An alternative approach: A highly selective dual responding fluoride sensor having active methylene group as binding site

Article abstract of DOI:10.1039/c2ob06815j

A newly designed phosphonium derivative (L) having active methylene functionality, shows unusual preference towards F^- over all other anions. The binding process through C-H...F^- hydrogen bond formation was probed by monitoring the c

Full text of DOI:10.1039/c2ob06815j

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Organic &
Biomolecular
Chemistry
Cite this: Org. Biomol. Chem., 2012, 10, 2263
www.rsc.org/obc
PAPER
An alternative approach: a highly selective dual responding fluoride sensor
having active methylene group as binding site†
Priyadip Das, Manoj K. Kesharwani, Amal K. Mandal, Eringathodi Suresh, Bishwajit Ganguly* and
Amitava Das*
Received 29th October 2011, Accepted 4th January 2012
DOI: 10.1039/c2ob06815j
A newly designed phosphonium derivative (L) having active methylene functionality, shows unusual
preference towards F− over all other anions. The binding process through C–H⋯F− hydrogen bond
formation was probed by monitoring the changes in either electronic or luminescence spectra. Changes in
both cases are significant enough to allow visual detection. The loss of molecular flexibility of L on
forming L·F− effectively interrupts the non-radiative deactivation pathway and accounts for the observed
switch on fluorescence response. The results of the time-resolved emission studies for L and L·F− using a
time-correlated single photon counting technique further corroborate this presumption. The excellent
preference of L towards F− is attributed to an efficient hydrogen bonding interaction between the strongly
polarized methylene protons and F−, which delineates the subtle difference in the affinity among other
competing anionic analytes like CN−, H2PO4−, CH3CO2−, etc. The relative affinities of various anions
and the preferential binding of F− to reagent L are also rationalized using computational studies.
significance in the areas of molecular recognition, environ-
Introduction
mental/biological sample analysis and diagonistics.2 Over the
The binding of an ionic guest molecule to a molecular receptor
is expected to induce an appreciable perturbation in the elec-
tronic environment of the host molecule. These binding-induced
processes influence the spectral and/or redox behaviour of mol-
ecular receptors. Often these responses are sensitive enough to
provide information about concentration gradient and relative
affinity towards a specific analyte. Considering the fundamental
roles of anions in various industrial, chemical, biological and
environmental processes, research in the area of anion recog-
nition and sensing has experienced exponential growth during
the past decade. The design and synthesis of anion sensors
capable of converting the anion binding event into a readable
signal output through optical, electrochemical and magnetic res-
onance responses have received considerable attention.1 In this
regard, ligand engineering has introduced more systematic
approaches for the development of tailor-made artificial receptors
for anion detection, which are expected to have enormous
past decades various neutral receptors, like urea, thiourea, imida-
zole, indole, amine, amide, pyrrole, phenol, and sulphonamide
moieties with hydrogen bond donor functionalities and a Lewis
acid in the recognition sites have been used for anion sensing
studies.3 On the other hand, cationic receptors for anion recog-
nition4 are generally derived from ammonium, guanidium, qui-
nolinium, and protonated quinoxaline salts, where electrostatic
interaction between the anionic guest and cationic receptors has
been utilized. More recently, metal–anion coordination in appro-
priately functionalized metal complexes has also been used for
the quest of finding receptors with desired specificity for certain
anionic analytes.5 A recent report also reveals that weaker inter-
actions, like anion–π interaction, could also account for an
improved sensitivity towards anions.6 In certain anion recog-
nition processes, where weaker interactions like hydrogen
bonding and/or anion–π interaction(s) is/are operational, the rela-
tive affinity of a receptor towards different anions could be corre-
lated based on the geometry of the receptor and/or anion itself
and the basicity of the anions.7 Because of the recent emphasis
on the development of chromogenic sensors for anions with
obvious ease in the detection processes, most synthetic sensor
molecules generally involve the covalent linking of an optical
signalling fragment to a neutral or cationic receptor for anions,
Central Salt & Marine Chemicals Research Institute, Bhavnagar,
364002 Gujarat, India. E-mail: ganguly@csmcri.org, amitava@
csmcri.org; Fax: +91 2782 567562; Tel: +91 2782 5677601
†Electronic supplementary information (ESI) available: Characterization
data for L, analysis of the spectroscopic titration of L with different
anions (Benisi–Hildebrand and titration plots), different excitation
spectra in presence of F− and CN−, ESI-Mass spectra for binding sto-
chiometry, 1H and 31P NMR study. CCDC 827147. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
c2ob06815j
which provides one or more H-bond donor sites for selective
binding and sensing of some anions, especially F−, CH3CO2
,
−
−
−
CN−, H2PO4 and PhCO2 in organic aprotic solvents.2,3,8
Unfortunately, few colorimetric anion sensors are able to
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2263–2271 | 2263

View Article Online
delineate between these anionic substrates with comparable basi-
city and surface charge density.9
Among various anionic analytes, studies on the design of an
efficient receptor for F− have received considerable attention due
to its deleterious influence in environmental pollution and sig-
nificance in biology. F− is a common ingredient in anaesthetic/
hypnotic/psychiatric drugs and military nerve gases and is a con-
taminant in drinking water. Excess fluoride exposure may cause
collagen breakdown, bone disorders, thyroid activity depression,
immune system disruption and anaemia.10 Thus, the need for
fast and reversible binding for the real time monitoring of F−
and its accurate and specific recognition over all other competi-
Fig. 1 (A) PLUTO diagram depicting the single crystal X-ray structure
of one of the L moieties present in the asymmetric unit. (B) Packing
diagram of the hexafluorophosphate salt of L viewed down the a-axis
(hydrogen atoms are omitted for clarity).
tive anions (e.g. CH3CO2−, CN−, H2PO4 and PhCO2−) has
−
become an unfortunate reality.
In contrast to the well acknowledged hydrogen bond donor
motifs which are generally used for anion recognition studies,
we are reporting herein a simple phosphonium ion derivative (L)
with acidic methylene groups, which acts as atypical hydrogen
bond donors towards various anions. This receptor, L has a
naphthalene skeleton as the building block as well as the signal-
ling unit. The preferential binding and recognition of F− are
achieved through the strong H-bonding interaction (C–H⋯F−)
between the acidic methylene hydrogens (–CH2–) of L and F−.
The present study reveals a rare example of the use of an active
methylene functionality as a binding site for anion recognition.9e
The selectivity for F− over other anions, especially CN−,
characterized by standard spectroscopic techniques (see the
Experimental section). The proposed molecular structure for L
was also confirmed from the single crystal X-ray structure
(Fig. 1).11
The receptor L crystallizes in a monoclinic system (P21/c
space group) with two molecules of the organic phosphonium
−
dication and PF6 as counter anions in the asymmetric unit
along with two CHCl3 molecules as solvent of crystallization.
The packing diagram viewed down a-axis is shown in Fig. 1B.
As depicted in Fig. 1B, the organic dication present in the asym-
metric unit is aligned down the a-axis and oriented diagonal to
the bc-plane. [PF6]− anions and lattice CHCl3 molecules are
between the adjacent phosphonium dications oriented diagonally.
In fact, no classical H-bonding interaction is observed in L.
However, C–H⋯F interaction between the fluorine atom as an
CH3CO2 and H2PO4−, is important as many reported F−
−
sensors suffer from the deleterious interference of these anions.
This newly synthesized reagent (L) showed unique selectivity
towards F− over other common anionic analytes, including CN−,
and other oxygen-containing anions e.g. CH3CO2−, PhCO2− and
H2PO4−. The influence of the spatial orientation of the methyl-
ene hydrogens on the binding mode was rationalized on the
basis of DFT studies.
+
acceptor from all PF6 anions and phenyl hydrogens of the PPh3
do exist with C⋯F distance ranging from 3.254(10) to 3.338(13)
Å and C–H⋯F angles ranging from 149 to 176°, stabilizing the
organic salt in the crystal lattice.
Results and discussion
Receptor L was synthesized following the methodology shown
in Scheme 1. 2,3-Dimethyl naphthalene (1) was allowed to react
with N-bromosuccinimide (NBS) and converted to its corre-
sponding di-bromo derivative. The crude reaction product was
further purified by column chromatography to isolate 2 in the
pure form with reasonable yield. This (2) on treatment with tri-
phenylphosphine yielded 3 with Br− as two counter anions,
which was subsequently converted to the hexafluorophosphate
salt (L) for further use. All intermediates and L were
Fig. 2 (A) UV-Vis spectra of L (4.0 × 10−5 M) in the presence of
[Bu4N]-salts of various anions (8.0 × 10−4 M) (X− = Cl−, Br−, I−, CN−,
SCN−, HSO4−, NO2−, NO3−, N3−, CH3COO−, ClO4−, IO4−, PhCO2−);
inset: colour of the solution of L in acetonitrile with different anions: (a)
L, (b) Cl− or Br−, (c) F−, (d) HSO4−, (e) N3−, (f) CH3CO2−, (g) CN−/
H2PO4−, (h) PhCO2−; (B) UV response of L (4.0 × 10−5 M) in CH3CN
medium on addition of the solution of tetrabutyl ammonium salt of
various anions (8.0 × 10−4 M): (a) CN−, (b) ClO4−, (c) PhCO2−, (d)
IO4−, (e) N3−, (f) H2PO4−, (g) I−, (h) Br−, (i) CH3CO2−, ( j) Cl−, (k)
NO3−, (l) F−, (m) HSO4−, (n) L with λmon = 358 nm.
Scheme 1 Methodology adopted for the synthesis of L.
2264 | Org. Biomol. Chem., 2012, 10, 2263–2271
This journal is © The Royal Society of Chemistry 2012

View Article Online
UV-Vis study
Electronic spectra for L were recorded in CH3CN solution in the
absence and presence of tetrabutyl ammonium salts of common
anionic analytes such as F−, H2PO4−, CH3CO2−, Cl−, Br−, I−,
−
CN−, SCN−, HSO4−, NO3−, NO2−, N3−, ClO4−, PhCO2 and
−
IO4 (Fig. 2A). The absorption spectrum for L was featureless
and no distinct absorption band was observed. A shoulder at
288 nm was observed and this could be ascribed to a charge
transfer transition, which is typical for naphthalene-based chro-
mophores.12 Electronic spectra for L remained unchanged in the
presence of excess (20 mole equivalents) of the above mentioned
anionic analytes, except for F−, CN− and H2PO4−. Among these
anions, the changes were less significant for CN− and H2PO4
.
−
Changes in the absorption spectra for CH3COO− could barely be
detected. For F−, an appreciable change was observed. A new
absorption band at 357 nm, a shoulder at 283 nm and a relatively
weaker, but broad, absorption band appeared at 541 nm upon
addition of the solution of [Bu4N]F (TBAF) (Fig. 2A). The
283 nm band was ascribed to the naphthalene-based charge
transfer (CT) transition, while new absorption bands at 357 and
541 were attributed to two different CT transitions involving
naphthalene and F− as donor fragment and the cationic [PPh3]+
moiety as the acceptor unit. Substantial changes in the absorp-
tion spectra signified a stronger interaction between the receptor
Fig. 3 Systematic changes in UV-Vis spectra of L (4.0 × 10−5 M) in
CH3CN with varying (A) [F−] (0.0–6.0 × 10−4 M), (B) with varying
[CN−] (0.0–1.78 × 10−3 M); Benesi–Hildebrand plot of L with (C)
fluoride ion, monitoring absorbance changes at 358 nm; (D) cyanide
ion, monitoring absorbance changes at 420 nm. Good linear fit confirms
the 1 : 1 binding stoichiometry.
for CH3COO− was less by two orders of magnitude (∼260
times).
−
L and F−; however, a much weaker interaction for CN−, H2PO4
and CH3COO− was observed (Fig. 2A). For all other anions, the
absence of any change in absorption spectral pattern either
suggests no interaction with L or that the interaction of the
respective anions with L is too weak to perturb the energies of
the frontier orbitals of the receptor molecule. Such a situation
was further examined by 1H NMR studies and the results of such
studies are discussed latter. Spectral changes for L in the pres-
ence of TBAF were significant enough in the visible region to
induce a visually detectable change in solution colour from col-
ourless to yellow. The observed changes in solution (in aceto-
nitrile) colour for L upon addition of 10 molar equivalents of
tetrabutyl ammonium salts of respective anions are shown as an
inset in Fig. 2A. This clearly reveals that aside from F−, no col-
orimetric response was observed for addition of any other anions
in CH3CN solution of L in comparable concentration.
The weaker binding of CN− and H2PO4− was not sufficient to
perturb the energies of the frontier orbitals of L, which accounts
for the less or insignificant spectral changes. Hence, the dramatic
combination of visually detectable colour change and specific
response towards F− makes this reagent ideally suited as a
specific colorimetric sensor for F− under the solution phase con-
ditions employed. The observed unique specificity for F− could
be attributed to the highest charge density for F− and conse-
quently more efficient H-bond formation. Presumably, the
increase in negative charge density on binding to F− is expected
to modify the dipoles associated with the charge transfer tran-
sition and/or stabilize the excited state of the chromophore,
which could result in the significant changes in the observed
UV-Vis spectrum and lead to a visually detectable colour
change.
In order to evaluate the relative affinity of the respective
−
anions (e.g. F−, CN−, H2PO4 and CH3COO−) towards L, sys-
tematic spectrophotometric titrations were carried out in aceto-
nitrile medium with varying concentration of each one of these
four anionic analytes, while maintaining the concentration of L
at 4.0 × 10−5 M. On the basis of the UV-Vis titration profile
(Fig. 3A) in the presence of externally added F−, an association
constant (KA) of (1.18 0.05) × 104 M−1 was obtained and 1 : 1
binding stoichiometry was obtained from the Benesi–Hildebrand
plot. This was further confirmed from the Job’s plot and ESI-MS
analysis (ESI†). Analogously, the binding affinities for CN−,
Luminescence study
We also studied the luminescence response of the receptor L in
the absence and presence of F−, H2PO4−, CH3CO2−, Cl−, Br−,
H2PO4 and CH3COO− towards L were also evaluated and
−
respective values were found to be (3.22 0.2) × 102 M−1, (1.56
0.11) × 102 M−1 and 47 7 M−1. The binding of 1 : 1 for
−
CN− and H2PO4 was also confirmed from the Benesi–Hilde-
brand plot and Job’s plot analysis (ESI†). Comparison of these
Scheme 2 Schematic representation of the increase in molecular rigid-
ity on binding to an anion with associated changes in observed lumines-
cence intensity for L.
−
data revealed that the binding affinities of CN− and H2PO4
towards L were 37 and 75 times less than that for F−, while that
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2263–2271 | 2265

View Article Online
Fig. 4 (A) Fluorescence scanning of L (2.0 × 10−5 M) with [Bu4N]
salts of various anions (4.0 × 10−4 M) (X− is Cl−, Br−, I−, HSO4
,
−
NO2−, NO3−, N3−, CH3CO2−, ClO4−, IO4−, PhCO2−) and (B) lumines-
cence response of L (2.0 × 10−5 M) in CH3CN medium on addition of a
solution of the tetrabutyl ammonium salt of various anions: (4.0 × 10−4
M): (a) CN−, (b) ClO4−, (c) PhCO2−, (d) IO4−, (e) N3−, (f) H2PO4−, (g)
I−, (h) Br−, (i) CH3CO2−, ( j) Cl−, (k) NO3−, (l) F−, (m) HSO4−, (n) L
with λmon = 366 nm and λext = 280 nm.
Fig. 5 Emission titration spectra (λext: 280 nm, slit width 2/2 nm) of L
(2.0 × 10−5 M) in CH3CN with varying (A) [F−] (0.0–3.15 × 10−4 M),
(B) [CN−] (0.0–9.0 × 10−4 M; Benesi–Hildebrand plot of L with (C)
fluoride, monitoring emission intensity changes at 366 nm, and (D)
cyanide ion, monitoring emission intensity changes at 370 nm. Good
linear fit confirms the 1 : 1 binding stoichiometry.
I−, CN−, SCN−, HSO4−, NO3−, NO2−, N3−, ClO4−, PhCO2
−
and IO4− in acetonitrile medium. For the free receptor L, a weak
emission band with emission maximum at 351 nm was observed
on excitation at 280 nm. The emission quantum yield (Φ) for L
was evaluated as 0.009 at λmax = 351 nm with respect to
naphthalene. The poor emission quantum yield for L could be
explained based on its inherent molecular flexibility (Scheme 2),
which favoured the non-radiative deactivation pathway of the
naphthalene-based excited states. However, emission spectra
recorded for L (2.0 × 10−5 M) in the presence of 20 mole
equivalents of F− in CH3CN solution showed a “turn-on” emis-
sion response with 10 fold increase in the emission intensity at
max
λems
= 366 nm (for λext = 280 nm) (Fig. 4A), while these
responses were much weaker for a solution of comparable con-
centration of either H2PO4− or CN−.
The increase in emission intensity for an analogous exper-
iment with CH3COO− was insignificant. Other anions failed to
induce any detectable change in the emission spectral pattern of
L (Fig. 4A). We further recorded the excitation spectra for L·F−
and L·CN− using 366 nm as the emission band, which generated
excitation spectra with absorption maxima at ∼280 nm in both
cases. Excitation at 357 nm resulted very weak and broad emis-
sion spectra with a maximum at ∼440 nm, while no emission
was observed on excitation at 540 nm, the other CT absorption
band maximum (Fig. 2A). The results of the systematic analysis
of the fluorescence titrations (Fig. 5A) also confirmed the
binding stoichiometry of 1 : 1 (L : F−) (Fig. 5C) with a binding
constant of (1.25 0.8) × 104 M−1. Equilibrium constants for
Fig. 6 Time-resolved decay profile for L (λext = 280 nm, λmon
366 nm) in the absence and presence of [Bu4N]F (5 mole equivalents) in
CH3CN medium. Plot shown in green is laser source response.
=
KL·CN− ∼ KL·H PO ≫ KL·CH COO−) are reflected in their respect-
−
2
4
3
ive binding constants. Thus, detailed spectral studies revealed
that the receptor L had a distinct preference towards F−, as com-
pared to CN−, H2PO4− and all other common anions studied. An
appreciable decrease in the molecular flexibility and associated
increase in the emission quantum yield for the naphthalene-
based emission band at 366 nm were also reflected in the results
of time-correlated single photon counting studies (TCSPC) for L
in the absence and presence of F− using a 280 nm LED as an
excitation source in an air equilibrated CH3CN solution.
−
the formation of L·CN−, L·H2PO4 and L·CH3COO− were also
evaluated from the emission titration profiles of the respective
anions using an acetonitrile solution of L (2.0 × 10−5 M) in the
presence of increasing [A−] (A− is CN−/H2PO4−/CH3COO−)
and the respective values were found to be (3.32 0.17) × 102
Emission decay traces for free L at λmon = 366 nm could be
best fitted with bi-exponential time constants of τ1 = 0.674
0.008 ns (75%) and τ2 = 4.013 0.006 ns (25%) (Fig. 6). The
longer and minor component could be attributed to the radiative
decay of the naphthalene-based excited states, while the major
component reflects the non-radiative deactivation of the excited
state. However in the presence of 5 mole equivalents of F−,
decay traces could be best fitted to τ1 = 1.35 0.07 ns (20%)
and τ2 = 9.95 0.08 ns (80%). The longer component became
M−1, (1.76
0.13) × 102 M−1 and 59
7 M−1. Thus, the
binding affinity of the respective ions toward L, obtained from
emission titrations profiles for each of these four anions,
matched well with the value obtained from the spectrophoto-
metric analysis.
Relatively weaker binding affinities for ions like CN−,
H2PO4 and CH3COO− as compared to that for F− (KL·F−
≫
−
2266 | Org. Biomol. Chem., 2012, 10, 2263–2271
This journal is © The Royal Society of Chemistry 2012

View Article Online
Fig. 7 Partial 1H NMR spectra (A) of compound L in the absence and presence of 10 and 50 mole equivalents of F− in CD3CN, (B) showing
changes in the position of the methylene proton signal in the absence and presence of varying [F−] in CD3CN; inset: showing changes in the methyl-
ene proton signal in the absence and presence of 10 and 50 mole equivalents of F−.
the major one in the presence of F− and this was attributed to the
radiative decay pathway of the naphthalene-based excited states.
This clearly reveals that the enhancement in luminescence
intensity could be attributed to the restricted vibrational and
rotational relaxation modes of the non-radiative decay of L·F−.
Scheme 3 Two probable binding modes for F− for the L·F− adduct
formation.
1H NMR study
appearance of another more downfield shifted (δ ≥ 4.7 ppm)
doublet signal with increasing concentration of F−. This change
was associated with a decrease in intensity of the primary
doublet signal with concomitant increase in intensity of the
newly appeared downfield shifted doublet signal at δ ≥ 4.7 ppm,
until [F−] became 15 mole equivalents with respect to [L]. For
even higher [F−] (≥20 mole equivalents), the primary methylene
proton signal was too broad to be studied properly. The coupling
constant for the new doublet (δ ≥ 4.7) was also found to be
15 Hz, characteristic for the protons of the –[CH2]– functionality
coupled to adjacent P-atoms. Thus, four methylene protons
became magnetically anisotropic upon binding to F− and gener-
ated two sets of signals. The asymmetric influence tends to
suggest an asymmetric binding of two of the four methylene
protons to the F−. Signals for several aromatic protons moved as
multiplets to δ = 7.670–7.363 ppm (Fig. 7A) in the presence of
excess F−. Two different binding models could be envisaged that
satisfy the 1 : 1 binding stoichiometry for L·F− adduct formation
(Scheme 3). Among these two possibilities, model B explained
well the emission enhancement through gain in molecular rigid-
The anion recognition property of L was also examined using
1H NMR spectroscopy in CD3CN medium, primarily by moni-
toring the changes in the signal of methylene proton (Fig. 7). 1H
NMR spectra of L were recorded in the absence and presence of
various anions. For L, the signal for the –[CH2]– functionality
appeared as a doublet at δ = 3.90 ppm (J = 15 Hz) due to coup-
ling with adjacent P-atoms and all aromatic protons appeared as
multiplets within the region 7.917–7.374 ppm (Fig. 7A). The
most noticeable change was observed only upon addition of 10
equiv. of [Bu4N]F, while changes observed for comparable con-
−
centration of CN− and H2PO4 were much less. For all other
anions used in this study, either no change or insignificant
changes were observed (ESI†). For F−, significant downfield
shifts of the methylene protons were evident, while the extent of
−
those changes for CN− and H2PO4 were small. In the case of
Br−, no change in the signals for the aromatic protons of L were
observed and a slight upfield shift for the methylene protons was
noticed (ESI†). This observation tends to suggest a strong ion-
pair formation between the two cationic PPh3-moieties and Br−.
Such an interaction was reported earlier for a phosphonium ion
derivative and Br−.8b,13 This was further confirmed by 31P NMR
studies and is discussed latter. The H-bonding interaction
1
ity and fits well with the observed shifts in the H and 31P NMR
spectra. Formation of the C–H⋯F− hydrogen bond in the adduct
L·F− would polarize the vicinal C–H bond of the methylene
functionality, which is incidentally proximal to the cationic
1
between F− and L was also investigated by systematic H NMR
titration (Fig. 7B).
+
PPh3 moiety. All these are expected to impart a certain partial
As mentioned earlier, the signal for the four protons of two –
[CH2]– functionalities in L appeared as a doublet at δ =
3.90 ppm. This signifies that all four protons of the two –[CH2]–
functionalities are magnetically isotropic. The signal (doublet)
for the four protons of the two –[CH2]– functionalities shifted
downfield (from 3.891 to 3.934 ppm) when one equivalent of F−
was added to a CD3CN solution of L. This characteristic signal
double bond character for the bond that links the methyl C-atom
and the P-atom of the [PPPh ]+ unit, which consequently would
3
have caused through-space deshielding effects on the other
vicinal methylene protons and its observed distinct downfield
shift.9c This result not only indicated that the receptor L is
bound to F− as an incipient hydrogen-bonded complex in the
primary stage, but also supports the model B as the probable
for H[–CH –]showed a continuous downfield shift along with the
2
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2263–2271 | 2267

View Article Online
Fig. 8 1H NMR spectra of L in the absence and presence of varying
−
concentrations of TBAF in CD3CN, revealing the generation of HF2
due to deprotonation of L in the presence of 50 mole equivalents of
TBAF in CD3CN medium. Deprotonation of L or the generation of
HF2− was not evident with 10 mole equivalents of TBAF.
1
Fig. 10 A plot of Δ = (δc − δu) vs. 1/[F−] for the H NMR titration of
L with F− in CD3CN.
equivalents of KOtBu; however, noticeable spectral changes
were observed in the spectrum that was recorded in the presence
of 50 mole equivalents of KOtBu. Signals for –[CH2]– protons
(at δ ∼ 3.90 ppm) completely disappeared, an observation which
was analogous to that in the presence of F− of comparable con-
centration. These results suggest the deprotonation of L in the
presence of 50 mole equivalents of F− as well as KOtBu.
The binding constant of F− to receptor L through the H-
1
bonded adducts formation was also evaluated from the H NMR
Fig. 9 Changes in the UV-Vis spectra of L (4.26 × 10−5 M) in the
presence of (A) 10 mole equivalents of F− and KOtBu, (B) 50 mole
equivalents of F − and KOtBu, in acetonitrile medium.
titration profile (Fig. 7B) using the following expression with L
as host (H) and F− as guest (G).
Kass ¼ ðΔ=ðΔ0 ꢀ ΔÞÞ=ð½G0ꢁ ꢀ ðΔ=Δ0Þ½H0ꢁÞ
binding mode. For binding mode A (Scheme 3), one would not
expect any significant downfield shift for the second set of
methylene protons that are not hydrogen bonded to the F−, a
situation which is contrary to our experimental observation.
However, in the presence of large excess (≥50 mole equivalents)
of [F−] deprotonation occurred and this was confirmed from the
Based on these NMR data, Δ0, the difference in δ values for –
[CH2]– protons (δu for L) in the free form (i.e. L) and fully com-
plexed species (δc for L·F−) under solution phase, was deter-
mined by extrapolating the Δ = (δc − δu) vs. 1/[F−] plot
(Fig. 10). The value for Δ0 was found to be 0.204 ppm and this
F−
was used for evaluation of KA (8589 900 M−1). This was
1
−
broad appearance of the H NMR signal corresponding to HF2
at around 16 ppm14 along with the disappearance of the charac-
teristic doublet methylene signals (inset Fig. 7 and 8). The
within the error limit of the binding constant obtained from
UV-Vis and emission titrations (vide supra).
The interaction of receptor L with F− was also studied
through 31P NMR spectroscopy. The 31P NMR spectrum of L in
the absence and presence of 10 mole equivalents of F− were
−
characteristic signal at around 16 ppm for HF2 was absent for
[F−] ≤15 mole equivalents.
This reflects the fact that in the presence of a large excess of
F− (≥50 mole equivalents) deprotonation of L occurs and the
deprotonated species [L−] gets stabilized in a polar solvent like
CH3CN.3e,7 The deprotonation process was also confirmed by
UV-Vis spectroscopy. The absorption spectrum recorded for L
showed two absorption maxima at 357 and 559 nm in the pres-
ence of 10 mole equivalents of F− (Fig. 9A) due to the formation
of the H-bonded adduct L·F−. On further addition of excess F−
(50 mole equivalents), the absorption maxima shifted to 405 nm
and 604 nm respectively (Fig. 9B). These changes were attribu-
ted to the deprotonation of L. This was further corroborated by
the similarity of this spectrum to one recorded for L in the pres-
ence of 50 mole equivalents of KOtBu (Fig. 9B), which is
certain to induce deprotonation. No significant spectral or colour
change was observed when the spectrum for L was recorded in
the presence of 10 mole equivalents of KOtBu (Fig. 9A). In
order to clarify this further, 1H NMR studies for L in the
absence and presence of varying concentrations of KOtBu in
DMSO (d6) were also carried out. No change was observed in
the spectrum that was recorded in the presence of 10 mole
recorded. A downfield shift of ∼3 ppm for the P[PPh ]+ signal of
3
L was observed in the presence of F− (Fig. 11). No such signifi-
cant shift in the P[PPh ]+ signal was observed in the presence of
3
other common anions (Fig. 11 and ESI†). Contrary to our
Fig. 11 31P NMR spectra of receptor L before and after addition of F−
(10 equiv.) and other anionic analytes H2PO4−, OAc− and CN− (30
equiv.) in CD3CN at room temperature.
2268 | Org. Biomol. Chem., 2012, 10, 2263–2271
This journal is © The Royal Society of Chemistry 2012

View Article Online
receptor L compared to CN− and H2PO4−. This is in accordance
with the experimental observations (vide supra). The F− ion was
positioned in a distorted square planar manner showing inter-
action with two active methylene hydrogens and two phenyl ring
hydrogen atoms. The F− ion interaction with the phenyl hydro-
gens is also supported by the change in 1H NMR spectral pattern
with F− (vide supra).
Conclusions
In summary, we have successfully employed a phosphonium
derivative of naphthalene L as a selective and sensitive sensor
for F−. Using this reagent (L) we have demonstrated a rational
strategy, where acidic methylene functionality is effectively used
for the selective detection of F− over all other common anions
(including CN− and other oxy anions). This reagent was also
found to be amenable to “colour changing” with associated
“turn-On” fluorescence response. The higher selectivity for F−
among various anionic analytes can be attributed to the acidity
of the methylene protons, and the high electronegativity and
basicity of F−. We have also investigated the interaction modes
by detailed analysis of the NMR (1H and 31P NMR) and
TCSPC experiments. The initial interaction modes of L with F−
in CH3CN led to the formation of a hydrogen bonded adduct
with associated changes in the absorption and emission spectral
pattern; however, for F− concentration > 50 mole equivalents
with respect to the receptor L, deprotonation phenomena prevail.
Fig. 12 GGA/BLYP/DNP optimized geometries and important dis-
tances (Å) of (A) L, (B) L·F−, (C) L·CN− and (D) L·H2PO4−. Calcu-
lated binding energies (EB) of anion complexes with L are also given.
(yellow: C; red: O; cyan: F−; orange: P; blue: N; white: H).
observations, an earlier report revealed an upfield shift of the
P[PPh ]+ signal of a phosphonium ion based receptor upon anion
3
binding, and the upfield shift was attributed to a tight ion pair
formation between Br− and cationic phosphonium ion.13 Thus,
shifts observed in the present study described a well defined
example of the use of acidic methylene functionality for anion
recognition through H-bonded adduct formation and explain
well the rigidity imparted on the receptor structure upon F−
coordination and thereby the increase in the naphthalene-based
emission quantum yield for L.
Thus, these results indicate that the receptor L acts as a
specific chromogenic sensor for F− and presents an atypical
example where methylene hydrogen atoms of the receptors L are
involved in the formation of an effective hydrogen bonded
adduct (L·F−). The tendency of F− to form stronger hydrogen
Experimental section
NBS, 2,3-dimethyl naphthalene, dibenzoyl peroxide, tetrabutyl
ammonium salt of various anions and ammonium hexafluoro-
phosphate were obtained from Sigma-Aldrich and were used as
received without any purification. Triphenyl phosphine and all
the other reagents used were of reagent grade (S. D. Fine Chemi-
cal, India) and were used as received. Various analytical and
spectroscopic data obtained for these intermediates provided
necessary support for the proposed formulation and required
purity. HPLC grade acetonitrile (Fisher Scientific) was used as a
solvent. Chloroform, methanol and carbon tetrachloride used for
different synthetic procedures and studies were purified through
distillation following standard procedures prior to use. Microana-
lyses (C, H, N) were performed using a Perkin–Elmer 4100
elemental analyser. FTIR spectra were recorded as KBr pellets
bonds leads to its specific recognition among all other anions.
2−
The preferential binding of F−, as compared to CN− and H2PO4
,
was also rationalized using computational studies.
To examine and rationalize the preferential binding of receptor
L towards F− anions as compared to CN− and H2PO4−, DFT cal-
culations were performed. All geometries were optimized with
GGA/BLYP/DNP methods15 using the DMol3 density functional
program (version 4.1.2) of Accelrys Inc.16 The optimized geo-
metry of receptor L is given in Fig. 12A. This was found to be
in good agreement with the observed single crystal X-ray struc-
ture (Fig. 1A). The calculated and single crystal X-ray structures
of L revealed that the active methylene hydrogens were posi-
tioned in such a way that only one analyte could effectively bind
to the receptor. The other two active methylene hydrogens were
oriented away from each other, and so were less likely to partici-
pate in the cooperative binding with the anions. The calculated
binding affinity in the gas phase for 1 : 1 complexes of L with
F− ion was found to be −182.5 kcal mol−1, which was higher
than binding affinity with CN− (−147.2 kcal mol−1) and
1
using Perkin-Elmer Spectra GX 2000 spectrometer. H and 31P
NMR spectra were recorded on Bruker 200 MHz (Avance-DPX
200)/500 MHz (Bruker Avance II 500) FT NMR. Electronic
spectra were recorded with Shimadzu UV-3101 PC spectropho-
tometer, while fluorescence spectra, as well as TCSPC studies
were carried out using the Fluorolog HRIBA JOBIN YVON and
Edinburgh Instruments, Model H5773-03, fitted with a blue-sen-
sitive photomultiplier.
UV-Vis and fluorescence study
A 2.0 × 10−4 M solution of compound L (Scheme 1) in aceto-
nitrile was prepared and stored in dark conditions. This solution
was used for all spectroscopic studies after appropriate dilution.
−
H2PO4 (−129.7 kcal mol−1) ions (Fig. 12). These results
suggest that F− should have the preferential binding with
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2263–2271 | 2269

View Article Online
2.0 × 10−3 M solutions of the tetrabutyl ammonium salt of the
respective anions were prepared in predried and distilled aceto-
nitrile and each solution was stored in an inert atmosphere. The
solution of compound L was further diluted for spectroscopic
titrations, and the effective final concentration was adjusted to
4.0 × 10−5 M, while the final analyte concentration for scanning
was 8.0 × 10−4 M. The effective concentration of the solution of
compound L used for the fluorescence study was 2.0 × 10−5 M,
while the final analyte concentration during emission spectral
scanning was 4.0 × 10−4 M. The [Bu4N]F solution was intro-
duced in an incremental fashion during both the UV-Vis and
luminescence titrations and their corresponding spectra were
recorded at 298 K. Benesi–Hildebrand analyses were used to
determine the association constant and binding stoichiometry.
diethyl ether and on constant stirring afforded a white solid
residue, which was filtered off, dried properly and finally recrys-
tallized from the minimum volume of chloroform to get the pure
product 3 (341.8 mg, 88%). 1H NMR (200 MHz, CDCl3, 25 °C,
TMS) δ (ppm): 7.970–7.871 (m, 11H; ArH), 7.776–7.677 (m,
17H; ArH), 7.439 (s, 2H; ArH), 7.366–7.269 (m, 6H; ArH),
5.739 (d, J = 14.8Hz, 4H; Ar–CH2P). IR (KBr) νmax/cm−1
:
3461, 3052, 2360, 1588, 1484, 1437, 1330, 1111, 1163, 996,
841, 747, 719, 690, 542, ESI-MS (m/z): 759 ((M+–Br−), 30%),
677.48 ((M+ − 2Br−), 48%). Elemental analysis: C48H40P2Br2:
calculated C (68.75), H (4.81); found C (67.85), H (4.73).
Synthesis of L (2,3-bis(triphenylphosphoniomethyl)naphthalene
di(hexafluorophosphate)
To a solution of 0.40 g (0.109 mmol) of 2,3-bis(triphenylpho-
sphoniomethyl)anthraquinone dibromide (3) in 20 mL of
MeOH, NH4PF6 (0.465 g, 2.85 mmol) was added. The reaction
mixture was stirred for 5 h, which afford a white precipitate. The
white solid was collected by filtration and dried properly to give
Time-correlated single photon counting experiment
Time-resolved fluorescence measurements were carried out using
Edinburgh Instruments, Model H5773-03. The instrument works
on the principle of the TCSPC technique. The relative fluor-
escence quantum yields (ϕf) were estimated using eqn (1) in
acetonitrile by using the integrated emission intensity of
naphthalene (ϕf = 0.23 in cyclohexane at RT) as a reference:17
1
L 366 mg (79.5%). H NMR (500 MHz, CD3CN, 25 °C, TMS)
δ (ppm): 7.91–7.88 (m, 6H; ArH), 7.70–7.66 (m, 12H; ArH),
7.48–7.44 (m, 16H; ArH), 7.38 (d, J = 3Hz 2H; ArH) 3.894 (d,
J = 15Hz, 4H; Ar –CH2P). 13C NMR (500 MHz, CD3CN,
25 °C, TMS) δ (ppm): 26.52 (d, Ar –CH2P), 116.394, 123.838,
123.873, 127.205, 128.129, 130.234, 130.88, 132.436, 132.879,
Φf ¼ Φf ðIsample=IstdÞðAstd=AsampleÞðη2sample=η2stdÞ ð1Þ
0
where Φf′ is the absolute quantum yield for the naphthalene,
used as reference; Isample and Istd are the integrated emission
intensities; Asample and Astd are the absorbances at the excitation
wavelength, and ηsample and ηstd are the respective refractive
indices.
134.102, 134.178, 135.676 (Ar– and P–Ar). IR (KBr) νmax
/
cm−1: 3433, 1589, 1440, 1112, 836, 741, 689, 556, 510,
ESI-MS (m/z): 823 ((M+ − PF6−), 49%). Elemental analysis:
C48H40F12P4: calculated C (59.51), H (4.16); found C (60.12), H
(4.53).
Synthesis of 2 (2,3-dimethyl bromonaphthalene)
Computational methods
NBS (626 mg, 3.516 mmol) and a catalytic amount of recrystal-
lized dibenzoylperoxide were added to a solution of 1 (250 mg,
1.60 mmol dissolved in 80 mL of CCl4). The resulting mixture
was refluxed for 4 h with irradiation from a 100 W lamp. The
decomposed product of NBS was then separated by filtration and
the filtrate was evaporated to dryness to afford an off-white
coloured solid residue, which was subjected to column chrom-
atography on silica gel as stationary phase and chloroform :
hexane solvent mixture (1 : 1, v/v) as the eluent to get 2 as a
pure product (380 mg, 75.5%). 1H NMR (200 MHz, CDCl3,
25 °C, TMS) δ (ppm): 7.86 (s, 2H; ArH), 7.815–7.809 (m, 2H;
ArH), 7.519–7.512 (m, 2H; ArH), 4.882 (s, 4H; –CH2). IR
(KBr) νmax/cm−1: 3056, 1268, 1220, 1201, 773, 641, 633,
ESI-MS (m/z): 338 ((M+ + Na+), 50%). Elemental analysis:
C12H10Br2: calculated C (45.90), H (3.21); found C (46.21), H
(3.01).
All geometries were fully optimized with generalized gradient
approximation (GGA) using BLYP functional integrated in
density functional program DMol3 (version 4.1.2) of Accelrys
Inc.15,16 The physical wave functions are expanded in terms of
numerical basis sets. We used a DNP double numerical polarized
basis set which is comparable to the 6-31G** basis set. All cal-
culations were performed in the gas phase.
Acknowledgements
DST and CSIR (India) have supported this work. P. D. and
A. K. M. acknowledge CSIR, New Delhi, and M. K. K.
acknowledges UGC, New Delhi for Sr. Research Fellowship.
Notes and references
Synthesis of 3 (2,3-bis(triphenylphosphoniomethyl)naphthalene
dibromide)
1 (a) F. P. Schmidtchen and M. Berger, Chem. Rev., 1997, 97, 1609;
(b) P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486;
(c) C. Suksai and T. Tuntulani, Chem. Soc. Rev., 2003, 32, 192;
(d) R. Martínez-Máñezl and F. Sancenón, Chem. Rev., 2003, 103, 4419;
(e) R. Martínez-Máñez1 and F. J. Sancenón, J. Fluoresc., 2005, 15, 267;
(f) T. Gunnlaugsson, M. Glynn, G. M. Tocci, P. E. Kruger and
F. M. Pfeffer, Coord. Chem. Rev., 2006, 250, 3094; (g) J. W. Steed,
Chem. Commun., 2006, 2637; (h) M. H. Filby and J. W. Steed, Coord.
Chem. Rev., 2006, 250, 3200; (i) P. A. Gale and R. Quesada, Coord.
A solution of 2 (145 mg, 0.462 mmol) and triphenyl phosphene
(233 mg, 1.016 mmol) in 50 mL dry chloroform was refluxed
for 3–4 h, then the reaction mixture was allowed to stir at room
temperature for 14 h. The reaction mixture was then evaporated
to dryness to afford a thick oily residue, which was treated with
2270 | Org. Biomol. Chem., 2012, 10, 2263–2271
This journal is © The Royal Society of Chemistry 2012

View Article Online
Chem. Rev., 2006, 250, 3219; ( j) B. T. Nguyen and E. V. Anslyn, Coord.
Chem. Rev., 2006, 250, 3118.
Mol. Model., 2010, 16, 1441; (d) A. D. Jose, D. K. Kumar, P. Kar,
S. Verma, A. Ghosh, B. Ganguly, H. N. Ghosh and A. Das, Tetrahedron,
2007, 63, 12007; (e) K. B. James, Acc. Chem. Res., 2005, 38, 671;
(f) E. Galbraith and T. D. James, Chem. Soc. Rev., 2010, 39, 3831.
8 (a) E. J. Cho, B. J. Ryu, Y. J. Lee and K. C. Nam, Org. Lett., 2005, 5,
2627; (b) C. Bhaumik, S. Das, D. Saha, S. Dutta and S. Baitalik, Inorg.
Chem., 2010, 49, 5049; (c) L. Wang, X. He, Y. Guo, J. Xu and S. Shao,
Org. Biomol. Chem., 2011, 9, 752; (d) E. Quinlan, S. E. Matthews and
T. Gunnalaugsson, J. Org. Chem., 2007, 72, 7497.
9 (a) Q. Wang, Y. Xie, Y. Ding, X. Li and W. Zhu, Chem. Commun., 2010,
46, 3669; (b) S. V. Bhosale, S. Bhosale, M. B. Kalyankar and
S. J. Langford, Org. Lett., 2006, 11, 5418; (c) C-I. Lin, S. Selvi,
J-M. Fang, P-T. Chou, C-H. Lai and Y-M. Cheng, J. Org. Chem.,
2007, 72, 3537; (d) Z-H. Lin, S-J. Ou, C. Duan, B. Zhang and
Z. Bai, Chem. Commun., 2006, 624; (e) P. Das, A. K. Mondal, M.
K. Kesharwani, E. Suresh, B. Ganguly and A. Das, Chem. Commun.,
2011, 47, 7398.
2 (a) P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486;
(b) J. L. Sessler, P. A. Gale and W. S. Cho, Anion Receptor Chemistry,
RSC, Cambridge, 2006; (c) P. A. Gale, Coord. Chem. Rev., 2001, 213,
79; (d) S. O. Kang, J. M. Llinares, V. W. Day and K. B. James, Chem.
Soc. Rev., 2010, 39, 3980; (e) R. M. Manez and F. Sancenon, Chem. Rev.,
2003, 103, 4419; (f) J. W. Steed, Chem. Soc. Rev., 2009, 38, 506;
(g) R. M. Duke, E. B. Veale, F. M. Pfeffer, P. E. Kruger and
T. Gunnlaugsson, Chem. Soc. Rev., 2010, 39, 3936; (h) L. A. Joyce, S.
H. Shabbir and E. V. Anslyn, Chem. Soc. Rev., 2010, 39, 3621;
(i) Supramolecular Chemistry of Anions, ed. A. Bianchi, K. Bowman-
James and E. Garcka-Espala, Wiley-VCH, New York, 1997;
( j) F. P. Schmidtchen, Coord. Chem. Rev., 2006, 250, 2918;
(k) V. Amendola, M. Vonizzoni, D. Esteban-Gomez, L. Fabbrizzi,
M. Licchelli, F. Sancenón and A. Taglietti, Coord. Chem. Rev., 2006,
250, 1451.
3 (a) M. Cametti and K. Rissanen, Chem. Commun., 2009, 2809;
(b) P. A. Gale, S. E. G. Garrido and S. E. Garlic, Chem. Soc. Rev., 2008,
37, 151; (c) Q. Wang, Y. Xie, Y. Ding, X. Li and W. Zhu, Chem.
Commun., 2010, 46, 3669; (d) D. A. Jose, D. K. Kumar, B. Ganguly and
A. Das, Org. Lett., 2004, 6, 3445; (e) D. A. Jose, P. Kar, D. Koley,
B. Ganguly, W. Thiel, H. N. Ghosh and A. Das, Inorg. Chem., 2007, 46,
5576; (f) X. He, S. Hu, Y. Guo, J. Xu and S. Shao, Org. Lett., 2006, 8,
331; (g) M. J. Chmielewski and J. Jurczak, Chem.–Eur. J., 2005, 11,
6080; (h) J. Yoo, M. S. Kim, S. J. Hong, J. L. Sessler and C. H. J. Lee, J.
Org. Chem., 2009, 74, 1065; (i) C. Caltagirone, G. W. Bates, P. A. Gale
and M. E. Light, Chem. Commun., 2008, 61; ( j) D. Esteban-Gomez,
L. Fabbrizzi and M. Licchelli, J. Org. Chem., 2005, 70, 5717;
(k) V. Thiagarajan and P. Ramamurthy, J. Lumin., 2007, 126, 886.
4 (a) M. D. Best, S. L. Tobey and E. V. Anslyn, Coord. Chem. Rev., 2003,
240, 3; (b) J. M. Llinares, D. Powell and K. Bowman-James, Coord.
Chem. Rev., 2003, 240, 57; (c) S. K. Kim, N. J. Singh, S. J. Kim, H,
G. Kim, J. K. Kim, J. W. Lee, K. S. Kim and J. Yoon, Org. Lett., 2003,
5, 2083; (d) M. Melaimi and F. P. Gabbaie, J. Am. Chem. Soc., 2005,
127, 9680; (e) V. Amendola, L. Fabbrizzi and E. Monzani, Chem.–Eur.
J., 2004, 10, 76; (f) R. Badugu, J. R. Lakowicz and C. D. Geddes, J.
Fluoresc., 2004, 14, 693; (g) B.-g. Zhang, P. Cai, C. Y. Duan, R. Miao,
L. G. Zhu, T. Niitsu and H. Inoue, Chem. Commun., 2004, 2206;
(h) S. Mizukami, T. Nagano, Y. Urano, A. Odani and K. Kikuchi, J. Am.
Chem. Soc., 2002, 124, 3920; (i) M. S. Han and D. H. Kim, Angew.
Chem., Int. Ed., 2002, 41, 3809.
10 (a) P. Connet, Fluoride, 2007, 40, 155; (b) R. J. Carton, Fluoride, 2006,
39, 163; (c) E. B. Bassin, D. Wypij and R. B. Davis, Cancer, Causes
Control, 2006, 17, 421; (d) Y. Yu, W. Yang, Z. Dong, C. Wan, J. Zhang,
J. Liu, K. Xiao, Y. Huang and B. Lu, Fluoride, 2008, 41, 134;
(e) R. P. Schwarzenbach, B. I. Escher, K. Fenner, T. B. Hofstetter, C.
A. Johnson, U. V. Gunten and B. Wehrli, Science, 2006, 313, 1072.
11 Crystal data. Compound 1 (CCDC 827147†): C98H82Cl6F24P8, formula
weight = 2176.10, Monoclinic, P21/c; a = 20.4538(12), b = 25.3853(15),
c = 19.2348(12) Å, α = 90, β = 90.2290(10), γ = 90°, U = 9987.1(10)
Å
−3, T = 293(2) K, Z = 4, Dc = 1.447 Mg m−3, μ(Mo Kα) = 0.391 mm−1
,
F(000) = 4432, colourless block 0.46 × 0.32 × 0.20 mm; 43 845 reflec-
tions measured of which 14 757 were unique (Rint = 0.1229), 1225 par-
ameters, R1 = 0.1336, wR2 = 0.2789 (with I > 2σ(I)), R1 = 0.2090, wR2 =
0.3180 for all data, GOF on F2 S = 1.144, Largest diff. peak and hole =
0.797 and –0.529 e Å−3
.
12 N. J. Turro, V. Ramamurthy and J. C. Scaiano, Modern Molecular Photo-
chemistry of Organic Molecules by University Science Books, Sausalito,
California (USA), 2010.
13 A. Hamadi, K. C. Num, B. J. Ryu, J. S. Kim and J. Viceness, Tetrahedron
Lett., 2004, 45, 4689.
14 (a) P. Ashokumar, V. T. Ramakrishnan and P. Ramamurthy, Chem.–Eur.
J., 2010, 16, 13271; (b) Z. Xu, S. Kim, H. N. Kim, S. J. Han, C. Lee,
J. S. Kim, X. Quan and J. Yoon, Tetrahedron Lett., 2007, 48, 9151;
(c) X. Peng, Y. Wu, J. Fan, M. Tian and K. Han, J. Org. Chem., 2005,
70, 10524; (d) A. Ghosh, S. Verma, B. Ganguly, H. N. Ghosh and
A. Das, Eur. J. Inorg. Chem., 2009, 2496.
15 (a) J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson,
M. R. Pederson, D. J. Singh and C. Fiolhais, Phys. Rev. B: Condens.
Matter, 1992, 46, 6671; (b) A. D. Becke, J. Chem. Phys., 1997, 107,
8554; (c) C. L. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
16 B. Delley, J. Chem. Phys., 2000, 113, 7756.
5 (a) P. D. Beer, D. P. Cormode and J. Davis, Chem. Commun., 2004, 414;
(b) A. Labande, J. Ruiz and D. Astruc, J. Am. Chem. Soc., 2002, 124,
1782; (c) S. Watanabe, M. Sonobe, M. Arai, Y. Tazume, T. Matsuo,
T. Nakamura and K. Yoshida, Chem. Commun., 2002, 2866.
6 S. Guha and S. Saha, J. Am. Chem. Soc., 2010, 132, 17674.
7 (a) A. Ghosh, B. Ganguly and A. Das, Inorg. Chem., 2007, 46, 9912;
(b) A. D. Jose, D. K. Kumar, B. Ganguly and A. Das, Tetrahedron Lett.,
2005, 46, 5343; (c) A. Ghosh, A. D. Jose, B. Ganguly and A. Das, J.
17 Reference materials for fluorescence measurement. D. F. Eaton, Pure
Appl. Chem., 1988, 60, 1107.
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2263–2271 | 2271