A new tetraarylcyclopentadienone based low molecular weight gelator: Synthesis, self-assembly properties and anion recognition

Article abstract of DOI:10.1039/c2nj40073a

A new class of tetraarylcyclopentadienones bearing 3-hydroxy-1-propynyl substituents has been synthesized. One of them, 3,4-bis (4-(3-hydroxy-3- methylbut-1-ynyl) phenyl)-2,5-diphenylcyclopenta-2,4-dienone, exhibits pronounced aggregation properties in various organic solvents responding to thermal and ultrasound stimuli and represents the first example of a tetraarylcyclopentadienone based low molecular weight organogelator. The hydroxydimethyl group on the ethynyl substituent proved to be essential to perform the gelation process. The ¹H NMR analysis and FT-IR spectroscopy suggested that the intermolecular π-π and hydrogen bonding interactions of the gelator with the solvent are the main driving forces for the supramolecular assembly. The SEM images of xerogels show the characteristic gelation morphologies of 3D fibrous network structures. Fluorescence and UV/Vis absorption studies provided more information to define the molecular packing model in the gelation state. In addition the obtained gels show selective response to the fluoride anion. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj40073a

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PAPER
A new tetraarylcyclopentadienone based low molecular weight gelator:
synthesis, self-assembly properties and anion recognitionwz
Mattia Boiani,a Andrea Baschieri,a Cristiana Cesari,b Rita Mazzoni,b
Stefano Stagni,b Stefano Zacchinib and Letizia Sambri*a
Received (in Montpellier, France) 31st January 2012, Accepted 10th April 2012
DOI: 10.1039/c2nj40073a
A new class of tetraarylcyclopentadienones bearing 3-hydroxy-1-propynyl substituents has been
synthesized. One of them, 3,4-bis (4-(3-hydroxy-3-methylbut-1-ynyl) phenyl)-2,5-
diphenylcyclopenta-2,4-dienone, exhibits pronounced aggregation properties in various organic
solvents responding to thermal and ultrasound stimuli and represents the first example of a
tetraarylcyclopentadienone based low molecular weight organogelator. The hydroxydimethyl
group on the ethynyl substituent proved to be essential to perform the gelation process.
1
The H NMR analysis and FT-IR spectroscopy suggested that the intermolecular p–p and
hydrogen bonding interactions of the gelator with the solvent are the main driving forces for the
supramolecular assembly. The SEM images of xerogels show the characteristic gelation
morphologies of 3D fibrous network structures. Fluorescence and UV/Vis absorption studies
provided more information to define the molecular packing model in the gelation state. In
addition the obtained gels show selective response to the fluoride anion.
carbamates,14 metal complexes15 and oligo (p-phenylene) vinylenes
Introduction
(OPVs),16 might behave as good gelators as they are capable of
Recently, supramolecular gels obtained from low molecular
such intermolecular interactions. However, the occurrence of
weight gelators (LMWGs) have attracted considerable attention in
gelation processes is ruled by further ‘‘external’’ factors, among
materials science1 since they represent a new class of smart
which the temperature17 and the nature of the solvents that are
materials sensitive to external stimuli,2 such as temperature,1
employed are of crucial importance. For example, some gelators
ultrasounds,3 light,4 chemical species5 and so on. Accordingly,
prefer aromatic or halogenated solvents18 and in some cases both
during the past few years a large variety of potentialities and
the gelation temperature and the type of the solvent affect the
applications of these soft materials in optoelectronics, as electronic
morphologies of the final aggregation.19
devices,6 light harvesting systems7 and sensors,8 in bio-materials1e,9
Functionalized cyclopentadienones are fascinating systems
and in drug delivery10 have been reported.
largely employed as building blocks for the synthesis of
Spontaneous self-assembly of low molecular weight mole-
polyphenylene derivatives.20 In addition, it is worth noting
cules is a powerful tool that allows complex supramolecular
that structures containing p-extended conjugated chromophores
nanoscale structures to be built.1 The weak and non-covalent
with enhanced absorption properties are of current interest in the
interactions such as hydrogen bonding, p–p stacking, coordination,
field of materials science since they can be used as ‘‘organic
electrostatic and van der Waals interactions are usually considered
metals’’, as semiconductors, and as emissive or absorbing layers
as the most important features for promoting sol–gel equilibria.1d
for OLEDs or photovoltaics.21
In principle, many different families of molecules, such as
The possibility to decorate the framework of such structures
amino acids,1a,11 sugars,12 long-chain hydrocarbons,13 ureas and
prompted us to study the synthesis of new hydroxy propargyl
arylcyclopentadienone derivatives. Considering the ability of
a Dipartimento di Chimica Organica ‘‘A. Mangini’’,
University of Bologna, v.le Risorgimento 4, 40136 Bologna, Italy.
E-mail: letizia.sambri@unibo.it; Fax: +39-051-2093654
b Dipartimento di Chimica Fisica ed Inorganica,
University of Bologna, v.le Risorgimento 4, 40136 Bologna, Italy
w Dedicated to Professor Giuseppe Bartoli on occasion of his 70th
birthday
z Electronic supplementary information (ESI) available: SEM images
and NMR spectra. CCDC-862499 (1a) and CCDC-862500 (6a). For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2nj40073a
such systems to give p–p stacking interactions, the introduction
on a polyaromatic structure of polar substituents able to
generate hydrogen bonding could open the possibility to form
gels, although any gelation properties have been never observed
for these extensively studied systems.
Herein we report the synthesis of a new class of functionalized
arylcyclopentadienones 1 (Fig. 1), and the study of the influence
of the substituents on the 3- and 4-phenyl rings on the ability to
give stimuli responsive organogels.
c
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tetraarylcyclopentadienones 1a and 1b in fair to good yields
(1a, 60%; 1b, 32%) (Scheme 1).
All the products were fully characterized by NMR, IR and
ESI-MS analysis. Moreover the molecular structures of 1a and
6a have been determined by X-ray diffraction.
The ORTEP diagrams of 1a and 6a and their main bond
lengths and angles are reported in Fig. S1 and S2 and in Tables S1
and S2 in ESI.z
The molecular structure of 6a is composed of two almost
identical –C(O)–C6H4–CRC–CMe2(OH) moieties bonded
together via C(10)–C(30) [1.538(6) A]. The latter contact is
in keeping with a pure single bond and rather elongated for a
C(sp2)–C(sp2) interaction. Each of the two –C(O)–C6H4–
CRC–CMe2(OH) moieties is almost planar (mean deviations
from the least squares planes 0.0499 and 0.0691 A) and forms
an angle of 79.91. The C(10)–O(10) [1.211(5) A] and
C(30)–O(30) [1.213(5) A] interactions are typical for double
bonds and both C(10) [sum angles 359.9(7)1] and C(30) [sum
angles 359.8(8)1] display an almost perfect sp2 hybridisation.
Conversely, C(1)–C(2)–C(3)–C(4) [angles at C(2) 179.2(5)1 and
at C(3) 177.4(5)1] and C(21)–C(22)–C(23)–C(24) [angles at
C(22) 175.6(13)1 and at C(23) 175.3(12)1] are nearly linear and
C(2)–C(3) [1.193(6) A] as well as C(22)–C(23) [1.171(11) A] are
triple bonds.
Fig. 1 Tetraarylcyclopentadienones 1a–b and 2a–c.
Results and discussion
Synthesis of tetraarylcyclopentadienones
Derivatives 2a22 and 2b20a can be easily obtained by condensation
of the appropriate diaromatic diketone with 1,3-diphenylacetone
4 (Scheme 1), as previously reported. In order to functionalize
tetraphenylcyclopentadienones with a polar pending chain to
obtain 1a and 1b, 2a has been at first reacted with 2-methyl-3-
butyn-2-ol (5a) or 1,1-diphenyl-2-propyn-1-ol (5b) under
Sonogashira coupling reaction conditions in THF at reflux.
Unfortunately, these reactions gave 1a and 1b only in traces,
selectively leading to an almost quantitative formation of
2,7-dimethylocta-3,5-diyne-2,7-diol23 and 1,1,6,6-tetraphenyl-
hexa-2,4-diyne-1,6-diol,24 respectively, both derived from the
homocoupling between the two propargyl alcohols 5a or 5b.
This problem has been overcome by functionalizing the
precursor 4,40-dibromobenzil (3) through the Sonogashira
coupling with the desired alkynyl alcohols 5a and 5b. This
approach leads to 6a and 6b in good yields (77–78%) under
milder conditions (rt). In both cases the subsequent condensa-
tion with 1,3-diphenylacetone 4 allowed us to obtain the target
The molecular structure of 1a consists of a cyclopenta-2,4-
dien-1-one ring substituted on 2 and 5 by two Ph-groups and
on 3 and 4 by two –CRC–CMe2(OH) moieties. The cyclo-
penta-2,4-dien-1-one ring is perfectly planar (mean deviation
from the least square plane 0.0070 A) in agreement with the
sp2 hybridisation of all C-atoms [sum angles of the pentagon
540.0(16)1]. The C–C bonds within the ring are perfectly
alternated, with C(1)–C(2), C(3)–C(4) and C(5)–C(1) [1.520(10),
1.516(11) and 1.471(11) A, respectively] being single bonds, and
C(2)–C(3) and C(4)–C(5) [1.337(10) and 1.354(10) A, respectively]
double bonds. The C(1)–O(1) interaction [1.221(9) A] is typical
for a ketone. Conversely, the very short C(12)–C(13) and
C(21)–C(22) distances [1.186(11) and 1.189(12) A] are in keeping
with their triple bond character. Even if partially disordered,
the OH groups are clearly involved in H-bonds which generate
an infinite 2-D layer of H-bonded molecules. The OH groups
all point towards the interior of the layer, whereas the Ph
substituents are located on the external faces of the layer.
Consecutive layers are pillared and held together by weak
CH–p interactions (Fig. S3 in ESIz).
Gel formation
We investigated the ability of the obtained cyclopentadienones
to gel various solvents by means of the standard heating–cooling
method.
A weighted amount of compounds 1 and 2 in 0.5 mL of the
different solvents was heated in screw-capped glass vials with a
heat gun until a solution was obtained. Then they were
allowed to reach room temperature and after a certain time
the organogel formation was tested by the ‘‘upside-down’’ test.
As reported in Table 1, only 1a gave satisfactory results in
gelling toluene and polar solvents such as acetonitrile (ACN)
and MeNO2 (Table 1, entries 1–3), whereas the other derivatives
proved to be soluble at room temperature in the tested solvents,
Scheme 1 Synthesis of tetraarylcyclopentadienones.
c
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Table 1 Gelation ability of compounds 1a–b and 2a–c in various solvents
further decreased, in almost all cases, if the hot solutions are directly
submitted to ultrasounds (0.78 W cmꢀ2, 37 kHz) for 2 min in a
water bath at 25 1C. The gelation occurs within a few minutes after
the sonication. No effect of ultrasounds on the concentration was
observed in the case of benzene, while a precipitate was obtained in
the case of ACN.
State
of 1aa
State
of 1b
State
of 2a
State
of 2b
State
of 2c
Entry
Solvent
1
2
3
4
5
6
7
8
Toluene
MeNO2
ACN
G
G
G
P
S
P
P
I
S
S
S
S
P
P
S
I
P
P
S
P
I
P
S
I
S
P
P
P
P
P
P
I
S
P
P
—
—
P
—
I
Et2O
It is well known that ultrasounds can promote gelation
by inducing a certain order to molecules that can self-assemble.
In this case the rapid cooling together with sonication presumably
furnished 1a with the right energy to aggregate even in lower
concentrations.3 As expected, the lower CGCs obtained after
sonication resulted in a decrease even in the corresponding Tg
(see Table 2). Nevertheless, the ultrasound irradiation of a
suspension of 1a performed at rt did not give rise to a solution,
therefore the heating step was essential.
MeOH
iPrOH
tBuOH
H2O
a
G = gel; P = precipitate; S = soluble; I = insoluble.
or precipitated after cooling. Therefore, the presence of an
ethynyl-dimethyl-hydroxy group in the para-position of the
3- and 4-phenyl rings (as for derivative 1a) was necessary to
ensure organogelation. In fact, when the two methyls were
replaced by two phenyl groups, the obtained product 1b
proved to be much more soluble in aromatic hydrocarbons.
On the other hand, the presence of the alkynyl alcohol was
mandatory: the absence of substituent (2c) or the presence of a
bromide or a methoxy moiety (2a and 2b) made the products
totally unable to perform organogelation. Besides the results
reported in Table 1, all the tested compounds proved to be
soluble at room temperature in DMF, THF, acetone and
DCM, and insoluble in hexane.
It is interesting to note that 1a can gel both polar and apolar
aromatic solvents, furthermore the increase of the number
of methyl substituents on the aromatic ring, from toluene to
m-xylene and mesitylene, lowers the CGCs. This feature can be
explained in terms of steric, p–p-stacking and CH-p inter-
actions between 1a and the solvent molecules.
Moreover, we investigated how the concentration of 1a
influenced the Tg in different solvents, such as toluene and
nitromethane.
As expected, a non-linear correlation between the Tg values
and the corresponding gelator concentration was observed,
but a linear dependence of the reciprocal melting temperature
with the natural logarithm of the concentration was found
(Fig. 3, Tables S4 and S5, ESIz), as it was reported in other
cases.25 Through the relationship between the concentration
(c) and Tg, found by Borer et al.26 and shown in eqn (1) the
standard enthalpy DH1 and entropy DS1 for the sol–gel
transition of the gel in each solvent were determined to be
ꢀ83 kJ molꢀ1 and ꢀ208 J molꢀ1 Kꢀ1 in nitromethane and
ꢀ104 kJ molꢀ1 and ꢀ281 J molꢀ1 Kꢀ1 in toluene, respectively.
Afterwards, the gelling ability of 1a was tested on a wider set
of solvents. As reported in Table 2, by heating in various
aromatic hydrocarbons and polar solvents transparent
solutions are obtained, that, after slow cooling to room
temperature, formed stable and dark red transparent gels.
The sol–gel process is reversible (Fig. 2) and 1a-solutions
and gels are stable over four months, in contrast to solutions
of different tetraarylcyclopentadienones.21
The sol–gel phase transition temperatures (Tg) were measured
with the method of tube inversion and low critical gel concentra-
tions (CGCs) are observed (Table 2). Interestingly, CGCs can be
1
ln c
þ
DHo DHo
DSo
¼ R
ð1Þ
Tg
Table 2 Properties of 1a-gels in various solvents before and after
sonication
Considering that through aggregation constants gelation properties
and molecular aggregation tendencies can be evaluated25c and
good gelators generally show an aggregation constant in the range
of 103–104 Mꢀ1, we calculated the DG in MeNO2 and toluene
at 298 K (ꢀ21.2 kJ molꢀ1 and ꢀ20.4 kJ molꢀ1, respectively),
CGCa (mM) Tg (1C)
CGC (mM)
after cooling after cooling after sonication sonication
Tg (1C) after
b
Solvent
Benzene 19.5
Toluene 12.6
53
50
59
67
56
62
48
48
19.5
9.9
—
40
50
61
49
56
44
—
o-Xylene 14.3
p-Xylene 12.8
m-Xylene 8.5
Mesitylene 7.6
12.2
11.1
5.8
and the aggregation constants KMeNO and Ktoluene (5.21 ꢁ 103 Mꢀ1
2
and 3.81 ꢁ 103 Mꢀ1, respectively), which demonstrated that 1a
4.9
7.0
displays good aggregation properties in both solvents.
MeNO2
ACN
8.7
23.7
—
Morphological characterization of the gels
a
b
CGC = critical gel concentration. Tg = temperature of gelation.
The morphologies of superstructures created in the organogel
networks were observed with scanning electron microscopy
(SEM). In each case, well developed and organized structures
were observed, suggesting that gelator molecules tend to
self-assemble along the 1D fibrillar direction. Relatively to
the xerogel derived from MeNO2 and ACN (Fig. 4a and b), the
formation of typical fibrillar networks was observed, consisting
of intertwined fibres and bundles with a general length of
several micrometres and a variable width of approximately less
than 100 nanometres. In the xerogels from aromatic solvents
Fig. 2 Example of the stimuli-responsive behavior of 1a in MeNO2.
c
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aromatic solvents clearly indicated a remarkable solvent
dependence of the aggregation, at least at the larger hierarchical
1
level. Further investigations with H-NMR spectroscopy will
account for more accurate details.
1H NMR spectroscopy studies
Generally, NMR techniques provide a great deal of information
regarding the self-assembly process in the gel state.27
To investigate the gelator–gelator and the gelator–solvent
interactions in the gelation process, concentration and temperature-
1
dependent H-NMR spectra of 1a in various deuterated solvents
(CD3CN, CD3NO2 and D8-toluene) were recorded.
All the NMR experiments indicated that in each case the
variation in concentration, as expected,27 did not cause
dramatic effects on the spectra, whereas appreciable chemical shift
variations with the increase of the temperature were observed. The
1
analysis of the H NMR spectra revealed interactions not only
between the OH-groups, but also between the aromatic part of the
molecule, depending on the solvent employed.16a,28 In fact polar
solvents seem to play a crucial role in favouring H-bonding,
whereas toluene, and presumably other aromatic solvents,
influenced mainly the p–p stacking.
1
Fig. 5 shows the temperature-dependent H NMR spectra
of compound 1a in CD3CN. A clear upfield shift of the
OH-peak of the 1a-CD3CN gel (30 mM) was observed upon
heating the sample from 25 1C (dH = 3.42 ppm) to 70 1C
(dH = 3.19 ppm). A less pronounced effect was observed in the
aromatic region as far as the gel transformed into solution.
Ranging from room temperature to 70 1C multiplets around
6.90 ppm and 7.25 ppm appeared better resolved, and the
signals at 6.90 ppm underwent a downfield shift to around
6.95 ppm (Fig. 6, top). These results imply that the H-bonding
due to hydroxyl groups is the main responsible for the gel
formation of 1a in acetonitrile. A similar behaviour was shown
by the 1a-gel in CD3NO2, a polar solvent like ACN, where the
main aggregation forces involve the OH substituents. From
the detailed NMR spectra reported in the ESIz (Fig. S8) we
can observe that the OH singlet was upshifted from 2.99 to
2.70 ppm by heating from 25 1C to 85 1C, while the methyls’
signal underwent only a slight chemical shift change varying
from 1.54 to 1.57 ppm and the aromatic multiplets appeared
less broad with increasing temperature. These results imply
that hydrogen bonding due to OH-groups plays a key role in
Fig. 3 Plot of the 1/Tg versus the natural logarithm of the concen-
tration of 1a in toluene (top) and nitromethane (bottom).
the fibres obtained were flat and wider, giving rise to a sort of
overlapped sheets, as highlighted in the images of the xerogel
from p-xylene, o-xylene and mesitylene (Fig. 4d–f).
The fact that different fibre morphologies were observed
from the xerogels from polar solvents or the xerogels from
Fig. 4 SEM images of xerogels of 1a from: (a) MeNO2; (b) ACN;
(c) benzene; (d) p-xylene; (e) o-xylene and (f) mesitylene.
Fig. 5 Temperature dependence of the 1H NMR spectra of 1a-CD3CN
gel (30 mM).
c
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Fig. 7 FT-IR spectra of 1a: (a) in DCM solution, (b) precipitate and
(c) xerogel from o-xylene.
Fig. 6 Temperature-dependent 1H NMR spectra of 1a: in CD3CN
30.0 mM (top) and in D8-toluene 14.3 mM (bottom).
interactions of hydroxyl groups in different entities depending
on the solvent.
the gel formation of 1a both in ACN and MeNO2. The
chemical shift variation of the OH-signals pointed out that
intermolecular hydrogen bonds are formed between hydroxyl
groups and became weaker as the temperature increased.29
On the other hand a different situation was found in
D8-toluene. In this case, in fact, although no concentration
effects were registered (Fig. S9, ESIz), a strong temperature
effect on the aromatic region was observed. Increasing the
anisotropy of the sample by heating the gels up to the melting
temperature, a sharpening of the resonance lines of aromatic
peaks, together with changes in chemical shifts occurred
(Fig. 6, bottom). The doublet at 6.68 ppm underwent a
downfield shift to 6.73 ppm, whereas the multiplet between
6.96 and 7.12 ppm and the doublet at 7.37 ppm were shifted to
upfield at 6.94–7.08 ppm and 7.32 ppm respectively. Concerning
the methyl and OH-signals, only slight changes were observed
(Fig. S10c, ESIz, from 1.41 to 1.39 ppm and from 0.45 to
0.47 ppm, respectively).
FT-IR spectroscopy studies
In order to further detect the occurrence of 1D hydrogen-
bonding interactions between OH-groups, which represents
one of the main driving forces for the self-assembly of gelators,
we compared the FT-IR spectra of 1a in various solutions and
in the solid state.
As depicted in Fig. 7, a solution of 1a in non-coordinating
solvent such as DCM showed two bands at 3590 cmꢀ1 and
3690 cmꢀ1 typical of free hydroxyl groups. In contrast, the
xerogel spectrum displayed a strong broad band between 3300
and 3400 cmꢀ1, indicating the presence of hydrogen bonds.
Anyway, in the precipitate, the corresponding band is much
weaker. To investigate the effect of the solvent on FT-IR
spectra in solution, a comparison between 1a-solution in
DCM, ACN and o-xylene was performed. As observed in
the 1H-NMR studies, the hydrogen-bonding is much more
important in ACN gel than in aromatic solvents. The same trend
can be observed also in the FT-IR spectra of solutions of 1a
(Fig. S12, ESIz): the more the OH-bonds became important for
the self-assembly, the more the OH-bands became broader and
shifted at lower wavenumbers typical of bonded OH-groups.
As found by 2D NMR analysis, the chemical shifts of the
H1- and H3-doublet in toluene are at 6.68 and 7.37 ppm
respectively (Fig. 1 and Fig. S11, ESIz). The increase of the
temperature induced an opposite shift of the signals of the two
different aromatic substituents. Thus, the variation of
chemical shifts of the aromatic protons with the temperature
implies that p–p stacking between 1a molecules and the solvent
moieties exists both in the solution and in the gel state.
In addition, temperature coefficients were calculated, being
almost the same for both gel and diluted samples. However
these data further evidenced that interactions between 1a
molecules and the solvent concern mainly the OH-group in
polar solvents and the aromatics in apolar ones. In fact the
OH-singlet is shifted 2.0 Hz 1Cꢀ1 in CD3CN versus 0.25 Hz 1Cꢀ1
in D8-toluene, whereas aromatic signals moved 0.05–0.2 Hz 1Cꢀ1
in CD3CN and 0.3–0.4 Hz 1Cꢀ1 in D8-toluene.
UV/Vis absorbance and photophysical properties
UV/Vis absorption measurements on 1a-solutions and 1a-gels
have been performed in order to monitor the supramolecular
assembly process and investigate the eventual effects of environ-
mental factors. Due to the high concentration of the sample
(14–24 mM), which is required to obtain the gel, only the region
between 420–700 nm has been taken into consideration, while
the more intense characteristic bands expected in the UV
region20 have been omitted. The spectra in solution show a
characteristic transition approximately centred at 500–510 nm
(Table 3). This band, which has been attributed via a VB
description30 and by recent DFT studies21 to a charge-transfer
From the obtained data, it is clear that the p–p driving force
assists 1D self-assembly of the gelator molecules in combi-
nation with the directional intermolecular hydrogen-bonding
c
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Table 3 UV/Vis absorbance data of 1a-solution (1.0 mM) and 1a-gels
ACN (24.0 mM), MeNO2 (8.7 mM) and toluene (12.6 mM)
Table 4 Fluorescence data of 1 ꢁ 10ꢀ4 M solution of 1a in various
solvents and 1a-gels
Solvent
lmax (nm) 1a-solution
lmax (nm) 1a-gel
Solvent
Toluene
Emission: lmax/nm
Excitation: lmax/nm
ACN
499, 459
501, 458
513, 471
507, 461
499, 459
503, 466
513, 473
—
Solution: 438
Gel: 458
Solution: 398
Gel: 454
Solution: 340
Gel: 344
Solution: 334
Gel: 334
MeNO2
Toluene
CH2Cl2
ACN
MeNO2
Solution: 494
Gel: 466
Solution: 400
Gel: 360
(PL) measurements onto acetonitrile-, toluene- and nitromethane-
based diluted solutions of 1a and on the corresponding gels. At
room temperature, all the diluted solutions of 1a displayed
structureless emission centred in the wavelength range between
398 and 494 nm (see Table 4). As aggregation took place, it was
observed that the gels of 1a in ACN and toluene maintained their
fluorescent character and exhibited emission profiles with red
shifted maxima if compared to those of the corresponding
solutions. The presence of a similar effect – the red shift of the
emission on going from solution to gel – has been correlated
with34 the occurrence of p–p interactions that are responsible for
the intermolecular stacking of the aromatic groups within the
structure of the gel. An opposite behaviour was observed by
comparing the emission spectra of the gel of 1a obtained from
nitromethane and the corresponding 1a-solution. In this case,
indeed, the fluorescence spectrum of the nitromethane gel is blue
shifted with respect to that of the parent diluted solution. Even if
we cannot provide a clear explanation for this feature, it should be
noted that the use of nitromethane as a solvent, at odds with that
observed with toluene and acetonitrile, involves the excitation of
the gel at a markedly different and much bluer wavelength with
respect to that required by the corresponding diluted solution.
However, in an attempt to monitor the gelation processes
occurring from the solutions of 1a in toluene, ACN and
nitromethane, we recorded successive photoluminescence
(PL) spectra of the various 1a-solutions at concentration
values spanning from 10ꢀ6 M to an upper limit (10ꢀ3 M)
that roughly matched the CGC. Even though in some
cases auto quenching phenomena were encountered for high
Fig. 8 Normalized UV/Vis absorption spectra of 1a-solution (1.0 mM)
and 1a-gel (8.7 mM) in MeNO2 at room temperature.
(CT) transition, undergoes negative solvatochromism (hypso-
chromic shift) upon increasing the solvent polarity.31
An additional absorption in the visible region is present
both in 1a-solutions and 1a-gels around 460–470 nm (Table 3)
that has never been observed in previous UV/Vis character-
izations of functionalized tetraarylcyclopentadienones21,30 and
it is probably ascribable to the influence of propargyl para
substituents –CRCCMe2OH in 3,4-phenyl rings. It is important
to highlight that the red shift of this transition, which was clearly
observed in the case of MeNO2 (from 458 to 466 nm in solution
and in gel respectively, Fig. 8), might be attributed to the
pronounced attitude exhibited by MeNO2 for promoting the
formation of hydrogen bonded complexes both as ‘‘bridged’’ and
‘‘open’’ type arrays.32
Concerning the measurement in gel, some previously
reported experimental and theoretical works have demon-
strated that p–p interactions might turn in the shift of the
absorption bands.33 In our case, the red shift of the UV/Vis
bands that was observed in the spectrum of 1a in toluene
compared to 1a in CH3CN and MeNO2 might confirm what
deduced from the 1H-NMR experiments, from which the
occurrence of H-bonding was suggested as the key factor for
the formation of gels in polar solvents, while p–p interactions
were suggested to be primarily responsible for the gel formation
in toluene. In the latter case no significant shifts from solution
to gel were detected, likely indicating that p–p interactions are
already present in solution.
Since fluorescence spectroscopy has been reported as a
valuable tool for gaining important information about the
gelation processes and, in particular, for getting more insights
about the molecular organization of the fluorophores that
constitute the gels,34 we have performed photoluminescence
Fig. 9 Fluorescence emission spectra of 1a in ACN at different
concentrations and in the gel state (lex = 330 nm) at 25 1C.
c
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concentrated samples (Fig. 9), the red or blue-shifting trends
of the emission maxima were in agreement with those observed
from the previous experiments. Indeed, relative to the samples
in ACN and toluene, the expected red shift (see Table 4) of the
emission maximum was observed on going from diluted to
concentrated solutions, being this effect more pronounced for
the ACN-based sample with respect to the one containing
toluene as the solvent. On the other hand, the blue shift of the
emission maximum of the nitromethane solutions of 1a was
observed as its concentration increased.
overlapped ribbons. In both cases, employing various polar and
aromatic solvents, the propargyl substituted compound 1a shows
good aggregation properties and low critical gel concentrations.
Notably, we found that the organogel of 1a could allow
recognition of fluoride anion selectively, since the gel is
destroyed only in the presence of Fꢀ.
Experimental section
General
Anion sensing properties
NMR spectra were recorded by using a Varian Mercury
400 MHz or a Varian Inova 600 MHz spectrometer with
tetramethylsilane as the internal standard. ESI-MS analyses
were performed by direct injection of methanol solutions of the
compounds using a WATERS ZQ 4000 mass spectrometer.
Ultrasound irradiation was performed by using an Elmasonic
S10 H ultrasound cleaner (max. power 200 W, 37 KHz).
Melting points were taken with a Stuart Scientific SMP3
melting point apparatus and were uncorrected.
The importance of soft materials lies on their ability to
respond to external stimuli that can be also of chemical nature.
In particular, high attention has been recently devoted to
anion responsive properties of gels.5d,11d,35
Therefore the behaviour of organogels of 1a in toluene,
ACN and MeNO2 towards the addition of 1 equivalent of
various tetrabutylammonium salts was investigated.
Interestingly, only the addition of the tetrabutylammonium
fluoride (TBAF) led to a rapid transformation of the gels into
solutions, in all the examined solvents. In fact, the gels
remained unaltered upon the addition of other salts (Clꢀ, Brꢀ,
Iꢀ or ClO4ꢀ), and they kept their temperature response-ability
unchanged, even after several days.
Absorption spectra were measured on a Varian Cary 4
double beam UV/Vis spectrometer and baseline corrected.
Steady-state emission spectra were recorded on an Edinburgh
FLS920P spectrofluorimeter equipped with a 450 W Xenon
arc lamp, double excitation and single emission monochromators
and a peltier cooled Hamamatsu R928P photomultiplier tube
(185–850 nm). Emission and excitation spectra were corrected for
source intensity (lamp and grating) and emission spectral response
(detector and grating) by calibration curve supplied with the
instrument. The solid gels were measured by using the front face
illumination sample holder of the Edinburgh FLS920 P instrument
after being placed and solidified in a thin (1 mL) quartz cuvette.
SEM characterization: the morphology of the gels was
investigated using a Zeiss 1540 EsB dual beam focused ion
beam/field emission scanning electron microscope with a
working distance of 8 mm. The samples were prepared by
drop casting the gel samples on a silicon wafer and sputtering
4 nm of gold after solvent evaporation. The imaging was
performed at electronic high tension (EHT) values between
20 and 25 kV. The IR spectra were recorded with a FT-IR
spectrometer Perkin Elmer Spectrum 2000.
These results clearly indicate a selective chemical response
of 1a-gels towards the presence of a fluoride anion resulting in
a naked-eye gel to sol transition. The gel collapse can be
presumably ascribed to the disruption of hydrogen bonding
interactions that help to stabilize the gel suprastructure. In
particular, a colour change of the solution was observed, from
dark red to yellow-orange, probably due to a proton abstraction
from the gel by the fluoride ion that led to the decomposition of 1a
to a complex mixture of unidentified products. Studies are in
progress to understand the effect of Fꢀ on the examined substrates.
The present example represents one of the few reported examples
of fluoride anion recognition by an organogel derived from a
hydroxyl-derivative.36 In fact, in most cases fluoride-responsive
organogels are derived from amide- or urea-derivatives.34,37
Conclusions
Gel-to-sol phase transition temperatures (Tg) were determined
by the ‘‘tube inversion method’’. A sealed vial containing the gel
was immersed in a thermostated oil bath. The temperature of the
bath was raised at a rate of 1.0 1C minꢀ1. Tg was defined as the
temperature at which the gel started to flow. The experiment was
carried out up to three times and the average was taken as the
Tg value.
In summary, we have synthesized a new class of 3,4-bis
(4-(3-hydroxy-propynyl) phenyl)-2,5-diphenylcyclopentadienone
derivatives, one of which (1a) proved to be, for the first time, a
powerful organogelator. The experimental results indicated that the
hydroxydimethylalkynyl substituents are fundamental to guarantee
the gelation properties of the tetraarylcyclopentadienone unit.
Combining the results of FT-IR, 1H NMR, UV/vis and
fluorescence emission spectra, we believe that H-bonding
and p–p interactions are the driving forces played for the gel
formation. Interestingly, the nature of the solvent promotes a
sort of modulation of these two interactions influencing
the morphology of 1a stimuli-responsive gels. In fact, polar
solvents act on OH-groups driving the formation of typical
intertwined thin fibres and bundles several micrometres long
and less than 100 nanometres wide, while aromatic solvents
mainly interact via p–p stacking with the aromatic core of
the molecule giving rise to flat and wider fibres, similar to
The critical gelator concentration (CGC) is determined from
the minimum amount of gelator required for the formation of
gel at room temperature.
For the naked-eye sensing test of gelation versus different
anions, 1 equivalent of solid tetrabutylammonium salts
(Fꢀ, Clꢀ, Brꢀ, Iꢀ and ClO4ꢀ) was placed on top of a sample
of gel. The samples were inversed after 24 hours to determine
the gel–sol transition, and the reversibility of the gelation was
repeated three more times.
The chemicals 4,40-dibromobenzil, 2-methyl-3-butin-2-ol,
1,1-diphenyl-2-propyn-2-ol, 1,3-diphenylacetone, CuI and Et3N
c
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were used as purchased from Aldrich; dichloro-bis(triphenyl-
phosphine)-palladium(II) was purchased from Strem and used
as received.
3,4-Bis(4-(3-hydroxy-3,3-diphenyl-prop-1-ynyl)phenyl)-2,5-di-
phenylcyclopenta-2,4-dienone (1b). 4,40-Bis(2-phenyl-3-butin-
2-ol)benzyl 6b (0.76 g, 1.22 mmol) and 1,3-diphenylacetone 4
(0.26 g, 1.22 mmol) were dissolved in EtOH. The solution was
heated to 80 1C and then NaOH (0.049 g, 1.22 mmol) was
added. The reaction mixture was refluxed for 16 h. The solvent
was removed under vacuum. Chromatography on silica gel
(CH2Cl2) gave the desired product 1b (0.31 g, 32.4% yields)
as a dark red solid (mp = 117.0–118.0 1C). 1H-NMR
(399.9 MHz, CDCl3) d 7.67–6.89 (m, 38H, aromatics), 2.88
(s, 2H, OH); 13C-{1H}NMR (100.6 MHz, CDCl3) d 199.63
(CQO), 153.06 (C), 144.74 (C), 133.21 (C), 131.51 (CH),
130.27 (C), 130.10 (CH), 129.34 (CH), 128.33 (CH), 128.17
(CH), 127.81 (CH), 126.03 (CH), 125.91 (C), 122.68 (C), 93.10
Syntheses
4,40-Bis(2-methyl-3-butin-2-ol)benzil (6a). 4,40-Dibromobenzil
3 (3.03 g, 7.33 mmol) and 2-methyl-3-butin-2-ol 5a (1.43 mL,
14.7 mmol) were dissolved in 50 mL of THF. Dichlorobis-
(triphenylphosphine)palladium(II) (0.62 g, 0.882 mmol), CuI
(0.28 g, 1.47 mmol) and Et3N (5 mL) were added and the
mixture was stirred for 1 h at room temperature. Then the
solvent was removed under vacuum and the crude was purified
by chromatography on silica gel (CH2Cl2/ethylacetate 10:1)
giving the desired product 6a (2.39 g, 78.3% yields) as a white
(C), 86.79 (C), 74.83 (C). IR (CH2Cl2,): n(CQO): 1714 cmꢀ1
.
1
solid (mp = 148.5–149.2 1C). H-NMR (399.9 MHz, CDCl3)
ESI-MS (MeOH, m/z): 819 (100) [M + Na]+.
d 7.90 (d, JH,H = 8.4 Hz, 4H, CHaryl), 7.52 (d, JH,H = 8.4 Hz,
4H, CHaryl), 2.15 (s, 2H, OH), 1.63 (s, 12H, CH3); 13C-{1H}NMR
(100.6 MHz, CDCl3) d 193.43 (CQO), 132.33 (CH), 132.10 (C),
130.00 (CH), 129.9 (C), 98.95 (C), 77.61 (C), 65.83 (C), 31.48
X-ray crystallography
The diffraction experiments were carried out on a Bruker
APEX II diffractometer equipped with a CCD detector and
using Mo-Ka radiation. Data were corrected for Lorentz
polarization and absorption effects (empirical absorption
correction SADABS).38 Structures were solved by direct
methods and refined by full-matrix least-squares based on all
data using F2.39 All non-hydrogen atoms were refined with
anisotropic displacement parameters, unless otherwise stated.
H-atoms were placed in calculated positions and treated
isotropically. Similar U restraints were applied to C and O
atoms (s.u. 0.005) in both structures. The crystals of 1a were
very thin plates poorly diffracting and, therefore, data have
been collected only at low resolution. One of the two
–CMe2(OH) groups in 1a is disordered over two positions.
Disordered atomic positions were split and refined isotropically
using one occupancy parameter per disordered group. The same
type of disorder involves one of the two –CMe2(OH) groups in
6a. Moreover, the whole –C6H4–CRC–CMe2(OH) moiety
bonded to the other –CMe2(OH) group in 6a is disordered over
two positions. Disordered atomic positions were split and refined
in this case anisotropically using one occupancy parameter per
disordered group. Because of the large disordered present in 6a, it
has not been possible in this case to locate the H-bonds.
(CH3). IR (CH2Cl2,): n(CQO): 1673 cmꢀ1; n(CQC): 1601 cmꢀ1
.
ESI-MS (MeOH, m/z): 397 (100) [M + Na]+. Crystals of 6a for
X-ray diffraction have been obtained by slow evaporation of a
CH2Cl2 solution.
4,40-Bis(3,3-diphenyl-propyn-3-ol)benzil (6b). 4,40-Dibromo-
benzil 3 (3.00 g, 7.33 mmol) and 1,1-diphenyl-2-propyn-2-ol 5b
(3.39 g, 16.1 mmol) were dissolved in 50 mL of THF.
Dichlorobis(triphenylphosphine)palladium(II) (0.62 g, 0.882 mmol),
CuI (0.28 g, 1.47 mmol) and Et3N (5 mL) were added and the
mixture was stirred for 6 h at room temperature. Then the solvent
was removed under vacuum and the crude was purified by
chromatography on silica gel (CH2Cl2) giving the desired product
6b (3.52 g, 77.2% yields) as a yellow solid (mp = 139.5–140.1 1C).
1H-NMR (399.9 MHz, CDCl3)
d 7.95–7.28 (m, 28H,
aromatics), 3.01 (s, 2H, OH); 13C-{1H}NMR (100.6 MHz,
CDCl3) d 192.98 (CQO), 144.38 (C), 132.25 (CH), 129.78
(CH), 129.24 (C), 128.40 (CH), 128.39 (C), 127.96 (CH),
125.99 (CH), 96.38 (C), 86.02 (C), 74.85 (C). IR (CH2Cl2,):
n(CQO): 1674 cmꢀ1; n(CQC): 1600 cmꢀ1. ESI-MS (MeOH,
m/z): 645 (100) [M + Na]+.
3,4-Bis(4-(3-hydroxy-3-methylbut-1-ynyl)phenyl)-2,5-diphenyl-
cyclopenta-2,4-dienone (1a). 4,40-Bis(2-methyl-3-butin-2-ol)-
benzyl 6a (3.73 g, 9.95 mmol) and 1,3-diphenylacetone 4
(2.10 g, 9.95 mmol) were dissolved in EtOH. The solution
was heated to 80 1C and then NaOH (0.56 g, 9.95 mmol) was
added. The reaction mixture was refluxed for 2 h. The solvent
was removed under vacuum. Chromatography on silica gel
(CH2Cl2/ethylacetate: 95/5) gave the desired product 1a
(3.27 g, 60% yields) as a dark red solid (mp = 150.5–152.0 1C).
1H-NMR (399.9 MHz, CDCl3) d 7.20–6.70 (18H, aromatics), 1.89
(s, 2H, OH), 1.52 (s, 12H, CH3); 13C-{1H}NMR (100.6 MHz,
CDCl3) d 199.72 (CQO), 153.21 (C), 132.87 (C), 131.31 (CH),
130.33 (C), 130.09 (CH), 129.30 (CH), 128.15 (CH), 127.73 (CH),
125.95 (C), 122.95 (C), 95.21 (C), 81.79 (C), 65.63 (C), 31.41 (CH3).
IR (CH2Cl2): n(CQO): 1713 cmꢀ1. ESI-MS (MeOH, m/z): 571
(100) [M + Na]+. Crystals of 1a for X-ray diffraction have been
obtained by slow evaporation of a THF solution.
Acknowledgements
Authors are grateful to M.I.U.R. (Rome), University of
Bologna, for financial support, to Consorzio Interuniversitario
Nazionale Metodologie
(C.I.N.M.P.I.S.) Bari.
e Processi Innovativi di Sintesi
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