Photon-, electron-, and proton-induced isomerization behavior of ferrocenylazobenzenes

Article abstract of DOI:10.1021/ic051184r

3-, 4-, and 2-ferrocenylazobenzenes, 1, 2, and 3, respectively, and several derivatives of 1 were synthesized, and their photoisomerization behaviors were examined. The molecular structures of 1 and its derivatives, 2-chloro-5-ferrocenylazobenzene (5) and 3-ferrocenyl-4′-hydroxylazobenzene (11), were determined by X-ray diffraction analysis. 3-Ferrocenyl compound 1 undergoes reversible trans-to-cis isomerization with a single green light source and the Fe^III/Fe^II redox change. 4- and 2-Ferrocenyl compounds, 2 and 3, also respond to green light in addition to UV light, exciting the π-π* transition, but the cis molar ratio in the photostationary state (PSS) is lower than that of 1. The response to green light in 2 and 3 is caused by the MLCT (from Fe d orbital to azo π* orbital) band excitation, while the character of the MLCT band, as estimated by time-dependent density functional theory calculations, differs between 1 and 2. The oxidized form of 2 undergoes facile cis-to-trans thermal isomerization. Both 1 and 2 undergo facile protonation and show proton-catalyzed cis-to-trans isomerization. Among the derivatives of 1, 2-chloro-5-ferrocenylazobenzene (5) exhibits the highest cis molar ratio (47%) in the PSS of green light irradiation.

Full text of DOI:10.1021/ic051184r

Inorg. Chem. 2005, 44, 7547−7558
Photon-, Electron-, and Proton-Induced Isomerization Behavior of
Ferrocenylazobenzenes
Aiko Sakamoto,† Akira Hirooka,† Kosuke Namiki,† Masato Kurihara,‡ Masaki Murata,†
Manabu Sugimoto,§ and Hiroshi Nishihara*,†
Department of Chemistry, School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan, Department of Biological Chemistry, Faculty of Science, Yamagata
UniVersity, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan, and Department of Applied
Chemistry and Biochemistry, Faculty of Engineering, Kumamoto UniVersity, Kurokami,
Kumamoto 860-8555, Japan
Received July 15, 2005
3-, 4-, and 2-ferrocenylazobenzenes, 1, 2, and 3, respectively, and several derivatives of 1 were synthesized, and
their photoisomerization behaviors were examined. The molecular structures of 1 and its derivatives, 2-chloro-5-
ferrocenylazobenzene (5) and 3-ferrocenyl-4′-hydroxylazobenzene (11), were determined by X-ray diffraction analysis.
3-Ferrocenyl compound 1 undergoes reversible trans-to-cis isomerization with a single green light source and the
FeIII/FeII redox change. 4- and 2-Ferrocenyl compounds, 2 and 3, also respond to green light in addition to UV
light, exciting the π−π* transition, but the cis molar ratio in the photostationary state (PSS) is lower than that of
1. The response to green light in 2 and 3 is caused by the MLCT (from Fe d orbital to azo
π* orbital) band
excitation, while the character of the MLCT band, as estimated by time-dependent density functional theory
calculations, differs between 1 and 2. The oxidized form of 2 undergoes facile cis-to-trans thermal isomerization.
Both 1 and 2 undergo facile protonation and show proton-catalyzed cis-to-trans isomerization. Among the derivatives
of 1, 2-chloro-5-ferrocenylazobenzene (5) exhibits the highest cis molar ratio (47%) in the PSS of green light
irradiation.
Introduction
plexes have been reported in recent years.2 We have been
systematically investigating the unique properties of azo-
conjugated transition metal complexes.3 One unique property
is the reversible trans-to-cis isomerization of the azobenzene
moieties using a combination of a single light source and
the redox change of the metal complex moieties. We
previously reported three different systems showing this
phenomenon: tris(azobenzene-bound bipyridine)cobalt,3h,3i
bis(azobenzene-bound bipyridine)copper,3j and 3-ferrocenylazo-
Among photochromic molecules whose structures and
colors change reversibly by photoirradiation with alternating
two kinds of light, azobenzene is one of the most popular.
This compound has attracted much attention recently because
of its possibilities of versatile applications to molecular
memories and switches.1 Combining azobenzene with metal
complexes is noteworthy in view of the ability to exhibit
novel functions deriving from the combination of redox,
optical, and magnetic properties of the metal complexes with
the photoisomerization of the azobenzene moiety. A number
of photoisomerization studies of azo-conjugated metal com-
(2) (a) Barlow, S.; Marder, S. R. Chem. Commun. 2000, 1555-1562.
(b) Yam, V. W. W.; Lau, V. C. Y.; Wu, L. X. J. Chem. Soc., Dalton.
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1999, 121, 48-53. (e) Otsuki, J.; Harada, K.; Araki, K. Chem. Lett.
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Francescangeli, O. Eur. J. Inorg. Chem. 1999, 1367-1372.
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2000, 1334-1335. (i) Yam, V. W. W.; Yang, Y.; Zhang, J.; Chu, B.
W. K.; Zhu, N. Organometallics 2001, 20, 4911-4918. (j) Muraoka,
T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem. Soc. 2003,
125, 5612-5613. (k) Horie, M.; Sakano, T.; Osakada, K.; Nakao, H.
2004, 23, 18-20.
* Author to whom correspondence should be addressed. Phone:
+81-3-5841-4346. Fax:
+81-3-5841-8063. E-mail:
nisihara@
chem.s.u-tokyo.ac.jp.
† University of Tokyo.
‡ Yamagata University.
§ Kumamoto University.
(1) (a) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873-1875. (b) Liu,
Z. F.; Fujishima, A.; Hashimoto, K. Nature 1990, 347, 658-660.
(c) Blanchard, P. M.; Mitchell, G. R. Appl. Phys. Lett. 1993, 63, 2038-
2040.
10.1021/ic051184r CCC: $30.25
Published on Web 09/13/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 21, 2005 7547

Sakamoto et al.
Chart 1. Ferrocenylazobenzenes
of 1, 5-10 were synthesized from 3-ferocenylaniline deriva-
tives and nitrosobenzene or nitrosophenol p-toluenesulfonate
(-OTs). The deprotection of the tosyl group of 10 was
conducted by KOH to afford a phenol derivative, 11, and
the following Williamson’s ether synthesis with 1,8-dibro-
mooctane gave the bromo-terminated alkoxy derivative, 12.
Then, the bromo group-terminated alkyl chain was converted
to phosphonic acid and disulfide diethyl ester to give 13 and
14, respectively. 3,5-Diferrocenylazobenzene, 4, was ob-
tained through a reaction with boronic acid, and other
derivatives were obtained by mixing 3-ferrocenylaniline
derivatives with nitrosobenzene in acetic acid. These new
compounds are highly stable in the air, and they are soluble
in most relatively polar solvents, such as dichloromethane,
chloroform, and acetonitrile. They were characterized by 1H
NMR and UV-vis spectra and by elemental analyses. The
molecular structures of 1, 2-chloro-5-ferrocenyl-azobenzene
(5), and 3-ferrocenyl-4′-hydroxy-azobenzene (11) were de-
termined by single-crystal X-ray crystallography.
The UV-vis absorption spectra of ferrocenylazobenzenes
in acetonitrile or dichloromethane are shown in Figure 1,
and the data are summarized in Table 1. Compounds 2 and
3, in which the azo and ferrocene moieties are fully
benzene.3l 3-Ferrocenylazobenzene is especially interesting
because it undergoes reversible isomerization not only with
the usual UV light but also with green light. Since making
this discovery, we have been investigating the effects of the
degree of electronic interaction between azobenzene and
ferrocene moieties on isomerization behavior by changing
the position of ferrocenyl groups on azobenzene and
introducing various substituents on the phenyl rings. In this
paper, we describe in detail the isomerization behavior of
ferrocenylazobenzenes, which can be controlled with pho-
tons, electrons, and protons.
π-conjugated, showed a strong MLCT band at λmax
492 nm with molar extinction coefficient ꢀmax
3690 M-1cm-1 and at λmax ) 478 nm with ꢀmax
)
)
)
2190 M-1cm-1, respectively, in addition to an azo π-π*
band at λmax ) 320 nm [nearly the same wavelength as that
of azobenzene (317 nm)] and at λmax ) 352 nm, respectively.
Compound 1 and its derivatives also exhibited a band around
390-492 nm ascribable to MLCT (or π-π*) transitions
(vide infra) in addition to an azo π-π* band at 319-
352 nm, while the MLCT band intensity was lower than
those of 2 and 3 because of the weaker electronic interaction
between the azo and ferrocenyl groups. Substitution of an
electron-donating group at the 4′ position of 1 caused a
significant shift of the azobenzene moieties’ π-π* bands
to lower energy.
Results and Discussion
Synthesis, Characterization, and X-ray Crystallo-
graphic Analysis. 3-Ferrocenylazobenzene (1) and 2-ferro-
cenylazobenzene (3) were prepared by a reaction of ferro-
cenylanilines and nitrosobenzene (see Chart 1). 4-Ferro-
cenylazobenzene (2) was prepared by a reaction of 4-amino-
azobenzene and ferrocenium ions. 4′-substituted compounds
The molecular structures of 1, 5, and 11 were determined
by single-crystal X-ray crystallography. The crystal data and
refinement parameters are given in Table 2, and an ORTEP
diagram of 1 is displayed in Figure 2 (ORTEP diagrams of
5 and 11 are shown in Figure S1 of the Supporting
Information). The bond lengths of N1-N2 for 1, 5, and 11
are 1.257(2), 1.253(3), and 1.271(2) Å, respectively, and the
dihedral angles between plane C6C7C8C9C10 and plane
C11C12C13C14C15C16 are 16.540°, 24.524°, and 9.060°,
respectively. The N1-N2 bond for 11 is longer than those
for the others, but the length does not correlate well with
the cis molar ratio in the photostationary state (PSS) upon
green light irradiation (vide infra). On the other hand, the
dihedral angle for 5 is larger than those for the others, and
seems to correlate with the largest cis molar ratio in the PSS
(vide infra). However, this result, which we attribute to the
packing in crystals, does not affect the cis molar ratio because
the single bond between ferrocene and azobenzene freely
(3) (a) Yutaka, T.; Kurihara, M.; Nishihara, H. Mol. Cryst. Liq. Cryst.
2000, 343, 193-198. (b) Yutaka, T.; Kurihara, M.; Kubo, K.;
Nishihara, H. Inorg. Chem. 2000, 39, 3438-3439. (c) Nishihara, H.
Bull. Chem. Soc. Jpn. 2001, 74, 19-29. (d) Yutaka, T.; Mori, I.;
Kurihara, M.; Mizutani, J.; Kubo, K.; Furusho, S.; Matsumura, K.;
Tamai, N.; Nishihara, H. Inorg. Chem. 2001, 40, 4986-4995.
(e) Nihei, M.; Kurihara, M.; Mizutani, J.; Nishihara, H. Chem. Lett.
2001, 852-853. (f) Nihei, M.; Kurihara, M.; Mizutani, J.; Nishihara,
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M.; Nishihara, H. Chem. Commun. 2001, 1656-1657. (h) Kume, S.;
Kurihara, M.; Nishihara, H. J. Kor. Electrochem. Soc. 2002, 189-
191. (i) Kume, S.; Kurihara, M.; Nishihara, H. Inorg. Chem. 2003,
42, 2194-2196. (j) Kurihara, M.; Nishihara, H. Coord. Chem. ReV.
2002, 226, 125-135. (k) Nishihara, H. Macromol. Symp. 2002, 186,
93-98. (l) Kurihara, M.; Hirooka, A.; Kume, S.; Sugimoto, M.;
Nishihara, H. J. Am. Chem. Soc. 2002, 124, 8800-8801. (m) Yutaka,
T.; Mori, I.; Kurihara, M.; Mizutani, J.; Tamai, N.; Kawai, T.; Irie,
M.; Nishihara, H. Inorg. Chem. 2002, 41, 7143-7150. (n) Yutaka,
T.; Mori, I.; Kurihara, M.; Tamai, N.; Nishihara, H. Inorg. Chem.
2003, 42, 6306-6313. (o) Nagashima, S.; Nihei, M.; Yamada, T.;
Murata, M.; Kurihara, M.; Nishihara, H. Macromol. Symp. 2003 204,
93-101. (p) Kume, S.; Murata, M.; Ozeki, T.; Nishihara, H. J. Am.
Chem. Soc. 2005, 127, 490-491. (q) Sakamoto, R.; Murata, M.; Kume,
S.; Sampei, H.; Sugimoto, M.; Nishihara, H. Chem. Commun. 2005,
1215-1217.
7548 Inorganic Chemistry, Vol. 44, No. 21, 2005

Isomerization BehaWior of Ferrocenylazobenzenes
Figure 1. (a) Overlay plots of UV-vis spectra of ferrocenylazobenzene derivatives 1 (7.8 × 10-5 M, solid line), 2 (6.0 × 10-5 M, short-dashed line), and
3 (3.6 × 10-5 M, dotted-dashed line) in acetonitrile. (b) 1-introduced substituents to the phenyl ring, which is bonded with ferrocene, 4 (3.6 × 10-5 M, solid
line), 5 (4.6 × 10-5 M, short-dashed line), 6 (2.9 × 10-5 M, dotted-dashed line), 7 (6.1 × 10-5 M, dotted line), 8 (3.1 × 10-5 M, dashed-dashed line), and
9 (3.6 × 10-5 M, long-dashed) in acetonitrile. (c) 4′-substituted 1 derivatives 10 (3.0 × 10-5 M, solid line), 11 (3.0 × 10-5 M, short-dashed line), and 12
(2.8 × 10-5 M, dotted-dashed line) in acetonitrile and 13 (3.0 × 10-5 M, dotted line) and 14 (3.0 × 10-5 M, dashed-dashed line) in dichloromethane.
Table 1. UV-Vis Absorption Spectral Data of Ferrocenylazobenzene
light (vide infra). Organic azobenzenes also have the ability
to shift the π-π* absorption wavelength up to around
500 nm by the introduction of donor and acceptor units, so-
called push-pull azobenzenes, but such azobenzenes gener-
ally have very short half-lives, owing to the low stability of
the cis form.4 In contrast, the photogenerated cis form of 1
has a high stability for thermal cis-to-trans isomerization.
The rate constant for the thermal cis-to-trans isomerization
of 1 was estimated to be 1.3 × 10-4 s-1 at 70 °C, consistent
with that of azobenzene (k ) 1.3 × 10-4 s-1 at 70 °C).
Derivatives
λmax,1
(nm)
ꢀmax,1
(M-1 cm-1
λmax,2
(nm)
ꢀmax,2
(M-1 cm-1
)
)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
319
352
320
331
319
325
326
324
347
320
346
348
352
348
21 400
24 500
25 900
21 100
22 900
30 500
22 400
23 100
21 100
25 200
25 800
26 000
29 800
30 100
444
492
478
446
448
444
440
390
451
440
428
430
429
420
1851
3677
2190
22698
1512
1432
1235
5915
5038
1600
1969
2050
2493
2271
1
The H NMR signals of 1 were shifted to upper-field
positions after UV and green light irradiation (Figure S2,
Supporting Information), indicating the generation of the cis
form.3d,5 The cis molar ratio of 1 in the PSS, estimated from
1
the integral ratio of the H NMR signals of the ferrocenyl
rotates in solution. It should be noted that there are no
hydrogen bonds between hydroxyl groups in the crystal of
11.
moiety, was 35% upon green light (546 nm) irradiation, and
the PSS was changed into a more cis-rich state (61% cis
molar ratio) upon UV (313 nm) irradiation. The quantum
yield for the trans-to-cis photoisomerization of 1 was
estimated to be 0.021 for UV (313 nm) irradiation, which is
smaller than that of azobenzene [Φtfc ) 0.12 (313 nm
excitation)],6 and a much higher value, 0.51, was obtained
for green light (546 nm) irradiation. The PSS reached a more
trans-rich state (17% cis molar ratio) upon blue light
(436 nm) irradiation, which effectively generates the n-π*
excitation state of the cis form.
Photon-, Electron-, and Proton-Coupled Isomerization
Behavior of 1. Figure 3 shows the spectral change of 1 in
acetonitrile by photoirradiation. A decrease in the intensity
of the π-π* band, derived mainly from the azo moiety, was
observed with isosbestic points at 270 and 395 nm through
UV (313 nm) irradiation, whose wavelength corresponds to
the maximum in the π-π* band. The same spectral change
was observed through green (546 nm) irradiation, whose
wavelength corresponds to the edge of the visible band.
Subsequent irradiation with blue light (436 nm) recovered
the intensity of the azo π-π* band. This spectroscopic
behavior is characteristic of the reversible trans-to-cis
isomerization of azobenzene, indicating that the trans-to-cis
photoisomerization of 1 proceeds not only through UV but
also through green irradiation. It is expected that an MLCT
character plays an important role in isomerization with green
(4) (a) Rau, H. Photochromic Molecules and Systems; Elsevier: Amster-
dam, 1990; Chapter 4. (b) Nishimura, N.; Kosako, S.; Sueishi, Y. Bull.
Chem. Soc. Jpn. 1984, 57, 1617-1625. (c) Nishimura, N.; Sueyoshi,
T.; Yamanaka, H.; Imai, E.; Yamamoto, S.; Hasegawa, S. Bull. Chem.
Soc. Jpn. 1976, 49, 1381-1387. (d) Sueyoshi, T.; Nishimura, N.;
Yamamoto, S.; Hasegawa, S. Chem. Lett. 1974, 1131-1134.
(5) Rudolph-Bo¨hner, S.; Kruger, M.; Oesterhelt, D.; Moroder, L.; Na¨gel,
T.; Wachtveitl, J. J. Photochem. Photobiol., A 1997, 105, 235-248.
(6) Rau, H. J. Photochem. 1984, 26, 221-225.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7549

Sakamoto et al.
Table 2. Crystal Data and Refinement Parameters for 1, 5, and 11
1
5
11
empirical formula
fw
C22H18FeN2
366.24
C22H17ClFeN2
400.69
C22H19FeN2O
382.24
cryst dimensions
cryst syst
lattice params
0.20× 0.20 × 0.20 mm
monoclinic
a ) 11.157(11) Å
b ) 7.528(7) Å
c ) 20.34(2) Å
â ) 96.805(12)°
V ) 1696.3(28) Å3
P21/c (#14)
4
0.40 × 0.20 × 0.10 mm
monoclinic
a ) 10.61(1) Å
b ) 7.825(10) Å
c ) 21.57(3) Å
â ) 92.808(8)°
V ) 1788.3(39) Å3
P21/c (#14)
4
0.40 × 0.30 × 0.10 mm
orthorhombic
a ) 25.55(2) Å
b ) 11.318(7) Å
c ) 5.848(4) Å
V ) 1690.9(18) Å3
Pna21 (#33)
4
space group
Z value
µ (Mo KR)
D (calcd)
F000
8.94 cm-1
9.99 cm-1
9.04 cm-1
1.501 g/cm3
792
1.434 g/cm3
760
0.047
0.090
1.115
1.488 g/cm3
824
0.042
0.099
1.112
a
R1
0.041
0.092
1.000
b
wR2
GOFc
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )2]/∑w(Fo )2]1/2
.
c GOF ) {∑[w(Fo - Fc )2]/∑(No - Nv)2]1/2, where No ) number of observations
2
2
2
2
2
and Nv ) number of variables.
Figure 2. ORTEP diagram of 1 with 50% probability. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å), dihedral angles (deg),
and torsion angles (deg) are as follows: N(1)-N(2) 1.2572, N(1)-C(15)
1.4306, N(2)-C(17) 1.4295, C(11)-C(6) 1.4774, Fe(1)-C(6) 2.0353,
C(6-10)-C(11-16) 16.540, C(15)-N(1)-N(2)-C(17) 175.0(2).
Figure 3. UV-vis absorption spectral change of 1 (5.5 × 10-5 M) in
acetonitrile upon photoirradiation with a monochromatic light at 546 nm
(solid lines) for 21 min and subsequent irradiation with a monochromatic
light at 320 nm (dotted lines) for 4 min. Inset shows the spectral changes
around the isosbestic points.
A cyclic voltammogram of trans-1 in 0.1 M n-Bu4NClO4-
acetonitrile solution exhibited a reversible oxidation wave
due to FeIII/FeII at E0′ ) 0.059 V versus Fc+/Fc (Figure S3,
Supporting Information). The sample solution was photo-
irradiated to generate a trans-cis mixture (35% cis molar
ratio), the oxidation potential of which was determined to
be 0.054 V versus Fc+/Fc (Figure S3), indicating that the
isomerization caused no significant shift in the redox
potential.
The electrochemical oxidation of trans-1 in 0.1 M n-Bu4-
NClO4-acetonitrile from the ferrocene to the ferrocenium
state was realized upon controlled-potential electrolysis at
0.45 V versus Ag/Ag+. Upon oxidation, an absorption
spectral change of the compound was observed. Namely, the
π-π* band shifted to higher energy from λmax ) 319 to
311 nm, an MLCT band around 500 nm decreased, and a
weak LMCT band appeared at 730 nm (Figure S4a, Sup-
porting Information).7 Subsequent controlled-potential elec-
trolysis at -0.1 V versus Ag/Ag+ recovered the initial
absorption spectrum (Figure S4b), indicating the high re-
versibility of this redox reaction. The chemical oxidation of
trans-1 with [Fe(η5-C5H4Cl)2]PF6 8 was also realized, and it
gave the same spectroscopic behavior as was observed upon
electrochemical oxidation.
The trans-to-cis photoisomerization behavior in the fer-
rocenium state was much different from that in the ferrocene
state. UV light irradiation of trans-1+ (FeIII state) in
acetonitrile caused a decrease in the π-π* band intensity,
indicating trans-to-cis isomerization, whereas almost no
spectral change was observed upon exposure to green light.
Photoirradiation to a newly appearing LMCT band did not
cause any photoreaction. These results indicate that the
drastic change in the response to green light is caused by
the elimination of an MLCT band upon oxidation of the
(7) Kurosawa, M.; Nankawa, T.; Matsuda, T.; Kubo, K.; Kurihara, M.;
(8) Horikoshi, T.; Kubo, K.; Nishihara, H. J. Chem. Soc., Dalton Trans.
1999, 3355-3360.
Nishihara, H. Inorg. Chem. 1999, 38, 5113-5123.
7550 Inorganic Chemistry, Vol. 44, No. 21, 2005

Isomerization BehaWior of Ferrocenylazobenzenes
minutes; the azo π-π* band was slightly blue-shifted, the
intensity of the visible band was increased, and a new band
appeared around 730 nm (Figure S5, Supporting Informa-
tion). Although there is a report investigating the spectral
behavior of ferrocenylazobenzenes in acidic solutions,9 to
our knowledge, there are no reports that the stoichiometric
amount of acid completely changes the absorption spectra
of ferrocenylazobenzenes. We have not isolated and char-
acterized the product, but its structure is likely to be one in
which the azo nitrogen is protonated, judging from the
protonation reaction of azobenzene-conjugated metalladi-
thiolenes.3f,3g The cis form of 1 is changed into the trans
form, which is catalyzed by a small amount of CF3SO3H
(Scheme 1; Figure S6, Supporting Information). The rate
constant for the cis-to-trans isomerization of 1 after the
addition of 0.08 equiv of CF3SO3H was estimated to be
4.8 × 10-2 s-1 at 20 °C.
Figure 4. UV-vis absorption spectral change of the following sample
solution upon photoirradiation with a monochromatic light at 546 nm; (inset)
time course change in absorbance at 305 nm of the sample solution in the
dark (a) or upon photoirradiation with a monochromatic light at 546 nm
(b). The sample solution was prepared by photoirradiation with a mono-
chromatic light at 546 nm to 3-ferrocenylazobenzene in acetonitrile
(5.62 × 10-5 M) to reach a photostationary state followed by oxidation
with 1 equiv of 1,1′-dichloroferrocenium hexafluorophosphate.
Isomerization Behavior of 2. The photochemical behavior
of 2 is much different from that of 1. In the UV-vis spectra
of 2 in acetonitrile, a decrease in the azo π-π* band intensity
of this sample solution was observed with an isosbestic point
at 304 nm through UV light (365 nm) irradiation (Figure
5a). Subsequent blue light (436 nm) irradiation, which may
excite the azo n-π* band of the cis form, recovered the azo
π-π* band intensity (Figure 5b), indicating reversible trans-
cis photoisomerization of the azobenzene moiety. A decrease
in the azo π-π* band intensity was also observed through
green light (546 nm) irradiation to a low-lying MLCT band,
as in the case of 1, but the absorption spectrum of the
resulting sample solution was only slightly different from
that of the trans form. The photogenerated cis form has a
relatively high stability for thermal cis-to-trans isomerization,
and the rate constant for the thermal cis-to-trans isomerization
of 2 was estimated to be 1.2 × 10-3 s-1 at 70 °C, an order
of magnitude higher than that of azobenzene or 1. The cis
molar ratios of 2 in the PSS upon UV light (365 nm) and
green light (546 nm) irradiation were 42% and 6%, respec-
tively, which were estimated from the integral ratio of
1H NMR signals of the ferrocenyl moiety (Figure S7,
Supporting Information). The quantum yield for the trans-
to-cis photoisomerization, Φtfc, of 2 was estimated to be
0.0033, which is smaller than that of 1, for UV light
(365 nm) irradiation.
The reversible redox reaction of the FeIII/FeII couple of
trans-2 occurs at E0′ ) 0.16 V versus Ag/Ag+ in 0.1 M
n-Bu4NClO4-acetonitrile solution. Electrochemical oxidation
caused an absorption spectral change in which the π-π*
band shifted to higher energy from λmax ) 352 to 338 nm;
an MLCT band around 500 nm decreased, and a weak LMCT
band appeared at 769 nm (Figure S8, Supporting Informa-
tion).
When the oxidizing agent, [Fe(η5-C5H4Cl)2]PF6, was added
to the acetonitrile solution of the photogenerated trans-cis
mixture of 2 at room temperature, the observed absorption
spectrum was the same as that of the trans form. This result
metal center and that the MLCT character plays an important
role in isomerization with green light in the ferrocene state.
The cis-to-trans thermal isomerization rate in the ferrocenium
state estimated by UV-vis spectral change was 8.7 ×
10-4 s-1 at 70 °C (Figure 4).
It is expected that the MLCT character makes no contribu-
tion to the ferrocenium state, and thus, cis-to-trans isomer-
ization through the n-π* excitation state proceeds mainly
upon green light irradiation, achieving a very trans-rich state
with green light irradiation in the PSS. The large difference
in the cis molar ratio in the PSS between the FeII and FeIII
states suggests the possibility of reversible trans-to-cis
conversion upon irradiation with a single green light by
changing the oxidation state of the iron center. The following
experiments proved this possibility. An acetonitrile solution
of trans-1 of FeII was irradiated with green light (546 nm)
to reach the PSS (35% cis molar ratio), and the resulting
mixture of trans and cis forms was oxidized to the FeIII state
immediately after a stoichiometric amount of [Fe(η5-C5H4-
Cl)2]PF6 was added. After the oxidation, the recovery of the
π-π* band absorbance was not pronounced in intensity after
several hours in the dark at room temperature, and the
thermal isomerization to the trans form proceeded very
slowly in the FeIII state, as noted above. Green light
irradiation promoted the increase in absorbance, which
reached the trans-rich PSS characteristic of the ferrocenium
state, suggesting that almost all of the trans form was
recovered (Figure 4). These results indicate that reversible
trans-to-cis isomerization can be achieved by a combination
of “on-off switching” of the MLCT character due to the
FeIII/FeII redox change and irradiation with a single green
light source (Scheme 1).
The protonation behavior of 1 was investigated. When 1
equiv of CF3SO3H was added to an acetonitrile solution of
1, the UV-vis absorption spectrum changed within a few
(9) Little, W. F.; Berry, R. A.; Kannan, P. J. Am. Chem. Soc. 1962, 84,
2525-2529.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7551

Sakamoto et al.
Figure 5. (a) UV-vis absorption spectral change of 2 (5.46 × 10-5 M) in acetonitrile upon photoirradiation with a monochromatic light at 365 nm for
14 min and (b) subsequent irradiation with a monochromatic light at 405 nm.
Scheme 1. Redox- and Proton-Conjugated Photoisomerization Pathway
Scheme 2. Resonance Structures of 2
The protonation behavior of 2 was also examined. An
acetonitrile solution of 2 showed a UV-vis absorption
spectral change upon the addition of 1.1 equiv of CF3SO3H.
Immediately after the addition of CF3SO3H, the absorbance
around 500 nm was increased in intensity and a new band
appeared around 800 nm (Figure S9, Supporting Informa-
tion), probably forming the monoprotonated form, as was
the case for 1. Proton-catalyzed cis-to-trans isomerization
was also observed. The rate constant for cis-to-trans isomer-
ization after the addition of 0.08 equiv of CF3SO3H was
estimated to be 1.4 × 10-2 s-1 at 20 °C (Figure S10,
Supporting Information).
Isomerization Behavior of 3. Another geometric isomer
in the ferrocenylazobenzene family, 3, was also employed
for the isomerization study. The absorption spectral change
of 3 in acetonitrile was observed upon UV light irradiation
(313 nm; Figure S11, Supporting Information), but the
decrease in intensity of the π-π* band was extremely small.
Subsequent violet light irradiation (405 nm) caused a back
reaction. Green light irradiation (546 nm) to the MLCT band
also caused trans-to-cis isomerization, though only to a small
extent. The cis molar ratios in the PSS upon UV light
(313 nm) and green light (546 nm) irradiation were estimated
to be 5% and 7%, respectively, judging from the integral
ratio of the 1H NMR signal (Figure S12, Supporting
Information).
implies very fast cis-to-trans thermal isomerization in the
ferrocenium state. When chemical oxidation was carried out
at 5 °C, the absorption spectra of the cis-trans mixture
changed, and the rate constant of this change was estimated
to be 3.8 × 10-3 s-1. The low stability of cis-2+ (FeIII state)
is attributed to the large delocalization of electrons on the
conjugated ligand, causing a canonical conformation with a
η6-fulvene-type structure10 and decreasing the double-bond
character of NdN (Scheme 2). This is reasonable because
the cis form of 1 in the ferrocenium state, which cannot give
a canonical structure, has a higher thermal stability than 2,
as noted above.
(10) (a) Murata, M.; Fujita, T.; Yamada, M.; Kurihara, M.; Nishihara, H.
Chem. Lett. 2000, 1328-1329. (b) Murata, M.; Yamada, M.; Fujita,
T.; Kojima, K.; Kurihara, M.; Kubo, K.; Kobayashi, Y.; Nishihara,
H. J. Am. Chem. Soc. 2001, 123, 12903-12904.
Excited-State Calculations of 1 and 2. It is of interest
to understand why green light irradiation caused a much
7552 Inorganic Chemistry, Vol. 44, No. 21, 2005

Isomerization BehaWior of Ferrocenylazobenzenes
Table 3. Experimental and Theoreticala Excitation Energies to the
Lowest Excited State among Those of Similar Character
the oxidized form with green light occurs through the n-π*
excited state.
excitation energy/eV
Photoisomerization Behaviors of Derivatives of 1.
Among the three geometric isomers of ferrocenylazobenzene,
1 exhibits the most significant isomerization behavior in
response to green light. But the cis molar ratio obtained
through green light irradiation was moderate, 35%. To
improve the cis molar ratio in the PSS, we examined the
effects of electron-withdrawing and -donating substituents
on two phenyl rings. The results are summarized in Table
4.
As for the derivatives (12, 13, 14) in which an electron-
donating group is introduced at the 4′ position, the cis molar
ratio at 365 nm irradiation was higher than it was with 1,
but that at 546 nm irradiation was lower than that with 1. In
contrast, the electron-withdrawing -OSO2C6H4CH3 group
at the 4′ position did not affect the cis molar ratio
significantly. It should be noted that the OH-substituted
derivative, 11, exhibited no spectral changes by irradiation
with either UV or visible light. It has been reported that the
cis-to-trans isomerization of hydoxyazo compounds was
extremely fast in polar solvents, because the tautomerism
of azo and hydrazone structures lowers the activation
energy.4,11,12
oscillator strength
(theor)
assignment
exptl
theor
(CI coefficient)b
trans-1
2.43
3.02
3.49
0.001
63cf 67 (0.55)
65 f 67 (-0.33)
64 f 67 (0.65)
62 f 67 (0.19)
62 f 67 (0.59)
2.79
3.90
0.060
0.693
cis-1
2.45
3.16
3.96
0.047
64 f 67 (0.56)
63 f 67 (-0.21)
62 f 68 (0.35)
63 f 67 (0.31)
58 f 67 (0.58)
0.001
0.103
trans-2
<0.001
0.113
2.42
2.63
63 f 67 (0.66)
66 f 67 (0.57)
66 f 68 (0.22)
61 f 68 (-0.23)
64 f 67 (0.55)
2.51
3.52
3.03
0.752
a TD-DFT calculations with three-parametrized Becke-Lee-Yang-Parr
(B3LYP) hybrid exchange-correlation functional. b The expansion coef-
ficient for the excited-state wave function. c The numbers of molecular
orbitals refer to those displayed in Figure 6.
higher cis molar ratio in 1 than it did in 2. Time-dependent
density functional theory (TD-DFT) calculations for 1 in the
trans and cis forms and for 2 in the trans form were carried
out to investigate the singlet excited state in which the
isomerization occurs. The calculated excitation energies in
the trans form were in reasonable agreement with the
experimental values, and the observed trends in the experi-
mental absorption spectra were correctly reproduced (Table
3). The noticeable features in the nature of the trans-1 excited
states are that the azo n-π* strongly mixes with the MLCT
configuration and that the ground state orbital for the
3.02 eV MLCT state (number 64) is delocalized over Fe and
the Cp ring rather than localized on the iron (Figure 6). The
presence of the MLCT character is the reason the molar
extinction coefficient of the visible band (λmax ) 444 nm,
ꢀ ) 1.86 × 103 M-1 cm-1) is much larger than that of the
n-π* band of trans-azobenzene (λmax ) 444 nm, ꢀ )
5.15 × 102 M-1 cm-1). The origin of the visible band in 1
is different from that of 2, because the ground state orbital
for the 2.51 eV MLCT state of the latter is localized on the
iron (Figure 6). It seems that isomerization through the “pure”
MLCT excitation state barely proceeds; the initial orbital of
the MLCT of 1 (number 66) is not a pure metal-based d
orbital, but the ligand character, as in the azobenzene moiety,
is mixed. It is possible that the green-irradiation-induced
trans-to-cis isomerization of 1 occurs on the potential energy
surface for the MLCT excited state. In fact, the almost
complete absence of a response to green light for the cis
formation in the ferrocenium state is caused by the disap-
pearance of the MLCT character in the ferrocene state upon
oxidation to the ferrocenium state. The appearance of a n-π*
band at 2.45 eV for cis-1 in the calculation (see Table 3)
supports the idea that the cis-to-trans isomerization of 1 in
When the ortho position to the ferrocenyl group was
substituted with an electron-donating methyl group (7) or
an electron-withdrawing chloro group (6), the UV and green
light photoisomerization behaviors were similar to those of
1. In contrast, when the para position to the ferrocenyl group
was substituted with the chloro group (5), the highest value
of the cis molar ratio at 546 nm was attained. As the
difference in energies between π-π* and MLCT bands
(νmax1 - νmax2 in Table 3) is largest for 5 among all the
derivatives in this study, we assume that the n-π* and
MLCT bands are less overlapped, provided that the energies
of the π-π* and n-π* bands are shifted in parallel.
Consequently, the trans-to-cis isomerization promoted by the
MLCT excitation is least perturbed by the cis-to-trans
isomerization by the n-π* excitation.
A derivative with two ferrocenyl moieties, 4, was also
employed. The intensity of the MLCT band was greater than
that with 1 (the molar extinction coefficient at 446 nm, ꢀ446
,
is 2698 for 4, whereas ꢀ444 ) 1851 for 1), but the cis molar
ratio in the PSS upon green light irradiation (546 nm) was
29%, which was not improved compared to that of 1. This
is probably because the π-π* band shifted to a longer
wavelength, and accordingly, the n-π* band shifted in the
same direction and overlapped more effectively with the
MLCT band.
Conclusion
3-Ferrocenylazobenzene, 1, showed reversible photo-
isomerization with UV and blue light irradiation. Excitation
of the MLCT band by green light irradiation also caused
(11) Hartley, G. S. J. Chem. Soc. 1938, 633-642.
(12) Brode, W. R.; Gould, J. H.; Wyman, G. M. J. Am. Chem. Soc. 1952,
74, 4641-4646.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7553

Sakamoto et al.
Figure 6. The molecular orbitals contributing to the main UV-vis absorption bands of (a) trans-1, (b) cis-1, and (c) trans-2.
Table 4. cis Molar Ratiosa of Ferrocenylazobenzene Derivatives in the
responses to green light between the Fe(II) and Fe(III) states
allowed us to realize redox-conjugated reversible isomer-
ization with a single green light source. Compound 1 has a
large affinity to protons, and cis-to-trans isomerization was
accelerated by a small amount of TfOH. A variety of
responses to external perturbations such as photons, redox,
and protons created a multimodal isomerization route with
a combination of those responses.
PSS on Monochromatic Light Irradiation
excitation wavelength/nm
313
365
436
17%
546
1
61 (45)%
35 (26)%
6%
7%
2
42%
3
5%
4
35%
(7)%
29 (17)%
47 (36)%
22 (19)%
20 (17)%
∼0%
5
6
7
39 (38)%
56 (50)%
67 (52)%
16 (15)%
15 (13)%
19 (16)%
∼0%
4-Ferrocenylazobenzene, 2, showed reversible trans-cis
photoisomerization with UV and visible light irradiation. The
cis form of 2 has a high stability in the Fe(II) state, and its
thermal stability was controlled by an oxidation state of the
metal center, which affected the electronic state of the azo
moiety through the conjugated chain. In the Fe(III) state,
the rate constant for thermal cis-to-trans isomerization was
drastically larger than that in the Fe(II) state. The azo moiety
of 2 also showed a large affinity to protons because of the
strong electron-donating ability of the ferrocenyl moiety. A
proton-coupled cis-to-trans isomerization route was con-
structed for 2. 2-Ferrocenylazobenzene, 3, underwent revers-
ible photoisomerization with UV and visible light irradiation,
but the amount of the cis form photogenerated was extremely
8
9
∼0%
∼0%
∼0%
∼0%
10
11
12
13
14
(48)%
(18)%
(∼0)%
(8)%
(∼0)%
(68)%
(77)%
(81)%
(∼0)%
(22)%
(22)%
(35)%
(9)%
(2)%
a The cis molar ratios were obtained from the integral ratio of 1H NMR
signals of the ferrocenyl moiety. The numbers in parentheses were obtained
using UV-vis spectral changes.
trans-to-cis photoisomerization. Only trans-to-cis efficient
isomerization with MLCT excitation was observed for the
case of 1. The response to green light was altered by
switching the oxidation state of the ferrocenyl moiety, which
controls the existence of an MLCT band. The different
7554 Inorganic Chemistry, Vol. 44, No. 21, 2005

Isomerization BehaWior of Ferrocenylazobenzenes
temperature for 2 h, then poured into water (100 mL) cooled on an
ice bath. A solution of sodium nitrite (1.06 g, 15.4 mmol) in water
at 0 °C was added dropwise with stirring to a solution of 2-chloro-
5-nitroaniline (2.50 g, 14.5 mmol) in 1:1 water/hydrochloric acid
(10 mL) kept at 0 °C by an ice bath, and the mixture was stirred
for 30 min to ensure complete diazotization. The diazonium solution
was added to a ferrocenium solution at 0 °C with vigorous stirring.
Then, copper powder (1.00 g) was added to the solution as a catalyst
and allowed to warm to room temperature. After 24 h of stirring,
the nitrogen effervescence ceased, and ascorbic acid (5.00 g) was
added to reduce the unreacted ferrocenium to ferrocene. Dichlo-
romethane was added, and the solution was filtered through Celite.
The organic layer was separated, and the aqueous layer was
extracted with dichloromethane. The combined organic extracts
were dried over sodium sulfate and filtered, and the solvent was
removed. The dark solid was purified by column chromatography
with an increasing proportion of dichloromethane in hexane as an
eluent. The first yellow-orange band was eluted with pure hexane
and yielded unchanged ferrocene. The second band, eluted with
50% dichloromethane/hexane, was collected. Removal of the
solvent, followed by recrystallization from dichloromethane/hexane,
afforded a5. Yield: 32% (1.61 g, 4.7 mmol). 1H NMR (270 MHz,
CDCl3): δ 7.91 (d, J ) 2.2 Hz, 1H, C6H3), 7.57 (dd, J ) 8.1 Hz,
J ) 2.2 Hz, 1H, C6H3), 7.43 (d, J ) 8.1 Hz, C6H3), 4.66 (t, J )
1.9 Hz, 2H, C5H4), 4.42 (t, J ) 1.9 Hz, 2H, C5H4), 4.07 (s, 5H,
C5H5).
small. The introduction of an electron-withdrawing substi-
tuent to 1 improved the efficiency of photoisomerization with
green light.
Experimental Section
Materials. Ferrocene was purchased from Kanto Chemicals and
recrystallized from hexane before use. 1,8-Dibromooctane was
purchased from Aldrich. 2-Methyl-5-nitroaniline, 4-chloro-3-nitro-
aniline, 2-chloro-5-nitroaniline, 4-nitro-2-aminophenol, 2-nitro-4-
aminophenol, 2,6-dibromo-4-nitroaniline, and dichlorobis(triphenyl-
phosphine)palladium(II) were purchased from Tokyo Kasei Kogyo
and used as received. Sodium nitrite and nitrobenzene were
purchased from Wako Pure Chemical Industries and used as
received. Zinc powder was purchased from Koso Chemical, washed
with hydrochloric acid, and then evaporated before use. Tin powder
was purchased from Koso Chemical and used as received. Other
reagents and solvents were purchased from Kanto Chemicals and
used as received. Spectroscopic-grade acetonitrile (Aldrich) and
spectroscopic-grade dichloromethane (Kanto Chemicals) were used
for UV-vis absorption spectroscopy. The acetonitrile used for the
electrochemical measurements was of HPLC grade (Kanto Chemi-
cals). n-Tetrabutylammonium perchlorate (lithium battery grade)
was purchased from Tomiyama Chemical and recrystallized from
HPLC-grade ethanol before use. 3-Ferrocenylnitrobenzene (a1),13-15
2-ferrocenylnitrobenzene (a3),14,16-18 3,5-diferrocenylnitrobenzene
(a4),19 2-ferrocenyl-4-nitrotoluene (a7),20 ferrocenylboronic acid,21
3,5-dibromonitrobenzene,19 3-ferrocenylaniline (b1),5,20 2-ferro-
cenylaniline (b3),18 3,5-di-ferrocenylaniline (b4),15 nitrosobenzene,22
and 1,1′-dichloroferrocenium hexafluorophosphate23,24 were pre-
pared according to the methods in the literature.
In the same way, 4-chloro-3-ferrocenylnitrobenzene (a6), 4-fer-
rocenyl-2-nitrophenol (a8), and 2-ferrocenyl-4-nitrophenol (a9)
were prepared in 20% yield (1.24 g) from 4-chloro-3-nitroaniline
(2.42 g, 14.0 mmol), 36% yield (1.54 g) from 2-nitro-4-aminophenol
(2.06 g, 13.4 mmol), and 22% yield (0.99 g) from 2-amino-4-
nitrophenol (2.17 g, 14.1 mmol), respectively. Data for a6. 1H NMR
(400 MHz, CDCl3): δ 8.51 (d, J ) 2.68 Hz, 1H, C6H3), 7.97 (dd,
J ) 8.9 Hz, J ) 2.81 Hz, 1H, C6H3), 7.49 (d, J ) 8.78 Hz, C6H3),
4.83 (s, 2H, C5H4), 4.44 (s, 2H, C5H4), 4.18 (s, 5H, C5H5). Data
Apparatus. UV-vis absorption spectra were obtained on an
8453 UV-visible Spectroscopy System (Hewlett-Packard) or a
1
V-570 spectrophotomer (Jasco). H NMR spectra were obtained
on an EX270 or JNM-AL400 spectrometer (both by JEOL).
Photoirradiation experiments were carried out using a Ushio 500W
super-high-pressure mercury lamp, the USH-500D, as a light source.
The monochromatic light was provided by a JASCO CT-10T
monochromator.
Ferrocenylnitrobenzene Derivatives. (Scheme S1A, Supporting
Information, illustrates the synthesis of new ferrocenylazobenzenes.)
2-Ferrocenyl-5-chloronitrobenzene (a5) was prepared according to
a report on the synthesis of 3-ferrocenylnitrobenzene (a1).13-15
Ferrocene (4.00 g, 21.5 mmol) was added to sulfuric acid (15 mL).
The resulting deep blue ferrocenium solution was stirred at room
1
for a8. H NMR (270 MHz, CDCl3): δ 10.53 (s, 1H, OH), 8.13
(d, J ) 2.1 Hz, 1H, C6H3), 7.71 (dd, J ) 8.9 Hz, J ) 2.1 Hz, 1H,
C6H3), 7.11 (d, J ) 8.9 Hz, 1H, C6H3), 4.62 (t, J ) 1.9 Hz, 2H,
C5H4), 4.36 (t, J ) 1.9 Hz, 2H, C5H4), 4.06 (s, 5H, C5H5). Data
1
for a9. H NMR (270 MHz, CDCl3): δ 8.14, 8.10, 8.01 (s, 1H,
OH), 7.03 (d, J ) 8.1 Hz), 4.57 (t, J ) 1.6 Hz, 2H, C5H4), 4.51 (t,
J ) 1.6 Hz, 2H, C5H4), 4.28 (s, 5H, C5H5).
Ferrocenylaniline Derivatives. 2-Chloro-5-ferrocenylaniline
(b5) was prepared according to the literature.15 A mixture of a5
(1.00 g, 2.93 mmol) and tin (3.94 g, 33.2 mmol) in 1:1 hydrochloric
acid/ethanol (100 mL) was stirred under reflux for 1 h. The resulting
orange solution was then cooled to room temperature and neutral-
ized with aqueous sodium hydroxide. Dichloromethane was added,
the solution was filtered with Celite, and the organic layer was
separated. The aqueous layer was extracted with dichloromethane.
The combined organic extracts were dried over sodium sulfate and
filtered. Removal of the solvent followed by recrystallization from
dichloromethane/hexane yielded 3-ferrocenylaniline as a yellow
solid. Yield: 69% (0.63 g, 2.0 mmol). 1H NMR (270 MHz,
CDCl3): δ 7.15 (d, J ) 8.4 Hz, 1H, C6H3), 6.88-6.82 (m, 2H,
C6H3), 4.57 (t, J ) 1.9 Hz, 2H, C5H4), 4.29 (t, J ) 1.9 Hz, 2H,
C5H4), 4.04 (s, 5H, C5H5).
(13) Nesmeyanov, A. N.; Perevalova, E. G.; Golovnya, R. V.; Shilovtseva,
L. S. Dokl. Akad. Nauk S. S. S. R. 1955, 102, 535-538.
(14) Roberts, R. M. G.; Silver, J.; Yamin, B. J. Organomet. Chem. 1984,
270, 221-228.
(15) Campo, J. A.; Cano, M.; Heras, J. V.; Lo`pez-Garabito, C.; Pinilla, E.;
Torres, R.; Rojo, G.; Agullo´-Lo´pez, F. J. Mater. Chem. 1999, 9, 899-
907.
(16) Roberts, R. M. G.; Silver, J.; Yamin, B. M.; Drew, M. G. B.; Eberhardt,
U. J. Chem. Soc., Dalton Trans. 1988, 1549-1556.
(17) Lanez, T.; Pauson, P. L. J. Chem. Soc., Perkin Trans. 1 1990, 2437-
2442.
(18) Patoux, C.; Coudret, C.; Launay, J.-P.; Joachim, C.; Gourdon, A. Inorg.
Chem. 1997, 36, 5037-5049.
(19) Little, W. F.; Reilley, C. N.; Johnson, J. D.; Lynn, K. N.; Sanders, A.
P. J. Am. Chem. Soc. 1964, 86, 1376-1381.
Similarly, 4-chloro-3-ferrocenylaniline (b6) and 3-ferrocenyl-4-
toluidine (b7) were prepared in yields of 91% (1.34 g) from a6
(1.61 g, 4.71 mmol) and 60% (0.85 g) from a7 (1.42 g, 4.41 mmol),
respectively. Data for b6. 1H NMR (270 MHz, CDCl3): δ 7.11 (d,
J ) 8.6 Hz, 1H, C6H3), 6.96 (d, J ) 2.7 Hz, 1H, C6H3), 6.50 (dd,
J ) 8.6, 2.7 Hz, 1H, C6H3), 4.72 (t, J ) 1.9 Hz, 2H, C5H4), 4.31
(20) Nesmeyanov, A. N. Proc. R. Soc. London 1955, 246, 495-503.
(21) Sawamura, M.; Sakaki, H.; Nakata, T.; Ito, Y. Bull. Chem. Soc. Jpn.
1993, 66, 2725-2729.
(22) Lutz, R. E.; Lytton, M. R. J. Org. Chem. 1937, 2, 68-75.
(23) Conway, B. G.; Rausch, M. D. Organometallics 1985, 4, 688-693.
(24) Horikoshi, T.; Kubo, K.; Nishihara, H. J. Chem. Soc., Dalton Trans.
1999, 3355-3360.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7555

Sakamoto et al.
(t, J ) 1.9 Hz, 2H, C5H4), 4.12 (s, 5H, C5H5), 3.66 (br, 2H, NH2).
Data for b7. H NMR (270 MHz, CDCl3): δ 7.04 (d, J ) 13.5
Hz, 1H, C6H3), 6.71 (d, J ) 13.5 Hz, 2H, C6H3), 4.64 (t, J ) 1.9
Hz, 4H, C5H4), 4.31 (t, J ) 1.9 Hz, 4H, C5H4), 4.08 (s, 10H, C5H5),
3.76 (br, 2H, NH2).
Anal. Calcd for C22H18N2Fe: C, 72.15; H, 4.95; N, 7.65. Found:
C, 72.16; H, 5.01; N, 7.68. 1H NMR (500 MHz, CD3CN): δ 7.90
(d, J ) 8.0 Hz, 2H), 7.85 (d, J ) 8.5 Hz, 2H), 7.70 (d, J ) 8.5 Hz,
2H), 7.59-7.50 (m, 3H), 4.82 (t, J ) 2.0 Hz, 2H, C5H4), 4.43 (t,
J ) 2.0 Hz, 2H, C5H4), 4.06 (s, 5H, C5H5).
1
5-Ferrocenyl-2-hydroxyazobenzene (8). A mixture of b8
(492 mg, 1.49 mmol) and 5 (160 mg, 1.50 mmol) in a small portion
of triethylamine and glacial acetic acid was stirred at room
temperature for 24 h. The solvent was removed in vacuo, and the
dark solid was purified by column chromatography with an
increasing proportion of chloroform in hexane as an eluent. The
second band, which was eluted with hexane/chloroform (1:2), was
collected and recrystallized from dichloromethane/hexane. Yield:
23% (131 mg, 0.343 mmol). Anal. Calcd for C22H18N2OFe: C,
2-Amino-4-ferrocenylphenol Hydrochloride (b8). A mixture
of a8 (1.00 g, 3.09 mmol) and tin powder (1.00 g, 8.42 mmol) in
1:1 hydrochloric acid/ethanol (20 mL) was stirred under reflux for
1 h. The resulting deep reddish solution was cooled, and orange
solid was precipitated. The solid was filtered off and washed with
a small portion of cold methanol and chloroform. Yield: 74%
(0.75 g, 2.28 mmol). H NMR (270 MHz, CDCl3): δ 7.47-7.43
(m, 2H, C6H3), 6.94 (d, J ) 8.1 Hz, 1H, C6H3), 4.63 (br, 2H, C5H4),
4.32 (br, 2H, C5H4), 4.03 (s, 5H, C5H5).
4-Amino-2-ferrocenylphenol Hydrochloride (b9). This com-
pound was prepared using a9 (0.99 g, 3.06 mmol) in a manner
similar to that described for b8. Yield: 49% (492 mg, 1.49 mmol).
1H NMR (270 MHz, CD3OD): δ 7.47 (d, J ) 2.7 Hz, 1H, C6H3),
7.03 (dd, J ) 8.6, 2.7 Hz, 1H, C6H3), 6.89 (d, J ) 8.6 Hz, 1H,
C6H3), 4.35 (br, 2H, C5H4), 4.06 (s, 5H, C5H5).
3-Ferrocenylazobenzene (1) and its Isomer and Derivatives.
A mixture of b1 (1.40 g, 5.05 mmol) and nitrosobenzene (1.02 g,
9.53 mmol) in glacial acetic acid (30 mL) was stirred at room
temperature for 24 h. The solvent was removed in vacuo, and the
dark solid was purified by column chromatography with 1:1 hexane/
dichloromethane as the eluent. After the first band was collected,
a deep-purple solid obtained upon evaporation was recrystallized
from dichloromethane/hexane. Yield: 24% (437 mg, 1.19 mmol).
Anal. Calcd for C22H18N2Fe: C, 72.15; H, 4.95; N, 7.65. Found:
C, 72 44; H, 5.09; N, 7.83. 1H NMR (270 MHz, CD3CN): δ 7.94-
7.87 (m, 3H), 7.63-7.42 (m, 5H), 7.30 (t, J ) 8.1 Hz, 1H), 4.80
(t, J ) 1.9 Hz, 2H, C5H4), 4.40 (t, J ) 1.9 Hz, 2H, C5H4), 4.02 (s,
5H, C5H5).
1
1
69.13; H, 4.75; N, 7.33. Found: C, 69.04; H, 4.83; N, 7.33. H
NMR (270 MHz, CDCl3): δ 12.85 (s, 1H, OH), 8.04 (d, J ) 2.2
Hz, 1H), 7.91 (dd, J ) 8.1, 1.6 Hz, 2H), 7.59-7.48 (m, 4H), 6.99
(d, J ) 8.6 Hz, 1H), 4.67 (t, J ) 1.9 Hz, 2H, C5H4), 4.33 (t, J )
1.9 Hz, 2H, C5H4), 4.08 (s, 5H, C5H5).
3-Ferrocenyl-4-hydroxyazobenzene (9). 9 was prepared from
b9 (488 mg, 1.48 mmol) in a manner similar to that of 8. Yield:
30% (170 mg, 0.44 mmol). Anal. Calcd for C22H18N2OFe: C, 69.13;
1
H, 4.75; N, 7.33. Found: C, 68.46; H, 4.86; N, 7.12. H NMR
(270 MHz, CD3CN): δ 8.07 (d, J ) 2.4 Hz, 1H), 7.89-7.85 (m,
2H), 7.71 (dd, J ) 8.4, 2.4 Hz, 1H), 7.59-7.46 (m, 3H), 7.02 (d,
J ) 8.4 Hz, 1H), 4.83 (t, J ) 1.9 Hz, 2H, C5H4), 4.38 (t, J )
1.9 Hz, 2H, C5H4), 4.07 (s, 5H, C5H5).
4-(3-Ferrocenyl-phenylazo)phenyl p-Toluenesulfonate (10).
4-Nitrosophenol p-toluenesulfonate was prepared from 4-nitro-
phenol p-toluenesulfonate21 (3 g, 10.2 mmol), and a mixture
containing unreacted 4-nitrophenol p-toluenesulfonate was used in
the next step. 10 was obtained from mixing 4-nitrosophenol
p-toluenesulfonate and 3-ferrocenylaniline in acetic acid and
chloroform. The crude product was purified using silica column
chromatography in hexane/chloroform (1:1) and then was recrystal-
lized from dichloromethane/hexane. Anal. Calcd for C29H25-
FeN2O3S: C, 64.93; H, 4.51; N, 5.22. Found: C, 64.82; H, 4.68;
N, 5.18. 1H NMR (CDCl3): δ 2.441 (s, 3H), 4.05 (s, 5H), 4.35 (s,
2H), 4.72 (s, 2H), 7.13 (d, J ) 6.93, 2H), 7.31 (d, J ) 8.25, 2H),
7.40 (t, J ) 7.918, 1H), 7.58 (d, J ) 8.25, 2H), 7.68 (d, J ) 7.92,
1H), 7.86 (d, J ) 8.91, 2H), 7.97 (s, 1H).
Similarly, 4, 5, 6, and 7 were prepared in 30% yield (242 mg)
from b4 (625 mg, 2.01 mmol), 19% yield (327 mg) from b5 (1.34
g, 4.30 mmol), 20% yield (202 mg) from b6 (850 mg, 2.65 mmol),
and 21% yield (478 mg) from b7 (191 mg, 0.41 mmol), respec-
tively. Data for 4. Anal. Calcd for C32H26N2Fe2: C, 69.85; H, 4.76;
1
N, 5.09. Found: C, 69.77; H, 4.76; N, 5.10. H NMR (270 MHz,
CD3CN): δ 7.98-7.59 (m, 8H), 4.90 (t, J ) 1.9 Hz, 4H, C5H4),
4.41 (t, J ) 1.9 Hz, 4H, C5H4), 4.09 (s, 10H, C5H5). Data for 5.
Anal. Calcd for C22H17N2ClFe: C, 65.95; H, 4.28; N, 6.99.
Found: C, 65.77; H, 4.48; N, 6.84. 1H NMR (270 MHz, CD3CN):
δ 8.00-7.96 (m, 2H), 7.78 (d, J ) 2.4 Hz, 1H), 7.69-7.53 (m,
5H), 4.76 (t, J ) 1.9 Hz, 2H, C5H4), 4.38 (t, J ) 1.9 Hz, 2H, C5H4),
4.05 (s, 5H, C5H5). Data for 6. Anal. Calcd for C22H17N2ClFe: C,
4-(3-Ferrocenylphenylazo)-phenol (11).25 Under a nitrogen
atmosphere, a mixture of 10 (1 g, 1.9 mol) and potassium hydroxide
(2.2 g, 22.2 mol) in ethanol (36 mL) and water (36 mL) was stirred
under reflux for 1 h. The resulting orange solution was then cooled
to room temperature and neutralized with hydrochloric acid. Diethyl
ether was added and then filtered with Celite, and the organic layer
was separated. The aqueous layer was extracted with diethyl ether.
The combined organic extracts were dried over sodium sulfate and
filtered. Removal of the solvents followed by recrystallization from
dichloromethane/hexane yielded an orange solid. Yield: 79%
(0.56 g, 0.44 mmol). Anal. Calcd for C22H18FeN2O: C, 66.02; H,
5.04; N, 7.00. Found: C, 66.11; H, 4.95; N, 6.90. 1H NMR
(CDCl3): δ 4.05 (s, 3H), 4.34 (s, 5H), 4.72 (s, 2H), 5.28 (s, 2H),
6.95 (d, J ) 8.58, 2H), 7.39 (t, J ) 7.59, 1H), 7.54 (d, J ) 7.59,
1H), 7.67 (d, J ) 8.58, 1H), 7.89 (d, J ) 8.91, 2H), 7.96 (s, 1H).
[4-(2-Bromo-octyloxy)-phenyl]-(3-ferrocenyl-phenyl)diaz-
ene (12).26 Under a nitrogen atmosphere, 11 (500 mg, 1.3 mol),
potassium carboxide (0.5 g, 5.0 mol), and 1,8-dibromooctane
1
65.95; H, 4.28; N, 6.99. Found: C, 65.97; H, 4.41; N, 6.96. H
NMR (270 MHz, CD3CN): δ 8.32 (d, J ) 2.4 Hz, 1H), 7.95 (m,
2H), 7.71 (dd, J ) 8.6, 2.4 Hz, 1H), 7.63-7.53 (m, 4H), 4.86 (t,
J ) 1.9 Hz, 2H, C5H4), 4.42 (t, J ) 1.9 Hz, 2H, C5H4), 4.18
(s, 5H, C5H5). Data for 7. Anal. Calcd for C23H20N2Fe: C, 72.65;
1
H, 5.30; N, 7.37. Found: C, 72.44; H, 5.40; N, 7.34. H NMR
(270 MHz, CD3CN): δ 8.30 (d, J ) 1.9 Hz, 1H, C6H3), 7.95-
7.90 (m, 2H, C6H5), 7.67 (dd, J ) 8.1, 1.9 Hz, 1H, C6H3), 7.62-
7.50 (m, 3H, C6H5), 7.33 (d, J ) 8.1 Hz, 1H, C6H3), 4.63 (t, J )
1.9 Hz, 2H, C5H4), 4.38 (t, J ) 1.9 Hz, 2H, C5H4), 4.16 (s, 5H,
C5H5), 2.46 (s, 3H, CH3).
4-Ferrocenylazobenzene (2). This compound was prepared in
the way described for a5 but starting from 4-aminoazobenzene.
Because of 4-aminoazobenzene’s low solubility in the water/
hydrochloric acid mixture, 4.20 g of 4-aminoazobenzene was
dissolved in a mixture of 10 mL of acetic acid and 10 mL of sulfuric
acid, whereupon it was diazotized. Yield: 17% (1.33 g, 3.63 mmol).
(25) Wolfrom, M. L.; Koos, E. W.; Bhat, H. B. J. Org. Chem. 1967, 32,
1058-1060.
(26) Boden, N.; Bushby, R. J.; Martim, P. S.; Evans, R. W.; Smith, D. A.
Langmuir 1999, 15, 3790-3797.
7556 Inorganic Chemistry, Vol. 44, No. 21, 2005

Isomerization BehaWior of Ferrocenylazobenzenes
λiIλiΘ(tn)
Aλ (tn,λo) ) Aλ (∞,λo) - [Aλ (∞,λo) - Aλ (0,λo)] e-p
(25 mL, 62.7 mmol) in dried ethanol (100 mL) were stirred and
kept at 60 °C for 24 h. The resulting orange solution was then
cooled to room temperature. Dichloromethane was added and then
filtered with Celite, and the organic layer was separated. The
aqueous layer was extracted with dichloromethane. The combined
organic extracts were dried over sodium sulfate and filtered. The
dark solid was purified by column chromatography with an
increasing proportion of dichloromethane in hexane as an eluent.
Removal of the solvent followed by recrystallization from dichlo-
romethane/hexane yielded an orange solid. Yield: 81% (0.60 g,
0.44 mmol). Anal. Calcd for C30H33BrFeN2O: C, 62.85; H, 5.80;
N, 4.89. Found: C, 63.06; H, 5.81; N, 4.70. 1H NMR (CDCl3): δ
3.40 (s, 2H), 4.03 (s, 5H), 4.35 (s, 2H), 4.73 (s, 2H), 5.28 (s, 1H),
6.99 (d, J ) 8.91, 1H), 7.39 (t, J ) 7.59, 1H), 7.53 (d, J ) 6.93,
1H), 7.68 (d, J ) 7.56, 1H), 7.92 (d, J ) 8.90, 1H), 7.953 (s, 1H).
MALDI-TOF MS (m/z): 571.77 ) M+ (Mw ) 572.11).
i
i
i
i
λi(t,λi)
1 - 10-A
tn
Θ(t ) )
dt
∫
n
0
Aλ (t,λi)
i
with light of wavelength λi measured at the observation wavelength
λo as a function of the irradiation time t. Furthermore, ꢀt(λi) and
ꢀc(λi) denote the molar absorption coefficients of the trans and cis
forms, respectively, measured at wavelength λi, and Iλ is the
i
intensity of the incident light. From these equations, the parameter
pλ can be obtained. And we can obtain the equation ct/cc ) ꢀcΦcft
ꢀtΦtfc, where ct and cc denote the trans and cis molar ratios in the
PSS, respectively. If the cis molar ratio in the PSS is known, ꢀc
can be calculated from the absorption spectrum of the trans form
/
i
and from that in the PSS, so that only the parameter pλ is needed
i
to obtain Φtfc. The intensity of the light was determined by using
8-[4-(3-Ferrocenylazo)phenoxy]octyl Disulfide (13).27 Under
a nitrogen atmosphere, 12 (100 mg, 0.17 mmol) and thiourea
(141.2 mg, 1.8 mmol) were stirred into dried ethanol (10 mL) and
kept at 80 °C for 72 h. Then, tetraethylenepentamine (0.35 mL,
1.9 mmol) was added and stirred for 8 h. The resulting orange
solution was then cooled to room temperature. Dichloromethane
was added and then filtered with Celite, and the organic layer was
separated. The aqueous layer was extracted with dichloromethane.
The combined organic extracts were dried over sodium sulfate and
filtered. The orange solid was purified by column chromatography
with an increasing proportion of dichloromethane in hexane as an
eluent. Removal of the solvent followed by recrystallization from
hexane yielded an orange solid. Yield: 18% (32.2 mg, 0.44 mmol).
Anal. Calcd for C60H66FeN2OS2‚H2O: C, 67.41; H, 6.41; N, 5.24.
K3[Fe(C2O4)3] as a chemical actinometer.29
Electrochemical Measurements. A glassy carbon rod (outside
diameter 5 mm, Tokai Carbon GC-20) was embedded in Pyrex
glass, and the cross-section was used as a working electrode. Cyclic
voltammetry was carried out in a standard one-compartment cell
under an argon atmosphere equipped with a platinum-wire counter
electrode and an Ag/Ag+ reference electrode [10 mM AgClO4 in
0.1 M n-Bu4ClO4-acetonitrile solution, E0′(ferrocenium/ferrocene)
) 0.210 V vs Ag/Ag+] with a BAS CV-50W or an ALS 750A
voltammetric analyzer. Controlled-potential electrolysis was carried
out with an electrochemical spectroscopic quartz cell equipped with
a Pt mesh working electrode, a Pt wire counter electrode, and the
Ag/Ag+ reference electrode with a Toho Technical Research
PS-14 Potentiostat.
1
Found: C, 67.66; H, 6.46; N, 4.78. H NMR (CDCl3): δ 2.62 (t,
Crystal Structure Determination. All measurements were made
on a Rigaku AFC8 diffractometer coupled with a CCD area detector
J ) 7.4, 2H), 3.97 (t, J ) 6.4, 2H), 4.33 (s, 2H), 4.72 (s, 2H), 6.74
(d, J ) 9.2, 1H), 7.33 (t, J ) 7.8, 1H), 7.47 (d, J ) 7.6, 1H), 7.58
(d, J ) 8.00, 1H), 7.84 (d, J ) 8.80, 2H), 7.87 (s, 1H).
with
graphite-monochromated
Mo
KR
radiation
(0.7107 Å). The structure was solved by direct methods and
expanded using Fourier techniques. Hydrogen atoms were not
included in the calculations. All calculations were performed using
the Crystal Structure crystallographic software package (Rigaku
Corp. and Molecular Structure Corp.). 1 was formed as a red-plate
crystal by evaporation of dichloromethane. This crystal, with
approximate dimensions of 0.20 × 0.20 × 0.20 mm, was mounted
on a glass loop. The crystals of 11 were grown as red plates by
vapor-diffusion evaporation of an ethanol/hexane solution (1:1).
These crystals, with approximate dimensions of 0.40 × 0.30 ×
0.10 mm each, were mounted on a glass loop. 5 was crystallized
as a dark red block by vapor-diffusion evaporation of dichlo-
romethane and hexane (1:1). This crystal, with approximate
dimensions of 0.40 × 0.20 × 0.10 mm, was mounted on a glass
loop.
DFT Calculations. The three-parametrized Becke-Lee-Yang-
Parr (B3LYP) hybrid exchange-correlation functional was em-
ployed. For comparisons with the UV-visible absorption spectra
observed in acetonitrile, the solvent effect was taken into account
by means of the polarized continuum model. The core electrons of
Fe and the first-row elements were replaced with effective core
potentials (ECPs), and their valence orbitals were described with
the double-ú basis set prepared for the ECPs. For hydrogen, the
4-31G basis set was used. The geometries of these compounds were
optimized with the DFT(B3LYP) method without the solvent effect.
The present calculations were implemented with the Gaussian98
(rev. A7) program.
{8-[4-(3-Ferrocenyl-phenylazo)-phenoxy]-octyl}-phosphon-
ic Acid Diethyl Ester (14). 12 (200 mg, 0.35 mmol) was refluxed
at 170 °C in triethyl phosphite (20 mL) for 3 days. After cooling
to room temperature, the triethyl phosphite was removed under a
vacuum. The orange solid was purified by column chromatography
with dichloromethane. Removal of the solvent followed by re-
crystallization from dichloromethane/hexane (1:1) yielded a
yellow solid. Yield: 46% (102 mg, 0.16 mmol). Anal. Calcd for
C34H43FeN2O4P: C, 64.76; H, 6.78; N, 4.44. Found: C, 64.61; H,
6.82; N, 4.36. 1H NMR (CDCl3): δ 2.62 (t, J ) 7.4, 2H), 3.97 (t,
J ) 6.4, 2H), 4.33 (s, 2H), 4.72 (s, 2H), 6.74 (d, J ) 9.2, 1H),
7.33 (t, J ) 7.8, 1H), 7.47 (d, J ) 7.6, 1H), 7.58 (d, J ) 8.00, 1H),
7.84 (d, J ) 8.80, 2H), 7.87 (s, 1H).
Determination of Photoisomerization Quantum Yields. A
10 mm light path length quartz cell was used to measure
photoisomerization. The sample solution was degassed by N2
bubbling before photoirradiation. According to the literature,28 the
quantum yield of trans-to-cis photoisomerization (Φtfc) and that
of cis-to-trans (Φcft) are described in the following equations:
p ) ꢀt(λi)Φtfc + ꢀc(λi)Φcft
λi
where Aλ (t,λo) designates the absorbance of the solution irradiated
i
(27) Wang, R.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A.
Langmuir 1997, 13, 4644-4651.
(28) Wettermark, G.; Langmuir, M. E.; Anderson, D. G. J. Am. Chem.
Soc. 1965, 87, 476-481.
(29) Gade, R.; Porada, T. J. Photochem. Photobiol., A 1997, 107, 27-34.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7557

Sakamoto et al.
spectral change upon oxidation (Figure S4) and upon protonation
of 1 (Figures S5 and S6), 1H NMR spectra (Figure S7) and cyclic
voltammogram (Figure S8), UV-vis spectral change upon oxidation
(Figure S9) and upon protonation of 2 (Figures S10 and S11), UV-
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research [Grants 15033215 (area 417)
and 14204066] and by a grant from The 21st Century COE
Program for Frontiers in Fundamental Chemistry from
MEXT, Japan.
1
vis spectral change upon photoirradiation (Figure S12), and H
NMR spectra of 3 (Figure S13). Crystallographic and geometric
data in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
Supporting Information Available: A scheme for the synthesis
of compounds, ORTEP diagrams of 5 and 11 (Figure S1), 1H NMR
spectra (Figure S2), cyclic voltammogram (Figure S3), UV-vis
IC051184R
7558 Inorganic Chemistry, Vol. 44, No. 21, 2005