Trans-cis isomerization of arylether dendrimers with azobenzene core and terminal hydroxy groups

Article abstract of DOI:10.1016/j.saa.2010.06.022

Azobenzene-cored arylether dendrimers 2a-2c with polar hydroxy groups at the periphery have been prepared from hydroboration-oxidation reaction of azobenzene-cored arylether dendrimers 1a-1c with nonpolar vinyl groups at the periphery. Trans → cis photois

Full text of DOI:10.1016/j.saa.2010.06.022

Spectrochimica Acta Part A 77 (2010) 478–484
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Trans–cis isomerization of arylether dendrimers with azobenzene core and
terminal hydroxy groups
Ji-hye Lee, Daeock Choi∗, Eun Ju Shin∗
Department of Chemistry, Sunchon National University, Suncheon, Jeonnam 540-742, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Azobenzene-cored arylether dendrimers 2a–2c with polar hydroxy groups at the periphery have been
prepared from hydroboration–oxidation reaction of azobenzene-cored arylether dendrimers 1a–1c with
nonpolar vinyl groups at the periphery. Trans → cis photoisomerization of central azo group has been
investigated by UV–vis spectral changes after photolysis. The results show that dendrimers 1a–1c and
2a–2c in methanol/dichloromethane (1/1, v/v) have carried out trans–cis isomerization. Reaction rate for
trans → cis photoisomerization of 2a–2c with polar hydroxy groups increases with increasing generation
upon irradiation of 350 nm. On the other hand, the reaction rate of cis → trans thermal back isomerization
for 2a–2c decreases with increasing the generation. This result is somewhat different from that for 1a–1c
with nonpolar vinyl groups. Both the reaction rates of trans → cis photoisomerization and cis → trans
thermal back isomerization for 1a–1c decrease in higher generation.
Received 20 January 2010
Received in revised form 13 May 2010
Accepted 4 June 2010
Keywords:
Azobenzene
Arylether dendrimer
Hydroxy group
Trans–cis photoisomerization
Absorption spectrum
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Dendrimers bearing the polar peripheral groups are espe-
cially important for the pharmaceutical, medicinal, and biological
Recently, dendritic functional molecules [1–18] have received
considerable attention in the viewpoint of the potential appli-
cations of numerous fields including drug-delivery systems,
diagnostic agents, nanoscale catalysts, enzyme mimics, nano-sized
molecular electronic device, molecular photoswitch, chemical
sensor, display, solar cell, etc. Because of precisely controlled func-
tionality/size/shape and interior void space, dendrimers may play
an important role in nanotechnology including the preparation of
functional nanomaterials.
Dendrimers including photochromic moiety are especially
interesting because those may accomplish the light-controlled
specific function. Light-driven simple geometrical change of core
molecule or concurrent geometrical changes of many peripheral
molecules may result in the large conformational change through-
out the dendritic entity [19–22].
applications. For example, one of the strategy for the use of
dendrimers as drug-delivery agents is the design of dendrimers
having nonpolar interior for solubilizing various hydrophobic
organic drug and polar hydrophilic exterior for being suscepti-
ble in aqueous solution. The internal cavity of an appropriately
designed dendrimer could be used for the entrapment of
drugs with the possibility of subsequent controlled release.
However, only a few dendrimers with nonpolar interior and
polar exterior behaving as unimolecular micelles were reported
[33,34].
Recently, we have reported the preparation and isomerization
behavior of azobenzene-cored arylether dendrimers with a photoi-
somerizable azobenzene unit in the core and terminal vinyl groups
in the periphery [35,36]. For this azobenzene-cored arylether den-
drimers, the introduction of various functional groups by the
reaction of terminal vinyl group could allow the easy modification
of periphery. Hydroboration–oxidation of terminal vinyl groups
may lead to the preparation of azobenzene-cored dendrimer with
polar hydroxyl groups in exterior.
Azobenzene is
a typical photochromic molecule to show
clean reversible isomerization [23–26]. Reversible transforma-
tion between more stable trans-azobenzene and less stable
cis-azobenzene can be controlled by UV or visible light irradiation
or thermally in the dark. Intense investigation has been carried out
for the dendrimer with a photoisomerizable azobenzene unit in the
core [27–29] or periphery [30–32].
Thus, herein we report the synthesis and photochemi-
cal/thermal isomerization behavior of azobenzene-cored arylether
dendrimers 2a–2c with polar hydroxyl groups in the periph-
ery in methanol/dichloromethane (1/1, v/v), in comparison with
azobenzene-cored arylether dendrimers 1a–1c with nonpolar vinyl
groups in the periphery. Structures of 1a–1c and 2a–2c are shown
in Fig. 1.
∗
Corresponding authors. Tel.: +82 61 750 3635; fax: +82 61 750 3508.
E-mail address: ejs@sunchon.ac.kr (E.J. Shin).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.06.022

J.-h. Lee et al. / Spectrochimica Acta Part A 77 (2010) 478–484
479
Fig. 1. Structures of 1a–1c and 2a–2c.
2. Experimental
syringe at −20 ◦C. After stirring at −20 ◦C for 12 h, 2 mL of
CH3OH was added followed by the simultaneous addition of
2 mL of 3 M NaOH aqueous solution and 2 mL of 30% H2O2
aqueous solution. The resulting solution was stirred overnight
at −5 ◦C. The reaction mixture was diluted by ethyl acetate
(20 mL), then washed by water and brine and dried over
MgSO4. The solvent was removed in vacuo and the reaction
mixture was purified by preparative TLC using MeOH/CHCl3
(1/9, v/v) as eluents to give 2a as a yellow solid (50 mg, 41%
yield). 1H NMR (400 MHz, DMSO-d6) ı 7.83 (d, 4H, J = 9.0 Hz,
–N N–ArH), 7.18 (d, 4H, J = 9.0 Hz, –N N–ArH), 6.60 (d, 4H,
J = 2.1 Hz, –O–CH2–ArꢀH), 6.43 (t, 2H, J = 2.1 Hz, –O–CH2–ArꢀH),
5.13 (s, 4H, –O–CH2–Arꢀ–), 4.44 (brs, 4H, –OCH2CH2CH2CH2OH),
4.03–3.94 (m, 8H, –OCH2CH2CH2CH2OH), 3.45–3.42 (m, 8H,
–OCH2CH2CH2CH2OH), 1.76–1.69 (m, 8H,–OCH2CH2CH2CH2OH),
1.58–1.51 (m, 8H, –OCH2CH2CH2CH2OH) ppm; MALDI-TOF MS m/z
746.89 (C42H54N2O10 requires 746.38).
2.1. Apparatus
1H NMR spectra were measured on a 400 MHz Bruker Avance
400 NMR spectrometer in DMSO-d6. MALDI-TOF Mass Spectra
were measured on Applied Biosystem Voyager-DE’STR System
4407 Mass Spectrometer using 2,5-dihydroxybenzoic acid in THF
as a matrix. Absorption spectra were recorded on a Shimadzu
UV-2401PC spectrophotometer. Photoirradiation was carried out
in a Rayonet RPR 100 photochemical reactor equipped with a
Southern Ultraviolet 3500 Å lamp using pyrex reaction tube in
methanol/dichloromethane (1/1, v/v). Reaction progress of photoi-
somerization on UV irradiation and reverse thermal isomerization
in the dark was monitored by the change of absorption spectra.
2.2. Reagents
All materials used in the present investigation were of analytical
reagent grade and purchased from Aldrich Chemical Co. They were
used as received.
The preparation of azobenzene-cored arylether dendrimers
1a–1c of first, second, and third generations with vinyl groups
in the periphery has been previously reported [36]. Photore-
sponsive azobenzene-cored arylether dendrimers 2a–2c of first,
second, and third generations with an azobenzene unit at the
core and hydroxy groups at the periphery have been synthesized
by hydroboration–oxidation reaction of corresponding 1a–1c with
terminal vinyl groups in the periphery as shown in Fig. 2. Their
structures have been identified by 1H NMR and MALDI-TOF Mass
Spectra.
Preparation of azobenzene-cored arylether dendrimers 2a of
first generation with an azobenzene unit at the core and hydroxy
groups at the periphery.
To a solution of 1a (0.05 g, 0.074 mmol) in 30 mL dry THF,
2.0 M BH3·THF (0.5 mL, 1.0 mmol) was added dropwise via
Fig. 2. Synthesis of 2a–2c from 1a–1c and trans–cis isomerization of 1a–1c and
2a–2c.

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J.-h. Lee et al. / Spectrochimica Acta Part A 77 (2010) 478–484
Fig. 3. Absorption spectra of 1a–1c (a) and 2a–2c (b) in methanol/dichloromethane (1/1, v/v).
Preparation of azobenzene-cored arylether dendrimers 2b of
3.2. Trans → cis photochemical isomerization
second generation with an azobenzene unit at the core and
hydroxy groups at the periphery 2b was synthesized according
to the same procedure as in the synthesis of 2a described above.
Yield 26% (35 mg starting from 1b (0.15 g, 0.11 mmol)); 1H NMR
(400 MHz, DMSO-d6) ı 7.83 (d, 4H, J = 8.8 Hz, –N N–ArH), 7.16 (d,
4H, J = 8.8 Hz, –N N–ArH), 6.72 (d, 4H, J = 1.9 Hz, –O–CH2–ArꢀH),
6.63 (brs, 2H, –O–CH2–ArꢀH), 6.56 (d, 8H, J = 1.9 Hz, –O–CH2–ArꢀꢀH),
6.41 (brs, 4H, –O–CH2–ArꢀꢀH), 5.13 (s, 4H, –O–CH2–Arꢀ–), 5.01
(s, 8H, –O–CH2–Arꢀꢀ–), 4.44 (brs, 8H, –OCH2CH2CH2CH2OH),
4.02–3.93 (m, 16H, –OCH2CH2CH2CH2OH), 3.45–3.42 (m, 16H,
–OCH2CH2CH2CH2OH), 1.74–1.68 (m, 16H, –OCH2CH2CH2CH2OH),
1.56–1.51 (m, 16H, –OCH2CH2CH2CH2OH) ppm; MALDI-TOF MS
m/z 1523.51 (C86H110N2O22 requires 1522.76).
Preparation of azobenzene-cored arylether dendrimers 2c of
third generation with an azobenzene unit at the core and hydroxy
groups at the periphery 2c was synthesized according to the same
procedure as in the synthesis of 2a described above. Yield 22%
(35 mg starting from 1c (0.15 g, 0.054 mmol)); 1H NMR (400 MHz,
DMSO-d6) ı 7.81 (d, 4H, J = 8.9 Hz, –N N–ArH), 7.16 (d, 4H, J = 8.9 Hz,
–N N–ArH), 6.74 (brs, 4H, –O–CH2–ArꢀH), 6.70 (d, 8H, J = 1.8 Hz,
–O–CH2–ArꢀꢀH) 6.65 (brs, 2H, –O–CH2–ArꢀH), 6.62 (t, 4H, J = 1.8 Hz,
–O–CH2–ArꢀꢀH), 6.61 (d, 16H, J = 1.9 Hz, –O–CH2–ArꢀꢀꢀH), 6.40 (t,
8H, J = 1.9 Hz, –O–CH2–ArꢀꢀꢀH) 5.13 (s, 4H, –O–CH2–Arꢀ–), 5.03 (s,
8H, –O–CH2–Arꢀꢀ–), 4.99 (s, 16H, –O–CH2–Arꢀꢀꢀ–) 4.44 (brs, 16H,
–OCH2CH2CH2CH2OH), 3.95–3.92 (m, 32H, –OCH2CH2CH2CH2OH),
3.45–3.39 (m, 32H, –OCH2CH2CH2CH2OH), 1.73–1.67 (m, 32H,
–OCH2CH2CH2CH2OH), 1.55–1.51 (m, 32H, –OCH2CH2CH2CH2OH)
ppm; MALDI-TOF MS m/z 3075.81 (C174H222N2O46 requires
3075.51).
The absorption spectral changes of 1a–1c and 2a–2c in
methanol/dichloromethane (1/1, v/v) on irradiation at 350 nm were
measured by 5 s interval for monitoring trans → cis photoisomer-
ization of azobenzene moiety. On 350 nm irradiation, intensities
of absorption bands at 358 nm of dendrimers remarkably decrease
and new band at 440–445 nm appears, as azobenzene moiety at
the core of dendrimers 1a–1c and 2a–2c converts photochemically
from trans to cis form. Fig. 4 shows trans–cis isomerization of 1c and
2c.
The absorption spectral changes of azobenzene-cored
arylether dendrimers 1a–1c containing vinyl peripheral groups in
methanol/dichloromethane (1/1, v/v) upon irradiation of 350 nm
are shown in Fig. 5.
As azobenzene moiety at the core of dendrimers trans 1a–1c
converts photochemically from trans to cis form on 350 nm irra-
diation, intensity of absorption band at 358 nm of dendrimers
remarkably decreases with appearing new band at 444 nm.
In methanol/dichloromethane (1/1, v/v), absorption spectrum
of 1a is not changed any more after 120 s irradiation. It is sup-
posed that trans 1a converts completely to the cis isomer in 120 s.
It takes 120 s up to reach to the photostationary state in trans → cis
conversion of 1b and 1c, too.
Absorption spectral changes of 2a–2c, azobenzene-cored
arylether dendrimers containing hydroxy peripheral groups, with
irradiation time (0–120 s, 5 s interval) on irradiation at 350 nm in
methanol/dichloromethane (1/1, v/v) are shown in Fig. 6.
Trans → cis photoisomerization reactions of azobenzene-cored
arylether dendrimers 2a–2c reach to the photosta-
tionary state when irradiated for ∼100 s at 350 nm in
methanol/dichloromethane (1/1, v/v). Absorption band inten-
sity at 358 nm remarkably decreases with appearing new band
at 440 nm, as trans 2a–2c converts from cis 2a–2c on 350 nm
irradiation,
3. Results and discussion
3.1. Absorption spectra
The absorption spectra of trans-azobenzene-cored arylether
dendrimers 1a–1c and 2a–2c in methanol/dichloromethane (1/1,
v/v) are shown in Fig. 3. Their absorption maxima are similar
regardless of dendrimer generation and appear around 277–282,
358 nm for trans isomers trans 1a–1c and trans 2a–2c, around 282,
319, 444 nm for cis 1a–1c, and around 259, 275, 320, 440 nm for
cis 2a–2c (Table 1). While the intensities at 358 nm absorption
band of azobenzene moiety are similar for all six trans den-
drimers, the absorbance at 277–282 nm absorption band of aryl
ether dendrimer moiety is increased on going from first gener-
ation dendrimers 1a and 2a to third generation dendrimers 1c
and 2c. In cis isomers, broad long-wavelength band at 440–444 nm
appears.
Table 1
Absorption maxima ꢀa
trans
and ꢀacis, and rate constants kt→c and kc→t for trans → cis
photoisomerization and cis → trans thermal back isomerization of 1a–1c and 2a–2c
in methanol/dichloromethane (1/1, v/v).
Compound
ꢀatrans, nm
ꢀacis, nm
kt→c, s−1
kc→t, h−1
1a
1b
1c
2a
2b
2c
283, 357
282, 358
282, 359
277, 358
277, 358
277, 358
283, 319, 445
282, 318, 444
282, 319, 444
259, 275, 320, 440
259, 275, 320, 440
261, 277, 319, 443
3.2 × 10−2
2.5 × 10−2
2.6 × 10−2
2.3 × 10−2
2.5 × 10−2
3.2 × 10−2
2.7 × 10−2
2.9 × 10−2
1.9 × 10−2
2.7 × 1 × 10−2
2.9 × 10−2
2.4 × 10−2

J.-h. Lee et al. / Spectrochimica Acta Part A 77 (2010) 478–484
481
Fig. 4. Trans–cis isomerization of 1c and 2c.
Regardless of peripheral groups, all six azobenzene-cored den-
drimers carried out very fast trans → cis photoisomerization on
irradiation of 350 nm and reached to the photostationay state
within 120 s. Trans → cis photoisomerization rates kt → c were cal-
culated from the absorbance changes with irradiation time t using
the following equation.
their slopes. As shown in Table 1, trans → cis photochemical iso-
merization rates (kt→c) of 1a–1c, which are dendrimers bearing
terminal vinyl groups, are similar to those of 2a–2c, which are
dendrimers bearing terminal hydroxy groups.
The initial reaction rate of trans → cis photochemical isomer-
ization (kt→c) in methanol/dichloromethane (1/1, v/v) decreases
slightly with increasing the dendrimer generation for 1a–1c
(1a (3.2 × 10−2 s−1) > 1b (2.5 × 10−2 s−1) ≈ 1c (2.6 × 10−2 s−1)), but
increases with increasing the dendrimer generation for 2a–2c (2a
(2.3 × 10−2 s−1) ≈ 2b (2.5 × 10−2 s−1) < 2c (3.2 × 10−2 s−1)).
The reaction rate of trans → cis photochemical isomerization of
1c is slowest among 1a–1c because of its bulkiness. Meanwhile,
probably, the enhancement of intramolecular hydrogen bonding
in cis isomer of higher generation for 2a–2c bearing terminal
A0 − A
∞
∞
ln
= kt→c
t
At − A
where A , At, and A represent absorbances before irradiation, at
∞
0
irradiation time t, and at the photostationary state gained after the
prolonged irradiation, respectively.
Plots of ln[(A − A )/(At − A )] vs. t for 1a–1c and 2a–2c in
∞
∞
0
methanol/dichloromethane (1/1, v/v) give the values of kt→c from

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J.-h. Lee et al. / Spectrochimica Acta Part A 77 (2010) 478–484
Fig. 6. Absorption spectral changes of 2a (a), 2b (b), and 2c (c) in
methanol/dichloromethane (1/1, v/v) with irradiation time (0–120 s, 5 s interval)
on irradiation at 350 nm.
Fig. 5. Absorption spectral changes of 1a (a), 1b (b), and 1c (c) in
methanol/dichloromethane (1/1, v/v) with irradiation time (0–120 s, 5 s interval)
on irradiation at 350 nm.
hydroxyl groups in methanol/dichloromethane (1/1, v/v) leads to
faster trans → cis photoisomerization in higher generation. Gener-
ation dependency of trans → cis photochemical isomerization rate
(kt→c) is significantly altered by different exterior groups (hydroxy
or vinyl groups) in dendrimer structure.
increases remarkably while absorbance at 440–445 nm decreases.
It indicates that azobenzene moiety at the core of dendrimers 1a–1c
and 2a–2c reverts thermally from cis to trans form.
Absorption spectral changes of 1a–1c in methanol/dichloro-
methane (1/1, v/v) with dark incubation time (0–75 h) after 350 nm
irradiation for 3 min are shown in Fig. 7.
3.3. Cis → trans thermal isomerization
During the dark incubation of dendrimers 1a–1c, absorption
band intensity at 358 nm increases remarkably, while absorbance
at 445 nm decreases. For 1a–1c, dark incubation time of 72–75 h is
required for recovery to the original spectra of corresponding trans
isomers in methanol/dichloromethane (1/1, v/v).
Absorption spectral changes of 2a–2c in methanol/
dichloromethane (1/1, v/v) with dark incubation time (0–75 h)
after 350 nm irradiation for 3 min are shown in Fig. 8.
Thermal reversion reactions of azobenzene-cored dendrimers
1a–1c and 2a–2c are slow at room temperature and monitored
by absorption spectral changes in methanol/dichloromethane (1/1,
v/v) during the incubation in the dark. As the solutions of 1a–1c
and 2a–2c were kept in the dark after irradiated at 350 nm for
3 min, slow thermal back reversion from cis form to trans form
in methanol/dichloromethane (1/1, v/v) occurs. During the dark
incubation, absorption band intensity at 358 nm of dendrimers
Similarly, during the dark incubation of dendrimers 2a–2c,
absorption band intensity at 358 nm increases remarkably, while

J.-h. Lee et al. / Spectrochimica Acta Part A 77 (2010) 478–484
483
Fig. 8. Absorption spectral changes of 2a (a), 2b (b), and 2c (c) in
methanol/dichloromethane (1/1, v/v) with dark incubation time (0–75 h)
after 350 nm irradiation for 3 min.
Fig. 7. Absorption spectral changes of 1a (a), 1b (b), and 1c (c) in
methanol/dichloromethane (1/1, v/v) with dark incubation time (0–75 h)
after 350 nm irradiation for 3 min.
absorbance at 440 nm decreases. Isomerization reactions from cis
2a–2c to corresponding trans isomers are completed after the dark
incubation of 72–74 h in methanol/dichloromethane (1/1, v/v). The
reaction time (72–75 h) of cis → trans thermal isomerization for
1a–1c and 2a–2c is independent of both the dendrimer generation
and the exterior groups (hydroxy or vinyl groups).
However, as shown in Fig. 8(b) and (c), absorption spectra of
2b and 2c are not completely recovered to the original spectra
of corresponding trans isomers even after dark incubation of 75 h.
Hydrogen bonding between adjacent terminal hydroxyl groups in
folded geometry of cis isomers for 2b and especially 2c may inhibit
the formation of unfolded and fully stretched geometry of trans
isomers, even if central azo group isomerizes during the dark incu-
bation. Trans 2c obtained after 75 h dark incubation of cis 2c shows
very structured absorption spectra, apart from the original spec-
tra of trans 2c. Therefore, it is plausible that trans 2c obtained after
75 h dark incubation of cis 2c has twisted geometry, as shown in
Fig. 4.
Cis → trans reverse thermal isomerization rates (kc→t) were cal-
culated from the absorbance changes with dark incubation time t
using the following equation.
A
∞ − A0
ln
= kc→t
t
A
∞ − At
where A , At, and A represent absorbances just after 350 nm irra-
∞
0
diation for 3 min, at dark incubation time t, and at the stationary
state gained after the prolonged dark incubation, respectively.
The values of kc→t for 1a–1c and 2a–2c are evaluated from the
slopes of the plots of ln[(A∞ − A0)/(A∞ − At)] vs. dark incubation
time t and are summarized in Table 1.
As shown in Table 1, the initial reaction rate of cis → trans ther-
mal isomerization (kc→t) is slow in higher generation dendrimers 1c

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J.-h. Lee et al. / Spectrochimica Acta Part A 77 (2010) 478–484
(1a (2.7 × 10−2 h−1) ≈ 1b (2.9 × 10−2 h−1) > 1c (1.9 × 10−2 h−1)) and
Economy (MKE) through the fostering project of the Industrial-
Academic Cooperation Centered University and by the Ministry of
Knowledge Economy (MKE) through the project of GTFAM Regional
Innovation Center (RIC).
2c (2a (2.7 × 10−2 h−1) ≈ 2b (2.9 × 10−2 h−1) > 2c (2.4 × 10−
h
−1)).
The enhancement of bulkiness in higher generation leads to
slower cis → trans thermal isomerization. Cis → trans thermal iso-
merization rate is affected by the generation of azobenzene-cored
dendrimers, but not much altered with exterior group of den-
drimer. Cis → trans thermal isomerization rate of 1a–1c bearing
terminal vinyl groups in methanol/dichloromethane (1/1, v/v) is
similar to that of 2a–2c bearing terminal hydroxy groups.
References
[1] R.W.J. Scott, O.M. Wilson, R.M. Crooks, J. Phys. Chem. B 109 (2005) 692–704.
[2] D.V. McGrath, Mol. Pharm. 2 (2005) 253–263.
[3] A.-M. Caminade, J.-P. Majoral, Acc. Chem. Res. 37 (2004) 341–348.
[4] J.M.J. Fréchet, J. Polym. Sci. A 41 (2003) 3713–3725.
[5] G.R.
Newkome,
C.N.
Moorefield,
F.
Vögtle,
Dendrimers
and
4. Conclusions
Dendrons—Concepts, Synthesis Applications, Wiley-VCH, New York, 2001.
[6] S.M. Grayson, J.M.J. Fréchet, Chem. Rev. 101 (2001) 3819–3867.
[7] M.W.P.L. Baars, E.W. Meijer, Top. Curr. Chem. 210 (2000) 131–182.
[8] D.K. Smith, F. Diederich, Top. Curr. Chem. 210 (2000) 183–227.
[9] A.W. Bosman, H.M. Janssen, E.W. Meijer, Chem. Rev. 99 (1999) 1665–1688.
[10] G.R. Newkome, E. He, C.N. Moorefield, Chem. Rev. 99 (1999) 1689–1746.
[11] O.A. Matthews, A.N. Shipway, J.F. Stoddart, Prog. Polym. Sci. 23 (1998) 1–56.
[12] F. Zeng, S.C. Zimmerman, Chem. Rev. 97 (1997) 1681–1712.
[13] J.M.J. Fréchet, Science 263 (1994) 1710–1715.
[14] D.A. Tomalia, Adv. Mater. 6 (1994) 529–539.
[15] D.A. Tomalia, A.M. Naylor, W.A. Goddard III, Angew. Chem., Int. Ed. Engl. 29
(1990) 138–175.
[16] Y.-S. Shon, D. Choi, Chem. Lett (2006) 644–645.
[17] O.M. Wilson, M.R. Knecht, J.C. Garcia-Martinez, R.M. Crooks, J. Am. Chem. Soc.
128 (2006) 4510–4511.
[18] S.-y. Lim, D. Choi, E.J. Shin, Bull. Korean Chem. Soc. 29 (2008) 1353–1358.
[19] A. Momotake, T.J. Arai, Photochem. Photobiol. C: Photochem. Rev. 5 (2004)
1–25.
[20] S. Hecht, J.M.J. Fréchet, Angew. Chem., Int. Ed. Engl. 40 (2001) 74–91.
[21] Y.-J. Mo, D.-L. Jiang, M. Uyemura, T. Aida, T. Kitagawa, J. Am. Chem. Soc. 127
(2005) 10020–10027.
[22] T. Hasobe, P.V. Kamat, M.A. Absalom, Y. Kashiwagi, J. Sly, M.J. Crossley, K.
Hosomizu, H. Imahori, S. Fukuzumi, J. Phys. Chem. B 108 (2004) 12865–12872.
[23] S. Shinkai, O. Manabe, Top. Curr. Chem. 121 (1984) 67–104.
[24] G.S. Kumar, D.C. Neckers, Chem. Rev. 89 (1989) 1915–1925.
[25] R. Cimiraglia, H.-J. Hofmann, Chem. Phys. Lett. 217 (1994) 430–435.
[26] Z. Sekkat, J. Wood, W. Knoll, J. Phys. Chem. 99 (1995) 17226–17234.
[27] A. Momotake, T. Arai, Tetrahedron Lett. 45 (2004) 4131–4134.
[28] D.-L. Jiang, T. Aida, Nature 388 (1997) 454–456.
[29] D.M. Junge, D.V. McGrath, Chem. Commun. (1997) 857–858.
[30] A. Archut, F. Vögtle, L. De Cola, G.C. Azzellini, V. Balzani, P.S. Ramanujam, R.H.
Berg, Chem. Eur. J. 4 (1998) 699–706.
[31] A. Archut, G.C. Azzellini, V. Balzani, L. De Cola, F. Vögtle, J. Am. Chem. Soc. 120
(1998) 12187–12191.
[32] K.-Y. Kay, K.-J. Han, Y.-J. Yu, Y.D. Park, Tetrahedron Lett. 43 (2002) 5053–5056.
[33] C.J. Hawker, K.L. Wooley, J.M.J. Fréchet, J. Chem. Soc. Perkin Trans. 1 (1993)
1287–1297.
[34] H. Tatewaki, N. Baden, A. Momotake, T. Arai, M. Terazima, J. Phys. Chem. B 108
(2004) 12783–12789.
Photoresponsive arylether dendrimers 2a–2c with azobenzene
unit at the core and hydroxyl groups at the periphery were prepared
from arylether dendrimers 1a–1c with azobenzene unit at the core
and vinyl groups at the periphery. Comparative study of reversible
trans–cis isomerization between dendrimers containing polar exte-
rior groups and dendrimers containing nonpolar exterior groups
was accomplished. Both the reaction rate for trans → cis photoi-
somerization (kt→c) and the reaction rate of cis → trans thermal
back isomerization (kc→t) of dendrimers 1a–1c containing nonpolar
exterior groups in methanol/dichloromethane (1/1, v/v) decrease
with increasing generation. This result is somewhat different from
that for 2a–2c. For dendrimers 2a–2c containing polar exterior
groups, the reaction rate of trans → cis photoisomerization (kt→c
)
increases in higher generation, but the reaction rate of cis → trans
thermal back isomerization (kc→t) decreases in higher generation.
The reaction rates of trans → cis photochemical isomerization and
cis → trans thermal isomerization for azobenzene-cored arylether
dendrimers are affected by both the generation and the exterior
group. It is noteworthy that trans 2c obtained after 75 h dark incu-
bation of cis 2c shows very structured absorption spectra, apart
from original spectra of trans 2c. For 2c, hydrogen bonding between
adjacent terminal hydroxyl groups in cis isomer of folded geometry
may inhibit the formation of unfolded and fully stretched geometry
of trans isomers. It is plausible that twisted trans 2c is formed from
the cis → trans thermal isomerization of central azo group.
Acknowledgements
This work is financially supported by the Ministry of Educa-
tion, Science and Technology (MEST), the Ministry of Knowledge
[35] D. Choi, J.-h. Lee, K.-h. Shin, E.J. Shin, Bull. Korean Chem. Soc. 28 (2007) 983–989.
[36] J.-H. Lee, D. Choi, J.E. Park, E.J. Shin, Bull. Korean Chem. Soc. 29 (2008) 761–766.