Li+-Induced fluorescent metallogel: A case of ESIPT-CHEF and ICT phenomenon

Article abstract of DOI:10.1039/c8cp04579h

A fluorescent metallogel (1percent w/v) has been synthesized from non-fluorescent ingredients viz. the smallest possible low molecular weight aromatic symmetrical ligand H₂SA (1) and LiOH in a chloroform and methanol mixture. The chelation of Li⁺ is not only responsible for the inhibition of excited state intramolecular proton transfer (ESIPT) or the origin of fluorescence through chelation enhanced fluorescence (CHEF) in 1, but also for aggregation leading to gelation. The metallogel obtained from 1/Li⁺ reveals a fibrous morphology while 1 with other, bigger size, alkali metal ions like Na⁺/K⁺/Cs⁺ demonstrates the growth of crystals with different shapes. The effect of the size of the alkali metal ion over gel formation is well explored by FTIR, UV-vis, fluorescence, average lifetime measurements, SEM and PXRD. The metallogel shows multi-stimuli responsive behaviour towards thermal and mechanical stress as well as reswelling properties. The regioisomer H₂PBA (2) also shows emission upon treatment with LiOH due to the presence of intramolecular charge transfer (ICT), this is well established by various experiments. The mechanism of gel formation is well established by FTIR, ¹H NMR, UV-vis, fluorescence, lifetime measurements, SEM and single crystal and powder XRD instrumental techniques. The involvement of various phenomena in gel formation has been further supported by other synthesized model compounds viz. H₂MBA (3), PMO (4), H₂SEA (5) and H₂SPA (6). True gel phase material is proved by detailed rheological experiments.

Full text of DOI:10.1039/c8cp04579h

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Li+-induced Fluorescent Metallogel: a case of ESIPT-CHEF and ICT
phenomenon
Received 00th January 20xx,
Accepted 00th January 20xx
Manish Kumar Dixitb and Mrigendra Dubeya
∗
DOI: 10.1039/x0xx00000x
A fluorescent metallogel (1%, w/v) has been synthesized from the non fluorescent ingredients viz. smallest possible low
molecular weight aromatic symmetrical ligand H2SA (1) and LiOH in chloroform and methanol mixture. The chelation of Li+
is not only responsible for the inhibition of Excited State Intramolecular Proton Transfer (ESIPT) or origin of fluorescence
through Chelation Enhanced Fluorescence (CHEF) in 1, but also for aggregation leading to gelation. Metallogel obtained
from 1/Li+ reveals the fibrous morphology while 1 with other bigger size alkali metal ions like Na+/K+/Cs+ demonstrates the
crystals growth with different shapes. Effect of size of alkali metal ions over gel formation are well explored by FTIR, UV-
vis, fluorescence, average lifetime measurments, SEM and PXRD. Metallogel shows multi-stumuli responsive behaviour
towards thermal and mechanical stress as well as reswelling property. Regioisomer H2PBA (2) also shows emission upon
treatment with LiOH due to presence of intramolecular charge transfer (ICT) is well established by various experiments.
Mechanism of gel formation is well established by FTIR, 1H NMR, UV-vis, fluorescence, life time measurements, SEM, single
crystal and powder XRD intrumental techniques. The involvement of various phenomenon in gel formation has been
further supported by other synthesized model coumpounds viz. H2MBA (3) and PMO (4), H2SEA (5) and H2SPA (6). True
gel phase material is proved by a detailed rheological experiments.
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compared to organic-gelators due to variety of interesting
coordination modes of ligand-metal and thereby influencing
the gel properties.6
Introduction
Metal-organic-materials (MOM) particularly metallogels are
the most attractive soft and smart material in the recent past
owing to their wide variety of interesting properties and
applications such as optical, morphological, rheological,
magnetism, redox, catalysis, conductance, optoelectronics,
nano materials, crystal growth media, pharmaceutics and
stimuli responsive materials.1,2,3 In the past few decades, gels
were synthesized usually from high molecular weight gelators
based on traditional long chain hydrophobic groups like lipids,
steroids, porphyrins and sugars.4 But in recent few years, low
molecular weight gelators (LMWG) have evoked the interest of
scientific community due to worthwhile ability to construct an
extensive molecular network from small molecules through
non-covalent interactions for instance electrostatic, π−π
stacking, H-bonding, Van der Waals as well as their wide-
spread applications.5 Following the increased use of LMWG,
development of metallogelators has consequently increased as
In recent past, metallogels were synthesized based on the
concept of strong metal-ligand interaction using well explored
moieties, for instance- amide (particularly urea or other -NH
containing compounds)1b,3d
,
pincer3e, analogous metal
chelating compound,3f calix[4]arene derivatives3g and crown
ether tagged ligands2d. Incorporation of metals like Pd2+, Pt2+,
Eu2+, Tb2+ and Au2+ (Zn2+ and Cd2+ for some extent) etc. to
aforementioned ligands may originate emission property
under various fluorescence phenomenon. But, the design of
LMWG based ligand with an objective to comprise of
fluorescence property after metal interaction as well as
morphological tuning during metallogel formation is still
challenging.
Chelation
enhanced
fluorescence
(CHEF)7
and
intramolecular charge transfer (ICT)8 phenomenon triggered
by metal ion has been widely utilized in the field of sensing,
aggregation and biological imaging, but rarely applied in case
of metallogels. In general, CHEF is introduced in ligands using
traditional moieties such as terpyridyl, amides or high
molecular weight ligand based on porphyrin moeity.9,7d By
utilizing the concept of CHEF and ICT, we recently reported
aSoft Materials Research Laboratory, Discipline of Metallurgy Engineering and
Materials Science, Indian Institute of Technology Indore, Simrol, Indore 453552,
India. E-mail: mdubey@iiti.ac.in; mrigendradubey@gmail.com; Tel: +91
9044413301.
two chiral metallogels based on
L-tartaric acid derived
bDepartment of Chemistry, Indian Institute of Technology (Banaras Hindu
University) Varanasi- 221005, U.P., India.
symmetrical gelators.10 Therein, we explored that the metal
binding to ligand enroots the appearance of fluorescence
(CHEF) as well as conformational change which provides a
planar platform for π−π stacking leading to aggregation
†Electronic Supplementary Informaꢀon (ESI) available: [Experimental procedures,
Schemes, Morphology, UV-vis titrations, fluorescence, 1H NMR titration and
mechanism.]
See DOI: 10.1039/x0xx00000x
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prepared gels (1% w/v). Linear viscoelastic regions of the gel
samples were determined by measuring the storage modulus,
followed by gelation. Further, the metallogel also showed
interesting nano-cluster morphology, optical and rheological
properties.10a Conversely, we could successfully switch the
optical (CHEF to ICT) and morphological (nano-cluster to fibres)
properties of metallogel by substituting the –OH group by
G′ (associated with energy storage) and the loss modulus, G
ʺ
(associated with the loss of energy) as a function of stress
amplitude (Dynamic oscillatory frequency of 1 rad s-1). The
following tests were performed: increasing amplitude of
oscillation up to 100 % apparent strain on shear, time and
frequency sweeps at 20 °C (20 min and from 0.1 to 100 rad s-1,
respectively) and a heating run to 90 °C at a scan rate of 5 °C
−
NO2 in aforesaid L-tartaric acid derived ligand (Scheme S1,
ESI†).10b Taking all this into consideration, herein, we report
the synthesis of fluorescent metallogel obtained from lowest
possible symmetrical aromatic ligand and LiOH.
min-1. All these measurements were conducted in triplicate
.
Synthetic procedure
EXPERIMENTAL SECTION
Salicylaldehyde Azine or [bis-(2-hydroxybenzylidene)-
hydrazine] (H2SA) 1: methanolic solution of 2-
Materials and Physical Methods:
A
hydroxybenzaldehyde (0.501 g, 4.10 mmol) was added drop
wise to a solution of Hydrazine monohydrate (0.100 g, 2.05
mmol) in methanol and resulting solution was refuxed for 10-
15 min. and then stirred for 4-6 hours at room temperature to
The solvents were distilled and dried by standard procedures
prior to their use. Reagents and solvents used in experiment
were purchased from Avra Synthesis Pvt. Ltd., Merck
Specialities Private Limited and S. D. Fine Chem. Ltd, Mumbai,
India and used as received without further purification.
complete the reaction. It afforded
a yellow crystalline
compound which was washed with methanol, hexane and
diethyl ether and dried in vacuum. Yield 0.400 g (87%). Anal.
calcd for C14H12N2O2: C, 69.99; H, 5.03; N, 11.66. Found C,
69.87; H, 5.11; N, 11.73. 1H NMR (CDCl3, 500 MHz, δH, ppm)
6.952 (t, 2H, Ar), 7.0 (d, 2H, Ar), 7.3 (d, 2H, Ar), 7.4(t, 2H, Ar),
Salicylaldehyde,
benzaldehyde,
m-hydroxy
p-methoxy
benzaldehyde,
benzaldehyde
p-hydroxy
and p-
phenylenediamine were purchased from Avra Synthesis Pvt.
Ltd., Hyderabad, India. Hydrazine monohydrate and Ethylene
diamine was purchased from Merck Specialities Private Limited
and S. D. Fine Chem. Ltd, Mumbai, India respectively. All the
compounds were synthesized by slight modification of the
literature procedures.11,12,13,14,15 Elemental analyses were
acquired on an Exeter CHN Analyser CE-440. FT-IR and UV-vis
study of ligands, dried solutions and xerogel were done on
PerkinElmer- spectrum 2 and Thermo scientific EVOLUTION
200 spectrophotometer, respectively. Photoluminescence
8.7 (s, 2H,
241.09 (calcd. 241.09). Liquid state IR (CHCl3, cm-1)
N)strech 1618. UV-vis spectrum (chloroform) [ , nm (
, M-1
cm-1)]: 295 (25800), 363 (20700).
=CH) and 11.4 (s, 2H,−
OH). m/z, ESI-MS: [M+H]+,
υ
(C=
λ
ε
bis(4-hydroxybenzylidene)hydrazine (H2PBA) 2: Methanolic
solution of 4-hydroxybenzaldehyde (0.501 g, 4.10 mmol) was
added drop wise to a solution of Hydrazine monohydrate
(0.100 g, 2.05 mmol) in methanol and resulting solution was
stirred for 4-6 hours to complete the reaction. It afforded a
yellow precipitate by in vacuo solvent removal which was
washed with hexane and diethyl ether and dried in vacuum.
Yield 0.377 g (83%). Anal. calcd for C14H12N2O2: C, 69.99; H,
5.03; N, 11.66. Found C, 70.11; H, 5.07; N, 11.34. 1H NMR
(DMSO-d6, 500 MHz, δH, ppm) 6.8 (d, 2H, Ar), 7.7 (d, 2H, Ar),
spectra were acquired on
a
Perkin Elmer LS 55
spectrophotometer. The lifetime measurements were made
using a time-correlated single photon counting (TCSPC) system
from Horiba Yovin (Model: Delta Flex) and data analysis was
performed using EzTime (HORIBA Scientific) decay analysis
software (ESI†). SEM images were captured by Carl Zeiss
EVO/18 Research 2045 and Oxford Instruments 5IN1000EDS,
8.5 (s, 2H,
Liquid state IR (CHCl3/DMSO, cm-1), υas
spectrum (CHCl3/DMSO, 4:1) [ , nm (
, M-1 cm-1)]: 323 (19800).
=
CH). m/z, ESI-MS: [M+H]+, 241.09 (calcd. 241.09).
N)strech 1606. UV-vis
respectively. Powder XRD data was collected on Rigaku
1
SmartLab between angle 2θ
obtained on a Bruker AVANCE III HD 500 spectrometer.
= 10–40o. H NMR spectra were
(C=
λ
ε
X-ray Crystallography:
bis(3-hydroxybenzylidene)hydrazine (H2MBA) 3: Methanolic
solution of 3-hydroxybenzaldehyde (0.501 g, 4.10 mmol) was
added drop wise to a solution of Hydrazine (0.100 g, 2.05
mmol) in methanol and resulting solution was stirred for 4-6
hours to complete the reaction. Then this reaction mixture was
filterated and kept for crystallization. It afforded a shiny yellow
crystals in 24 hrs which were washed with Chloroform and
ether and dried in vacuum. Yield 240 mg (82%). Anal. calcd for
C14H12N2O2: C, 69.99; H, 5.03; N, 11.66. Found C, 70.14; H,
4.99; N, 11.39. 1H NMR (CDCl3, 500MHz, δH, ppm) 6.9 (d, 2H,
Ar), 7.3 (m, 6H, Ar), 8.6 (s, 2H, =CH), 9.7 (s, 2H, -OH). m/z, ESI-
Single crystals of C7H7NO (H2SA) were obtained from the
reaction mixture of
1 (chloroform) with CsOH (methanol). A
suitable crystal was selected and mounted on glass fibre on a
SuperNova, Single source at offset/far, HyPix3000
diffractometer. The crystal was kept at 293 K during data
collection. Using Olex2 the structure was solved with the
ShelXT structure solution program using Intrinsic Phasing and
refined with the olex2.refine refinement package using Gauss-
Newton minimisation.16,17,18
Rheological Study:
MS: [M+H]+, 241.09 (calcd. 241.09). Liquid state IR (CHCl3, cm-
1
)
υas
(C
=
N)strech 1666. UV-vis spectrum (CHCl3) [
λ
, nm (
ε
, M-1
Measurements were performed using
a stress-controlled
cm-1)]: 303 nm (18300).
Rheometer (Anton Paar TWINE Rheometer MCR 702)
equipped with stainless steel parallel plates (20 mm diameter,
1.0 mm gap). Experiments were carried out on freshly
N,N’-bis(4-methoxybenzylidene)hydrazine
(PMO)
4:
Methanolic solution of 4-methoxybenzaldehyde (0.557 g, 4.10
2 | J. Name., 2012, 00, 1-3
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mmol) was added drop wise to a solution of Hydrazine (0.100 N,O metal binding site and the importance of middle core (N-N
g, 2.05 mmol) in methanol and resulting solution was stirred bond) of towards the origin of fluorescence (CHEF) as well as
1
for 4-6 hours to complete the reaction. It afforded a yellow gelation. Thus, herein, we present a fluorescent metallogel
compound which was washed with methanol, hexane and obtained from the mixture of non-fluorescent components
diethyl ether and dried in vacuum. Yield 0.423g (79%). Anal. H2SA (1) and LiOH, exhibiting interesting optical, morphological
calcd for C14H12N2O2: C, 71.62; H, 6.01; N, 10.44. Found C, and rheological properties. The ligand H2SA (
1
),11 and its
71.24; H, 5.98; N, 10.37
3.8 (s, 6H, -CH3), 6.9 (d, 4H, Ar), 7.7 (d, 4H, Ar), 8.6 (s, 2H,
m/z, ESI-MS: [M+H]+, 269.013 (calcd. 269.13). Liquid state IR
(CHCl3, cm-1) υas
N)strech 1666. UV-vis spectrum (CHCl3) [
nm (
, M-1 cm-1)]: 330 (19500).
.
1H NMR (CDCl3, 500MHz, δH, ppm)
OH
HO
OH
N
=
CH).
N
N
N
N
N
HO
OH
(C
=
λ,
2 (H PBA)
1(H SA)
2
3 (H MBA)
2
HO
2
OH
ε
HO
OMe
N, N’-bis(Salicylidene)Ethylenediamine (H2SEA) 5: Methanolic
solution of 2-hydroxybenzaldehyde (0.488 g, 4 mmol) was
added drop wise to a solution of Ethylene diamine (0.120 g, 2
mmol) in methanol and resulting solution was stirred for 4-6
hours to complete the reaction. It afforded a shiny yellow
compound which was washed with methanol, hexane and
N
N
N
N
N
N
OH 5 (H SEA)
MeO
OH
6 (H SPA)
2
4 (PMO)
2
Scheme 1: Structures of ligand 1
6) along with their abbreviations.
and corresponding model ligands (2, 3, 4, 5 and
two regioisomers H2PBA (
2
),12 H2MBA (
),11 PMO (
3
),13 as well as other
),15 H2SEA ( ),14 were
diethyl ether and dried in vacuum. Yield 0.334g (83%). Anal.
calcd for C14H12N2O2: C, 71.62; H, 6.01; N, 10.44. Found C,
71.82; H, 5.98; N, 10.35. 1H NMR (CDCl3, 500MHz, δH, ppm) 3.9
model compounds H2SPA (
6
4
5
synthesized in good yield (80-90 %) with slight modification in
literature procedures. The characterization data was found to
be in full agreement with their proposed formulations (Scheme
(t, 4H,
(t, 2H, Ar), 8.3 (s, 2H,
H]−, 267.11 (calcd. 267.11). Liquid state IR (CHCl3,cm-1)
N)strech 1620 UV-vis spectrum (CHCl3) [ , nm (
, M-1 cm-
1)]: 257 (27000), 319 (10500).
−
CH), 6.8 (t, 2H, Ar), 6.9 (d, 2H, Ar), 7.2 (d, 2H, Ar), 7.3
=
CH), 13.2 (s, 2H, OH). m/z, ESI-MS: [M-
1, S2 and experimental secꢀon, ESI†). Ligand
1
(~4x10-2 M) was
dissolved in CHCl3 followed by addition of methanolic solution
of LiOH (~8x10-2 M) turned into an opaque green-blue
fluorescent gel (1%, w/v) instantly at ambient room
temperature (Fig. 1, S1 and S3, ESI†). The gelaꢀon was
preliminarily confirmed by conventional inverted vial test and
further supported by falling ball method (Fig. S2, ESI†).
υ
(C
=
.
λ
ε
2,2'-[1,4-phenylenebis{(E)-nitrilomethylidyne}]bisphenol
(H2SPA) 6: Methanolic solution of 2-hydroxybenzaldehyde
(0.226 g, 1.84 mmol) was added drop wise to a solution of p-
phenylenediamine (0.100 g, 0.92 mmol) in methanol and
resulting solution was stirred for 4-6 hours to complete the
reaction. It afforded a shiny yellow compound which was
washed with methanol, hexane and diethyl ether and dried in
vacuum. Yield 0.240 g (82%). Anal. calcd for C14H12N2O2: C,
Further, optimization of gelation with
1 and methanolic LiOH
in other common solvents leads to only suspension or solution
rather than metallogel formation under aforementioned
similar condiꢀons (Table S1, ESI†). The other alkali bases like
NaOH, KOH and CsOH also failed to produce metallogel under
aforesaid similar conditions, which indicates the selectivity of
ionic radius of Li+ ion towards gelation (Table S2, Fig. S1,
ESI†).10 In addition, the role of higher solvation ability of Li+
compared to other alkali metal ions towards gelation cannot
be ruled out. To ensure about the role of alkali metal ion in
gelation, we performed the gelation test with strong bases like
tetrabutylammonium hydroxide (TBAOH) and triethyl amine
(Et3N), in turn, obtained a clear non-fluorescent solution which
proves the importance of alkali base (Fig. S1, ESI†).
75.93; H, 5.10; N, 8.86. Found C, 75.24; H, 5.13; N, 8.75
NMR (CDCl3, 500MHz, δH, ppm) 6.9 (t, 2H, Ar), 7.0 (d, 2H, Ar),
~7.4 (s, 4H, Ar), 7.4 (t, 2H, Ar), 8.7 (s, 2H, CH), 13.2 (s, 2H,
OH). m/z, ESI-MS: [M-H]-, 315.11 (calcd. 315.11). Liquid state
IR (CHCl3, cm-1)
N)strech 1606 UV-vis spectrum (CHCl3) [
nm (
, M-1 cm-1)]: 274 (16150), 370 (25000).
.
1H
=
υ
(
C=
.
λ,
ε
Results and discussion
In order to monitor the role of L-tartaric acid based flexible
Interestingly, regioisomer
upon addition of LiOH and forms notable orange-yellow
fluorescent solution under similar gelation conditions to
indicating the importance of the position of –OH group in gel
formation. Further, position isomer , supporting ligand and
structurally flexible ligand afforded non fluorescent solution
while the stiff imparts the fluorescent suspension at the
gelation conditions to . The gelation ability of compounds
and in other solvents with methanolic LiOH has
2 could not pass the gelation test
chiral core in gelation as well as its contribution towards the
gel properties,10a herein, we synthesized the smallest possible
1
,
symmetrical aromatic low molecular weight ligand H2SA (1)
(Scheme 1, Schemes S1 and S2 ESI†). The new ligand molecule
contains two N, O donor metal chelation sites, and it is
expected to achieve planar structure upon chelation which
provides a platform for growth of building block/aggregation
via π−π stacking followed by formation of CHEF based
3
4
5
6
1
1,
metallogel.19 Thus, it is worth mentioning that ligand
1 is
2,
3
,
4
,
5
6
been summarized in table S1 and figure S3 (ESI†).
rationally devoid of any well-known traditional gelating units
like long hydrophobic chain, amide, pseudo amide or other
linkages.4,5 Further, we synthesized some other model
The above anomalous facts directly associated with the
position of –OH group and type of central core (spacer);
attracted our attention towards ligands being probable good
candidates for the study of optical as well as aggregation
compounds like H2PBA (
2), H2MBA (3), PMO (4), H2SEA (5) and
H2SPA (6) with an objective to demonstrate the necessity of
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Figure 1: Responses of 1 upon addition of two equivalents of (a) LiOH, fluorescent gel; (b) NaOH, fluorescent sol; (c) KOH, non fluorescent sol and (d) CsOH, non-
fluorescent sol. The corresponding models (a’ to d’) represent the possible molecular arrangement in respective gel or solution.
to 200 nm, while a turbid solution of
1
/Na+ reveals broken
/K+
fibrous aggregates. Further, the clear solution of
demonstrates the crystals along with random aggregates,
while combination of
/Cs+ turn out single crystal growth. The
1
1
morphological tuning from long range fibres to crystals with a
sequential increment in size of alkali metal ions clearly
indicates that the gelation is strongly dependent on the size of
alkali metal ions10,6b-c and this type of morphological variation
has not been reported particularly with ultra-LMWG systems.
In order to support distinct morphological features,
powder X-ray diffraction pattern was analyzed between
2
θ
range of 10-40o over various combinations of xerogel/dried
sols and found that loses its crystalline property and
transforms into an amorphous material upon gelation ( ;
Li+
1
1/
xerogel) as it is evident from broadening of peaks (Fig. 4). The
crystallization of the Li+ as LiOH in an insignificant fraction
during drying cannot be ruled out (Fig. 4). Further, appearance
of sharp peaks in case of
crystalline nature which is comparable with peak patterns of
1/ 1/
Na+ and K+ signifies the
1.
The observation may be correlated with the presence of very
weak electrostatic interaction between N, O donor and Na+ or
K+ compared to Li+, indicates crystallization of
1 without alkali
metal ion. Pattern has no significant change in NaOH to KOH
and CsOH which may be due to reversible binding of alkali
Figure 2. Multi-stimuli responses shown by gel under UV light irradiation,
ultrasound, mechanical and temperature. Note that heating of gel simply
produces xerogel followed by gel in presence of solvent.
metal ion to the ligand
right by structural and crystallographic parameters
determination of crystal obtained from the solution of /CsOH
which is in full agreement with the parameters reported for
hence signifying the crystallization of without alkali metal
ion.19 Furthermore, to verify the crystallization of
without
Cs+, we performed PXRD on bulk crystals of Cs+ which
matches nicely with simulated PXRD pattern of
(Fig. 4). 11a, 20
In addition, 1H NMR of crystals dissolved in CDCl3 confirms the
presence of phenolic protons. Further, pKa of phenolic protons
could not be estimated due to poor solubility in water even in
1. Further, the assumption was proved
1
properties. The freshly prepared gel demonstrated the stimuli
response towards mechanical (shaking) or ultrasound (30 kHz)
in a gel-sol reversible manner as well as the notable reswelling
property as demonstrated in figure 2.
1,
1
1
1/
Morphology of vacuum dried metallogel and effect of
other alkali metal ions obstructing gelation were monitored by
SEM (Fig. 3 and S4, ESI†). Xerogel (1
/Li+) shows long range
1
fibrous network with an average diameter in the range of 100
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A
B
c
D
C‘
Figure 3: SEM images of xerogel and dried solutions of (A) 1/Li+, (B) 1/Na+, (C) 1/K+ and (D) 1/Cs+ showing the fibre, broken fibre, crystalline morphology and crystals, respectively.
(c) Magnified image of C reveals the shape is similar to natural flower.
A
KOH
(101)
1+CsOH
B
K CO
2
3
1 (simulated)
(103)
LiOH
(101)
(212)
(212)
(213)
(102)
1+CsOH
1+KOH
10
20
30
40
B
1+NaOH
θ
2
(degree)
C
O7
(212)
(212)
(101)
(102)
(103)
(101)
(101)
Xerogel (1+LiOH)
H7
N9
(213)
(213)
(102)
(103)
ligand (1)(experimental)
1 (simulated)
(101)
N9
H7
O7
10
20
30
40
θ
2
(degree)
Figure 4: Powder X-ray Diffraction pattern of (
line) and /CsOH (green line), respectively. (B) Comparision between and
drawn using 35% ellipsoids probability and hydrogen bonding between –OH and N atom.
A
)
1
(black line, experimental; blue dotted line, simulated), xerogel (orange line), gelator
1/NaOH (red line), 1/KOH (blue
1
1
(silmulated) and crystals obtained from /CsOH solution and (C) ORTEP view of gelator 1
1
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0.3
A
A
60
40
20
0
1
1.0
0.6
0.2
a
+
1/Li
0.2
450
550
wavelength
650
1
0.1
+
1/Li
650
450
600
500 550
wavelength (nm)
250
300
350
400
450
500
B
400
300
200
100
0
wavelength (nm)
2
+
1
2/Li
B
0.3
0.2
0.1
+
1/Li
+
1/Na
+
1/ K
+
1/ Cs
500
550
wavelength (nm)
450
600
650
C
6
+
300
350 400
wavelength (nm)
450
500
1/Li
+
1/Na
4
2
0
+
1/ K
Figure 5: (A) UV-vis titration spectra of 1 (1x10-5 M, CHCl3, red line) vs. LiOH (1x10-3 M,
CH3OH, black dotted and blue) and (B) A comparative UV-vis spectrum of 1 (1x10-5 M,
CHCl3, red line) vs. LiOH (blue line), NaOH (green line), KOH (black) and CsOH (magenta
line).
+
1/ Cs
presence of DMF-water mixture also. However, the presence
of minor impurities likes hydroxides and carbonates of
respective alkali metal cations can not be ruled out (Fig. 4).
Among the prominent peaks in PXRD pattern of dried
metallogel, the presence of four periodic diffraction peaks at
400
650
450
500 550
wavelength (nm)
600
+
4
t= 1.277ns(1+Li
)
10
D
+
t= 1.221ns(1+Na
+
t= 2.167ns(2+Li
)
3
)
2θ = 11.3, 22.5, 33.9 and 45.8 were analyzed and
+
10
t= 0.331ns(6+Li
)
corresponding d-values found to be 7.8, 3.9, 2.6 and 1.9 nm,
respectively, which followed a ratio of d : d/2 : d/3 : d/4 (Fig.
S5, ESI†). The periodic pattern of interspacing suggests the
layered organization within aggregate of metallogel and the
interlayer distance was found to be 7.8 nm.21
2
10
1
10
UV-vis experiments were performed to monitor the optical
behaviour of 1
with respect to the alkali metal ions viz; Li+, Na+,
K+ and Cs+ (Fig. 5 and S6, ESI†). A light yellow coloured solution
0
10
30
40
50
Time (ns)
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Figure 6: (A) Fluorescence titration spectrum of
1
(
λ
ex= 363 nm, 1x10-3 M, CHCl3,
2 (λ
ex= 323 nm, 1x10-3 M,
red line) vs. LiOH (1x10-1 M, MeOH, black dotted and blue line), (b) Inset of B
due to two well known phenomenon viz. excited-state
intramolecular proton-transfer (ESIPT) and chelation enhanced
fluorescence (CHEF), respectively.7a,23 Among the reported
examples, nearly all ESIPT-based molecules exhibit emission in
the visible region (400–650 nm) with large Stokes shift (8500 -
10500 cm–1) and upon deprotonation of phenolic proton
followed by metal binding inhibits the ESIPT permanently.24a-d
shows initial change; (B) Fluorescence titration of
CHCl3) vs. LiOH (1x10-1 M, MeOH, black dotted line and blue line) under similar
conditions to A; (C) Fluorescence spectrum demonstrated the effect of size of
alkali metal ions over
1 and (D) Fluorescence lifetime plot (τav) of 1 1
/Li+, /Na+,
2
/Li+, /Li+ shown in red, green, blue and yellow lines, respectively.
6
of
20700 M-1cm-1) and 295 nm (
π∗ and π* transitions, respectively.20c,22 Further, upon
aliquot addition of LiOH (1x10-3 M, MeOH) to solution of
1
(1x10-5 M; CHCl3) displayed two sharp bands at 363 nm (
, 25800 M-1 cm-1) attributed to
ε,
ε
n
−
π−
Further, upon NaOH addition to
1
also leads to enhancement
= 30
1,
in intensity but with rather lesser blue shift at 504 nm (Δ
λ
concomitant decrease of band 363 nm and appearance of new
band at 414 nm along with a broad hump at 390 through an
isosbestic point at 386 nm was observed which suggests the
deprotonation of phenolic proton as well as chelation of Li+
nm, Stokes shift= 7700 cm–1) indicates the weaker interaction
than Li+ (vide supra, Fig. S9, ESI†).
Moreover, addition of bigger size alkali metal ion K+ or Cs+
to
504 nm (Δλ= 30 nm) and 519 nm (Δλ= 15 nm), respectively, but
with very weaker emission intensity than 1/Li+ and 1/Na+
suggesting weak interaction with
compare to Li+ or Na+ (Fig.
S9, ESI†). In extension, TBAOH could not produce any
significant change in emission spectra of (Fig. S10, ESI†).
1, the ensuing entity 1/ 1/
K+ or Cs+ exhibited blue shift at
with
1 in N,O fashion. In addition, band at 295 nm
hypochromically blue shifted to 286 nm (Δλ= 11 nm) due to
,
the origin of π−π stacking interaction. On the other hand,
1
addition of NaOH to
1 under aforementioned similar
conditions also showed the simultaneous transformation of
363 nm band into 401 nm through an isosbestic point at
377nm, while band at 295 nm showed only a hypochromic
shift. Similarly, 1/K+ and 1/Cs+ demonstrated the conversion of
363 nm band into 414 and 417 nm, respectively, through an
isosbestic point at ~383 nm. Remarkably, the band at 295 nm
did not offer any significant shift except the decrease in
absorbance at 295 nm only upon addition of K+ or Cs+ ions
1
Thus, the successive decrease in blue shift and emission
intensity with respect to increment in size of alkali metal ions
exclusively support the observation made in UV-vis and
morphology.
To probe CHEF phenomenon, we synthesized regioisomer
2
by shifting –OH group from ortho to para position with an
objective to obstruct the chelation site. Surprisingly, weak
which was due to lack of
π
-
π
stacking in solution (vide infra).
with Li+ relative to
fluorescent solution of
2
(λex = 323 nm, λem = 530 nm, 1x10-3
The difference in spectral behaviour of
1
M, CHCl3/DMSO, 4:1) turned into highly emissive upon
addition of LiOH (1x10-1M, CH3OH) and exhibited red shifted
Na+, K+ and Cs+ ions due to difference in size of alkali metal
ions or the interaction/suitability to fit in the N,O binding site.
In other words, Li+ only showed the ability to make effective
band at 565 nm (ꢀλ= 35 nm) with notable large Stokes shift of
13300 cm–1, albeit, in the absence of chelation site. At this
stage, we assumed that the phenomenon of intramolecular
charge transfer (ICT) occurs from the lone pair of N to para O-
Li+ in a synergic or push-pull manner which is supported by
aforementioned large Stokes shift (Fig. 7).25 Further to justify
our assumption, we prepared meta positioned –OH
interaction with
1
and helped in formation of gel, while Na+, K+
and Cs+ turned out as solutions which were well supported by
their morphology and PXRD pattern. Further, bigger size non-
alkali strong base TBAOH also showed spectral changes similar
to Cs+ while weak bases like NEt3 or NH3 failed to initiate
changes suggests the vital role of strength of base as well as
size of alkali metal ion in gelation (Fig. S7, ESI†). Upon aliquot
addition of LiOH under aforesaid gelation conditions,
regioisomer
3 with an intention to inhibit ICT due to the
absence of conjugation which obstruct the flow of charge from
imine N to –O-Li+ (Fig. 7). Moreover, our strategy worked well
regioisomer
2 as well as other supporting ligands 5 and 6
and
CHCl3) upon addition of LiOH under similar conditions to
3
remained non-fluorescent (λex = 303 nm, 1x10-3 M,
exhibited almost similar kind of concomitant conversion of
their one band 323/319/370 nm into other new 380/337/397
nm through isosbestic point at 347/324/387 nm, respectively
2. In
addition, the push-pull or synergic mechanism behind ICT is
also attested by addition of LiOH to the solution of para –OMe
(Fig. S8, ESI†). Regioisomer
3 and auxiliary ligand 4 produce
containing
4
(λex = 330 nm, 1x10-3 M, CHCl3), in turn, exhibited
almost unchanged spectra with their λmax at 303 nm and 330
nm, respectively (Fig. S8, ESI†). Thus, it is expected that LiOH
first deprotonates the –OH and then Li+ interacts with O or N,
O donor atoms via electrostatic interaction.
lack of emission (Fig. S11, ESI†).
In order to elucidate the Li+ triggered ICT generation in
2,
we performed titration between ICT based complex 2 =
/Li+ (λex
323 nm, 1x10-3 M, CHCl3/DMSO/CH3OH) and [12]-crown-4
ether (1x10-1 M, CH3OH) (Fig. 8, ESI†). Aliquot addition of [12]-
The fluorescence experiments were executed in order to
scrutinize the selectivity of ligand
over other alkali metal ions or other rationally structurally
close and ligands.
ex= 363 nm, 1x10-3 M, CHCl3)
1
and Li+ towards gelation
crown-4 to fluorescent solution of 2
/Li+ results in decrease in
intensity of 565 nm band and concomitant appearance of band
at 535 nm corresponding to ligand 2 through an isosbestic
point at 490 nm. Further, a plot between equivalent of [12]-
crown-4 added and emission intensity at 565 nm shows that
two equivalent [12]-crown-4 is required for entire trapping of
2
,
3,
4,
5
6
1 (λ
displayed very weak emission band at 534 nm (Stokes shift=
~8800 cm–1),5d but upon subsequent addition of methanolic
LiOH leads to appearance of the blue shifted (Δλ= 77 nm) band
at 457 nm with notable large (~1000 fold) increment in
emission intensity (Fig. 6). The almost non emissive
Li+ ions from the
2
/Li+ complex. Thus, generation of non
fluorescent ligand 2 from highly fluorescent solution of 2/Li+
advocated the role of Li+ in ICT generation.10b
characteristic of
1 and highly emissive nature of 1
/Li+ may be
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Figure 7: Sketch and model diagrams represents mechanism- 1) N, O donor ligand
1
in protonated form active for ESIPT and Li+ interaction trigger the blue green
produced yellow fluorescent
and treatment with LiOH shows non-fluorescent solution due to disturbance in conjugation proves
also supports the presence of ICT in
/Li+. 5) Ethylene moiety was introduced as a spacer to disturb ICT and the
effect of CHEF due to high degree of free rotation. 6) Again to prove ESIPT in neutral form and CHEF with Li+ complexation, a benzene ring was introduced as a
spacer, in turn,
/Li+ shows yellow fluorescent precipitate.
fluorescence followed by metallogel formation due to CHEF and aggregation, respectively. 2) Movement of –OH to para position in
2
precipitate with Li+ due to ICT. 3) Reshuffling of –OH to meta in
the presence of ICT in . 4) Para –Ome containing
3
2
4
2
6
radiationless deactivation from its excited state.24e-g Further, to
ascertain the CHEF phenomenon and role of middle core in the
appearance of fluorescence, we synthesized structurally rigid
The non-fluorescent nature of
5 (λ
ex= 319 nm, 1x10-3 M,
CHCl3) remains unchanged upon addition of methanolic LiOH
except minor enhancement in emission intensity, albeit, it is
fully decorated with two chelation sites (Scheme 1 and Fig.
conjugation enriched ligand
ex= 370 nm, 1x10-3 M, CHCl3, Stokes
may be due to the flexible middle core which restricts itself shift= ~6200 cm–1) upon addition of LiOH (Fig. S11, ESI†). It is
from achieving rigid structure followed by the enhancement in worth mentioning that forms precipitate rather than gel
6, and as expected; it exhibited
S11, ESI†).25a The lack of fluorescence even after Li+ chelation emission band at 481 nm (
λ
6
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unlike , notwithstanding both of which are rationally close
ARTICLE
800
-2
Gel (~10 M)
-3
10
1
M
M
-4
10
systems in terms of binding sites, rigidity and fluorescence
behaviour possibly due to less planar structure.26 Average
600
lifetimes determined for the
found to be = 1.277, 1.221, 2.167 and 0.331 ns, respectively
(Fig. and lifeꢀme measurement secꢀon, ESI†). The
aforementioned different fluorescence to considerable
1 1 2 6
/Li+, /Na+, /Li+ and /Li+ and
τ
400
200
6
a
decrease in the energy gap between the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO), resulting in a red shift in emission spectra (Fig.
S11, ESI†).27 Thus, the small change in middle core and
position of –OH affects the gelation and fluorescence
properties.
0
500
wavelength (nm)
600
400
450
550
650
To achieve better mechanistic insights of gelation, a
dilution experiment of freshly prepared metallogel was carried
out within range of 10-2M to 10-4M. A 2.3 fold enhancement in
fluorescence intensity with a significant blue shift of 25 nm
was observed within dilution range 10-2 -10-3M, due to the
presence of Aggregation Caused Quenching (ACQ)
phenomenon. Further dilution (10-3-10-4M) leads to complete
quenching of intensity due to excessive dilution of 1/Li+ species
(Fig. 9). Further, variable temperature experiments were
performed to demonstrate the reswelling nature of metallogel
(Fig. S12, ESI†). Upon heating up to 70oC, a significant decrease
(three fold) in the emission intensity was observed due to
vaporization of the solvent from gel network followed by
xerogel formation. Further, in situ addition of a mixture of
CHCl3 and methanol (4:1) solvents to the xerogel, fluorescence
intensity was recovered up to ~85% of freshly prepared gel
which supports the observation made about the reswelling
Figure 9: (A) Effect of dilution (with CHCl3) on fluorescence spectra of metallogel
1
complete quenching from 10-3 to 10-4M.
/Li+ ex= 363 nm, ~10-2 M; CHCl3/CH3OH, red line) displayed two step spectral
(λ
changes i.e., 10-2 to 10-3M fluorescence intensity enhancement followed by
To probe the weak interactions involved in gelation
1
process, we performed H NMR titration experiment between
1
and LiOH under similar gelation conditions in CDCl3/CD3OD
(Fig. S14, ESI†). 1H NMR spectrum of
showed labile –OH
protons appears at = 11.4 ppm, aldimine protons at 8.72 ppm
and aromatic protons between 7.45 to 7.0 ppm. Upon aliquot
addition of CD3OD solution of LiOH to , it was observed that
1
δ
1
the slightly higher degree of upfield shifting (ꢀδ= 0.1 ppm) for
aldimine protons (-N=CH) as well as disappearance of –OH
protons signifies the chelation of Li+ to ‘–CH=N’ and phenolate
site. The broadening of peaks upon further addition of LiOH (2
equiv.) may be due to aggregation. The noticeable upfield shift
of other aromatic protons due to π-π interactions during
gelation cannot be ignored in titration experiment. Thus,
nature of metallogel (Fig. 2 and S12, ESI†).
conclusion drawn based on experimental results indicates that
the chelation of Li+ as well as
π-π staking factors are mainly
responsible for gel formation as illustrated in figure S15 (ESI†).
On the other hand, a detailed mechanism behind appearance
of fluorescence as well as possible molecular arrangement in
solution of various other supporting ligands is depicted in
figure 7.
+
400
300
200
100
0
2/Li
+
2/Li +[12]-crown-4
400
300
200
100
0
In order to support the true gel phase material, detailed
dynamic rheological experiments were carried out with freshly
prepared gel (1%, w/v) and results demonstrated in figure 10.
Stress sweep experiment (oscillation frequency 1Hz at 25°C)
shows that storage modulus (G′) is found to be approximately
0.0 2.0
equivalent of [12]-crown-4
1.5
1.0
2.5
0.5
1 order higher than that of loss modulus (Gʺ) up to 1.5 Pa,
above which deformation takes place and both the moduli
cross each other at a certain point of 2.3 Pa which indicates
the soft solid like characteristic of 1
/Li+ containing metallogel.
In strain sweep experiment, at constant oscillation frequency
1Hz, gel was stable up to ~10% strain with negligible effect
over elasticity but further strain leads to the phase transition
from gel to sol phase. Dynamic frequency sweep measurement
500
550
wavelength (nm)
600
450
650
Figure 8: Fluorescence titration between 2
/Li+ (λex = 323nm, λem = 565nm, 1x10-
3M, CHCl3/MeOH) and [12]-crown-4 (1x10-1M, CH3OH). Inset plot between
fluorescence intensity (at 565nm) and equivalent of crown ether ([12]-crown-4)
added. The strong band at 646 nm is the twofold overtone wavelength for the
excitation wavelength (323 nm).
shows that G′ and G
long range
0.75- 1.6 rad sec-1) which supports the
predominant elastic nature. The G′ and have been
measured in the range 20–90 C at constant frequency 1 Hz
ʺ are linearly parallel to each other up to
(−
G
ʺ
°
strain. The metallogel shows nearly unchanged viscoelastic
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mechanical and ultrasound. Metallogel demonstrated the
alkali metal size dependent morphological variance (fibre to
A
-1
)
B
log(Frequency/ rad sec
-0.5 0.0 0.5 1.0 1.5
4.0
3.5
3.0
2.5
crystal) as well as fluorescence properties. CHEF and
π-π
4.0
4.0
3.5
3.0
2.5
stacking played crucial role in appearance of fluorescence and
metallogel formation. Moreover, the slight variation in spacer
and functional groups in such small systems result into
3.5
3.0
2.5
gel/sol/precipitate
and
corresponding
interesting
phenomenon of ESIPT-CHEF to ICT. True gel phase material
was attested by rheological studies. The present gel may find
application in designing of other lower molecular weight gel
and can be used for metal ions sensing.
-0.5 0.0 0.5 1.0 1.5
log(Frequency/ rad sec
-4
-3
log (Stress / %)
-2
-1
-1
)
6.2
D
C
4.0
3.5
3.0
2.5
6.0
5.8
5.6
5.4
5.2
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
M. Dubey acknowledges the Department of Science and
Technology, New Delhi, India, for financial support under DST-
INSPIRE Faculty award. MKD acknowledges IIT (BHU) for MHRD
fellowship. We thank SIC IIT Indore, Dr. S. C. Sahoo (PU
Chandigarh) and Anton Paar, Gurugram for extending the
instrumental facilities.
30
50 70
Temperature (°C)
90
0.0 0.5 1.0 1.5 2.0 2.5
log (Strain / %)
4.0
3.6
3.2
2.8
0.90
0.85
0.80
0.75
0.70
F
E
Notes and References
1
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50 70
Temperature (°C)
90
20 30 40 50 60 70
Temperature (°C)
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at 25 °C, (B) Primary axis: Dynamic frequency sweep measurements of G′ and G
at strain of 0.5%. Secondary axis: complex viscosity measurements, (C) Dynamic
oscillation strain sweep of G′ and G
at frequency of 10 rad s-1 and temperature
at 25 C, (D) Dynamic temperature ramp of comp lex viscosity measurement at 5
°C min-1, (E) Dynamic temperature ramp G′ and G at a heating rate of 5°C min-1
strain of 0.5 % and frequency of 10 rad s-1 and (F) Dynamic temperature ramp of
the loss tangent (tan = G /G′) plot at 5
C min-1 which indicates the phase
transition temperature (Tgel) at ~70 C.
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ʺ
°
ʺ
,
δ
ʺ
°
3
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δ
= Gʺ/G′) and temperature, which confirms the Tgel at 50°C.
Further, complex viscosity on temperature ramp also supports
the aforesaid critical temperature of gel. Thus, the rheological
characterization of present gel suggests the formation of a
typical ‘soft-solid like’ gel phase material.28
4
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Conclusions
In conclusion, through this work we have described the
synthesis and properties of fluorescent metallogel obtained
from non-fluorescent constituents viz. an ultra low molecular
weight symmetrical ligand and LiOH. Metallogel showed multi-
stimuli responsive behaviour towards temperature,
10 | J. Name., 2012, 00, 1-3
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ARTICLE
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