Synthesis and photochemical reactions of photochromic terarylene having a leaving methoxy group

Article abstract of DOI:10.1021/ol802969b

Photochromic terarylene having a methoxy group and hydrogen as the leaving units at the photochemical reaction center carbon atoms has been synthesized. This molecule shows irreversible photochemical reaction affording a highly fluorgscent condensed aroma

Full text of DOI:10.1021/ol802969b

ORGANIC
LETTERS
2009
Vol. 11, No. 7
1475-1478
Synthesis and Photochemical Reactions
of Photochromic Terarylene Having a
Leaving Methoxy Group
Hisako Nakagawa, Shigekazu Kawai, Takuya Nakashima, and Tsuyoshi Kawai*
Graduate School of Materials Science, Nara Institute of Science and Technology,
NAIST, 8916-5 Takayama-Cho, Ikoma, Nara 630-0192, Japan
tkawai@ms.naist.jp
Received December 26, 2008
ABSTRACT
Photochromic terarylene having a methoxy group and hydrogen as the leaving units at the photochemical reaction center carbon atoms has
been synthesized. This molecule shows irreversible photochemical reaction affording a highly fluorescent condensed aromatic molecule.
Recently, considerable interest has been focused on organic
photochromic molecules whose molecular structure changes
upon absorption of light.1 Some of them have been practically
used as light modulating materials, which change their colors
under sunlight and bleach spontaneously in dark conditions.2
Since the discovery of photochromic diarylethene3 and flu-
guides4 that hardly show spontaneous thermal bleaching at room
temperature, these “P-type” photochromic molecules have been
expected to be used as the active materials for future rewritable
data storage.5 Especially, photochromic diarylethene shows
unique properties suitable for practical materials, i.e., high
fatigue durability and relatively high photochromic coloration
reactivity even in the solid state.6 Some photochromic fluores-
cent molecules have been proposed based on diarylethene by
connecting fluorescent units with the diarylethene backbone,
and their clear reversible fluorescence modulation has been
useful as the fluorescence-mode recording material.7 One of
the authors has studied some fluorescent diarylethene molecules
and demonstrated their fluorescence switching even at the single
molecule level and also the possibility of 3D high-density
recording.8 However, there is still a serious problem in using
(5) PhotoreactiVe Materials for Ultrahigh-Density Optical Memory; Irie,
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Jpn. J. Appl. Phys. 1997, 36, 3736. (b) Kawai, T.; Koshido, T.; Katsumi,
Y. Appl. Phys. Lett. 1995, 67, 795. (c) Tanio, N.; Irie, M. Jpn. J. Appl.
Phys. 1994, 33, 1550. (d) Biteau, J.; Chaput, F.; Lahlil, K.; Boilot, J.-P.;
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Mater. 1998, 10, 1945–1950. (e) Kawai, T.; Fukuda, N.; Gro¨schl. D.;
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Irie, M. Jpn. J. Appl. Phys. 1994, 33, 3942. (g) Kim, M.-S.; Kawai, T.;
Irie, M. Chem. Lett. 2001, 702. (h) Kim, M.-S.; Maruyama, H.; Kawai, T.;
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10.1021/ol802969b CCC: $40.75
Published on Web 03/04/2009
2009 American Chemical Society

fluorescent photochromic molecules as practical optical record-
ing materials. That is, the excitation light for the fluorescence
readout usually induces photochromic bleaching or coloration
reactions, and recorded information would easily be destroyed.
To the best of our knowledge, the nondestructive readout
capability and reversible fluorescence-mode recording have
never been achieved simultaneously in the photomemory
molecules.
Because of the difficulty to achieve the nondestructive
read-out capability, some write-once optical memory systems
based on the organic fluorescence molecules have recently
been reported. Irie et al.9 and Misawa et al.10 have individu-
ally studied fluorescence-mode write-once molecular memory
systems. In both systems, the emission properties of the acid-
sensitive fluorescent molecules are modulated by photoacid
generation reactions. That is, the information is stored as
the local acidity and readout with the modulated fluorescence
signal. Because of irreversibility of the photoacid generation,
they performed as the write-once memory, and nondestruc-
tive readout could be achieved. In these systems, however,
the long-range diffusion of acid in the polymer matrix causes
degradation of the stored information, while the short-range
diffusion of acid is essentially required. This problem comes
from the bimolecular nature of these systems. It is thus
required to develop new unimolecular recording systems
whose fluorescent nature can be irreversibly modulated by
light irradiation.
ecules having fairly high quantum yields in the photochemi-
cal cyclization reaction (60-70%) and high photochromic
reactivity even in the solid states.11 The methoxy group
(CH3O-) and hydrogen (H-) are introduced as the leaving
units at the carbon atoms of the photochemical reaction
center. Compound 1 would be converted to the benzo[1,2-
b:3,4-b′]bis-[1]-benzothiophene structure, which has been
reported as a luminescent molecule.12 If a diarylethene
structure with hexafluoro cyclopentene is used, the elimina-
tion of the fluorine would be induced.13 In this letter, we
report the synthesis and photochemical properties of com-
pound 1.
The synthetic procedure is illustrated in the Supporting
Information. To dibromo phenyl thiazole, benzothiophene
derivatives having a hydrogen and a methoxy group at the
R- and ꢀ-carbon atoms, respectively, were connected by the
Pd-catalyzed coupling reactions. Chemical characterization
1
of 1a was performed with H NMR and MS spectra as
summarized in the Supporting Information (Figure S1).
Photochemical reactivity of 1a was studied in acetone and
in hexane as high and low polarity solvents, respectively.
The original colorless acetone solution of 1a was observed
to turn yellow in color after UV-light irradiation of 365 nm
in wavelength. A fair amount of yellow precipitates formed
after a few minutes of UV-light irradiation. Acetone was
removed from the yellow solution, and the residue was
washed with acetone and hexane many times. The yellow
solid was thus separated. 1H NMR and HRMS measurements
of the residue were carried out, and the formation of 1c was
identified as presented in the Supporting Information (Figure
S2). This suggests that the photoreaction of 1a generated
1c. 1c did not return to the original 1a by irradiation of light
or heating.
In the present study, we propose novel photoresponsive
fluorescent molecules, whose backbone structure is illustrated
in Scheme 1. The hexatriene analogues having leaving groups
Scheme 1
NMR measurement was also performed after UV-light
irradiation in d6-acetone solution of 1a to confirm the
conversion of 1a to 1c. Since the solution contained a small
amount of yellow precipitates after the UV-light irradiation,
1H NMR signals in the aromatic region were rather diffused.
Nevertheless, the signal of the methoxy proton of 1a (4.0
ppm) was apparently decreased, and that of methanol (3.3
ppm, s, 3H) was clearly observed after the UV-light
irradiation, indicating elimination of CH3OH during forma-
tion of 1c. No 1H signals other than those of 1a and methanol
were observed in the aliphatic region. The ratio of 1a and
X- and Y- are expected to show pericyclization reactions
with relatively high efficiency and to give stable condensed
aromatic moiety after liberation of the X-Y molecule. We
here synthesized compound 1, whose molecular structure is
shown in Scheme 2.
Scheme 2
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Compound 1 is a derivative of “triangle terarylene” which
the authors have recently proposed as photochromic mol-
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44, 1640.
1476
Org. Lett., Vol. 11, No. 7, 2009

methanol was typically 3:1, although signal intensity in the
aromatic region could not be analyzed because of the diffused
signals.
We also studied photoreactivity of 1a in hexane as a low
polarity solvent. Figure 1 shows the changes in the absorption
was completed within less than one minute even after
exposure to TFA vapor, which suggests progress of the acid-
catalyzed elimination reaction. The yellow precipitate was
1
also collected and identified to be 1c by H NMR and MS
measurements. As shown in Figure 1, the hexane solution
did not show any absorption band in the visible region after
the addition of TFA, indicating a complete reaction of 1b to
1c. In the hexane solution, 1b seemed to be relatively stable,
and the acid-catalyzed elimination of the methoxy group
resulted in 1c. These changes of color upon the photochemi-
cal and acid-catalyzed reactions are presented in the Sup-
porting Information (Figure S3).
The absorption spectrum of 1c showed a characteristic
bathochromic shift from that of 1a, and the threshold
wavelength of the absorption band of 1c was slightly longer
than that of 1a. As summarized in the Supporting Information
(Figure S4), the TD-DFT calculation of the B3LYP/6-31G(d)
level supported this small bathochromic shift of 1c in
comparison with 1a. That is, absorption peak wavelengths
of 1a and 1c were calculated to be 295 and 380 nm,
respectively. The fused aromatic ring structure of 1c would
be responsible for this bathochromic shift.
Optical properties of 1c were examined in toluene after
careful treatment with hexane for removing 1a. Figure 2
Figure 1. Changes of the absorption spectrum of 1a in hexane
solution: (red -) original state (1a), (blue ----) after UV irradiation
(1b + 1a), (green - - -) after acid addition (1c + 1a).
spectra of the hexane solution of 1a (4.8 × 10-5 mol dm-3)
upon light irradiation. The colorless 1a solution turned to
red upon UV-light irradiation. This red solution showed an
absorption band at 530 nm. The red solution returned to the
original state 1a quasi-reversibly upon visible light irradia-
tion. An isosbestic point at 395 nm was observed in the
photocoloration and the photobleaching reactions, indicating
the photochemical reactions consisting of well-defined two
components. The visible absorption band at about 500 nm
is typical for the ring-closed form of the terarylene deriva-
tives, and Scheme 2 shows the ring-closed form isomer 1b
as a possible structure of the red component. We assumed
1b and evaluated its π-π* wavelength by the quantum-
chemical calculation. The TD-DFT calculation in B3LYP/
6-31G for 1b predicted a visible absorption band of 504 nm,
which agreed well with the experimental observation. The
evaluated HOMO and LUMO distributions and also the
stable geometries of 1a and 1b are given in the Supporting
Information (Figure S5). For the calculation, the Gaussi-
an0314 suite of programs was used. The photochemical
reaction from 1a to 1b would be specified as the pericy-
clization reaction typical for the terarylene derivatives, whose
photochemical quantum yields are typically about 60%.11
The isosbestic point was maintained during the first several
minutes of UV-light irradiation and visible light irradiation,
and further UV-light irradiation resulted in the irreversible
reaction, which was characterized to be the elimination
reaction from 1b to 1c. Indeed, the red colored hexane
solution gradually changed to yellow, and thus the separation
of 1b from the red solution failed.
Figure 2. Emission (-) and excitation (---) spectra of 1c in toluene
solution (4.8 × 10-5 mol dm-3) obtained with excitation at 320
nm and detection at 430 nm, respectively.
shows fluorescence emission and excitation spectra of 1c in
the toluene solution (6.75 × 10-6 mol dm-3) under nitrogen
atmosphere. 1c showed a characteristic emission peak at 420
nm. The peak wavelength of the excitation spectrum at about
360 nm agreed well with that of the absorption spectrum.
1c showed a relatively high fluorescence quantum yield of
10% at ambient temperature in toluene. Fluorescence of 1c
can thus be measured under excitation at 400-410 nm where
1a showed no absorbance. 1c is expected to show fairly high
stability in fluorescent emission because of its stable
condensed aromatic structure with the 26π electron system.
If 1a would be used in the write-once memory systems, 365
nm UV laser light would be available for recording as it
The hexane solution containing the red compound changed
to yellow immediately after addition of TFA. This reaction
Org. Lett., Vol. 11, No. 7, 2009
1477

induces photochemical reaction to 1c in the hydrophilic
medium, and 405 nm violet laser-light could be used for
readout of the memory as it excites the fluorescence emission
of 1c but not any photochemical reaction of 1a; thus,
nondestructive readout memory should be achieved.
In summary, we synthesized a new photoresponsive
molecule having a terarylene type structure and a methoxy
group as the leaving unit, which shows a relatively high
fluorescence quantum yield after UV-light irradiation, indi-
cating the possibility of future application to an active
material in write-once type optical data-storage systems.
Acknowledgment. The authors thank Ms. Nishikawa and
Ms. Yamamura in NAIST for their contribution to measur-
ments of MS spectra. This work has been partly supported
by Grant-in-Aid for Scientific Research in Priority Area
“Photochromism” and “Super-Hierarchical Structure” from
the Ministry of Education, Culture, Sports, Science and
Technology, MEXT, Japan.
Supporting Information Available: Synthetic scheme,
TD-DFT calculation data, and spectral data (NMR and MS).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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