Fluorescence-enhanced organogels and mesomorphic superstructure based on hydrazine derivatives

Article abstract of DOI:10.1021/la100325c

New low-molecular-mass organic gelators (LMOGs) bearing hydrazine linkage and end-capped by phenyl, namely 1,4-bis[(3,4-bisalkoxyphenyl)hydrozide] phenylene (BPH-n, n = 4, 6, 8, 10), were designed and synthesized. These organogelators have shown great ability to gel a variety of organic solvents to form stable organogels with the critical gelation concentration as low as 8.7 × 10^-4 mol L^-1 (0.06 wt %). The formed gel has a high gel-sol transition temperature (T_gel) at low gelation concentration. Aggregation-induced emission (AIE) has been observed after gelation though conventional chromophore units not incorporated inBPH-n. The fluorescence quantumyields of xerogel are 2 orders higher than that of dilute solution. In addition, the BPH-n (n=6, 8, 10) exhibited thermotropic hexagonal column (Col_h) mesophase, which are stable at room temperature as revealed by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD) studies.

Full text of DOI:10.1021/la100325c

pubs.acs.org/Langmuir
© 2010 American Chemical Society
Fluorescence-Enhanced Organogels and Mesomorphic Superstructure
Based on Hydrazine Derivatives
Peng Zhang, Haitao Wang, Huimin Liu, and Min Li*
Key Laboratory of Automobile Materials, Ministry of Education, Institute of Materials Science and En-
gineering, Jilin University, Changchun 130012, People’s Republic of China
Received January 23, 2010. Revised Manuscript Received March 4, 2010
New low-molecular-mass organic gelators (LMOGs) bearing hydrazine linkage and end-capped by phenyl, namely 1,4-
bis[(3,4-bisalkoxyphenyl)hydrozide]phenylene (BPH-n, n = 4, 6, 8, 10), were designed and synthesized. These organo-
gelators have shown great ability to gel a variety of organic solvents to form stable organogels with the critical gelation
concentration as low as 8.7 Â 10-4 mol L-1 (0.06 wt %). The formed gel has a high gel-sol transition temperature (Tgel) at
low gelation concentration. Aggregation-induced emission (AIE) has been observed after gelation though conventional
chromophore units not incorporated in BPH-n. The fluorescence quantum yields of xerogel are 2 orders higher than that of
dilute solution. In addition, the BPH-n (n = 6, 8, 10) exhibited thermotropic hexagonal column (Colh) mesophase, which
are stable at room temperature as revealed by differential scanning calorimetry (DSC), polarized optical microscopy
(POM), and X-ray diffraction (XRD) studies.
Introduction
ing thermotropic mesomorphic behaviors, whereas this is the
usual case for some wedge-shaped or disklike molecules.4-13
Among the noncovalent interactions, hydrogen bonding was
most commonly used to direct the self-assembling process because
of their strength, directionality, reversibility, and selectivity. Pep-
tide, amino acid, amide, and urea groups have been widely
employed as building blocks to afford supramolecular gels and
liquid crystals. In earlier reports, a star-shaped gelator containing
amide tethers exhibited thermotropic cubic and columnar meso-
phases.4 It was confirmed that the aggregation in the organogels
and microsegregation in the mesophase could be tuned by inter- to
intramolecular hydrogen bonds. Replacing the metallic lumino-
phores by purely organic fluorophores,5 these star-shaped mole-
cules exhibited strong fluorescence in the gels and mesophases.
Li et al. reported a series of bisurea-functionalized naphthalene
derivatives to develop thermotropic liquid crystal and switchable
fluorescent organogel systems, which are sensitive to temperature
and chemical stimuli.6 Furthermore, a new type of LMOGs,
namely 5-cyano-2-(3, 4, 5-trialkoxybenzoylamino)tropones, ex-
hibited hexagonal columnae order with identical lattice para-
meters in both their liquid crystal phase and gel states.7 As
noted by Mori, intermolecular hydrogen bonding between the
tropone carbonyl group and NH of the amide part plays a crucial
role. Analogously, amino acid based dendrons have been con-
firmed to give rise to gel, lyotropic, and thermotropic liquid crystal
states with a hexagonal columnar arrangement.8
Self-assembled systems, such as supramolecular gels1 and
liquid crystals (LCs),2 are fascinating organized soft materials
that can respond to external stimuli such as temperature, electrical
pulses, light, and chemicals. In recent years, low-molecular-mass
organic gelators (LMOGs) which could both gelate solvents and
exhibit thermotropic mesomorphic behaviors received much
attention from both theoretical and practical viewpoints. Gene-
rally speaking, there seems to be some inherent relationship
between the LMOGs and LCs.3,4 To achieve a gel/mesomorphic
state, a balance is required between the tendency of the molecules
to dissolve/melt and to aggregate. The shape of a molecule has an
important effect on self-assembly. It is relatively rare to find
rodlike compounds capable of both gelling solvents and exhibit-
*Corresponding author: e-mail minli@jlu.edu.cn; Fax 86 431 85168444.
(1) (a) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1218.
(b) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (c) Jong de, J.
J. D.; Lucas, L. N.; Kellogg, R. M.; Esh van, J. H.; Feringa, B. L. Science 2004, 304,
278–281.
(2) Recent examples: (a) Sagara, Y.; Yamane, S.; Mutai, T.; Araki, K.; Kato, T.
Adv. Funct. Mater. 2009, 19, 1869–1875. (b) Teng, M. J.; Kuang, G. C.; Jia, X. R.; Gao,
M.; Lia, Y.; Wei, Y. J. Mater. Chem. 2009, 19, 5648–5654. (c) Pal, S. K.; Agarwal, A.;
Abbott, N. L. Small 2009, 5, 2589–2596. (d) Fong, W. K.; Hanley, T.; Boyd, B. J.
J. Controlled Release 2009, 135, 218–226.
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Prange, T. J. Am. Chem. Soc. 2004, 126, 12403–12413.
In contrast, little attention has been paid to LMOGs and LCs
based on hydrazide derivatives. In our previous work, it was
demonstrated that intermolecular hydrogen bonding was still
interacting in the SmA phase and played important roles in
stabilizing the mesophase9 and organogels10 of hydrazide deriva-
tives. Furthermore, achiral wedge-shaped11 and twin-tapered12,13
compounds containing dihydrazide groups showed thermotropic
mesophase and strong gelation ability in organic solvents. Both
left- and right-handed helical ribbons with nonuniform helical
pitch were observed after the formation of organogels.11,13
Many efforts have been devoted to the development of gels
with optical absorption or fluorescence because of the inherent
(5) Camerel, F.; Bonardi, L.; Schmutz, M.; Ziessel, R. J. Am. Chem. Soc. 2006,
128, 4548–4549.
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Huang, C. H. Langmuir 2007, 23, 8224–8230.
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(8) Kuang, G. C.; Ji, Y.; Jia, X. R.; Li, Y.; Chen, E. Q.; Wei, Y. Chem. Mater.
2008, 20, 4173–4175.
(9) Pang, D. M.; Wang, H. T.; Li, M. Tetrahedron 2005, 61, 6108–6114.
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Cryst. 2006, 33, 439–443.
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Qu, S. N.; Li, M. New J. Chem. 2007, 31, 401–408.
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Langmuir 2010, 26(12), 10183–10190
Published on Web 03/16/2010
DOI: 10.1021/la100325c 10183

Article
Zhang et al.
Scheme 1. Synthetic Route for BPH-n (n = 4, 6, 8, 10) Compounds
Table 1. Thermal Transitional Properties of BPH-na Transitional
Temperatures (°C) and the Enthalpies of Transition (kJ/mol, in
Parentheses)
compound
first heating
first cooling
BPH-4
BPH-6
Cr 253 (72.53) I
Cr 207 (55.64) Colh1
254 (5.51) I
Cr 193 (65.48) Colh1
264 (6.69) I
I 243 (65.04) Cr
I 247 (5.12) Colh1 185 (17.22)
Colh2
I 257 (6.51) Colh1 176 (9.11)
Colh2
I 241 (4.98) Colh1 163 (7.53)
Colh2
BPH-8
BPH-10
Cr 197 (49.03) Colh1
253 (4.96) I
a Cr, Colh, and I indicate crystalline state, hexagonal columnar phase,
and isotropic liquid, respectively.
optoelectronic properties.4-6,14-18 For example, Shinkai and co-
workers reported a novel visible-light-harvesting organogel sys-
tem utilizing the 1D alignment nature of perylene-containing
cholesterol-based gelators.15a In a recent report, Ajayaghosh et al.
demonstrated gelation-assisted trapping of fluorescent supramo-
lecular architectures in a polymer film and its application in
erasable thermal imaging.15b Fluorescent gelators possessed the
AIE feature attracted extremely much attention since organogels
are thermoreversible, and accordingly thermal fluorescence
modulation can be realized.4-6,16 As a result, these fluores-
cent organogels based on molecular assemblies can be applied
as model systems for light-emitting diodes,15a fluorescence
sensors,1a,17,21 and other optoelectronic devices. A number of
possible mechanistic pathways, including excimers/exciplex
formation,16a,16e,19 conformational planarization,16b,18 J-aggre-
gate formation,16c,16d,18 restriction of intramolecular rotation
(RIR),16d,16e,20 and twisted intramolecular charge transfer
(TICT),20 were considered to be responsible for the AIE effect.
In this context as part of our continuing effort in hydrazide
derivatives, a novel class of symmetric dumbbell-shaped mole-
cules, namely 1,4-bis[(3,4-bisalkoxyphenyl)hydrozide]phenylene
(BPH-n, n = 4, 6, 8, 10) (see Scheme 1), was designed. The com-
pounds showed thermotropic columnar mesophase and excellent
gelation ability in organic solvents. The formed gel has a high Tgel
at low gelation concentration. Interestingly, enhanced fluores-
cence emission was observed after gelation, although the solution
of BPH-n was almost nonfluorescent. It was attributed to the
combination of the partial double-bond properties of C-N bonds
in the hydrazide group, the restricted intramolecular rotational
motions in aggregate state, and J-aggregation by hydrogen-
bonding interactions between the hydrazide groups.
Figure 1. Pseudo-focal-conic fan-shaped textures of the hexago-
nal columnar phase of BPH-10 at 205 °C (400Â).
(TMS) as an internal standard. UV-vis absorption spectra were
recorded on a Shimazu UV-160 spectrometer, and photolumines-
cence was measured on a Perkin-Elmer LS 55 spectrometer. Field
emission scanning electron microscopy (FE-SEM) images were
taken with a JSM-6700F apparatus. Samples for FE-SEM mea-
surement were prepared by wiping a small amount of gel onto a
silicon plate followed by evaporating the solvent at ambient
temperature. Phase transitional properties were investigated
with a Netzsch DSC 204. The rate of heating and cooling was
10 °C min-1; the weight of the sample was about 3 mg, and indium
and zinc were used for calibration. The peak maximum was taken
as the phase transition temperature. Texture observation was
conducted on a Leica DMLP polarizing optical microscope
equipped with a Leitz 350 microscope heating stage. X-ray
diffraction was carried out with a Bruker Avance D8 X-ray
diffractometer. FT-IR spectra were recorded with a Perkin-Elmer
spectrometer (Spectrum One B); the sample was in the form of a
pressed tablet with KBr. The gelator was mixed in a capped sealed
test tube [3.5 cm (height) Â 0.5 cm (radius)] with the appropriate
amount of solvent, and the mixture was heated until the solid was
dissolved. The sample vial was cooled to 4 °C and then turned
upside down. When a clear or slightly opaque gel formed, the
solvent therein was immobilized at this stage.
Experimental Section
Characterization. 1H NMR spectra were recorded with a
Mercury-300BB 300 MHz spectrometer, using tetramethylsilane
(14) (a) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008,
37, 109–122. (b) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644–656.
(15) (a) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43,
1229–1233. (b) Srinivasan, S.; Babu, P. A.; Mahesh, S.; Ajayaghosh, A. J. Am. Chem.
Soc. 2009, 131, 15122–15123.
(16) For example: (a) Babu, S. S.; Praveen, V. K.; Prasanthkumar, S.;
Ajayaghosh, A. Chem.;Eur. J. 2008, 14, 9577–9584. (b) Kim, T. H.; Choi, M. S.;
Sohn, B. H.; Park, S. Y.; Lyoo, W. S. K.; Lee, T. S. Chem. Commun. 2008, 21, 2364–
2366. (c) Xue, P. C.; Lu, R.; Chen, G. J.; Zhang, Y.; Nomoto, H.; Takafuji, M.; Ihara, H.
Chem.;Eur. J. 2007, 13, 8231–8239. (d) Chen, P.; Lu, R.; Xue, P. C.; Xu, T. H.; Chen,
G. J.; Zhao, Y. Y. Langmuir 2009, 25, 8395–8399. (e) Wang, C.; Zhang, D. Q.; Xiang,
J. F.; Zhu, D. B. Langmuir 2007, 23, 9195–9200.
(17) Esch van, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266.
(18) (a) Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y. Angew.
Chem., Int. Ed. 2004, 43, 6346–6350. (b) An, B. K.; Kwon, S. K.; Park, S. Y. Angew.
Chem., Int. Ed. 2007, 46, 1978–1982.
(19) (a) Zhao, Q.; Li, L.; Li, F. Y.; Yu, M. X.; Liu, Z. P.; Yi, T.; Huang, C. H.
Chem. Commun. 2008, 6, 685–687. (b) Huang, K. W.; Wu, H. Z.; Shi, M.; Li, F. Y.; Yi,
T.; Huang, C. H. Chem. Commun. 2009, 10, 1243–1245.
Synthesis. The synthesis of BPH-n is similar to that of
FH-Tn.12 The detailed description of the synthesis and purifica-
tion of the intermediate compounds can be found in our previous
work.12,13,22 The obtained dihydrazide derivatives were purified
by repeated recrystallization from tetrahydrofuran for further 1H
NMR, FT-IR measurements, and elemental analysis; yield
1
>60%. (Because of the poor solubility, the H NMR measure-
ment of BPH-10 was not performed).
1,4-Bis[(3,4-bisbutyloxyphenyl)hydrozide] phenylene (BPH-4).
1H NMR (300 MHz, DMSO) (ppm, from TMS): 10.61 (s, 2H);
(20) Hong, Y. N.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009, 29, 4332–
4353.
(21) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159.
(22) Qu, S. N.; Li, M. Tetrahedron 2008, 64, 10890–10895.
10184 DOI: 10.1021/la100325c
Langmuir 2010, 26(12), 10183–10190

Zhang et al.
Article
Figure 3. Chemical shifts of NH-1 and NH-2 of BPH-6 (1.78 Â
10-3 mol L-1) in 20% DMSO-d6/CDCl3 vs temperature (500
MHz; NH-1: near to alkoxyphenyl; NH-2: near to central phenyl).
Figure 4. Temperature-dependent FT-IR spectra of BPH-10 dur-
.
ing first heating run in the range of 3500-1200 cm-1
Figure 2. X-ray diffraction patterns of BPH-6 (a), -8 (b), and -10 (c).
10.41 (s, 2H); 8.04(s, 4H); 7.57-7.53 (m, 4H); 7.08 (d, 2H, J = 8.4
Hz); 4.07-4.01 (m, 8H); 1.77-1.68 (m, 8H); 1.53-1.40 (m, 8H);
0.97-0.93 (m, 12H). FT-IR (KBr, pellet, cm-1): 3192, 3020, 2972,
1659, 1601, 1573, 1509, 1459, 1393, 1340, 1273, 1223, 1146, 1124,
1067, 997, 867, 717, 642, 507. Elemental analysis: calculated for
C38H50N4O8 (%): C, 66.07; H, 7.30; N, 8.11. Found: C, 66.23; H,
7.44; N, 8.02.
1,4-Bis[(3,4-bishexyloxyphenyl)hydrozide]phenylene (BPH-6).
1H NMR (300 MHz, DMSO) (ppm, from TMS): 10.61 (s, 2H);
10.41 (s, 2H); 8.04(s, 4H); 7.56-7.52 (m, 4H); 7.07 (d, 2H, J = 8.7
Hz); 4.06-4.0 (m, 8H); 1.78-1.69 (m, 8H); 1.47-1.43 (m, 8H);
1.35-1.30 (m, 16H); 0.91-0.86 (m, 12H). FT-IR (KBr, pellet,
cm-1): 3192, 3020, 2954, 2929, 2872, 2858, 1659, 1602, 1573, 1509,
1459, 1393, 1339, 1271, 1222, 1147, 1124, 1070, 1001, 867, 716,
640, 507. Elemental analysis: calculated for C46H66N4O8 (%): C,
68.80; H, 8.28; N, 6.98. Found: C, 68.89; H, 8.44; N, 6.77.
Figure 5. Plot of v(N-H) wavenumbers and hydrogen bond
length of N-H3 3 3 OdC of BPH-10 vs temperature.
1,4-Bis[(3,4-bisoctyloxyphenyl)hydrozide]phenylene (BPH-8).
1H NMR (300 MHz, DMSO) (ppm, from TMS): 10.61 (s, 2H);
10.41 (s, 2H); 8.04 (s, 4H); 7.56-7.52 (m, 4H); 7.07 (d, 2H, J = 8.4
Hz); 4.05-3.99 (m, 8H); 1.78-1.69 (m, 8H); 1.47-1.43 (m, 8H);
Langmuir 2010, 26(12), 10183–10190
DOI: 10.1021/la100325c 10185

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Zhang et al.
Table 2. Gelation Properties of BPH-n (n = 4, 6, 8, 10)a,b
compound
ethanol
THF
chloroform
DCE
benzene
toluene
DMF
DMSO
BPH-4
BPH-6
BPH-8
BPH-10
5.80 Â 10-3
2.24 Â 10-3
1.64 Â 10-3
1.36 Â 10-3
6.52 Â 10-3
1.12 Â 10-2
7.87 Â 10-3
4.38 Â 10-3
7.58 Â 10-3
1.86 Â 10-3
1.31 Â 10-3
8.73 Â 10-4
1.35 Â 10-2
1.09 Â 10-3
1.02 Â 10-3
P
P
P
S
S
S
S
S
P
8.75 Â 10-3
5.76 Â 10-3
3.42 Â 10-3
6.48 Â 10-3
3.79 Â 10-3
1.69 Â 10-3
8.26 Â 10-3
5.52 Â 10-3
a Values denote the minimum gel concentration (mol L-1) necessary for organogelation. I: insoluble, S: soluble, P: precipitation. b THF, DCE, DMF,
and DMSO indicate tetrahydrofuran, 1,2-dichloroethane, dimethylformamide, and dimethyl sulfoxide.
Variable temperature X-raydiffraction was performed onthese
tetracatenar dihydrazide derivatives to get further information on
molecular arrangements in their mesophases. Figure 2a shows the
XRD patterns of BPH-6 in its Colh1 (at 220 °C), Colh2 (at 25 °C),
and Colh2 phase hold for 5 months. The characteristic patterns of
Colh1 phase at 220 °C during first cooling run consists of two
˚
sharp peaks (28.1, 14.0 A) in the low-angle region and a diffuse
˚
broad halo with maximum at about 4.5 A, which is characteristic
of the liquidlike arrangement of the aliphatic. The peaks in the
small-angle region were assigned to (100) and (200) reflection,
thus the two-dimensional hexagonal arrangement of the columns
˚
with a lattice parameter of a = 33.5 A. At room temperature,
˚
some additional diffraction peaks at both low-angle (29.5 A) and
˚
wide-angle regions (17.0, 14.7, 8.9, and 4.9 A) were observed in the
XRD patterns, which was assigned to Colh2 phase. The d values
are slightly larger than that in its Colh1 phase, which revealed that
the packing modes of the aggregates in both cases are the same.
Figure 6. Concentration-dependent melting temperature of BPH-
8 gels in 1,2-dichloroethane, chloroform, and tetrahydrofuran.
Interestingly, the columnar order of the Colh2 phase could be
retained even after being held at room temperature for more than
5 months (Figure 2a). Likewise, X-ray diffraction measurements
of BPH-n (n = 8, 10) was performed (Figure 2b,c). Similar phase
structures were also observed for BPH-n (n = 8, 10), and the
XRD results of BPH-n (n = 6, 8, 10) in the columnar phases are
listed in Table S1.
1.30-1.27 (m, 32H); 0.88-0.84 (m, 12H). FT-IR (KBr, pellet,
cm-1): 3187, 3020, 2954, 2924, 2872, 2851, 1660, 1601, 1572, 1510,
1459, 1393, 1339, 1271, 1224, 1147, 1124, 1067, 997, 867, 716, 641,
509. Elemental analysis: calculated for C54H82N4O8 (%): C,
70.86; H, 9.03; N, 6.12. Found: C, 70.67; H, 9.08; N, 6.23.
1,4-Bis[(3,4-bisdodecyloxyphenyl)hydrozide]phenylene (BPH-
10). FT-IR (KBr, pellet, cm-1): 3182, 3022, 2954, 2922, 2872,
2850, 1659, 1602, 1572, 1510, 1460, 1394, 1341, 1273, 1224, 1147,
1124, 1071, 997, 867, 718, 643, 504. Elemental analysis: calculated
for C62H98N4O8 (%): C, 72.48; H, 9.61; N, 5.45. Found: C, 72.53;
H, 9.36; N, 5.68.
Tetracatenar columnar mesophases was considered a slice of
column contain more than one molecules. According to these
information mentioned above, the average number of molecules
(μ) in a column slice can be calculated through the equation μ =
(31/2NAa2hF)/2M for the hexagonal phase, where NA is Avoga-
dro’s number, ar, br, and a are lattice parameters, h is the
intracolumnar periodicity, F is the density (assumed to be 1 g
cm-3), and M is the molecular weight of the compound.27 For
these materials, it can be estimated that approximately three
molecules (3.0-3.3, as calculated) are included in a unit cell on
average (Table S1).
Results and Discussion
Mesophase Properties of the Compounds. The phase be-
havior of BPH-n was studied by POM, DSC, and XRD. The
phase transition temperatures and the associated enthalpic
changes for BPH-n (n = 4, 6, 8, 10) in first heating and cooling
are summarized in Table 1. It can be seen that the phase behaviors
were strongly affected by the length of the flexible terminal chains.
BPH-4 is nonmesomorphic. However, BPH-6 showed two transi-
tions at 207 °C (55.64 kJ/mol) and 254 °C (5.51 kJ/mol)
corresponding to Cr-Colh1 and Colh1-I transitions during
heating, while two exothermic peaks at 247 °C (5.12 kJ/mol)
and 185 °C (17.22 kJ/mol) corresponding to I-Colh1 and
Colh1-Colh2 transitions upon cooling from its isotropic phase.
Similar phase behaviors were observed for the higher homologues
of BPH-n (n = 8, 10) (Table 1 and Figure S1). It should be
mentioned that no crystallization was observed for the higher
homologues of BPH-n (n = 6, 8, 10) even when they were cooled
to 20 °C at cooling rate of 10 °C/min. Figure 1 shows the
optical texture of BPH-10 in its Colh1 phase on the cooling run
(10 °C/min). The typically pseudo focal-conic texture with linear
birefringent defects and the black areas, which are homeotropic
domains with the columns aligned perpendicular to the glass
substrates, suggesting the characteristic of hexagonal columnar
phase of BPH-10.
Intermolecular Hydrogen Bonding in BPH-n. 1H NMR
spectroscopic study can give more information about the self-
1
assembly process. Unfortunately, the H NMR diluting experi-
ments were not performed for BPH-n in CDCl3 because of its
poor solubility at room temperature and strong gelation tendency
at low concentration. Temperature-dependent 1H NMR spectro-
scopic experiments were performed to explore the hydrogen-
bonding motif. Generally, internally hydrogen-bonded amides
are expected to show a much smaller shift with temperature
(<3.0 Â 10-3 ppm K-1) compared to those directed externally
and accessible for hydrogen bonding to a polar solvent (>4.0 Â
10-3 ppm K-1).36 As shown in Figure 3, both NH-1 (near to
alkoxyphenyl) and NH-2 (near to central phenyl) of BPH-6
showed large shifts (5.45 Â 10-3 and 5.43 Â 10-3 ppm K-1
,
respectively) with temperature in 20% DMSO-d6/CDCl3, sug-
gesting the primary involvement of N-H protons in intermole-
cular hydrogen bonding.
Intermolecular hydrogen bonding in BPH-n was further con-
firmed by temperature-dependent FT-IR spectroscopy. Table S2
10186 DOI: 10.1021/la100325c
Langmuir 2010, 26(12), 10183–10190

Zhang et al.
Article
Figure 8. XRD patterns of BPH-8: (a) xerogel from chloroform
(0.5 wt %) and (b) crystals.
wavenumbers of v(N-H) of BPH-10 are at around 3185, 3233,
and 3309 cm-1 in the crystalline state, Colh phase, and isotropic
phase, respectively. The calculated hydrogen-bonding length of
˚
N-H3 3 3 OdC was 1.97 A at 25 °C, which is in the range of
moderate hydrogen bonds.28 It increased slowly with tempera-
ture within the crystalline and columnar phase, while jumped to
˚
2.26 A, which indicated a weak hydrogen bonding in the isotropic
state.
Gelation Properties. The presence of two hydrazide tethers
on the amphiphilic molecules and the formation of columnar
aggregates in the mesomorphic state prompted us to probe their
gelation ability. Their gelation potential was examined for eight
different organic solvents, and the minimum gel concentrations
are summarized in Table 2. It can be found that the length of the
terminal alkoxy chains greatly affected the gelation ability.
Compounds BPH-6, -8, and -10 bearing the longer terminal
chains can gelate 6-7 kinds of solvents among the 8 solvents
tested herein. The gelation ability is superior in some solvents with
minimum gelation concentrations as low as 8.7 Â 10-4 mol L-1
(0.06 wt %), and therefore, compounds BPH-6, -8, and -10 can be
considered as supergelators. In contrast, the compound BPH-4
with shorter terminal chains formed the organogels in three kinds
of organic solvents, and the required minimum gel concentration
is much higher. The only exception is tetrahydrofuran, where
BPH-4, -6, -8, and -10 formed organogels with the minimum
Figure 7. SEM images of xerogel from (a) BPH-4 in 1,2-dichloro-
ethane(0.9 wt %), (b) BPH-6 in1,2-dichloroethane (0.15 wt%), (c)
BPH-8in1,2-dichloroethane(0.15wt%), (d) BPH-8inchloroform
(0.15 wt %), (e) BPH-8 in benzene (0.7 wt %), (f) BPH-8 in
dimethylformamide (0.9 wt %), (g) BPH-8 in tetrahydrofuran
(0.9 wt %), and (h) BPH-8 in toluene (0.6 wt %). The original
larger SEM images are in the Supporting Information.
gelation concentration of 6.52 Â 10-3, 1.12 Â 10-2, 7.87 Â 10-3
,
and 4.38 Â 10-3 mol L-1, respectively. The reason is not conclu-
sive but likely to be the poor solubility of BPH-4 in THF.
Moreover, the sol-gel transition is fully thermoreversible even
after several cycles of heating and cooling. The organogels are
remarkably stable which can be stored for months showing no
sign of decomposition.
presents the assignments of infrared frequencies for BPH-10 at 25
°C.9,12 At room temperature, BPH-10 exhibits characteristic
bands of N-H stretching vibrations of hydrazide group at 3182
cm-1 and amide I bands at 1659 (weak) and 1602 cm-1 (strong),
indicating that the majority of the N-Hs of the hydrazide group
are associated with the CdO groups via N-H3 3 3 OdC hydrogen
bonding in the crystalline phase.9,12,24 Moreover, these conclu-
sions were further supported by the fact that the N-H stretching
vibration band and amide I bond became weaker and shifted to
higher frequencies upon heating. Figure 4 shows the FT-IR
spectra of BPH-10 at different temperatures during first heating
run. The blue shift of CdO stretching vibration was companied
by an increase in intensity at around 1689 and 1663 cm-1, while
the absorption at 1602 cm-1 diffused and decreased, and a sharp
decrease of the absorption intensity at columnar-isotropic tran-
sition. The v(N-H) became weaker and broader and shifted to
higher frequencies as well as the hydrogen-bonding length28 of
N-H3 3 3 OdC increasing upon heating, as shown in Figure 5. The
Figure 6 shows the gel-sol transition temperature (Tgel) of
BPH-8 gels in tetrahydrofuran, 1,2-dichloroethane, and chloro-
form as a function of concentration. Tgel values were determined
by the “falling drop” method.23 The Tgel of BPH-8 in DCE
increases from 60 °C (1.42 Â 10-3 mol L-1) to 108 °C (2.73 Â 10-2
mol L-1), which are higher than the boiling point of 1,2-
dichloroethane (83.5 °C). The Tgel of BPH-8 in THF and chloro-
(23) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352–355.
(24) (a) Xue, C.; Jin, S.; Weng, X.; Ge, J. J.; Shen, Z.; Shen, H.; Graham, M. J.;
Jeong, J. K.; Huang, H.; Zhang, D.; Guo, M.; Harris, F. W.; Cheng, S. Z. D. Chem.
Mater. 2004, 16, 1014–1025. (b) Shen, H.; Jeong, J. K.; Xiong, H.; Graham, M. J.;
Leng, S.; Zheng, J. X.; Huang, H.; Guo, M.; Harris, F. W.; Cheng, S. Z. D. Soft Matter
2006, 2, 232–242.
(25) Esch, J.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266.
Langmuir 2010, 26(12), 10183–10190
DOI: 10.1021/la100325c 10187

Article
Zhang et al.
Figure 9. Photographs showing progressive gelation of0.1 wt % (1.63Â 10-3 mol L-1) BPH-8inchloroformunder ultraviolet light (365 nm)
illumination at different time periods (left: xerogel of BPH-8 from chloroform; middle: gel of BPH-8 in chloroform; right: the sol-gel
transition of BPH-8 in chloroform).
form have similar behaviors, and among them, the Tgel values
appear in the order of 1,2-dichloroethane > chloroform >
tetrahydrofuran.
In order to investigate the aggregation morphology of the
organogels, the xerogels of BPH-n were prepared and subjected to
scanning electron microscope (SEM). As shown in Figure 7a-c,
xerogel of BPH-4 from 1,2-dichloroethane consists of flexible
ribbons with the width of 0.5-2 μm, while BPH-6 and BPH-8
xerogel showed 3D cross-linking network, in which bundles of
straight wirelike fibers with the width of 0.2-1 μm are observed.
The more entangled and dense fiber morphology is observed for
BPH-6 and -8 compared to that of BPH-4 in 1,2-dichloroethane,
indicating that the interactions between individual fibers are
stronger in BPH-6 and -8 than BPH-4. Thus, the gelation
efficiency and stability are qualitatively in agreement with the
observed morphology. The SEM images of BPH-8 in different
solvents are shown in Figure 7c-h. All SEM images show
Figure 10. Absorption (a) and emission (b) spectra of BPH-8 in
chloroform at 1.0 Â 10-5 mol L-1. Emission spectra of BPH-8 gel
interconnected networks of fibers with the width of 0.2-3 μm
in chloroform at 0.1 wt % (c) and xerogel from chloroform (d).
and tens to hundreds of micrometers in length. The formation of
Excitation wavelength (λex) = 320 nm.
fibers in the gels indicates that the self-assembly is driven by
strong directional intermolecular interactions.
Figure 8 shows the X-ray diffraction patterns of BPH-8 crystal
units in stacking.31 A similar arrangement can also be observed
for the BPH-8 xerogel from chloroform, indicating a similar
supramolecular arrangement in both cases. At room tempera-
ture, BPH-8 exhibits characteristic bands of N-H stretching
vibrations of hydrazide group (Figure S2) at 3187 cm-1 (the
absence of free N-H, a relatively sharp peak with the frequency
higher than 3400 cm-1) and amide I bands at 1660 (weak) and
1601 cm-1 (strong), indicating that the N-H groups are asso-
ciated with CdO groups via N-H3 3 3 OdC hydrogen bonding in
crystal and xerogel states.9,12,24
and xerogel from chloroform. The XRD pattern of BPH-8
˚
crystal showed one sharp peak (30.6 A) in low-angle range,
˚
several broad weak peaks (20.5, 15.5, 9.2, 8.0 6.7, and 5.1 A) in
wide-angle regions, and two diffuse broad peaks with a max-
˚
imum at about 4.2 and 4.5 A, suggesting the coexistence ordered
˚
and disordered feature. The broad peak at 4.5 A is characteristic
of the liquidlike arrangement of the aliphatic chains,32 while that
˚
at 4.2 A might be attributed to the repeat distance of hydrazide
Aggregation-Induced Emission. Interestingly, although the
dilute solution of BPH-n was almost nonfluorescent, a strongly
enhanced fluorescence emission was induced by the gelation
process. The gel-sol transition of the systems, as well as the
fluorescent emission, is reversibly controlled by a change of the
temperature. Herein, we typically discuss the fluorescent proper-
ties of BPH-8 gel (chloroform, 0.1 wt %) in detail. Figure 9 shows
progressive gelation-induced-fluorescence enhancement under
ultraviolet light (365 nm) illumination at different time periods,
e.g. from 2 to 10 min. BPH-8 was completely dissolved in
chloroform when heated to its boiling point. The fluid solution
was almost nonfluorescent under the illumination of UV light.
Upon cooling to room temperature, immobile gels formed within
10 min, showing macroscopical fluorescence emission in contrast
to its isotropic solution.
(26) (a) Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L.; Chen, H. Y.; Qiu, C. F.;
Kwok, H. S.; Zhan, X. W.; Liu, Y. Q.; Zhu, D. B.; Tang, B. Z. Chem. Commun.
2001, 18, 1740–1741. (b) Freemantle, M. Chem. Eng. News 2001, 79, 21–25. (c) Chen,
J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y. P.; Lo, S. M. F.; Williams, I. D.; Zhu, D. B.;
Tang, B. Z. Chem. Mater. 2003, 15, 1535–1546. (d) Yu, G.; Yin, S. W.; Liu, Y. Q.; Chen,
J. S.; Xu, X. J.; Sun, X. B.; Ma, D. G.; Zhan, X. W.; Peng, Q.; Shuai, Z. G.; Tang, B. Z.;
Zhu, D. B.; Fang, W. H.; Luo, Y. J. Am. Chem. Soc. 2005, 127, 6335–6346. (e) Tong,
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10188 DOI: 10.1021/la100325c
Langmuir 2010, 26(12), 10183–10190

Zhang et al.
Article
Table 3. Photophysical Properties of BPH-n Dilute Solution (Chloroform, 1.0 Â 10-5 mol L-1) and Xerogels at Room Temperature
fluorescence
compound
absorption λabs (nm)a
λ
em (nm)a
quantum yielda
λ
em (nm)b
quantum yieldb
BPH-4
BPH-6
BPH-8
BPH-10
304
305
304
306
372, 392
371, 392
373, 394
373, 393
0.13 Â 10-2
0.17 Â 10-2
0.16 Â 10-2
0.16 Â 10-2
438
439
439
438
22 Â 10-2
19 Â 10-2
18 Â 10-2
21 Â 10-2
a In chloroform solution. b Xerogels from chloroform. The quantum yields of xerogels were obtained in a calibrated integrating sphere.29a The
quantum yields of solution were determined by using quinine sulfate in 0.1 M sulfuric acid as a reference.29b
The absorption spectrum of BPH-8 in dilute solution
(chloroform, 1.0 Â 10-5 mol L-1) exhibited a maximum at 304
nm (Figure 10a). As shown in Figure 10b, BPH-8 in the dilute
solution (chloroform, 1.0 Â 10-5 mol L-1) exhibited a weakly
split structural emission band (373 and 394 nm) which was
attributed to the characteristic monomer emission. The structure-
less broad emission centered at 439 nm was observed in the gel
state (0.1 wt % in chloroform), with an impressive increase in
the photoluminescence intensity compared with that in the solu-
tion (Figure 10c). As shown in Figure 10d, the xerogel develop-
ing from chloroform (0.1 wt %) exhibited blue fluorescence
emission (λem ≈ 439 nm) with the fluorescence quantum yield
ΦF about 18.0%, whichis 113-fold higherthanthat(ΦF ≈ 0.16%)
of the chloroform dilute solution (1.0 Â 10-5 mol L-1). Herein,
the decrease of the fluorescent intensity in a high concentration
by the inner filter effect33 was neglected. The remarkable fluor-
escence enhancement from gels was unambiguously assigned to
the phenomenon of aggregation-induced emission (AIE).4-6,16,20
Figure 11. PL spectra of BPH-8 in 1,2-dichloroethane solution (at
80 °C), partial gel (from 45 to 75 °C), and gel state (below 40 °C) at
The absorption and fluorescence data along with fluorescence
quantum yields of BPH-n (n = 4, 6, 8, 10) are summarized in
the same concentration (1.2 mg mL-1) excited at 320 nm.
Table 3. All compounds studied here exhibit weak fluorescence in
rings were joined by the flexible linking group (-CO-NH-),
chloroform dilute solution with quantum yields lower than
which generally suppresses the radiative decay channel.21,25
0.17%, whereas 2 orders higher in quantum yields were observed
Through density functional theory (DFT) geometry optimiza-
for xerogels compared to the corresponding solution quantum
yields.
To obtain insights into a correlation relationship between the
emission and the aggregation mode along with the sol-gel
transition, temperature-dependent fluorescence spectra of BPH-
tion,30 we found that the bond length of C-N in the hydrazide
˚
group is 1.35 A, which is much shorter than the theoretical value
˚
of the C-N single bond (1.47 A), while almost the same as that of
˚
CdN (1.34 A). This indicates that the C-N bonds in the
hydrazide group possess partial double bond properties and can
8 with a high concentration in the 1,2-dichloroethane (1.2 mg
mL-1) were measured from 80 to 25 °C. Its solution was almost
be effectively conjugated with the aromatic rings, which must be
the origin of the light emission. The distorted geometry makes the
nonfluorescent under the illumination of UV light, whereas its gel
backbone deviate from a conjugated plane, which means that free
was highly fluorescent. As shown in Figure 11, the emission
intramolecular torsional motion in the solution can lead to fast
spectrum obtained from the gel of 1,2-dichloroethane at 25 °C
displayed a strong peak at 439 nm. Upon heating, the emission
nonradiative relaxation and reduce the PL quantum yield in the
solution.19,20 In the aggregate state, however, the free torsional
spectrum obtained from the partial gel at 45 °C shown an
motion is obviously restricted by supramolecular interactions
additional emission band at 415 nm. Above 75 °C, the gels turned
resulting in the closure of the nonradiative decay channel and
into fluid solution. A hypsochromic shift of the emission bands
enhanced fluorescence emission.19,20,26 Consequently, aggrega-
was observed from 439 nm in gel state (at 25 °C) to 402 and 423
tion-induced emission phenomenon in the gelation process was
attributed to the synergistic effect of the restricted intramolecular
nm in solution (at 80 °C) with the evident decreases of the
fluorescence intensity. These results are in line with the formation
of molecular J-aggregations.6,16c,16d,34 This interesting property
rotational motions and J-aggregate formation.
would provide an opportunity to modulate the fluorescent emis-
sion by conveniently controlling the aggregation degree.35
Conclusion
Isolated BPH-n molecules in the dilute chloroform solution are
considered to be significantly twisted because the two aromatic
We have prepared a new series of hydrazine-based derivatives
BPH-n (n = 4, 6, 8, 10). BPH-n (n = 6, 8, 10) with longer terminal
chains displayed Colh mesophase, in which the microsegregation
effect and intermolecular hydrogen bonding between hydrazide
groups played a crucial role in the formation of the columnar
phase. The derivatives are capable of gelling a variety of organic
solvents to form stable organogels via cooperative intermolecular
hydrogen-bonding forces between hydrazine groups, with mini-
mum gelation concentration as low as 8.7 Â 10-4 mol L-1 (0.06
wt %). BPH-n showed gelation-induced enhanced fluorescence
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C. Anal. Chem. 2002, 74, 5217–5226.
(34) Camerel, F.; Bonardi, L.; Schmutz, M.; Ziessel, R. J. Am. Chem. Soc. 2006,
128, 4548–4549.
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7529–7541. (b) Hamuro, Y.; Geib, S. J.; Hamilto, A. D. J. Am. Chem. Soc. 1997, 119,
10587–7541.
Langmuir 2010, 26(12), 10183–10190
DOI: 10.1021/la100325c 10189

Article
Zhang et al.
emission, and it was attributed to the restricted intramolecular
rotational motions and J-aggregate formation.
Engineering of Jilin University for their financial support of this
work.
Acknowledgment. The authors are grateful to the National
Science Foundation Committee of China (Project No.
50873044), Program for New Century Excellent Talents in
Universities of China Ministry of Education, Special Foundation
for PhD Program in Universities of China Ministry of Educa-
tion (Project No. 20050183057), and Project 985-Automotive
Supporting Information Available: DSC curves of BPH-n
(n = 6, 8, 10) (Figure S1); FT-IR spectra of BPH-8 (Figure
S2); WAXD results for BPH-n (Table S1); assignments of
infrared frequencies for BPH-10 (Table S2); original larger
SEM images of Figure 7a-h. This material is available free
of charge via the Internet at http://pubs.acs.org.
10190 DOI: 10.1021/la100325c
Langmuir 2010, 26(12), 10183–10190