Unsurpassed cage effect for the photolysis of dibenzyl ketones in water-soluble dendrimers

Article abstract of DOI:10.1039/c1ob05244f

Amphiphilic water-soluble poly(alkyl aryl ether) dendrimers Gn (n = 1-3) with charge-neutral tetraethylene glycol monomethyl ethers at their periphery were synthesized as microreactors to control the photochemical reactions of dibenzyl ketone derivatives in aqueous solutions. Photophysical studies demonstrated that Gn can encapsulate organic molecules and provide a hydrophobic microenvironment. The product distribution of photolysis of dibenzyl ketone derivatives can be successfully controlled by encapsulating the substrates within dendrimers, and an unsurpassed cage effect of 1.00 is reached in high generation dendrimers, revealing that a thick and compact "shell" was formed at the periphery of the dendrimers. The cage effect is also significantly influenced by the substituent at the para-position of the guest molecules. The higher generation dendrimers exhibit a better confined microenvironment and the aggregates possess more compact cavities to "lock" the guests than the corresponding unimolecular dendrimers. After photolysis, the separation of products can be easily achieved by extracting from the dendrimer solutions and the dendrimers are simply recovered and reused.

Full text of DOI:10.1039/c1ob05244f

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PAPER
Unsurpassed cage effect for the photolysis of dibenzyl ketones in water-soluble
dendrimers†
Zhao Yuan, Jinping Chen,* Yi Zeng, Ying-Ying Li, Yongbin Han and Yi Li*
Received 16th February 2011, Accepted 6th June 2011
DOI: 10.1039/c1ob05244f
Amphiphilic water-soluble poly(alkyl aryl ether) dendrimers Gn (n = 1–3) with charge-neutral
tetraethylene glycol monomethyl ethers at their periphery were synthesized as microreactors to control
the photochemical reactions of dibenzyl ketone derivatives in aqueous solutions. Photophysical studies
demonstrated that Gn can encapsulate organic molecules and provide a hydrophobic
microenvironment. The product distribution of photolysis of dibenzyl ketone derivatives can be
successfully controlled by encapsulating the substrates within dendrimers, and an unsurpassed cage
effect of 1.00 is reached in high generation dendrimers, revealing that a thick and compact “shell” was
formed at the periphery of the dendrimers. The cage effect is also significantly influenced by the
substituent at the para-position of the guest molecules. The higher generation dendrimers exhibit a
better confined microenvironment and the aggregates possess more compact cavities to “lock” the
guests than the corresponding unimolecular dendrimers. After photolysis, the separation of products
can be easily achieved by extracting from the dendrimer solutions and the dendrimers are simply
recovered and reused.
faster reaction and higher conversion rate. The same dendrimers
were also used for the photosensitized oxygenation of sulfides
Introduction
by Shiraishi’s group, giving a similar conclusion.12 Recently, we
used carboxylic acid terminated poly(aryl ether) dendrimers as
microreactors to conduct photooxidation of olefins in aqueous
media, and demonstrated that the photooxidation pathways can
be successfully controlled by encapsulating the substrate and
sensitizer molecules in the same or different sets of dendrimers.13
Ramamurthy and co-workers14,15 synthesized several poly(alkyl
aryl ether) dendrimers with hydroxyl or carboxyl groups at the
peripheries and applied them as microreactors to conduct photore-
actions. It has been demonstrated that these dendrimers can act
as “unimolecular micelles” and offer much better constrainment
than traditional micelles, although their hydrophobic pockets are
not totally “leak-proof” for encapsulated guests.
The interest in “leak-proof” dendritic microreactors urges us
to develop new dendritic systems with “tight and closed walls”
keeping guests inside the hydrophobic cavities. In the present
work, we synthesized aseries of water-soluble poly(alkyl aryl ether)
dendrimers (Gn, n = 1–3) with a charge-neutral tetraethyleneglycol
monomethyl ether as the hydrophilic group at the periphery
(Fig. 1). Pyrene was used as a probe to evaluate the encapsulation
and microenvironment properties of the dendrimers and the
photolysis of dibenzyl ketone derivatives within Gn was studied,
affording an unsurpassed cage effect of 1.00 for high generation
dendrimers. Based on these results, we established that the tangled
tetraethyleneglycol monomethyl ether chains of dendrimers can
provide a compact and thick shell, acting as a “wall” to prevent
the escape of guest molecules. In addition, isolation of guests from
The selectivity of photoreactions continues to be one of the main
topics in organic chemistry and the development of catalytic
hosts capable of extraordinary efficiency and specificity in organic
photochemical reactions has been attracting much interest toward
this goal.1 Different kinds of hosts have been synthesized and
used as microreactors to conduct photoreactions of encapsulated
guests. Among these, zeolites,2,3 micelles,4,5 vesicles,6 cyclodextrins7
and so on,8,9 are commonly applied, and the photochemical
reactions inside them proceed with enhanced rates or a special
product selectivity, which is generally attributed to entropic
effects.10 Dendrimers as a new series of synthetic microreactors
have been attracting more attention in photochemistry.11–15
Amphiphilic dendrimers with a hydrophilic exterior and a
hydrophobic interior contain analogous microenvironments to
micelles. The initial example of the use dendrimers as microre-
actors for photoreactions was reported by Fre´chet’s group.11 They
designed and synthesized a series of amphiphilic dendrimers with
a benzophenonyl core as oxygen sensitizer, apolar interior and
polar exterior, and applied them to the photoinduced oxidation
of cyclopentadiene in O2-saturated methanol. The experiment
results showed that the higher generation dendrimers lead to
Key Laboratory of Photochemical Conversion and Optoelectronic Mate-
rials,Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing, 100190, P. R. China. E-mail: yili@mail.ipc.ac.cn,chenjp@
mail.ipc.ac.cn
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c1ob05244f
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Fig. 1 Structures of the amphiphilic dendrimers of generation 1 to 3 (G1–G3).
dendrimer solutions after the photolysis was easily realized by
extracting with n-hexane and the dendrimers could be recovered
and reused without loss of selectivity, which accords with the
concept of “green chemistry”.16
section. All the compounds have been purified by column
chromatography and characterized by H NMR, IR, and mass
spectrometry (MALDI-TOF or ESI).
1
With tetraethylene glycol monomethyl ether chains at the
periphery, all these dendrimers Gn (n = 1–3) are soluble in
water. Based on the fact that the amphiphilic dendrimers are
capable of forming aggregates in water,18 dynamic light scattering
(DLS) experiments were carried out to illustrate the aggregation
behavior of dendrimers Gn (n = 1–3) in aqueous solution. During
the DLS experiments the periphery group concentration of Gn
remains same to give a similar amount of hydrophobic moiety for
different generations. No measurable formation of particle was
observed for Gn (n = 1–3) as the periphery group concentration
below 9.6 ¥ 10-4 M, suggesting that all these dendrimers afford
unimolecular micelles at these experimental conditions. When the
periphery group concentration reaches 9.6 ¥ 10-3 M, particles with
hydrodynamic radius (Rh) of 944, 1380, 817 nm and polydispersity
of 0.45, 0.50, 0.40 were detected in G1, G2 and G3 aqueous
solution, respectively, indicative of the formation of the Gn
assembly. The DLS experimental results of Gn in aqueous solution
at different concentrations are summarized in Table 1 and a figure
for the DLS measurement on these dendrimers is provided in
supplementary material (see supplementary material Figure S14).
The formation of spherical aggregates was also observed by an
optical microscope in Gn aqueous solution. The size of the G2
aggregate is larger than those of G1 and G3. The proportion of
Results and discussion
Synthesis and aggregation behavior of dendrimers in aqueous
solution
The dendrimers (Gn, n = 1–3) are constituted with 1,1,1-tris(4-
hydroxyphenyl)ethane, 3,5-dihydroxybenzyl alcohol, a hexam-
ethylene chain and a tetraethylene glycol monomethyl ether,
acting as the core, branching juncture, spacer and the hydrophilic
periphery, respectively. The tetraethylene glycol monomethyl ether
functionalized poly(alkyl aryl ether) dendritic benzyl alcohols
(2, 4, 6, Gn-OH, n = 1–3) were synthesized by an adapted conver-
gent synthetic methodology17 as shown in Scheme 1. Alkylation
of 1,1,1-tris(4-hydroxyphenyl)ethane with 1,6-dibromohexane was
first performed to afford the alkyl core (7). The poly(alkyl aryl
ether) dendrimers (Gn, n = 1–3) were obtained by coupling the
alkyl core (7) with three pieces of Gn-OH (n = 1–3). In this
constitution, the first, second and third generation dendrimers
possess 6, 12 and 24 tetraethylene glycol monomethyl ether groups
at their peripheries, respectively. The details of the synthesis and
characterization of the compounds are described in Experiment
Scheme 1 Synthesis of water-soluble dendrimers.
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Table 1 DLS data of Gn in aqueous solution at different concentrations
of pyrene were measured in water and aqueous solutions of
the unimolecular/aggregated dendrimer Gn, and several of them
normalized to the first emission band are shown in Fig. 3. An
obvious increase of the third emission band relative to the first
one is observed in Gn (n = 1–3) dendrimer aqueous solutions,
whether in unimolecular or aggregate form, and the ratio I1/I3 vs.
generation is plotted in the inset of Fig. 3. There is a steep decrease
of I1/I3 between water and G1 aqueous solutions, demonstrating
that the dendrimers Gn (n = 1–3) are capable of encapsulating
molecules and provide a hydrophobic microenvironment. The
trend of the ratio I1/I3 with dendrimer generation in Gn (n = 1–3)
aqueous solution indicates that the higher generation dendrimer
provides a more hydrophobic cavity. The smaller value of I1/I3 in
aggregated G1 to G3 aqueous solution in comparison with that
in the corresponding unimolecular dendrimer solution shows that
the dendrimer aggregates can isolate the encapsulated molecules
from the surrounding water better.
Generation
Concentration (mM)
Rh (nm)
Polydispersity
Diluted
G1
16 ¥ 10-2
8.0 ¥ 10-2
4.0 ¥ 10-2
Not found
Not found
Not found
—
—
—
G2
G3
Concentrated
G1
G2
G3
1.6
0.80
0.40
944
1380
817
0.45
0.50
0.40
tetraethylene glycol groups in the dendritic architecture at a certain
generation, which leads to the difference of hydrophilic–lipophilic
balance, and the conformation of dendritic molecules might be the
cause for the observed anomalous behavior of the G2 aggregate.
Encapsulation ability and interior micropolarity of dendrimers
Prior to utilizing the dendrimers as reaction media, understand-
ing the encapsulation ability and interior micropolarity of the
dendrimers is essential. Pyrene is used as one of the most common
probes to detect the environment polarity, and is chosen as the
probe to detect the interior of dendrimers as others and we did
before.14,18,19 Fig. 2 illustrates the UV-VIS absorption spectra of
pyrene in water and aqueous solutions of Gn dendrimers. The
solubility of pyrene is very low in water, and is evidently enhanced
in the presence of Gn (n = 1–3) dendrimers. According to the UV-
VIS data, the solubility of pyrene increases by a factor about
37, 65, and 91 times higher for G1, G2, and G3 dendrimers,
respectively, indicative of an increase of the hydrophobic interior
with increasing generation. The average numbers of pyrene
molecules encapsulated within each dendrimer are estimated from
the extinction coefficient, which are 0.8, 1.4 and 2.0 for G1, G2 and
G3, respectively. These results indicate that dendrimers Gn (n =
1–3) are capable of providing a hydrophobic microenvironment
which sequesters lipophilic molecules within their interior cavities.
Fig. 3 Fluorescence spectra of pyrene in water and aqueous solution of
G3 dendrimers (c = 1.0 ¥ 10-5 M for unimolecular G3and 4.0 ¥ 10-4
M
for G3 aggregate). lex = 335 nm. (inset: plot of the ratio of I1/I3 bands of
pyrene vs. dendrimer generation).
Photolysis of dibenzyl ketone derivatives within dendritic
microreactors
Since the initial discovery by Turro and co-workers that the
product distribution resulting from photolysis of dibenzyl ketone
could be controlled by micelles,21 this reaction has been selected
as the benchmark to assess the efficacy of a medium as the “cage”.
Therefore, this Norrish type I reaction was selected to evaluate the
mobility of guests within the hydrophobic pockets of dendrimers
in this work. Irradiation of dibenzyl ketone derivatives results in
an a-cleavage followed by a decarbonylation to give the secondary
radical pair.3,22 These photolysis reactions are known to give a
range of products, and the product distributions are sensitive to
the surrounding reaction environment. In general, the three diaryl
ethanes AA, AB, and BB resulting from the radical pair are formed
in the ratio of 1 : 2 : 1 in homogenous solutions as shown in Scheme
2. When the secondary radical pair is held within a restrained
medium and can not move freely, the only expected product is AB.
The cage effect for this reaction can be calculated according to
eqn (1).
Fig. 2 Absorption spectra of pyrene in water and aqueous solutions of
dendrimers (c = 1.0 ¥ 10-5 M).
The ratio of the first and third emission bands of pyrene,
I1/I3, is relatively larger in polar environments and smaller in
less polar solvents, and this is usually utilized to characterize
the polarity of the microenvironment.20 The fluorescence spectra
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Scheme 2 Photolysis of dibenzyl ketones derivatives.
in Gn (n = 1–3) dendrimers gave a distribution of products
strikingly different from that in homogeneous solution. The cage
effect for 8a increased dramatically as the generations advanced,
demonstrating a much more confined microenvironment provided
by higher generation dendrimers. Excitingly, only AB product was
obtained within G2 and G3 dendrimers, affording an ideal cage
effect of 1.00. The photochemistry of 8a in restrictive media such
as traditional micelles and polymeric micelles has been extensively
investigated by Ramamurthy et al.,5,9 and the cage effects were
never over 0.80. The cage effects obtained from our tetraethylene
glycol monomethyl ether terminated dendrimers are also much
higher than the corresponding carboxylic acid terminated or
phenolic hydroxyl terminated poly(alkyl aryl ether) dendrimers,
as listed in Table 2.14,15 The impressive cage effect results for G2
and G3 indicate that the tetraethylene glycol monomethyl ether
terminated dendrimers possess extremely confined hydrophobic
cavities for the photolysis of 8a. Their “leak-proof” character of
G2 and G3 may be attributed to a thick and compact “shell” at
their periphery resulting from intertwisting of tetraethylene glycol
monomethyl ether chains, which prevents the reactants and radical
pairs escaping from hosts into the outer aqueous phase within their
lifetimes.
Previous studies have demonstrated that dendrimers show
different selectivityof encapsulatingsubstrates withvariedsizeand
shape,23 suggesting that smaller substrates are more conveniently
encapsulated in the inner cavities of dendrimers than the bulky
substrates. To further investigate the Gn dendrimers as confined
microreactors for conducting photoreactions of encapsulated
guest molecules, a series of dibenzyl ketone derivatives with
different size substituents (8a–8d) were selected as the substrates,
and their structures are shown in Table 3. The photolysis of these
compounds was conducted in aqueous solutions of unimolecular
dendrimers Gn (n = 1–3) and in hexane as a comparison. All
the different substituted dibenzyl ketones exhibit the same cage
effect of zero in homogeneous solution of hexane, but showed
an obvious substituent effect on photoproduct distributions in
dilute aqueous solutions of dendrimers Gn (n = 2–3). As the
size of the substituent of the dibenzyl ketones increases from
methyl (8a) to tert-butyl (8d), the cage effect drops from 1.00
to 0.58 and 0.72 in G2 and G3, respectively. In consideration
of the flexible backbone of dendrimers, the lower cage effects in
Gn (n = 2–3) can be explained by incomplete encapsulation of
the bulky dibenzyl ketones. Although the hydrophobicity of the
guest molecules increases significantly as the substituents change
from methyl to tert-butyl, the size of the guest molecule plays
a more important role for the observed cage effect than that of
(1)
The Norrish I reaction of 1-phenyl-3-p-tolyl-propan-2-one (8a)
has been widely used as the standard to estimate the cage effect
of various media, and was first investigated in dilute aqueous
solution of dendrimers Gn (unimolecular dendrimers, n = 1–3).
The substrate molecules were encapsulated in the inner cavities of
dendrimers by sonicating for 10 min and then stirring the mixture
of the substrate molecules (c = 4 ¥ 10-5 M) and the Gn (n = 1–3)
aqueous solution for 24 h. Then dialyses were performed in Gn (n =
1–3) aqueous solutions to remove the unencapsulated substrate
molecules. In order to get a similar amount of hydrophobic
moiety for different generation dendrimers, the concentrations
of dendrimers used in the photolysis were set to 0.16, 0.08 and
0.04 mM for the G1 to G3 unimolecular system, respectively.
According to the ratio of the concentrations of guest and
dendrimers, the average loading number per dendrimer is no more
than one for the substrate molecule. The photolysis was performed
in deaerated Gn aqueous solutions by exposing them to l > 300 nm
light. After irradiation, the products were extracted with hexane
or dichloromethane and analysed by gas chromatography. As a
comparison, the photochemical behavior of 8a in hexane was also
investigated. Analysis of the product distribution gave the cage
effects of photolysis in different media, which are summarized in
Table 2.
As expected, irradiation of 8a in hexane resulted in diaryl
ethane products AA, AB and BB in a 1 : 2 : 1 ratio with a cage
effect of zero. Photolysis of the substrate molecules encapsulated
Table 2 Photolysis of 8a in different media
Relative ratio
Medium
AA
AB
BB
Cage effect
Hexane
H2O
23
21
16
0
50
53
63
100
100
54
67
80
32
44
61
27
26
21
0
0.00
0.06
0.26
1.00
1.00
0.11
0.42
0.77
0.09
0.18
0.50
unimolecular G1a
unimolecular G2a
unimolecular G3a
0
0
b
G1–(OH)6
G2–(OH)12
G3–(OH)24
G1–(CO2H)6
G2–(CO2H)12
G3–(CO2H)24
15
11
4
16
18
10
28
17
6
10
10
10
b
b
c
c
c
a [G1] = 1.6 ¥ 10-4 M, [G2] = 8.0 ¥ 10-5 M, [G3] = 4.0 ¥ 10-5 M. b From
literature.14 c From literature.15
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Table 3 Photolysis of dibenzyl ketone derivatives in hexane and aqueous solutions of dendrimers
Cage effect of different guest moleculea,b
Medium
Hexane
0.00
0.00
0.00
0.00
0.00
0.00
Unimolecularc
G1
G2
G3
0.26
1.00
1.00
0.26
0.80
1.00
0.28
0.78
0.95
0.28
0.58
0.72
0.88
1.00
1.00
0.46
1.00
1.00
Aggregated
G1
0.67
1.00
1.00
0.62
0.96
1.00
0.60
0.84
0.96
0.56
0.60
0.92
0.92
1.00
1.00
0.47
1.00
1.00
G2
G3
a [guest molecule] ª 4 ¥ 10-5 M for unimolecular Gn and 4 ¥ 10-4 M for Gn aggregate. b Error range ~ 0.02. c [G1] = 1.6 ¥ 10-4 M, [G2] = 8.0 ¥ 10-5 M,
[G3] = 4.0 ¥ 10-5 M. d [G1] = 1.6 ¥ 10-3 M, [G2] = 8.0 ¥ 10-4 M, [G3] = 4.0 ¥ 10-4 M.
the hydrophobicity of the guest. The steric hindrance of guests
prevents dendrimers from forming a dense “shell”, resulting in a
leakage of the bulky substrates and the formation of radical pairs
from host dendrimers. A visual expression of the substituent effect
is illustrated in Scheme 3.
The photolysis of two alkyl ether substituted dibenzyl ketone
substrates (8e, 8f) in an aqueous solution of unimolecular
dendrimers Gn (n = 1–3) was also investigated. The methoxyl
substituted dibenzyl ketone (8e) with a similar size to 8b exhibits
cage effects of 0.88 in G1 dendrimer and 1.00 in Gn (n = 2,3),
which are much higher than the ethyl substituted ketone (8b) in
the corresponding conditions. Obviously, the higher cage effect
comes from the substituent at the para-position of guest molecule,
which interacts with the hydrophilic periphery ether chains of
dendrimers, and consequently, the encapsulation of the guest
molecule within dendrimers is stabilized and the cage effects are
enhanced. The effect of substituents was further strengthened by
the photolysis of 8f, which has a relatively long ether substituent.
Although the substituent possesses five atoms, the cage effects for
8f can still reach to 1.00 in aqueous solution of unimolecular
dendrimers Gn (n = 2, 3), while the lower cage effect in G1
can be attributed to the incompatible sizes of host and guest
molecules.
The DLS experiments have demonstrated that the amphiphilic
dendrimers Gn (n = 1–3) can form aggregates at high concentration
in aqueous solutions, and the photochemical behavior of 8a–8f
in dendrimer aggregates was examined. The concentrations of
dendrimers used in the aggregated system were set to 1.6, 0.8
and 0.4 mM for G1 to G3, respectively, and the concentration of
substrate molecules was about 4 ¥ 10-4 M. According to the average
diameter of dendrimer aggregate, every aggregate contains at least
several hundreds of dendrimer molecules, indicating that each
aggregate was occupied by several substrate molecules. The results
of the photolysis were also summarized in Table 3. Apparently,
Scheme 3 Representations of substituent effect: (a) 8a and (b) 8b in G3 dendrimer.
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the cage effects are generally higher in the aggregates than those in
corresponding unimolecular dendrimer unless it already reaches
1.00, revealing that the aggregates possess more compact cavities
to “lock” guests than the corresponding unimolecular dendrimers.
The guest molecules are kept isolated by each cavity of aggregates,
which makes the radical pairs formed in the cavity facilitate the
recombination within their lifetime.
Fluorescence spectra of pyrene
A pyrene stock solution was made in dichloromethane. The
required amount of this solution was transferred into a vial,
and the dichloromethane was evaporated. Then the necessary
amount of water or dendrimer solution was added to make the
concentration of pyrene 1 ¥ 10-6 M, and the solution was stirred
at room temperature for 24 h. Fluorescence spectra were obtained
by exciting the pyrene solution at 335 nm.
Conclusion
Inclusion of reactants within dendrimers
The amphiphilic water-soluble poly(alkyl aryl ether) dendrimers
(Gn, n = 1–3) with tetraethyleneglycol monomethyl ether as
the terminal groups can act as microreactors to control the
photochemical reaction of dibenzyl ketone derivatives, and an
unsurpassed cage effect of 1.00 is reached in high generation
dendrimers, which is much better than that of hydroxyl or carboxyl
group terminated dendrimers. The cage effect is also significantly
influenced by the substituent of the guest molecule. The higher
generation dendrimers exhibit better confined microenvironments,
and the aggregates possess more restricted cavities than the
corresponding unimolecular dendrimers. It is worth noting that
the products of photoreactions can be easily extracted from the
dendrimer solution and the dendrimer is readily recovered and
reused, which makes dendrimers a better choice of photochemical
microreactors.
The procedures adopted for all substrates were similar and one of
them is described below. A certain amount of substrate was added
to a glass reactor and a known amount of dendrimer in aqueous
solution was added to the reactor. After sonication for 10 min, the
solution was stirred for 24 h in the dark. At the same time dialysis
was performed to remove the substrate molecules located outside
of the dendrimers in solution.
Photolysis and product analysis in aqueous dendrimer solution
The samples were purged with nitrogen for 30 min prior to
use, and nitrogen was bubbled through the solution during the
photolysis. A 500 W medium-high pressure Hg lamp was employed
as the light source, and the glass reactor was able to cut off the
light with the wavelength below 300 nm. Irradiation for 30 min
resulted in ~80% conversion in all cases. After photolysis, reactants
and products were extracted from aqueous solution using n-
hexane or CH2Cl2 and the organic layer was dried over anhydrous
MgSO4, concentrated, and analyzed by gas chromatography. All
the photoreaction products were analyzed and characterized by
GC-MS.
Experimental section
Materials
All reagents were purchased from Acros, Alfa Aesar, or Aldrich
and used without further purification unless otherwise noted.
Milli-Q water was used in aqueous experiments. Dichloromethane
(CH2Cl2) was distilled from CaH2.
Synthesis of dendrimers
Bromide (1). To a stirred solution of the tetraethylene glycol
monomethyl ether (6.8 g, 32.8 mmol) in tetrahydrofuran (100 mL)
was added carbon tetrabromide (13.6 g, 41.0 mmol) followed
by the portionwise addition of triphenylphosphine (10.7 g,
41.0 mmol). The reaction mixture was stirred at room temperature
for 30 min, filtered, evaporated to dryness, and partitioned between
water and dichloromethane. The aqueous layer was then extracted
with dichloromethane (2 ¥ 100 mL), and the combined extracts
were dried with Na2SO4 and evaporated to dryness. The crude
product was purified by column chromatography, eluting with a
2 : 1 mixture of hexane and dichloromethane. After evaporation of
the solvents, the bromide (1) was obtained as a colorless oil (8.0 g,
90.0% yield).
Instruments
1H NMR (400 MHz) spectra were obtained from a Bruker Avance
P-400 spectrometer. IR spectra were recorded on a Varian Excal-
ibur 3100 spectrometer. MALDI-TOF mass spectra were carried
out on a Bruker Microflex spectrometer. ESI mass spectra were
recorded on a Waters LCT Premier XE spectrometer. Dynamic
Light Scattering measurements were performed on a Malvern
Zetasizer 3000HS. Absorption and emission spectra were run
on a Shimadzu UV-1601PC spectrometer and a Hitachi F-4500
spectrometer, respectively. Gas chromatography (GC) experiments
were carried out on a BeiFen 3420 gas chromatograph fitted with
3% OV-17 column and FID detector. GC-MS experiments were
run on a Waters GCT Premier GC mass spectrometer with a J&W
DB-5MS column.
G1–OH (2). To a stirred solution of bromide (1) (7.9 g,
29.2 mmol) and 3,5-dihydroxybenzyl alcohol (1.9 g, 13.3 mmol)
in acetone (200 mL) were added potassium carbonate (9.2 g,
66.5 mmol) and 18-crown-6 (0.70 g, 2.7 mmol). The reaction
mixture was heated at reflux under nitrogen for 18 h, filtered,
evaporated to dryness, and partitioned between water and
dichloromethane. The aqueous layer was then extracted with
dichloromethane (2 ¥ 100 mL), and the combined extracts were
dried (Na2SO4) and evaporated to dryness. The crude material
was then purified by column chromatography, eluting with a 50 : 1
mixture of dichloromethane and methanol to give the G1–OH
Absorption spectra of pyrene
An excess of pyrene was added to water or an aqueous solution of
the dendrimer (c = 1 ¥ 10-5 M), and stirred at room temperature for
24 h. Then the solution was filtered through a filter paper (medium
porosity) to remove any floating particles. The absorption spectra
of these solutions were recorded using quartz cells.
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1
(2) as a colorless oil (6.6 g, 95.4% yield). H NMR (400 MHz,
H), 1.45 (m, CH2CH2(CH2)2CH2CH2, 12 H). IR (KBr pellet) nmax:
CDCl3, ppm) d 6.51 (s, o-Ar, 2 H), 6.37 (s, p-Ar, 1 H), 4.56 (s,
ArCH2OH, 2 H), 4.07 (t, J = 4.7 Hz, OCH2CH2OAr, 4 H), 3.80
(t, J = 4.7 Hz, OCH2CH2OAr, 4 H), 3.70–3.50 (m, OCH2CH2O,
24 H), 3.34 (s, CH3O, 6 H). IR (KBr pellet) nmax: 3442, 2925, 2875,
1597, 1450, 1353, 1294, 1171, 1107 cm-1. MALDI-TOF MS: calcd.
m/z 520.3, found [M + Na]+: 543.2.
2925, 2857, 1596, 1450, 1353, 1294, 1168, 1107 cm-1. MALDI-
TOF MS: calcd. m/z 1506.8, found [M + Na]+: 1529.6.
G3–OH (6). This compound was prepared from 1.0 equiv
of 3,5-dihydroxybenzyl alcohol and 2.2 equiv of the G2–Br (5),
according to the general procedure for Gn–OH with potassium
carbonate and 18-crown-6 in acetone. The crude product was
purified by column chromatography, eluting with a 35 : 1 mixture
of dichloromethane and methanol, to give the G3–OH (6) as a
G1–Br (3). To a stirred solution of G1–OH (2) (3.8 g,
7.3 mmol) and 1,6-dibromohexane (14.2 g, 58.4 mmol) in THF
(200 mL) was added sodium hydride (1.75 g, 73.0 mmol). The
reaction mixture was heated at reflux under nitrogen for 24 h,
filtered, evaporated to dryness, and partitioned between water
and dichloromethane. The aqueous layer was then extracted with
dichloromethane (2 ¥ 100 mL), and the combined extracts were
dried (Na2SO4) and evaporated to dryness. The crude material
was then purified by column chromatography, eluting with a 50 : 1
mixture of dichloromethane and methanol to give the G1–Br
1
colorless oil (75.5% yield). H NMR (400 MHz, CDCl3, ppm) d
6.49 (s, o-Ar, 8 H), 6.46 (s, o-Ar, 6 H), 6.40 (s, p-Ar, 4 H), 6.35
(s, p-Ar, 3 H), 4.60 (d, J = 6.0 Hz, ArCH2OH, 2 H), 4.41 (s,
ArCH2OCH2, 12 H), 4.09 (t, J = 4.8 Hz, OCH2CH2OAr, 16 H),
3.92 (t, J = 6.5 Hz, CH2CH2CH2OAr, 12 H), 3.83 (t, J = 4.8 Hz,
OCH2CH2OAr, 16 H), 3.73–3.53 (m, OCH2CH2O, 96 H), 3.44
(m, ArCH2OCH2CH2, 12 H), 3.37 (s, CH3O, 24 H), 1.75 (quintet,
J = 7.0 Hz, CH2CH2CH2OAr, 12 H), 1.63 (quintet, J = 7.4 Hz,
CH2OCH2CH2CH2, 12 H), 1.45 (m, CH2CH2(CH2)2CH2CH2, 24
H). IR (KBr pellet) nmax: 3443, 2926, 2857, 1596, 1450, 1354, 1293,
1168, 1108 cm-1. MALDI-TOF MS: calcd. m/z 2993.8, found
[M + K]+: 3032.0.
1
(3) as a colorless oil (4.2 g, 84.4% yield). H NMR (400 MHz,
CDCl3, ppm) d 6.49 (s, o-Ar, 2 H), 6.40 (s, p-Ar, 1 H), 4.41 (s,
ArCH2OCH2, 2 H), 4.09 (t, J = 4.8 Hz, OCH2CH2OAr, 4 H), 3.84
(t, J = 4.8 Hz, OCH2CH2OAr, 4 H), 3.73–3.53 (m, OCH2CH2O,
24 H), 3.45–3.39 (m, ArCH2OCH2CH2 and CH2CH2Br, 4 H),
3.37 (s, CH3O, 6 H), 1.86 (quintet, J = 7.0 Hz, CH2CH2CH2Br, 2
H), 1.61 (quintet, J = 7.4 Hz, CH2OCH2CH2CH2, 2 H), 1.45 (m,
CH2CH2(CH2)2CH2CH2, 4 H). IR (KBr pellet) nmax: 2930, 2869,
1596, 1450, 1353, 1294, 1171, 1110 cm-1. MALDI-TOF MS: calcd.
m/z 682.3, found [M + Na]+: 705.2 and [M + K]+: 721.2.
Core (7). To a stirred solution of 1,6-dibromohexane (6.5 g,
26.4 mmol), potassium carbonate (1.7 g, 12.4 mmol) and 18-
crown-6 (0.17 g, 0.66 mmol) in acetone (100 mL) was added
1,1,1-tris(4-hydroxyphenyl)ethane (1.0 g, 3.3 mmol). The reac-
tion mixture was heated at reflux under nitrogen for 18 h,
filtered, evaporated to dryness, and partitioned between water
and dichloromethane. The aqueous layer was then extracted with
dichloromethane (2 ¥ 100 mL), and the combined extracts were
dried (Na2SO4) and evaporated to dryness. The crude material
was then purified by column chromatography, eluting with a
15 : 1 mixture of hexane and ethyl acetate to give the core (7)
as a colorless oil (2.2 g, 82.1% yield). 1H NMR (400 MHz,
CDCl3, ppm) d 6.98 (d, J = 6.9 Hz, m-Ar, 6 H), 6.77 (d, J = 6.9 Hz,
p-Ar, 6 H), 3.93 (t, J = 6.6 Hz, CH2CH2O, 6 H), 3.42 (t, J = 6.6 Hz,
CH2CH2Br, 6 H), 2.10 (s, CH3C, 3 H), 1.89 (quintet, J = 7.0 Hz,
CH2CH2CH2Br, 6 H), 1.78 (quintet, J = 6.7 Hz, CH2CH2CH2OAr,
6 H), 1.50 (m, CH2CH2(CH2)2CH2CH2, 12 H). IR (KBr pellet)
G2–OH (4). This compound was prepared from 1.0 equiv
of 3,5-dihydroxybenzyl alcohol and 2.2 equiv of the G1–Br (3),
according to the general procedure for Gn–OH with potassium
carbonate and 18-crown-6 in acetone. The crude product was
purified by column chromatography, eluting with a 40 : 1 mixture
of dichloromethane and methanol, to give the G2–OH (4) as a
1
colorless oil (78.1% yield). H NMR (400 MHz, CDCl3, ppm) d
6.49 (s, o-Ar, 6 H), 6.40 (s, p-Ar, 2 H), 6.35 (s, p-Ar, 1 H), 4.60 (d, J =
6.0 Hz, ArCH2OH, 2 H), 4.41 (s, ArCH2OCH2, 4 H), 4.09 (t, J =
4.8 Hz, OCH2CH2OAr, 8 H), 3.93 (t, J = 6.5 Hz, CH2CH2CH2OAr,
4 H), 3.82 (t, J = 4.8 Hz, OCH2CH2OAr, 8 H), 3.72–3.53 (m,
OCH2CH2O, 48 H), 3.44 (t, J = 6.5 Hz, ArCH2OCH2CH2, 4 H),
3.37 (s, CH3O, 12 H), 1.76 (quintet, J = 7.0 Hz, CH2CH2CH2OAr,
4 H), 1.63 (quintet, J = 7.4 Hz, CH2OCH2CH2CH2, 4 H), 1.45 (m,
CH2CH2(CH2)2CH2CH2, 8 H). IR (KBr pellet) nmax: 3449, 2925,
2856, 1597, 1451, 1353, 1294, 1168, 1108 cm-1. MALDI-TOF MS:
calcd. m/z 1344.8, found [M + Na]+: 1367.5.
n
max: 2937, 2860, 1608, 1508, 1473, 1291, 1248, 1181, 1119 cm-1.
MALDI-TOF MS: calcd. m/z 794.1, found [M + Na]+: 817.2.
G1. To a stirred solution of core (7) (0.76 g, 0.96 mmol) and
G1–OH (3) (1.65 g, 3.2 mmol) in THF (20 mL) was added sodium
hydride (0.76 g, 32 mmol). The reaction mixture was heated at
reflux under nitrogen for 48 h, filtered, evaporated to dryness, and
partitioned between water and dichloromethane. The aqueous
layer was then extracted with dichloromethane (2 ¥ 20 mL),
and the combined extracts were dried (Na2SO4) and evaporated
to dryness. The crude material was then purified by column
chromatography, eluting with a 40 : 1 mixture of dichloromethane
and methanol to give the G1 as a colorless oil (1.5 g, 74.0% yield).
1H NMR (400 MHz, CDCl3, ppm) d 6.97 (d, J = 8.8 Hz, m-Ar,
6 H), 6.76 (d, J = 8.8 Hz, p-Ar, 6 H), 6.50 (s, o-Ar, 6 H), 6.40
(s, p-Ar, 3 H), 4.41 (s, ArCH2OCH2, 6 H), 4.09 (t, J = 4.7 Hz,
OCH2CH2OAr, 12 H), 3.91 (t, J = 6.4 Hz, CH2CH2CH2OAr, 6
H), 3.84 (t, J = 4.7 Hz, OCH2CH2OAr, 12 H), 3.73–3.53 (m,
OCH2CH2O, 72 H), 3.44 (t, J = 6.6 Hz, ArCH2OCH2CH2, 6
H), 3.37 (s, CH3O, 18 H), 2.09 (s, CH3C, 3 H),1.77 (quintet,
G2–Br (5). This compound was prepared from the G2–OH (4)
according to the general procedure for Gn–Br with sodium hydride
in tetrahydrofuran. The crude product was purified by column
chromatography eluting with a 50 : 1 mixture of dichloromethane
and methanol to give the G2–Br (5) as a colorless oil (91.2%
1
yield). H NMR (400 MHz, CDCl3, ppm) d 6.50 (s, o-Ar, 4 H),
6.46 (s, o-Ar, 2 H), 6.40 (s, p-Ar, 2 H), 6.35 (s, p-Ar, 1 H), 4.41
(s, ArCH2OCH2, 6 H), 4.09 (t, J = 4.8 Hz, OCH2CH2OAr, 8 H),
3.92 (t, J = 6.5 Hz, CH2CH2CH2OAr, 4 H), 3.83 (t, J = 4.8 Hz,
OCH2CH2OAr, 8 H), 3.73–3.53 (m, OCH2CH2O, 48 H), 3.46–3.40
(m, ArCH2OCH2CH2 and CH2CH2Br, 6 H), 3.37 (s, CH3O, 12 H),
1.86 (quintet, J = 7.1 Hz, CH2CH2CH2Br, 2 H), 1.77 (quintet, J =
7.0 Hz, CH2CH2CH2OAr, 4 H), 1.63 (m, CH2OCH2CH2CH2, 6
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J = 7.0 Hz, CH2CH2CH2OAr, 6 H), 1.63 (quintet, J = 7.4 Hz,
CH2OCH2CH2CH2, 6 H), 1.45 (m, CH2CH2(CH2)2CH2CH2, 12
H). 13C NMR (100 MHz, CDCl3, ppm) d 159.8, 156.9, 141.5,
140.9, 129.4, 113.4, 106.0, 100.6, 72.6, 71.8, 70.7, 70.5, 70.4, 70.2,
acidified with 10% aqueous hydrochloric acid. The product was
extracted with ether. The combined organic layer was washed
with aqueous sodium bicarbonate solution, followed by water,
dried over anhydrous sodium sulfate and concentrated. To a 60%
sulfuric acid solution, the above obtained product was added
69.6, 67.6, 67.3, 58.9, 29.6, 29.2, 29.0, 26.0. IR (KBr pellet) nmax
:
◦
2932, 2869, 1596, 1508, 1450, 1353, 1294, 1248, 1176, 1110 cm-1.
MALDI-TOF MS: calcd. m/z 2113.2, found [M + Na]+: 2136.4
and [M + K]+: 2152.4. UV-vis (in H2O) lmax(e): 280 nm (4.48 ¥ 103
M-1 cm-1).
and stirred at 60–65 C for 12–15 h. The reaction was cooled to
room temperature and diluted with ice-cold water. It was extracted
with three portions of ether and the combined organic layer was
washed with aqueous sodium bicarbonate solution, dried and
concentrated. Column chromatography (silica gel, hexane/ethyl
acetate) was performed to obtain the pure ketones (30–40% yield,
>99% purity by GC).
8a:1H NMR (400 MHz, CDCl3, ppm) d 7.31 (m, 3 H), 7.15 (d,
J = 7.8 Hz, 4 H), 7.07 (d, J = 7.9 Hz, 2 H), 3.72 (s, 2 H), 3.69 (s, 2
H), 2.38 (s, 3 H).
8b:1H NMR (400 MHz, CDCl3, ppm) d 7.31 (m, 3 H), 7.16 (d,
J = 7.8 Hz, 4 H), 7.07 (d, J = 7.9 Hz, 2 H), 3.72 (s, 2 H), 3.69 (s, 2
H), 2.64 (q, J = 7.6 Hz, 2 H), 1.23 (t, J = 7.6 Hz, 3 H).
8c:1H NMR (400 MHz, CDCl3, ppm) d 7.31 (m, 3 H), 7.17 (m,
4 H), 7.08 (d, J = 8.0 Hz, 2 H), 3.72 (s, 2 H), 3.69 (s, 2 H), 2.89
(quintet, J = 6.9 Hz, 1 H), 1.24 (d, J = 6.9 Hz, 6 H).
8d:1H NMR (400 MHz, CDCl3, ppm) d 7.33 (m, 5 H), 7.15 (d,
J = 7.0 Hz, 2 H), 7.09 (d, J = 8.2 Hz, 2 H), 3.72 (s, 2 H), 3.69 (s, 2
H), 1.31 (s, 9 H).
8e:1H NMR (400 MHz, CDCl3, ppm) d 7.32 (m, 3 H), 7.15 (d,
J = 6.8 Hz, 2 H), 7.07 (d, J = 8.6 Hz, 2 H), 6.85 (d, J = 8.6 Hz, 2
H), 3.80 (s, 3 H), 3.71 (s, 2 H), 3.66 (s, 2 H).
8f:1H NMR (400 MHz, CDCl3, ppm) d 7.32 (m, 3 H), 7.14 (d,
J = 6.9 Hz, 2 H), 7.05 (d, J = 8.6 Hz, 2 H), 6.88 (d, J = 8.6 Hz, 2
H), 4.11 (t, J = 4.8 Hz, 2 H), 3.75 (t, J = 4.8 Hz, 2 H), 3.70 (s, 2
H), 3.65 (s, 2 H), 3.45 (s, 3 H).
G2. This compound was prepared from 1.0 equiv of core (7)
and3.3 equiv of G2–OH(4), accordingto the generalprocedure for
Gn with NaH in THF. The crude product was purified by column
chromatography, eluting with a 30 : 1 mixture of dichloromethane
and methanol, to give the G2 as a colorless oil (41.3% yield). 1H
NMR (400 MHz, CDCl3, ppm) d 6.96 (d, J = 8.8 Hz, m-Ar, 6 H),
6.75 (d, J = 8.8 Hz, p-Ar, 6 H), 6.50 (s, o-Ar, 12 H), 6.46 (s, o-Ar, 6
H), 6.40 (s, p-Ar, 6 H), 6.35 (s, p-Ar, 3 H), 4.41 (s, ArCH2OCH2, 18
H), 4.09 (t, J = 4.8 Hz, OCH2CH2OAr, 24 H), 3.92 (t, J = 6.5 Hz,
CH2CH2CH2OAr, 18 H), 3.83 (t, J = 4.8 Hz, OCH2CH2OAr, 24 H),
3.73–3.53 (m, OCH2CH2O, 144 H), 3.44 (m, ArCH2OCH2CH2, 18
H), 3.37 (s, CH3O, 36 H), 2.09 (s, CH3C, 3 H), 1.77 (quintet, J =
7.0 Hz, CH2CH2CH2OAr, 18 H), 1.63 (m, CH2OCH2CH2CH2, 18
H), 1.44 (m, CH2CH2(CH2)2CH2CH2, 36 H). 13C NMR (100 MHz,
CDCl3, ppm) d 160.3, 159.9, 157.0, 141.6, 141.0, 129.5, 113.5,
106.1, 105.7, 100.7, 100.3, 72.8, 71.9, 70.7, 70.5, 70.4, 70.3, 69.6,
67.8, 67.7, 67.4, 58.9, 50.4 29.6, 29.2, 25.9. IR (KBr pellet) nmax
:
2927, 2863, 1596, 1508, 1453, 1353, 1293, 1248, 1168, 1110 cm-1.
MALDI-TOF MS: calcd. m/z 4589.7, found [M + Na]+: 4612.1.
UV-vis (in H2O) lmax(e): 280 nm (9.32 ¥ 103 M-1 cm-1).
G3. This compound was prepared from 1.0 equiv of core (7)
and3.3 equiv of G3–OH(6), accordingto the generalprocedure for
Gn with NaH in THF. The crude product was purified by column
chromatography, eluting with a 20 : 1 mixture of dichloromethane
and methanol, to give G3 as a colorless oil (25.2% yield). 1H NMR
(400 MHz, CDCl3, ppm): d 6.96 (d, J = 8.8 Hz, m-Ar, 6 H), 6.75
(d, J = 8.8 Hz, p-Ar, 6 H), 6.50 (s, o-Ar, 24 H), 6.46 (s, o-Ar, 18 H),
6.40 (s, p-Ar, 12 H), 6.35 (s, p-Ar, 9 H), 4.41 (s, ArCH2OCH2, 42
H), 4.09 (t, J = 4.8 Hz, OCH2CH2OAr, 48 H), 3.92 (t, J = 6.5 Hz,
CH2CH2CH2OAr, 42 H), 3.83 (t, J = 4.8 Hz, OCH2CH2OAr, 48 H),
3.72–3.53 (m, OCH2CH2O, 288 H), 3.44 (m, ArCH2OCH2CH2, 42
H), 3.37 (s, CH3O, 72 H), 2.09 (s, CH3C, 3 H), 1.76 (quintet, J =
7.0 Hz, CH2CH2CH2OAr, 42 H), 1.64 (m, CH2OCH2CH2CH2, 42
H), 1.44 (m, CH2CH2(CH2)2CH2CH2, 84 H). 13C NMR (100 MHz,
CDCl3, ppm) d 160.4, 160.1, 160.0, 159.8, 157.1, 141.6, 141.1,
132.3, 129.6, 113.6, 108.1, 108.0, 107.7, 106.6, 106.2, 105.8, 100.8,
100.5, 72.9, 72.0, 70.8, 70.7, 70.5, 70.4, 69.7, 69.6, 67.8, 67.5, 67.4,
59.1, 29.8, 29.3, 26.1, 25.9. IR (KBr pellet) nmax: 2934, 2868, 1596,
1508, 1451, 1353, 1294, 1248, 1168, 1108 cm-1. MALDI-TOF MS:
calcd. m/z 9533.7, found [M + Na]+: 9557.0. UV-vis (in H2O)
Acknowledgements
Financial support from the National Natural Science Foundation
of China (Grant Nos. 20772134, 20733007, 21002109, 21073215,
21004072), the National Basic Research Program (Grant Nos.
2010CB934500 and 2007CB808004), and the Chinese Academy of
Sciences is gratefully acknowledged. J. P. Chen thanks the Beijing
Nova Program for the financial support.
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