Synthesis and photochemical characteristics of amphiphilic enediyne dendrimers

Article abstract of DOI:10.1246/bcsj.82.723

Amphiphilic enediyne-cored dendrimers were prepared as pure cis and trans isomers. All dendrimers were fluorescent to give fluorescence quantum yield of 0.100.64, and they underwent efficient photochemical isomerization with quantum yields of 0.220.50 in

Full text of DOI:10.1246/bcsj.82.723

© 2009 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 82, No. 6, 723–729 (2009)
723
Synthesis and Photochemical Characteristics
of Amphiphilic Enediyne Dendrimers
Nao Yoshimura,1 Atsuya Momotake,1 Yoshihiro Shinohara,2 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
Received November 10, 2008; E-mail: arai@chem.tsukuba.ac.jp
Amphiphilic enediyne-cored dendrimers were prepared as pure cis and trans isomers. All dendrimers were
fluorescent to give fluorescence quantum yield of 0.10-0.64, and they underwent efficient photochemical isomerization
with quantum yields of 0.22-0.50 in THF. In water, other photochemical reactions took place. Fluorescence or triplet
lifetimes were also strongly affected by dendrimer generation and solvent polarity. The structure-photochemical property
relationship of new photoresponsive dendrimers is discussed.
The photochemical behavior of arylethenes and related
compounds has been the subject of intense research.1-3 Among
those, we have focused our attention on the effect of aromatic
groups on the photoisomerization of C=C double bonds
between one-way and two-way isomerization.2,4-6 These
studies are quite important in view of the photoisomerization
mechanism of chromoproteins such as rhodopsin and phyto-
chrome,7-10 or development of photoresponsive materials
having photoisomerizable cores such as C=C (or N=N)
double bonds. In the latter case, an approach using dendrimers
is interesting because when large dendrons are directly
or indirectly connected to a photoresponsive dye, a small
structural change at C=C double bonds by photoirradiation
could be amplified to a whole structure change. In this respect,
several dendrimers with stilbene core have been prepared and
the effect of the large dendron group has been explored.11-13
Thus, stilbene dendrimers with benzyl ether dendra at the
periphery effectively underwent mutual photoisomerization
in organic solvent such as THF.11 Figure 1 shows dendrimers
with hydrophobic benzyl ether dendra (a) and also shows
a dendrimer with a hydrophilic group at the terminus of
a phenyl ring (b). The hydrophilic stilbene dendrimers
underwent specific photoisomerization from the trans isomer
to the cis isomer. Furthermore, a fast process on a 10 ns time
scale and a slow process on a time scale of ¯s to ms was
observed.14
the fluorescence emission not only in trans isomer but also in
cis isomer and the contribution of 3c* in the equilibration in the
excited triplet state with t* and p* support the almost planar
structure due to the absence of steric effects even in the cis
isomer.
We have been interested in the effect of large substituents of
the dendritic structure in enediynes compounds. The photo-
chemical and photophysical properties of dendritic enediyne
in organic solvent have been studied.16 Studies on photo-
chemistry and GPC analysis of dendrimers in organic solvent
have been reported. We wish to report here the photochemistry
of enediyne type dendrimers in aqueous solution at physio-
logical pH.
3
3
Results and Discussion
Synthesis. Amphiphilic enediyne-cored dendrimers, cis-
and trans-Gn (n = 1, 3, and 4 for cis form, n = 1 and 3 for
trans form) were newly synthesized as illustrated in Scheme 1.
Their structures were identified by MALDI-TOF MS and NMR
spectroscopy.
Optical Properties. Figures 2a and 2b show the absorption
spectra of cis- and trans-G1 and cis- and trans-G3 in THF,
MeOH, and water at room temperature under Ar. For all
dendrimers, the absorption band around 300-400 nm which
is almost the same between these two dendrimers is mainly
due to the ³-³* band of the enediyne unit, and that around
280 nm is higher in absorbance for G3 than for G1 and is
assigned mainly due to the dendron units. In water, G1 showed
red-shift of the absorption maximum and smaller extinction
coefficient at 300-400 nm compared to those in THF and
MeOH. However, G3 gave similar absorption spectra in MeOH
and water, but showed different absorption spectrum in THF.
These results indicate that surrounding dendron units of G3
provided a greatly different environment of the core unit
compared to those of G1.
As to the photoisomerization of enediyne compounds, we
have reported the mutual isomerization of bis-phenylethynyl-
ethene where both cis and trans isomers gave fluorescence
emission and intersystem crossing to the triplet state: the
isomerization took place both in the excited singlet state and
triplet state.15 The potential energy surface of the triplet state
was estimated by quenching experiments with azulene and
oxygen: the triplet state is equilibrated among the trans (3t*),
cis (3c*), and twisted conformation (3p*). The observation of
Published on the web June 12, 2009; doi:10.1246/bcsj.82.723

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Bull. Chem. Soc. Jpn. Vol. 82, No. 6 (2009)
Properties of Amphiphilic Enediyne Dendrimers
Figure 1.
Figure 2. Absorption spectra of cis-G1 and cis-G3 (a) and trans-G1 and trans-G3 (b) in THF, MeOH, and water.
Fluorescence and fluorescence excitation spectra of cis-G1
spherical structure of the dendrimer surrounding the core
enediyne unit at the center. Therefore, one can expect that G3
exists as a globular structure in water solution and this structure
may inhibit self aggregation between enediyne unit for G3.
Figure 4 shows the change of the absorption spectra of cis-
G3 on excitation at maximum wavelength in THF (a) and water
(b). In THF cis-G3 underwent cis-trans isomerization to give
the trans/cis isomer ratio at the photostationary state, ([t]/
[c])pss = 42/58. However, in water, the absorbance decreased
due to the cis-trans isomerization accompanied by other
photochemical reactions as revealed by the appearance of a
new band at longer wavelength than the absorption band
derived from cis-trans isomerization of the enediyne unit. The
and cis-G3 are shown in Figure 3. The fluorescence values
of trans isomers are shown in Table 1. The fluorescence
spectrum derived from the enediyne unit of cis-G1 in water
(-max(FL) = 408 nm) showed large red-shift (34 nm) compared
to THF (-max(FL) = 374 nm) (Figure 3a). On the other hand,
those of cis-G3 showed similar shape and small spectral shift
between THF (-max(FL) = 374 nm) and water (-max(FL) =
385 nm) (Figure 3b).
The fluorescence quantum yield was determined to be 0.34
and 0.38 in THF and 0.11 and 0.23 in water for cis-G1 and cis-
G3, respectively (Table 1). 32 triethylene glycol units at the
termini of peripheral dendron groups in cis-G3 forced the

N. Yoshimura et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 6 (2009)
725
Scheme 1. a) cis-1,2-Dichloroethylene, Pd(PPh3)2Cl2, CuI, n-BuNH2, benzene, b) irradiation (308 nm) in benzene, and c) AGn-Br
(n = 1, 3, or 4), K2CO3, 18-crown-6-ether, cis- or trans-2, THF.
photochemical reaction other than the cis-trans isomerization
can be followed by GPC analysis (Figure 5), where the
retention time of the product is shorter than the starting
compound cis-G3 indicating that the molecular weight of the
product is higher than cis-G3.
cis-G1 underwent cis-trans photoisomerization in THF to
give the photostationary state isomer composition of ([t]/[c])pss
to be 48/52 as shown in the change of the absorption spectra
upon photoirradiation. In water cis-G1 underwent cis-trans
photoisomerization and also other photochemical reactions as
revealed by the slow rise of the absorbance at a longer
wavelength region at 380 nm. Actually, one can determine the
molecular weight of the main product as high as 3913.5 which
is almost double of that of cis-G1 by MALDI-TOF MS. The
photoinduced reaction other than photoisomerization of cis-G3
and cis-G1 observed in water may not neccessarily be

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Properties of Amphiphilic Enediyne Dendrimers
Figure 3. Fluorescence and fluorescence excitation spectra of cis-G1 and cis-G3 in THF, MeOH, and water under argon at room
temperature.
Table 1. Absorption Maximum (-max(abs)), Extinction Coefficient (¾), Fluorescence Maximum (-max(FL)), Fluorescence
Quantum Yield (¯f), Quantum Yield for Photoisomerization (¯iso), Quantum Yield for Intersystem Crossing (¯isc), Lifetime for
Excited Singlet State (¸S), and Lifetime for Excited Triplet State (¸T) for Amphiphilic Enediyne-Cored Dendrimers
Substrate
Solvent
-
max(abs)/nm
¾/cm¹1 mol¹1 dm3
-
max(FL)/nm
¯
f
¯
iso
¯
isc
¸S/nsa)
¸T/μs
cis-G1
THF
267, 334, 361
264, 332, 358
272, 324
277, 336, 362
277, 334
278, 327, 371
286, 333, 358
284, 330, 354
284, 324
283, 334, 359
282, 338, 364
282, 338, 367
277, 335, 361
278, 335, 369
278, 339
22500, 21700, 13300
22000, 20200, 12300
19800, 14800
75800, 20800, 13300
71000, 15500
67200, 15700, 9100
23400, 51400, 34500
25000, 53000, 35800
18600, 30900
80000, 52700, 35200
85200, 40900, 24500
64100, 35600, 20000
374, 396
371, 393
408, 428
374, 396
382, 402
385, 407
371, 392
368, 391
409, 432
372, 394
378, 403
383, 406
0.34 0.50 0.37
0.14 0.27 0.25
1.07
0.81
1.17
1.43
1.56
1.44
0.90
0.47
1.24
0.94
0.93
1.01
0.57
0.98
®
MeOH
Water
THF
MeOH
Water
THF
MeOH
Water
THF
MeOH
Water
THF
0.11
®
0.05
cis-G3
0.38 0.42 0.43
0.21 0.18 0.24
0.72
2.1, 11
8.2, 54
0.60
1.0
®
0.75
0.23
®
0.16
trans-G1
trans-G3
cis-G4
0.64 0.22 0.16
0.19 0.22 0.09
0.10
®
0.03
0.64 0.25 0.20
0.26
0.20
0.39
0.18
0.25
®
®
0.11
0.11
1.4, 14
8.5, 53
140000, 22600, 14200 374, 396
132000, 15900, 8700
123000, 15600
MeOH
Water
379, 401
382, 405
a) Average fluorescence lifetime.
intermolecular dimerization between two enediyne units,
because the reaction rate of cis-G3 was not very much slower
than that of cis-G1 (Figures 4d and 4e), despite the large
dendron groups in cis-G3. Thus, this photochemical reaction is
at the moment not totally elucidated, but one could at least
mention that it should be a photochemical oligomerization such
as dimerization.
Figure 6 shows the transient spectra and decay profiles of the
transient species of cis- and trans-G3 and cis- and trans-G1
observed on 308 nm laser excitation in THF, MeOH, and water.
In THF, the observed transient from cis- and trans-G3 with 720
and 750 ns, respectively, was quenched by oxygen and
assigned to the triplet state of enediyne unit (Figures 6a-6c).
The transient spectra and decay profile of cis- and trans-G1
(Figures 6d-6f) is almost the same to those of G3, but the
decay time constant is slightly shorter in G1 than in G3: the
triplet lifetime of cis- and trans-G1 is 570 and 600 ns and these
values should be the same within the experimental uncertainty.
The increase of the triplet lifetime in THF with increasing
generation indicates that the surrounding dendron group may
affect the molecular motion of the deactivation. One can expect
the potential energy surface of isomerization of these enediynes
to be similar to that of the parent compound bisphenyleth-
ynylethene (BEE).15 Thus, 3t*, 3p*, and 3c* should be populat-
ed in the excited triplet state and be equilibrated. The equilib-
rium constant among 3c*, 3p*, and 3t* in BEE is estimated to be
35:11:54.15 The triplet lifetime of G1 is longer than that of BEE
indicating that the alkoxy substituent increases the triplet state
3
lifetime probably increasing the population of the planar c*
3
and t*. Further increase in the triplet lifetime in G3 compared

N. Yoshimura et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 6 (2009)
727
Figure 4. Change in the absorption spectra upon 334 nm irradiation for cis-G3 in THF (a), for cis-G3 in water (b), and for cis-G1 in
water (c). The time-dependence of the absorbance during 334 nm light irradiation for cis-G3 (d) and for cis-G1 (e) in water.
was not quenched by oxygen. The observed transient with
54 ¯s lifetime could be assigned to the radical species of the
enediyne unit and subsequent reactions such as hydrogen
abstraction to give dimerization product. Such photoinduced
intermolecular reaction of enediyne in water has not been
reported, but the GPC analysis to observe the photochemical
product of higher molecular weight may support the above
discussion.
As to the triplet lifetime, both cis and trans isomers of the
same generation gave quite similar values within experimental
uncertainty. However, the triplet lifetime increased with
increasing generation from 580 « 20 ns of G1 to 730 « 20 ns
Figure 5. Overlay of GPC chromatograms of cis-G3 before
of G3 in THF and 1 to 1.7 ¯s in MeOH. Because of the lower
irradiation (dash line), irradiated (334 nm) for 30 min (thin
solubility of G1 in water, we could not successfully observe the
line), and for 400 min (solid line) in water.
transient spectra in water. If we compare the triplet lifetime in
to G1 indicates the further increase of the population of the
planar 3c* and 3t* by introduction of the large dendron group or
the suppression of the deactivation from the excited triplet state
to the ground state or the suppression of the rate constant of the
twisting around the double bond from the planar 3c* and 3t* to
different solvent, the lifetime increased from 580 ns to 1 ¯s
in G1 from THF to MeOH. But the triplet lifetime increased
from 730 ns to 1.7 ¯s in G3 from THF to MeOH and further
increases to 8.5 ¯s in water.
The solvent effect to increase the triplet lifetime should be
caused by the hydrogen-bonding interaction with protic solvent
at the periphery to induce hydrophobic interaction of the
molecule.
3
twisted p*.
The triplet lifetime of G3 (1.7 ¯s) (Figure 6g) in MeOH is
also longer than that of G1 (1 ¯s) (Figure 6h). The decay curve
in G3 fits two exponential functions with 1.7 ¯s due to the
triplet state and ca. 10 ¯s due to the ground state.
Conclusion
The observed transient of G3 in water showed double
exponential function to be 8.2 and 54 ¯s (Figure 6i). While the
shorter lived transient was quenched by oxygen and assigned to
the triplet state of the enediyne unit, the longer lived transient
Amphiphilic enediyne-cored dendrimers were successfully
prepared as pure cis and trans isomers. All dendrimers were
fluorescent in each solvent to give fluorescence quantum yield
of 0.10-0.64. The fluorescence spectrum of cis-G1 was red-

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Properties of Amphiphilic Enediyne Dendrimers
Figure 6. Transient spectra for cis-G3 (a) and trans-G3 (b) and decay profiles of cis- and trans-G3 (c) in THF. Transient spectra for
cis-G1 (d) and trans-G1 (e) and decay profiles of cis- and trans-G1 (f) in THF. Decay profiles of cis- and trans-G3 (g) and cis- and
trans-G1 (h) in MeOH. Decay profiles of cis- and trans-G3 (i) in water.
fluorescence spectra were recorded on a Shimadzu UV-1600 UV-
visible spectrophotometer and on a Hitachi F-4500 fluorescence
spectrometer, respectively. MALDI-TOF MS measurements were
taken on using 2-(4-hydroxyphenylazo)benzoic acid (HABA) as a
matrix without adding any salts.
Quantum yield of fluorescence emission and quantum yield of
photoisomerization were determined by a procedure previously
reported.11 Laser flash photolysis was also performed according to
a previous paper.11
shifted by 34 nm in water, compared to those in organic
solvents. On the other hand, spectral shift in cis-G3 was small
between organic solvent and water. In THF all dendrimers
underwent efficient photochemical isomerization even with
large dendron substituents. The quantum yields of photo-
chemical isomerization in THF were 0.22-0.50. In water, other
photochemical reactions took place, as revealed by the
appearance of a new band at longer wavelength than the
absorption band of cis and trans isomers. The triplet lifetime in
THF increased in higher generation indicating that the
surrounding dendron group may affect the molecular motion
of the deactivation. The observed transient of G3 in water
showed double exponential function. The shorter lived tran-
sient was assigned to the triplet state of the enediyne unit, the
longer lived transient was probably due to formation of ground
state intermediates such as radical species.
cis-1,6-Bis[3,5-di(tert-butyldimethylsilyl)oxy]hex-1,5-diyne-3-
ene (cis-2).
A mixture of 117 (1.29 g, 3.56 mmol), cis-1,2-
dichloroethylene (158 mg, 1.63 mol), Pd(PPh3)2Cl2 (20 mg, 0.03
mmol), CuI (32 mg, 0.18 mmol), n-BuNH2 (710 mg, 9.63 mmol) in
deaerated benzene (10 mL) was stirred at rt for 3 h. The reaction
was quenched by addition of water (50 mL) and the mixture
was extracted with dichloromethane (50 mL © 3). The organic
layer was washed with brine and was dried over MgSO4. After
evaporation, the residue was purified by silica gel chromatography
(eluent: hexane/dichloromethane = 5/1) to give cis-2 as colorless
oil (910 mg, 75%).
1H NMR (CDCl3, 400 MHz, Me4Si): ¤ 6.59 (4H, d, J = 2.4 Hz,
ArH), 6.32 (2H, t, J = 2.4 Hz, ArH), 6.06 (2H, s, CH=CH), 0.95
(36H, s, C(CH3)3), 0.16 (24H, s, Si(CH3)2); 13C NMR (CDCl3,
100 MHz, Me4Si): ¤ 156.36, 123.89, 119.20, 116.66, 113.55,
Experimental
Solvents and commercially available compounds were pur-
chased from standard suppliers and purified by standard methods.
1H NMR spectra were measured with
(400 MHz for H NMR) spectrometer in solution of CDCl3 with
tetramethylsilane as an internal standard. UV absorption and
a Bruker ARX-400
1

N. Yoshimura et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 6 (2009)
729
97.42, 86.38, 25.59, 18.14, ¹4.47; Elemental analysis. Anal. Calcd
for C42H68O4Si4: C, 67.32; H, 9.15; N, 0.00%. Found: C, 67.20; H,
9.30; N, 0.00%.
(16H, t, J = 4.8 Hz, den-CH2), 3.75-3.62 (48H, m, den-CH2),
3.57-3.53 (16H, m, den-CH2), 3.37 (24H, s, den-O-CH3);
13C NMR (CDCl3, 100 MHz, Me4Si): ¤ 160.12, 159.63, 138.82,
124.17, 120.84, 110.54, 106.06, 103.75, 101.19, 94.92, 87.80,
71.93, 70.83, 70.66, 70.58, 70.05, 69.67, 67.52, 59.05; MALDI-
TOF MS (m/z) [M + Na]+ calcd for C102H148O36Na, 1971.96;
found, 1971.53.
trans-G3. 1H NMR (CDCl3, 400 MHz, Me4Si): ¤ 6.76 (4H, d,
J = 2.0 Hz, core-ArH), 6.71-6.67 (26H, m, core + den-ArH),
6.60-6.51 (44H, m, den-ArH), 6.43 (16H, t, J = 2.2 Hz, den-
ArH), 6.27 (2H, s, core-CH=CH), 4.97-4.80 (56H, m, den-CH2-
Ar), 4.08 (64H, t, J = 4.8 Hz, den-CH2), 3.81 (64H, t, J = 4.8 Hz,
den-CH2), 3.73-3.56 (192H, m, den-CH2), 3.54-3.44 (64H, m,
den-CH2), 3.34 (96H, s, den-O-CH3); MALDI-TOF MS (m/z)
[M + Na]+ calcd for C438H628O156Na, 8407.14; found, 8410.60.
trans-1,6-Bis[3,5-di(tert-butyldimethylsilyl)oxy]hex-1,5-diyne-
3-ene (trans-2).
Laser flash photolysis was performed to a
solution of cis-2 (600 mg 0.80 mmol) in benzene (5 mL) in a quartz
cell. After irradiation time reached 2.5 h, solvent was evaporated.
The residue was purified by column chromatography eluting with
hexane-dichloromethane 10:1 to give trans-2 as a white solid
(230 mg, 38%).
1H NMR (CDCl3, 400 MHz, Me4Si): ¤ 6.57 (4H, d, J = 2.0 Hz,
ArH), 6.33 (2H, t, J = 2.0 Hz, ArH), 6.25 (2H, s, CH=CH), 0.98
(36H, s, C(CH3)3), 0.20 (24H, s, Si(CH3)2); 13C NMR (CDCl3, 100
MHz, Me4Si): ¤ 156.43, 123.82, 120.71, 116.70, 113.57, 94.79,
87.60, 25.64, 18.19, ¹4.41; Elemental analysis. Anal. Calcd for
C42H68O4Si4: C, 67.32; H, 9.15; N, 0.00%. Found: C, 67.13; H,
9.12; N, 0.00%. mp 84-86 °C.
cis-G1 (Typical Procedure). A mixture of cis-2 (248 mg,
0.33 mmol), amphiphilic dendron AG1-Br18 (760 mg, 1.54 mmol),
K2CO3 (380 mg, 2.75 mmol), 18-crown-6-ether (79 mg, 0.30
mmol) in THF (30 mL) was refluxed under nitrogen for 15 h.
After reaction was completed, the mixed solution was filtered and
evaporated. The residue was purified by silica gel chromatography
(eluent: dichloromethane/MeOH = 25/1) followed by GPC chro-
matography (CHCl3) to give cis-G1 as pale yellow oil (348 mg,
54%).
This work was supported by a Grant-in-Aid for Scientific
Research in a Priority Area “New Frontiers in Photochromism
(No. 471) and (No. 19550176) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
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J = 2.4 Hz, core-ArH), 6.66-6.49 (70H, m, core + den-ArH), 6.41
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core-ArH), 6.70-6.44 (150H, m, core + den-ArH), 6.38 (32H, t,
J = 2.0 Hz, den-ArH), 6.06 (2H, s, core-CH=CH), 4.96-4.82
(120H, m, den-CH2-Ar), 4.02 (128H, t, J = 4.8 Hz, den-CH2), 3.75
(128H, t, J = 4.8 Hz, den-CH2), 3.70-3.54 (384H, m, den-CH2),
3.52-3.42 (128H, m, den-CH2), 3.31 (192H, s, den-O-CH3);
MALDI-TOF MS (m/z) [M + K]+ calcd for C886H1268O316K,
17003.42; found, 17003.10.
trans-G1. 1H NMR (CDCl3, 400 MHz, Me4Si): ¤ 6.68 (4H, d,
J = 2.0 Hz, core-ArH), 6.57 (10H, m, core-ArH + den-ArH), 6.45
(4H, t, J = 2.2 Hz, den-ArH), 6.26 (2H, s, core-CH=CH), 4.94
(8H, s, den-CH2-Ar), 4.11 (16H, t, J = 4.8 Hz, den-CH2), 3.84
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