Full Paper
On the basis of the above experimental results and our pre-
vious study on peripherally dimethyl isophthalate functional-
ized dendritic analogues,[11] we assume that the formation of
supramolecular organogels may be mainly driven by the multi-
ple p–p aromatic stacking interactions, together with disper-
sive halogen, CH–p, and weak CÀH···X hydrogen-bonding in-
teractions.
a subtle influence on gelation properties. Gelation cannot
happen unless the solvophobic interaction is strong enough.
Moreover, gelation also cannot occur when the dendritic gela-
tor is over-solvophobic and leads to macroscopic phase sepa-
ration between the gelator and the solvents. For these den-
drons, the solubility decreased in the order Gn-F>Gn-Cl>Gn-
Br>Gn-I (n=1, 2). Dendron Gn-F exhibited higher solubility in
the tested solvents than dendrons Gn-I and Gn-Br.
On the basis of the above discussion, we propose that the
significant halogen effect on the gelation properties of these
dendritic gelators may be the consequence of a subtle balance
of the various noncovalent interactions and the solubility of
dendrons. For example, for dendron Gn-F, multiple p–p interac-
tions, CH–p interactions, strong CÀH···F hydrogen bonding,
and weak solvophobic interactions synergistically enable the
formation of stable organogels. However, the multiple p–p in-
teractions, dispersive halogen interactions (halogen–arene and
halogen–halogen interactions), cooperative CH–p, and strong
solvophobic interactions are likely to be the key contributors
in the formation of stable self-assembled gels for Gn-I. The hal-
ogen effect is especially significant for these dendrons with
moderate solvophobicity and noncovalent interactions. Den-
dron G2-Cl exhibited excellent gelation properties, and den-
dron G2-Br showed poor gelation properties. Therefore, these
results demonstrated that the gelation efficiency is highly de-
pendent on the peripheral halogen substituents. Owing not
only to tuning of the molecular solubility and gelator–solvent
interactions, but also to balancing of the intermolecular forces
(including p–p, dispersive halogen, CH–p, and weak CÀH···X
hydrogen-bonding interactions), dendrons G2-Cl and G2-I
turned out to be the most efficient gelators.
The effects of halogen substituents on dendritic gelators
As an effective methodology for structural alteration, modifica-
tion of the dendritic periphery with different substituents af-
fects physicochemical properties of dendrons, such as chemical
reactivity, melting point, noncovalent interactions, and espe-
cially solubility. It has been demonstrated that the gelator solu-
bility has a subtle influence on gelation properties in dendritic
organogel systems, such as Tgel and CGC, as well as on the
degree of cooperative self-assembly.[23] To probe the possibility
of utilizing halogen-substituent effects to tune the organogela-
tion propensity, a number of peripheral halogen-functionalized
poly(benzyl ether) dendrons Gn-X (n=1, 2) were studied in
detail. It was found that the halogen substituents on the pe-
riphery of the dendrons have a profound effect on the organo-
gelation propensity. The gelation abilities, thermotropic behav-
iors, aggregation microstructures, and mechanical properties
of the self-assembled organogels could be fine-tuned by pe-
ripheral halogen substituents.
In view of the observed profound effects of halogen sub-
stituents on the organogelation propensity of these dendritic
gelators, the question arises which factors are responsible for
the observed differences in gelation behavior of peripherally
halogenated dendrons. There are two possible reasons for the
effects of halogen substituents on these dendritic gelators:
1) the various noncovalent interactions between dendrons and
2) the solubility of the dendrons.
Structural considerations for dendritic gelators
To probe the structural requirements for efficient dendritic ge-
lators, a number of chloro-functionalized dendritic gelators, 3-
G2-Cl, 4-G2-Cl, G2-Ph, 3,4-G2-Cl, and G3-Cl, were synthesized
and studied. The number and position of halo substituents
and the dendritic generation may synergistically and subtly
balance the molecular packing, solubility, and gelator–solvent
interactions in gels formed by G2-Cl.[26]
The number and position of halo substituents also played
a crucial role in balancing the solubility and gel-formation
properties of G2-Cl in organic solvents. Dendrons 3-G2-Cl and
4-G2-Cl with one chloro group at the periphery were soluble in
most solvents tested, and no gelation was observed. In addi-
tion, dendron G2-Ph without chloro groups at the periphery
exhibited no gelation ability in all tested solvents. Interestingly,
dendron 3,4-G2-Cl with chloro substituents at the 3,4-positions
also showed excellent gelation properties, which suggested
that a slight change in position of the halo substituents did
not have a significant effect on gelation ability. Moreover, 3,4-
G2-Cl is different from our previously reported dendron 3,4-G2-
Me with peripheral methyl ester groups. Gelation was not ob-
served for 3,4-G2-Me in all tested solvents.[11a] However, the
3,4-G2-Cl gel exhibited slightly lower Tgel value and storage
modulus G’ than the G2-Cl gel at the same concentration.
Previous work indicated that the magnitude of the electron-
ic perturbation of the substituted aromatic system decreases
as a function of the electronegativity of the halogen substitu-
ent (F>Cl>Br>I).[6d] However, it has been suggested that the
larger orbitals of iodine and bromine are more easily polarized
than those of fluorine and chlorine, which allows them to par-
ticipate in more stabilizing p–p interactions.[13b] Moreover, ex-
perimental results and theoretical calculations have consistent-
ly shown that the ability to generate dispersive halogen inter-
actions (halogen–arene and halogen–halogen interactions) in-
creases with increasing polarizability of the halogen.[6i,24] Since
iodine is the most polarizable atom in the halogen series, it
would have a higher polarizability (5.410À24 cm3) than bro-
mine (3.110À24 cm3) or chlorine (2.210À24 cm3). Therefore,
dendrons Gn-I have a greater propensity for the formation of
halogen–arene and/or halogen–halogen interactions than den-
drons Gn-Br and Gn-Cl. Furthermore, as the electronegativity of
the substituent decreases (F>Cl>Br) the strength of the weak
CÀH···halogen hydrogen-bonding interaction decreases.[25]
Hence, the propensity for the formation of CÀH···halogen hy-
drogen bonds is greater for dendrons Gn-F and Gn-Cl than den-
drons Gn-Br and Gn-I. In addition, the gelator solubility has
Chem. Eur. J. 2016, 22, 4980 – 4990
4988
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Feng, Yu
