Naphthalene-cholesterol conjugate as simple gelator for selective sensing of CN– ion

Article abstract of DOI:10.1080/10610278.2016.1236926

Cholesterol-based Schiff base 1 has been designed and synthesised. The Schiff base 1 forms yellow coloured gel in DMF:H₂O (2:1, v/v) and the gel is anion responsive. Among different anions, the gel phase of 1 is selectively transformed into sol in the presence of CN^– ions and validates its visual sensing. ¹H NMR, FTIR and HRMS spectroscopic techniques were adopted to study the gelation of 1 and its responsive behaviour towards anions.

Full text of DOI:10.1080/10610278.2016.1236926

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Naphthalene-cholesterol conjugate as simple
–
gelator for selective sensing of CN ion
Kumaresh Ghosh & Santanu Panja
To cite this article: Kumaresh Ghosh & Santanu Panja (2016): Naphthalene-cholesterol
–
conjugate as simple gelator for selective sensing of CN ion, Supramolecular Chemistry, DOI:
10.1080/10610278.2016.1236926
To link to this article: http://dx.doi.org/10.1080/10610278.2016.1236926
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Date: 19 October 2016, At: 09:38

Supramolecular chemiStry, 2016
ꢁꢁꢂ://dx.dꢃꢄ.ꢃꢅg/10.1080/10610278.2016.1236926
ꢀ
Naphthalene-cholesterol conjugate as simple gelator for selective sensing of
–
CN ion
Kumaresh Ghosh and Santanu Panja
Dꢆꢂꢇꢅꢁꢈꢆnꢁ ꢃf cꢀꢆꢈꢄsꢁꢅꢉ, unꢄvꢆꢅsꢄꢁꢉ ꢃf Kꢇꢊꢉꢇnꢄ, Kꢇꢊꢉꢇnꢄ, indꢄꢇ
ABSTRACT
ARTICLE HISTORY
Cholesterol-based Schiff base 1 has been designed and synthesised. The Schiff base 1 forms yellow
rꢆꢋꢆꢄvꢆd 20 Jꢌꢊꢉ 2016
aꢋꢋꢆꢂꢁꢆd 11 Sꢆꢂꢁꢆꢈbꢆꢅ 2016
coloured gel in DMF:H O (2:1, v/v) and the gel is anion responsive. Among different anions, the gel
2
–
phase of 1 is selectively transformed into sol in the presence of CN ions and validates its visual
sensing. H NMR, FTIR and HRMS spectroscopic techniques were adopted to study the gelation of 1
1
KEYWORDS
cꢀꢃꢊꢆsꢁꢆꢅꢃꢊ; ꢋꢉꢇnꢄdꢆ sꢆnsꢄng;
sꢌꢂꢅꢇꢈꢃꢊꢆꢋꢌꢊꢇꢅ gꢆꢊꢇꢁꢃꢅ;
ꢇnꢄꢃn ꢅꢆsꢂꢃnsꢄvꢆ gꢆꢊ
and its responsive behaviour towards anions.
Kumaresh Ghosh* and Santanu Panja
O
N
H
H
O
H
OH
1
Cholesterol-based Schiff base 1 has been designed and synthesized. The Schiff base 1 forms yellow
colored gel in DMF: H O (2:1, v/v) and the gel is anion responsive. Among different anions, the gel
2
-
phase of 1 is selectively transformed into sol in the presence of CN ions and validates its visual
sensing.
Introduction
–
organic ligand (19) are known to act as CN detector.
Due to interference of other competitive anions, most
of them suffer from selectivity. Moreover, nucleophilicity
During the past decades, in the field of molecular recog-
nition, building of artificial sensors for anions has brought
considerable interest due to the imperative role of anions
in environment and biological systems (1–4). The compet-
itive nature among basicity, nucleophilicity and hydro-
gen bonding capability of anions needs precise design
of receptor modules that are to provide suitable binding
sites for selective recognition of anions (5–9). Of the dif-
–
of CN enabled to develop reaction-based probes that
include addition of CN towards cationic boranes, α,
β-unsaturated systems, coumarins, hydrazones, amides,
aldehydes, ketones, salicyl-imine and so on (20–22), where
the interference of other anions are efficiently minimised.
All these methods involved either fluorometric or colori-
metric changes of the receptors in solution. Beside these
solution phase studies, the detection of CN ion using suit-
able supramolecular gelators is another approach which
is less explored (23–25).
Gels are viscoelastic semisolid materials in which
the gelator molecules undergo self-association to form
three-dimensional matrices. Non-covalent interactions like
hydrogen bonding, van der Waals interaction, π–π stack-
ing, etc. assist the molecular aggregation of the gelators by
–
–
ferent anions, the sensing of CN ion draws attention as it
–
is considered as the most hazardous material for environ-
ment due to its slow decomposition rate upon degrada-
tion (10) and extreme toxicity for living cell (11) that may
cause death of human beings within several minutes. It
can affect adversely the vascular, visual, central nervous,
cardiac, endocrine and metabolic systems (12).
Till today, synthetic receptors based on hydrogen-bond-
ing principle (13–18) as well as metal-anchored simple
CONTACT Kꢌꢈꢇꢅꢆsꢀ Gꢀꢃsꢀ
gꢀꢃsꢀ_k2003@ꢉꢇꢀꢃꢃ.ꢋꢃ.ꢄn
Sꢌꢂꢂꢊꢆꢈꢆnꢁꢇꢊ dꢇꢁꢇ fꢃꢅ ꢁꢀꢄs ꢇꢅꢁꢄꢋꢊꢆ ꢋꢇn bꢆ ꢇꢋꢋꢆssꢆd ꢀꢆꢅꢆ ꢀꢁꢁꢂ://dx.dꢃꢄ.ꢃꢅg/10.1080/10610278.2016.1236926.
©
2016 infꢃꢅꢈꢇ uK lꢄꢈꢄꢁꢆd, ꢁꢅꢇdꢄng ꢇs tꢇꢉꢊꢃꢅ & Fꢅꢇnꢋꢄs Gꢅꢃꢌꢂ

2
K. GHOSH AND S. PANꢀA
O
O
O
(
i)
(ii)
R
OH
R
Cl
R
N3
H
N
96%
89%
O
O
O
H
2
3
O
H
OH
(
iii)
,
7
4%
1
H
N
R =
(iv)
7%
R
Figure 1. Sꢁꢅꢌꢋꢁꢌꢅꢆ ꢃf ꢋꢃꢈꢂꢃꢌnd 1.
1
O
X
7
4
a: X = Boc
: X= H
4
immobilising large volume of solvents (26–28). During our
ongoing research in designing anion responsive supramo-
lecular gelators (29–33), herein we wish to report a simple
naphthalene–cholesterol coupled Schiff base 1 that forms
Scheme 1. (ꢄ) cꢀꢊꢃꢅꢃꢇꢋꢆꢁꢉꢊ ꢋꢀꢊꢃꢅꢄdꢆ, ꢂꢉꢅꢄdꢄnꢆ, dꢅꢉ ch cꢊ , ꢅꢁ,
2
2
1
0 ꢀ; (ꢄꢄ) ch cN, NꢇN , rꢆflꢌx, 5 ꢀ; (ꢄꢄꢄ) (ꢇ) ppꢀ , thF, h o, ꢅꢁ, 4 ꢀ;
3 3 3 2
(
b) (Boc) o, eꢁ N, dꢅꢉ thF, ꢅꢁ, 6 ꢀ; (ꢋ) tFa, dꢅꢉ ch cꢊ , ꢅꢁ,1 ꢀ; (ꢄv)
2
3
2
2
gel in DMF:H O (2:1, v/v) solvent. The gel of 1 exhibits an
efficient visual response towards CN ion through gel to sol
2-ꢀꢉdꢅꢃxꢉ-1-nꢇꢂꢁꢀꢇꢊdꢆꢀꢉdꢆ, dꢅꢉ bꢆnzꢆnꢆ, ꢅꢆflꢌx 12 ꢀ.
2
–
transition which is associated with a colour change from
yellow to brown (Figure 1).
be 62 °C and a sharp increase in Tgel was observed with
higher gelator concentration (Figure 2S). The morphol-
ogy of the gel was characterised by SEM images that
revealed closely spaced plate like matrix (Figure 2(A)).
The rheology experiment (Figure 2(B)) was carried out to
examine the mechanical properties of the gel. Variation
of stress amplitudes and frequencies for the gel exposed
that the dynamic storage modulus G′ was always greater
than the corresponding loss modulus G′′ over the range
studied. The stress sweep experiment at a fixed frequency
(ω = 10 rad/s) gave a non-linear response of both G′ and
G′′ but no prominent crossover point was found within
the experimental stress region. In frequency sweep exper-
iment at a constant strain of 1%, both G′ and G′′ were
found to be frequency independent and exhibited more
than five order higher magnitude of G′ than those of G′′ at
any frequency. These results corroborated the viscoelastic
nature of the gel.
Results and discussion
Synthesis
Compound 1 was synthesised according to the Scheme 1.
Reaction of cholesterol with chloroacetyl chloride afforded
compound 2 which on reflux with NaN in CH CN gave
3
3
the cholesterol azide 3 (31). Reduction of the azide 3
using PPh /H O yielded corresponding amine 4 which on
3
2
reaction with 2-hydroxy-1-napthaldehyde in dry benzene
introduced the compound 1 in appreciable yield.
Gelation study
Structure 1 is the fascinating integration between naph-
thalene chromophore and the cholesterol phase via imine
bond formation and these different segments play crucial
role during gelation. The naphthylaldimine group locks
the molecular conformation in planar arrangement by
forming intramolecular hydrogen bond that results in
extensive π–π stacking between the naphthalene rings
To our opinion, the gel formation of 1 may be attrib-
uted to the aggregation of the gelators according to the
probable mode shown in Figure 3(A). In this regard, FTIR
spectra of 1 in its amorphous and gel states (Figure 3S)
were recorded to get some idea about the mechanism of
gelation. In both amorphous and gel states, the stretching
(
34–36). The cholesteryl part with large hydrophobic sur-
−1
face triggers the intermolecular association through van
der Waals interaction (37, 38). The gelation ability of 1 was
examined in a wide range of solvents with different polar-
ities (Table 1S). While in non-polar solvents compound 1
remained either soluble or partially soluble, it formed
for –C=N appeared at 1636 cm remained unaffected. This
suggested the existance of strong intramolecular H-bond
with the –OH group. In addition, the stretching frequency
−
1
for ester carbonyl of 1 appeared at 1750–1743 cm in
amorphous state moved to 1755–1742 cm− in gel state.
This small shift is presumably due to involvement of ester
carbonyl in hydrogen bonding with water of the medium
probably via water linking during gel formation. We also
believe that π–π stacking between the naphthalene rings
and the hydrophobic–hydrophobic interaction between
the cholesterol units stabilize the network. A shoulder-
ing in the region 310–350 nm in UV–vis spectrum of 1 in
gel state with respect to its sol state and 4 nm red shift
of the absorption at 400 nm in solution corroborates
1
yellow coloured gel in DMF:H O (2:1, v/v) at a minimum
2
concentration of 12 mg/mL.
Morphology, rheology studies and thermal stability
of gels
The gel was stable at room temperature and exhibited
thermally activated gel–sol reversible transition (Figure
1
S). The gel melting temperature at mgc was recorded to

SUPRAMOLECULAR CHEMISTRꢁ
3
(
A)
(
B)
5
5
10
10
G'
G''
G'
G''
4
10
4
3
2
1
1
0
3
10
10
2
10
10
1
10
10
1
10
100
0.1
1
10
100
%
strain
Ang. Frequency (rad/sec)
Figure 2. (a) Sem ꢄꢈꢇgꢆs ꢃf xꢆꢅꢃgꢆꢊ ꢃf 1 ꢇnd (B) rꢀꢆꢃꢊꢃgꢉ ꢆxꢂꢆꢅꢄꢈꢆnꢁ ꢃf gꢆꢊ 1 ꢄn DmF:h2o (2:1, v/v).
Figure 3. (a) Sꢌggꢆsꢁꢆd ꢈꢃdꢆs ꢃf ꢄnꢁꢆꢅꢇꢋꢁꢄꢃn ꢃf 1 ꢄn gꢆꢊ sꢁꢇꢁꢆ; (B) cꢃꢈꢂꢇꢅꢄsꢃn ꢃf (ꢇ) nꢃꢅꢈꢇꢊꢄsꢆd uV–vꢄs ꢇnd (b) flꢌꢃꢅꢆsꢋꢆnꢋꢆ sꢂꢆꢋꢁꢅꢇ
λ = 300 nꢈ) ꢃf 1 ꢄn ꢁꢀꢆ sꢃꢊ ꢇnd gꢆꢊ sꢁꢇꢁꢆs [sꢃꢊꢌꢁꢄꢃn ꢃf 1 wꢇs ꢂꢅꢆꢂꢇꢅꢆd ꢄn ch cN ꢋꢃnꢁꢇꢄnꢄng 1% chcꢊ ].
(
ꢆx
3
3
the aggregation (Figure 3(B-a)). Moreover, a red shift of
0 nm in the absorption band at 418 nm during gelation
the observation of Niu et al. in other related systems (36).
Furthermore, in fluorescence, the quenching of emissions
1
is a clear indication of π–π stacking (ꢀ aggregates) between
the naphthalene rings (39–41). This is in accordance with
centred at 366 and 438 nm in solution (CH CN containing
3
1% CHCl ) with the appearance of new band at 565 nm
3

4
K. GHOSH AND S. PANꢀA
Figure 4. pꢀꢃꢁꢃgꢅꢇꢂꢀ sꢀꢃwꢄng ꢁꢀꢆ ꢂꢀꢇsꢆ ꢋꢀꢇngꢆs ꢃf 1 (15 ꢈg/ꢈl) ꢄn DmF:h o (2:1, v/v) ꢄn ꢂꢅꢆsꢆnꢋꢆ ꢃf 5 ꢆqꢌꢄv. ꢇꢈꢃꢌnꢁs ꢃf dꢄffꢆꢅꢆnꢁ
2
ꢇnꢄꢃns [c = 1 m ꢄn h o ꢇs ꢂꢆꢅꢋꢀꢊꢃꢅꢇꢁꢆ sꢇꢊꢁ] ꢇfꢁꢆꢅ 8 ꢀ.
2
in the gel state (Figure 3(B-b)) explains the overlapping
of naphthalene rings to form higher order of π–π stack-
ing aggregation in gel matrix (42, 43). The quenching of
emission in such case is known as ‘aggregation-caused
quenching’ (44). In this context, the study of concentra-
tion-dependant fluorescence of 1 in solution is worth men-
tioning (Figure 4S). As the concentration of 1 is increased,
the monomer emission of naphthalene was reduced with
the appearance of red-shifted emission at 468 nm for
stacked naphthalenes. The emissions for stacked naph-
thalenes at 565 and 468 nm in the gel and sol states,
respectively, are presumably attributed to the difference
in energy of the excited states of naphthalene in two dif-
ferent states for which the energy gap for transitions varies.
(tetrabutylammonium cyanide) supported the deprotona-
tion phenomena (Figure 5(A)) (45, 46). On deprotonation,
the aromatic protons displayed small upfield chemical
shift. The imine proton (H ) underwent 0.11 ppm upfield
b
chemical shift which clearly explained the persistence of
the imine bond in the interaction process. Deprotonation
–
of H was also observed in the presence of F (Figure 6S)
a
(0.10 ppm upfield shift of the proton H ). In contrast, in the
b
–
presence of CN , the signal for H proton moved upfield
c
by 2.07 ppm. During the event, proton of type ‘d’ which
appeared at 4.74 ppm as multiplet also underwent small
upfield shift (0.15 ppm). Such change in positions of the
signals for the indicated protons recommended the forma-
–
tion of CN adduct on nucleophilic attack to ester carbonyl
of 1. FTIR study also revealed the disappearance of the
−1
signal at 1743 cm for the ester carbonyl stretching upon
Anion responsive behaviour in both gel and sol
states
–
interaction with CN ion in solution phase (Figure 7S).
–
The CN -adduct formation was also confirmed by mass
The stimuli responsive nature of the gel was investigated
by adding concentrated solutions of different anions
spectral analysis (Figure 5(B)). In HRMS spectrum, a major
+
peak at m/z 663.4462 that corresponds to (M + CH CN-1)
3
(
counter cations: tetrabutylammonium ions) on the top
[calcd. 663.4218] was observed upon treatment of 1 with
CN .
–
of the gel at room temperature (Figure 4). Among the dif-
ferent anions studied, the gel state of 1 was converted into
solution only in presence of 5 equiv. amounts of CN ions
Cyanide-induced gel to sol conversion of 1 due to
nucleophilic attack of CN onto the ester carbonyl was
–
–
after 8 h with a colour change from yellow to brown. Under
similar conditions, the gel phase remained unaffected in
also realised by FTIR study. In this case, the ester car-
bonyl stretching at 1742 cm− was abolished with the
1
–
–
–
–
–
−
−1
presence of other anions such as F , Cl , Br , I , AcO , H PO
appearance of a new stretching at 2169 cm for cyanide
2
4
3
−
−
−
,
HP O , HSO or NO ions (Figure 4). To understand the
motif (Figure 7S). In the event, negligible change in –C=N
stretching corroborated the persistence of the imine bond
in the presence of CN ion.
2
7
4
3
–
sensitivity of the gel towards CN ions, different amounts
of CN ions were added to the gel (Figure 5S). In presence
of 1 equiv. amount of CN , disruption of the gel into the sol
took longer time. These results suggest that the gelator 1
–
–
–
In order to understand the solution phase interac-
tions, we performed UV–vis and fluorescence titrations of
–
−5
is highly selective in visual detection of CN ion.
1 (c = 2.50 × 10 M) with the said anions (counter cati-
–
−3
The gel to sol transition of 1 in the presence of CN
ons:tetrabutylammonium ions; c = 1.0 × 10 M) in CH CN
3
may be due to either deprotonation of phenolic –OH or
containing 1% CHCl . In UV–vis, compound 1 initially
3
–
nucleophilic addition of CN ion to the activated –C=N
exhibits three absorbtion maxima at 302 nm, 404 nm and
418 nm. During titration, the intensity of these three peaks
was decreased by almost equal extent for all anions except
1
or ester carbonyl group. To establish this fact, H NMR
and FTIR of 1 were recorded before and after addition of
–
1
–
CN ions. In H NMR, recorded in CDCl solvent, the disap-
CN (Figure 8S). A weak ratiometric response with an isos-
3
pearance of the signal at 14.68 ppm for –OH (hydrogen
bonded) of 1 in the presence of equiv. amount of TBACN
bestic point at 430 nm was observed on gradual addition
of CN ions (Figure 6(a)). In fluorescence, compound 1 did
–

SUPRAMOLECULAR CHEMISTRꢁ
5
O
O
O
N
CN
H
O
O
N
O
N
R
R
R
Ha
H
Hc
O
CN
-
Hc
O
Hc
O
(
A)
Hb
Hb
Hb
CN-
H
R
R
O
O
NC
O
NC
O
He
H
H
Hd
R=
proton
transfer
Hc
N
O
H
Hc
N
O
Hb
Hb
HO CN
H
N
(
B)
H
O
O
H
M
-
C41H55N O
Exact Mass: 623.4218
2
3
1
−2
Figure 5. (a) pꢇꢅꢁꢄꢇꢊ h Nmr (400 mhz) sꢂꢆꢋꢁꢅꢇꢊ ꢃf (ꢇ) 1 [c = 1.18 × 10 m], (b) 1 wꢄꢁꢀ ꢆqꢌꢄv. ꢇꢈꢃꢌnꢁ ꢃf tBacN ꢇnd (ꢋ) 1 wꢄꢁꢀ 2 ꢆqꢌꢄv.
ꢇꢈꢃꢌnꢁ ꢃf tBacN ꢄn cDcꢊ wꢄꢁꢀ ꢇ ꢂꢅꢃbꢇbꢊꢆ ꢈꢆꢋꢀꢇnꢄsꢈ; (B) hrmS sꢂꢆꢋꢁꢅꢇ fꢃꢅ ꢁꢀꢆ ꢋꢉꢇnꢄdꢆ ꢇddꢌꢋꢁ ꢃf 1.
3
–
3−
F andHP O (Figure 10S) the intensity of the emission at
not show any selectivity towards anions (Figure 9S). Upon
excitation at 300 nm, compound 1 exhibited two emission
peaks at 366 nm and 438 nm for monomer and excimer
emissions, respectively. The monomer emission is attrib-
uted to the excited state proton transfer (47, 48) from the
phenol hydrogen to the imine nitrogen atom. In presence
2
7
460 nm underwent significant enhancement that resulted
in an overall red shift of 22 nm from the initial emission
maxima of 438 nm. Other anions taken in this study had
negligible effect on emission of 1 (Figure 10S). The Benesi–
Hildebrand plot (49) of the change in emission intensity
of 1 against the reciprocal of anion concentration gave a
linear fit, characteristic of 1:1 complexation from which the
–
–
3−
of the basic anions like CN , F andHP O , deprotonation of
2
7
phenolic –OH resulted in formation of weakly basic phenox-
ide ion that tuned the excited state charge transfer (47, 48)
2
−1
association constant was estimated to be 1.56 × 10 M ,
2
−1
2
−1
–
–
3−
7
(ESCT) process from the phenoxide ion to the imine bond.
1.59 × 10 M and 1.81 × 10 M for CN , F and HP O
2
–
Facilitation of ESCT resulted in generation of a new peak at
, respectively (Figure 11S). The detection limit (50) for CN
−
5
4
60 nm in the emission spectra of 1 in the presence of above
was determined to be 1.35 × 10 M (Figure 12S). However,
addition of Ca ions (c = 5.0 × 10 M) to the ensemble of
–
2+
−3
said anions. Upon gradual addition of CN (Figure 6(b)),

6
K. GHOSH AND S. PANꢀA
(
a)
(b)
0
0
0
.10
.05
.00
4
30 nm
5
5
1
.5x10
0
0
0
0
0
.20
.15
.10
.05
.00
1.0x10
420
425
430
435
440
445
450
4
5
.0x10
0
.0
300
400
500
3
50
400
450
500
550
Wavelength (nm)
Wavelength (nm)
−5
−3
Figure 6. cꢀꢇngꢆ ꢄn ꢇbsꢃꢅbꢇnꢋꢆ (ꢇ) ꢇnd ꢆꢈꢄssꢄꢃn (b) sꢂꢆꢋꢁꢅꢇ ꢃf 1 (c = 2.50 × 10 m) ꢌꢂꢃn ꢇddꢄꢁꢄꢃn ꢃf tBacN (c = 1.0 × 10 m) ꢄn ch cN
3
ꢋꢃnꢁꢇꢄnꢄng 1% chcꢊ3.
–
and fluorescence was marked and selective only to CN ion
when it was added in higher equiv. amounts to the solu-
tion of 1 (Figures 14S and 15S) and showed consistency
with the gel phase observations.
However, in order to correlate the bulk phase interac-
tions of 1 with the solution phase in CH CN, time-depend-
3
ent UV–vis and fluorescence spectra of 1 in the presence
–
of CN ions at high concentration were recorded in CH CN
3
containing 4% CHCl (CHCl was used for homogeneity).
3
3
In this case, a ratiometric nature in the absorption spec-
tra of 1 (c = 1.0 × 10− M) was observed with time in the
3
–
−3
presence of equiv. amount of CN (c = 5.0 × 10 M) ions.
Initially, a charge transfer band at 658 nm (within 1 min) in
the absorption spectra due to generation of naphthoxide
ion was noted and the colour of the solution changes from
yellow to deep green (Figure 7). With time the intensity
of this peak was decreased with simultaneous generation
of a new band at 490 nm (Figure 8(a)) and the colour of
Figure 7. pꢄꢋꢁꢃꢅꢄꢇꢊ ꢅꢆꢂꢅꢆsꢆnꢁꢇꢁꢄꢃn ꢃf ꢁꢀꢆ ꢋꢃꢊꢃꢌꢅ ꢋꢀꢇngꢆs ꢃf 1
c = 1.0 × 10− m) ꢄn ꢁꢀꢆ ꢂꢅꢆsꢆnꢋꢆ ꢃf ꢆqꢌꢄv. ꢇꢈꢃꢌnꢁ ꢃf ꢅꢆsꢂꢆꢋꢁꢄvꢆ
3
(
−3
ꢇnꢄꢃns (c = 5.0 × 10 m) wꢄꢁꢀ ꢁꢄꢈꢆ ꢄn ch cN ꢋꢃnꢁꢇꢄnꢄng 4% chcꢊ .
3
3
3
−
the solution was changed into rose red. In case ofHP O ,
2
7
–
1
with F resulted in almost recovery of the initial emission
similar spectral behaviour with colour change of the solu-
–
–
bands probably due to scavenging of F ions through the
formation of CaF (Figure 13S). Under similar conditions,
individual ensembles of 1 with CN and HP O exhibited
negligible changes in emission spectra upon addition of
Ca ions. Only 12 nm and 15 nm blue shifts were observed
for CN andHP O ensembles, respectively. These studies
clearly revealed that the basicity and the hydrogen bond-
ing ability of the anions seemed to play major role for the
changes in the emission spectra of 1, and therefore resulted
less selectivity for cyanide in solution.
These observations create contradiction with the gel
phase interactions of 1 with anions that resulted in selec-
tive phase transition in the presence of CN . This occurs
due to different solvent systems considered in gel and
solution phase interactions. To be confirmed, the solution
tion was observed as that of CN ions but in lesser extent
–
whereas F ions did not bring about any noticeable change
2
–
3−
in absorption spectrum of 1 (Figure 16S). Figure 8(b), in this
regard, represents time-dependent change in absorbance
of 1 in the presence of equiv. amount of above-mentioned
anions. In fluorescence, compound 1 initially exhibited
almost similar behaviour in emission spectra in the pres-
ence of said three anions (Figure 17S). On progression,
2
7
2+
–
3−
2
7
–
while the ensemble of 1 with CN showed fluorescence
quenching (Figure 8(c)), the other ensembles exhibited
turn on in emission at 468 nm with time (Figure 18S). A
plot of change in emission intensities against time (Figure
8(d)) clearly revealed that initially, due to accumulation of
large amounts of basic anions, rapid formation of naphth-
oxide ions favoured ESCT process which on progression is
repressed by the keto-enol equilibrium in solution. Close
proximity of –NH of keto form (Figure 5(A)) trigger the
–
phase interaction of 1 with the said anions in DMF/H O (2:1
v/v) was carried out. In this case, the change in absorbance
2

SUPRAMOLECULAR CHEMISTRꢁ
7
(
a)
-
(
b)
1 + 1 equiv. of CN
1.5
1.0
0.5
0.0
-
5
0 mins
1 + 1 equiv. of F
3-
1.6
1.2
0.8
0.4
0.0
1 + 1 equiv. of HP O
2 7
0
min 595 nm
500
600
Wavelength (nm)
700
0
20
40
Time (mins)
-
(
c)
1 + 1 equiv. of CN
(
d)
-
1
+ 1 equiv. of F
4
1 min
3-
3
x10
4
1 + 1 equiv. of HP O
2 7
7
.50x10
4
2x10
4
4
5
0 min
5.00x10
4
1x10
2.50x10
0
350
400
450
500
550
0
20
40
Wavelength (nm)
Time (mins)
–
Figure 8. (ꢇ) tꢄꢈꢆ-dꢆꢂꢆndꢆnꢁ uV–vꢄs sꢂꢆꢋꢁꢅꢇ ꢃf 1 ꢋꢃnꢁꢇꢄnꢄng ꢆqꢌꢄv. ꢇꢈꢃꢌnꢁ ꢃf cN ; (b) ꢋꢀꢇngꢆ ꢄn ꢇbsꢃꢅbꢇnꢋꢆ ꢃf 1 wꢄꢁꢀ ꢁꢄꢈꢆ ꢋꢃnꢁꢇꢄnꢄng
–
ꢆqꢌꢄv. ꢇꢈꢃꢌnꢁ ꢃf dꢄffꢆꢅꢆnꢁ ꢇnꢄꢃns; (ꢋ) ꢁꢄꢈꢆ-dꢆꢂꢆndꢆnꢁ ꢆꢈꢄssꢄꢃn sꢂꢆꢋꢁꢅꢇ ꢃf 1 ꢋꢃnꢁꢇꢄnꢄng ꢆqꢌꢄv. ꢇꢈꢃꢌnꢁ ꢃf cN ; (d) ꢋꢀꢇngꢆ ꢄn ꢆꢈꢄssꢄꢃn
−3
−3
ꢄnꢁꢆnsꢄꢁꢄꢆs ꢃf 1 wꢄꢁꢀ ꢁꢄꢈꢆ ꢋꢃnꢁꢇꢄnꢄng ꢆqꢌꢄv. ꢇꢈꢃꢌnꢁ ꢃf dꢄffꢆꢅꢆnꢁ ꢇnꢄꢃns; [ꢋꢃnꢋ. ꢃf 1 = 1.0 × 10 m, ꢋꢃnꢋ. ꢃf ꢇnꢄꢃns = 5.0 × 10 m ꢄn ch cN
3
ꢋꢃnꢁꢇꢄnꢄng 4% chcꢊ3].
–
CN addition to ester carbonyl of 1 and validates the cya-
been established as a good candidate for selective naked
eye detection of CN ion in both gel and sol states. Further
–
nide adduct formation as described in Figure 5.
insight along this direction is underway in our laboratory.
Conclusion
In conclusion, naphthalene–cholesterol conjugate 1 has
been designed and synthesised. Hydrogen bonding effect
of the salicylaldimine like group in 1 resulted in extensive
π–π stacking between the naphthalene rings which assists
gel formation through extended intermolecular aggrega-
Experimental
Materials
Cholesterol was purchased from Spectrochem. Pyridine,
chloroacetyl chloride, sodium azide, PPh , (Boc) O, Et N,
3
2
3
tion of 1 in DMF:H O (2:1, v/v) mixture solvent. The aggre-
gation is further stabilized by large hydrophobic surface
provided by cholesterol moiety in 1. The gel as obtained
were obtained from Spectrochem. Tetrabutylammonium
salts of anions used in the study were purchased from
Sigma-Aldrich and were carefully handled. All solvents
used in the synthesis were purified, dried and distilled
as required. Solvents used in NMR experiments were
obtained from Aldrich. Thin layer chromatography was
2
in DMF:H O (2:1, v/v) is found to be thermo reversible and
2
–
anion responsive. CN ion over a series of other anions
causes rapid gel to sol transition with a colour change from
1
1
yellow to brown and validates its visual sensing. H NMR,
performed on Merck precoated silica gel 60-F plates. H
2
54
FTIR and HRMS spectroscopic techniques unequivocally
suggested the formation of CN adduct via the nucleop-
and 13C NMR spectra were recorded using Bruker 400 MHz
instrument usingTMS as internal standard. High resolution
mass data were acquired by the electron spray ionisation
technique on XEVO GS-2 QTOf Waters mass spectrometer.
FTIR measurements of all the compounds and dried gels
(xerogels) were carried out using a Perkin-Elmer L120-
–
hilic addition to the ester carbonyl of 1. Fluorescence study
of 1 in CH CN containing 1% CHCl interprets the detection
3
3
–
of CN ions by showing turn-on emission but suffers from
less selectivity due to interference of other basic anions
like F andHP O7 . However, increase in concentration of 1
–
3−
−1
00A spectrometer (νmax in cm ) using KBr cell and KBr
2
–
in solution brought about selectivity towards CN ion with
pellets, respectively. Scanning electron microscopy (SEM)
images were obtained on EVO LS-10 ZEISS instrument.
Fluorescence and UV–vis studies were performed using
a distinguished colour change of the solution from yellow
to rose red. Thus, the simple design-based molecule 1 has

8
K. GHOSH AND S. PANꢀA
Horiba Fluoromax 4C spectrofluorimeter and Shimadzu
UV-2450 spectrophotometer, respectively.
addition of 0.62 mL of Et N (4.24 mmol). The reaction mix-
ture was stirred at the room temperature for 6 h. Then the
solvent was removed under vacuum and the crude mass
3
was extracted with CHCl . The Boc- protected crude mass
3
Syntheses
was purified by column chromatography using 40% ethyl
acetate – petroleum ether as eluent to afford compound
Chloro-acetic acid 17-(1,5-dimethyl-hexyl)-10,13-
dimethyl-2,3,4,7,8,9,10,11,12,13,14,15,16,17-
tetradecahydro-1H-cyclopenta[a]phenanthren-3-yl
ester (2)
To a stirred solution of cholesterol (0.5 g, 1.29 mmol)
in 20 mL dry CH Cl was added chloroacetyl chloride
1
4a in 96% yield (1.1 g). H NMR (CDCl , 400 MHz): δ 5.38
3
(d, 2H, J = 4 Hz), 5.01 (br t, 1H), 4.68 (m, 1H), 3.88 (d, 2H,
J = 4 Hz), 2.33 (d, 2H, J = 8 Hz), 2.02–0.67 (m, 50H, cho-
−1
lesteryl and methyl protons of Boc-group); FTIR (KBr, cm ):
3382, 2937, 1753, 1725, 1677.
2
2
(
0.16 mL, 1.93 mmol) and pyridine (0.05 mL, 0.65 mmol)
In the next step, 1.0 g of the Boc- protected compound
4a (1.83 mmol) was dissolved in 10 mL of dry CH Cl and
under nitrogenous atmosphere. The mixture was allowed
to stir for 10 h at room temperature. After completion
of reaction, the solvent was evaporated and the crude
was extracted with CHCl (3 × 30 mL). The organic layer
was washed several times with water and separated and
dried over Na SO . Evaporation of the solvent gave white
2
2
TFA (4.17 g, 36.6 mmol) in 10 ml of dry CH Cl was added
2
2
dropwise at 0 °C. The reaction mixture was stirred at room
temperature for 1 h. Then solvent was evaporated under
reduced pressure and the crude mass was extracted with
3
CHCl . The organic layer was washed with aq. NaHCO3
2
4
3
solid compound. Recrystallisation from petroleum ether
afforded pure product 2 (0.58 g, yield 96%), mp 148 °C. H
solution (15 mL × 3) and dried over anhydrous Na SO .
2
4
1
The solvent was concentrated under reduced pressure and
the compound 4 (0.70 g, yield 85%) was used directly in
the next step.
NMR (400 MHz, CDCl ) δ 5.37 (m, 1H), 4.72 (m, 1H), 4.03 (s,
3
2
H), 2.36 (m, 2H), 2.02–0.85 (m, 38H), 0.67 (s, 3H); FTIR (KBr,
−
1
cm ): 2939, 2907, 2821, 1753, 1620, 1195.
Compound 1
Azido-acetic acid 17-(1,5-dimethyl-hexyl)-10,13-
dimethyl-2,3,4,7,8,9,10,11,12,13,14,15,16,17-
tetradecahydro-1H-cyclopenta[a]phenanthren-3-yl
ester (3)
Compound 4 (0.1 g, 0.34 mmol) and 2-hydroxy-1-napthal-
dehyde (0.07 g, 0.41 mmol) were dissolved in 5 mL of dry
benzene and the reaction mixture was refluxed for 12 h.
Then the solvent was removed under reduced pressure and
the crude mass was thoroughly washed with diethyl ether
to afford pure yellow coloured compound 1 in an excellent
To a stirred solution of compound 2 (0.5 g, 1.08 mmol) in
CH CN (20 mL) NaN was added (0.11 g, 1.6 mmol) and
3
3
the reaction mixture was refluxed for 5 h. The progress of
reaction was monitored by TLC. After the total consump-
tion of the halide, solvent was evaporated off and water
was added. The reaction mixture was extracted with CHCl3.
Evaporation of the solvent gave the crude azide product 3
1
yield (0.097 g, 77%), mp 166 °C. H NMR (CDCl , 400 MHz):
3
δ 14.68 (brs, 1H), 8.93 (s, 1H), 7.94 (d, 1H, J = 8 Hz), 7.74 (d,
1H, J = 8 Hz), 7.68 (d, 1H, J = 8 Hz), 7.46 (t, 1H, J = 8 Hz), 7.28
(t, 1H, J = 8 Hz), 7.03 (d, 1H, J = 8 Hz), 5.38 (d, 1H, J = 4 Hz),
1
3
(
0.45 g, yield 89%, mp 116 °C), which after recrystallisation
4.78–4.70 (m, 1H), 4.39 (s, 2H), 2.39–0.67 (m, 43H); C NMR
from diethyl ether was directly used in the next step. FTIR
(CDCl , 100 MHz): 171.5, 168.2, 161.2, 139.2, 136.5, 133.2,
129.2, 127.9, 126.8, 123.1, 123.0, 122.9, 118.4, 107.8, 75.6,
3
−
1
(
KBr, cm ): 2938, 2107, 1747, 1213.
5
6.6, 56.1, 49.9, 42.3, 39.7, 39.5, 38.0, 36.9, 36.5, 36.1, 35.8,
(
8R,9R,10S,13S,14R,17S)-10,13-
31.9, 31.8, 28.2, 28.0, 27.7, 24.2, 23.8, 22.8, 22.5, 21.0, 19.3,
−1
dimethyl-17-((S)-6-methylheptan-2-yl)-
2
1
18.7, 11.8; FTIR (KBr) ν cm : 3385, 2936, 1750, 1743, 1636,
+
,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-
H-cyclopenta[a]phenanthren-3-yl 2-aminoacetate
1543, 1362. HRMS (TOF MS ES+): calcd. 598.4260 (M + H) ,
+
found 598.4290 (M + H) .
(4)
Compound 4 was obtained from compound 3 via a three
step reaction. Initially, to a stirred solution of compound
Gelation test
3
(1 g, 2.12 mmol) in 25 ml THF containing four drops of
The relevant amount of compound 1 was dissolved in
desired solvent (1 mL) forming a homogeneous solution,
slightly warmed and then allowed to cool slowly to room
temperature to form gel. Gel was tested by an inversion
of vial method.
water, PPh (1.66 g, 6.36 mmol) was added and the reac-
tion mixture was allowed to stir at room temperature for
4
3
h. A solution of (Boc) O (0.92 g, 4.24 mmol) in 10 mL of
2
THF was added to the reaction mixture followed by the

SUPRAMOLECULAR CHEMISTRꢁ
9
Determination of gel–sol transition temperature (Tg)
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The gel-to-sol transition temperature (T ) was measured
g
by the dropping ball method. The T was defined as the
g
ꢁork, 2005.
temperature at which the gel was melted and started to
flow. In this test, a small glass ball was carefully placed on
the top of the gel to be tested, which was present in a test
tube. The tube was slowly heated in a thermostated oil
bath until the ball fell to the bottom of the test tube. The
temperature at which the ball reaches the bottom of the
(
11) Vennesland, B.; Comm, E.E.; Knownles, C.ꢀ.; Westly, ꢀ.;
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1
981.
(
12) Koenig, R. Science 2000, 287, 1737–1738.
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483–3495.
1
(
(
test tube is taken as T of that system.
g
4
3
General procedure for fluorescence and UV–vis
titrations
(
(
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Stock solution of the compound 1 was prepared in
desired solvent in the concentration range of 10−5 M.
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2
Stock solutions of anions were also prepared in the same
solvent in the concentration range of 10− M. Solution
of 1 (2 mL) was taken in the cuvette and to this solution
different anions were individually added in different
amounts. Upon addition of anions, the change in emis-
sion of the compound was recorded. The same stock
solutions were used to perform the UV–vis titration
experiment in the same way.
(
19) Divya, K.P.; Sreejith, S.; Balakrishna, B.; ꢀayamurthy, P.; Anees,
3
P.; Ajayaghosh, A. Chem. Commun. 2010, 46, 6069–6071.
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(
(
(
21) Mohr, G.ꢀ. Anal. Bioanal. Chem. 2006, 386, 1201–1214.
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5
2, 768–771.
(
(
(
(
(
(
24) Lin, Q.; Lu, T.T.; Zhu, X.; Sun, B.; ꢁang, Q.P.; Wei, T.B.; Zhang,
ꢁ.M. Chem. Commun. 2015, 51, 1635–1638.
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ꢁ.M. Chem. Eur. J. 2014, 20, 11457–11462.
Supplementary material
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Chem. Soc. 2012, 134, 4834–4841.
27) Babu, S.S.; Praveen, V.K.; Ajayaghosh, A. Chem. Rev. 2014,
1
Gelation results, emission, absorption, FTIR and H NMR spectra,
1
13
binding curves and copies of H, C NMR and HRMS.
1
14, 1973–2129.
28) Kartha, K.K.; Sandeep, A.; Praveen, V.K.; Ajayaghosh, A.
Chem. Rec. 2015, 15, 252–265.
29) Ghosh, K.; Kar, D.; Panja, S.; Bhattacharya, S. RSC Adv. 2014,
4, 3798–3803.
Acknowledgments
We thank DST, West Bengal (Project No. 755(Sanc.)/ST/P/
S&T/4G-3/2014, dated 27.11.2014) for providing financial assis-
tance. SP thanks CSIR, New Delhi, India for a fellowship.
(30) Ghosh, K.; Panja, S.; Bhattacharya, S. RSC Adv. 2015, 5,
72772–72779.
(
(
31) Ghosh, K.; Panja, S. RSC. Adv. 2015, 5, 12094–12099.
32) Ghosh, K.; Kar, D. Org. Biomol. Chem. 2012, 10, 8800–8807.
Disclosure statement
(33) Ghosh, K.; Kar, D.; Bhattacharya, S. Supramol. Chem. 2014,
6, 313–320.
34) Datta, S.; Bhattacharya, S. Chem. Commun. 2012, 48, 877–
79.
2
No potential conflict of interest was reported by the authors.
(
(
8
35) Dubey, M.; Kumar, A.; Gupta, R.K.; Pandey, D.S. Chem.
Commun. 2014, 50, 8144–8147.
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