ChemComm
Communication
the array are stabilized by formation of several intermolecular The value of G0 was about 3 times larger than that of G00 over the
hydrogen bonds such as C–HꢀꢀꢀO, C–HꢀꢀꢀN(pyridine) and whole range (0.1–100 rad sꢁ1), suggesting that the gel is fairly
N–H(amide)ꢀꢀꢀN(pyridine) types, which have bond lengths of tolerant to external force. As observed by changes of dynamic strain
ca. 2.3–2.6 Å (Fig. S7C and Table S3, ESI†). Intermolecular hydrogen sweep, the values of both G0 and G00 of the gel 1 were also 100-times
bonds between an amide NH and CO were not observed. Instead, an larger than that of the gel 2. Furthermore, time-dependent oscilla-
offset face-to-face type of weak p–p stacking and CHꢀꢀꢀp interactions tion measurements were used to monitor the gelation processes of
between pyridine moieties in neighboring arrays were observed with gels 1 and 2 (Fig. S10C, ESI†). The time sweep shows the rapid
the shortest distance between two carbon atoms in each aromatic increase of G0 and G00 in the initial stage of gelation, followed by a
ring being 3.4 and 3.5 Å, and edge-to-centroid distance 2.6 Å slower long term approach to a final pseudo-equilibrium plateau. At
(Fig. S7D, ESI†).
meta-Substituted 2 is also formed by two different intermolecular magnitude higher than G00.
the end of the experiment, the value of G0 was about an order of
hydrogen bonding interactions. The crystal structure of 2 is stabilized
In conclusion, our comparisons of the crystal and gel structures of
primarily by the formation of intermolecular hydrogen bonds between 1 and 2 have provided complete information about their molecular
a hydrogen of pyridine and an amide, and the –CQO and –CH of the packing. The XRD patterns of both the crystals and the gels indicated
pyridine. These hydrogen bonds have bond lengths of ca. 2.2–2.8 Å the same crystal structure. Clearly, p–p stacking and hydrogen
(Fig. S8C and Table S4, ESI†). The offset face-to-face type p–p stacking bonding are crucial factors in the assembly of small-molecular
between pyridine moieties in neighboring arrays was also observed gelators. The gelation abilities of pyridine-based derivatives 1 and 2
with the shortest distance between two carbon atoms in each aromatic are strongly dependent on the pKb values. More interestingly, the
ring being 3.2 and 3.3 Å (Fig. S8D, ESI†), which is relatively stronger fluorescence intensity of meta-substituted 2 was relatively smaller
than the interactions obtained for para-substituted 1. In particular, the than that of the para-substituted 1, indicating that the emission
intermolecular H-bonds between –CQO and –NH in meta-substituted property is also dependent on the binding strength of the p–p
2 have a bond length of 2.26 Å. The relatively strong p–p stacking stacking. As a complementary armory of dynamic oscillation, steady
interaction in 2 induced the larger quenching effect.17
shear experiments indicated that the gel formed for the para-
Wide-angle X-ray diffraction (WAXD) was used to probe the substituted gel 1 is relatively strong and is a thermally resistant
molecular packing of 1 in either the xerogel or the bulk crystal. network as compared to the gel 2 bearing the meta-substituent. The
Powder XRD patterns of xerogels 1 and 2 prepared under neutral fluorescence property of the pyridine-based gels may be suitable for
conditions had the same diffraction pattern as that of the bulk crystal applications in optoelectronic devices.
prepared from water or methanol. Furthermore, the strong
This work was supported by a grant from World Class Project
similarities between the powder XRD patterns of 1 and 2 in the bulk (WCU: R32-2008-000-20003-0) and NRF (2012R1A4A1027750 and
crystal and those of the xerogels 1 and 2 suggested that the molecular 2012-002547) supported by Ministry of Education Science and
packing of 1 and 2 in the bulk crystal was the same as the molecular Technology, Korea. In addition, this work was partially
packing of gelators in the gel state (Fig. S9, ESI†). The results also supported by a grant from the Next-Generation BioGreen 21
suggest the view that the large quenching effect of the gel 2 is due to Program (SSAC, grant#: PJ009041022012), Rural development
relatively strong p–p stacking between the pyridine groups.
Rheological information is an indicator of the behavior of the
gels when they are exposed to mechanical stress. The ‘‘storage’’ (or
‘‘elastic’’) modulus G0 represents the ability of the deformed material
to ‘‘snap back’’ to its original geometry, and the ‘‘loss’’ (or ‘‘viscous’’)
modulus G00 represents the tendency of a material to flow under
stress. Two rheological criteria required for a gel are: (i) the
independence of the dynamic elastic modulus, G0, with respect to
the oscillatory frequency, and (ii) G0 must exceed the loss modulus
G00 by about 1 order of magnitude.
Administration, Korea.
Notes and references
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¨
We first used dynamic strain sweep to determine the proper
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.