1,3,5-tristyrylbenzene dendrimers: A novel model system to explore oxygen quenching in a highly organized environment

Article abstract of DOI:10.1039/b303412g

Control of the efficiency of photoisomerization by oxygen was demonstrated for aryl ether type dendrimers containing photoresponsive 1,3,5-tristyrylbenzene. The large dendrons do not affect the photochemistry itself, due to volume-conserving isomerization

Full text of DOI:10.1039/b303412g

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1,3,5-Tristyrylbenzene dendrimers: a novel model system to explore
oxygen quenching in a highly organized environment
Mayuko Uda, Atsuya Momotake and Tatsuo Arai*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: arai@chem.tsukuba.ac.jp; Fax: ϩ81-298-53-6503; Tel: ϩ81-298-53-4315
Received 28th March 2003, Accepted 22nd April 2003
First published as an Advance Article on the web 25th April 2003
Control of the efficiency of photoisomerization by oxygen
was demonstrated for 1,3,5-tristyrylbenzene dendrimers 1–
3; the large dendrons do not significantly affect the photo-
chemistry itself due to volume-conserving isomerization
mechanisms but may affect the mobility of the molecules.
basic unit for comparison with other dendrimers, was prepared
from reaction of the Wittig–Horner reagent10 with 3,5-
dimethoxybenzaldehyde. A coupling reaction of 1,3,5-tris(3,5-
dihydroxystyryl)benzene with second and fourth generations
of aryl ether dendric bromides11,12 (G2-Br and G4-Br) gave
dendrimers 2 and 3, respectively. After purification by column
chromatography or GPC, dendrimers 1–3 were characterized
Previously we reported on the synthesis and photochemical
properties of stilbene dendrimers.1 The photoisomerization of
the dendrimers may proceed not by a one-bond rotation of 180Њ
but by a volume-conserving process. Water-soluble stilbene
dendrimers2 were also synthesized and showed unusual one-
way trans-to-cis photoisomerization. We are currently inter-
ested in a model system to understand the dynamic process of
quenching by oxygen in dendrimers. Incidentally, oxygen
quenching of pyrene in SDS micelles with polymers has been
investigated by van Stam et al.3 As appropriate molecules for
this study, we chose the photoresponsive 1,3,5-tristyrylbenzene
dendrimers. Dendrimers are attracting increasing attention due
to their three-dimensional, well-defined structure and ready
availability through iterative synthesis.4–9 The advantage of
using dendrimers for this study is that the production of singlet
oxygen and the dynamic quenching process by oxygen can be
revealed as the quenching rate of the singlet state is measured
with each generation of dendrimer. 1,3,5-Tristyrylbenzene was
chosen as the dendrimer core due to its long-lived excited sing-
let state and so as to be able to observe the triplet state after
quenching the singlet state by oxygen. Herein we report the
synthesis and photochemistry of novel 1,3,5-tristyrylbenzene
dendrimers. We have found that oxygen accelerates the effi-
ciency of E-to-Z isomerization and the quenching rate constant
of the excited state of the tristyrylbenzene core by oxygen
decreases with increasing generation.
1
by H NMR, MALDI-TOF MS, and UV absorption spectro-
scopy. Fluorescence and fluorescence excitation spectra of 1–3
are shown in Fig. 1. Fluorescence spectra of 1–3 were almost the
same whilst the fluorescence excitation spectra were different,
especially in the shorter wavelength region. The difference is
due to singlet energy transfer from the dendron group to the
core.
Zeroth, second and fourth generations of E,E,E-tristyryl-
benzene dendrimers 1–3 were synthesized for this study
(Scheme 1).† Zeroth generation dendrimer 1, which serves as a
Fig. 1 Fluorescence (FL) and fluorescence excitation (FLE) spectra of
1 (red), 2 (green), and 3 (blue) in THF. The spectra were measured with
the absorption maximum set to be < 0.1 and were normalized.
Scheme 1 Structures of tristyrylbenzene dendrimers 1–3.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 6 3 5 – 1 6 3 7
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Table 1 Quantum yields, fluorescence lifetimes, rate constants for quenching, fluorescence emission, intersystem crossing, and quantum yield of
intersystem crossing of tristyrylbenzene dendrimers 1–3
kisc(Ar) ϩ
kisc(O2)
kisc(Ar)
kisc(O2)
ΦE
Z(O2) ΦE
Z(O2)/
Z(Ar) Φf
τs(Ar) τs(air) τs(O2) kq/1010
kf/107
M
/106 MϪ1
/108 MϪ1 /108 MϪ1
ΦE
Z(Ar) ΦE
0.58
/ns
/ns
/ns
M
Ϫ1 sϪ1
Ϫ1 sϪ1 sϪ1
sϪ1
sϪ1
Φisc
1
2
3
0.08
0.08
0.07
7.3
6.3
3.3
0.68 20.3
0.64 18.6
0.61 17.5
13.2
14.0
13.9
4.6
5.8
8.0
1.71
1.21
0.67
3.3
3.4
3.5
7.9
8.6
8.0
1.68
1.17
0.68
1.76
1.26
0.76
0.81
0.73
0.61
0.50
0.23
To investigate oxygen quenching of the excited core of these
temperature did not affect the efficiency of the photoisomeriz-
ation which must therefore occur in the excited triplet state. The
photoisomerization was dramatically accelerated under oxygen
and the quantum yield of E-to-Z isomerization enhanced from
0.08 to 0.58, 0.08 to 0.50, and 0.07 to 0.23, respectively for 1, 2,
and 3 (Table 1). This result shows that the oxygen effect
decreased with increasing generation and is in good agreement
with the observed quenching rate constant by oxygen.
dendrimers, fluorescence lifetimes were measured at room
temperature in THF. Dendrimers 1–3 had fluorescence lifetimes
of 17–18 ns under argon. Under oxygen, these were 4.6, 5.8 and
8.0 ns for 1, 2 and 3, i.e. less than half of the corresponding
value under argon. Although the fluorescence lifetime under
argon was almost the same for each generation, those under
oxygen were different. Thus, the quenching rate constant
decreased from 1.71 × 1010
M
Ϫ1 sϪ1 for 1 to 1.21 × 1010
M
Ϫ1 sϪ1
The rate constant for fluorescence emission, kf, estimated
fromthefluorescencequantumyieldandthefluorescencelifetime
was almost constant at ca. (3.4 0.1) × 107 sϪ1 (Table 1). The
quantum yield of intersystem crossing, Φisc, was estimated from
the quantum yield of isomerization and the fluorescence life-
time, assuming that the isomerization takes place in the excited
triplet state after intersystem crossing to the triplet state as sug-
gested from the lack of a temperature effect as described above.
kisc is also almost constant at ca. (8.3 0.4) × 106 sϪ1 (Table 1).
These results indicate a very interesting finding, viz. that
the intrinsic photochemical behaviour was scarcely affected by
the presence of a large dendron group. As described above, the
increase of the quantum yield of E-to-Z isomerization should
be a consequence of acceleration of the intersystem crossing to
the triplet state by oxygen. Therefore, one can estimate the rate
constant of the intersystem crossing from the singlet excited
state to the triplet excited state accelerated by oxygen, kisc(O2),
from the fluorescence lifetime as shown in Table 1. The kisc(O2)
values are 1.68 × 108, 1.17 × 108, and 0.68 × 108 sϪ1, respectively
for 1–3. In addition, we could assume that most of the oxygen
quenching results in the production of singlet oxygen and the
triplet excited state of stilbene dendrimers.
In conclusion, control of the efficiency of photoisomeriz-
ation by oxygen was demonstrated for aryl ether type den-
drimers containing photoresponsive 1,3,5-tristyrylbenzene. The
large dendrons do not affect the photochemistry itself, due to
volume-conserving isomerization mechanisms, but do affect the
mobility of the molecules. In addition, the results provide
essential information for understanding the behavior of photo-
responsive large dendric macromolecules. The present finding
that oxygen can interact with photoexcited states in large con-
gested molecules—and be monitored by the quenching rate
constant and the reaction efficiency of the excited state mole-
cule—should be important in the construction of artificial
biomolecular systems and may open a new approach to
reveal dynamic quenching of excited states by oxygen in highly
organized and congested biological environments. Further
investigation into practical applications and the synthesis of
corresponding higher generation dendrimers is underway.
for 2 to 0.67 × 1010 Ϫ1 sϪ1 for 3 (Table 1). This result, in which
M
higher generations underwent oxygen quenching less effectively
than lower generations, suggests two simple effects of macro-
molecules on the interaction of oxygen with the excited core
moiety. One is the so-called eggshell effect, which derives from
the presumption that higher generation dendrimers have
harder outer layers (or higher density) thereby inhibiting inter-
action between the core moiety and oxygen. Another is the
effect of the molecular size which reduces the mobility of
higher generation dendrimers resulting in lower reaction rates.
On irradiation with 330 nm light in THF under argon, den-
drimers 1–3 exhibited a decrease in their absorption spectra in
the region 300–350 nm. The absorption change for irradiation
of 3 is shown in Fig. 2. Changes in the spectral profile among
1–3 are almost the same. Therefore, 1–3 should undergo the
same reaction on photoirradiation. The photochemical reac-
1
tion of 1 was followed by H-NMR spectroscopy and revealed
that the isomerization of 1 took place only between E,E,E
and Z,E,E isomers. Since the spectral changes upon photo-
irradiation of 2 and 3 are almost the same, we can surmise that
2 and 3 also undergo isomerization only between E,E,E and
Z,E,E isomers. The quantum yields of photoisomerization were
0.07–0.08. The efficiency of the isomerization is almost the
same among all generations in spite of huge differences in
volume between 1 and 3.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (417) and the 21st Century COE
Program from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of the Japanese Government
and by the Asahi Glass Foundation. We thank Dr. Ritsuko
Nagahata of the Reseach Center of Macromolecular Tech-
nology, National Institute of Advanced Industrial Science
and Technology (AIST) for carrying out the MALDI-TOF
mass spectroscopy.
Fig. 2 Change of the absorption spectrum of 3 on irradiation with 330
nm light in THF under argon: before (dotted line) and after irradiation
(solid line).
Usually, E-olefins deactivate from the excited singlet state by
three processes: intersystem crossing to the triplet state, fluor-
escence emission and E–Z isomerization.13,14 Of these deactiv-
ation processes, only E–Z isomerization is expected to be
affected by temperature. In all cases, however, changing the
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 6 3 5 – 1 6 3 7
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3: MALDI-TOF MS found: m/z 9962.7. Calc. for C660H564O90Na:
Notes and references
[M ϩ Na]ϩ, 9958.7; δH (600 MHz, CDCl3) 4.84–5.03 (180H, m,
Ph–CH2O–Ph), 6.49–6.63 (135H, m, Ar–H), 7.22–7.35 (249H, m,
† [3,5-Bis(diethoxyphosphorylmethyl)benzyl]phosphonic acid diethyl
ester: A mixture of 1,3,5-tris-bromomethylbenzene (2.2 g, 5.7 mmol)
and triethylphosphite (3.1 g, 18.6 mmol) was heated under nitrogen at
160 ЊC for 4 h. The mixture was dried under reduced pressure to give
[3,5-bis(diethoxyphosphorylmethyl)benzyl]phosphonic acid diethyl
ester (yellow oil, yield 2.96 g, 98.6%), which was used in the next reac-
tion without further purification. δH (200 MHz, CDCl3) 1.26 (18H, t,
J = 7.0 Hz, –OCH2CH3), 3.06 (3H, s, Ph–CH2PO–), 3.17 (3H, s,
Ph–CH2PO–), 3.95–4.14 (12H, q, J = 7.0 Hz, –OCH2CH3), 7.14–7.15
(3H, m, C6H3).
–CH᎐CH– and Ar–H).
᎐
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a Volume-conserving Isomerization
Mechanism, Photochem. Photobiol., 2002, 76, 13–22.
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assembly of dendrimer electrolytes: negatively and positively
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1: A solution of potassium tert-butoxide (2.08 g, 18.6 mmol) in THF
(15 ml) was added to the solution of [3,5-bis(diethoxyphosphoryl-
methyl)benzyl]phosphonic acid diethyl ester (2.96 g, 5.6 mmol) in THF
(15 ml) under nitrogen at 0 ЊC and the mixture was stirred for 1 h. 3,5-
Dimethoxybenzaldehyde (3.08 g, 18.5 mmol) in THF (15 ml) was added
and stirred for 1 h at room temperature. The reaction mixture was
poured into water and extracted with ether. The organic layer was dried
over MgSO4 and concentrated in vacuo. The residue was purified by
silica-gel column chromatography (eluent: hexane/ethyl acetate = 5/1) to
give 0.68 g (22%) of 1,3,5-tri(3,5-dimethoxystyryl)benzene 1 as white
needles; mp 182–183 ЊC; δH (200 MHz, CDCl3) 3.86, (12H, s, OCH3),
6.43 (3H, t, J = 2.2 Hz, Ar–H), 6.72 (6H, d, J = 2.2 Hz, Ar–H), 7.14 (6H,
s, –CH᎐CH–), 7.56 (3H, s, Ar–H); δC (50 MHz, CDCl3) 55.34, 100.17,
᎐
104.69, 129.27, 139.62, 161.10.
E,E,E-1,3,5-Tri(3,5-dihydroxystyryl)benzene: Boron tribromide
(1.7 g, 6.8 mmol) in dichloromethane (10 ml) was added to the solution
of 1 (0.39 g, 0.69 mmol) in dichloromethane (40 ml) at 0 ЊC and was
stirred at room temperature for 8 h. After the reaction completed, water
was added slowly to the reaction mixture, and then extracted with ether
(100 ml × 3). The combined extracts were dried over Na2SO4 and con-
centrated in vacuo. Purification by silica-gel column chromatography
(eluent hexane/ethyl acetate = 1/3) gave 150 mg (45%) of 1,3,5-tri(3,5-
dihydroxystyryl)benzene. δH (200 MHz, CD3OD) 4.87 (6H, s, Ph–OH),
6.22 (3H, t, J = 2.2 Hz, p-H in outer C6H3), 6.55 (6H, d, J = 2.2 Hz, o-H
in outer C6H3), 7.12 (6H, s, –CH᎐CH–), 7.58 (3H, s, inner C6H3).
᎐
2 (typical procedure): A mixture of G2-Br (0.76 g, 1.98 mmol),
E,E,E-1,3,5-tri(3,5-dihydroxystyryl)benzene (0.13 g, 0.28 mmol), 18-
crown-6-ether (0.06 g, 0.21 mmol) and K2CO3 (0.52 g, 3.76 mmol) in
2-butanone (30 ml) were refluxed under argon for 29 h. The solvent was
evaporated and the residue was dissolved in dichloromethane (50 ml).
After washing with water, the aqueous layer was further extracted with
another 50 ml of dichloromethane. The combined extracts were dried
over Na2SO4, then filtered and evaporated. Purification was achieved by
silica-gel column chromatography (eluent hexane/chloroform = 1/2) fol-
lowed by GPC to give 226 mg of 2 (35%) as a white powder. MALDI-
TOF MS found: m/z 2317.4. Calc. for C156H132O18Na: [M ϩ Na]ϩ,
2317.8; δH (200 MHz, CDCl3) 5.01–5.03 (36H, m, Ph–CH2O–Ph), 6.40–
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6.79 (27H, m, Ar–H), 7.08–7.17 (6H, m, –CH᎐CH–), 7.28–7.38 (60H,
m, Ar–H), 7.54 (3H, s, Ar–H).
᎐
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 6 3 5 – 1 6 3 7
1637