Organogel based on β-diketone-boron difluoride without alkyl chain and H-bonding unit directed by optimally balanced π-π Interaction

Article abstract of DOI:10.1039/c0cc03448g

Rigid boomerang shaped triphenylamine functionalized bis-β-diketone- boron difluoride without alkyl chain and H-bonding unit exhibits good gelation ability in some mixed organic solvents directed by optimally balanced π-π interactions.

Full text of DOI:10.1039/c0cc03448g

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Organogel based on b-diketone-boron difluoride without alkyl chain and
H-bonding unit directed by optimally balanced p–p interactionw
Xiaofei Zhang, Ran Lu,* Junhui Jia, Xingliang Liu, Pengchong Xue, Defang Xu and
Huipeng Zhou
Received 24th August 2010, Accepted 23rd September 2010
DOI: 10.1039/c0cc03448g
Rigid boomerang shaped triphenylamine functionalized
bis-b-diketone-boron difluoride without alkyl chain and H-bonding
unit exhibits good gelation ability in some mixed organic
solvents directed by optimally balanced p–p interactions.
Therefore, the construction of an organogel based on
b-diketone-boron difluoride will be significant not only in
the concept of designing new p-gelators but also in extending
gel-phase material with appealing photonic and electronic
functionality. In this communication, we synthesized a series
of new bis- and tri-b-diketone-boron difluorides linked to the
benzene ring in different positions using triphenylamine as the
terminal groups. This would induce increased solubility in
organic solvents due to the umbrella conformation (Scheme 1).
It was found that only the boomerang shaped compound 1,
which has two b-diketone-boron difluoride moieties linked to
benzene in meta-positions, could form a stable gel with strong
red emission in some mixed solvents.
Recently, p-conjugated low molecular mass organic gelators
(LMOGs), especially with rigid functional moieties, have
attracted substantial interest from the viewpoint of acting as
building blocks of supramolecular assemblies due to their
unique optical and optoelectronic properties, such as multi-
color emission, enhanced fluorescence, and sensing abilities.1
In general, most of the p-gelators bear long carbon chains or
steroidal groups, which prefer to balance their solubility and
crystallization but might be unimportant in the functionality
of the organogels. Therefore, the strategy to design rigid
p-gelators without long carbon chains or steroidal groups is
consistent with the current point of view of atom economy in
green chemistry. Until recently, only a few such rigid LMOGs
have been reported. Hisaki et al. have synthesized an organo-
gelator derived from dehydrobenzoannulene with methyl ester
groups.2a An et al. have designed a gelator of a trifluoromethyl
based cyanostilbene.2b Ishi-I et al. have reported the gelators
based on discotic hexaazatritriphenylene with six aromatic
side chains.2c Our group have found that tert-butyl substituted
carbazole derivatives can self-assemble into organogels
directed by p–p interaction and H-bonding, and the tuning
of the strength of p–p interaction appropriately is helpful for
the fabrication of functional gels.2d,e Therefore, we deem that
p-conjugated compounds without alkyl group and H-bonding
unit would arrange into organogels driven by balanced p–p
interactions.
The synthetic routes for 1–4 were shown in Scheme S1
(see ESIw). Claisen condensation between 1-(4-(diphenylamino)-
phenyl) ethanone and the corresponding dimethyl phthalates
or trimethyl benzene-1,3,5-tricarboxylate catalyzed by sodium
hydride in dry THF afforded b-diketones,5 which were
complexed with BF3ÁEt2O in dry CH2Cl2 under an atmosphere
of nitrogen to give 1–4 in the yields of 45–70%.3c Their
molecular structures were characterized with FT-IR,
1H NMR, elemental analysis, and MALDI/TOF mass
spectroscopy, and the detailed synthetic methods and the
characterizations were shown in the ESI.w
The gelation properties of 1–4 were tested in various solvents
by means of the ‘‘stable to inversion of a test tube’’ method. It
was found that 2–4 could not form gel in the tested solvents, such
as cyclohexane, hexane, CHCl3, THF, and some mixed solvents
by typical heating-cooling processes (see Table S1 ESIw). As for
1, it could not form a gel in THF, toluene, 1,2-dichloroethane or
In addition, b-diketone-boron difluoride compounds have been
extensively studied in recent years due to their intriguing spectro-
scopic properties, such as large molar absorption coefficients,
two-photon absorption activity, controllable emission wave-
length and high fluorescence quantum yields.3 Many applications
have been found in sensors, OLED, and OFET, etc.4
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun 130012, China.
E-mail: luran@mail.jlu.edu.cn; Fax: +86 431 88499179;
Tel: +86 431 88499179
w Electronic supplementary information (ESI) available: Synthetic
methods and characterization data for new compounds, gelation
properties, fluorescence spectra, optimal geometry configurations.
Experimental details for the structural determination of single crystal
are presented in the cif file. CCDC 790916. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI:
10.1039/c0cc03448g
Scheme 1 Molecular structures of compounds 1–4.
Chem. Commun., 2010, 46, 8419–8421 8419
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chlorobenzene in which the solubility was too large. It could not
gel the solvents of hexane, cyclohexane or ethanol due to poor
solubility. In order to balance the precipitate and solubility a
series of mixed solvents were employed. It was found
that organogel 1 could be generated in CHCl3/cyclohexane
(v/v = 2/3), and some mixed solvents (see Table S1 ESIw). The
critical gelation concentration (CGC) values were in the range of
1.9 and 2.7 mM. Moreover, we found that 1,2-dichloroethane/
cyclohexane gel and chlorobenzene/hexane gel formed from 1
could be preserved for several weeks at room temperature, while
CHCl3/cyclohexane gel 1 could be stable for serval days. The
above results suggested that the molecular conformation had a
significant effect on the self-assembling properties of compounds
1–4.6 Accordingly, the optimal geometry configurations were
calculated by the semi-empirical quantum mechanical method
(AM1 force field). As shown in Fig. S1 (see ESIw), 1 possessed a
boomerang shape in which the dihedral angle between the
benzene rings connected to the b-diketone-boron difluoride
groups was small so that the extended p-conjugated system
favoured molecular self-assembly via p–p interaction.7 This could
be further confirmed by the change in the absorption spectrum of
1 in gel phase compared with that in solution. As shown in
Fig. 1a, the absorption band of 1 appeared at 494 nm in dilute
solution, which might be partly attributed to charge-transfer
(CT) transitions due to its solvent dependence (see Fig. S5
ESIw).8 It blue-shifted to 484 nm in the gel phase, meaning that
the gelator molecules 1 might self-assemble into H-aggregates in
the gel state.9 As for 2, the dihedral angle between the benzene
rings connected to b-diketone-boron difluoride groups was much
larger than that in 1 (see Fig. S2 ESIw) due to steric hindrance,
leading to a smaller degree of p-conjugation in 2 than in 1.
For example, the absorption band of 2 was located at 473 nm
(see Fig. S6 ESIw), which emerged in the high-energy region
compared with 1. Hence, it is understandable that 2 cannot form
gels in organic solvents on account of its good solubility resulting
from very weak or minimal p–p interactions (see Table S1 ESIw).
On the contrary, the conjugated degree of 3 and 4 is larger than 1
(see Fig. S3 and S4 ESIw). On the one hand, the conjugated
degree would be enhanced for the compounds with the functional
groups in para-positions compared with those bearing the groups
in meta-positions, so the absorption of 3 appeared at 522 nm
(see Fig. S6 ESIw). On the other hand, the conjugated degree of 4
can be extended due to more b-diketone-boron difluoride
units connected to meta-positions on the central benzene ring
compared with 1, and the absorption of 4 was located at 511 nm
(see Fig. S6 ESIw). As a result, 3 and 4 preferred to precipitate
from organic solvents because of strong p–p interactions. Thus, it
is reasonable that the gelation ability of the rigid conjugated
molecules could be tuned by the degree of conjugation.
Moreover, the ‘‘boomerang’’ shape might be another factor,
which might favour the gelation of 1.2a
The fluorescence emission spectral changes of 1 from sol to
gel phases were shown in Fig. 1b. It was clear that 1 gave an
emission band at 646 nm in solution, which blue-shifted to
618 nm in gel state. In order to reveal the origin of the emission
of 1, the fluorescence spectra in solvents with different polarity
were recorded (see Fig. S7 ESIw). The emission band was
broadened and red-shifted significantly with increasing solvent
polarity, for example, from 530 nm in cyclohexane to 642 nm
in CHCl3. Based on the evidence of a large Stokes shift
(142 nm in CHCl3) and the obvious broadening of the
emission bands in polar solvents, we suggested that the gelator 1
gave a twisted intramolecular charge-transfer (TICT) emission.8
In addition, from the plotting of the emission maximum energy
as a function of the Lippert solvent polarity (see Fig. S9 ESIw),
we could find a deviation of the emission maximum energy in
cyclohexane from the linear relationship followed by those in
other solvents. This means that a locally excited (LE) state was
responsible for the emission in nonpolar solvents and the
TICT state contributed to the emission in polar solvents.10
Therefore we deduced that the blue-shift of the emission of 1 in
CHCl3/cyclohexane gel compared with that in solution could
be attributed to the suppression of TICT since conformational
rotation might be inhibited when the molecules are packed
into an ordered superstructure in the gel phase.11 Moreover,
the concentration dependent fluorescence spectra of 1 in
CHCl3/cyclohexane (v/v = 2/3) further proved this point
because the aggregation of 1 at high concentration could lead
to a red-shift of the emission (see Fig. S10 ESIw), suggesting
that p–p interaction was irrelevant to the blue-shift of the
emission in the gel state. Meanwhile, the fact that the emission
intensity of 1 decreased with increasing concentration, when
the concentration was higher than 10 mM, illustrated that the
lower emission intensity of gel 1 compared with that of the
solution at the same concentration (Fig. 1b) were due to
the formation of H-aggregates in a gel state.12 Although the
emission intensity of the gel was lower than that of the
solution, the gel can still emit intense red light as shown in
the inset in Fig. 1b as well as in the fluorescence microscopy
(FM) image of the gel (see Fig. S8 ESIw).
The morphology of the organogel 1 was determined by
SEM, TEM and AFM techniques. From the SEM image, we
can find a three-dimensional network consisting of lots of
entangled fibres (Fig. 2a). These fibre bundles are built up
from thin fibrils, which intertwine around each other as shown
in TEM image (Fig. 2b). The AFM image revealed that the
thin fibrils were of 80–90 nm in width, 20–25 nm in height and
several micrometres in length (Fig. 2c).
To illustrate how the molecules pack, we obtained the single
crystal of 1 (see ESI w). Crystallographic analysis revealed that
two molecules packed into J-aggregate in each unit cell
(Fig. 3a), which was confirmed by the red-shift of the absorption
of 1 the single crystal compared with that in dilute solution
(see Fig. S12 ESIw). However, from the results of absorption
Fig. 1 (a) Normalized UV/Vis spectra of 1 in solution (black, 1.0 mM)
and in gel (red) in CHCl3/cyclohexane (v/v = 2/3, 3.5 mM).
(b) Variational fluorescence spectra of 1 with cooling the hot solution
to room temperature in CHCl3/cyclohexane (v/v = 2/3, 3.5 mM)
excited at 490 nm. The inset in (b) shows the photograph of 1 in sol
(left) and gel (right) states illuminated by 365 nm light.
c
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In conclusion, a series of rigid b-diketone-boron difluoride
derivatives have been synthesized, and it is found that only the
boomerang shaped compound 1 exhibits good gelation ability
in some mixed solvents. It suggests that the balanced p–p
interaction, which can be tuned by the molecular conjugation,
can lead to the gel formation from rigid p-conjugated
molecules without alkyl chain and H-bonding unit. This
strategy provides a new perspective not only in designing
new rigid p-gelators from the point of view of atomic economy
but also in extending gel-phase material with appealing photonic
and electronic functionality.
This work is financially supported by the National Natural
Science Foundation of China (20874034 and 51073068), 973
Program (2009CB939701), Open Project of State Key Labora-
tory of Supramolecular Structure and Materials
(SKLSSM200901).
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8419–8421 8421