ever, the closed-ring isomer 1b does not have this ability be-
cause the p-conjugated system between the pyridinium and
phenol units has been disconnected by the formation of
a sp3-hybridized carbon atom at the 2-position of the
thiophene ring.[9,10] In compound 2, the phenol unit inter-
acts with the electron-donating methoxy group in the open-
ring isomer, however, in the closed-ring isomer the electron-
accepting pyridinium group interacts with the phenol group.
This switch from the methoxyphenyl to the pyridinium
group is anticipated to affect the pKa of the phenol
group.
The band at l=660 nm was slightly redshifted.
Compounds 2 and 4: Upon irradiation with UV light, the
solution of 4 in methanol changed from colorless to blue
and a new absorption maximum was observed at l=600 nm.
On the other hand, the solution of N-methylated compound
2 changed from light yellow to green. As a result of the N-
methylation, the absorption maximum of the photoirradiat-
ed solution shifted to a longer wavelength by as much as l=
66 nm (see the Supporting Information).
Although diarylethene 4 underwent reversible, photochro-
mic reactions in methanol, the closed-ring isomer 2b decom-
posed on irradiation with l=365 nm UV light even under
neutral conditions. The decomposition was strongly sup-
pressed by the addition of acid. It is inferred from this acid
effect that the deprotonated closed-ring isomer 2b(OÀ) is
not stable on being irradiated with UV light. A similar phe-
nomenon has also been reported for a diarylethene with
phenol groups.[5]
Diarylethenes 1 and 2 were synthesized according to the
routes shown in Scheme 3. We were unable to synthesize 3
from route B, therefore, we employed route A. The depro-
tection of the methoxymethyl (MOM) group and the N-al-
kylation were carried out with CF3SO3CH3.[11,1]
Upon addition of base, the absorption spectra of 2a and
the photostationary-state solution changed as shown in
Figure 2. The absorption band of 2a at l=300 nm de-
creased. In the photostationary state, the bands at l=300
and 666 nm decreased and the band at l=732 nm increased.
Photochromic reactions
Compounds 1 and 3: Diarylethenes 1 and 3 underwent re-
versible photochromic reactions by means of alternative ir-
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pKa Switching
FULL PAPER
Scheme 3. Synthetic routes to compounds 1 (above) and 2 (below). Reagents and conditions: a) N-bromosuccinimide (NBS) and THF; b) nBuLi,
(OBu)3, and then [Pd(PPh3)4], 4-bromopyridine hydrochloride, 20 wt% Na2CO3 (aq), and THF; c) nBuLi, and then octafluorocyclopentene and THF;
B
A
ACHTREUNG
d) nBuLi and THF; e) dichloromethane.
On the addition of base, the absorption maximum at l=
666 nm showed a redshift of as much as l=66 nm.
To reveal the difference in behavior between 1 and 2, the-
oretical calculations were carried out by using the B3LYP/6-
31G(d)[15] method in Gaussian 03.[16] The results are summar-
ized in Table 1 and Figure 5. E(OH) and E(OÀ) are the total
energies of the OH and OÀ forms, respectively, and DE-
Photochemical switching of the pKa: pKa values of the
open- and closed-ring isomers were determined spectroscop-
ically for compounds 1 and 2 in a mixed solvent of metha-
nol/water (5:2). The colored isomers were very stable. Ther-
mal cycloreversion was not observed during the acid–base ti-
trations at room temperature. Figures 3 and 4 illustrate the
acid–base titration curves for 1 and 2, respectively. The
curves were obtained by measuring the absorption changes
at l=475 nm for 1a and at l=350 nm for 2a. The absorp-
tion changes at l=660 and 770 nm were followed for 1b
and 2b, respectively. The pKa value of 1a was determined to
be 9.8, whereas it increased to 10.2 for 1b. The pKa value of
1 increased upon photoirradiation. The difference in the pKa
values between 1a and 1b is very small. To increase the
change in pKa upon photoirradiation, we designed com-
pound 2 and measured the photoirradiation effect. The
switch from the donor methoxyphenyl group to the acceptor
pyridinium group is anticipated to strongly perturb the
proton dissociation of phenol in 2. The pKa values of 2a and
2b were determined to be 10.2 and 9.0, respectively. In com-
pound 2, a reversible pKa change upon irradiation with UV
and visible light was observed.
A
DE(prot) is small, a proton readily dissociates even at low
E
pH.[17] This result indicates that the pKa of 1a is smaller
than that of 1b. On the other hand, the pKa of 2b is much
smaller than that of 2a. Moreover, the difference in DE-
A
larger than that for 1. The calculated results agree well with
the experimental results.
Although both 1 and 2 showed pKa changes upon photo-
A
for 1 was much smaller than expected. The pKa of 1a(OH)
was larger than that of 2b(OH). To reveal the reason for
this, theoretical calculations of the most stable conforma-
tions of 1a(OÀ) and 2b(OÀ) were carried out. Figure 5
shows the conformations of 1a(OÀ) and 2b(OÀ). The molec-
ular planarity of the p-conjugated chains, that is, pyridini-
um–thiophene–phenoxide rings, is of interest here. The pla-
narity is strongly perturbed in the case of 1a(OÀ), whereas
in 2b(OÀ), the pyridinium and phenoxide groups are almost
coplanar. The phenoxide group rotates due to the steric hin-
Chem. Eur. J. 2006, 12, 4283 – 4288
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4285

M. Irie et al.
Figure 1. a) Absorption spectral change of 1a in
a mixed solvent
(CH3OH/water 5:2) following the addition of aqueous KOH. b) Absorp-
tion spectral change of the photostationary-state solution under irradia-
tion with l=365 nm light in a mixed solvent (CH3OH/water 5:2) follow-
ing the addition of aqueous KOH.
Figure 2. a) Absorption spectral change of 2a in
a mixed solvent
(CH3OH/water 5:2) following the addition of aqueous HCl and aqueous
KOH. b) Absorption spectral change of the photostationary-state solu-
tion under irradiation with l=365 nm light in mixed solvent (CH3OH/
water 5:2) following the addition of aqueous KOH.
drance between the phenylthiophene group and the phenox-
ide group. This is the reason why 1a cannot have a low pKa.
On the other hand, the planar conformation is attained in
form 2b(OÀ), which results in the formation of quinoid
structure 2b(O). The steric hindrance prevents a large pKa
change in compound 1.
The photochromic reaction was also affected by the addi-
tion of base. Under high-pH conditions, that is, pH values
greater than the pKa of 1a(OH), the photocyclization reac-
tion of 1a(OÀ) was strongly prohibited. This is ascribed to
the formation of the quinoid-type resonance structure
1a(O).[18]
Figure 3. a) Titration curve of 1a. The absorption change at l=475 nm
was monitored in the pH range from 12.94 to 1.88 in a mixed solvent
(CH3OH/water 5:2). b) Titration curve of 1b. The absorption change at
l=660 nm was monitored in the pH range from 13.11 to 2.05 in a mixed
solvent (CH3OH/water 5:2).
Conclusion
Diarylethene derivatives 1 and 2 showed reversible, photoin-
duced changes in pKa based on photoisomerization between
open- and closed-ring isomers. A change in the p-conjugated
system in one of the aryl groups caused the pKa to change.
The pKa change of compound 1 from 9.8 to 10.2 upon UV
irradiation was smaller than that observed for compound 2,
in which the pKa value decreased from 10.2 to 9.0. To ex-
plain the small effect observed for 1, theoretical calculations
were carried out. The small difference in pKa for 1 is attrib-
uted to the nonplanar conformation of the pyridinium–thio-
phene–phenoxide rings in the open-ring isomer. The confor-
mation of the molecules also plays an important role in con-
trolling the pKa in addition to the configurational change be-
tween the open- and closed-ring isomers. The concept of
changing the p-conjugated system in one of the aryl groups
by means of the photocyclization reaction is useful for con-
trolling molecular properties, such as, magnetic interactions,
electric conduction, and energy transfer, among others.
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pKa Switching
FULL PAPER
using the Henderson–Hasselbach equation derived from the absorbance
spectral data.
In this work, all acid–base titrations were carried out from pHꢀ13 to 2
by using aqueous HCl and aqueous KOH. The pKa values of the closed-
ring isomers were determined from the absorbance in the photostation-
ary state under irradiation with l=365 nm light.
Syntheses
4-[4-(2,3,3,4,4,5,5-heptafluorocyclopent-1-enyl)-5-(4-methoxymethoxyphe-
nyl)thiophen-2-yl]pyridine (9): Under an argon atmosphere, a solution of
15% n-butyllithium in hexane (7.6 mL, 1.2 mmol) was added to a stirred
solution of 3-bromo-2-(4-methoxymethoxyphenyl)-5-(4-pyridyl)thiophene
8 (4.00 g, 10.6 mmol) in dry THF (250 mL) at À788C. After 30 min, octa-
fluorocyclopentene (5.7 mL, 42.4 mmol) was added to the reaction mix-
ture at À788C. The mixture was left to reach room temperature. After
1.5 h, the reaction was stopped by the addition of water. The product was
extracted with diethyl ether and washed with brine three times. The or-
ganic layer was dried over MgSO4, filtered, and concentrated. The resi-
due was purified by using silica gel column chromatography with hexane/
ethyl acetate (1:2) as the eluent. Compound 9 was obtained as a yellow
wax (1.45 g, 28%).
Figure 4. a) Titration curve of 2a. The absorption change at l=350 nm
was monitored in the pH range from 12.37 to 2.91 in a mixed solvent
(CH3OH/water 5:2). b) Titration curve of 2b. The absorption change at
l=770 nm was monitored in the pH range from 12.40 to 2.59 in a mixed
solvent (CH3OH/water 5:2).
1H NMR (CDCl3, 400 MHz): d=3.51 (s, 3H), 5.23 (s, 2H), 7.10 (d, J=
8.8 Hz, 2H), 7.31 (d, J=8.8 Hz, 2H), 7.48 (d, J=6 Hz, 2H), 7.52 (s, 1H),
8.65 ppm (d, J=6 Hz, 2H); FAB HRMS: m/z calcd for C22H15F7NO2S
[M+H]+: 490.0712; found: 490.0698.
Table 1. Relative energies calculated at the B3LYP/6-31G(d) level of
theory and the experimental pKa values for 1 and 2.
E(OH) [a.u.][a]
E(OÀ) [a.u.][a]
DE
A
]
pKa
1a
1b
2a
2b
À2757.8168
À2757.7816
À2872.3421
À2872.3149
À2757.3767
À2757.3057
À2871.8701
À2871.8866
66.01
71.37
70.79
64.24
9.8
10.2
10.2
9.0
4-{4-[3,3,4,4,5,5-hexafluoro-2-(2-methyl-5-phenylthiophen-3-yl)cyclopent-
1-enyl]-5-(4-methoxymethoxyphenyl)thiophen-2-yl}pyridine (3a): Under
an argon atmosphere,
a solution of 15% n-butyllithium in hexane
(1.1 mL, 1.84 mmol) was added to a stirred solution of 2-methyl-3-
bromo-5-phenylthiophene (0.41 g, 1.6 mmol) in dry THF (50 mL) at
À788C. After 30 min, a solution of 9 (1.19 g, 2.4 mmol) in dry THF
(20 mL) was added to the reaction mixture at À788C. The mixture was
left to reach room temperature. After 3 h, the reaction was stopped by
the addition of water. The product was extracted with diethyl ether and
washed with brine three times. The organic layer was dried over MgSO4,
filtered, and concentrated. The residue was purified by using silica gel
column chromatography with hexane/ethyl acetate (1:1) as the eluent
and reversed-phase HPLC with methanol/acetonitrile (1:1). Compound
3a was obtained as a colorless solid (80 mg, 8%).
1H NMR (CDCl3, 400 MHz): d=1.83 (s, 3H), 3.33 (s, 3H), 4.70 (s, 2H),
6.26 (s, 1H), 6.77 (d, J=8.4 Hz, 2H), 6.91 (d, J=8.4 Hz, 2H), 7.34 (m,
5H), 7.49 (d, J=6.8 Hz, 2H), 7.52 (s, 1H), 8.65 ppm (d, J=6 Hz, 2H);
FAB HRMS: m/z calcd for C33H24F6NO2S2 [M+H]+: 644.1153; found:
644.1146.
[a] a.u.=atomic units.
Experimental Section
General: 1H NMR spectra were recorded on a Bruker AVANCE-400
spectrometer operating at 400 MHz. UV/Vis spectra were recorded on a
Hitachi U-3500 absorption spectrophotometer. Fast-atom bombardment
(FAB) high-resolution mass spectrometry (HRMS) data were obtained
on an JEOL JMS mate II instrument. Photoirradiation was carried out
by using a 500 W super-high-pressure mercury lamp as the light source.
Determination of pKa: Measurements of the pH values were carried out
by using a HORIBA pH meter F-51. The pKa values were calculated by
Figure 5. Conformational structures of 1a(OÀ) and 2b(OÀ) calculated at the B3LYP/6-31G(d) level of theory.
Chem. Eur. J. 2006, 12, 4283 – 4288
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M. Irie et al.
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1-enyl]-5-(4-hydroxyphenyl)thiophen-2-yl}-1-methylpyridinium
(1):
Methyl trifluoromethanesulfonate (1.0 mL, 6.1 mmol) was added to a stir-
red solution of 3 (80 mg, 0.12 mmol) in dry dichloromethane (80 mL).
The reaction was stirred at ambient temperature in the dark under a ni-
trogen atmosphere. After 36 h, the resulting suspension was filtered. The
reaction solution was concentrated. The residue was purified by using re-
verse-phase silica gel column chromatography with acetonitrile/methanol
(1:1) as the eluent and reversed-phase HPLC with methanol/acetonitrile
(1:1). Compound 1a was obtained as a yellow solid (50 mg, 68%).
1H NMR (CDCl3, 400 MHz): d=1.77 (s, 3H), 4.25 (s, 3H), 6.21 (s, 1H),
6.54 (d, J=8.4 Hz, 2H), 6.82 (d, J=8.8 Hz, 2H), 7.20 (m, 5H), 7.25 (s,
1H), 8.23 (d, J=6 Hz, 2H), 8.69 ppm (d, J=6.4 Hz, 2H); UV/Vis
(CH3OH/water
5:2):
lmax
(e)=287
(19870),
380 nm
(13419 molÀ1 dm3 cmÀ1); FAB HRMS: m/z calcd for C32H22F6NOS2 [M]+:
614.1041; found: 614.1013.
3-(2,3,3,4,4,5,5-heptafluorocyclopent-1-enyl)-5-(4-methoxymethoxyphen-
yl)-2-(4-methoxyphenyl)thiophene (11): This compound was prepared
from 2.73 g (6.73 mmol) of compound 10 by using a similar procedure to
that used to prepare 9. The product was purified by using silica gel
column chromatography with dichloromethane/hexane (1:1) as the
eluent. Compound 11 was obtained as a yellow wax (2.62 g, 75%).
1H NMR (CDCl3, 400 MHz): d=3.50 (s, 3H), 3.85 (s, 3H), 5.21 (s, 2H),
6.94 (d, J=8.8 Hz, 2H), 7.08 (d, J=8.8 Hz, 2H), 7.22 (s, 1H), 7.30 (d, J=
8.8 Hz, 2H), 7.53 ppm (d, J=8.8 Hz, 2H); FAB HRMS: m/z calcd for
C24H17F7O3S [M]+: 518.0787; found: 518.0780.
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phenyl)thiophen-3-yl]cyclopent-1-enyl}-5-methylthiophen-2-yl)pyridine
(4): This compound was prepared from 1.22 g (2.35 mmol) of compound
11 by using a similar procedure to that used to prepare 3.
The product was purified by silica gel column chromatography with
hexane/ethyl acetate (1:1) as the eluent and reversed-phase HPLC with
methanol. Compound 4 was obtained as a white solid (50 mg, 4%).
1H NMR (CDCl3, 400 MHz): d=1.86 (s, 3H), 3.42 (s, 3H), 3.50 (s, 3H),
5.23 (s, 2H), 6.59 (d, J=8.8 Hz, 2H), 6.88 (d, J=8.4 Hz, 2H), 7.09 (d, J=
8.4 Hz, 2H), 7.20 (d, J=6.4 Hz, 2H), 7.37 (s, 1H), 7.55 (d, J=8.8 Hz,
2H), 8.56 ppm (d, J=6 Hz, 2H); FAB HRMS: m/z calcd for
C34H25F6NO3S2 [M]+: 673.1180; found: 673.1184.
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mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004..
4-(4-{3,3,4,4,5,5-hexafluoro-2-[5-(4-hydroxyphenyl)-2-(4-methoxyphe-
nyl)thiophen-3-yl]cyclopent-1-enyl}-5-methylthiophen-2-yl)-1-methylpyri-
dinium (2): This compound was prepared from 50 mg (0.07 mmol) of
compound 4 by using a similar procedure to that used to prepare 1. The
product was purified by using silica gel column chromatography with ace-
tonitrile/methanol (1:1) as the eluent and reversed-phase HPLC with
methanol/water (20:1). Compound 2 was obtained as a yellow solid
(10 mg, 22%).
1H NMR (CD3OD, 400 MHz): d=1.98 (s, 3H), 2.91 (s, 3H), 4.33 (s, 3H),
6.67 (d, J=8.8 Hz, 2H), 6.85 (d, J=8.8 Hz, 2H), 6.91 (d, J=8.8 Hz, 2H),
6.99 (s, 1H), 7.42 (s, 1H), 7.51 (d, J=8.8 Hz, 2H), 7.99 (d, J=7.2 Hz,
2H), 8.74 ppm (d, J=6.8 Hz, 2H); UV/Vis (CH3OH/water 5:2): lmax
(e)=350 nm (21987 molÀ1 dm3 cmÀ1); FAB HRMS: m/z calcd for
C33H24F6NO2S2 [M]+: 644.1147; found: 644.1115.
´
[17] J. Makowska, M. Makowski, L. Chmurzynski, J. Phys. Chem. A
2004, 108, 10354–10358.
[18] N. Tanifuji, K. Matsuda, M. Irie, Org. Lett. 2005, 7, 3777–3780.
Received: October 19, 2005
Revised: February 16, 2006
Published online: April 4, 2006
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research
(no. 15105006) and the 21st century COE program from MEXT, Japan.
This work was also supported by PRESTO, JST.
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