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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
7077
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Feng, Yu
