A saponification-triggered gelation of ester-based Zn(ii) complex through conformational transformations

Article abstract of DOI:10.1039/c4cc03537b

Novel saponification-triggered gelation in an ester-based bis-salen Zn(ii) complex (1) is described. Strategic structural modifications induced by NaOH in 1 tune the dipolar-/π-interactions leading to J-aggregation and the creation of an inorganic gel mat

Full text of DOI:10.1039/c4cc03537b

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A saponification-triggered gelation of ester-based
Zn(II) complex through conformational
transformations†
Cite this: Chem. Commun., 2014,
0, 10086
5
Received 10th May 2014,
Accepted 4th July 2014
Ashish Kumar, Mrigendra Dubey, Amit Kumar and Daya Shankar Pandey*
DOI: 10.1039/c4cc03537b
www.rsc.org/chemcomm
Novel saponification-triggered gelation in an ester-based bis-salen Zn(II) simple salen core on an isophthalate ring, was chosen for the
ꢀ
complex (1) is described. Strategic structural modifications induced by present study, and thus a zinc complex [Zn(L )] (1) imparting L
2
NaOH in 1 tune the dipolar-/p-interactions leading to J-aggregation and was prepared. The complex 1 exhibits excellent photophysical
the creation of an inorganic gel material (IGM), which has been properties and greater tunability for planar p-interactions owing
ꢀ
established by photophysical, DFT and rheological studies.
to the presence of two L units. Through this contribution, we
demonstrate the gelation in 1 triggered by saponification. To our
Gel chemistry has attracted remarkable attention over the past knowledge, this is the first report detailing the gelation of an
couple of decades, because of the significant contributions gel inorganic complex (MOG) produced by saponification.
1
materials can make in diverse areas. Due to their fascinating
Complex 1 is soluble in weakly polar aprotic solvents like
structures and usage, numerous methodologies, including soni- CHCl , THF and DCM, but it does not form a gel. It requires an
3
cation, photo-chemical, pH, temperature and analyte-assisted additive to enforce the appropriate conformational changes to
approaches have been developed for the synthesis of gel materials, facilitate the p–p/weak interactions for the gelation to occur.
1
b,2
particularly from low-molecular-weight gelators called LMWGs.
To work out this strategy, saponification, typically employed for
ꢀ
+
+
In addition, gelation properties and the potential use of the the hydrolysis of fatty acid esters into –COO Na /–K salts in
LMWGs are usually fine-tuned by their design and molecular the presence of NaOH/–KOH, has been undertaken with a slight
architecture via alteration of the non-covalent/weak (p-stacking, modification. Herein, NaOH has been used to convert the esters
H-bonding, hydrophobic and dipole–dipole) interactions.
of 1 into highly ‘polar groups’ (in the least polar solvent, CHCl3)
Usually, gelators create gels in an appropriate solvent or to facilitate significant conformational changes suitable for gel phase
1a
solvent mixture, sometimes with the addition of an additive for the oriented weak p-interactions, and in turn gelation (Scheme 1).
optimization of gelation behavior, such as metals, forming metal– Moreover, methanolic instead of an aqueous solution of NaOH
3
organic gels (MOGs). However, to date, no rational methods for the has been employed to avoid possible hydrolysis of the aldimine
creation of MOGs directly from an inorganic complex via a chemical moiety and to hold the basic skeleton of the molecule intact,
transformation have yet been reported. Moreover, the gelation
properties of ‘ether/ester’-based LMWGs are often optimized by
creating alterations in the alkyl chain through a sophisticated
1c,4
multi-step synthesis.
These issues motivated us to develop an
improved, single-step, convenient and cost-effective gelation process
for such systems. Furthermore, peripherally substituted ether/
ester(s) are suitable for gelation, and indeed, the phenyl ring
of the salen unit may favour this process by increasing the
1
b
planar p-interactions. Considering these points, the ligand
HL), which has two m-substituted ethyl ester groups and a
(
Department of Chemistry, Faculty of science, Banaras Hindu University,
Varanasi - 221005, U.P., India. E-mail: dspbhu@bhu.ac.in; Fax: +91 542 2368174;
Tel: +91 542 6702480
†
Electronic supplementary information (ESI) available: Experimental proce-
dures, morphology, photophysical, NMR titration, DFT, rheological data and
mechanism of gelation. See DOI: 10.1039/c4cc03537b
Scheme 1 Synthetic strategy adopted for IGM demonstrating the crea-
tion of IGM from complex 1.
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and also to address the miscibility criterion. Furthermore, the
end product, which forms the gel, is a bis-salen Zn(II) complex
ꢀ
+
with four –COO Na moieties (2). All the attempts to synthesize
directly by reacting unprotected ligand (H L) with metal salts
2
3
failed; moreover, such a reaction always afforded only an
insoluble product (Scheme 1, red arrowed pathway).
The addition of methanolic NaOH (2.0 M) to a solution of 1
3
in CHCl afforded IGM at rt (1.4%, w/v) (Fig. S1, ESI†). The
ꢀ
5
gelation was also scrutinized using other common solvents Fig. 1 (a) UV/vis titration spectra of 1 (c, 5.0 ꢂ 10 M in CHCl ) vs. NaOH;
3
ꢀ2
with the resulting data given in Table S1, ESI.† Complexes (b) fluorescence titration spectra of 1 (c, 2.0 ꢂ 10 M in CHCl
3
) vs. NaOH.
= 526 nm),
2
+
2+
2+
2+
Inset shows the variable temperature spectra of IGM (l
em
exploring the AIEE effect and showing the B80% thermoreversibility.
based on Cd , Cu , Co and Ni were also synthesized under
analogous conditions, and it was observed that these afforded a
2
+
gelatinous precipitate, except for Cd , which required a longer
gelation time (Fig. S2, ESI†). The gelation of 1 was examined (Fig. S7a, ESI†). Aliquot additions of NaOH resulted in signifi-
with other bases, namely, NH , Et O, KOH and cant quenching, followed by a gradual red shift (B16 nm, at
3
3
N, LiOHꢁH
2
+
+
+
CsOH, but it only occurred with Na /K and not with Li /other saturation; limit of quantification, B1.0 : 0.2 to B1.0 : 4.0 for
+
bases. The inability of Li to induce gelation may be attributed 1 : NaOH; Fig. 1b). These spectral changes signify the conver-
+
to 4CQOꢁ ꢁ ꢁLi interactions, as evidenced by a shift of the sion of 1 into 2 and the appreciable conformational changes
ꢀ
1
4
CQOn band (B9 cm ) in the vibrational spectra of 1 on pertaining to it with a p-stacking like excimer formation. The
str
5
the addition of LiOHꢁH O (Fig. S5, ESI†). This observation decrease in the fluorescence intensity may be attributed to
2
strongly indicates that gelation is governed by saponification. the enhanced excitation energy transfer within and/or between
Under heating and cooling, the IGM showed thermoreversibility the stacked species, i.e. self-quenching. Furthermore, the small
up to B63 1C.
increase in the fluorescence intensity observed during the last
+
Technically, saponification may create a mixture of Na -salt, few additions of NaOH may be associated with various weak
due to several ester sites on 1. Therefore, it has been optimized interactions in the gel matrix (due to solvated Na ions and
by varying the ratio of 1 : NaOH (i.e. 1 : 1 to 1 : 6), and it was –COO polar groups), attributable to an aggregation-induced
+
ꢀ
5
a
established that 1 : 4 is the most appropriate ratio for a strong emission enhancement (AIEE) effect (Fig. S7c, ESI†). Fluores-
gel (based on creating a compact and transparent gel in cence studies established the existence of the remarkable
the minimum time, Fig. S2, ESI†), which indirectly signified conformational changes leading to J-aggregates and are consistent
saponification as the basis for gelation. Morphology of the IGM with the conclusions drawn from the UV/vis studies. The spectral
was followed by scanning electron microscopy, which revealed studies carried out at high concentrations were well optimized and
aggregated gel fibres that were further confirmed by AFM their authenticity justified by other experiments (Fig. S7, ESI†).
ꢀ4
images. TEM images of the diluted gel (B10 M) indicated Variable temperature fluorescence experiments on IGM displayed a
intermediate assembly, as evident from the aggregated nano-sized continual decrease of the fluorescence intensity in the temperature
structures exhibiting some fibre like directional alignments range 25–70 1C, attributable to segregation of the aggregates. Based
2
a
(
Fig. S3 and S4, ESI†).
To gain a deeper insight into the gelation mechanism, definite change in the conformation may be ruled out. Further-
UV/vis and fluorescence studies were undertaken (ESI†). The more, upon cooling to rt (B25 1C), the intensity regained much of
on only an insignificant shift in the position of the bands, any
ꢀ
5
addition of NaOH to a solution of 1 (c, 5.0 ꢂ 10 M) leads to a its initial value (B80%), which indicates the re-aggregation
bathochromic shift (Dl, 41 nm) and the appearance of a new (Fig. 1b). These observations strongly indicate the thermoreversible
band at 384 nm. Moreover, there was a concurrent decrease nature of the IGM and also the AIEE effect, due to the repeated
in the optical density of the characteristic bands due to 1 (l, 343 segregation and re-aggregation in the gel matrix.
and 275 nm) (Fig. S6, ESI†). The band at 384 nm may be
ascribed to the creation of a new species by cleavage of the idea about the formation of the aggregates, H NMR titration
ester linkages after saponification (complex 2). Ratiometric was carried out by taking 1 in CDCl (0.02 M), NaOH in CD OD
To achieve clear mechanistic insights and also to have an
1
3
3
changes were observed with a noticeable isosbestic point at (2.0 M), and adding NaOH in four steps (1.0 equiv. each).
B364 nm, which signifies the formation of 2 by the gradual It was observed that the isophthalate ring protons H1 (2H,
ꢀ
degradation of 1. A large bathochromic shift (41 nm) indicates d 8.46 ppm) displayed a significant downfield shift (Dd, 0.16),
appreciable conformational changes leading to J-aggregation which most likely indicated p-stacking and/or cation–p inter-
6
a
+
via p-stacking (Fig. 1a). This observation was supported by actions due to the presence of Na ions. The resonance corres-
variable temperature (VT) UV/vis measurements on diluted gel, ponding to proton H1 displayed a criss-cross over the aldimine
ꢀ
which showed a significant hypo- and hypso-chromic shift signal H4 (2H, d 8.49 ppm), which does not indicate any shift. It
ꢀ
(
B10 nm) in the absorption band associated with aggregation simply indicates that the added NaOH does not hydrolyse the
6
b
(B384 nm) (Fig. S6c, ESI†).
confined aldimine moiety. The criss-crossing was further
ꢀ
2
The fluorescence spectrum of 1 (c, 1.0 ꢂ 10 M) displayed a affirmed by the merger of both the signals (H1 and H4) into a
ꢀ
ꢀ
strong band at 510 nm (lex, 343 nm; Stokes shift, B165 nm) single broad peak after the addition of 1.0 equiv. of NaOH and
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the separation upon further additions (2.0–4.0 equiv.) of the
base. Furthermore, the H2 and H3 (4H, d 8.00 ppm) protons
ꢀ
ꢀ
from the same ring may not be involved in stacking, as these
hardly exhibited any shift. Moreover, amongst phenolate ring
protons (8H; H5, H6, H7 and H8, d 7.44–6.66 ppm) only H6 and
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
H8 (o- and p- to phenolate oxygen) displayed an upfield shift
ꢀ
with broad features. This may be due to their involvement in
6
b,7
C–Hꢁ ꢁ ꢁp type interactions.
The concomitant disappearance Fig. 2 (a) DFT-optimized structure of 2 showing the involvement of
the isophthalate ring planes in p–p/cation–p interactions; (b) dynamic
temperature ramp of the loss of tangent (tan d = G /G ) at 5 1C min
showing the phase transition temperature (Tgel) 84 1C for gel.
of signals in the aliphatic region corroborated well with clea-
00
0
ꢀ1
,
vage of the ester linkage (–CH
and –CH , 12H; H10 and H12, d 1.28 ppm) (Fig. S8, ESI†).
2
–, 8H; H9 and H11, d 4.28 ppm
ꢀ
ꢀꢀ
3
ꢀꢀ
ꢀꢀ
Involvement of the H1 in p-stacking was confirmed by a VT
ꢀ
1
H NMR experiment, as it successively exhibited a high field shift Complex viscosity measurements at increasing temperature
(Dd, 0.09) at 55 1C. Thus, the NMR studies undoubtedly demon- supported the phase transition at 80 1C. This shows phase
strated the conversion of esters into a carboxylate moiety (1 - 2) transition via gel–semi sol–solid, which is expected for a mixed
with an appropriate conformation for p-interactions favourable solvent-based inorganic gel, and it indicates formation of a
for aggregation, and consecutively, for gelation.
typical ‘soft-solid’ like gel phase material. This observation is
To firmly validate the significant conformational reorgani- consistent with the VT fluorescence studies. A double logarithmic
zation, DFT optimizations were undertaken on the model plot between the complex viscosity (Z*) and the angular frequency
structures of 1 and 2. The optimized structures clearly showed (o) with a gradient close to ꢀ1 suggests a regular decrease in
substantial geometrical differences from the square planar (1) viscosity with increasing frequency.
0
to the tetrahedral (2) (Fig. S9, ESI†). Distant isophthalate rings
The mechanical strength of the gel was measured using G
2
+
00
in 1 (like the Cu analogue reported in our previous work) are and G as a function of shear stress at 22 1C and a frequency
8
ꢀ1
0
00
close to each other in 2. In doing so, 1 might have undergone of 1 rad s . Over a large range of shear stress, G supersedes G
significant conformational changes, most likely by an enormous by B1 order of magnitude, which clearly indicates a gel phase
ꢀ
+
9
increase (B185 fold) in the dipole moment due to the –COO Na
material. Notably, with a gradual increase in the applied
+
ꢀ
0
00
moiety. Two Na ions are flanked with the –COO moieties of stress, both G and G remained almost invariant and cross
the native isophthalate rings, while the other two are flanked by each other only at a certain yield stress (1.3 Pa), showing non-
ꢀ
+
the –COO of both isophthalate rings. Na ions are also linear properties and indicating mechanical breaking of the gel
involved in significant interactions with the subsequent iso- beyond the yield stress following a phase transition (gel–sol).
phthalate rings, which provide strong evidence of the dominant On the other hand, IGM is mechanically stable for an applied
cation–p interactions (2.13–2.76 Å). The optimized structure of strain up to 1.2% (Fig. S10, ESI†). Frequency sweep measure-
0 00
2
favours p–p stacking interactions involving isophthalate ments showed G and G to be frequency independent in the
+
ꢀ1
0
00
rings, even after involvement of the Na ion in close proximity, frequency region (ꢀ0.2–1.5 rad s ). As G 4 G , this suggests
and thereby supports the downfield shift for the H1 proton. that the IGM behaves as a gel phase material. The gelation
ꢀ
Moreover, it confirms the intermolecular p-interactions that are property was also probed by comparative PXRD patterns for 1
indispensable for gelation due to the position of the fourth Na , and its xerogel. The loss of the peaks due to xerogel established
+
1
0
which is most probably involved in interplanar cation–p interac- its amorphous nature (IGM) (Fig. S11, ESI†). Based on the
tions. On the contrary, phenolate rings are distant enough that they above results, it is concluded that saponification is the key
can only get involved in stabilization of the supramolecular net- observable step that induces significant conformational change
work (gel matrix) via weak C–Hꢁꢁꢁp interactions for aggregation, in 1 by the in situ conversion of the ester into carboxylate,
and therefore support unusual chemical shifts for the phenolate thereby increasing the p-interactions by manifolds to create a
1
1
ring protons (H6 and H8). Thus, the conclusions drawn from the true phase gel material (IGM) (Fig. S14, ESI†).
spectral studies are consistent with the observations elicited
ꢀ
ꢀ
In summary, through this work the applicability of the
from the DFT-optimized structures (Fig. 2a). The inability of saponification process to trigger gelation in a Zn(II) complex
2
+
2+
2+
Cu , Ni and Co analogues to form a gel can also be explained has been demonstrated. Gelation is accompanied by substan-
by performing DFT calculations (Fig. S9, ESI†). tial conformational changes encompassing gel-phase oriented
To exclude the artefacts of the gel phase material, freshly p-stacking/cation–p interactions. This has been thoroughly
1
prepared IGM was directly subjected to rheological studies. At demonstrated by various spectral (UV/vis, fluorescence, H NMR)
0
fixed concentrations of IGM (1.4%, w/v), the storage (G ) and and DFT studies, and the true gel phase was also affirmed by
00
loss modulus (G ) were measured in the temperature range rheological investigations. Therefore, through this work a novel
0
00
2
2–100 1C, and it was observed that G 4 G up to 84 1C. The gel methodology based on saponification-triggered gelation for
started to deform at 70 1C and ended at 84 1C, where both inorganic complexes has been established.
0
00
the moduli meet each other (G = G ), suggesting a phase
The authors thank DST and UGC, New Delhi, India for
transformation. A sharp change in the loss tangent value providing the Fluorescence Spectrometer through the scheme
00
0
5b
(
tan d = G /G ) at 84 1C specifies Tgel for IGM (Fig. 2b).
SR/S1/IC-25/2011 (DSP), and Senior Research Fellowship through
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the scheme R/Dev./Sch.-(UGC–SRF)/S–01 (AK). The authors also
thank Indian Institute of Technology, Indore, India for extend-
ing various facilities.
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