Interfacial regions governing internal cavities of dendrimers. Studies of poly(alkyl aryl ether) dendrimers constituted with linkers of varying alkyl chain length

Article abstract of DOI:10.1021/jo200464g

This report deals with a study of the properties of internal cavities of dendritic macromolecules that are capable of encapsulating and mediating photoreactions of guest molecules. The internal cavity structures of dendrimers are determined by the interfacial regions between the aqueous exterior and hydrocarbon like interior constituted by the linkers that connect symmetrically sited branch points constituting the dendrimer and head groups that cap the dendrimers. Phloroglucinol-based poly(alkyl aryl ether) dendrimers constituted with a homologous series of alkyl linkers were undertaken for the current study. Twelve dendrimers within first, second, and third generations, having ethyl, n-propyl, n-butyl, and n-pentyl groups as the linkers and hydroxyl groups at peripheries in each generation, were synthesized. Encapsulation of pyrene and coumarins by aqueous basic solutions of dendrimers were monitored by UV-vis and fluorescence spectroscopies, which showed that a lower generation dendrimer with an optimal alkyl linker presented better encapsulation abilities than a higher generation dendrimer. Norrish type I photoreaction of dibenzyl ketone was carried out within the above series of dendrimers to probe their abilities to hold guests and reactive intermediate radical pairs within themselves. The extent of cage effect from the series of third generation dendrimers was observed to be higher with dendrimers having an n-pentyl group as the linker.

Full text of DOI:10.1021/jo200464g

ARTICLE
pubs.acs.org/joc
Interfacial Regions Governing Internal Cavities of Dendrimers.
Studies of Poly(alkyl aryl ether) Dendrimers Constituted with
Linkers of Varying Alkyl Chain Length
Baskar Natarajan,† Shipra Gupta,‡ Vaidhyanathan Ramamurthy,*,‡ and Narayanaswamy Jayaraman*,†
†Department of Organic Chemistry, Indian Institute of Science, Bangalore 560118, India
‡Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States
S Supporting Information
b
ABSTRACT: This report deals with a study of the properties of
internal cavities of dendritic macromolecules that are capable of
encapsulating and mediating photoreactions of guest molecules.
The internal cavity structures of dendrimers are determined
by the interfacial regions between the aqueous exterior and
hydrocarbon like interior constituted by the linkers that connect
symmetrically sited branch points constituting the dendrimer and
head groups that cap the dendrimers. Phloroglucinol-based poly-
(alkyl aryl ether) dendrimers constituted with a homologous
series of alkyl linkers were undertaken for the current study.
Twelve dendrimers within first, second, and third generations,
having ethyl, n-propyl, n-butyl, and n-pentyl groups as the linkers and hydroxyl groups at peripheries in each generation, were
synthesized. Encapsulation of pyrene and coumarins by aqueous basic solutions of dendrimers were monitored by UVꢀvis and
fluorescence spectroscopies, which showed that a lower generation dendrimer with an optimal alkyl linker presented better
encapsulation abilities than a higher generation dendrimer. Norrish type I photoreaction of dibenzyl ketone was carried out within
the above series of dendrimers to probe their abilities to hold guests and reactive intermediate radical pairs within themselves. The
extent of cage effect from the series of third generation dendrimers was observed to be higher with dendrimers having an n-pentyl
group as the linker.
’ INTRODUCTION
binding affinities through co-operative effects to investigate multi-
valent interactions between ligands and receptors.4 Interfacial
regions provide a surface for manipulation of dynamic inner cavity
structures and sizes. Linker moieties connecting the branch points
define interface regions that separate the inner cavities from an
aqueous exterior. Given that each branch point is arranged symme-
trically about a core in a dendritic structure, the interfacial regions
and the interior cavities are highly symmetrical and repetitive. Thus
the dynamic inner cavities of dendrimers are micro- or nanocontai-
ners capable of encapsulating guest molecules and controlling
reactions occurring within them. Reactions in such restricted spaces,
in general, are visualized in terms of a model based on “reaction
cavity” a term used commonly to define a constrained microenvir-
onment generated by container molecules, micelles, liposomes,
cyclodextrins and related cavitands, zeolites, polymers and
crystals.24ꢀ28 In assessing the dynamic inner cavities suitable
for photochemical reactions, it was demonstrated previously that
phloroglucinol-based poly(alkyl aryl ether) dendrimers mediated
such reactions at the microenvironments generated as a result of
the polar exterior and relatively nonpolar interior in an aqueous
Monodispersed macromolecules, namely, dendrimers, pro-
vide well-defined, inner cavities, resulting from branches-upon-
branches structural feature.1ꢀ6 The cavities provided by dendri-
mers could be visualized to be dynamic as it is not a cavity of fixed
size as in the case of, for example, cyclodextrins. Because of
fluctuations of the alkyl chain that constitutes the dendrimer, the
cavity sizes and shapes are expected to change with time, similar
to that in micelles, and hence we call the cavities provided by
dendrimers “dynamic cavities”. As a result of dense peripheries,
higher generation dendrimers exhibit better-defined, dynamic
inner cavities than lower generation ones. Exploring properties of
the inner cavities of dendrimers is a continuing theme in diverse
studies associated with dendritic macromolecules.7,8 Polar exter-
ior and nonpolar interior assisted by hydrophobicꢀhydrophilic
balances control the microenvironment of the inner cavities of
a dendritic structure.9ꢀ18 With the presence of well-defined,
dynamic inner cavities, more profoundly in the higher genera-
tions, dendrimers are similar to other container molecules,19ꢀ23
wherein the interior, exterior, and the interface have been utilized
for defined purposes. For example, the interior of dendritic struc-
tures, presenting dynamic inner cavities, are capable of encapsulating
small molecules. Exterior peripheries of dendrimers offer enhanced
Received: March 3, 2011
Published: April 13, 2011
r
2011 American Chemical Society
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ARTICLE
Scheme 1a
a Reagents and conditions: (i) NaH, H2O, DMF, 24 h; (ii) 2, K2CO3, 18-crown-ether (catalytic), 2-butanone/DMF (2:1), 90 °C, 24ꢀ72 h; (iii) Pd/C
(10%), H2, THF, reflux, 12ꢀ15 h.
medium.29,30 In order to assess how changes in the interface regions
modify the structures of dynamic inner cavities, we undertook to
study dendrimers with systematically changing linker lengths. With
this objective, we report herein synthesis of a homologous series of
poly(alkyl aryl ether) dendrimers, wherein the spacer lengths
between the branch points were varied systematically. The reaction
cavity properties inherent in these homologous series of dendrimers,
from one to three generations, presenting 6ꢀ24 hydroxyl group
functionalities at their peripheries, were investigated in an aqueous
basic solution. Each generation presented a set of homologous series
of dendrimers, with linker lengths varying from an ethyl to n-pentyl
group. Microenvironments resulting from dynamic inner cavities of
dendrimers were assessed with pyrene and coumarins as photo-
physical probes. Following this, photochemical reaction pertaining
to the cleavage of dibenzyl ketone was used to study changing
microenvironments in the case of third generation dendrimers with
differing linker lengths. Details of synthesis and photochemical
studies are reported below.
groups was performed by adopting the method established
previously.31 Required monomers with varying spacer length
were synthesized by O-alkylation of di-O-benzyl phloroglucinol
with excess dibromoalkanes, namely, 1,2-dibromoethane, 1,3-
dibromopropane, 1,4-dibromobutane, and 1,5-dibromopentane,
in the presence of K2CO3 and 18-crown-ether (catalytic). First
generation dendrimers with varying spacer lengths were synthe-
sized by a 3-fold alkylation of phloroglucinol triacetate (1)31 with
monomer 2, in the presence of NaH and water (Scheme 1).
Subsequent removal of benzyl groups (Pd/C, H2) led to hydro-
xyl group terminated first generation dendrimers, C2G1, C3G1,
C4G1, and C5G1. Synthesis of second generation dendrimers,
CnG2, was performed through 6-fold alkylation of CnG1 den-
drimers with monomer 2, in the presence of K2CO3 and
18-crown-ether (catalytic) in 2-butanone/DMF (2:1). Deprotec-
tion of benzyl groups afforded hydroxyl group terminated second
generation dendrimers CnG2 in good yields. Repeating the
sequence of reactions with CnG2 dendrimers, 12-fold O-alkyla-
tion followed by deprotection afforded third generation dendri-
mers CnG3 in a moderate yield, even when the reaction was
conducted for a longer period. The benzyl-protected dendrimers
were colorless gums, whereas hydroxyl group terminated dendri-
mers were white foamy solids. Further, benzyl-protected dendrimers
’ RESULTS AND DISCUSSION
Synthesis of Dendrimers. Synthesis of phloroglucinol-based
poly(alkyl aryl ether) dendrimers with various alkyl spacer
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Table 1. Solubilization of Pyrene and Its Relative I3/I1
Fluorescence Band Intensity in Aqueous Basic Solutions of
Dendrimers
dendrimers possess fewer alkyl spacers than third generation
dendrimers. On the other hand, hydrophobicities generated at
the interior voids of the dendritic structure are related to the alkyl
chain length. Thus, C5G2 appeared to provide higher hydro-
phobicity than C2G3, even when the latter is a third generation
dendrimer.
aq solution
solubilized
ΔGtr
of dendrimer
pyrene (μM)
(cal/mol)
I3/I1
In order to assess polarities of microenvironments generated at
the dendritic interior cavities, fluorescence spectra of pyrene in
aqueous basic solutions of dendrimer were recorded (Figure 1).
The ratio of vibrational band intensities I3/I1 of pyrene fluores-
cence is known to depend on the microenvironment polarity.34,35
Pyrene solubilized aqueous basic dendrimer solutions (200 μM)
were prepared, and the I3/I1 values arising from pyrene fluores-
cence were measured (Table 1). A higher I3/I1 value indicates a
hydrophobic environment and vice versa. It was found that across
the dendrimer generations (G1ꢀG3) with uniform spacer lengths,
I3/I1 increased with increasing generations. On the other hand,
within a dendrimer generation with various spacer units, I3/I1
was found to increase progressively from ethyl to n-pentyl spacer.
The third generation C5, C4, and C3 spacered dendrimers
exhibited higher hydrophobic interior (I3/I1 = 0.84, 0.75, 0.79,
respectively), whereas third generation C2 spacered dendrimer
showed lower microenvironment polarity (I3/I1 = 0.61). Among
second generation dendrimers, I3/I1 for C5 spacered dendrimer
was 0.78, which was ∼1.2ꢀ1.5 times higher than propyl (C3) and
ethyl (C2) spacered dendrimers. In the case of first generation
dendrimers, there was no increment in the I3/I1 values, which
reflected the absence of a hydrophobic interior arising from this
series of dendrimers, except for the C5G1 dendrimer (I3/I1 =
0.67). A broad excimer emission of pyrene was detected for the
first generation dendrimers (Figure 2), which weakened further
for the second generation dendrimers. Third generation dendri-
mers did not exhibit detectable excimer emission. Excimer
emission results from the association of an excited state pyrene
molecule with ground state molecule. This also could result from
ground state aggregation of pyrene molecules. Aggregation is
more likely in a hydrophilic environment and the appearance of
an excimer in G1 but not in G3 dendrimer is in agreement with
the micropolarity reported by pyrene molecules. The higher the
polarity (G1 dendrimer; lower I3/I1), the higher are the chances
of aggregation, and excimer emission is more probable. The local
concentration of pyrene is likely to be more in G1 than in the G3
dendrimer, which could also be the cause for excimer emission in
the G1 dendrimer. Further, as observed in the solubilization
experiments, C5G2 showed higher microenvironmental polarity
than C2G3 and to an extent C3G3, thereby indicating that the
role of the alkyl chain constituting the interfacial region is as
important as a higher generation of the dendrimer.
G1
C2
C3
C4
C5
C2
C3
C4
C5
C2
C3
C4
C5
1.33
1.58
1.73
2.32
2.80
3.48
4.40
11.3
5.71
8.36
17.9
33.2
ꢀ709
ꢀ812
0.52
0.56
0.56
0.67
0.53
0.63
0.64
0.78
0.61
0.79
0.75
0.84
ꢀ866
ꢀ1040
ꢀ1151
ꢀ1280
ꢀ1418
ꢀ1976
ꢀ1572
ꢀ1798
ꢀ2248
ꢀ2614
G2
G3
were soluble in many organic solvents, whereas hydroxyl group
terminated dendrimers were soluble in EtOAc, THF, MeOH,
DMF, and DMSO and insoluble in PhMe, Et2O, and CH2Cl2.
Characterizations of dendrimers CnG1ꢀCnG3 were performed
1
by H and 13C NMR spectroscopies. The generation growth,
and monodispersities of dendrimers were also confirmed by ESI-
MS or MALDI-TOF mass spectrometry and elemental composi-
tion analyses.
Assessing Dynamic Inner Cavities with Probes. Studies of
the microenvironments attendant in each dendrimer in the series
CnG1ꢀCnG3 were assessed initially by identifying the extent of
pyrene solubilization in aqueous basic solutions, which was
prepared by addition of the required molar equivalents of NaOH.
To assess solubilization, pyrene was admixed with an aqueous
basic solution of dendrimers and stirred at 25 °C for 12 h in dark,
after which the solution was filtered, and the extent of pyrene
solubilization in each dendrimer was determined by UVꢀvis
absorption spectroscopy (ε335 50,730 molꢀ1 cmꢀ1).32,33 The
amount of pyrene solubilized in water (pH ∼10) was estimated
to be 0.4 μM, whereas that in aqueous basic third generation C2,
C3, C4, and C5 dendrimer solutions (100 μM) was found to
be 5.7, 8.4, 17.9, and 33.2 μM, respectively. A similar pyrene
solubilization trend was observed with second generation den-
drimers, with varying alkyl linker lengths. Across three genera-
tions, first generation dendrimers CnG1 showed the least pyrene
solubilization. The solubility of pyrene in aqueous basic solutions
of dendrimers was also adjudged by the free energy (ΔGtr) of
transfer associated with solubilization (Table 1). The free energy
of transfer became favorable progressively as the dendrimer
generations advanced and as the linker lengths progressed from
ethyl to n-pentyl group. From the solubilization assessments, a
relation of interfacial regions constituted by alkyl group versus
generation number emerges. The most distinct observation is the
higher solubilization of pyrene by C5G2 compared with that
by C2G3 and C3G3, i.e., although C2G3 and C3G3 are third
generation dendrimers, they solubilize pyrene less than C5G2,
which is a second generation dendrimer with a pentamethylene
linker between the branch points. Qualitatively, the number of
aromatic rings and the number of hydroxyl groups remain
uniform within a generation, leaving differences in the solubility
profiles to remain with the alkyl chain length. Second generation
Assessing Dynamic Inner Cavity with Coumarins as
Probes. Subsequent to pyrene solubilization experiments,
microenvironments of the dendrimers were assessed with coumarin
dyes, comprising coumarin 480 (C480), coumarin 1 (C1) and
coumarin 153 (C153) (Figure 3). Coumarins belong to a family
of laser dyes whose fluorescence quantum yield and lifetime
increase with the decrease in solvent polarity.36 These probes are
used extensively in various supramolecular assemblies.37,38 Fluores-
cence spectra of C1, C480 (20 μM), and C153 (40 μM) in
water and in aqueous basic solutions containing dendrimers
(300ꢀ800 μM) (pH ∼10) were recorded. In a titration
experiment, aqueous basic dendrimer solution was added to
aqueous coumarin solutions, until no further spectral change
was observed. Table 2 shows λem of C480, C1, and C153 after
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ARTICLE
Figure 1. Emission spectra of pyrene in the presence of dendrimers. [pyrene] = 0.01 mM and [dendrimer] = 0.2 mM in aq NaOH (0.1 M), excitation
wavelength = 335 nm.
indicated a decrease in the polarity of the microenvironment
possessing the probe.39,40 It was observed that across the
dendritic generations G1ꢀG3 with uniform spacer lengths, blue
shift increased with increasing dendrimer generations. In the case
of C480, blue shifts for C5G3 and C5G2 dendrimers were
observed to be 15 and 8 nm, respectively, whereas C5G1 did
not exhibit a shift in λem. Similarly, for C153, blue shifts for C5G3
and C5G2 dendrimers were observed to be 36 and 26 nm,
respectively, and there was no shift with the C5G1 dendrimer. In
the case of C1 dye, addition of C5G3 and C5G2 dendrimers
afforded a blue shift of 18 and 14 nm, respectively, with no shift in
the case of C5G1 dendrimer. Negligible blue shift in the emission
spectra of probes on addition of G1 dendrimers was in accor-
dance with the results obtained for pyrene as probe.
On the other hand, within a dendrimer generation with various
Figure 2. Emission spectra of pyrene in the presence of first generation
spacer units, blue shift was found to be higher for C5, C4 spacers,
dendrimers C2G1ꢀC5G1 in aq alkaline medium (0.2 mM), λex = 335 nm.
when compared to C3 and C2 spacer dendrimers. For example,
for C480, within the third generation dendrimers, blue shift was
15 nmfor C5G3, whereas the shift was 10nm for C2G3 dendrimer.
Similarly, for C153, a blue shift of 36 nm was observed for C5G3,
whereas that for C2G3 was 22 nm (Figure 4). The same trend was
observed in case of C1 dye. As observed with pyrene probe, the
above results indicated that across the generations, third genera-
tion dendrimers possess more hydrophobic environments,
whereas within a generation, there was a continuous decrease
in hydrophobicity of the environment with systematic decrease
in the spacer length.
Figure 3. Molecular structures of coumarins used for solubilization
studies.
the addition of CnG1ꢀCnG3 dendrimers. As an example, the
emission spectra of C153 recorded in C5G1, C5G2, and C5G3
are shown in Figure 4. A blue shift in λem was observed in
general, upon addition of a dendrimer to coumarin, which
Analysis of results with three different coumarins also revealed
that the blue shift was higher in case of C153 (36 nm), in
comparison to C1 and C480 (18 and 15 nm, respectively),
presumably due to the nonpolar nature of C153,41 having a CF3
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Table 2. Blue Shifts in Emission λmax and ET 30 Values of Coumarins in Various Dendrimer Solutions
coumarin 480a,b
coumarin 1a,c
coumarin 153d
aq dendrimer
solution
emission (λmax
)
blue shift
(nm)
ET value
emission (λmax
)
blue shift
(nm)
ET value
emission (λmax
)
blue shift
(nm)
ET value
(nm)
(kcal/mol)
(nm)
(kcal/mol)
(nm)
(kcal/mol)
C2G1
C3G1
C4G1
C5G1
C2G2
C3G2
C4G2
C5G2
C2G3
C3G3
C4G3
C5G3
487
487
487
487
487
483
479
479
477
475
472
472
0
0
63.1
63.1
63.1
63.1
63.1
60.3
58.5
58.5
57.7
57.0
55.4
55.4
470
470
470
470
466
462
459
456
456
456
454
452
0
0
63.1
63.1
63.1
63.1
53.0
51.8
50.8
49.9
49.9
49.9
49.2
48.5
550
550
550
550
548
534
530
524
528
524
524
514
0
0
63.1
63.1
63.1
63.1
51.7
48.1
47.1
45.5
46.6
45.5
45.5
43.0
0
0
0
0
0
0
0
4
2
4
8
16
20
26
22
26
26
36
8
11
14
14
14
16
18
8
10
12
15
15
a [CnG1] = 0.4 mM in 0.1 M aq NaOH; [CnG2] and [CnG3] = 0.3 mM in 0.1 M aq NaOH. b [C480] = 0.02 mM; λem for C480 in water = 487 nm.
c [C1] = 0.02 mM; λem for C1 in water = 470 nm. d [CnG1] = 0.8 mM in 0.1 M aq NaOH; [CnG2] and [CnG3] = 0.6 mM in 0.1 M aq NaOH; [C153] =
0.04 mM; λem for C153 in water = 550 nm.
Figure 4. Emission spectra of coumarin 153 in the presence of various generation dendrimers (CnG1ꢀCnG3, n = 2, 3, 4, 5) [CnG1] = 0.8 mM in 0.1 M
aq NaOH; [CnG2] and [CnG3] = 0.6 mM in 0.1 M aq NaOH; [C153] = 0.04 mM; λex = 400 nm and λem for C153 in water = 550 nm.
group and the attendant higher affinity of this dye to occupy
hydrophobic sites within dendritic interior.
ET 30 value for C5G3 was 55.4 kcal/mol (methanol-like),
whereas that for C2G3 was 57.7 kcal/mol (glycerol-like). The
above assessment of microenvironments at dendritic interiors
using coumarins reiterated previous observations that alkyl chain
length connecting the branch junctures of dendritic structure is
an important criterion, in addition to dendrimer generations. As
observed with pyrene, experiments with coumarins also showed
higher microenvironmental polarity for second generation C5G2
dendrimer than third generation C2G3 and C3G3 dendrimers.
Photolysis of 1-Phenyl-3-p-tolyl-propane-2-one Included
within the CnG3 Dendrimers. Studies with polarity probes
Apart from the magnitude of blue shifts, the polarity of the
microenvironments located inside the dendrimers was also
judged on an ET 30 scale for all the three coumarins. Usually
the more hydrophobic the environment, the lesser is the ET 30
value.42 The ET 30 values obtained in all the cases corresponded
to the observed magnitude of the blue shifts. For example, across
generations, for C480, ET 30 value for C5G3 was 55.4 kcal/mol
(methanol-like), whereas for C5G1, the value was 63.1 kcal/mol
(water-like). Similarly, within a generation, with varying spacers,
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Scheme 2. Photolysis of Dibenzylketone inside Aqueous Basic Solutions of Dendrimers
length decreased to C4G3, the cage effect also decreased to 0.62.
The cage effect for C3G3 was observed to be 0.54, whereas
negligible cage effect was observed in the case of C2G3 dendrimer.
These results indicated that the third generation dendrimers with
the pentamethylene linker possessed higher hydrophobicity, as
concluded from the above solubilization and photophysical studies.
As the spacer length decreased, the hydrophobicity also decreased.
Table 3. Photolysis of 1-Phenyl-3-p-tolyl-propan-2-one in
Aqueous Basic Dendrimer Solutionsa,b
medium
AA
AB
BB
4
cage effect
hexane
C2G3
C3G3
C4G3
C5G3
21
30
10
9
51
46
77
81
69
29
24
13
10
0.05
0.54
0.62
1.0
’ CONCLUSION
31
a [CnG3] = 1.0 mM in 0.01 M aq NaOH. b Cage effect = (AB ꢀ BB ꢀ
The study presented herein attempts to assess interfacial
regions constituting the dynamic inner cavities of dendrimers.
For this purpose, phloroglucinol-based poly(alkyl aryl ether)
dendrimers were synthesized with varying alkyl spacers up to
three generations. Each generation of dendrimer possessed
linkers varying between ethyl and n-pentyl alkyl spacers con-
necting the branch points, thereby changing the sizes of the
dynamic inner cavities of dendrimers. Upon synthesis, solubili-
ties of pyrene in aqueous basic solutions were assessed first,
followed by a series of studies to identify the microenvironmental
properties arising from dendritic interiors. Fluorescence studies
of encapsulated dye molecules, namely, pyrene and coumarins,
showed that dendrimers containing longer alkyl chain length
exhibited significant hydrophobic microenvironments compared
with dendrimers with shorter alkyl chain length, within each
dendrimer generation. The role of alkyl chain in endowing higher
hydrophobicity could be observed for a lower generation den-
drimer with longer alkyl chain compared with a higher generation
dendrimer with shorter alkyl chain. A photoreaction pertaining
to photocleavage of dibenzyl ketone at the interior of third
generation dendrimer series showed that dendrimer with longer
alkyl chain afforded higher rigidities to reactive intermediates,
thereby reducing the “leakiness” of intermediates to bulk envir-
onment and facilitating product formation with high selectivities.
AA)/(AA þ AB þ BB)
pyrene and coumarins showed pronounced hydrophobic inter-
iors, especially with third generation dendrimers. An effort was
thus undertaken to verify the ability of third generation dendri-
mers to act as reaction media. In this instance, abilities of
poly(alkyl aryl ether) dendrimers to mediate various photoche-
mical reactions were assessed previously.29,30 In continuation, it
was deemed necessary to probe the mobility of guest molecules
in the hydrophobic environments of dendrimers with varying
interfacial regions resulting from different spacer groups within a
dendrimer generation. The series within third generation den-
drimers, constituted with C2ꢀC5 alkyl chain length, was chosen,
in order to conduct photolysis of dibenzyl ketone (Scheme 2).43
Irradiation of 1-phenyl-3-p-tolyl-propane-2-one 3 in hexane solu-
tion resulted in an R-cleavage, yielding the primary radical pairs D
and E, followed by decarbonylation, to afford a secondary radical pair
A and B. In hexane solution, no product from the radical pairs D and
E was detected. Three diaryl ethanes 5 (AA), 6 (AB) and 7 (BB),
resulting from the radical pairs A and B, were formed in the ratio
1:2:1. In restricted environments, the rearrangement product 4 is
formed from the radical pair E.44 When the radical pairs A and B
were held within a cage with little translational mobility, the only
product expected was 6 (AB).45ꢀ48 The cage effect (AB ꢀ AA ꢀ
BB)/(AA þ AB þ BB) and the yield of rearrangement product 4
provide information concerning the “leakiness” of the reaction
cavity with respect to radical pairs D, E, A, and B.
’ EXPERIMENTAL SECTION
General Procedure for Alkylation Reaction. A mixture of dendritic
phenol (1 molar equiv), monomer 2 (1.2 molar equiv per OH group),
K2CO3 (1.2 molar equiv per OH group), and 18-crown-ether (catalytic)
in 2-butanone/DMF (2:1 v/v) was refluxed for 24ꢀ72 h. Solvents were
then removed in vacuo, and the resulting residue was dissolved in EtOAc,
washed with water, dried (Na2SO4), concentrated, and purified (SiO2) to
afford benzyl-protected poly(alkyl aryl ether) dendrimers, as gums.
The results of photolysis of 1-phenyl-3-p-tolyl-propane-2-one 3
in hexane and in aqueous basic third generation dendrimers with
various spacer groups are summarized in Table 3. It was observed
that in case of C5G3 dendrimer the cage effect was 1.0 with the
formation of rearrangement product 4 in 31% yield. As the spacer
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General Procedure for Debenzylation Reaction. A mixture
of benzyl-protected dendrimer and PdꢀC (10%) in THF was refluxed
in the presence of H2 (g) for 24 h. The reaction mixture was filtered
through Celite, concentrated, and purified (SiO2) to afford hydroxyl
group-terminated poly(alkyl aryl ether) dendrimers, as white foams.
C2G1. A mixture of 1 (0.45 g, 1.78 mmol), 2 (ethyl spacer) (3.0 g, 7.15
mmol), and NaH (60% in mineral oil, 0.6 g) in DMF (30 mL) was stirred
at 0 °C for 15 min, followed by addition of aq DMF (2%) (6 mL), over a
period of 2 h. The reaction mixture was stirred for 24 h at rt, solvents
were removed in vacuo, and the resulting residue dissolved in EtOAc,
washed with water, dried (Na2SO4), concentrated, and purified (SiO2,
PhMe/EtOAc = 98:2) to afford benzyl-protected dendrimer C2G1
(1.45 g, 72%), as a white gum.
A mixture of the above intermediate (1.3 g, 1.16 mmol) and Pd/C
(10%) (0.3 g) in THF (30 mL) was refluxed for 24 h and treated as
described in the general procedure to afford C2G1 (0.6 g, 87%), as a
foam. 1H NMR (DMSO-d6, 400 MHz) δ 4.12 (br, 6 H), 4.21 (br, 6 H),
5.81 (s, 6 H), 5.83 (s, 3 H), 6.19 (s, 3 H), 9.20 (s, 6 H); 13C NMR
(DMSO-d6, 100 MHz) δ 65.9, 66.4, 93.1, 94.0, 95.7, 159.0, 160.1, 160.2;
ESI-MS m/z calcd for C30H30O12 [M þ Na]þ 605.1635, found 605.1635.
Anal. Calcd for C30H30O12: C, 61.85; H, 5.19. Found: C, 61.43; H, 6.13.
C3G1. A mixture of 1 (0.5 g, 2.0 mmol), 2 (n-propyl spacer) (3.4 g,
7.9 mmol), and NaH (60% in mineral oil, 0.65 g) in DMF (35 mL) was
stirred at 0 °C for 15 min, followed by addition of aq DMF (2%) (6 mL),
over a period of 2 h. The reaction mixture stirred for 12 h at rt, solvents
were removed in vacuo, and the resulting residue was dissolved in
EtOAc, washed with water, dried (Na2SO4), concentrated, and purified
(SiO2, PhMe/EtOAc = 98:2), to afford benzyl-protected dendrimer
C3G1 (1.8 g, 76%), as a white gum.
A mixture of the above intermediate (1.5 g, 1.3 mmol) and Pd/C
(10%) (0.3 g) in THF (35 mL) was refluxed for 24 h and treated as
described in the general procedure to afford C3G1 (0.75 g, 91%), as a
foam. 1H NMR (DMSO-d6, 400 MHz) δ 2.01 (m, J = 6.2 Hz, 6 H), 3.90
(m, J=6.2Hz, 6H), 3.97(m, 6H), 5.73(s, 6H), 5.75(s, 3H), 6.05(s, 3H),
9.10 (s, 6 H); 13C NMR (DMSO-d6, 100 MHz) δ 28.6, 63.7, 64.2,
93.1, 93.8, 95.5, 159.0, 160.3, 160.4. ESI-MS m/z calcd for C33H36O12
[M þ Na]þ 647.2104, found 647.2106. Anal. Calcd for C33H36O12:
C, 63.45; H, 5.81. Found: C, 63.60; H, 6.89.
C4G1. A mixture of 1 (0.83 g, 3.3 mmol), 2 (n-butyl spacer) (5.8 g,
13.2 mmol) and NaH (60% in mineral oil, 1.1 g, 26.3 mmol) in DMF
(50 mL) was stirred at 0 °C for 15 min, followed by addition of aq DMF
(2%) (6 mL), over a period of 2 h. The reaction mixture stirred for 12 h
at rt, solvents were removed in vacuo, and the resulting residue was
dissolved in EtOAc, washed with water, dried (Na2SO4), concentrated,
and purified (SiO2, PhMe/EtOAc = 98:2), to afford benzyl-protected
dendrimer C4G1 (2.8 g, 71%), as a white gum.
100 MHz) δ 65.9, 66.4, 93.1, 94.0, 95.7, 159.0, 160.1, 160.2. ESI-MS m/z
calcd for C78H78O30 [M þ Na]þ 1517.4476, found 1517.4468. Anal.
Calcd for C78H78O30: C, 62.65; H, 5.26. Found: C, 62.55; H, 6.43.
C3G2. A mixture of C3G1 (1.15 g, 1.84 mmol), 2 (n-propyl spacer)
(5.9 g, 13.8 mmol), K2CO3 (2.3 g, 16.5 mmol), and 18-crown-ether
(catalytic) in 2-butanone (45 mL) and DMF (15 mL) was stirred at
90 °C for 30 h and treated as described in the general procedure to afford
benzyl-protected dendrimer C3G2 (3.6 g, 73%), as a colorless gum.
A mixture of the above intermediate (3.5 g, 1.3 mmol) and Pd/C
(10%) in THF (50 mL) was refluxed for 24 h and treated as described in
the general procedure to afford C3G2 (1.8 g, 85%), as a white foam. 1H
NMR (DMSO-d6, 400 MHz) δ 2.05 (m, J = 6.0 Hz, 18 H), 3.94 (t, J =
6.0 Hz, 12 H), 4.02 (m, 24 H), 5.78 (s, 12 H), 5.80 (s, 6 H), 6.10 (s, 12 H),
9.15 (s, 12 H); 13C NMR (DMSO-d6, 100 MHz) δ 28.6, 63.7, 64.2, 93.1,
93.8, 95.5, 159.0, 160.2, 160.3. MALDI-TOF-MS m/z calcd for C87H96O30
[M þ H]þ 1622.68, found 1622.50. Anal. Calcd for C87H96O30: C, 64.44;
H, 5.97. Found: C, 63.63; H, 5.13.
C4G2. A mixture of C4G1 (1.2 g, 1.8 mmol), 2 (n-butyl spacer) (6.0 g,
13.5 mmol), K2CO3 (2.3 g, 16.2 mmol), and 18-crown-ether (catalytic)
in 2-butanone (45 mL) and DMF (15 mL) was stirred at 90 °C for 30 h
and treated as described in the general procedure to afford benzyl-
protected dendrimer C4G2 (3.7 g, 73%), as a colorless gum.
A mixture of the above intermediate (3.5 g, 1.24 mmol) and Pd/C (10%)
in THF (50 mL) was refluxed for 24 h and treated as described in the
1
general procedure to afford C4G2 (1.75 g, 81%), as a foam. H NMR
(DMSO-d6, 400 MHz) δ 1.77 (br, 36 H), 3.85 (br, 12 H), 3.93 (br, 24 H),
5.77 (s, 12 H), 5.80 (s, 6 H), 6.06 (s, 12 H), 9.13 (s, 12 H); 13C NMR
(DMSO-d6, 100 MHz) δ 25.5, 66.7, 67.1, 93.1, 93.7, 95.7, 158.9, 160.4.
MALDI-TOF-MS m/z calcd for C96H114O30 [M þ H]þ 1748.74, found
1748.43. Anal. Calcd for C96H114O30: C, 65.97; H, 6.57. Found: C, 65.26;
H, 5.85.
C2G3. A mixture of C2G2 (1.1 g, 0.75 mmol), 2 (ethyl spacer) (4.5 g,
10.8 mmol), K2CO3 (1.8 g, 13.0 mmol), and 18-crown-ether (catalytic)
in 2-butanone (30 mL) and DMF (20 mL) was stirred at 90 °C for 48 h
and treated as described in the general procedure to afford benzyl-
protected dendrimer C2G3 (2.7 g, 65%), as a colorless gum.
A mixture of the above intermediate (2.5 g, 0.45 mmol) and Pd/C
(10%) in THF (50 mL) was refluxed for 24 h and treated as described in
the general procedure to afford C2G3 (1.1 g, 73%), as a foam. 1H NMR
(DMSO-d6, 400 MHz) δ 4.10 (br, 24 H), 4.20 (br, 60 H), 5.80 (s, 24 H),
5.82 (s, 12 H), 6.20 (s, 30 H), 9.19 (s, 24 H); 13C NMR (DMSO-d6, 100
MHz) δ 65.9, 66.4, 93.1, 94.0, 95.7, 159.0, 160.0, 160.2. MALDI-TOF-
MS m/z calcd for C174H174O66 [M]þ 3321.03, found 3321.82. Anal.
Calcd for C174H174O66: C, 62.92; H, 5.28. Found: C, 62.28; H, 6.31.
C3G3. A mixture of C3G2 (1.15 g, 0.73 mmol), 2 (n-propyl spacer)
(4.7 g, 11.0 mmol), K2CO3 (1.8 g, 13.0 mmol), and 18-crown-ether
(catalytic) in 2-butanone (35 mL) and DMF (25 mL) was stirred at
90 °C for 48 h and treated as described in the general procedure to afford
benzyl-protected dendrimer C3G3 (2.9 g, 68%), as a colorless gum.
A mixture of the above intermediate (2.7 g, 0.46 mmol) and Pd/C
(10%) in THF (50 mL) was refluxed for 24 h and treated as described in
the general procedure to afford C3G3 (1.2 g, 71%), as a foam. 1H NMR
(DMSO-d6, 400 MHz) δ 2.04 (m, J = 6.0 Hz, 42 H), 3.93 (t, J = 6.0 Hz,
24 H), 4.00 (br, 60 H), 5.78 (s, 24 H), 5.81 (s, 12 H), 6.10 (s, 30 H), 9.14
(s, 24 H); 13C NMR (DMSO-d6, 100 MHz) δ 28.6, 63.7, 64.2, 93.1, 93.8,
95.5, 159.0, 160.3, 160.4. MALDI-TOF-MS m/z calcd for C195H216O66
[M þ H]þ 3616.36, found 3616.93. Anal. Calcd for C195H216O66: C,
64.77; H, 6.02. Found: C, 64.67; H, 5.29.
A mixture of the above intermediate (2.5 g, 2.01 mmol) and Pd/C
(10%) in THF (50 mL) was refluxed for 24 h and treated as described in
the general procedure to afford C4G1 (1.15 g, 87%), as a foam. 1H NMR
(DMSO-d6, 400 MHz) δ 1.78 (br, 12 H), 3.86 (br, 6 H), 3.94 (br, 6 H),
5.77 (s, 6 H), 5.80 (s, 3 H), 6.06 (s, 3 H), 9.13 (s, 6 H); 13C NMR
(DMSO-d6, 100 MHz) δ 25.4, 66.7, 67.1, 93.1, 93.7, 95.4, 158.9, 160.4.
ESI-MS m/z calcd for C36H42O12 [M þ Na]þ 689.2574, found 689.2578.
Anal. Calcd for C36H42O12: C, 64.85; H, 6.35. Found: C, 63.75; H, 6.03.
C2G2. A mixture of C2G1 (1.41 g, 2.42 mmol), 2 (ethyl spacer) (7.5 g,
18.2mmol), K2CO3 (3.0 g, 21.8 mmol), and 18-crown-ether(catalytic) in
2-butanone (40 mL) and DMF (20 mL) was stirred at 90 °C for 30 h and
treated as described in the general procedure to afford benzyl-protected
dendrimer C2G2 (5.0 g, 79%), as a colorless gum.
C4G3. A mixture of C4G2 (0.76 g, 0.43 mmol), 2 (n-butyl spacer) (2.9 g,
6.5 mmol), K2CO3 (1.1 g, 7.8 mmol), and 18-crown-ether (catalytic) in
2-butanone (35 mL) and DMF (15 mL) was stirred at 90 °C for 48 h and
treated as described in the general procedure to afford benzyl-protected
dendrimer C4G3 (1.65 g, 63%), as a colorless gum.
A mixture of above intermediate (4.0 g, 1.55 mmol) and Pd/C (10%)
(1.0 g) in THF (50 mL) was refluxed for 24 h and treated as described in
the general procedure to afford C2G2 (1.9 g, 81%), as a foam. 1H NMR
(DMSO-d6, 400 MHz) δ 4.12 (br, 12 H), 4.21 (br, 24 H), 5.81 (s, 12 H),
5.83 (s, 6 H), 6.20 (s, 12 H), 9.19 (s, 12 H); 13C NMR (DMSO-d6,
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The Journal of Organic Chemistry
ARTICLE
A mixture of the above intermediate (1.6 g, 0.26 mmol) and Pd/C
(10%) in THF (50 mL) was refluxed for 24 h and treated as described in
the general procedure to afford C4G3 (0.68 g, 67%), as a foam. 1H NMR
(DMSO-d6, 400 MHz) δ 1.76 (br, 84 H), 3.84 (br, 24 H), 3.98 (br,
60 H), 5.77 (s, 24 H), 5.80 (s, 12 H), 6.05 (s, 30 H), 9.13 (s, 24 H); 13C
NMR (DMSO-d6, 100 MHz) δ 25.4, 66.6, 67.1, 93.1, 93.6, 95.4, 158.9,
160.4. MALDI-TOF-MS m/z calcd for C216H258O66 [M þ H]þ 3910.69,
found 3911.08. Anal. Calcd for C216H258O66: C, 66.35; H, 6.65. Found: C,
65.26; H, 5.85.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jayaraman@orgchem.iisc.ernet.in; vramamurthy@mail.
as.miami.edu.
Encapsulation of Pyrene in Dendritic Interior. A solution of
dendrimer (2 μmol) and pyrene (5 mg) in THF (1 mL) was prepared,
and the solvent was removed in vacuo. The resulting residue was added
with aq NaOH (1.1 molar equiv per hydroxyl group) solution (10 mL)
(pH ∼10), the mixture was stirred for 12 h in the dark, filtered (0.45 μ),
and extracted with PhMe (4 ꢁ 5 mL), and the organic portion was
evaporated in vacuo. The resulting residue was dissolved in EtOH (3 mL),
and the amount of pyrene was determined by UVꢀvis spectroscopy (ε335
50,734 molꢀ1 cmꢀ1).32,33 The free energy transfer (ΔGtr)49 of pyrene
solubilization was calculated from equation ΔGtr = ꢀRT ln(Cs/Cw),
where Cs and Cw were the solubilities of pyrene in aqueous basic solution
of dendrimer and in water (pH ∼9.5), respectively.
’ ACKNOWLEDGMENT
N.J. thanks Department of Science and Technology, New
Delhi for a financial support. Council of Scientific and Industrial
Research, New Delhi is acknowledged for a research fellowship
to B.N. V.R. is grateful to the National Science Foundation, USA,
for generous financial support (CHE-0848017).
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