A new class of organogelators based on triphenylmethyl derivatives of primary alcohols: Hydrophobic interactions alone can mediate gelation

Article abstract of DOI:10.3762/bjoc.13.17

In the present work, we have explored the use of the triphenylmethyl group, a commonly used protecting group for primary alcohols as a gelling structural component in the design of molecular gelators. We synthesized a small library of triphenylmethyl deri

Full text of DOI:10.3762/bjoc.13.17

A new class of organogelators based on triphenylmethyl
derivatives of primary alcohols: hydrophobic interactions
alone can mediate gelation
Wangkhem P. Singh and Rajkumar S. Singh*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 138–149.
Organic Materials Research Laboratory, Department of Basic
Sciences & Social Sciences, North-Eastern Hill University,
Shillong-793022, Meghalaya, India
doi:10.3762/bjoc.13.17
Received: 30 September 2016
Accepted: 30 December 2016
Published: 23 January 2017
Email:
Rajkumar S. Singh* - rksunil3000@yahoo.co.in
Associate Editor: S. C. Zimmerman
* Corresponding author
© 2017 Singh and Singh; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
hydrophobic interactions; organogelator; SEM; triphenylmethyl group;
xerogel
Abstract
In the present work, we have explored the use of the triphenylmethyl group, a commonly used protecting group for primary alco-
hols as a gelling structural component in the design of molecular gelators. We synthesized a small library of triphenylmethyl deriva-
tives of simple primary alcohols and studied their gelation properties in different solvents. Gelation efficiency for some of the deriv-
atives was moderate to excellent with a minimum gelation concentration ranging between 0.5–4.0% w/v and a gel–sol transition
temperature range of 31–75 °C. 1,8-Bis(trityloxy)octane, the ditrityl derivative of 1,8-octanediol was the most efficient
organogelator. Detailed characterizations of the gel were carried out using scanning electron microscopy, FTIR spectroscopy,
rheology and powder XRD techniques. This gel also showed a good absorption profile for a water soluble dye. Given the non-polar
nature of this molecule, gel formation is likely to be mediated by hydrophobic interactions between the triphenylmethyl moieties
and alkyl chains. Possible self-assembled packing arrangements in the gel state for 1,8-bis(trityloxy)octane and (hexadecyl-
oxymethanetriyl)tribenzene are presented. Results from this study strongly indicate that triphenylmethyl group is a promising
gelling structural unit which may be further exploited in the design of small molecule based gelators.
Introduction
Small organic molecules capable of forming gels are called low mental conditions. Gels, so formed are supramolecular in nature
molecular weight gelators (LMWGs) [1-3]. These LMWGs can as they result from self-assembly of the gelator molecules
immobilize organic solvents (forming organogels) and water or through secondary interactions like H-bonding, π-stacking,
aqueous solvents (forming hydrogels) under different experi- donor–acceptor interaction, electrostatic, metal coordination,
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Beilstein J. Org. Chem. 2017, 13, 138–149.
hydrophobic forces or van der Waals’ interactions [4-6]. We of gelators based on hydrophobic interactions alone (not in
know a great deal about the aggregation behaviour of gelator combination with other types of secondary interactions) is
molecules from many studies conducted during the past several underexplored compared to the overwhelming use of other
years. However, rational design and accurate prediction of the types of interactions [27,28]. Hence, we were motivated to
gel forming ability of a given molecule is still a formidable search for new hydrophobic structural components which can
challenge, if not altogether impossible [7,8]. Often, the be incorporated in the design of new gelators. Another impor-
discovery of new gelators relies on an empirical approach tant aspect we also considered is the accessibility of gelator
wherein structural components capable of forming non-cova- molecules in large quantities from cheap starting materials in as
lent interactions are incorporated into different molecular scaf- few synthetic steps as possible. Keeping these two aspects in
folds and tested for their gelation abilities. In another approach, mind, we explored and identified the triphenylmethyl group
modifications are made to known gelator scaffolds in the search [29], a commonly used protecting group for primary alcohols as
for new gelators. These approaches have been used widely with a potential gelling structural component in the design of new
a fair degree of success giving rise to gelator molecules with gelators. To the best of our knowledge, the use of the triphenyl-
diverse molecular structures [9-13]. They are being increas- methyl (trityl) group as a potential gelling structural component
ingly explored for a variety of uses in the fields of materials in the design of molecular gelators has not been reported before.
science, biomedicines, environment science, etc. Hence, the We reasoned that the triphenylmethyl group (having three
discovery of new functional gelators continues to be an impor- benzene rings) will be a good candidate for hydrophobic inter-
tant focus of research worldwide.
actions mediated gelation. We also noted that since the triph-
enylmethyl group as a whole exhibits a non-planar geometry
Since most gels are formed by non-covalent interactions, they (unlike the planar geometry of say anthracene, pyrene, etc.),
can be assembled or disassembled in response to appropriate π–π stacking will be favoured only between benzene units of
external stimuli. Functional gels have been reported that are different trityl groups. A small library of triphenylmethyl deriv-
sensitive to physical stimuli like UV–vis light [14], ultrasound atives was synthesized from the corresponding primary alco-
[15], and mechanical forces [16]. On the other hand, chemical hols employing a single step reaction and detailed gelation
stimuli-sensitive gels respond to stimuli such as acids or bases studies carried out. Remarkably, we found that some of these
[17], metal ions [18], oxidation and reduction [19], reactive triphenylmethyl derivatives can act as efficient gelators of some
species enzymes, etc. Such responsive gel systems are highly polar solvents thereby validating our approach.
desirable in stimuli-responsive sensor materials, drug delivery,
Results and Discussion
catalysis, nano- and mesoscopic assemblies, light harvesting
systems, and many others [20-22].
Synthesis
We synthesized a small library of triphenylmethyl derivatives of
As mentioned above, developing a new gelator is still largely a easily available simple primary alcohols (Scheme 1, TPM-
trial and error method. The most widely used design strategy G1–TPM-G15). Trityl derivatives of simple aliphatic alcohols
involves the inclusion of structural components favouring inter- afforded the Series 1 compounds. Simple dihydroxy com-
molecular non-covalent interactions (like H-bonding, pounds can generate both mono-trityl (Series 2) and ditrityl de-
π-stacking, donor–acceptor interaction, metal coordination, rivatives (Series 3). The mono-trityl derivatives were synthe-
ionic, hydrophobic forces, etc.) into the gelator molecular struc- sized in one step by the reaction of the corresponding alcohols
ture and studying their gelation behaviour. H-bonding interac- (1 equivalent) with trityl chloride (1 equivalent) in the presence
tion (present in amide, urea, carbamate linkages, etc) alone or in of triethylamine (1 equivalent) in dichloromethane (DCM) at
combination with other interactions is the most extensively used room temperature (Scheme 1, TPM-G1–TPM-G10). On using
strategy in the design of gelators [23,24]. Similar is the case for two equivalents of trityl chloride and triethylamine with one
π-stacking interaction (seen in naphthalene, anthracene, pyrene, equivalent of the dihydroxy compounds, the respective ditrityl
perylene, etc.) which has been mostly used in combination with derivatives were obtained (Scheme 1, TPM-G11–TPM-G15).
other types of secondary interactions in the discovery of many
gelators [25,26]. This holds true for many other non-covalent Gelation behaviour
interactions as well. As a consequence of this approach, numer- We evaluated the gelation behaviour of the 15 triphenylmethyl
ous gelator molecules with highly varied structures have been derivatives in 21 different organic solvents (both polar and non-
reported. Because of the structural diversity, gelation behav- polar solvents) at a concentration of 2% w/v. The gelation be-
iours also vary significantly. Taken together, the development haviour is summarized in Table 1. Three of the compounds
of new gelators using this empirical approach still continues to (TPM-G4, TPM-G5, TPM-G12) formed organogels in polar
be quite successful. Interestingly, we observed that the design solvents like dimethylsulfoxide (DMSO), propan-1-ol, propan-
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Beilstein J. Org. Chem. 2017, 13, 138–149.
Scheme 1: Chemical structures of triphenylmethyl-based organogelators.
2-ol, butan-1-ol, butan-2-ol, diethylene glycol, and triethylene ethylene glycol (Figure 1). TPM-G12 (the ditrityl derivative of
glycol. TPM-G5 (trityl derivative of 1-hexadecanol) formed an 1,8-octanediol) turned out to be an excellent gelator of some
organogel in DMSO while TPM-G4 (trityl derivative of polar solvents. It formed gels in DMSO, propan-1-ol, propan-2-
1-tetradecanol) was able to gel diethylene glycol and tri- ol, butan-1-ol, and butan-2-ol (Figure 1). The gels formed are
Table 1: Gelation properties of TPM-G1 – TPM-G15a,b.
Solvent
TPM TPM TPM TPM
-G1 -G2 -G3 -G4
TPM
-G5
TPM TPM TPM TPM TPM TPM TPM
-G6 -G7 -G8 -G9 -G10 -G11 -G12 -G13 -G14 -G15
TPM TPM TPM
benzene
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
T
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
T
T
S
S
S
S
S
S
S
S
S
S
S
S
S
T
S
S
S
S
S
T
T
T
S
S
P
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
T
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
T
T
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
T
S
S
S
S
S
S
S
S
S
S
S
S
P
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
T
S
S
S
S
S
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
S
S
S
S
S
I
toluene
S
S
S
o-xylene
S
S
S
nitrobenzene
chlorobenzene
1,2-dichlorobenzene
cyclohexane
THF
S
S
S
S
P
S
S
S
S
S
S
S
S
S
S
DMF
S
S
S
acetonitrile
1,4-dioxane
DMSO
S
T
I
S
S
S
T
I
S
S
S
P
P
P
S
P
P
P
P
S
P
G(2.0)
P
G(0.5)
methanol
P
I
propan-1-ol
propan-2-ol
butan-1-ol
S
S
I
G(0.5)
G(0.5)
G(0.6)
G(0.6)
T
S
S
S
S
S
T
P
S
S
S
S
S
S
S
S
S
S
S
I
S
S
P
I
butan-2-ol
S
S
ethyleneglycol
diethyleneglycol
triethyleneglycol
benzylalcohol
T
S
S
S
S
S
G(4.0)
G(8.0)
S
T
S
T
S
S
S
aValues given inside the brackets denote the minimum gel concentration (MGC, % w/v ) to form organogels at room temperature. G: gel; S: solution;
P: precipitate; I: insoluble; T: turbid. bTHF, DMSO, and DMF indicate tetrahydrofuran, dimethyl sulfoxide and N,N-dimethylformamide, respectively.
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Beilstein J. Org. Chem. 2017, 13, 138–149.
opaque and not transparent or translucent. Since these triphenyl-
methyl derivatives are non-polar, the formation of gels in polar
solvents (and not in non-polar solvents) is understandable. Gel
formation in polar solvents will be driven mostly by hydro-
phobic interactions between the alkyl chains and the triphenyl-
methyl moieties. Compared to other planar aromatic molecules
(such as naphthalene, anthracene, perylene, etc.), the triphenyl-
methyl moiety as a whole exhibits a non-planar geometry,
hence π-stacking interaction between triphenylmethyl moieties
may not be optimal. The π-stacking interaction may, however,
be present between the benzene units of different triphenyl-
methyl moieties. It is interesting to note here that the presence
of hydrophobic interactions alone (in the absence of H-bond
forming structural components) is sufficient for gel formation in
polar solvents. For Series 1 compounds, the presence of longer
alkyl chains (tetradecyl and hexadecyl) favoured the formation
of gels. None of the Series 2 derivatives, containing one
hydroxy group and one triphenylmethyl group, formed gels in
the solvents tested. The contrasting gelation behaviour of TPM-
G1 (non-gelator), TPM-G7 (non-gelator), and TPM-G12
(gelator) is worth highlighting. TPM-G1 contains a triphenyl-
methyl moiety at one end of an octyl chain, TPM-G7 has a
triphenylmethyl moiety and a hydroxy group at the two ends of
an octyl chain, whereas TPM-G12 has two triphenylmethyl
moieties at the two ends of an octyl chain. The presence of an
additional triphenylmethyl group (in TPM-G12, and not in
TPM-G1 and TPM-G7) has a profound effect on gelation be-
haviour as shown in Table 1 and Figure 1. Although TPM-G12
and TPM-G15 (with three oxyethylene units flanked by two
triphenylmethyl groups) are of similar molecular size and
shape, TPM-G15 did not form a gel in any solvents. The pres-
ence of the more polar oxyethylene unit (compared to non-polar
octyl chain) had a negative effect on the gelation behaviour of
TPM-G15. TPM-G11 (the ditrityl derivative of 1,6-hexane-
diol) and TPM-G13 (the ditrityl derivative of 1,10-decanediol)
did not form a gel implying that an octyl chain is the optimal
length for gel formation.
Figure 1: Organogels formed by TPM-G12 in (a) propan-1-ol;
(b) DMSO at 0.5% w/v; (c) organogel from TPM-G5 in DMSO at
2% w/v; (d) organogel from TPM-G4 in diethylene glycol at 4% w/v.
The minimum gelation concentration (MGC) of the three gela-
tors were determined [30]. They are in the range of 0.5–4.0%
w/v (Table 1). TPM-G12 is an excellent gelator with an MGC
value of 0.5% in both propan-1-ol and DMSO. MGC values for
TPM-G5 and TPM-G4 are 2.0% (DMSO) and 4.0% (diethyl-
eneglycol), respectively. Figure 2 shows the gel–sol transition
temperature (Tgel) of the gels which were determined using the
dropping ball method [31]. Tgel values (at MGC) for TPM-G12
are 73 °C (DMSO) and 75 °C (propan-1-ol). Tgel values for
TPM-G5 and TPM-G4 were much lower at 38 °C and 31 °C,
Figure 2: Plot of Tgel (gel–sol transition temperature) versus gelators
at different concentrations. TPM-G12 in (a) DMSO and (b) propan-1-ol;
(c) TPM-G5 in DMSO; (d) TPM-G4 in diethylene glycol.
respectively, at the MGC. As expected, with an increase in Stability of gels
gelator concentration there is an increase in Tgel for all the three The room temperature stability of the gels was also monitored.
gelators.
The organogel from TPM-G12 (1% w/v in propan-1-ol and
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Beilstein J. Org. Chem. 2017, 13, 138–149.
DMSO) remained stable for a long time (more than a month, Differential scanning calorimetry
Figure S4 in Supporting Information File 1) without any appre- Differential scanning calorimetry (DSC) of gels prepared from
ciable change in its structural integrity. However, on prolonged TPM-G12 (in propan-1-ol and DMSO) was carried out to study
storage at room temperature, there is a gradual loss of the the gel-sol transition (Tgel) behaviour. Figure 3 shows the DSC
entrapped solvent from the gels. TPM-G5 also formed a strong heating curves of TPM-G12 prepared at 2% w/v. When a
gel in DMSO which is stable at room temperature for a long system changes from an ordered to a disordered state such as
time (≈2 weeks). In contrast, organogels formed from TPM-G4 the transition from gel to sol, an endothermic peak in the DSC
in diethylene glycol and triethylene glycol were weak and not scan is expected which is observed here [32]. For gel prepared
stable at prolonged storage (≈2 days). This is reflected in its in propan-1-ol, the Tgel (melting of gel) is observed to be 97 °C,
higher MGC and lower Tgel values.
whereas for DMSO gel the value is 104 °C. Gel melting profile
for propan-1-ol gel is much sharper compared to DMSO gel. It
Generally, triphenylmethyl protecting groups of alcohols are may also be noted here that the Tgel determined from DSC is
easily removed under acidic conditions in solution [29]. We somewhat higher in comparison to values obtained from the
wanted to see if this is still true for triphenylmethyl derivatives dropping ball method (Figure 2).
present in the gel state (e.g., gels obtained from triphenyl-
methyl derivatives in this study). Hence, the stability of Gel morphologies
TPM-G12 gel (propan-1-ol) under acidic conditions was tested. To understand the microscopic structures and morphologies of
Interestingly, acidification of the organogel (by adding 100 µL the gels formed by the gelators, scanning electron microscopy
of 2 N HCl solution to the organogel prepared in 0.5 mL) did (SEM) studies were performed with the dried gels [33]. Dried
not bring about any visible change in its integrity. It remained gels (xerogels) were obtained from the gels by evaporation of
stable for days without any apparent structural disintegration. If the solvent. The morphologies for TPM-G12 and TPM-G5 are
the triphenylmethyl protecting group was easily removed in the drastically different (Figure 4). For TPM-G12 (propan-1-ol and
gel state, there would have been a discernible change in the DMSO), ‘rod-like’ structures with varying sizes (in width and
structural integrity of the gel (visually at least). The control test length) were observed. These ‘rod-like’ structures are mostly
in solution (gelator solubilised in DCM followed by addition of separated and not very interconnected. They are unlike highly
acid) showed that the triphenylmethyl group is easily removed interconnected structures which are commonly observed in
as expected (as monitored by TLC). This observation clearly fibrous networks seen for many gelators reported in the litera-
demonstrates that the triphenylmethyl derivative of a primary ture. The dimensions of these rods are approximately in the
alcohol in the gel state (self-aggregated into nano- and micro- range of tens of nanometers to low micrometer in width and
scale structures) is impervious to an acidic solution in contrast several tens of micrometers in length. Interestingly, smaller
to its unstable solution-phase behavior.
sized rod-like structures are seen in DMSO gel while larger
Figure 3: DSC thermograms of gels prepared from TPM-G12 in (a) Propan-1-ol and (b) DMSO.
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Figure 4: SEM images of the dried gels. TPM-G12 in propan-1-ol (a) and (b), in DMSO (c) and (d); TPM-G5 in DMSO (e) and (f). (a), (c), (e) are of
lower magnification while (b), (d), (f) are of higher magnification.
ones are observed for propan-1-ol gel. In stark contrast, of the organogels over a certain range of applied stress. At the
TPM-G5 formed irregular-shaped ‘sheet-like’ structures which low-stress region it shows a somewhat linear response, howev-
are highly interconnected in nature. Width and length of these er, it shows deviation from linearity at higher stress values.
sheet-like structures are in the range of several tens of microme- Beyond a certain stress point, G‘‘ becomes higher than G‘
ters.
which is defined as the yield stress of the viscoelastic material.
For these gels, similar yield stress values (≈6–7 Pa) were ob-
tained. These values are somewhat on the lower side.
Rheological studies
Rheological studies of TPM-G12 gels (propan-1-ol and
DMSO) were performed to understand their viscoelastic behav- FTIR studies
iour. We measured the two parameters, G‘ (storage modulus) FTIR is one of the techniques used to study the influence of
and G“ (loss modulus) of these gels. While G‘ relates to the non-covalent interactions in the self-assembly of gelator mole-
ability of a deformed material to store energy, G“ concerns with cules [35]. We carried out the FTIR analysis of TPM-G12 in
the flow behaviour of the material under stress [34]. In the gel solution (CHCl3, non-self-assembled state) and xerogel (KBr,
state, the value of G‘ should be greater than G“ while this is self-assembled state) and compared their differences. Since
reversed in the sol state. We carried out stress amplitude sweep there is no structural component capable of forming relatively
experiment wherein G‘ and G“ were measured as a function of strong intermolecular non-covalent interactions in TPM-G12
oscillatory shear stress at a constant oscillation frequency (e.g., hydrogen bond, ionic bond), we did not expect much
(Figure 5). As expected for gels, G‘ is greater than G“ for both difference between the two IR spectra. Indeed, this is the case
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Beilstein J. Org. Chem. 2017, 13, 138–149.
Figure 5: Stress sweep rheological experiment of TPM-G12 gel (1.5% w/v) in (a) propan-1-ol (b) DMSO.
as shown in Figure 6. Aromatic ring peaks for C–H str (2θ value), one medium-intensity peak at 11.86°, and two low-
(3019 cm−1), out-of-plane C–H def (768 cm−1) and C=C str intensity peaks at 20.15° and 22.02° were observed. The corre-
(1402 cm–1, 1448 cm–1 and 1490 cm–1) are seen in the solution sponding d-spacing values are 8.12 Å, 7.50 Å, 4.48 Å and
state. Except for out-of-plane C–H def (759 cm–1), almost iden- 4.10 Å, respectively (Figure 7a). In the energy-minimized state,
tical peak values are observed in the xerogel too. The peak TPM-G12 adopts a symmetrical ‘dumbbell-shaped‘ structure
value for C–O str (1216 cm–1) also remains the same in both the (Figure 8a). Based on the observed d-spacing values and energy
states. So, FTIR studies strongly suggest that there is an absence minimized molecular size, we propose a possible molecular
of strong intermolecular non-covalent interactions in the gel packing arrangement of TPM-G12 in the self-assembled gel
state.
state (Figure 9a). Self-assembly in the gel state will be driven
predominantly by hydrophobic interactions between the octyl
alkyl chains and triphenylmethyl groups. The gelator molecules
XRD studies and molecular packing
We have carried out powder XRD analysis using dried gel of may be arranged linearly in a head-to-tail (end-to-end) direc-
TPM-G12 (propan-1-ol) and TPM-G5 (DMSO) to get an tion. These linear arrays can then interact laterally with two
understanding of the molecular packing in the self-assembled triphenylmethyl groups in close proximity to neighbouring octyl
gel state [36]. For TPM-G12, one high-intensity peak at 10.94° chains resulting in a roughly two-dimensional planar packing
Figure 6: FTIR spectra of gelator TPM-G12 in (a) CHCl3 and (b) xerogel in KBr.
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Figure 7: Powder X-ray diffraction patterns of xerogel of (a) TPM-G12 from propan-1-ol and (b) TPM-G5 from DMSO.
arrangement. The distance between any two adjacent linear tures will be similar to the width of TPM-G12 (8.50 Å). The
arrays will be approximately equal to (or less than) the width of observed d-spacing value of 8.12 Å from powder XRD experi-
the TPM-G12 molecule (8.50 Å). Subsequently, different ment correlates closely with this value. The weak intensity peak
layers of these two-dimensional planar structures can aggregate at 22.02° (4.10 Å) may result from π–π stacking between
together to give the three-dimensional microstructure of the gel. benzene rings of adjacent triphenylmethyl groups which are
The distance between any two planar two-dimensional struc- likely to be involved in self-assembly of the gelator molecules.
Figure 8: Energy minimized conformational structure of (a) TPM-G12 and (b) TPM-G5 obtained using B3LYP/6-31G (d, p) level of computations.
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Beilstein J. Org. Chem. 2017, 13, 138–149.
Figure 9: Possible molecular packing arrangement in the self-assembled gel state of (a) TPM-G12 and (b) TPM-G5.
For TPM-G5, two high-intensity peaks at 9.56° (2θ value) and minimized conformational structure of TPM-G5. Based on the
12.75°, and two low-intensity peaks at 19.17° and 22.39° were observed d-spacing values and energy minimized molecular
observed (Figure 7b). The d-spacing values are 9.24 Å, 6.93 Å, conformation of TPM-G5, a possible molecular packing
4.62 Å and 3.96 Å, respectively. Figure 8b shows the energy arrangement in the self-assembled gel state is presented
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Beilstein J. Org. Chem. 2017, 13, 138–149.
(Figure 9b). One possible arrangement is an interdigitated bilay- with Direct Red 80 may facilitate dye absorption and removal.
er structure. The molecules are oriented side by side such that Figure 10a shows the time-dependent dye absorption profile of
each hexadecyl alkyl chain is sandwiched between two such Direct Red 80 by TPM-G12 gel (propan-1-ol) as measured by
chains. The high-intensity peak at 12.75° (d-spacing value of UV–vis technique. The dye absorption efficiency is 58, 63 and
6.93 Å which is comparable to the width of TPM-G5, 6.99 Å) 69% after 6, 12, and 24 h of incubation, respectively. So,
can be attributed to the distance between adjacent linear molec- TPM-G12 gel showed a fairly good absorption capacity for
ular arrays and also between two-dimensional planar arrays. Direct Red 80. In contrast to this observation, the same gel
The weak intensity peak at 22.39° (3.96 Å) can be attributed to showed very low absorption (9% after 24 h) of crystal violet
the presence of π–π stacking between benzene rings of adjacent dye (much polar compared to Direct Red 80) which reinforces
triphenylmethyl groups.
our hypothesis that hydrophobic interactions between the gel
microstructures and dye molecules play an important role in the
absorption process (Figure 10b).
Dye absorption studies
Dyes are commercially important and are widely used in many
industries. But most of these dyes are toxic and harmful to the Conclusion
environment. Discharge of waste water generated during use of We have developed a new class of organogelators based on
these water-soluble dyes without proper pretreatment creates a triphenylmethyl derivatives of simple and easily available pri-
serious environmental and health hazard. Hence, development mary alcohols. One big advantage of these gelators is their
of simple and reliable methods to remove dyes from the indus- straightforward synthesis and easy accessibility. Overall, gela-
trial waste water is becoming increasingly important. Conven- tion efficiency of some of the triphenylmethyl derivatives was
tionally, this has been achieved through the use of materials like moderate to excellent with minimum gelation concentration in
clay, porous silica, polymers, charcoals, etc. [37]. More the range of 0.5–4.0%, w/v. 1,8-Bis(trityloxy)octane, the ditrityl
recently, some gels derived from low molecular weight com- derivative of 1,8-octanediol was found to be an excellent gelator
pounds have been shown to possess the ability to absorb dyes of some polar solvents. Detailed characterizations of the gel
[38,39]. Hence, small molecules based molecular gels have were carried out using scanning electron microscopy, FTIR
recently emerged as a new class of materials which can be used spectroscopy, rheology and powder XRD techniques. Based on
in the removal of water-soluble toxic dyes. We reasoned that its absorption profile for a water soluble dye, this gel can poten-
TPM-G12 gel (being non-polar) can perhaps be utilized for tially be used as a dye removal agent from waste water. The
removal of water-soluble dyes having appreciable hydrophobic results strongly suggest that hydrophobic interactions alone can
character (perhaps with a large π surface area, e.g., Direct mediate gelation of polar solvents and this approach can be
Red 80). The hypothesis being hydrophobic interactions be- exploited in the design of new gelators. We are further pursuing
tween the non-polar self-assembled TPM-G12 microstructures this line of investigation by exploring for hitherto undiscovered
Figure 10: Time-dependent UV–vis absorption profile of (a) Direct Red 80 (b) Crystal Violet aqueous dye solution (0.02 mM) by TPM-G12 gel
(1% w/v in propan-1-ol).
147

Beilstein J. Org. Chem. 2017, 13, 138–149.
17.Maitra, U.; Chakrabarty, A. Beilstein J. Org. Chem. 2011, 7, 304–309.
doi:10.3762/bjoc.7.40
hydrophobic structural components which can be incorporated
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Supporting Information
Supporting Information File 1
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-17-S1.pdf]
doi:10.1002/anie.201106767
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Acknowledgements
W.P.S gratefully acknowledges UGC, India for providing a
Non-NET fellowship. This work was partially supported by the
Plan grant money of NEHU, Shillong (to R.S.S). We sincerely
thank SAIF-NEHU (Shillong), SAIF-CDRI (Lucknow), SAIF-
GU (Guwahati), STIC-CUSAT (Cochin), AIRF-JNU (New
Delhi) and SIC-IIT Indore for providing different analytical
services. Shougaijam Premila Devi is thanked for help in energy
minimization studies of TPM-G12 and TPM-G5.
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