Correlation between the molecular structure and trans ? cis isomerization characteristics of azobenzenes

Article abstract of DOI:10.1007/s11426-011-4413-9

Photochemical and thermal isomerization of various azobenzenes was systematically investigated to understand the correlation between the molecular structure and trans ? cis isomerization characteristics of azobenzenes. A blue shift in π-π*absorption band of ortho-alkylated azobenzenes (1o and 2o) was observed together with a reduction in molar extinction coefficient ( ε) in comparison with both meta-alkylated azobenzenes (4m and 5m) and 7p lacking the meta and ortho substituents. For orthoalkylated azobenzene, photochemical trans-to-cis isomerization and thermal back cis-to-trans isomerization in solution occurred slowly when compared with 4m, 5m and 7p. The half-life time of the cis form of 2o was found to be 380 h, which is about 8-50 times longer than those of comparable 4m, 5m (43-13 h) and 7p (7 h). Furthermore, comparison of the molecular structure and isomerization characteristics of azobenzene thiol (2o and 5m) self-assembled monolayers on flat gold surfaces indicates that the trans-to-cis photoconversion in monolayer systems is influenced by steric hindrance and strong intermolecular interaction between azobenzene units.

Full text of DOI:10.1007/s11426-011-4413-9

SCIENCE CHINA
Chemistry
December 2011 Vol.54 No.12: 1955–1961
doi: 10.1007/s11426-011-4413-9
• ARTICLES •
Correlation between the molecular structure and trans ↔ cis
isomerization characteristics of azobenzenes
HAN Mina* & HONDA Takumu
Department of Chemistry and Department of Electronic Chemistry,
Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
Received August 17, 2011; accepted September 15, 2011
Photochemical and thermal isomerization of various azobenzenes was systematically investigated to understand the correlation
between the molecular structure and trans ↔ cis isomerization characteristics of azobenzenes. A blue shift in –* absorption
band of ortho-alkylated azobenzenes (1o and 2o) was observed together with a reduction in molar extinction coefficient () in
comparison with both meta-alkylated azobenzenes (4m and 5m) and 7p lacking the meta and ortho substituents. For ortho-
alkylated azobenzene, photochemical trans-to-cis isomerization and thermal back cis-to-trans isomerization in solution oc-
curred slowly when compared with 4m, 5m and 7p. The half-life time of the cis form of 2o was found to be 380 h, which is
about 8–50 times longer than those of comparable 4m, 5m (43–13 h) and 7p (7 h). Furthermore, comparison of the molecular
structure and isomerization characteristics of azobenzene thiol (2o and 5m) self-assembled monolayers on flat gold surfaces
indicates that the trans-to-cis photoconversion in monolayer systems is influenced by steric hindrance and strong intermolecu-
lar interaction between azobenzene units.
azobenzene, isomerization, lifetime of cis form, molecular structure
the time scale of minutes to hours and the lifetime of the cis
1 Introduction
form is influenced by the surrounding environment (sol-
vents, polymer matrix, temperature, etc.) as well as the mo-
lecular structure [15–23]. In particular, the steric factor in
azobenzene can significantly vary the rate of thermal
cis-to-trans isomerization. As an example the steric strain in
azobenzenophanes, in which two azobenzenes are cyclically
connected by relatively short atomic chains, influences the
lifetime of the cis form, from seconds to years [24–28].
Tamaoki et al. have reported that the lifetime of the cis-cis
isomer in a xanthene-based cyclic azobenzene dimer was
increased up to 6.4 years, and that an azobenzenophane, an
azobenzene dimer connected at 3- and 3′-positions without
spacers, formed thermodynamically stable cis form [27, 28].
Other sterically hindered azobenzene molecules are pro-
duced by introducing bulky substituents to the benzene ring
[29, 30]. Wheeler and Gore have reported the results of
spectroscopic investigation of ortho- and para-substituted
azobenzenes [31]. Bunce et al. have also showed spectro-
Photochromic azobenzene molecules undergo trans-to-cis
isomerization upon irradiation with UV light and this pro-
cess can be reversed by either heating or irradiation with
visible light [1–7]. The trans form is thermodynamically
more stable by about 12 kcal mol1 than the cis form, and
the cis form is kinetically stabilized by an activation barrier
of isomerization. There are two possible isomerization
mechanisms in azobenzene: via rotation of the phenyl ring
about the N=N bond or in-plane inversion around one of the
nitrogen atoms [1–9]. Experimental data and theoretical
calculations predominantly support the inversion mecha-
nism for thermal isomerization of both the parent azoben-
zene and its substituted azobenzene derivatives [10–14].
In general, thermal cis-to-trans isomerization occurs on
*Corresponding author (email: han.m.ab@m.titech.ac.jp)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com www.springerlink.com

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Han M, et al. Sci China Chem December (2011) Vol.54 No.12
scopic data and the results of quantum mechanical calcula-
tions for a series of azobenzenes [32, 33]. Despite these
investigations, systematic information regarding photo-
chemical and thermal isomerization of ortho-, meta-, and
para-substituted azobenzenes in solution as well as in mon-
olayers is desirable. Here we prepared ortho- (1o and 2o)
and meta-alkylated (4m and 5m) azobenzenes, and 7p
lacking the meta and ortho substituents (Scheme 1) to clari-
fy the relationship between the molecular structural and
spectroscopic features. Photochemical and thermal isomeri-
zation as well as the related thermodynamic parameters was
also investigated to obtain insight into molecular structure
factors responsible for controlling trans ↔ cis isomerization
and the lifetime of the cis form. Furthermore, we examined
reversible photoswitching of the molecular conformation in
self-assembled monolayers (SAMs) on a flat gold surface.
(E)-1-(4′-butoxy-3,5-diethylbiphenyl-4-yl)-2-(4-(decyloxy)-3-
isopropylphenyl)diazene (1o)
1o was prepared by reacting (E)-4-((4′-butoxy-3,5-diethyl-
biphenyl-4-yl)diazenyl)-2-sec-butylphenol (3.00 g, 6.5
mmol) with 1-bromodecane (4.33 g, 19.6 mmol) in 60 mL
acetone in the presence of K2CO3 (2.71 g, 19.6 mmol) and a
catalytic amount of tetrabutylammonium bromide. The
reaction mixture was stirred at 60 °C for 7 h and then cooled
to room temperature, followed by addition of water and ethyl
acetate. The organic layer was collected and the solvent was
evaporated. The residue was purified by silica gel column
:
chromatography (hexane dichloromethane, v/v = 3/1).
1
Yield: 55%. H NMR (270 MHz, CDCl3, ppm) 0.88 (t,
3H, CH3), 0.99 (t, 3H, CH3), 1.1–1.6 (m, 28H, CH2 and
CH3), 1.82 (m, 4H, CH2), 2.74 (q, 4H, ArCH2CH3), 3.37 (m,
1H, ArCH), 4.02 (tt, 4H, ArOCH2), 6.94 (m, 3H, Ar-H),
7.31 (s, 2H, Ar-H), 7.53–7.83 (m, 4H, Ar-H). 13C NMR
(300 MHz, CDCl3) 13.9, 14.1, 15.6, 19.3, 22.4, 22.7, 25.4,
26.2, 27.2, (–CH2–; 29.72, 29.31, 29.54, 29.57), 31.3, 31.9,
67.8, 68.3, 110.9, 114.7, 120.4, 122.1, 125.8, 128.1, 133.4,
136.8, 137.8, 140.0, 147.0, 150.1, 158.7, 159.1. Anal. calcd:
C, 80.09%; H, 9.65%; N, 4.79%. Found: C, 80.03%; H,
9.75%; N, 4.72%. FAB-MS (m/z): [M+H]+ found 585, calcd
for C39H56N2O2 = 584.43.
(E)-1-(4′-butoxy-2-methylbiphenyl-4-yl)-2-(3-sec-butyl-4-
(hexadecyloxy)phenyl)diazene (4m)
1H NMR (270 MHz, CDCl3)  0.81 (m, 6H, CH3), 0.90 (t,
3H, CH3), 1.2–1.8 (m, 37H, CH2 and CH3), 2.30 (s, 3H,
ArCH3), 3.10 (m, 1H, ArCH), 3.96 (tt, 4H, ArOCH2),
6.84–6.90 (m, 3H, Ar-H), 7.18–7.28 (m, 3H, Ar-H),
7.64–7.75 (m, 4H, Ar-H). 13C NMR (300 MHz, CDCl3)
12.3, 13.9, 14.1, 19.3, 20.3, 20.7, 22.7, 26.1, (–CH2–; 29.28,
29.33, 29.35, 29.58, 29.65, 29.66, 29.69, 29.77), 31.4, 31.9,
33.9, 67.7, 68.3, 111.0, 114.1, 120.0, 121.8, 122.0, 124.2,
130.2, 130.6, 133.5, 136.3, 136.7, 143.6, 146.8, 151.8,
158.4, 159.2. Anal. calcd: C, 80.57%; H, 10.06%; N, 4.37%.
Found: C, 80.21%; H, 10.16%; N, 4.39%. FAB-MS (m/z):
[M+H]+ found 641, calcd for C43H64N2O2 = 640.50.
Scheme 1 Molecular structures of 1o–2o, 4m–5m and 7p.
2 Experimental section
2.1 Instrumentation
Dimethylformamide (DMF) and dichloromethane (DCM) of
spectroscopic grade were used to dissolve the azobenzenes.
Azobenzene solutions were exposed to UV light (365 nm, 1–2
mW cm2, Mineralight lamp, Model UVGL-25) or visible
light (436 nm, 1–2 mW cm2, a high-pressure UV lamp, Ushio
Inc., combination of Toshiba color filters, Y-43+V-44).
Absorption spectra were recorded on a Shimadzu UV-
3100PC UV-vis-NIR scanning spectrophotometer. NMR
spectra were obtained using JEOL JNM-EX270 (270 MHz)
and JEOL JNM-ECP300 (300 MHz) spectrometers.
(E)-1-(4′-butoxybiphenyl-4-yl)-2-(4-(decyloxy)phenyl)diazene
(7p)
1
Yield: 45%. H NMR (300 MHz, CDCl3)  0.89 (t, 3H,
CH3), 0.99 (t, 3H, CH3), 1.1–1.7 (m, 16H, CH2), 1.82 (m,
4H, CH2), 4.04 (tt, 4H, ArOCH2), 6.97 (m, 4H, Ar-H), 7.58
(d, 2H, Ar-H), 7.67 (d, 2H, Ar-H), 7.91 (m, 4H, Ar-H). Anal.
calcd: C, 78.97% ; H, 8.70%; N, 5.76%. Found: C, 78.81%;
H, 8.63%; N, 5.75%. FAB-MS (m/z): [M + H]+ found 488,
calcd for C32H43N2O2 = 487.33.
3 Results and discussion
2.2 Synthesis
All azobenzenes (including 2o and 5m [34]) were synthesized
according to the procedure in the literature [35, 36].
The photophysical data corresponding to the azobenzenes
used in this study are presented in Table 1. Varying the sub-

Han M, et al. Sci China Chem December (2011) Vol.54 No.12
1957
stituents at one end of the tail part for the same azobenzene
unit (1o–2o and 4m–5m) has little effect on the transition
bands in UV-vis absorption spectra; that is, strong spectral
overlap between the bands can be seen in Figure 1. The se-
ries of meta-methylated azobenzenes in dichloromethane
(DCM) possessed strong –* absorption bands at 368 nm
and negligibly weak n–* bands in the as-prepared trans-
rich state. The max of –* absorption band at 354 nm of
ortho-alkylated azobenzenes showed a blue shift, together
with a considerable reduction of about 30%–40% in molar
extinction coefficient (), in comparison with those of
4m–5m and 7p (Table 1) [37].
The observed spectroscopic features are closely related to
the steric effect arising from the substitution of two ethyl
groups at the ortho positions with respect to the azo group.
It is known that unsubstituted trans-azobenzene adopts a
planar conformation, and that methyl substituents even at
the ortho positions have little influence on the planar struc-
ture of azobenzene in the crystal state [13, 38]. By contrast,
our X-ray crystal structure analysis revealed that the intro-
duction of two ethyl groups at the ortho positions led to a
large distortion of the phenyl ring from coplanarity [35, 39].
3.1 Photochemical trans-to-cis isomerization
Figure 2 shows typical examples of changes in the absorb-
Figure 2 (a) Changes in the normalized absorbance at max as a function
of 365 nm light irradiation time. Inset: changes in absorption spectra of 5m
in DCM. 1H NMR spectra of (b) all-trans 5m and (c) after UV light irradi-
ation.
Table 1 Photophysical data of azobenzenes in dichloromethane (DCM)
–* (nm)
n–* (nm)
(L mol1 cm1)a)
448 (2,000)
Cpd
t1/2 (h)b)
(L mol1 cm1)
353.5 (25,000)
ance at max during trans-to-cis isomerization of the azo-
benzenes as a function of 365 nm light irradiation time. UV
light irradiation for 6 min caused sufficient trans-to-cis
isomerization for 5m, as shown with the significant reduc-
tion in –* absorption band. By contrast, 2o reached a
cis-rich photostationary state within 10 min of UV light
irradiation and showed slower trans-to-cis isomerization.
More than 95% of the cis form for all azobenzenes was
generated at a photostationary state of UV light irradiation,
as confirmed by NMR and UV-vis absorption spectroscopy
measurements.
c)
380
43
13
7
1o
2o
354 (26,000)
368 (36,000)
368 (37,000)
373 (41,000)
449 (2,100)
447.5 (3,400)
447 (3,400)
450 (4,200)
4m
5m
7p
a) After UV light irradiation (cis-rich state); b) at 293 K; c) not deter-
minable.
3.2 Photochemical and thermal cis-to-trans isomeriza-
tion
Reverse cis-to-trans isomerization can be induced either
photochemically or thermally. First, upon irradiation of the
UV-exposed solution with visible light at 436 nm, the –*
absorption band was increased up to about 70% with respect
to the initial –* absorbance (Figure 3). This result indi-
cates that about 30% of the cis form still existed in the pho-
tostationary state of visible light [40]. Further alternating UV
and visible light irradiation caused reversible trans-to-cis and
cis-to-trans isomerization processes, reaching respective
Figure 1 UV-vis absorption spectra of as-prepared azobenzenes (1o–2o,
4m–5m and 7p) in DCM.

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Han M, et al. Sci China Chem December (2011) Vol.54 No.12
preexponential factor, A2o, was one order larger in magni-
tude and the corresponding activation entropy (S‡2o) was
less negative (faster component) when compared with those
for 5m and 7p. On the other hand, the activation energies of
ortho-alkylated 2o were about 2–4 kcal mol1 higher than
those of 5m and 7p in solution (Ea(2o) > Ea(5m) > Ea(7p)).
Figure 3 Changes in absorption spectra of 2o in DCM. Inset: Normalized
absorbance at max upon alternating UV and visible light irradiation.
photostationary states of UV and visible light.
Second, dark incubation of the UV-exposed solution re-
sulted in a slow maximization of –* absorption band as a
consequence of thermal cis-to-trans isomerization, which
followed the first-order kinetics according to the following
equation:
Figure 4 First-order plots for thermal cis-to-trans isomerization of azo-
benzenes in DCM at 20 °C.






Ao  A
ln
 kt
A  A
t
where Ao, A∞, and At are the absorbance at max of –*
band of trans-azobenzene, before exposure to UV light, at a
photostationary state of UV light, and at time t, respectively.
k is the first-order rate constant for thermal cis-to-trans
isomerization. The absorbances are substantially propor-
tional to the concentrations of the trans form at the moni-
toring wavelength and the ratio of the trans form was cor-
rected based on NMR data. Obviously, thermal back
cis-to-trans isomerization of the sterically hindered 2o pro-
ceeded much more slowly at room temperature and the
half-life time of 2o (380 h) was approximately 8–50 times
longer than those of the comparable 4m–5m (43– h) and
7p (7 h), as shown in Figure 4 and Table 1. Such slow
isomerization rate of 2o is due to bulky substituents that in-
hibit large-scale distortion of the azo group for trans ↔ cis
isomerization [33]. Further evidence for trans ↔ cis isom-
erization was provided by NMR experiments (Figure 5).
Approximately 50% of cis-to-trans conversion of 2o took
place after about 2 weeks in the dark and approximately 3%–
5% of the cis form still existed in solution after 2 months,
consistent with UV-vis absorption spectroscopic data.
Figure 5 Changes in the normalized absorbance at max of –* band (○)
of 2o, and the ratio of the trans form (●) of 2o obtained from H NMR
1
experiments as a function of dark incubation time at ambient temperature.
Calculation of the first-order rate constant at the various
temperatures allowed us to estimate thermodynamic activa-
tion parameters such as the activation energy (Ea), activa-
tion enthalpy (H‡), and activation entropy (S‡) for ther-
mal cis-to-trans reaction from Arrhenius and Eyring plots.
Figure 6 shows the first-order plot for thermal cis-to-trans
isomerization of 2o in DMF. The evaluated values for 2o
and 5m, and 7p in DMF and DCM are listed in Table 2. The
Figure 6 First-order plots for thermal cis-to-trans isomerization of 2o in
DMF at various temperatures.

Han M, et al. Sci China Chem December (2011) Vol.54 No.12
1959
Table 2 Thermodynamic activation parameters of thermal cis-to-trans isomerization of 2o, 5m and 7p
DCM
DMF
Cpd
Ea
H‡
(kcal mol1)
23.2
S‡
(cal K1 mol1)
8.5
Ea
H‡
(kcal mol1)
23.4
S‡
A (s1)
A (s1)
(kcal mol1)
(kcal mol1)
(cal K1 mol1)
8.3
2o
5m
7p
2.3×1011
0.14×1011
0.065×1011
23.8
20.4
19.4
2.6×1011
0.93×1011
0.81×1011
23.9
21.6
20.9
19.8
21.1
13.4
15.6
10.3
10.7
18.8
20.3
Figure 7 shows isokinetic plots for H‡ and S‡ based
on the rate constants at various temperatures for azoben-
zenes so as to evaluate the mechanism of thermal isomeri-
zation. All the plots for 2o, 5m, and 7p in different solvents
are around the straight line including azobenzenes known to
isomerize through the inversion mechanism [11, 13, 29].
Steric hindrance arising from additional substitutions at the
meta and ortho positions did not cause a large deviation
from the linear relationship. Our data thus suggest that
isomerization for the azobenzenes used in this study pro-
ceeds through the inversion mechanism rather than the rota-
tion mechanism requiring a larger volume change, irrespec-
tive of substituents attached to azobenzene units.
spectrum in solution (Figure 8(a)). By contrast, the –*
absorption band of 5m monolayer was substantially blue-
shifted (Figure 8(b)), which is explained in terms of
H-aggregation through strong intermolecular interaction
between planar 5m azobenzene units.
UV light irradiation caused the significant decrease in the
–* absorption band and emergence of a band at around
280 nm (Figure 8(a)). The trans-to-cis photoconversion was
found to be ~90% for sterically hindered 2o SAMs, almost
identical to those obtained with 2o dissolved in solution,
strongly indicative of reversible photoswitching of the mo-
lecular conformation between the trans and cis forms in
monolayer systems. Despite the high trans-to-cis pho-
toisomerization, the molecular photoswitches in smooth
azobenzene thiol monolayers could hardly be visualized by
atomic force microscopy.
3.3 Photoresponsive azobenzene-based self-assembled
monolayers (SAMs)
On the other hand, the estimated trans-to-cis photocon-
version yield of 5m SAMs was about 50%, even though the
area-per-molecule value is improved by the introduction of
alkyl groups at the meta position. UV light irradiation for
10–15 min decreased the contact angle to 91° from 94.6°
(Figure 8(c)). The contact angle did not increase to a higher
value after the first visible-light irradiation. Upon the se-
cond UV light irradiation, the contact angle on the SAM
was decreased to ~87°. This behavior might be associated
with the relatively low photoconversion and strong molecu-
lar interactions between the planar azobenzene units.
Thermal cis-to-trans isomerization of 5m SAM pro-
ceeded over 2 h at ambient temperature. The kinetics of the
photoisomerization reaction was characterized by use of the
first-order plot. As shown in Figure 8(d), the cis-to-trans
isomerization deviated from the linearity predicted by the
first-order, suggesting the strong tendency to form H-aggre-
gates between 5m molecules in monolayers. Nevertheless,
the origin of difference in the thermal cis-to-trans isomeri-
zation rate between in monolayer and in solution still re-
mains to be solved.
We fabricated SAMs of azobenzene thiols (2o and 5m) to
investigate the photoresponse of azobenzene units chemi-
sorbed on flat gold surfaces. Azobenzene SAMs were pre-
pared by immersion of sold substrates in azobenzene solu-
tion in DCM for 1–2 days. After immersion, the sample was
rinsed with DCM, and blown dry with nitrogen gas [34, 36,
41]. Single-component azobenzene thiol generated a homo-
geneously distributed smooth monolayer with the root-
meansquare surface roughness of 1.6±0.2 Å. The contact
angles of 2o and 5m SAMs for water on a flat gold substrate
were 94°±1° and 94.6°±1°, respectively. The –* absorption
band at 350 nm of ortho-alkylated 2o monolayer was similar
to that (max = 354 nm) of the monomer-like absorption
4 Conclusions
The spectroscopic features and trans ↔ cis isomerization
behavior of ortho-ethylated azobenzenes (1o and 2o) were
investigated in comparison with meta-methylated azoben-
zenes (4m and 5m) and 7p lacking the meta and ortho sub-
stituents. In contrast to 4m, 5m and 7p, the sterically hin-
dered 1o and 2o undergo distortion of the phenyl ring in the
Figure 7 Isokinetic plots for H‡ and S‡ of thermal isomerization of
azobenzenes. 2o (); 5m (▲); 7p (); unsubstituted azobenzene in dif-
ferent solvents [11, 13] (); para-substituted azobenzene [11] (); ortho-
alkylated azobenzenes [29] ().

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Han M, et al. Sci China Chem December (2011) Vol.54 No.12
Figure 8 UV-vis absorption spectral changes of (a) 2o and (b) 5m SAMs on gold substrates; (c) changes of the water contact angle of 5m SAM as a func-
tion of photoirradiation during four cycles; (d) first-order plot for thermal cis-to-trans isomerization of 5m in SAM.
5
Dugave C, Demange L. Cis-trans isomerization of organic molecules
and biomolecules: Implications and applications. Chem Rev, 2003,
103: 2475–2532
In: Sekkat Z, Knoll W, eds. Photoreactive Organic Thin Films. New
York: Academic Press, Elsevier Science, 2002
Ishi-i T, Shinkai S. Dye-based organogels: Stimuli-responsive soft
materials based on one-dimensional self-assembling aromatic dyes.
Top Curr Chem, 2005, 258: 119–160
Rau H. Spectroscopic properties of organic azo-compounds. Angew
Chem Int Ed Engl, 1973, 12: 224–235
azobenzene unit from coplanarity, which is likely responsi-
ble for spectroscopic features (such as a blue shift in –*
absorption bands and a reduction in the molar extinction
coefficient). Photochemical and thermal isomerization rates
and the lifetime of the cis form showed obvious dependence
on the molecular structure of azobenzene. For 2o slow
thermal isomerization was found when compared with 4m,
5m and 7p. The experimental results for thermal isomeriza-
tion indicate that the inversion mechanism is predominantly
operative in the azobenzenes, irrespective of additional sub-
stituents attached to the azobenzene unit. Furthermore, SAMs
of sterically hindered azobenzene thiols showed the reversi-
ble photoswitching nature on gold surface. Our investigation
suggests that understanding the correlation between the mo-
lecular structure and isomerization characteristics provides
insight into the molecular design strategy for controlling
photochemical and thermal isomerization behaviors.
6
7
8
9
Cembran A, Bernardi F, Garavelli M, Gagliardi L, Orlandi G. On the
mechanism of the cis-trans isomerization in the lowest electronic
states of azobenzene: S-0, S-1, and T-1. J Am Chem Soc, 2004,126:
3234–3243
10 Shinkai S, Nakaji T, Nishida Y, Ogawa T, Manabe O. Photoresponsive
crown ethers.1. Cis-trans isomerism of azobenzene as a tool to enforce
conformational-changes of crown ethers and polymers. J Am Chem
Soc, 1980, 102: 5860–5865
11 Asano T, Okada T, Shinkai S, Shigematsu K, Kusano Y, Manabe O.
Temperature and pressure dependences of thermal cis-to-trans
isomerization of azobenzenes which evidence an inversion mechanism.
J Am Chem Soc, 1981, 103: 5161–5165
12 Rau H, Lüddecke E. On the rotation-inversion controversy on
photo-isomerization of azobenzenes. Experimental proof of inversion. J
Am Chem Soc, 1982, 104: 1616–1620
13 Ruslim C, Ichimura K. Spectroscopic and thermal isomerization
characteristics of 3,3′-dialkoxy and dialkanoyloxy azobenzenes. J
Mater Chem, 2000, 10: 2704–2707
We are grateful to the Center for Advanced Materials Analysis (Suzukake-
dai), Technical Department, Tokyo Institute of Technology, for FAB-MS
and elemental analyses. This work was supported by the Global COE Pro-
gram from the Ministry of Education, Culture, Sports, Science, and Tech-
nology of Japanese Government.
14 Sokalski WA, Gora RW, Bartkowiak W, Kobylinski P, Sworakowski J,
Chyla A, Leszczynski J. New theoretical insight into the thermal cis-
trans isomerization of azo compounds: Protonation lowers the activation
barrier. J Chem Phys, 2001, 114: 5504–5508 and references therein
15 Irick G, Pacifici JG. Photochemistry of azo compounds. I. Photo-
reduction of 4-diethylamino-4′-nitroazobenzene. Tetrahedron Lett,
1969, 10: 1303–1306
16 Wildes PD, Pacifici JG, Irick G, Whitten DG. Solvent and substituent
effects on thermal isomerization of substituted azobenzenes. Flash
spectroscopic study. J Am Chem Soc, 1971, 93: 2004–2008
1
2
3
4
Dürr H, Bouas-Laurent H. Photochromism: Molecules and Systems.
Amsterdam: Elsevier, 1990
Zollinger H. Color Chemistry: Synthesis, Properties and Applications
of Organic Dyes and Pigments, 2nd eds. Weinheim: VCH, 1991
Ichimura K. Photoalignment of liquid-crystal systems. Chem Rev,
2000, 100: 1847–1873
Pieroni O, Fissi A, Angelini N, Lenci F. Photoresponsive polypeptides.
Acc Chem Res, 2001, 34: 9–17

Han M, et al. Sci China Chem December (2011) Vol.54 No.12
1961
17 Gabor G, Fischer E. Spectra and cis-trans isomerism in highly
1898–1900
bipolar derivatives of azobenzene. J Phys Chem, 1971, 75: 581–583
18 Paik CS, Morawetz H. Photochemical and thermal isomerization of
azoaromatic residues in side chains and backbone of polymers in bulk.
Macromolecules, 1972, 5: 171–177
19 Brown EV, Granneman GR. Cis-trans isomerism in pyridyl analogs
of azobenzene. Kinetic and molecular-orbital analysis. J Am Chem
Soc, 1975, 97: 621–627
20 Nerbonne JM, Weiss RG. Elucidation of thermal-isomerization
mechanism for azobenzene in a cholesteric liquid-crystal solvent. J
Am Chem Soc, 1978, 100: 5953–5954
29 Ortruba JP, Weiss RG. Liquid Crystalline Solvents as Mechanistic
Probes. 11. The syn→anti thermal isomerization mechanism of some
low-“bipolarity” azobenzenes. J Org Chem, 1983, 48: 3448–3453
30 Rau H, Shen YQ. Photoisomerization of sterically hindered azoben-
zenes. J Photochem Photobiol A: Chem, 1988, 42: 321–327
31 Wheeler OH, Gore PH. Absorption spectra of aromatic azo and
related compounds. IV. Benzanils. J Org Chem, 1961, 26: 3295–3301
32 Forber CL, Kelusky EC, Bunce NJ, Zerner MC. Electronic spectra of
cis-azobenzenes and trans-azobenzenes: consequences of ortho
substitution. J Am Chem Soc, 1985, 107: 5884–5890
21 Haitjema HJ, Tan YY, Challa G. Thermal isomerization of azoben-
zene-based acrylic-monomers and (co)polymers with dimethylamino
substituents in solution, influence of addition of (poly)acid,
copolymer composition, spacer length, and solvent type.
Macromolecules, 1995, 28: 2867–2873
22 Wei WH, Tomohiro T, Kodaka M, Okuno H. Selective synthesis and
kinetic measurement of 1:1 and 2:2 cyclic compounds containing
1,4,7,10-tetraazacyclododecane and azobenzene units. J Org Chem,
2000, 65: 8979–8987
23 Wang CH, Weiss RG. Thermal cis→trans isomerization of covalently
attached azobenzene groups in undrawn and drawn polyethylene films.
Characterization and comparisons of occupied sites. Macromolecules,
2003, 36: 3833–3840
24 Tamaoki N, Yamaoka T. Light-intensity dependence in the photo-
chromism of dibenzo[2.2](4.4′)-azobenzenophane. J Chem Soc Perk
T 2, 1991, 6: 873–878
25 H. Rau, D. Röttger, Photochromic azobenzenes which are stable in the trans
and cis forms. Mol Cryst Liq Cryst Sci Technol A, 1994, 246: 143–146
26 Schmiegel J, Grutzmacher HF. A macrocyclic 2,19-dioxo[3.3](3,3′)
azobenzolophane by transition-metal carbonyl complex-mediated CO
insertion and cyclization. Chem Ber, 1990, 123: 1749–1752
27 Nagamani SA, Norikane Y, Tamaoki N. Photoinduced hinge-like
molecular motion: Studies on xanthene-based cyclic azobenzene di-
mers. J Org Chem, 2005, 70: 9304–9313
33 Bunce NJ, Ferguson G, Forber CL, Stachnyk GJ. Sterically hindered
azobenzenes: Isolation of cis isomers and kinetics of thermal
cis-trans isomerization. J Org Chem, 1987, 52: 394–398
34 Han M, Ishikawa D, Honda T, Ito E, Hara M. Light-driven molecular
switches in azobenzene self-assembled monolayers: Effect of molecular
structure on reversible photoisomerization and stable cis state. Chem
Commun, 2010, 46: 3598–3600
35 Han M, Hirade T, Hara M. A reversibly photoswitchable mononuclear
palladium(II) complex with ortho-diethylated azobenzene ligands. New
J Chem, 2010, 34: 2887–2891
36 Tamada K, Akiyama H, Wei TX, Kim SA. Photoisomerization reac-
tion of unsymmetrical azobenzene disulfide self-assembled monolay-
ers: Modification of azobenzene dyes to improve thermal endurance
for photoreaction. Langmuir, 2003, 19: 2306–2312
37 For instance, solubility of 7p in DMF is < 0.1 mg/1mL
38 Gabe EJ, Wang Y, Barclay LRC, Dust JM. Structures of hindered
azobenzenes. 3. 2,2′,4,4′,6,6′-Hexamethylazobenzene. Acta Crystallogr
B, 1981, 37: 978–979
39 Han MR, Hashizume D, Hara M. 1-[(E)-3-sec-butyl-4-(4′-ethoxy-3,5-
diethylbiphenyl-4-yldiazenyl)phenoxy]hexadecane. Acta Crystallogr
E, 2006, 62: O3001–O3003
40 Han M, Ichimura K. Tilt orientation of p-methoxyazobenzene side
chains in liquid crystalline polymer films by irradiation with
nonpolarized light. Macromolecules, 2001, 34: 82–89
28 Norikane Y, Katoh R, Tamaoki N. Unconventional thermodynamically
stable cis isomer and trans to cis thermal isomerization in reversibly
photoresponsive [0.0](3,3′)-azobenzenophane. Chem Commun, 2008:
41 Tamada K, Nagasawa J, Nakanishi F, Abe K, Ishida T, Hara M,
Knoll W. Structure and growth of hexyl azobenzene thiol SAMs on
Au(111). Langmuir, 1998, 14: 3264–3271
HAN Mina was born in Gyeonggi-do, Korea, received her B.S. degree in
Chemistry Education from Kongju National Teachers University and her M.S.
degree in Chemistry from Yonsei University. In 1997, she joined Professor
Kunihiro Ichimura’s group at Tokyo Institute of Technology in Yokohama, ob-
taining her Ph.D. in 2000 in the area of photofunctional organic materials and
liquid crystalline polymers. She undertook a postdoctoral research with Professor
Georges Belfort at Rensselaer Polytechnic Institute from 2001 to 2003, and then
moved to RIKEN as a Japan Society for the Promotion of Science (JSPS) fellow.
In 2008, she returned to Tokyo Institute of Technology, where she is currently an
assistant professor of Chemistry and Electronic Chemistry. Her research interests
include photochromic materials, self-assembly, organic nano/micro-structured
materials.
HONDA Takumu was born in Fukushima Prefecture, Japan, in 1984. He re-
ceived his B.S. degree (2007) in applied chemistry from Shibaura Institute of
Technology and his M.S. degree (2009) in electronic chemistry from Tokyo In-
stitute of Technology, where he worked on the synthesis and characterization of
photofunctional organic materials. He is currently a researcher at Kansai Paint
Co., Ltd., Japan.