A systematic study of peripherally multiple aromatic ester-functionalized poly(benzyl ether) dendrons for the fabrication of organogels: Structure-property relationships and thixotropic property

Article abstract of DOI:10.1002/chem.201400157

A new class of peripherally multiple aromatic ester-functionalized poly(benzyl ether) dendrons and/or dendrimers with different focal point substituents, surface groups, interior structures, as well as different generations have been synthesized and their structure-property relationships with respect to their gelation ability have been investigated systematically. Most of these dendrons are able to gel organic solvents over a wide polarity range. Evident dendritic effects were observed not only in gelation capability but also in thermotropic, morphological, and rheological characterizations. It was disclosed that subtle changes in peripheral ester functionalities and interior dendritic structures affected the gelation behavior of the dendrons significantly. Among all the dendrons studied, the second- and third-generation dendrons G₀G₂-Me and G₀G₃-Me with dimethyl isophthalates (DMIP) as peripheral groups exhibited the best capability in gelation, and stable gels were formed in more than 22 aromatic and polar organic solvents. The lowest critical gelation concentration (CGC) reached 2.0 mg mL^-1, indicating that approximately 1.35×10⁴ solvent molecules could be entrapped by one dendritic molecule. Further study on driving forces in gel formation was carried out by using a combination of single-crystal/powder X-ray diffraction (XRD) analysis and concentration- dependent (CD)/temperature-dependent (TD) ¹H NMR spectroscopy. The results obtained from these experiments revealed that the multiple π-π stacking of extended π-systems due to the peripheral DMIP rings, cooperatively assisted by non-conventional hydrogen-bonding, is the key contributor in the formation of the highly ordered supramolecular and fibrillar network. In addition, these dendritic organogels exhibited unexpected thixotropic-responsive properties, which make them promising candidates with potential applications in the field of intelligent soft materials. s

Full text of DOI:10.1002/chem.201400157

DOI: 10.1002/chem.201400157
Full Paper
&
Gelators
A Systematic Study of Peripherally Multiple Aromatic Ester-
Functionalized Poly(benzyl ether) Dendrons for the Fabrication of
Organogels: Structure–Property Relationships and Thixotropic
Property
Yu Feng,[a] Zhi-Xiong Liu,[a] Hui Chen,[a] Zhi-Chao Yan,[b] Yan-Mei He,*[a] Chen-Yang Liu,[b] and
Qing-Hua Fan*[a]
Abstract: A new class of peripherally multiple aromatic
ester-functionalized poly(benzyl ether) dendrons and/or den-
drimers with different focal point substituents, surface
groups, interior structures, as well as different generations
have been synthesized and their structure–property relation-
ships with respect to their gelation ability have been investi-
gated systematically. Most of these dendrons are able to gel
organic solvents over a wide polarity range. Evident dendrit-
ic effects were observed not only in gelation capability but
also in thermotropic, morphological, and rheological charac-
terizations. It was disclosed that subtle changes in peripheral
ester functionalities and interior dendritic structures affected
the gelation behavior of the dendrons significantly. Among
all the dendrons studied, the second- and third-generation
dendrons G0G2-Me and G0G3-Me with dimethyl isophthalates
(DMIP) as peripheral groups exhibited the best capability in
gelation, and stable gels were formed in more than 22 aro-
matic and polar organic solvents. The lowest critical gelation
concentration (CGC) reached 2.0 mgmLÀ1, indicating that ap-
proximately 1.35ꢀ104 solvent molecules could be entrapped
by one dendritic molecule. Further study on driving forces in
gel formation was carried out by using a combination of
single-crystal/powder X-ray diffraction (XRD) analysis and
concentration-dependent (CD)/temperature-dependent (TD)
1H NMR spectroscopy. The results obtained from these ex-
periments revealed that the multiple p–p stacking of ex-
tended p-systems due to the peripheral DMIP rings, cooper-
atively assisted by non-conventional hydrogen-bonding, is
the key contributor in the formation of the highly ordered
supramolecular and fibrillar network. In addition, these den-
dritic organogels exhibited unexpected thixotropic-respon-
sive properties, which make them promising candidates with
potential applications in the field of intelligent soft
materials.
Introduction
ture and diversity in functionalization, and thus have become
a new type of promising candidate in the research of gel
phase materials.[2–16] The dendritic architectures render them
capable of being incorporated with different functional groups
into the core, the inner and/or outer layers, providing multi-
recognition sites for self-assembly. However, due to their more
complex molecular structures (compared with the low-molecu-
lar-weight gelators), it is still difficult to manipulate and bal-
ance various weak interactions, including hydrogen bonding,
electrostatic interactions, solvophobic forces, p–p interactions,
and van der Waals forces, to meet the requirements for gel for-
mation. In addition, the highly structural tunability of dendritic
molecules provides an ideal platform for the systematic study
of the structure–property relationships of dendritic gel sys-
tems. However, an in-depth understanding of the gelation
mechanism, which will guide the rational design of new den-
dritic gelators, is less reported. Therefore, the discovery of new
types of dendritic organogelators has been a great challenge.
Among the successful dendritic organogelators reported to
date, a typical hydrogen-bonding-dominated self-assembling
protocol has been mainly utilized in the molecular design by
selecting an amide unit as the main gelation motif.[5–11] In
As one of the most efficient approaches for the construction of
highly ordered nanoscale functional self-assemblies, the dis-
covery and application of new gelators has gained extraordina-
ry attention.[1] Dendritic gelators, the newest member in gel
family, exhibit unique properties different from those of the
small molecule gel and traditional polymer gel systems due to
their well-defined three-dimensional macromolecular architec-
[a] Dr. Y. Feng, Z.-X. Liu, H. Chen, Y.-M. He, Prof. Dr. Q.-H. Fan
Beijing National Laboratory for Molecular Sciences
and CAS Key Laboratory of Molecular Recognition and Function
Institute of Chemistry Chinese Academy of Sciences (CAS)
Beijing 100190 (P.R. China)
Fax: (+86)10-62554472
E-mail: heym@iccas.ac.cn
fanqh@iccas.ac.cn
[b] Z.-C. Yan, Prof. Dr. C.-Y. Liu
Beijing National Laboratory for Molecular Sciences
and CAS Key Laboratory of Engineering Plastics
Institute of Chemistry, CAS, Beijing 100190 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400157.
Chem. Eur. J. 2014, 20, 7069 – 7082
7069
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2000, Aida and co-workers reported the first example of den-
dritic organogel by introducing a dipeptide into the focal
point of ester-terminated Frꢂchet-type dendrons.[5] It was dem-
onstrated that the gelation was mainly induced by hydrogen-
bonding interactions. The role of the ester-terminated Frꢂchet-
type dendron in the self-assembly processes was described as
supplementary van der Waals forces. From 2001, Smith and co-
workers have systematically studied both one-component and
two-component dendritic organogels based on poly(lysine)
dendrimers with amide dendritic branches, and disclosed that
the intermolecular hydrogen bonding played an important
role in the gel-phase assembly.[7] However, these efficient den-
dritic gelators were limited to low generations because nega-
tive steric factors might become increasingly dominant for
high-generation dendrimers, hindering the anisotropic self-as-
sembly.[7c] Distinguished from these core- and skeleton-amide
functionalized dendritic gelators,[5–11] self-assembly dominated
by the peripheral functional groups of dendritic molecules
allows straight and cooperative multivalent interactions be-
tween intermolecular multiple gelation motifs, which has
proven to be beneficial for highly efficient hydrogel forma-
tion.[3,4] More importantly, the capability to accommodate
a functional group at the core or the focal point renders the
peripherally functionalized dendritic gelators more attractive
for applications in the fabrication of functional nanomaterials.
Surprisingly, in sharp contrast to dendritic hydrogels, dendritic
organogelators with gelation motifs at the periphery are rarely
reported.[14,17]
Results and Discussion
Molecular design and synthesis
To elucidate the effect of peripheral ester groups, poly(benzyl
ether) skeleton, and focal-point functionalities on gelation
properties, four categories of peripherally aromatic ester-func-
tionalized poly(benzyl ether) dendrons were designed as illus-
trated in Scheme 1, based on the molecular structure of the
best dendritic organogelator G0G2-Me demonstrated in our
preliminary research. To evaluate the effect of different periph-
eral ester substituents on gelation properties, methyl, ethyl, or
benzyl ester groups were introduced into the outer phenyl
rings of G0G2 dendron, in a 3-mono-, 4-mono-, 3,4-di-, or 3,5-
disubstituted manner (category 1). Frꢂchet dendrons of gener-
ation 1 to 3 were attached to the focal point of G0G2-Me to
see how bigger dendritic wedges affect the gelation properties
(category 2). Dendrons in the same molecular formula of G0G2-
Me with variations in backbone structure, such as the “re-
versed” Frꢂchet-type dendron G0G2’-Me bearing peripheral
DMIP motifs and 3,4-branching analogues of G0G2-Me (3,4-
G0G2-Me-N1 and 3,4-G0G2-Me-N2) compose the third category.
The fourth category includes the dendrons with similar archi-
tecture as G0G2-Me, but of different generation from 1 to 4.
According to our previously reported method,[17,18] sixteen
peripherally aromatic ester-functionalized poly(benzyl ether)
dendrons/dendrimers, as described above, were divergently
synthesized by using benzyl alcohol or Frꢂchet dendritic
benzyl alcohols as a starting material and commercially avail-
able dimethyl 5-hydroxylisophthalate and/or dimethyl 4-hy-
droxylphthalate as the generation growing agent, through a re-
petitive Mitsunobu coupling and ester reduction sequence.
Dendrons with different aromatic ester functionalities other
than DMIP were also synthesized by using diethyl 5-hydroxyli-
sophthalate, dibenzyl 5-hydroxylisophthalate, dimethyl 4-hy-
droxylphthalate, methyl 3-hydroxybenzoate, or methyl 4-hy-
droxybenzoate as the reactant in the final Mitsunobu reaction.
In addition, poly(benzyl ether) dendron G0G2-Ph without sur-
face ester groups and a linear oligo(benzyl ether) G1-8-COOMe
bearing 4 DMIP groups as pendants were also prepared for
comparison. Chemical structures and purities of all the com-
In our previous communication on the serendipitous discov-
ery of a new kind of dendritic organogelator,[17] peripherally di-
methyl isophthalates (DMIP)-functionalized poly(benzyl ether)
dendrons without any conventional gelation motifs, such as
amides, long alkyl side chains, and steroidal groups, showed
unprecedented high efficiency in gelating aromatic and polar
organic solvents, and even aqueous media. Different from the
gel-phase self-assembly of the core- and skeleton-amide func-
tionalized dendritic gelators, which is dominated by typical hy-
drogen-bonding,[5–11] such peripherally functionalized poly(ben-
zyl ether) dendrons represent a new type of dendritic gelators
whose assembly is dominated by cooperative multi-types of
non-covalent-bonding interactions resulting from peripheral
groups. To give an in-depth insight of this new dendritic gel
system, herein, we report the design and synthesis of series of
peripherally aromatic ester-functionalized poly(benzyl ether)
dendrons by altering the component and/or structure of the
peripheral aromatic ester groups, dendritic branches, and the
focal point moiety. A systematic study on structure–property
relationships was pursued to obtain fundamental structural pa-
rameters for molecular design, which focused on the mecha-
nism elucidation of the self-assembly processes. Furthermore,
the unexpected thixotropic-responsive properties of these den-
dritic organogels were also investigated.
pounds synthesized were confirmed by using H/13C NMR spec-
1
troscopy and MALDI-TOF mass spectrometry as well as ele-
mental analysis (see the Supporting Information for details).
Gelation abilities of dendritic organogelators
In general, gelation properties change dramatically along with
the small structural variation in the gelator molecule depend-
ing on the solvent used. For better understanding of the struc-
ture–property relationships, the gelation abilities of the synthe-
sized dendritic macromolecules were investigated in various
organic solvents and mixed solvents by a “stable to inversion”
method; the results are summarized in Table 1, and some key
features are discussed.
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7070
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Scheme 1. Schematic structures of poly(benzyl ether) dendritic macromolecules and oligomer.
Impact of peripheral ester functionalities
tigated the role of the peripheral ester functionalities of the
poly(benzyl ether) dendrons on gelation properties, including
ester species, substituent number, and substitution position of
the ester groups.
As one important methodology for structural alteration, modi-
fication of the dendritic periphery with different functionalities
will obviously affect the properties of dendrons/dendrimers,
such as chemical reactivity, glass transition temperature, and
especially, the solubility. Considering the nature of the compli-
cated phase-separation system of the gel, it is of special inter-
est to fine-tune the gelator solubility upon balancing the am-
phiphilic molecular structure. It has been demonstrated in the
dendritic organogel systems that the gelator solubility has
a subtle influence on gelation properties, such as Tgel and criti-
cal gelation concentration (CGC), as well as the cooperative
self-assembling level.[19] Therefore, in this work, we firstly inves-
As shown in Table 1 and Table S1 (the Supporting Informa-
tion), the critical gelation concentrations of dendrons G0G2 in
category 1 in various organic and mixed solvents were collect-
ed. Dendron G0G2-Ph without ester groups at the periphery
was soluble in most solvents tested, and no gelation phenom-
enon was observed. In sharp contrast to dendron G0G2-Ph, the
DMIP-functionalized dendron G0G2-Me is the most efficient ge-
lator among the dendrons screened, which could gel 15 aro-
matic and polar organic solvents and 7 mixed solvents (includ-
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7071
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Table 1. Gelation properties and critical gelator concentrations (CGCs) of dendrons and dendrimers in various organic solvents and mixed solvents at
258C.[a]
G0G1-Me G0G2-Me 3,4-G0G2-Me-N1 3,4-G0G2-Me-N2 G0G2’-Me G0G3-Me G0G4-Me G1G2-Me G2G2-Me G3G2-Me
Solvents
Toluene
Benzene
Anisole
Phenyl ether
G (30.3)
G (52.6)
PG
PG
S
PG
S
PG
PG
S
P
S
G (47.3)
G (43.6)
S
G (3.3)
G (2.2)
G (8.2)
G (3.7)
G (11.5)
G (14.5)
G (32.7)
G (18.0)
G (10.7)
G (3.9)
G (6.6)
G (2.2)
G (4.3)
G (2.4)
G (10.0)
G (3.2)
G (2.7)
G (7.3)
G (4.4)
G (2.0)
G (5.1)
G (2.8)
PG
G (22.2)
S
PG
PG
PG
G (22.8)
S
PG
S
S
S
S
PG
G (35.6)
G (16.8)
G (33.3)
G (18.9)
G (13.0)
PG
G (21.8)
G (7.1)
PG
G (20.0)
G (20.8)
G (18.0)
G (11.5)
G (16.8)
G (29.6)
PG
G (16.0)
G (23.3)
G (12.5)
G (11.3)
G (19.1)
G (8.4)
G (3.6)
PG
G (19.7)
G (22.3)
G (15.5)
G (8.1)
G (10.9)
G (42.0)
G (10.6)
G (5.2)
G (10.1)
G (2.6)
G (2.8)
G (6.9)
G (13.0)
G (18.0)
G (6.9)
G (3.4)
G (3.2)
G (4.4)
G (2.7)
G (8.0)
G (2.0)
G (12.2)
G (2.9)
G (2.5)
G (6.0)
G (5.1)
G (5.1)
G (5.5)
G (2.1)
G (33.8)
G (52.7)
G (33.8)
PG
G (39.0)
S
PG
PG
G (50.8)
P
P
P
G (33.0)
G (21.0)
G (36.0)
P
G (5.0)
G (4.5)
G (33.5)
G (16.7)
G (28.0)
S
G (16.2)
G (52.0)
S
S
S
S
S
PG
PG
S
S
S
S
S
S
PG
Pyridine
Cyanobenzene
Benzaldehyde
Ethyl acetate
Cyclohexanone
Acetone
S
S
PG
G (14.0)
PG
G (32.8)
G (16.5)
G (11.6)
P
G (6.6)
G (3.6)
G (3.1)
PG
G (12.0)
G (3.7)
G (16.8)
G (12.4)
G (1.8)
G (21.0)
G (38.2)
PG
S
G (6.2)
G (12.3)
G (8.1)
G (9.6)
G (13.7)
PG
G (8.4)
G (16.6)
PG
G (12.0)
G (27.5)
G (12.9)
G (20.0)
G (3.1)
S
G (21.0)
G (36.8)
PG
G (8.5)
I
Acetonitrile
2-Methoxyethanol
Benzyl alcohol
Tetrachloromethane
1,2-Dichloroethane
CHCl3/CH3CN=1:9
CHCl3/CCl4 =1:9
THF/CH3OH=3:1
THF/H2O=3:1
Pyridine/H2O=4:1
Anisole/CCl4 =3:2
G (6.3)
G (10.7)
G (31.5)
S
G (10.0)
PG
PG
PG
G (15.8)
S
P
PG
PG
P
P
P
P
PG
PG
G (33.0)
PG
PG
G (6.0)
PG
P
S
G (7.3)
G (32.5)
PG
G (28.5)
S
G (13.7)
P
Anisole/Hexane=1:1 G (36.0)
G (26.6)
P
[a] G: gelation; PG: partial gelation; S: soluble (>60.0 mgmLÀ1); I: insoluble upon heating; P: precipitation. The value given in parentheses is the CGC in
mgmLÀ1
.
ing water-containing mixtures) with the lowest CGC of
2.0 mgmLÀ1. When methyl ester groups were replaced by
ethyl esters (G0G2-Et) or benzyl esters (G0G2-CH2Ph), decreased
gelation abilities were noticed. Stable gels were only formed in
five solvents and mixed solvents with high CGC values. Surpris-
ingly, gelation was not detected for the peripherally dissym-
metric dendron 3,4-G0G2-Me in all the selected solvents. Re-
ducing the substituent number of methyl ester to one per
phenyl ring also caused a decrease in the gelation efficiency.
The positive effect of molecular symmetry was also found for
dendron 4-G0G2-Me, exhibiting a better performance than 3-
G0G2-Me, which was more soluble in most organic solvents. All
these results suggest, as we expected, that the gelation ability
of the poly(benzyl ether) dendrons could be finely tuned by
peripheral modification, and DMIP is demonstrated to be the
most efficient motif for gelation.
point also exhibited efficient gelation in 6 solvent systems, in-
cluding hydrogen-donating solvents, such as acetone, 2-me-
thoxyethanol, and benzyl alcohol. The lowest CGC of G3G2-Me
reached 6.3 mgmLÀ1 in 2-methoxymethanol. Considering the
high molecular weight of the co-dendrimer, this CGC value is
comparable to those of the best low-molecular-weight gela-
tors. The highly structural tolerance of focal point attachment
from the viewpoint of gelation ability indicates that these den-
dritic gelators with peripheral functionalities could be used as
an efficient gelation motif for the development of new func-
tional nanomaterials by easy post-functionalization at the focal
point.[20]
Impact of the backbone structure
To elucidate the effect of the structure of dendritic backbone
on gelation ability, dendrons identical in chemical composition
of G0G2-Me were synthesized for comparison, including G0G2’-
Me, a Frꢂchet-type dendron possessing peripheral DMIP motifs
with a “reversed” dendritic backbone, and 3,4-branching ana-
logues of G0G2-Me (3,4-G0G2-Me-N1 and 3,4-G0G2-Me-N2) at
different branching points of generation growth. A linear oli-
go(benzyl ether) G1-8-COOMe with DMIP motifs as pendant
groups was also prepared. As shown in Table 1 and Table S1
(the Supporting Information), the monodispersed dendron
G0G2’-Me exhibited comparable gelation behavior as G0G2-Me
from the view-point of solvent scope, but the CGC values were
a little bit higher. The symmetry of the dendritic architecture
was noticed to influence the gelation ability of the gelators. In-
creases of CGC values in most aromatic solvents were ob-
Impact of focal point attachment of Frꢀchet dendrons
Peripheral DMIP-functionalized co-dendrimers bearing a Frꢂchet
dendron at the focal point, GmG2-Me (m=1–3), were then
tested in selected solvents and compared to the focal benzyl
ether-substituted analogue G0G2-Me. An obvious trend of gela-
tion ability variation was found. When increasing the genera-
tion of the focal Frꢂchet dendron from 1 to 3, the gelator solu-
bility increased gradually in aromatic solvents accompanied
with the decrease in gelation efficiency. As shown in Table 1,
G1G2-Me formed gels in 18 tested solvents, whereas G2G2-Me
only gave gels in 11 solvents. More importantly, the co-dendri-
mer G3G2-Me bearing a bulky dendritic wedge at the focal
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7072
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
served for both 3,4-G0G2-Me-N1 and 3,4-G0G2-Me-N2. In sharp
contrast to dendron G1G2-Me, a stable gel of linear analogue
G1-8-COOMe could only exist in THF-CH3OH (3:1, v/v) mixture,
suggesting that the dendritic architecture is critical in forming
the self-assembled gels.
Impact of the dendritic generation
The impact of dendron generation on gelation ability is much
more difficult to quantify than it is to describe due to the con-
siderable changes of multi-parameters related to the genera-
tion growth, including the size of dendron, the configuration
of molecule, and more importantly for peripherally functional-
ized poly(benzyl ether) dendrons, the amount of peripheral
functionalities. In our study, dendron generation exhibited an
intriguing effect on gelation ability (Table 1 and Table S1, the
Supporting Information). Similar to the reported results of
other dendritic gelators, moderate generation (generally 2 to
3) of the dendron is superior to low generation for gel forma-
tion, dendrons G0G2-Me and G0G3-Me displayed much better
gelation ability than G0G1-Me. Both dendrons formed stable
gels in all organic solvents investigated, and proved to be the
most efficient gelators with CGC values mostly less than
1 wt.%. It is of particular interest to witness that dendron
G0G4-Me with a globular shape[21] also exhibited better gela-
tion behavior than G0G1-Me, gels were formed in 7 solvents,
though the CGC values were relatively higher. This is quite dif-
ferent from the performance of amides or peptide-containing
dendritic gelators, whose self-assembly is dominated by typical
hydrogen bonding.
Figure 1. Effect of concentration on Tgel of G0G2-Me, G0G3-Me, G1G2-Me,
G2G2-Me, and G0G2’-Me as measured by inverse flow method (solvent: tolu-
ene).
lar increase in Tgel was observed as the concentration of gelator
molecule was elevated, suggesting an increase in the thermo-
stability of the gel. The generation effects were found to be
similar to those of xerogels. As shown in Figure 1, the second-
generation dendron G0G2-Me had a Tgel value of 668C at the
concentration of 10 mm, the value increased to 718C for G0G3-
Me and 738C for G0G4-Me. The first-generation dendron G0G1-
Me was a solution at this concentration. As the generation of
the focal Frꢂchet dendron increased from 0 to 2, the Tgel of co-
dendrimers GmG2-Me decreased dramatically with the gel sam-
ples at the same concentration. For example, the Tgel for G0G2-
Me was 75.08C at 20 mm, whereas the Tgel of G1G2-Me drop-
ped to 49.08C, and the lowest Tgel of 348C was observed for
G2G2-Me. The co-dendrimer G3G2-Me bearing the biggest poly-
(benzyl ether) dendron as the focal point attachment was a so-
lution at this concentration. The wet gel of dendron G0G2’-Me
with a reversed backbone structure showed lower thermosta-
bility compared with that of G0G2-Me.
Thermotropic behavior of dendritic gels
The structure–property relationships were also investigated in
the thermotropic behavior of dendritic xerogels and native
gels with focuses on the generation influence based on two
series of dendrons, G0Gn-Me and GmG2-Me, showing apparent
“dendritic effects”. A positive “dendritic effect” of dendrons
G0Gn-Me has been demonstrated along with the generation in-
creasing for both xerogels and wet gels, whereas, a negative
“dendritic effect” of the focal point-functionalized co-dendrim-
ers GmG2-Me has been identified.
The glass transition temperatures (Tg) of xerogels[5] prepared
by a freeze-drying method in benzene were determined by dif-
ferential scanning calorimetry (DSC) at a heating rate of
108CminÀ1 from 0 to 1408C (Figure S1, the Supporting Infor-
mation). For dendrons G0Gn-Me, upon increasing the dendron
generation from 1 to 4, a steady enhancement of Tg was in-
spected from 36.48C for G0G1-Me to 55.4, 71.8, and 73.18C for
G0G2-Me, G0G3-Me, and G0G4-Me, respectively. On the contrary,
the Tg for co-dendrimers GmG2-Me gradually decreased upon
generation elevation of the focal point-attached Frꢂchet den-
dron; 55.4, 49.5, and 47.28C for G0G2-Me, G1G2-Me, and G2G2-
Me were recorded, respectively.
Morphological properties of dendritic gels
The morphological properties of xerogels of these dendrons/
dendrimers were studied in detail by scanning electron micros-
copy (SEM), environmental scanning electron microscopy
(ESEM), and transmission electron microscopy (TEM; Figure 2
and Figures S2–S5, the Supporting Information). Although fi-
brillar assemblies were found to be essential for most gel sam-
ples, the generation effect on the gel morphology was still dis-
tinctive. For example, in the xerogels of dendron series of
G0Gn-Me (n=1, 2, 3, 4) from benzene, the generation growth
caused a notable variation in morphology. Dendron G0G1-Me
was distributed into aggregates of straight ribbons with large
diameters of 200–600 nm. Long fibers with a homogeneous di-
ameter of 50–150 nm were observed for dendron G0G2-Me,
and bundles of entangled fibers composed a crossed network.
Fibers as the exclusive aggregates were also confirmed in
G0G3-Me samples, but the fiber diameter and shape were dif-
The effect of concentration on gel–sol phase-transition tem-
perature (Tgel) of wet gels in toluene was further examined by
using the tube-inversion method. For all gelators tested, a regu-
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7073
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
(Figure 3). First, to assess the dy-
namic nature and relaxation
modes of these dendritic gels,
the oscillatory frequency sweep
measurements were carried out
at the same concentration (Fig-
ure 3A and 3B). The frequency
sweep curves of these dendritic
gels were consistent with the
behaviors as true gels: both the
storage modulus (G’) and the
loss modulus (G’’) exhibited
a slight frequency dependence
in the frequency range of 0.1–
100 radsÀ1 with G’ an order of
magnitude larger than G’’, imply-
ing that the gels are typical elas-
Figure 2. SEM and ESEM images of freeze-dried dendritic xerogels from benzene: A) and E) G0G1-Me; B) and
F) G0G2-Me; C) and G) G0G3-Me; D) and H) G0G4-Me. Scale bar is 2.0 and 5.0 mm, respectively.
ferent from those of G0G2-Me. In addition to the curled fiber
bundles of 80–200 nm that intertwined to construct the net-
work skeleton, imperceptibly thin fibers of 30–80 nm in diame-
ter filled in to connect the network, which was only observed
in ESEM images possibly due to the mild conditions for sample
preparation. In contrast, the essential morphology of G0G4-Me
aggregates were three-dimensional globular particles of diam-
eter 2–3 mm, which were porous with a dense network of
branched fibrillar structures inside. Imperceptibly thin fibers
were also found on the surface of these particles to facilitate
the conjugation of particles. This distinct morphology behavior
is probably ascribed to the special dendritic molecular shape
and large amount of the peripheral functionalities (16 DMIP
groups with 32 methyl esters per dendron). The morphology
changes were also confirmed by TEM images (Figure S2, the
Supporting Information). In addi-
tic rather than viscous soft matter. In addition, the variation of
dynamic moduli (G’ and G’’) versus time at 208C are shown in
Figure 3C and 3D. For all the dendritic gelators tested, the
modulus values kept constant within a long period of time, in-
dicating the stability of the gel network was maintained once
it was formed. More importantly, dendritic generation effects
were also observed for both series of dendrons, G0Gn-Me and
GmG2-Me. The organogel rigidity determined by G’ followed
the increasing trend as: G0G2’-Me<G2G2-Me<G1G2-Me<
G0G2-Me<G0G3-Me. The trends for G0Gn-Me and GmG2-Me are
in agreement with those acquired from the study of thermo-
tropic behavior as mentioned above. The dendritic gel of G0G3-
Me displayed better rigidity associated with an elastic gel
structure than that of G0G2-Me. For example, the storage mod-
ulus G’ for G0G3-Me at 10 mm is 32 kPa, which is higher than
tion, no evident differences in
morphology behaviors of den-
drimers GmG2-Me (m=0, 1, 2)
were discovered (Figures S3 and
S4, the Supporting Information).
The morphologies of the xero-
gels with respective solvents
were also investigated by using
SEM. For example, a very dense
fiber-like morphology for G0G3-
Me in different organic solvents
was observed (Figure S5, the
Supporting Information).
Rheological behavior of den-
dritic gels
To receive additional information
of the structure–property rela-
tionships, rheological experi-
ments with dendritic gels of
G0Gn-Me (n=2, 3), GmG2-Me
Figure 3. Storage modulus G’ and loss modulus G’’ for dendritic organogels formed in toluene (10 mm) at 208C as
a function of A) and B) oscillation frequency with a strain of 0.03%, C) and D) time with a frequency of 1 Hz and
(m=1, 2), and G0G2’-Me formed
in toluene were conducted a strain of 0.03%.
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7074
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
19 kPa for G0G2-Me, suggesting a multivalent effect. As the
generation of the focal Frꢂchet dendron increased from 0 to 2,
the storage modulus G’ of GmG2-Me decreased at the same
concentration. For example, the G’ for G0G2-Me was 19 kPa at
10 mm, whereas G’ of G1G2-Me dropped to 16 kPa, and the
lowest G’ of 10.8 kPa was observed for G2G2-Me. Surprisingly,
the gel G0G2’-Me showed the weakest rigidity among the gela-
tors tested, which is different from the results of the thermo-
tropic experiments, in which G0G2’-Me showed better thermo-
stability than the focal point-functionalized co-dendrimers
G1G2-Me and G2G2-Me.
Detailed measurements of rheological properties for deter-
mining the influence of gelator concentration on storage and
loss moduli were also carried out. Figures S6 and S7 (the Sup-
porting Information) show the time and frequency sweep
curves of dendritic gels of G0G2-Me and G0G3-Me formed in
toluene at different concentrations. For both organogels, the
storage modulus G’ and the loss modulus G’’ increased as ex-
pected with the increasing of dendritic gelator concentration
from 2 to 12 mm, which is in accordance with the rheological
property of a true gel. The mechanical properties of the result-
ing gels from G0G3-Me are substantially superior to those of
G0G2-Me. These differences should be ascribed to the different
mode and level of self-assembly relating directly to the varia-
tion of the dendritic structure.[22] From the SEM images of xero-
gels from toluene (see Figure S8, the Supporting Information),
well-intertwined thicker fibers in a curved shape were found
for dendron G0G3-Me, whereas G0G2-Me showed a straight and
looser network structure.
p-systems of the peripherally ester-functionalized phenyl rings,
consisting of a phenyl ring and C=O double bond, is essential
for p–p stacking interactions. Although differing in composi-
tion and size, conjugated p-systems were observed in most
crystal structures, except for 3,4-G0G1-Me, which could not
form gel in all selected solvents; 2) At least one type of inter-
molecular p–p stacking interactions between two intermolecu-
lar peripheral extended p-systems was commonly observed for
most single crystals, except for 4-G0G1-Me, which is crucial for
efficient gel formation; 3) Besides p–p stacking, intermolecular
weak CÀH···p interactions (2.81–2.90 ꢃ) and non-conventional
hydrogen bonding (2.31–2.71 ꢃ) were ubiquitous in all crystal
structures. The hydrogen atoms of CH3 or CH2 of ester groups,
different benzyl hydrogens, and phenyl hydrogens of all den-
dron layers could be hydrogen donors, whereas carbonyl
oxygen atoms, benzyl oxygen atoms, and even the alcoholic
oxygen atoms of ester groups could be acceptors for hydrogen
bonding. At the same time, the p-systems involved in the CÀ
H···p interactions could be any substituted phenyl rings at the
periphery or in the middle layer of the dendrons.
For the best gelator analogue G0G1-Me, it was noted that
two types of peripheral extended p-systems existed. For one
of the two peripheral substituted phenyl rings in the molecule,
both carbonyl groups were coplanar with the phenyl ring, and
constituted a conjugated p-system. The two carbonyl oxygen
atoms pointed outside, forming cooperative hydrogen-bond-
ing interactions with the ortho hydrogen atom between the
two ester groups. This peripheral DMIP ring (electron-deficient
“head”) was coplanar with the internal benzyl ring (electron-
rich “tail”). The two neighboring extended planes were stacked
in head-to-tail pairs with interplanar distances of about 3.42 ꢃ
(Figure S15, the Supporting Information) indicative of p–p in-
teractions. In the case of the other peripheral phenyl ring, the
directions of the two carbonyls were opposite, but the copla-
nar extended p-system was still constructed including the
phenyl ring and both the carboxylates. This peripheral DMIP
ring adopted the “J-aggregation” p–p interaction mode with
another peripheral DMIP ring of the same type in another den-
dron molecule with interplanar distances of about 3.42 ꢃ. So,
both peripheral DMIP rings and the internal phenyl ring all par-
ticipate in the p–p stacking interactions, and compact lamellar
aggregation of dendrons dominated by p–p interactions with
the assistance of multiple intermolecular weak CÀH···p interac-
tions (2.59–2.87 ꢃ) between methyl groups of peripheral esters
and DMIP rings and multiple non-conventional hydrogen-
bonding interactions related to the carbonyl oxygen atoms of
the peripheral ester groups (2.42–2.71 ꢃ) explains why periph-
erally DMIP-functionalized dendrons are the most effective in
gel formation (Figure S15, the Supporting Information).
Driving forces in gel formation
To gain better insight into the possible relationships between
dendritic structure parameters and molecular recognition, the
key issue is to reveal the driving forces responsible for the
spontaneous supramolecular process of self-assembly. X-ray
1
diffraction experiments and elaborate H NMR studies were ap-
plied to this investigation.
X-ray diffraction experiments of low generation dendrons
In our previous work,[17] it has been demonstrated that the
multiple p–p stacking interactions due to the peripheral DMIP
motifs are the key contributor in forming the self-assembled
gel for dendritic gelator G0G2-Me. To shed light on the driving
forces in gel phase aggregation from the view point of a struc-
ture–property relationship study, single crystals of G0G1 den-
drons with different peripheral ester groups and dendron skel-
eton, including G0G1-Me, 3-G0G1-Me, 4-G0G1-Me, 3,4-G0G1-Me,
G0G1-Et, and G0G1’-Me were acquired through slow evapora-
tion of acetonitrile/methanol mixed solutions at room temper-
ature (see the Supporting Information), and the molecular
structures were determined unambiguously by using X-ray
crystallography analysis. From the packing analysis of the
single crystals as shown in Figure S15 (the Supporting Informa-
tion) and Figure 4, several intriguing features were noticed and
summarized as follows: 1) The formation of extended coplanar
In the crystal structures of 3-G0G1-Me and 4-G0G1-Me, only
one of the two peripheral ester-functionalized phenyl rings
formed carbonyl conjugated p-systems. “J-aggregation” p–p
interactions (3.60 and 3.70 ꢃ) between these extended p-sys-
tems were observed for 3-G0G1-Me, whereas instead, only mul-
tiple sets of CH···p interactions (2.82–2.90 ꢃ) were found in the
crystal structure of 4-G0G1-Me. For dendron 3,4-G0G1-Me,
whose analogue 3,4-G0G2-Me could not form a gel in all of the
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7075
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 4. Molecular structure and interactions of A) 3-G0G1-Me, B) 4-G0G1-Me, C) G0G1-Et, D) 3,4-G0G1-Me, and E) G0G1’-Me in single crystals.
selected solvents, no coplanar extended p-system was ob-
served but weak p–p stacking and CH···p interactions were
found to exist in the crystal structure. In contrast with the two
types of extended p-systems found in G0G1-Me, only one of
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7076
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
the peripheral ester-functionalized phenyl rings of G0G1-Et
formed carbonyl conjugated p-systems with the two carbonyl
groups adopting an “opposite” conformation, and “J-aggrega-
tion” p–p interactions (3.43 ꢃ) between the same type of ex-
tended p-systems were proven to exist. In addition, two types
of peripheral extended p-systems similar to those found in
G0G1-Me were also observed in the crystal structure of G0G1’-
Me. Three kinds of p–p stacking interactions between periph-
eral extended p-systems were distinctly observed for G0G1’-Me,
including two types of “H-aggregation” p–p stacking (3.28 and
3.43 ꢃ) between the same p-systems and one type of “J-aggre-
gation” p–p stacking (3.30 ꢃ) between different p-systems. The
internal phenyl ring was not involved in p–p stacking, differing
from that found in the crystal structure of G0G1-Me. Such mul-
tiple p–p stacking interactions well explain the similar gelation
properties exhibited by G0G1’-Me and G0G1-Me. Relating the
p–p stacking interactions noted in different dendron crystal
structures with their gelation abilities, it seems that highly effi-
cient p–p stacking interactions of peripheral enlarged p-sys-
tems result in compact lamellar stacking of dendrons, thus
play the most important role in the highly ordered self-assem-
bly of these peripherally aromatic ester-functionalized
dendrons.
25.4, 24.4, and 25.28, corresponding to a d-spacing of 3.50,
3.64, and 3.54 ꢃ, respectively, were observed, suggesting typi-
cal p–p stacking interactions. In contrast, no apparent signal
indicative for p–p stacking was found for xerogels of dendrons
4-G0G1-Me and 3,4-G0G1-Me (Figure S16, the Supporting
Information).
1H NMR spectroscopic studies in the sol and gel states
To further investigate the role of p–p stacking in gelation pro-
cess, the molecular aggregation behaviors of G0G1 dendritic
derivatives, including G0G1-Me, G0G1’-Me, 3-G0G1-Me, 4-G0G1-
Me, 3,4-G0G1-Me, and G0G1-Et, in solution were confirmed by
concentration-dependent (CD) and temperature-dependent
1
(TD) H NMR spectroscopy studies.[17,24] In the CD-1H NMR ex-
periments in CD3CN (Figure 5), the increase in solution concen-
tration from 0.13 to 100 mm resulted in slight upfield shifts of
the resonance signals for the aromatic protons on the periph-
eral ester-substituted phenyl rings (H(a) and H(b)) and the in-
ternal benzyl rings (H(c) and H(d)) for most dendrons tested,
indicating crescent p–p stacking induced by molecular aggre-
gation, except for 3,4-G0G1-Me (no extended conjugated p-
system was observed in its crystal structure). For example, the
resonance signal of the aromatic proton H(a) on the peripheral
phenyl rings of dendron G0G1-Me showed the largest upfield
shift (Dd=0.097 ppm), whereas dendrons G0G1’-Me, G0G1-Et,
3-G0G1-Me, and 4-G0G1-Me gave upfield chemical shifts of Ha
as Dd=0.063, 0.084, 0.016, and 0.020 ppm, respectively. A neg-
ligible upfield chemical shift of Dd=0.006 ppm was noticed
for dendron 3,4-G0G1-Me, showing quite weak p–p stacking. In
addition, the resonance signals of all the benzyl protons (H(h)
and H(i)) were also found slightly upfield shifted in the experi-
ments. It should be also noted that obvious peak broadening
was observed for dendron G0G1’-Me as the concentration in-
creased to higher than 14.3 mm. So, p–p stacking interactions
are proven universally existent in the solution phase aggrega-
tion of the peripherally aromatic ester-functionalized poly(ben-
zyl ether) dendritic gelators.
TD-1H NMR studies of G0G1-Me in solution (CD3CN, 14.3 mm)
and G0G2-Me in the gel state (CD3CN, 8 mgmLÀ1) were also
carried out separately. Regarding the TD-1H NMR experiments
of G0G1-Me in solution, the resonance signals related to the ar-
omatic protons (H(a), H(b), H(c), and H(d)) were found to grad-
ually shift downfield when the temperature increased from 10
to 558C (Figure S18, the Supporting Information), indicating
the weakening of p–p stacking. Notably, the signals corre-
sponding to the benzyl ring at the focal point remained con-
stant. A similar signal-shift trend was also observed in the gel–
sol phase transition experiments of G0G2-Me upon warming
from 20 to 908C (Figure S19, the Supporting Information). In
addition to the downfield shifts of the related aromatic signals,
all proton signals experienced a dramatic broadening below
508C, which is consistent with the virtually steady gel being re-
tained under these conditions.
Although the solvophobic effect may be the most conceiva-
ble motive for gel formation, the cooperative effects of multi-
ple intermolecular and intramolecular non-covalent interac-
tions, including various p–p stacking, CÀH···p interactions, and
hydrogen bonding, are responsible for the highly ordered and
efficient self-assembly of these specific dendrons. For 3-G0G1-
Me and G0G1-Et, one of the peripheral phenyl rings is enlisted
in the p–p stacking, whereas the other peripheral phenyl ring
usually participates in the CÀH···p interactions and/or hydro-
gen bonding. For G0G1-Me and G0G1’-Me, both peripherally
ester-substituted phenyl rings participated in multiple p–p
stacking, meanwhile, CÀH···p interactions and hydrogen bond-
ing were also related to these peripheral aryl groups. Generally,
most parts of the dendritic molecule were involved in the mo-
lecular-recognition process in a cooperative manner, which
probably is the conceivable reason why these peripherally
ester-functionalized poly(benzyl ether) dendrons without any
conventional gelation motifs become efficient gelators in or-
ganic solvents, and why the dendritic structure has a high tol-
erance for gelation. Of course, the flexible molecular skeleton
and versatile functionalities of the dendritic molecules are ben-
eficial for the management of all these supramolecular recog-
nitions to form gel phase material in the presence of plentiful
solvent molecules. In conclusion, these peripherally multiple
aromatic ester-functionalized poly(benzyl ether) dendrons are
a new kind of dendritic gelators whose gel phase assembling
is dominated by multiple p–p stacking interactions and co-
operative CÀH···p interactions and hydrogen bonding due to
the functionalized peripheral groups.
In addition to the single crystal structures, the p–p stacking
interactions were further evidenced by powder X-ray diffrac-
tion (PXRD) studies.[17,23] In the PXRD patterns of dendron xero-
gels of G0G1-Me, G0G1’-Me, and G0G1-Et from benzene (Fig-
ure S16, the Supporting Information), distinctive peaks at 2q=
Based on all the above experimental evidences, multiple p–
p stacking interactions, together with hydrogen bonding, CÀ
H···p interactions, and solvophobic interactions might be the
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7077
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
1
Figure 5. Partial H NMR spectra (600 MHz, [D3]CH3CN) of dendrons at different concentrations (298 K) showing the aromatic region (using TMS as the internal
standard): A) G0G1-Me, B) 3-G0G1-Me, C) 4-G0G1-Me, D) 3,4-G0G1-Me, E) G0G1-Et, F) G0G1’-Me.
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7078
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
main contributors for the unique gelation capability of these
peripherally aromatic ester-functionalized poly(benzyl ether)
dendrons without bearing any conventional gelation motifs.
Furthermore, the dendritic periphery dominated self-assembly
of these poly(benzyl ether) dendrons in the gel phase renders
them new dendritic gelation “motifs” awaiting further applica-
tions in the field of nanomaterials.
known as the upper of the linear viscoelastic regime, repre-
senting a partial collapse of the gel to a quasi-liquid state. In-
terestingly, this gel exhibited a very quick recovery of mechani-
cal strength after the large amplitude oscillatory breakdown
(Figure 7C). For example, the initial value of G’ was about
12500 Pa; it dropped to 64 Pa under a large amplitude oscilla-
tory force (g=100%; w=6.28 radsÀ1 (1.0 Hz)), indicating the
transformation to a quasi-liquid state. However, when the am-
plitude oscillation was decreased (g=0.05%) at the same fre-
quency (1.0 Hz), G’ immediately recovered to about 80% of its
initial value (9900 Pa) within 10 s, representing the recovery of
a quasi-solid state. However, with prolonged time of more
than one hour, G’ and G’’ could reach their initial values (Fig-
ure 7D). Furthermore, this thixotropic process could be repeat-
ed several times. The thixotropic properties of dendritic orga-
nogels of other generations (G0G2-Me) and/or in different sol-
vents have also been investigated (Figure S20–S30 in the Sup-
porting Information), affording conformable conclusions. All
these results of rheological experiments indicate that G0G3-Me
and G0G2-Me are promising candidates for thixotropic soft ma-
terials with both large mechanical strength and a fast recovery
ability.
Thixotropic-responsive properties of the dendritic organo-
gels
Thixotropic gels, which exhibit reversible sol–gel transforma-
tions upon exposure to external mechanical stress, represent
an intriguing class of truly dynamic self-assembled supramolec-
ular systems. Among hundreds of functional low-molecular-
weight gelators (LMWG) reported to date, only very few of
them exhibited unique thixotropic properties.[25] Containing
multiple “adhesive termini” for non-covalent binding, dendritic
gelators are envisioned to be superior in thixotropic materials
than LMWG. However, only a few examples of thixotropic den-
dritic organogels have been reported so far.[26] Therefore, it re-
mains a great challenge to discover new dendritic organogel
systems possessing thixotropic and self-repairing properties.
Very excitingly, in the investigations of our peripherally multi-
ple aromatic ester-functionalized poly(benzyl ether) dendritic
gels, it was unexpectedly observed that the dendritic gels of
G0G3-Me and G0G2-Me, formed in most solvents tested, exhibit-
ed thixotropic properties (Table S8, the Supporting Informa-
tion). As shown in Figure 6, when treated with external me-
Conclusion
A series of peripherally aromatic ester-functionalized poly(ben-
zyl ether) dendritic gelators with structural variations in focal
point substitution, surface groups, dendron generations, and
dendritic skeleton configuration have been synthesized. A sys-
tematic study of the structure–property relationships of these
dendritic gelators, accompanied by the mechanism study by
using XRD and CD-/TD-1H NMR spectroscopic techniques,
afford some important conclusions, which are useful for the
molecular design of new dendritic gelators in the future:[27]
1) The gelation efficiency is highly dependent on the periph-
eral ester groups, not only through tuning the molecular
solubility, but also by affecting the self-assembly mode;
dendrons with DMIP motifs as peripheral layers turned out
to be the most efficient gelators.
Figure 6. Photographs of the G0G3-Me/1,2-dichloroethane gel (6 mm) upon
mechanical stress: A) original gel before mechanical stress; B) The solution
obtained after mechanical stress; C) The gel recovered upon resting for 1 h.
2) The generation effect of dendrons on gelation properties is
clearly demonstrated although it is complicated. The gela-
tion ability, the thermotropic behavior, the morphological
properties, and even the rheological behavior vary upon an
increase in generation.
chanical stress, the translucent G0G3-Me gel from 1,2-dichloro-
ethane lost most of its viscosity and transformed into a viscous
solution. After resting for at least 1 h at room temperature, the
gel could be recovered. This cycling process could be repeated
many times.
3) The attachment of Frꢂchet dendrons at the focal point
does not change the fact of stable gel formation for co-
dendrimers GmG2-Me, but the bigger the dendritic wedge,
the worse the gelation ability.
To verify the thixotropic behaviors of the dendritic organo-
gels, rheological experiments were carried out with G0G3-Me
and G0G2-Me gels. As shown in Figure 7A, the storage modu-
lus (G’) and loss modulus (G’’) as a function of angular frequen-
cy (w) of the gel G0G3-Me/1,2-dichloroethane at different con-
centrations were determined, showing that the gel behaved as
a true gel. Strain amplitude sweeps were also proceeded to
reveal the elastic response of the G0G3-Me/1,2-dichloroethane
gel (6 mm). As shown in Figure 7B, the value of G’ rapidly de-
creased in the region above the critical strain value (g>1%),
4) The symmetric monodisperse dendritic skeleton is much
more in favor of constructing highly efficient gel systems
compared with the linear analogue and dissymmetric den-
drons.
5) The multiple p–p stacking of extended p-systems due to
the peripheral ester-functionalized phenyl rings, coopera-
tively assisted by supplementary intermolecular CÀH···p in-
teractions and non-conventional hydrogen bonding, is the
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7079
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 7. Rheological data of G0G3-Me gel from 1,2-dichloroethane (158C): A) Frequency sweeps (w=0.1–100 radsÀ1) at different concentrations (c=4, 6, 8,
and 10 mm); B) Strain amplitude sweeps; C) Step strain; D) Time sweep measurement (c=6 mm). Step strain was a continuous measurement for four cycles.
key contributor in the formation of stable self-assembled
gels, which is supported firmly by single-crystal XRD analy-
sis and CD-/TD-1H NMR spectroscopy in solution and in the
gel state.
can tolerate a bulky substituent at the core, which renders
them efficient dendritic gelation motifs and provides a great
opportunity for incorporating a functional group into the core
to fabricate functional supramolecular materials.[20] These dis-
coveries thus open up a new avenue for the rational design of
new types of dendritic gelators and corresponding functional
soft materials.[27]
6) Furthermore, these dendritic organogelators were found to
exhibit unexpected multiple stimuli-responsive behaviors
upon exposure to environmental stimuli,[26,28] such as tem-
perature and mechanical stress, which makes them a new
kind of dendritic candidates with potential applications in
the field of intelligent soft materials.
Experimental Section
General
In summary, we have demonstrated that the peripherally
multiple aromatic ester, in particular, DMIP-functionalized, poly-
(benzyl ether) dendrons are a new class of efficient dendritic
organogelators. Distinguished from the core- and skeleton-
functionalized dendritic gelators, for example, dendrons bear-
ing amide groups at the core or skeleton backbone, our den-
dritic gelators are functionalized at the periphery, whose gel-
phase self-assembly is dominated by synergistic non-covalent
interactions due to the peripheral ester-functionalized benzyl
rings, instead of the conventional gelation motifs like long
alkyl chains or steroidal groups, leaving the focal point vacant
to facilitate post-functionalization. Furthermore, these gelators
All chemicals were obtained from Aldrich, Alfa, or Acros and used
as received unless otherwise mentioned. The organic solvents used
were dried according to published methods. Unless otherwise
noted, all reactions were performed under an atmosphere of dini-
trogen employing standard Schlenk techniques. 1H NMR and
13C NMR spectra were recorded on Bruker AMX 300 Spectrometer
(1H: 300 MHz; 13C: 75 MHz) or Bruker AMX-600 spectrometers (1H:
600 MHz; 13C: 150 MHz) at 298 K. Chemical shifts are reported in
parts per million (ppm) relative to the internal standards, partially
deuterated solvents or tetramethylsilane (TMS). Coupling constants
(J) are denoted in Hz and chemical shifts (d) in ppm. Multiplicities
are denoted as follows: s=singlet, d=doublet, m=multiplet, br=
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7080
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
broad. Matrix-assisted laser desorption-ionization (time of flight)
mass spectrometry (MALDI-TOF) was performed on a Bruker Biflex
III MALDI-TOF spectrometer with a-cyano-4-hydroxylcinnamic acid
(CCA) as the matrix. HRMS-FAB spectra were performed on
a Bruker APEX 47E FTMS spectrometer. HRMS-ESI mass spectra
were obtained on a Bruker APEX IV instrument. Elemental analyses
were performed on a Carlo–Erba-1106 instrument.
at 750 mm. A solvent-trapping device was placed above the plate
to avoid evaporation. When gel samples were mounted on the
plate, they were allowed to stand for at least 5 min to facilitate the
recovery of the structure. All measurements were made at 15 or
208C. A strain sweep at a constant frequency (6.28 radsÀ1) was per-
formed in the 0.01–200% range to determine the linear viscoelastic
region (LVER) of the gel sample. The frequency sweep was ob-
tained from 0.1 to 100 radsÀ1 at a constant strain of 0.03 or 0.05%,
well within the linear regime determined by the strain sweep. A
thixotropic study was conducted to examine the recovery behavior
of dendritic organogels after the strain sweep. The recovery of the
storage modulus of the destroyed gel is monitored at a constant
frequency (6.28 radsÀ1) under a low strain (0.05%) just after the
strain sweep progress. The storage modulus G’ and the loss modu-
lus G’’ are recorded as functions of time in the recovery process.
Scanning electron microscopy (SEM)
The morphologies and sizes of the dendron xerogels were charac-
terized by field emission scanning electron microscopy (FE-SEM, Hi-
tachi S-4800) at an accelerating voltage of 15 kV. Samples were
prepared by drop casting the suspension of freeze-dried gel in
hexane on a silicon substrate. To minimize sample charging, a thin
layer of Au was deposited onto the samples before SEM examina-
tion.
To investigate the recovery properties of the samples in response
to applied shear forces, we used the following programmed proce-
dure (applied shear force, expressed in terms of strain; duration in
parentheses): 1) Standing the gel sample for 5 min after mounting;
2) Linear small-amplitude oscillations (at g=0.05% and w=1 Hz)
for 5 min to monitor initial value of G’ and G’’; 3) Nonlinear, large-
amplitude oscillations (g=100%) at the same frequency for
1.5 min to break down the gel structure; 4) Linear small-amplitude
oscillations (g=0.05%) for 5 min at the same frequency to monitor
recovery of mechanical strength of gels; 5) An additional two
cycles of the breakdown and recovering processes described in (3)
and (4). The measurements were carried out with the solvent-trap
to prevent drying a gel sample during the long-time measurement.
The storage modulus G’ and the loss modulus G’’ are recorded as
functions of time in both processes.
Environmental scanning electron microscopy (ESEM)
The morphologies and sizes of the dendron xerogels were charac-
terized by FEI Quanta 200 ESEM FEG at an accelerating voltage of
10 kV. Samples were prepared by drop-casting the suspension of
freeze-dried gel in hexane on a silicon substrate.
Transmission electron microscopy (TEM)
TEM was performed on a JEOL JEM-2011 and JEOL JEM-1011 micro-
scope. Samples were prepared by drop-casting the suspension of
freeze-dried gel in hexane on carbon-coated copper grids and the
TEM pictures were obtained without staining.
Gelation test
Single-crystal X-ray diffraction
A weighed sample of a dendritic organogelator was mixed with se-
lected solvent (0.5 mL) in a septum-capped vial and heated in an
oil bath until the solid was dissolved. Then, the sample vial was
cooled to room temperature. The aggregation state was then as-
sessed. If no flow was observed when inverting the vial, a stable
gel was formed and noted as gelation (G). If part of the mixture
formed a gel but flow was still observed, the phenomenon was re-
corded as partial gelation (PG). If precipitation occurred, P was
noted, and if the clear solution (>60 mgmLÀ1) was retained, it was
marked as soluble (S). If the compound was unable to dissolve, it
was noted as insoluble (I). In some solvent systems (for example, 2-
A crystal suitable for X-ray diffraction was obtained by slow evapo-
ration of dendron solution at room temperature. All data were col-
lected on a Rigaku Saturn X-ray diffractometer with graphite-mon-
ochromator MoKa radiation (l=0.71073 ꢃ) at 113 K. Intensities
were collected for absorption effects by using the multi-scan tech-
nique SADABS. The structures were solved by direction methods
and refined by a full matrix least-squares technique based on F2
using SHELXL 97 program (Sheldrick, 1997). The extended packing
plots and data from crystal packing were obtained by using the
software Mercury 1.4.1. CCDC-980756 (G0G1-Me), CCDC-980757 (3-
G0G1-Me), CCDC-980758 (4-G0G1-Me), CCDC-980759 (3,4-G0G1-Me),
CCDC-980760 (G0G1-Et), and 980761 (G0G1’-Me) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
methoxyethanol, acetonitrile), short-term sonication (0.40 WcmÀ2
,
40 KHz, 1–2 min) was required at the beginning of the cooling pro-
cess. Repeated heating and cooling confirmed the thermoreversi-
bility of the gelation process. The critical gelation concentration
(CGC) of the organogelator was determined by measuring the min-
imum amount of gelator required for the formation of a stable gel
at room temperature.
Powder X-ray diffraction (PXRD)
The gel of the dendron in benzene was coated on a glass plate
and the solvent was slowly evaporated. Then, the glass plates with
dry gels were fixed on a sample holder and subjected to XRD anal-
ysis at room temperature on a Japan Rigaku D/MAX-2500/PC dif-
fractometer. XRD patterns were recorded at a scanning rate of
28minÀ1 in the 2q range of 0.5 to 30 with CuKa radiation (l=
1.54178 ꢃ).
Acknowledgements
The authors thank the National Natural Science Foundation of
China (No 21074140 and 91027046) and the National Basic Re-
search Program of China (973 program, No. 2013CB932800) for
financial support.
Rheological measurements
Rheological measurements were carried out with a stress-con-
trolled rheometer (TA Instruments, AR-G2) equipped with steel par-
allel-plate geometry (40 mm diameter). The gap distance was fixed
Keywords: dendrimers · gelators · organogels · self-assembly ·
structure–property relationships
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7081
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[1] For selected reviews, see: a) Molecular Gels: Materials with Self-Assem-
bled Fibrillar Networks (Eds.: R. G. Weiss, P. Terech), Springer, Amsterdam,
2006; b) Functional Molecular Gels (Eds.: B. Escuder, J. F. Miravet), The
Royal Society of Chemistry, Cambridge, 2014; c) R. G. Weiss, Langmuir
2009, 25, 8369–8369, and reviews from this special issue; d) P. Terech,
R. G. Weiss, Chem. Rev. 1997, 97, 3133–3160; e) N. M. Sangeetha, U.
Maitra, Chem. Soc. Rev. 2005, 34, 821–836; f) P. Dastidar, Chem. Soc. Rev.
2008, 37, 2699–2715; g) M. O. M. Piepenbrock, G. O. Lloyd, N. Clarke,
J. W. Steed, Chem. Rev. 2010, 110, 1960–2004; h) A. Dawn, T. Shiraki, S.
Haraguchi, S. Tamaru, S. Shinkai, Chem. Asian J. 2011, 6, 266–282;
i) A. Y. Y. Tam, V. W. W. Yam, Chem. Soc. Rev. 2013, 42, 1540–1567; j) G. C.
Yu, X. Z. Yan, C. Y. Han, F. H. Huang, Chem. Soc. Rev. 2013, 42, 6697–
6722; k) K. K. Kartha, R. D. Mukhopadhyay, A. Ajayaghosh, Chimia 2013,
67, 51–63; l) S. S. Babu, V. K. Praveen, A. Ajayaghosh, Chem. Rev. 2014,
114, 1973–2129.
[15] a) X. C. Yang, R. Lu, F. Y. Gai, P. C. Xue, Y. Zhan, Chem. Commun. 2010, 46,
1088–1090; b) Z. C. Ding, R. B. Xing, X. D. Wang, J. Q. Ding, L. X. Wang,
Y. C. Han, Soft Matter 2013, 9, 10404–10412.
[16] A. Pꢂrez, J. L. Serrano, T. Sierra, A. Ballesteros, D. Saꢄ, J. Barluenga, J.
Am. Chem. Soc. 2011, 133, 8110–8113.
[17] Y. Feng, Z. T. Liu, J. Liu, Y. M. He, Q. Y. Zheng, Q. H. Fan, J. Am. Chem.
Soc. 2009, 131, 7950–7951.
[18] Y. Feng, Y. M. He, L. W. Zhao, Y. Y. Huang, Q. H. Fan, Org. Lett. 2007, 9,
2261–2264.
[19] A. R. Hirst, I. A. Coates, T. R. Boucheteau, J. F. Miravet, B. Escuder, V. Cas-
telletto, I. W. Hamley, D. K. Smith, J. Am. Chem. Soc. 2008, 130, 9113–
9121.
[20] a) Q. Chen, Y. Feng, D. Q. Zhang, G. X. Zhang, Q. H. Fan, S. N. Sun, D. B.
Zhu, Adv. Funct. Mater. 2010, 20, 36–42; b) Q. Chen, D. Q. Zhang, G. X.
Zhang, X. Y. Yang, Y. Feng, Q. H. Fan, D. B. Zhu, Adv. Funct. Mater. 2010,
20, 3244–3251; c) Z. X. Liu, Y. Feng, Z. C. Yan, Y. M. He, C. Y. Liu, Q. H.
Fan, Chem. Mater. 2012, 24, 3751–3757; d) X. Y. Yang, G. X. Zhang, L. Q.
Li, D. Q. Zhang, L. F. Chi, D. B. Zhu, Small 2012, 8, 578–584; e) G. Z.
Zhao, L. J. Chen, W. Wang, J. Zhang, G. Yang, D. X. Wang, Y. H. Yu, H. B.
Yang, Chem. Eur. J. 2013, 19, 10094–10100; f) Z. X. Liu, Y. Feng, Z. Y.
Zhao, Z. C. Yan, Y. M. He, X. J. Luo, C. Y. Liu, Q. H. Fan, Chem. Eur. J.
2014, 20, 533–541.
[21] a) S. M. Grayson, J. M. J. Frꢂchet, Chem. Rev. 2001, 101, 3819–3867;
b) C. J. Hawker, J. M. J. Frꢂchet, J. Chem. Soc. Chem. Commun. 1990,
1010–1013; c) C. J. Hawker, J. M. J. Frꢂchet, J. Am. Chem. Soc. 1990, 112,
7638–7647; d) C. J. Hawker, J. M. J. Frꢂchet, Macromolecules 1990, 23,
4726–4729.
[22] a) N. Mohmeyer, H. W. Schmidt, Chem. Eur. J. 2005, 11, 863–872;
b) D. M. Ryan, S. B. Anderson, B. L. Nilsson, Soft Matter 2010, 6, 3220–
3231.
[23] a) A. Ajayaghosh, S. J. George, J. Am. Chem. Soc. 2001, 123, 5148–5149;
b) J. Hu, D. Zhang, S. Jin, S. Z. D. Cheng, F. W. Harris, Chem. Mater. 2004,
16, 4912–4915.
[24] a) T. Tu, W. Assenmacher, H. Peterlik, R. Weisbarth, M. Nieger, K. H. Dçtz,
Angew. Chem. 2007, 119, 6486–6490; Angew. Chem. Int. Ed. 2007, 46,
6368–6371; b) G. Fernꢄndez, L. Sꢄnchez, E. M. Pꢂrez, N. Martꢅn, J. Am.
Chem. Soc. 2008, 130, 10674–10683; c) W. Lu, Y. Law, J. Han, S. S. Chui,
D. Ma, N. Zhu, C. M. Che, Chem. Asian J. 2008, 3, 59–69.
[25] For selected examples, see: a) J. van Esch, F. Schoonbeek, M. de Loos, H.
Kooijman, A. L. Spek, R. M. Kellogg, B. L. Feringa, Chem. Eur. J. 1999, 5,
937–950; b) P. Terech, D. Pasquier, V. Bordas, C. Rossat, Langmuir 2000,
16, 4485–4494; c) J. B. Beck, S. J. Rowan, J. Am. Chem. Soc. 2003, 125,
13922–13923; d) X. Huang, S. R. Raghavan, P. Terech, R. G. Weiss, J. Am.
Chem. Soc. 2006, 128, 15341–15352; e) W. G. Weng, J. B. Beck, A. M. Ja-
mieson, S. J. Rowan, J. Am. Chem. Soc. 2006, 128, 11663–11672; f) J. Liu,
P. L. He, J. L. Yan, X. H. Fang, J. X. Peng, K. Q. Liu, Y. Fang, Adv. Mater.
2008, 20, 2508–2511; g) P. Mukhopadhyay, N. Fujita, A. Takada, T. Kishi-
da, M. Shirakawa, S. Shinkai, Angew. Chem. 2010, 122, 6482–6486;
Angew. Chem. Int. Ed. 2010, 49, 6338–6342; h) T. D. Hamilton, D. K.
Bucar, J. Baltrusaitis, D. R. Flanagan, Y. J. Li, S. Ghorai, A. V. Tivanski, L. R.
MacGillivray, J. Am. Chem. Soc. 2011, 133, 3365–3371; i) A. Dawn, T. Shir-
aki, H. Ichikawa, A. Takada, Y. Takahashi, Y. Tsuchiya, L. T. N. Lien, S. Shin-
kai, J. Am. Chem. Soc. 2012, 134, 2161–2171; j) M. M. Zhang, D. H. Xu,
X. Z. Yan, J. Z. Chen, S. Y. Dong, B. Zheng, F. H. Huang, Angew. Chem.
2012, 124, 7117–7121; Angew. Chem. Int. Ed. 2012, 51, 7011–7015.
[26] a) V. Percec, M. Peterca, M. E. Yurchenko, J. G. Rudick, P. A. Heiney,
Chem. Eur. J. 2008, 14, 909–918; b) J. Liu, Y. Feng, Z. X. Liu, Y. M. He,
Q. H. Fan, Chem. Asian J. 2013, 8, 572–581.
[2] For recent reviews, see: a) A. R. Hirst, D. K. Smith, Top. Curr. Chem. 2005,
256, 237–273; b) D. K. Smith, Adv. Mater. 2006, 18, 2773–2778; c) D. K.
Smith, Chem. Commun. 2006, 34–44.
[3] a) G. R. Newkome, G. R. Baker, M. J. Saunders, P. S. Russo, V. K. Gupta,
Z. Q. Yao, J. E. Miller, K. Bouillion, J. Chem. Soc. Chem. Commun. 1986,
752–753; b) G. R. Newkome, G. R. Baker, S. Arai, M. J. Saunders, P. S.
Russo, K. J. Theriot, C. N. Moorefield, L. E. Rogers, J. E. Miller, T. R. Lieux,
M. E. Murray, B. Phillips, L. Pascal, J. Am. Chem. Soc. 1990, 112, 8458–
8465; c) G. R. Newkome, C. N. Moorefield, G. R. Baker, R. K. Behera, G. H.
Escamilla, M. J. Saunders, Angew. Chem. 1992, 104, 901–903; Angew.
Chem. Int. Ed. Engl. 1992, 31, 917–919.
[4] a) C. Marmillon, F. Gauffre, T. Gulik-Krzywicki, C. Loup, A. M. Caminade,
J. P. Majoral, J. P. Vors, E. Rump, Angew. Chem. 2001, 113, 2696–2699;
Angew. Chem. Int. Ed. 2001, 40, 2626–2629; b) A. El Ghzaoui, F. Gauffre,
A. M. Caminade, J. P. Majoral, H. Lannibois-Drean, Langmuir 2004, 20,
9348–9353; c) C. Kojima, S. Tsumura, A. Harada, K. Kono, J. Am. Chem.
Soc. 2009, 131, 6052–6053; d) Q. G. Wang, J. L. Mynar, M. Yoshida, E.
Lee, M. Lee, K. Okuro, K. Kinbara, T. Aida, Nature 2010, 463, 339–343.
[5] a) W. D. Jang, D. L. Jiang, T. Aida, J. Am. Chem. Soc. 2000, 122, 3232–
3233; b) W. D. Jang, T. Aida, Macromolecules 2003, 36, 8461–8469.
[6] a) C. Kim, K. T. Kim, Y. Chang, H. H. Song, T. Y. Cho, H. J. Jeon, J. Am.
Chem. Soc. 2001, 123, 5586–5587; b) C. Kim, S. J. Lee, I. H. Lee, K. T. Kim,
H. H. Song, H. J. Jeon, Chem. Mater. 2003, 15, 3638–3642; c) J. Lee, J. M.
Kim, M. Yun, C. Park, J. Park, K. H. Lee, C. Kim, Soft Matter 2011, 7,
9021–9026.
[7] a) K. S. Partridge, D. K. Smith, G. M. Dykes, P. T. McGrail, Chem. Commun.
2001, 319–320; b) A. R. Hirst, D. K. Smith, M. C. Feiters, H. P. M. Geurts,
A. C. Wright, J. Am. Chem. Soc. 2003, 125, 9010–9011; c) B. Q. Huang,
A. R. Hirst, D. K. Smith, V. Castelletto, I. W. Hamley, J. Am. Chem. Soc.
2005, 127, 7130–7139; d) A. R. Hirst, J. F. Miravet, B. Escuder, L. Noirez,
V. Castelletto, I. W. Hamley, D. K. Smith, Chem. Eur. J. 2009, 15, 372–379;
e) W. Edwards, D. K. Smith, J. Am. Chem. Soc. 2013, 135, 5911–5920.
[8] a) Y. Ji, Y. F. Luo, X. R. Jia, E. Q. Chen, Y. Huang, C. Ye, B. B. Wang, Q. F.
Zhou, Y. Wei, Angew. Chem. 2005, 117, 6179–6183; Angew. Chem. Int.
Ed. 2005, 44, 6025–6029; b) G. C. Kuang, Y. Ji, X. R. Jia, E. Q. Chen, M.
Gao, J. M. Yeh, Y. Wei, Chem. Mater. 2009, 21, 456–462; c) G. C. Kuang,
X. R. Jia, M. J. Teng, E. Q. Chen, W. S. Li, Y. Ji, Chem. Mater. 2012, 24, 71–
80.
[9] a) H. F. Chow, J. Zhang, Chem. Eur. J. 2005, 11, 5817–5831; b) K. N. Lau,
H. F. Chow, M. C. Chan, K. W. Wong, Angew. Chem. 2008, 120, 7018–
7022; Angew. Chem. Int. Ed. 2008, 47, 6912–6916.
[10] M. Yang, Z. J. Zhang, F. Yuan, W. Wang, S. Hess, K. Lienkamp, I. Lieber-
wirth, G. Wegner, Chem. Eur. J. 2008, 14, 3330–3337.
[11] a) P. F. Duan, M. H. Liu, Langmuir 2009, 25, 8706–8713; b) P. F. Duan, L.
Qin, X. F. Zhu, M. H. Liu, Chem. Eur. J. 2011, 17, 6389–6395; c) L. Qin,
P. F. Duan, F. Xie, L. Zhang, M. H. Liu, Chem. Commun. 2013, 49, 10823–
10825.
[12] a) E. R. Zubarev, M. U. Pralle, E. D. Sone, S. I. Stupp, J. Am. Chem. Soc.
2001, 123, 4105–4106; b) E. R. Zubarev, E. D. Sone, S. I. Stupp, Chem.
Eur. J. 2006, 12, 7313–7327.
[27] Y. Feng, Z. X. Liu, L. Y. Wang, H. Chen, Y. M. He, Q. H. Fan, Chin. Sci. Bull.
2012, 57, 4289–4295, and unpublished results.
[28] a) Y. Ji, G. C. Kuang, X. R. Jia, E. Q. Chen, B. B. Wang, W. S. Li, Y. Wei, J.
Lei, Chem. Commun. 2007, 4233–4235; b) J. H. Kim, M. Seo, Y. J. Kim,
S. Y. Kim, Langmuir 2009, 25, 1761–1766; c) G. C. Kuang, Y. Ji, X. R. Jia, Y.
Li, E. Q. Chen, Z. X. Zhang, Y. Wei, Tetrahedron 2009, 65, 3496–3501;
d) P. Rajamalli, E. Prasad, Org. Lett. 2011, 13, 3714–3717.
[13] M. Yoshida, Z. M. Fresco, S. Ohnishi, J. M. J. Frꢂchet, Macromolecules
2005, 38, 334–344.
Received: January 14, 2014
Published online on April 17, 2014
[14] M. Seo, J. H. Kim, J. Kim, N. Park, J. Park, S. Y. Kim, Chem. Eur. J. 2010, 16,
2427–2441.
Chem. Eur. J. 2014, 20, 7069 – 7082
www.chemeurj.org
7082
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim