Photochemical properties of 3,3′- disubstituted stilbene dendrimers

Full text of DOI:10.1246/bcsj.82.603

Short Articles
Bull. Chem. Soc. Jpn. Vol. 82, No. 5, 603–605 (2009)
603
Photochemical Properties of 3,3¤-
Disubstituted Stilbene Dendrimers
Kazumi Kato,1 Atsuya Momotake,1
Yoshihiro Shinohara,2 Tomoo Sato,1
Kayori Takahashi,3 Ritsuko Nagahata,3
Yoshinobu Nishimura,1 and Tatsuo Arai*1
1Graduate School of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8571
2Research Facility Center for Science and Technology,
University of Tsukuba, Tsukuba 305-8577
3National Institute of Advanced Industrial Science
and Technology (AIST), Tsukuba 305-8565
Figure 1. Structures of stilbene dendrimers trans-3 and -4,
their standard molecules trans-1 and -2, and dendritic
bromide G3-Br.
Received November 14, 2008; E-mail: arai@chem.tsukuba.
ac.jp
Table 1. Fluorescence Quantum Yield, Lifetime of Excited
Singlet State, Quantum Yield of Trans-to-Cis Photo-
chemical Isomerization for trans-1-4
The quantum yield of isomerization and the fluores-
cence lifetime of newly synthesized 3,3¤-disubstituted water-
soluble stilbene dendrimer trans-4 were significantly differ-
ent from those of model compounds. The results indicate that
not only properties of dendrimer surfaces but also substituent
positions are important for dendrimer photochemistry.
Solvents
¯
¸s/ns
<0.5
<0.5
0.73
0.70 (0.97)
4.55 (0.03)
¯
t¼c
f
trans-1
trans-2
trans-3
trans-4
THF
1 mM KOHaq
THF
0.21
0.14
0.28
0.14
0.40
0.36
0.35
0.063
1 mM KOHaq
Photochemical properties of small molecular weight chro-
mophores in dendrimers have been studied.1-10 When a small
chromophore that changes structure under photoirradiation is
introduced as a dendrimer core, the small structural changes of
the core can be amplified to a large structural change of the
whole molecule. By this strategy, we have prepared several
dendrimers having a photoresponsive molecular structure.11-13
In organic solvents, stilbene dendrimers with benzyl ether
dendron groups at the 3,3¤,5,5¤-position of the core stilbene
have been studied.14,15 Trans-to-cis photoisomerization pro-
ceeds through the deactivation from the excited singlet state
perpendicular conformation, as revealed by the temperature
effect on the fluorescence lifetime. In aqueous solution,
photochemical behaviors of water-soluble stilbene dendrimers
have been studied. Interestingly, a third generation dendrimer
exhibited one-way trans-to-cis isomerization upon photoirra-
diation.16 The quantum yield of the isomerization as well as the
fluorescence quantum yields and fluorescence lifetimes of these
dendrimers have also been reported.17
Here, we report the photochemical properties of 3,3¤-
disubstituted stilbene dendrimers trans-3 and the corresponding
potassium salts trans-4 and their standard compounds trans-1
and trans-2 (Figure 1). In aqueous solution, the quantum yield
of isomerization (¯t¼c) for trans-4 was 0.063 and was much
lower than that for standard trans-2 (¯t¼c = 0.36) (Table 1). In
addition, the fluorescence lifetime of trans-4 in aqueous
solution was calculated to be two exponential functions to
give 0.70 and 4.55 ns.
Results and Discussion
Figures 2a-2d show the absorption, fluorescence, and
fluorescence excitation spectra of trans-1 and -3 in THF and
the corresponding potassium salts trans-2 and -4 in 1 © 10¹3 M
KOH aqueous solution, respectively. In Figures 2c and 2d, the
increase in the absorption maximum at 277-280 nm indicates
increments of the benzyl ether group. All compounds exhibited
absorption at 300-360 nm with -max at 310-315 nm due to the
trans-stilbene cores. The absorption band of the stilbene moiety
appeared in almost the same region.
In THF, the fluorescence spectra were similar to each other to
give -max = 363 and 365 nm, respectively, for trans-1 and -3.
The fluorescence maximum of their corresponding potas-
sium salts in aqueous solution slightly shifted to a longer
wavelength to give -max = 372 and 373 nm, for trans-2 and -4,
respectively. The fluorescence excitation spectra of the core
stilbene at 300-360 nm appeared at almost the same region
as the absorption spectra for trans-1 and -2, but the spectral
profiles of the fluorescence excitation spectra of trans-3 and -4
at 280 nm are quite different from those of the absorption
spectra. These results show that the energy transfer from the
dendron group to the core stilbene scarcely takes place in trans-
3 in THF, whereas the excited singlet energy transfer from the
dendron moiety to the core stilbene was observed in trans-4 in
aqueous solution. One can estimate the energy-transter effi-
ciency by comparing the absorption spectra and the fluores-
Published on the web May 13, 2009; doi:10.1246/bcsj.82.603

604
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
© 2009 The Chemical Society of Japan
cence spectra of the dendron part and the stilbene part to be
20% for trans-4. In THF, the fluorescence quantum yield (¯f)
of trans-1 (0.21) is slightly lower than that for trans-3 (0.28)
(Table 1). In aqueous solution, those of both trans-2 and -4 are
0.14. trans-1 and -2 gave a fluorescence lifetime of <0.5 ns and
that for trans-3 was 0.73 ns with single-exponential decay. On
the other hand, the fluorescence decay curves do not fit a
single-exponential function, but rather fit two exponential
functions to give 0.70 and 4.55 ns for trans-4. The shorter
lifetime (0.70 ns) and the longer lifetime (4.55 ns) could be
assigned to the monomer fluorescence and excimer or
aggregated form, respectively. These results indicate that the
structure and environment are not uniform in 4 in aqueous
solution. Furthermore, one can increase the fluorescence
lifetime of the stilbene core effectively by introducing a large
dendron group at the periphery of stilbene.
On irradiation with 320 nm light under argon, the trans
isomers 1-4 underwent isomerization around the double bond
to give the corresponding cis isomer (Figure 3). The quantum
yields of the trans-to-cis isomerization were similar among 1-3
(0.35-0.40), while that for trans-4 was much lower (0.063).
Figure 3b shows the time profile of the change of absorbance at
320 nm for trans-2 and -4.
One can compare the isomerization properties of these 3,3¤-
disubstituted dendrimers to another stilbene dendrimer series
having four dendron groups at the 3,3¤,5,5¤-positions on
stilbene (5 and 6 in Figure 4).15-17 In the series of 3,3¤,5,5¤-
tetrasubstituted dendrimers, the isomerization efficiency of
higher generations were similar to those of lower generations.
In those cases the time scale for rotation about the C=C double
bond does not change from lower to higher generations,
probably due to their volume conserving isomerization mech-
anism. It was found that the conformational change of the
stilbene core completes within the excited-state lifetime,
followed by the conformational change of the dendron part
on a different time scale. These isomerization properties may
be observed when the dendrimers exist as monomer in THF for
5 or unimolecular micelle in aqueous solution for 6, so that this
isomerization behavior can apply to 3,3¤-disubstituted stilbene
trans-3 in THF. On the other hand, the isomerization quantum
yield of trans-4 is much lower than the others. In addition, the
fluorescence lifetime of trans-4 was 0.70 and 4.55 ns. These
results suggest that trans-4 does not exist unimolecularly in
aqueous solution.
In summary the photochemical properties of 3,3¤-disubsti-
tuted stilbene dendrimers trans-3 and the corresponding
potassium salts trans-4 and their standard compounds trans-1
and -2 were studied. The fluorescence decay curves fit two
Figure 2. Absorption (solid line), fluorescence (dotted
line), and fluorescence excitation spectra (dashed line) of
trans-1-4.
Figure 3. (a) Change in the absorption spectra of trans-4 in KOH aq upon irradiation at 320 nm at room temperature under argon. (b)
Time dependence of the absorbance (320 nm) of trans-2 and -4 in KOH aqueous solution upon photoirradiation.

K. Kato et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
605
80 °C for 15 h. The aqueous solution was cooled down to room
temperature. 1 M HCl was slowly added dropwise to the solution
to acidify to pH 3 and the obtained precipitate was filtered and
washed five times with water to give 2 (35 mg, 81% yield).
1H NMR (400 Hz, DMSO-d6): ¤ 7.31-7.16 (m, 8H), 6.84-6.81 (m,
2H), 4.72 (s, 4H). Elemental analysis: Anal. Calcd for C18H16O6:
C, 65.85; H, 4.91; N, 0.00%. Found: C, 65.62; H, 5.03; N, 0.00%.
trans-3.
trans-3 was synthesized according to the above
procedure using G3-Br. 1H NMR (500 MHz, CDCl3, Me4Si): ¤
7.93 (d, J = 8.0 Hz, 16H), 7.36 (d, J = 8.0 Hz, 16H), 7.15 (t, J =
8.0 Hz, 2H), 7.00-6.99 (m, 4H), 6.94 (s, 2H), 6.76 (d, J = 8.0 Hz,
2H), 6.59-6.57 (m, 12H), 6.44-6.42 (m, 6H), 4.97 (s, 16H), 4.92
(s, 4H), 4.87 (s, 8H), 3.81 (s, 24H). MALDI-TOF MS Calcd for
C
128H112O30Na [M + Na]: 2151.7, found: 2151.1.
trans-4. trans-4 was synthesized according to the above
1
procedure using trans-3. H NMR (500 Hz, DMSO-d6): ¤ 7.92 (d,
J = 10 Hz, 16H), 7.50 (d, J = 10 Hz, 16H), 7.24-7.20 (m, 6H),
7.13 (d, J = 10 Hz, 2H), 6.87 (d, J = 10 Hz, 2H), 6.70 (s, 12H),
6.62 (s, 4H), 6.58 (s, 2H), 5.15 (s, 16H), 4.95 (s, 8H), 4.87 (s, 4H).
MALDI-TOF MS Calcd for C120H96O30Na [M + Na]: Calcd for
2041.0, found: 2040.1.
Figure 4. Structures of the stilbene dendrimer 5 and water-
soluble stilbene dendrimer 6.
This work was supported by a Grant-in-Aid for Science
Research in a Priority Area “New Frontiers in Photochromism
(No. 471)” from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan.
exponential functions to give 0.70 and 4.55 ns for trans-4,
indicating that the environment of trans-4 in aqueous solution
is not uniform. The quantum yield of isomerization (¯t¼c) for
dendrimer trans-4 was 0.063 and was much lower than that for
trans-2 (¯t¼c = 0.36), indicating that dendritic molecules
soluble in water may form a specific environment by varying
aggregated systems depending on the concentration to control
the efficiency of photochemical reaction and the excited state
properties.
References
1
C. Dugave, L. Demange, Chem. Rev. 2003, 103, 2475.
A. Momotake, T. Arai, J. Photochem. Photobiol., C 2004,
2
5, 1.
3
D. M. Junge, D. V. McGrath, Chem. Commun. 1997, 857.
D. M. Junge, D. V. McGrath, J. Am. Chem. Soc. 1999, 121,
4
Experimental
4912.
5
Measurement.
Absorption and fluorescence spectra were
D. Grebel-Koehler, D. Liu, S. D. Feyter, V. Enkelmann, T.
measured on a Shimadzu UV-1600 and on a Hitachi F-4000
fluorescence spectrometer, respectively. Fluorescence lifetimes
Weil, C. Engels, C. Samyn, K. Müllen, F. C. D. Schryver,
Macromolecules 2003, 36, 578.
were determined with
a
Horiba NAES-1100 time-resolved
6
L.-X. Liao, D. M. Junge, D. V. McGrath, Macromolecules
spectrofluorometer. The quantum yields of fluorescence emissions
and the quantum yield of the trans-to-cis isomerization were
determined by procedures in Ref. 15.
2002, 35, 319.
7
8
S. Li, D. V. McGrath, J. Am. Chem. Soc. 2000, 122, 6795.
L.-X. Liao, F. Stellacci, D. V. McGrath, J. Am. Chem. Soc.
trans-1. Ethyl bromoacetate (0.417 g, 2.50 mmol) was added to
a mixture of trans-3,3¤-dihydroxystilbene (0.0750 g, 0.353 mmol),
K2CO3 (0.233 g, 1.69 mmol), and 18-crown-6 (50 mg) in dry
acetone (50 mL) and the mixture was refluxed for 18 h. After
cooling down to room temperature, the mixture was filtered and
evaporated. The crude product was purified with column chroma-
tography eluting with dichloromethane/hexane = 1/1 and then
with gradually increasing proportion of dichloromethane (to
2004, 126, 2181.
9
S. Ghosh, A. K. Banthia, Z. Chen, Tetrahedron 2005, 61,
2889.
10 A. Ray, S. Bhattacharya, S. Ghorai, T. Ganguly, A.
Bhattacharjya, Tetrahedron Lett. 2007, 48, 8078.
11 M. Uda, A. Momotake, T. Arai, Photochem. Photobiol. Sci.
2003, 2, 845.
12 A. Momotake, T. Arai, Tetrahedron Lett. 2004, 45, 4131.
13 N. Yoshimura, A. Momotake, Y. Shinohara, Y. Nishimura,
T. Arai, Bull. Chem. Soc. Jpn. 2007, 80, 1995.
14 T. Mizutani, M. Ikegami, R. Nagahata, T. Arai, Chem. Lett.
2001, 1014.
dichloromethane 100%) to give product as
a white solid
1
(121 mg, 67% yield). H NMR (400 MHz, CDCl3, Me4Si): ¤ 7.28
(t, J = 8.0 Hz, 2H), 7.14 (d, J = 5.0 Hz, 2H), 7.07-7.06 (m, 2H),
7.04 (s, 2H), 6.83-6.81 (m, 2H), 4.66 (s, 4H), 4.29 (q, J = 10 Hz,
4H), 1.31 (t, J = 10 Hz, 6H). Elemental analysis: Anal. Calcd for
C22H24O6: C, 68.74; H, 6.29; N, 0.00%. Found: C, 68.54; H, 6.39;
N, 0.00%.
15 M. Uda, T. Mizutani, J. Hayakawa, M. Ikegami, R.
Nagahata, T. Arai, A. Momotake, Photochem. Photobiol. 2002,
76, 596.
trans-2. 1 M KOH aqueous solution (0.6 mL) was added to the
mixture of trans-1 (51 mg, 0.13 mmol) in ethanol/benzene (8 mL/
11 mL) and the mixture was refluxed for 2 h. After the solvents
were evaporated to remove benzene, water (10 mL) was added
to the residue to give a red solution, which was then stirred at
16 J. Hayakawa, A. Momotake, T. Arai, Chem. Commun.
2003, 94.
17 A. Momotake, J. Hayakawa, R. Nagahata, T. Arai, Bull.
Chem. Soc. Jpn. 2004, 77, 1195.