A selective fluoride encapsulated neutral tripodal receptor capsule: Solvatochromism and solvatomorphism

Article abstract of DOI:10.1039/c0cc05430e

The dinitrophenyl functionalized tris-(amide) receptor behaves as a selective chemosensor for fluoride by encapsulation within the tripodal pseudocavity in polar aprotic solvents exhibiting solvatochromism and solvatomorphism.

Full text of DOI:10.1039/c0cc05430e

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COMMUNICATION
A selective fluoride encapsulated neutral tripodal receptor capsule:
solvatochromism and solvatomorphismw
Sandeep Kumar Dey and Gopal Das*
Received 7th December 2010, Accepted 8th March 2011
DOI: 10.1039/c0cc05430e
The dinitrophenyl functionalized tris-(amide) receptor behaves
as a selective chemosensor for fluoride by encapsulation within
the tripodal pseudocavity in polar aprotic solvents exhibiting
solvatochromism and solvatomorphism.
Selective anion sensing and recognition has recently developed
into an area of immense research interest in supramolecular
and biological chemistry.1 Development of chemosensors for
fluoride is important due to their roles in the health, medical,
and environmental sciences.2 Fluoride is one of the most
Scheme 1 Molecular structures of receptors L and CL.
in dry chloroform6 (ESIw).z Structural study reveals that the
challenging targets for anion recognition because of its high
combined effect of intramolecular N–HꢁꢁꢁO hydrogen bonding
hydration enthalpy. Optical colour changes as a signalling
and aromatic pꢁ ꢁ ꢁp stacking between the flexible arms of L
event, employing synthetic receptors, are widely accepted due
to the low cost and easy detection of fluoride ions in solution.3
However, only a few colorimetric anion sensors are able
to differentiate selectively between Fꢀ and other anionic
substrates of similar basicity and surface charge density.
Synthetic anion chemosensors generally involve the covalent
linking of an optical signalling chromophoric fragment to a
neutral anion receptor containing mostly amide, urea or
thiourea units among others (indoles, pyrroles, etc.) which
can provide multiple H-bonding sites for selective binding and
sensing of some anions.4 Anion receptors in nature often
involve amide linkages as H-bond donors; hence, amide based
receptors are important for anion binding study.5
resist the opening of the triamide receptor (ESIw).
The highly ordered hydrogen donating cavity of L could
give rise to colorimetric changes as the dinitrophenyl moiety
being a part of the triamide receptor function can possibly
induce a charge-transfer (CT) band in the absorption spectra
upon anion recognition. The fluoride recognition chemistry is
immediately detected in solution by a dramatic increase in
solubility of the receptor in polar aprotic solvents (DMSO,
DMF, MeCN, acetone, THF and dioxane) with a concomitant
optical signalling from colourless to orange/purple upon
addition of TBAF to the suspension of L.
Receptor L, in the absence of any anionic guest, shows
no characteristic absorption in the visible spectrum when
recorded in a library of aprotic solvents. The UV/Vis study
suggests that L can selectively detect fluoride ions colori-
metrically even in the presence of other compeꢀtitive anions
Tripodal scaffolds offer a flexible and structurally preorganized
cavity, which has previously been explored in the area of anion
receptor chemistry and anion directed self-assembly formation.
In this study we report three solvatomorphs of a fluoride
encapsulated neutral molecular capsule of a p-acidic tripodal
triamide receptor (L) (Scheme 1) which efficiently serves as
an optical Fꢀ sensor with characteristic solvent dependent
absorptions in the optical spectrum.
like Clꢀ, Brꢀ, Iꢀ, AcOꢀ, NO3ꢀ, H2PO4ꢀ, HSO4 and ClO4
ꢀ
(Fig. 1a and c). Upon gradual addition of standard
Fꢀ solution (10 mM) to a solution of L (10 mM) in DMSO,
three new absorption bands appear at lmax = 388 (l1), 537 (l2)
and 665 (l3) nm and grow with increasing concentration of
Fꢀ (Fig. 1b). Intense colorations with emergence of new bands
in the optical spectral region can be attributed to the strong
anionꢁ ꢁ ꢁp charge-transfer interactions involving a Fꢀ ion and
p-acidic dinitrophenyl amide receptor, L.7 The CT absorptions
demonstrated a strong solvent dependence which can be
visually marked (Fig. 2c) and confirmed by optical spectro-
scopy (Fig. 2a). However, the CT bands did not exhibit a
linear correlation to solvent polarity probably due to the
complexity of solute–solvent interactions where the hydrogen
bond accepting nature of the solvents plays a critical role. The
Tripodal amide receptor L is synthesized in good yield by
the reaction of tris(2-aminoethyl)amine with three equivalents
of 3,5-dinitrobenzoyl chloride in the presence of triethylamine
Department of Chemistry, Indian Institute of Technology Guwahati,
Assam, 781 03, India. E-mail: gdas@iitg.ernet.in;
Fax: +91-361-258-2349; Tel: +91-361-258-2313
w Electronic supplementary information (ESI) available: Experimental
details, crystallographic tables, 1H, 13C, 19F NMR, ESI-MS and
FT-IR Spectra, UV-Vis and 1H NMR titration experiments. CCDC
795525, 795526, 803581, and 803582. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc05430e
c
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Chem. Commun., 2011, 47, 4983–4985 4983

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acetone, THF and DMSO. The shortest wavelength for l2
absorption was 498 nm recorded in MeCN and dioxane which
exhibit identical spectral patterns, and the longest wavelength
was 565 nm observed in acetone, indicating that the [Lꢁ ꢁ ꢁFꢀ]
complex is more stabilized in acetone followed by THF and
DMSO among other aprotic solvents. The origin of the multi-CT
bands observed can possibly be explained by the existence
of weak electron resonance between the lone pair electrons of
Fꢀ ions with two closely spaced unoccupied orbitals of the
p-acidic receptor.7
Efforts were made to examine the binding of Fꢀ with
receptor L in the solid state, following the selectivity and
solvatochromism of L towards Fꢀ in the UV/Vis study. Slow
evaporation of a solution mixture of L with excess TBAF
(10 equiv.) in aprotic solvents viz. MeCN, DMF and THF
resulted in solvatomorphs of fluoride-encapsulated complexes
TBA[L(F)]ꢁH2O (1a), TBA[L(F)]ꢁDMF (1b) and TBA[L(F)]ꢁ
3THF (1c) respectively. Structural analyses reveal that
irrespective of the solvent of crystallization, Fꢀ is encapsulated
within the tripodal cavity governed by six strong intramolecular
H-bonds involving the amide NH protons and three aryl CH
protons (Fig. 3). In solvatomorphs 1a, 1b and 1c the encapsulated
Fꢀ is H-bonded to the NH protons with average Nꢁ ꢁ ꢁFꢀ
distances of 2.706, 2.679 and 2.705 A, respectively, whereas
the coordinating CH protons interact with average Cꢁ ꢁ ꢁFꢀ
distances of 2.998, 2.929 and 2.998 A, respectively, demonstrating
the strong binding of Fꢀ in the solid state (ESIw). Unlike in 1a
and 1c, the encapsulated Fꢀ in 1b interacts with the arenes C1g
and C3g via weak anionꢁ ꢁ ꢁp interactions with distances of
4.007 and 4.054 A respectively. The FT-IR spectra of the
complexes show a considerable shift upto 20 cmꢀ1 for the
CQO frequency relative to free L due to the formation of
strong NHꢁ ꢁ ꢁF hydrogen bonds (ESIw). Following the solvato-
morphism of the [Lꢁ ꢁ ꢁFꢀ] complex, it can be vaguely argued
that crystallization of a lattice water in complex 1a suggests
weak interactions of the complex with MeCN and thereby, the
shortest lmax values for the complex were observed in MeCN.
In contrast, the presence of one DMF and multiple THF
molecules in the crystals of 1b and 1c, respectively, indicates
comparatively better interactions of the complex with the
respective solvents demonstrating a gradual bathochromic
shift of the CT bands relative to MeCN. With p-acidic arenes,
C–Hꢁ ꢁ ꢁXꢀ hydrogen bonds are often in competition with
other interaction types and by coordinating to the CH protons
they can possibly generate a delocalized anion in solution,
Fig. 1 (a) Changes in the UV/Vis spectrum of L in DMSO upon
addition of TBA salts of anions (25 equiv.); (b) UV/Vis titration of L
(10 mM) in DMSO upon addition of Fꢀ solution (10 mM); (c) colour
changes observed upon addition of anions (25 equiv.) to DMSO
solutions of L.
Fig. 2 (a) Changes in the UV/Vis spectrum of L in various aprotic
solvents upon addition of excess TBAF; (b) UV/Vis titration of L
(10 mM) in DMF upon addition of standard Fꢀ solution (10 mM) and
inset: UV/Vis titration of L (10 mM) in acetone with increasing Fꢀ
concentration (10 mM); (c) variable colour changes observed with the
addition of Fꢀ (25 equiv.) to the solutions of L in various polar aprotic
solvents.
selectivity and CT character of L in response to Fꢀ were
confirmed by comparison to a control receptor, CL (ethyl ester
of carboxy-dinitrobenzene), that cannot differentiate Fꢀ from
other basic anions, such as AcOꢀ and H2PO4ꢀ, producing an
optical signal from colorless to blue irrespective of solvent
choice and polarity indicating the worth of a preorganized
H-bonding cavity of the tripodal scaffold in L towards solvato-
chromic Fꢀ recognition. UV/Vis titration of L with TBAF in
acetone exhibited considerable bathochromic shifts of 17 and
28 nm in the l1 and l2 absorptions (l1 = 405 nm and l2 =
565 nm), respectively, whereas a hypsochromic shift of 19 nm
was observed for l3 (l3 = 646 nm), indicative of an additional
solute–solvent interaction acting as a mechanism to stabilize
the complex more in acetone relative to DMSO (Fig. 2b,
inset). Similarly in THF, appreciable bathochromic shifts of
12 and 20 nm were observed for the l1 and l2 absorption
maxima (l1 = 400 nm and l2 = 557 nm). However in DMF,
titration curves showed blue shifts of 7 and 20 nm in the
absorptions of l1 and l2 (l1 = 381 nm and l2 = 517 nm),
respectively (Fig. 2b), suggestive of a minor interaction of the
[Lꢁ ꢁ ꢁFꢀ] complex with the DMF molecules compared to
Fig. 3 (a) Crystal structure of 1a showing encapsulation of Fꢀ inside
the tripodal cavity where dotted lines represent (D–H)ꢁ ꢁꢁFꢀ interactions;
(b) spacefill representation depicting the formation of a Fꢀ encapsulated
neutral molecular capsule in 1a. TBA cations and solvent are omitted for
clarity.
c
4984 Chem. Commun., 2011, 47, 4983–4985
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support and DST-FIST for single crystal X-ray diffraction
facility. SKD acknowledges IIT Guwahati, India, for
fellowship.
Notes and references
ꢀ
z Crystal data for L: C29H30N10O16S, M = 806.70, triclinic (P1), a =
11.2942(3), b = 12.4744(4), c = 12.8671(4) A, a = 84.723(2)1, b =
79.650(3)1, g = 88.404(2)1, V = 1775.65(9) A3, Z = 2, Dc = 1.509 g cmꢀ1
,
m = 0.180 mmꢀ1, T = 298(2) K, 13 679 reflections, 8734 independent
(Rint = 0.0564), R(F) = 0.0508 [I > 2s(I)], wR(F2) = 0.1510 (all data),
GOF = 0.911. 1a: C43H60FN11O16, M = 1006.02, monoclinic (P21/c),
a = 16.129(3), b = 15.781(2), c = 25.393(4) A, b = 127.592(9)1, V =
5121.4(15) A3, Z = 4, Dc = 1.305 g cmꢀ1, m = 0.103 mmꢀ1, T = 298(2) K,
35 440 reflections, 8854 independent (Rint = 0.0935), R(F) = 0.0603
Fig. 4 Partial 1H NMR spectra of L (below) in DMSO-d6 and upon
[I
> = 0.2034 (all data), GOF = 1.008. 1b:
2s(I)], wR(F2)
C46H67FN12O16, M = 1063.12, triclinic (P1), a = 10. 0759(3), b =
addition of equivalent amount of TBA salt of a fluoride anion (above).
ꢀ
16.7929(6), c = 16.8986(5) A, a = 79.197(2)1, b = 78.461(3)1,
g = 89.753(2)1, V = 2750.23(16) A3, Z = 2, Dc = 1.284 g cmꢀ1
,
which may also be responsible for the intense coloration of L
in the presence of Fꢀ.
m = 0.100 mmꢀ1, T = 298(2) K, 29 276 reflections, 13732 inde-
pendent (Rint = 0.0321), R(F) = 0.0693 [I > 2s(I)], wR(F2) =
0.1841 (all data), GOF = 1.011. 1c: C43H60FN11O15, M = 990.02,
monoclinic (P21/c), a = 10.0293(9), b = 33.293(3), c = 19.4982(17) A,
It is worth mentioning that titration with TBAOH results in
similar spectral behaviour of L to those observed with Fꢀ.
Furthermore, progressive additions of protic solvent (such as
methanol or water) result in gradual attenuation of CT bands
perhaps due to the capability of protic solvents to compete for
Fꢀ with NH functions and p-acidic arenes, disfavoring the
formation of anionꢁ ꢁ ꢁp CT complexes3h,i (ESIw). Thus, the
nature of the chromophoric change can be attributed to Fꢀꢁ ꢁ ꢁp
CT interactions wherein Fꢀ is hydrogen bonded to the NH
groups as evident in the crystal structure of solvatomorph 1b.
The selectivity of L towards fluoride was further confirmed
by 1H NMR studies. Addition of an equivalent amount of
TBAF salt to a solution of L in DMSO-d6 resulted in a
significant downfield shift of the amide –NH and aryl –CHa
proton resonances with high Dd values of 3.55 and 0.72 ppm,
respectively, indicative of a structural alteration of L that
could influence both the –NH as well as –CH protons to
encapsulate Fꢀ ions (Fig. 4). However, in cases of other
halides and oxoanions (added as TBA salts) no appreciable
change in chemical shift values of the –NH and –CH
resonances of L is observed, suggesting the non-interacting
nature or very weak interactions of other anions with L
b = 90.382(6)1, V = 6510.4(10) A3, Z = 4, Dc = 1.010 g cmꢀ1
,
m = 0.079 mmꢀ1, T = 298(2) K, 73 179 reflections, 11177 independent
(Rint = 0.0837), R(F) = 0.0814 [I > 2s(I)], wR(F2) = 0.1952 (all data),
GOF = 1.018.
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1
(ESIw). The H NMR titration curve gives the best fit for a
1 : 1 binding model for host to guest, in agreement with Job’s
plots indicating a maximum Dd at 0.5 = [L]/([L] + [Fꢀ]). The
high binding constant value calculated using EQN MR 2
reveals that L binds very strongly with Fꢀ having log K >
7.0 Mꢀ1 (error limit r 15%).8 In the 19F NMR spectra,
addition of one equiv. of L to a solution of TBAF in
DMSO-d6 resulted in a significant upfield shift of 1.726 ppm
for the free fluoride resonance, indicating the participation of
the anion in H-bonding with L (ESIw). Whereas, the 1H NMR
titration of CL shows a gradual upfield shift of the acidic aryl
protons with increasing Fꢀ concentration as prevalent in the
CT complexes of nitroaromatics with halides (ESIw).7a
In conclusion, we have synthesized a tren-based triamide
receptor molecule bearing dinitroaryl functions which can
selectively sense fluoride ions over other anions by a visible
colour change with a remarkable display of solvatochromism
and solvatomorphism in various polar aprotic solvents.
GD gratefully acknowledges DST (SR/S1/IC-01/2008) and
CSIR (01-2235/08/EMR-II), New Delhi, India, for financial
8 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4983–4985 4985