A ferrocenylspiropyran that functions as a molecular photomemory with controllable depth

Article abstract of DOI:10.1002/anie.200600535

Deep memory: A photochromic molecule can control the stability of its merocyanine (MC) form by changing the oxidation state of the ferrocene (Fc) moiety (see picture). This ferrocenyl-spiropyran (SP) system works not only in solution but also in a polymer electrolyte. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200600535

Communications
Photochromism
DOI: 10.1002/anie.200600535
A Ferrocenylspiropyran That Functions as a
Molecular Photomemory with Controllable
Depth**
Sayoko Nagashima, Masaki Murata, and
Hiroshi Nishihara*
Spiropyran (SP) derivatives,[1] such as 1’,3’,3’-trimethyl-6-
nitrospiro[2H-1-benzopyran-2,2’-indoline], belongto a class
of organic photochromic compounds that have been inten-
sively investigated and have potential applications as molec-
ular electronic devices, optical switches, and memories.[2]
These dyes are converted into the colored merocyanine
(MC) form upon irradiation with UV light, but revert to the
colorless SP form when left in the dark or upon irradiation
with visible light. The open MC form has a larger dipole
moment than the closed SP form, and thus the stability of the
MC form strongly depends on the electronic effects of the
substituent(s) of the indolium and phenyl moieties, the
solvent polarity, and the metal complexation.[3] If the
donor–acceptor nature of the substituent can be controlled
by a redox reaction, we can switch the balance of thermody-
[*] S. Nagashima, Dr. M. Murata, Prof. H. Nishihara
Department of Chemistry, School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5841-8063
E-mail: nisihara@chem.s.u-tokyo.ac.jp
[**] This work was supported by Grants-in-Aid for Scientific Research
(nos. 16047204 (area 434) and 17205007), by a grant from The 21st
Century COE Program for Frontiers in Fundamental Chemistry from
MEXT, and by CREST, JST (Japan).
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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namic stability, and thus the memory depth, between the SP
and MC forms. Herein, we report a new system in which the
stability of the MC form can be completely and reversibly
switched by a combination of SP/MC photoisomerization and
a ferrocene/ferrocenium redox cycle.[4]
A 5-ferrocenylspiropyran (Fc-SP) was synthesized by
condensation of 2-hydroxy-5-nitrobenzaldehyde and 5-ferro-
cenyl-substituted Fischerꢀs Base in methanol (see the Sup-
portingInformation). A reversible Fe III/FeII redox wave is
observed at À0.03 V versus ferrocenium/ferrocene (Fc+/Fc) in
dichloromethane in the cyclic voltammogram of Fc-SP (see
the SupportingInformation). The UV/Vis spectrum of Fc-SP
in methanol shows a p–p* transition band at lmax = 345 nm
(e = 2.9 ” 104 mÀ1 cmÀ1; see the SupportingInformation). The
electron donation of the ferrocene moiety results in a lower
energy of the p–p* band of Fc-SP relative to unsubstituted
spiropyrans.
Irradiation of a pale-yellow solution of Fc-SP in methanol
with UV light (l = 365 nm) gave a red-purple solution with an
absorption maximum wavelength of 530 nm (see the Support-
ingInformation), which can be ascribed to the generation of
the MC form (Fc-MC; Scheme 1). The molar ratio of the MC
Figure 1. Sequential spectral changes of Fc-SP in dichloromethane:
a) Initial state (Fc-SP), b) after oxidation with 1.0 equivalent of [Fe(h5-
C5H4Cl)2]PF6, c) after irradiation with 365-nm light, d) after reduction
with 1.0 equivalent of [Fe(h5-C5Me5)2], and e) after standing at 208C.
Inset: UV/Vis–near-IR spectral changes from (a) to (b) (left) and from
(b) to(c) (right).
to reach the PSS, the 1250-nm band vanished and a new band
appeared at 445 nm (Figure 1c), that is, a marked blue shift
occurs upon photoirradiation. The 445-nm band was ascribed
to the isomerization from the SP to the MC form, based on
the findingthat subsequent reduction agve Fc-MC (see
below). It should be noted that the complete disappearance
of the 1250-nm band signifies that only the MC form is
present in the PSS of the oxidized state, whereas the MC
molar ratio in the PSS of the Fc state reaches only 56%
(Figure 2A).
Scheme 1. Photoisomerization of Fc-SP.
form was estimated from the photo-generated 1H NMR
spectral changes in [D4]MeOH (see the SupportingInforma-
tion) to be 75% in the photostationary state (PSS).
The lmax value, yield in the PSS, and the thermodynamic
stability of the MC form depend on the solvent polarity. Fc-
MC exhibits negative solvatochromism, that is, its lmax
decreases with an increase in the Reichard empirical solvent
polarity parameter, ET(30)[5] (see the SupportingInforma-
tion). The lmax value of Fc-MC is red-shifted relative to that of
the unsubstituted MC (see the SupportingInformation) as a
result of the ferrocene moiety actingas a donor with a
resonance stabilization effect. Also, the yield of the MC form
in the PSS duringirradiation with UV light is lower, and
thermal isomerization from the MC to the SP form is faster, in
solvents with lower polarity. In dichloromethane, for example,
Fc-MC (lmax = 578 nm) was formed with a yield of 56% in the
PSS, which is lower than the 75% obtained in methanol,
although it isomerizes to Fc-SP within a few hours at 208C,
much faster than in methanol.
Next, we attempted to combine the redox and photo-
chemical changes of Fc-SP in dichloromethane. When a
dichloromethane solution of Fc-SP was oxidized to Fc+-SP
with a stoichiometric amount of 1,1’-dichloroferrocenium
hexafluorophosphate (E0’ = 0.19 V versus Fc+/Fc),[6] new
absorption peaks appeared at 515 and 1250 nm (compare
Figure 1a and b and left inset). When this oxidized solution
was irradiated with 365-nm light for a length of time sufficient
Figure 2. A) Photo- and redox reactions of Fc-SP/Fc-MC. B) The
decrease in absorbance at lmax of the MC forms at 208C in CH2Cl2
before and after oxidation with [Fe(h5-C5H4Cl)2]PF6 under irradiation
with 365-nm light toreach the PSS.
The ratio of the MC form remained constant after
oxidation; irradiation of Fc+-MC with UV or visible light
could not cause reversion to the SP form (Figure 2B), and
thermal isomerization to the SP form hardly proceeded. In
other words, Fc+-MC is very stable towards light and heat.
Previous methods to stabilize the MC state are mainly based
on the Coulomb interaction of the phenolate anion with
nearby cationic species.[3] The reason for the unique MC
stabilization by Fc+ in the present study has not been fully
clarified, but the large blue shift of lmax for Fc+-MC relative to
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Fc-MC suggests a strong electronic effect of Fc+ as a result of
p conjugation.
Chemical reduction of Fc+-MC with a stoichiometric
only the film on the left-hand side turns from purple to orange
as a result of the oxidation of Fc-SP (c). When the films are
irradiated with visible light, the color goes back to yellow (d).
When the current is switched off, the re-reduction of Fc-SP
takes place in the left-hand film, as seen by a change in color
from yellow to brown (e). The brown color is due to a mixture
of the oxidized and re-reduced MC forms in the film. When
left without further treatment, the film reverts to yellow (f).
These color changes demonstrate the change in readiness of
the SP–MC conversion as a result of the redox change of the
ferrocene site.
amount of decamethylferrocene (E0’ = À0.59 V versus Fc+/
Fc)[7] allowed us to reach the MC PSS, as confirmed by the
appearance of the characteristic band of Fc-MC (Figure 1d),
and the MC form reverted thermally to the SP form
(Figure 1e). This reversible redox and photoisomerization
cycle (Figure 2A) indicates that the Fc state corresponds to a
shallow (deletable) photomemory and the Fc+ state to a deep
(undeletable) photomemory.
The reversible redox and photoisomerization cycle was
also studied in a polymer matrix. Poly(ethylene oxide) and
polyvinylchloride (PVC) films containingFc-SP show a
coloringto purple ( lmax = 545 nm) and blue (lmax = 578 nm),
respectively, upon irradiation with UV light (365 nm; see
Figure 3A and the Supporting Information), thus indicating
In conclusion, a photochromic molecule with an ability to
switch between two isomeric structures by a redox change has
been created. This phenomenon has been demonstrated both
in solution and in solid polymer electrolytes. The system is
expected to provide a new approach for the realization of
molecule-based devices, and should be important in the field
of semiconductor computer technology based on “random-
access memory” and “read-only memory”, in which the
control of memory depth is becomingincreasingly important.
Experimental Section
Synthesis of 1 (prepared accordingto a report on the synthesis of 3-
ferrocenylnitrobenzene): Ferrocene (6.69 g, 0.0361 mol) was added to
sulfuric acid (specific gravity = 1.84, 40 mL). The resultingdeep-blue
ferrocenium solution was stirred at room temperature for 3 h. A
solution of sodium nitrite (0.80 g, 0.0116 mol) in water (5 mL) at less
than 08C was added dropwise to a solution of 5-amino-1,3,3-
trimethyl-2-methyleneindoline (1.83 g, 0.0972 mol) in 1:1 water/
hydrochloric acid (specific gravity 1.18, 10 mL) with stirring. The
mixture was stirred at less than 08C for 3 h to ensure complete
diazotization. Copper powder (2.0 g) was added to the ferrocenium
solution as a catalyst, and the diazonium solution was then added
dropwise with vigorous stirring. The nitrogen effervescence ceased
after stirringfor 4 h, and ascorbic acid (5.0 g) was added to reduce the
startingferrocenium to ferrocene. The product was extracted with
dichloromethane. The combined organic extracts were dried with
sodium sulfate, filtered through Celite, and the solvent was elimi-
nated. The dark solid thus obtained was purified by column
chromatography on alumina with an increasing proportion of ethyl
acetate in n-hexane as the eluent. The first yellow-orange band was
eluted with pure hexane and yielded unchanged ferrocene. The third
yellow band, which eluted with ethyl acetate/hexane (50:50), was
collected. The yellow component was evaporated to yield 97.9 mg
(0.274 mmol, 2.81%) of 1 as a brown solid. 1H NMR (400 MHz,
CD2Cl2, room temperature): d = 1.37 (s, 6H), 3.03 (s, 3H), 3.84 (s,
2H), 4.03 (s, 5H), 4.23 (t, J = 2.0 Hz, 2H), 4.53 (t, J = 2.0 Hz, 2H),
6.45 (d, J = 8.0 Hz, 1H), 7.18 (d, J = 1.6 Hz, 1H), 7.25 ppm (dd, J =
8.0, 1.6 Hz, 1H); MALDI-TOF-MS: m/z 357 [M+].
Figure 3. A) Optical changes of Fc-SP in a) poly(ethylene oxide) and
b) PVC films with a photomask after irradiation at 365 nm. B) Optical
changes of Fc-SP/PVC films upon irradiation at 365 nm (a–b), applica-
tion of an electric field of 2 V (b–c), irradiation at 546 nm (c– d),
application of an electric field of 0 V (d– e), and in the absence of
further treatment (e– f). On the right-hand side an identical film
sample is shown that was not subjected to the redox process (that is,
to which the potential was not applied).
Synthesis of Fc-SP: Compound 1 (38.8 mg, 1.09 mmol) and 2-
hydroxy-5-nitrobenzaldehyde (18.7 mg, 1.11 mmol) were stirred at
room temperature in anhydrous methanol for 1.5 h. The resulting
mixture was separated by column chromatography on alumina using
ethyl acetate/hexane (1:5, v/v) as the eluent. The yellow band was
collected and the solvent evaporated to yield 20.3 mg(0.400 mmol,
36.9%) of Fc-SP as a greenish-yellow powder. Elemental analysis
(%) calcd for C29H26FeN2O3: C 68.79, H 5.18, N 5.53; found C 68.54, H
5.31, N 5.31; 1H NMR (400 MHz, CD2Cl2, room temperature): d =
1.21 (s, 3H), 1.32 (s, 3H), 2.74 (s, 3H), 4.05 (s, 5H), 4.26 (t, J = 1.9 Hz,
2H), 4.53 (t, J = 1.9 Hz, 2H), 5.89 (d, J = 10.4 Hz, 1H), 6.49 (d, J =
8.0 Hz, 1H), 6.82 (d, J = 10.0 Hz, 1H), 6.97 (d, J = 10.4 Hz, 1H), 7.21
(d, J = 1.7 Hz, 1H), 7.33 (dd, J = 10.0, 1.7 Hz, 1H), 8.00 (dd, J = 8.0,
the formation of the MC form; this color change can be
reversed thermally or by irradiation with 546-nm light.
Oxidation and re-reduction of Fc-SP in a PVC film containing
Bu4NBF4 as an electrolyte sandwiched by two ITO electrodes
occurs reversibly when the voltage between the two ITO
electrodes is controlled between 0 and 2 V.
Figure 3B shows the results for two identical films to only
one of which (the left one) an electric field was applied. Upon
irradiation at 365 nm, both of the films become purple (a to
b). When charged with electricity at the ITO electrode cell,
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2.9 Hz, 1H), 8.02 ppm (d, J = 2.9 Hz, 1H); MALDI-TOF-MS:
m/z 506 [M+].
Polymer-electrolyte experiment: Photoirradiation was carried
out with a USHIO high-pressure mercury lamp as the light source.
The bright lines were selected with a monochromator (Jasco, CT-10)
or with
a glass filter (Toshiba). Two kinds of polymer were
investigated. One polyelectrolyte (FB-011, DAISO Research Labo-
ratories; thickness: 40 mm) consisted of the ternary copolymer of
ethylene oxide (EO), 2-(2-methoxyethoxy)ethyl glycidyl ether (EM-
2), and allyl glycidyl ether (AGE), with an EO/EM-2/AGE molar
ratio of 82:12:1.8. Lithium bis(trifluoromethansulfonyl)imide
(LiTFSI) was added as a supportingsalt ([Li]/[O] = 0.06:1). The
other polymer (Toughnyl, Japan Wavelock Co.; thickness: 100 mm)
contained PVC. A solution of nBu4NBF4 was introduced into the
PVC film as an electrolyte and the solvents evaporated. The Fc-SP
solution was swollen in these polymer films and the solvent was
removed. The dried polymer films were pitched with two ITO-coated
glass electrodes. The voltage was controlled with a Hokuto Denko
HSV-100 automatic polarization system.
Received: February 9, 2006
Published online: May 31, 2006
Keywords: photochromism · photomemory · redox chemistry ·
.
sandwich complexes · spiro compounds
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