Photoisomerization and fluorescence properties of para-substituted benzyl ether-type stilbene dendrimers

Article abstract of DOI:10.1246/bcsj.80.586

Stilbene dendrimers were prepared by coupling 4,4′-dihydroxystilbene with first, second, third, or fouth generation benzyl ether-type dendrons. Despite a short lifetime of the core structure (<1 ns), all the generations of stilbene dendrimers underwent photoisomerization with the same efficiency as that of 4,4′-dimethoxystilbene.

Full text of DOI:10.1246/bcsj.80.586

586 Short Articles
Bull. Chem. Soc. Jpn. Vol. 80, No. 3, 586–588 (2007)
could influence the photoisomerization of para-substituted stil-
bene G0–G4 (Fig. 1) more strongly than that of meta-substitut-
ed stilbene, because a dendron may not keep up with the
movement of photoisomerization of the stilbene core with a
short singlet lifetime of hundreds of picosecond for para-sub-
stituted stilbene.
Photoisomerization and
Fluorescence Properties
of para-Substituted Benzyl
Ether-Type Stilbene Dendrimers
All the generations of para-substituted stilbene dendrimers
G0–G4 were synthesized by a coupling reaction between
trans-4,40-dihydroxystilbene and the corresponding benzyl-
type dendritic bromide in the presence of K2CO3 and 18-
crown-6 in 2-butanone.2,7 trans-G0, trans-G1, and trans-G2
were purified by recrystallization, and trans-G3 and trans-G4
were purified by gel permeation column chromatography. The
molecular weight of trans-G3 was measured by MALDI-TOF-
MS. However, the mass peak for trans-G4 could not be found
in the MS. HPLC analysis by using a gel permeation column
(Column TOSOH, G-2500H HR; eluent, chloroform) gave
the retention time of 9.46 and 8.75 min for trans-G3 and
trans-G4, respectively. In addition, 1H NMR, absorption, fluo-
rescence, and fluorescence excitation spectra support the struc-
ture of trans-G4.
Shinko Watanabe,1 Masashi Ikegami,1
Ritsuko Nagahata,2 and Tatsuo Arai
ꢀ1
1Graduate School of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8571
2Research Institute for Innovation in Sustainable Chemistry,
National Institute of Advanced Industrial Science and
Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565
Received August 7, 2006; E-mail: arai@chem.tsukuba.ac.jp
Figure 2 shows absorption spectra of trans-G1, -G2, -G3,
and -G4 compared with that of trans-4,40-dimethoxystilbene
(G0) in THF under argon atmosphere. An absorption band
appeared in the wavelength region from 300 to 340 nm with
a maximum of 328 nm for G1 to G4 is similar to that of G0,
and therefore, the absorption band is assigned to the core stil-
bene. On the other hand, the molar extinction coefficient at
284 nm increased with each generation of the dendrimer, and
therefore, the band was assigned to the benzyl ether-type den-
dron group.
Stilbene dendrimers, G1–G4, underwent trans–cis isomeri-
zation upon irradiation at 330 nm in THF solution under argon
atmosphere. Figure 3 shows the absorption spectral changes
for trans-G4 on irradiation at 330 nm in THF solution, and
the absorbance changed at 328 nm (the absorption maximum)
for trans-G0–G4 with irradiation time. The absorbance at
328 nm decreased upon irradiation with an isosbestic point in
the absorption spectra at around 278 nm to give a photostation-
ary state.
The quantum yield of trans-to-cis isomerization of G0 was
determined to be 0.32 in THF by HPLC analysis and by using
tris(oxalate)ferrate actinometry. In addition, the quantum yield
of trans-to-cis isomerization of G1, G2, G3, and G4 was deter-
mined to be 0.46, 0.37, 0.36, and 0.36, respectively. These re-
sults showed that the introduction of dendron substituents
scarcely affects the efficiency of photoisomerization.
Figure 4 shows fluorescence and fluorescence excitation
spectra of trans-G1, -G2, -G3, and -G4 compared with that
of G0 in THF under argon atmosphere. The spectral shape
and the maximum wavelength of the trans-form of dendrimers
upon irradiation at 330 and 280 nm were the same as those of
the trans-G0, indicating that the fluorescence emission was ob-
served from the excited singlet state of the stilbene core. The
quantum yield of fluorescence emission upon irradiation at
330 nm was 0.34, 0.41, 0.41, 0.46, and 0.47 for trans-G0,
-G1, -G2, -G3, and -G4, respectively. The result showed that
the introduction of a dendron group slightly increased the fluo-
rescence quantum yield.
Stilbene dendrimers were prepared by coupling 4,40-di-
hydroxystilbene with first, second, third, or fouth generation
benzyl ether-type dendrons. Despite a short lifetime of the
core structure (<1 ns), all the generations of stilbene dendri-
mers underwent photoisomerization with the same efficiency
as that of 4,40-dimethoxystilbene.
The movement of a large molecular weight molecules is re-
lated to signal transmission, such as vision. In this system, the
isomerization of a retinal chromophore induces a structural
change in the protein. We have been studying a series of stil-
bene dendrimers, which have large molecular weight and un-
dergo photoisomerization with considerably high efficiency.1
Photoisomerization of stilbene dendrimers takes place within
the singlet lifetime of stilbene followed by a structural change
in a part of the dendron to give a stable conformation.
In the course of our studies, we have found that the stilbene
dendrimers, in which a series of benzyl ether-type dendrons
are substituted at the meta position of stilbene, (m-Gn; n ¼
1, 2, 3, and 4), underwent photoisomerization with almost
the same efficiency as that of 3,30,5,50-tetramethoxystilbene
(m-G0).2,3 For example, m-G4 with a molecular weight over
6500 underwent trans–cis isomerization in the singlet excited
state within 10 ns.2 According to these results, the photoiso-
merization of m-G4 is supposed to take place via a volume
conserving mechanism such as Hula-twist mechanism,4,5 in
which the C=C double bond and the adjacent single bond ro-
tate simultaneously. If the photoisomerization of stilbene den-
drimers takes place via a conventional one-bond rotation
mechanism, isomerization would be accompanied by a large
structural change in dendrons.
The movement of the dendrons is expected to be much
slower than that of the stilbene core because of the large differ-
ence in molecular weight and steric hindrance. The singlet life-
time (350 ps)6 of trans-4,40-dimethoxystilbene, which is indi-
cated as trans-G0 in Fig. 1, is much shorter than that of
3,30,5,50-tetramethoxystilbene (10 ns).2 Therefore, the dendron
Published on the web March 13, 2007; doi:10.1246/bcsj.80.586

Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Ó 2007 The Chemical Society of Japan
587
MeO
OMe
O
O
O
O
G0
O
O
G2
O
O
G1
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
G3
G4
Fig. 1. Structures of stilbene dendrimers-substituted dendron at para position of stilbene.
5
4
3
2
1
0
G0
G1
G0
G1
G2
G3
G4
260
280
300
320
340
360
380
Wavelength / nm
G2
Fig. 2. Absorption spectra of trans-G0, -G1, -G2, -G3, and
-G4 in THF under argon atmosphere.
0.2
0.15
0.1
G0
G1
G2
G3
G4
0.3
0.2
0.1
0
G3
G4
0.05
0
0
1000 2000 3000 4000 5000
Time / s
250
300
350
400
Wavelength / nm
250
300
350
400
450
500
Fig. 3. Change in the absorption spectrum of trans-G4
upon irradiation at 330 nm in THF under argon atmo-
sphere. Inset shows the change in the absorbance at 330
nm of the optically matched THF solutions of G0, G1,
G2, G3, and G4 upon irradiation at 330 nm.
Wavelength / nm
Fig. 4. Fluorescence spectra and fluorescence excitation of
trans-G0, -G1, -G2, -G3, and -G4 in THF under argon at
room temperature.

588 S. Watanabe et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
stituted stilbene dendrimers, G1–G4, could take place simulta-
neously with a rotation of the ether bond at the para position of
the sitilbene due to higher symmetry than m-Gn (Path (2) in
Fig. 5). In any case, photoisomerization of stilbene dendrimers
should take place concomitant rotations of the C=C double
bond and a single bond.
In conclusion, benzyl ether-type dendrons slightly affected
the movement of a core molecule, indicating the occurrence
of a volume conserving isomerization mechanism. Thus, the
trans–cis isomerization takes place within subnano-second
time scale within the interior of the dendrimers. These results
will open a way to develop the photoresponsive macromole-
cules based on trans–cis isomerization of olefins and to show
how large molecules move.
Ha
O
Hc
= Dendron
O
Hb
Hd
(1)
(2)
Ha
O
Ha
Hb
O
Hb
Hc
Hd
Hd
O
Hc
O
Ha
Ha
O
O
Hb
Hb
Hd
Hc
Hc
Hd
O
O
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 417), a Grant-in-Aid for Sci-
entific Research (No. 16350005) and the 21st Century COE
Program from the Ministry of Education, Culture, Sports, Sci-
ence and Technology (MEXT) of the Japanese Government,
by Research Foundation for Opto-Science and Technology,
by University of Tsukuba Research Projects, and by the Asahi
Glass Foundation, and by JSR corporation.
Fig. 5. Postulated mechanism of trans–cis isomerization in
para-substituted stilbene dendrimer.
The dendrimers, trans-G1, -G2, -G3, and -G4, did not ex-
hibit the fluorescence emission attributed to the dendrons even
upon irradiation at 284 nm, where the dendron group mainly
absorbed the light.8,9 The efficiency of the singlet energy trans-
fer from a dendron to a stilbene core can be estimated from the
normalized fluorescence excitation spectrum and absorption
References
1
5, 1.
2
A. Momotake, T. Arai, J. Photochem. Photobiol., C 2004,
spectrum. Thus, the quantum yield of energy transfer (ꢀET
)
from a dendron to stilbene core was calculated to be 1, 0.65,
and 0.57 for trans-G2, -G3, and -G4, respectively. The value
of ꢀET was related by the distance between peripheral dendron
group and a stilbene core, and energy transfer showed to take
M. Uda, T. Mizutani, J. Hayakawa, A. Momotake, M.
Ikegami, R. Nagahata, T. Arai, Photochem. Photobiol. 2002, 76,
596.
3 T. Mizutani, M. Ikegami, R. Nagahata, T. Arai, Chem. Lett.
2001, 1014.
place by the Forster mechanism.
¨
The singlet lifetime of trans-3,30,5,50-tetramethoxystilbene
depends on the presence of oxygen, temperature, and solvent,
probably due to the long singlet lifetime of ca. 10 ns.2 In this
case, temperature affects the rate constant of trans–cis isomer-
ization in the excited singlet state having activation barrier.10
We tried to determine the fluorescence lifetime of trans-G0–
G4, but the lifetime was out of range of the detection limit
(1 ns) of our single photon counting apparatus (Horiba NAES-
1100). These results indicate that the fluorescence lifetime of
the stilbene core was scarcely affected by the dendron group,
and the dendrimers underwent trans–cis isomerization within
the singlet lifetime of hundreds of picosecond. A bulky den-
dron group may affect the isomerization due to the steric hin-
drance. However, the stilbene dendrimers G0–G4 underwent
photoisomerization with a high quantum yield of 0.3 as men-
tioned above. This means that the isomerization should take
place not by a one-bond rotation but by a concomitant twisting
of the single bond and the double bond.
4
5
6
R. S. H. Liu, G. S. Hammond, Chem. Eur. J. 2001, 7, 4536.
R. S. H. Liu, Acc. Chem. Res. 2001, 34, 555.
D. M. Zeglinski, D. H. Waldeck, J. Phys. Chem. 1988, 92,
692.
7
trans-G1: 1H NMR (400 MHz, DMSO): ꢀ 5.08 (4H, s),
6.96 (4H, d, J ¼ 8:8 Hz), 7.00 (2H, s), 7.25–7.49 (14H, m). Anal.
Calcd for C28H24O2: C, 85.68, H, 6.16%. Found: C, 85.01, H,
6.30%. trans-G2: 1H NMR (400 MHz, CDCl3): ꢀ 5.00 (4H, s),
5.02 (8H, s), 6.55 (2H, t, J ¼ 2:4 Hz), 6.67 (4H, d, J ¼ 2:0 Hz),
6.90–6.92 (6H, m), 7.26–7.43 (24H, m). Anal. Calcd for C56H48-
O6: C, 82.33, H, 5.92%. Found: C, 82.03, H, 6.22%. trans-G3:
1H NMR (400 MHz, CDCl3): ꢀ 4.96 (4H, s), 4.99 (8H, s), 5.02
(16H, s), 6.54 (2H, t, J ¼ 2:2 Hz), 6.56 (4H, t, J ¼ 2:2 Hz), 6.62–
6.69 (12H, m), 6.89 (4H, d, J ¼ 10:4 Hz), 6.93 (2H, s), 7.26–
7.44 (44H, m). MALDI-TOF-MS Calcd for C112H96O14 [M]þ
=
1
1664.7; Found 1664.2. trans-G4: H NMR (400 MHz, CDCl3): ꢀ
4.88–4.97 (28H, m), 5.04 (32H, s), 6.49–6.57 (14H, m), 6.55
(4H, d, J ¼ 8:8 Hz), 6.59–6.71 (28H, m), 6.89 (2H, s), 7.26–
7.41 (84H, m).
According to these results, the para-substituted stilbene
dendrimers seem to undergo trans–cis photoisomerization by
a Hula-twist mechanism (Path (1) in Fig. 5). Another possible
explanation is that trans–cis photoisomerization of para-sub-
8
M. Imai, M. Ikegami, A. Momotake, R. Nagahata, T. Arai,
Photochem. Photobiol. Sci. 2003, 2, 1181.
M. Imai, T. Arai, Tetrahedron Lett. 2002, 43, 5265.
10 D. H. Waldeck, Chem. Rev. 1991, 91, 415.
9