A terpyridyl-imidazole (tpy-HImzPh3) based bifunctional receptor for multichannel detection of Fe2+ and F- ions

Article abstract of DOI:10.1039/c1dt10965k

The X-ray crystal structures of the tridentate ligand, 4′-[4-(4,5- diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2′:6′,2′′] terpyridine (tpy-HImzPh₃) and its bis-homoleptic iron(ii) complex of composition [Fe(tpy-HImzPh₃)₂]²⁺

Full text of DOI:10.1039/c1dt10965k

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PAPER
A terpyridyl-imidazole (tpy-HImzPh3) based bifunctional receptor for
multichannel detection of Fe2+ and F- ions†
Chanchal Bhaumik, Shyamal Das, Dinesh Maity and Sujoy Baitalik*
Received 24th May 2011, Accepted 18th August 2011
DOI: 10.1039/c1dt10965k
The X-ray crystal structures of the tridentate ligand, 4¢-[4-(4,5-diphenyl-1H-imidazol-2-yl)-
phenyl]-[2,2¢:6¢,2¢¢]terpyridine (tpy-HImzPh3) and its bis-homoleptic iron(II) complex of composition
[Fe(tpy-HImzPh3)2]2+ have been determined, showing that the ligand crystallized in a monoclinic form
with the space group P21/c while its Fe(II) complex crystallizes in an orthorhombic form with space
group Fddd. Both the anion and cation binding properties of the receptor were thoroughly investigated
in dimethylformamide-acetonitrile (1 : 9) solution using absorption, emission, and 1H NMR spectral
studies which revealed that the receptor acts as a sensor for both F- and Fe2+. In the presence of excess
F- ion, deprotonation of the imidazole N–H fragment of the receptor occurs, an event which is signaled
by the development of a yellow color visible with the naked eye. The estimated value of the equilibrium
constant of the receptor with F- is 1.9 ¥ 104 M-1. Deprotonation is also observed in the presence of
hydroxide. The receptor also shows colorimetric and fluorimetric sensing ability towards Fe2+ ions. The
binding site for the metal ion in the system has been unambiguously established by single-crystal X-ray
diffraction studies of the Fe(II) complex of the receptor. The influence of solvents on the absorption and
fluorescence spectra of the receptor has been investigated in detail. Cyclic voltammetric (CV) and
square wave voltammetric (SWV) measurements carried out in dimethylformamide-acetonitrile (2 : 3)
provided evidence in favor of cation (Fe2+) and anion (F-) concentration dependent electrochemical
responses, enabling the ligand to act as a suitable electrochemical sensor for F- and Fe2+ ions.
problem of iron and fluoride ion detection one of considerable
Introduction
current interest. Consequently, much effort has been directed
The design of sensors capable of recognizing and sensing both
towards the design of receptors that can selectively recognize either
anions or cations.1–11 A number of compounds containing urea,
thiourea, amide, pyrrole or imidazole subunits that are capable
of providing hydrogen bond forming sites have been reported to
exhibit strong affinity and selectivity towards certain anions.1–11 On
the other hand, different types of ligands with N, O, or S donor
centers act as binding sites for several transition metal cations.12–22
The majority of these chemosensors are three-component systems
comprising a signaling unit, a guest binding unit, and a linker that
connects these two units.5–10 Although a number of chemosensors
have been developed for the detection of transition metal cations
and anions,15–29 sensing of both Fe2+ and F- ions by a single
receptor is still rather rare.
In recent years, the development of triple channel sensors, that is
sensors where interactions can be sensed by monitoring three dif-
ferent physicochemical outputs (absorption, emission and redox
properties), have emerged as a topic of considerable interest.30,31
To this end, in our search for appropriate ligands for the
construction of suitable multichannel sensors that are capable of
recognizing cations and anions, we have found that the 4¢-[4-(4,5-
diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2¢:6¢,2¢¢]terpyridine32 (tpy-
HImzPh3) system, wherein a terpyridine moiety has been fused
at its 4¢-position with the 2,4,5 triphenyl imidazole motif, has yet
cations and anions is one of the most challenging fields because
of the important role they play in biological, industrial and envi-
ronmental processes.1–11,15–21 The sensitive and selective detection
of ferrous and fluoride ions is important because of their crucial
biological roles. Among the transition metals, iron is the most
important element involved in living systems.12–15 Biologically, iron
plays crucial roles in the transport and storage of oxygen and also
in electron transport in diverse metalloenzymes.12–15 Iron is truly
ubiquitous in living systems and tracking its homeostasis using a
suitable technique is of great significance to clarify its biological
effects.22 On the other hand, fluoride plays an important role in
preventing dental caries and in the treatment of osteoporosis.1–5,16
High doses of this anion are, however, dangerous and can lead to
dental or skeletal fluorosis.16 This diversity of function makes the
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University,
Kolkata, 700 032, India. E-mail: sbaitalik@hotmail.com; Fax: + 91
3324146584; Tel: + 91 33 24146666
† Electronic supplementary information (ESI) available: Fig. S1–S14,
Table S1–S3 and X-ray crystallographic file in CIF format for the free
receptor and its Fe(II) complex. CCDC reference numbers 817562–817563.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1dt10965k
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to be exploited in the area of sensor development. Tridendate
ligands, such as terpyridine (tpy), are known to form complexes
with a large variety of transition metal ions.33 Moreover, these
binding moieties are versatile building blocks in supramolecular
and macromolecular chemistry.33–36 In contrast to the related
2,2¢-bipyridine (bpy) ligand, which gives rise to the formation
of M(bpy)32+-type metal complexes with D/K chirality, tpy
527.63 ([L+H]+). Anal. Calcd for C36H25N5: C, 81.95; H, 4.78; N,
13.27. Found: C, 81.87; H, 4.81; N, 13.25.
Synthesis of [Fe(tpy-HImzPh3)2](ClO4)2. To
a
stirred
chloroform-methanol (1 : 1) solution (30 mL) of the ligand
(0.11 g, 0.21 mmol) was added solid Fe(ClO4)2·6H2O (0.36 g, 0.1
mmol). The colorless solution changed immediately to violet, and
during stirring at room temperature for ~1 h a microcrystalline
compound deposited. The compound was filtered, washed
with water, and dried in a vacuum. On recrystallization from
acetonitrile-methanol (1 : 1) a violet crystalline compound was
forms achiral octahedral coordination compounds of the type
2+ 34
M(tpy)2
. In order to accomplish our goal, we have exploited
the fluoride ion signaling potential of the imidazole NH group and
transition metal ion binding ability of the terpyridine moiety. To
our knowledge, no such studies have been reported in literature for
multichannel detection of both Fe2+ and F- ions using a terpyridyl-
imidazole type of ligand. As will be seen, as a consequence of
cation/anion interaction, remarkable changes in color (that can
be followed by the naked eye) along with similar absorption,
photoluminescence and redox responses occur.
1
obtained: yield 0.10 g (76%). H NMR (DMSO, 300 MHz): d =
13.02 (s, 2H, NH(imidazole)), 9.73 (s, 4H, H3¢), 9.08 (d, 4H, J =
8.0 Hz, H3), 8.70 (d, 4H, J = 8.4 Hz, H8), 8.50 (d, 4H, J = 8.3
Hz, H7), 8.04 (t, 4H, J = 7.6 Hz, H4), 7.63–7.29 (m, 24H (20
phenyl-H + 4H6)), 7.20 (t, 4H, J = 6.4 Hz, H5). Anal. Calcd for
C72H50N10Cl2O8Fe: C, 66.00; H, 3.84; N, 10.69. Found: C, 65.95;
H, 3.81; N, 10.71. ESI-MS (positive, CH3CN) m/z = 554.75 (100%)
[Fe(tpy-HImzPh3)2]2+; 1209.12 (5%) [Fe(tpy-HImzPh3)2(ClO4)]+.
Caution! Perchlorate salts of different metals and of the Fe(II)
complex used in this study are potentially explosive and therefore
should be handled in small quantities with care.
Physical measurements
Elemental (C, H, and N) analyses were performed on a Perkin-
Elmer 2400II analyzer. Electrospray ionization mass spectra
(ESI-MS) were obtained on a Micromass Qtof YA 263 mass
1
spectrometer. 1H and { H–1H} COSY spectra were obtained on a
Experimental
Bruker Avance DPX 300 spectrometer using DMSO-d6 solutions.
Electronic absorption spectra were obtained with a Shimadzu
UV 1800 spectrophotometer at room temperature. The sensing
studies of the receptor with different anions and cations were
carried out in dimethylformamide-acetonitrile (1 : 9) solution. For
a typical titration experiment, 2 mL aliquots of a given cation
or anion (2.0 ¥ 10-2 M) were added to a 2.5 mL solution of the
ligand (2.0 ¥ 10-5 M). TBA salts of different anions and hydrated
perchlorate salts of the metals were used for titration experiments.
The binding/equilibrium constants of the anions were evaluated
from the absorbance data using eqn (1).42
Materials
Reagent grade chemicals obtained from commercial sources
were used as received. Solvents were purified and dried
according to standard methods.37 Benzil and tetrabutylam-
monium (TBA) salt of the anions were purchased from
Sigma-Aldrich. 4¢-(p-methylphenyl)-2,2¢:6¢,2¢¢-terpyridine (tpy-
PhCH3), 4¢-(p-dibromomethylphenyl)-2,2¢:6¢,2¢¢-terpyridine(tpy-
PhCHBr2), 4¢-formyl-2,2¢:6¢,2¢¢-terpyridine (tpy-PhCHO), were
synthesized according to the literature procedures.31d,38–41
Preparation of the ligand 4¢-[4-(4,5-diphenyl-1H-imidazol-2-yl)-
phenyl]-[2,2¢:6¢,2¢¢] terpyridine (tpy-HImzPh3). The synthesis of
the compound was undertaken by following a method previ-
ously described.32 A mixture of 4¢-(p-formylphenyl)-2,2¢:6¢,2¢¢-
terpyridine (tpy-PhCHO) (1.00 g, 2.97 mmol), benzil (630 mg,
3.00 mmol), and ammonium acetate (2.3 g, 30 mmol) in acetic
acid (30 mL) was heated at reflux for 2 h. The resulting orange-red
solution was cooled to room temperature and poured into crushed
ice (250 mL) with vigorous stirring. A grey colored precipitate
thus obtained was filtered off. It was then slurried with water (ca.
200 mL) and slowly treated with 25% aqueous ammonia to pH ª
8 when the color changed to light pink. The solid was collected
by filtration and washed several times with water. The residue was
purified by silica gel column chromatography in chloroform and
recrystallized from chloroform-methanol (1 : 1) to give the desired
compound as a light green crystalline solid (1.41 g, 2.68 mmol,
yield 87%). 1H NMR (DMSO, 300 MHz): d = 12.90 (s, 1H,
NH(imidazole)), 8.78 (m, 4H (2H3¢+2H6)), 8.68 (d, 2H, J = 7.9
Hz, H3), 8.30 (d, 2H, J = 8.0 Hz, H8), 8.09–8.00 (m, 4H (2H4 +
2H7)), 7.58–7.20 (m, 12H (10H (phenyl) + 2H5)). ESI-MS: m/z
Aobs = (A0 + A•K[G]T)/(1 + K[G]T)
(1)
where Aobs is the observed absorbance, A0 is the absorbance of
the free receptor, A• is the maximum absorbance induced by the
presence of a given anionic guest, [G]T is the total concentration of
the guest, and K is the binding constant of the host–guest entity.
Binding constants were performed in duplicate, and the average
value is reported.
Emission spectra were recorded on a Perkin-Elmer LS55 fluo-
rescence spectrophotometer. Photoluminescence titrations were
carried out with the same sets of solutions as were made for
spectrophotometry. Quantum yields of the free receptor and its
Zn(II) complexes were determined in different solvents by a relative
method using quinine sulfate for the free receptor and [Ru(bipy)3]2+
for the Zn(II) complexes as the standard.43 Time-correlated single
photon counting (TCSPC) measurements were carried out for
the luminescence decay of the ligand in different solvents at
room temperature. For TCSPC measurement, the photoexcitation
was performed at 300 nm for the free receptor and at 370 nm
for the Zn(II) complexes using a picosecond diode laser (IBH
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Table 1 Crystallographic data for the receptor tpy-HImzPh3 and the
Nanoled-07) in an IBH Fluorocube apparatus. The fluorescence
decay data were collected on a Hamamatsu MCP photomultiplier
(R3809) and were analyzed using IBH DAS6 software.
Fe(II) complex [Fe(tpy-HImzPh3)2]2+
Compounds
Tpy-HImzPh3
[Fe(tpy-HImzPh3)2]2+
The electrochemical measurements were carried out with a
BAS 100B electrochemistry system. A three-electrode assembly
comprising a Pt (for oxidation) or glassy carbon (for reduc-
tion) working electrode, Pt auxiliary electrode, and an aqueous
Ag/AgCl reference electrode was used. The cyclic voltammetric
(CV) and square wave voltammetric (SWV) measurements were
carried out at 25 ◦C in dimethylformamide-acetonitrile (2 : 3)
solution of the complexes (ca.1 mM) and the concentration of the
supporting electrolyte tetraethylammonium perchlorate (TEAP)
was maintained at 0.1 M. All of the potentials reported in this
study were referenced against the Ag/AgCl electrode, which under
the given experimental conditions gave a value of 0.36 V for
the ferrocene/ferrocenium couple. For electrochemical titrations
25 mL aliquots of a perchlorate salt of a cation and TBA salts of
the anions (4.0 ¥ 10-2 M in acetonitrile) were added to a 5 mL (1.0
¥ 10-3 M) solution of the ligand.
Formula
fw
T (K)
C36H25N5
527.61
273(2)
Monoclinic
P21/c
14.6483(15)
18.3958(19)
10.1403(11)
90
93.376(7)
90
2727.7(5)
1.285
C72H50N10Cl2O8Fe
1309.97
296(2)
Orthorhombic
Fddd
23.5948(16)
36.253(3)
38.053(3)
90
90
90
32 550(4)
1.069
16
0.303
10 816
1.55–25.20
7307/0/420
0.990
0.0664
0.2247
0.800/-0.377
Cryst. Syst.
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
3
˚
V (A )
Dc (g cm-3)
Z
4
m (mm-1)
0.077
1104
1.78–25.11
4830/0/370
0.909
0.0491
0.1617
0.187/-0.214
F(000)
q range (deg)
data/restraints/params
GOF on F2
R1 [I > 2s(I)],
wR2 a(all data)
Experimental uncertainties were as follows: absorption maxima,
2 nm; molar absorption coefficients, 10%; emission maxima,
5
˚
Drmax/Drmin (e A)
nm; excited-state lifetimes, 10%; luminescence quantum yields,
20%; redox potentials, 10 mV.
ꢀ
ꢀ
ꢀ
ꢀ
a R1(F) = [ ꢀ F0 | - |FC ꢀ/ | F0], wR2 (F2) = [ w(F0 - FC2)2/
b
2
w(F02)2]1/2
X-ray crystal structure determination
violet and the complex was isolated from the solution as a dark
violet crystalline solid. Both the ligand and its Fe(II) complex have
been characterized by elemental (C, H and N) analyses, ESI-MS,
UV-Vis and 1H NMR spectroscopic measurements and the results
are given in the Experimental section. The ESI-MS of the Fe(II)
complex in CH3CN (Fig. S1(b), ESI†) shows two abundant peaks
at m/z 554.75 and 1209.12, respectively. The isotopic patterns
of the original peak at m/z 554.75 separated by 0.5 Da fit very
well to the isotope distribution pattern calculated for [Fe(tpy-
HImzPh3)2]2+ (100%). The peak at m/z 1209.12 is assigned to
[Fe(tpy-HImzPh3)2(ClO4)]+ (5%).
Diffraction quality crystals of the ligand were obtained by slow
evaporation of a chloroform-ethanol (3 : 2) solution, whereas for
the iron complex they were formed by diffusing toluene into
a dichloromethane-acetonitrile (2 : 1) solution. X-ray diffraction
data for the crystals mounted on a glass fiber and coated with
perfluoropolyether oil were collected on a Bruker-AXS SMART
APEX II diffractometer at room temperature equipped with
CCD detector using graphite-monochromated Mo-Ka radiation
˚
(l = 0.71073 A). Crystallographic data and details of struc-
ture determination are summarized in Table 1. The data were
processed with SAINT44 and absorption corrections were made
with SADABS.44 The structure was solved by direct and Fourier
methods and refined by full-matrix least-square based on F2 using
the WINGX software which utilizes SHELX-97.45 For structure
solution and refinement the SHELXTL software package46 was
used. The nonhydrogen atoms were refined anisotropically, while
the hydrogen atoms were placed with fixed thermal parameters at
idealized positions. The electron density map showed the presence
of some unassignable peaks, which were removed by running the
program SQUEEZE.47
Description of crystal structures of tpy-HImzPh3 and
[Fe(tpy-HImzPh3)2]2+
The terpyridyl-imidazole ligand crystallized in a monoclinic form
with the space group P21/c while its Fe(II) complex [Fe(tpy-
HImzPh3)2]2+ crystallizes in an orthorhombic form with space
group Fddd. Structural projections of the ligand and its Fe(II)
complex are shown in Fig. 1. Selected bond distances and angles
of the Fe(II) complex are given in Table 2, while for the receptor
are listed in Table S1 and S2, ESI.†
The final least-squares refinement (I > 200s(I)) converged to
reasonably good R values (Table 1).
The X-ray crystal structure of the ligand showed that the
three pyridine rings of the terpyridine moiety adopt a transoid
arrangement about the interannular C–C bonds, which is in
agreement with the literature.48 This configuration minimizes
electrostatic interactions between the nitrogen lone pairs and
the van der Waals interactions between the meta protons. The
Results and discussion
Synthesis and characterization
˚
A mixture of benzil and 4¢-(p-formylphenyl)-2,2¢:6¢,2¢¢-terpyridine
(tpy-PhCHO) in a 1 : 1 molar ratio was subjected to undergo
condensation in acetic acid in the presence of excess of ammonium
acetate for the synthesis of the ligand, tpy-HImzPh3. When a
methanol-chloroform solution of the ligand (2 equivalents) was
treated with Fe(ClO4)2 (1 equivalent) the solution became intensely
interannular C–C bond length [1.488(3) A] is comparable with
those of the reported 2,2¢:6¢,2¢¢ terpyridine derivatives [1.480(1)–
22,48
˚
1.498(3) A].
The C–C and C–N bond lengths in the pyridyl
and phenyl rings, in the range 1.357(4)–1.492(3) and 1.328(3)–
˚
1.346(3) A, lie within the expected ranges. The dihedral angles
between the central pyridine plane and the two lateral ones are
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Fig. 1 Structural projections of the receptor tpy-HImzPh3 (a) and the complex cation [Fe(tpy-HImzPh3)2]2+ (b). Hydrogen atoms except imidazole N–H
(H1A) are omitted for clarity.
˚
Table 2 Selected bond distances (A) and angles (deg) for [Fe(tpy-
HImzPh3)2]2+
[Fe(tpy-HImzPh3)2]2+
Fe–N(3)
Fe–N(4)
1.968(3)
1.877(2)
Fe–N(5)
1.977(3)
N(3)–Fe–N(3)
N(4)–Fe–N(3)
N(4)–Fe–N(3)
N(5)–Fe–N(3)
N(5)–Fe–N(3)
N(4)–Fe–N(4)
N(5)–Fe–N(4)
N(5)–Fe–N(4)
N(5)–Fe–N(5)
89.26(15)
80.52(10)
99.60(10)
92.82(10)
161.59(10)
179.83(15)
81.09(11)
98.79(10)
90.96(14)
10.98 and 16.44◦, while the dihedral angle between the phenyl
group attached to the central pyridine and imidazole moiety is
14.42◦. Again the two phenyl groups are twisted heavily from the
plane of the imidazole ring and the dihedral angles are 42.75 and
66.54◦.
Fig. 2 Capped-stick projection of the receptor showing intermolecular
In the structure of the ligand, it is interesting to note that the NH
proton of the imidazole ring is involved in strong intermolecular
hydrogen bonding interactions with the N atom of a neighboring
hydrogen bonding interactions.
˚
ligand, with an N–H ◊ ◊ ◊ N distance of 2.172 A, forming infinite
for the formation of adducts with the anions via hydrogen bonding
chains (Fig. 2). The N–H proton of tpy-HImzPh3 could be used
interactions.
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The structure of the complex cation [Fe(tpy-HImzPh3)2]2+,
which has crystallographically imposed symmetry, is shown in Fig.
1(b). In the bis-chelated complex, the bivalent iron is coordinated
by the tridentate ligand and has a distorted octahedral geometry
having a meridional N3N3 chromophore. The chelate bite angles
span the range between 80.5 and 99.6◦. It is to be noted that
although the inter-ligand trans angle made by N4–Fe–N4 is
179.83◦, very close to linearity, the intra-ligand trans angle, N3–
Fe–N5, is 161.59◦ and deviates largely from linearity. The central
group of signals in the region 7.20–7.58 ppm, assigned on the
basis of coupling constants and chemical shifts. The terpyridine
protons H3–H6, on the other hand, lie in the range 7.20–8.78 ppm.
Fig. 3(a) shows that addition of F- leads to complete removal of
the N–H signal and small upfield shifts of H8 and phenyl ring
protons, although the chemical shifts for the tpy protons are far
less affected. Clearly, the F- ion acts as a proton abstractor. The
upfield shift of C–H8 and protons of two phenyl rings in the
ligand is due to augmentation of electron density at these sites
due to delocalization of the negative charge of the imidazole ring
brought about by N–H deprotonation.
˚
Fe–N bond, 1.877(2) A, is shorter than the two outer bonds
˚
of 1.968(3) and 1.977(3) A to each ligand, probably because of
*
1
efficient overlap of the metal t2g orbital with the p orbitals of the
The H NMR spectrum of [Fe(tpy-HImzPh3)2]2+ is consistent
central pyridyl group. Similar to that of the free receptor, each
phenyl ring in the Fe(II) complex is also twisted with respect to the
plane of the imidazole group (the corresponding dihedral angles
are 17.80 and 52.60◦ respectively).
with a symmetrical complex as shown by X-ray crystal structure
in the solid state. Fig. 3(b) illustrates the effect of coordination
of the Fe2+ center to the ligand on the chemical shift values of
its different protons. It can be seen that the chemical shifts of
H3, H7 and H8 protons are shifted downfield, while the phenyl
protons and H4 of the terpyridine moiety are almost unaffected
by coordination. Proton H6 of the tpy moiety is affected the most
and shifts significantly upfield, because this proton lies above the
shielding region of a pyridine ring of the other tpy ligand (Fig. 1).
1H NMR spectra
The 1H NMR spectra of both the receptor and [Fe(tpy-
HImzPh3)2](ClO4)2 complex recorded in DMSO-d6 (Fig. 3) show
the occurrence of a number of resonances in the aromatic region.
The COSY spectra have been particularly useful to locate spin
couplings of these protons. The numbering scheme used to assign
the observed resonances is given in Fig. 3. The distinct signal,
which is the most downfield-shifted, is observed as a singlet at
12.90 ppm for the ligand and 13.02 ppm for its Fe(II) complex and
is due to the imidazole NH proton. The protons of two phenyl
rings attached to the imidazole moiety are characterized by a
Absorption and emission spectral characteristics
The absorption and emission spectra of the receptor have been
recorded in several solvents and the relevant spectral data in
different solvents are given in Table 3. The absorption spectra
of the receptor obtained in chloroform and dimethylformamide
solutions are shown in Fig. 4. In all the solvents two intense
bands are observed in the UV region; the lowest energy band
ranges between 285 and 356 nm, while the higher energy band lies
between 269 and 286 nm. The higher energy band seems to be due
to a p–p* transition, while the lower energy band probably arises
due to an intra-ligand charge transfer transition of the terpyridyl-
imidazole ligand. It can be seen from Table 3 that the band maxima
of the receptor shifts to longer wavelengths with an increase in
polarity, as well as with the extent of the hydrogen bonding ability
of the solvent (Fig. S2, ESI†). The effect of different solvents on
the absorption and emission spectra of the Zn(II) complexes of
the receptor have also been studied (Fig. S3, ESI†). The Zn(II)
complex, of composition [Zn(tpy-HImzPh3)2]2+, is prepared in situ
by adding 0.5 equivalents of Zn(ClO4)2 to the tpy-HImzPh3 ligand.
It is shown in Fig. 4 that the lowest-energy absorption band of
Fig. 3 1H spectra of the receptor in the absence and presence of (a) 10
equiv. F- and (b) 0.5 equiv. Fe(ClO4)2 in DMSO-d6.
Fig. 4 Absorption spectra of the receptor and its Zn(II) complex in
chloroform and dimethylformamide solution.
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Table 3 Absorption and luminescence spectral data of the receptor and its Zn(II) complex in different solvents
Free receptor
Zn(II) complex
Emission
max, nm
Emission
max, nm
Absorption lmax
,
Absorption lmax
nm (e, M-1 cm-1)
Solvent
CHCl3
nm (e, M-1 cm-1)
l
t, ns
U
l
t, ns
U
335(br)(50 400)
286(br)(46 600)
448
450
487
492
515
472
415
1.61
0.084
427(15 800)
323(37 600)
287(42 600)
422(16 100)
325(39 700)
286(50 200)
389(36 800)
316(42 700)
286(62 500)
394(26 700)
317(34 200)
288(46 600)
389(23 700)
284(29 100)
620
613
612
630
610
640
—
0.89
&
3.67
1.12
&
3.37
1.03
&
3.34
0.17
&
4.18
0.55
&
2.55
0.92
&
0.126
CH2Cl2
DMF
335(br)(31 100)
283(br)(30 500)
1.83
2.56
2.63
0.72
2.60
1.25
0.106
0.096
0.1
0.08
0.059
0.035
0.03
0.058
—
353(br)(32 600)
278(br)(29 600)
DMSO
CH3OH
(CH3)2CO
Hexane
356(br)(40 350)
278(br)(37 100)
333(br)(34 000)
277(25 000)
0.012
0.11
350(br)(41 150)
402(32 500)
1.28
—
285(br)(7700)
269(33 830)
0.088
—
the free ligand is red-shifted upon complexation by Zn2+ ion: the
maximum in chloroform shifts from 335 to 427 nm. Coordination
of the Zn2+ ion serves to increase electron delocalization on the
complex backbone, causing a red shift in the absorption.35,49
Upon excitation of either of the two absorption maxima,
an intense emission band with a peak lying between 415 nm
(hexane) and 515 nm (methanol) is observed. Fig. 5(a) and
5(b) show the absolute and normalized emission spectra of the
receptor in different solvents. As compared with the absorption
spectra, the emission spectral behavior shows significantly larger
solvatochromism. The observed Stokes’ shifts [(nabs-nem)] can be
ascribed to the distinct PCT effect. Fig. 5(c) shows the photograph
of different emission colors (violet → blue → green → yellow) of
the free ligand in different solvents upon excitation with UV light.
It is interesting to note that as the emission colors were finely tuned
from violet to blue to green to finally yellow; the receptor can
act as a suitable solvent polarity indicator, e.g. a solvtaochromic
probe.49,50 We have also studied the fluorescence decay behavior of
the receptor in different solvents and the changes in decay profiles
of the receptor as a function of solvent is represented in Fig. 6(a).
In all cases, a single exponential fit to the decay profiles was found
single exponential decay of the free receptor, the Zn(II) complex
exhibited complex kinetics that adequately fit with a sum of two
exponentials. In general there is a gradual increase in the lifetime
(2.25 to 4.18 ns) of the longer component as the polarity of the
solvent increases, with the exception of methanol and acetone.
The electronic absorption spectrum of an acetonitrile solution
of [Fe(tpy-HImzPh3)2]2+ shows an intense band at 575 nm which
is due to the FeII(dp)–p*(tpy–HImzPh3) MLCT transition. The
absorption band is shifted to lower energy compared to the
MLCT absorption of the parent [Fe(tpy)2]2+ (551 nm) complex.51
The electronic spectra of the complex also exhibits a series
of higher energy absorptions arising from ligand centered p–
p and n–p transitions. On excitation of any of its absorption
bands, the iron complex does not exhibit luminescence at room
temperature. It is well known in the literature that the photophysics
of the polypyridine Fe(II) complex is complicated by the presence
of ligand field (LF) excited states.34,51 These states arise from
population of the empty metal ds* orbitals. Although optical
transitions to yield the LF excited states are not dipole-allowed
and are therefore of low probability, the LF states can become
populated by crossing from the MLCT states. This occurs readily
in polypyridine Fe(II) complexes where the LF state lies below the
MLCT state.34,51
*
*
2
satisfactory from the c values and the residuals and the measured
lifetime of the receptor lies in the range between 0.72 and 2.65 ns
at room temperature. It is of interest to note that the lifetime of
the receptor increases gradually with the increase in the polarity
of the solvent, except in methanol.
Colorimetric signaling
The effect of complexation of Zn2+ ion to the receptor is also
reflected in the emission profiles in different solvents. Compared
to the free receptor, its Zn(II) complex shows weak emission in
the long wavelength region which probably originates from the
ILCT state of [Zn(tpy-HImzPh3)2]2+ (Fig. S3(b), ESI†).35,49,50 The
effect of different solvents on Zn2+ complexes of the receptor
has also been investigated by time-resolved fluorescence studies
and the decay profiles are shown in Fig. 6(b). As compared with
The sensing ability of the receptor has been studied on a qualitative
basis by visual examination of the anion- and cation-induced
color changes in dimethylformamide-acetonitrile (1 : 9) solutions
before and after the addition of anion and cation, respectively.
Tetrabutylammonium salts of anions and hydrated perchlorate
salts of the cations have been used as substrates for the receptor.
The photograph in Fig. 7 shows the dramatic color changes of
the receptor in the presence of F- and Fe2+ ions in contrast to
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Fig. 6 Time-resolved fluorescence decay profiles of the free receptor (a)
and its Zn(II) complex (b) in different solvents.
Fig. 5 Fluorescence emission: absolute (a) normalized spectra (b) and
different emission colors (c) seen upon excitation of the receptor with UV
light in different solvents.
other anions and cations, respectively. The photograph shows that
among the anions, only F- and OH- ions can cause evolution of a
bright yellow color. On the other hand, addition of 0.5 equivalents
of various metal ions to a solution of the receptor shows an
appreciable change of color from colorless to deep violet only
with Fe2+. Other ions do not show such a dramatic color change,
except for the appearance of a very light pink color in the case of
Co2+ and a pale yellow color with the remaining metal ions. The
visible color change of the receptor in the presence of F- and Fe2+
can be useful for “naked-eye” detection of these ions in solution.
Fig. 7 Color changes that occur when the dimethylformamide-acetoni-
trile (1 : 9) solutions of the receptor are treated with (a) various anions as
their tetrabutylammonium (TBA) salts and (b) various metal cations as
their perchlorate salts.
Cation sensing
The sensing ability of the receptor for metal cations was studied
through UV-vis spectra in dimethylformamide-acetonitrile (1 : 9)
solutions. Fig. 8 shows the changes in the spectral profiles of the
receptor following the addition of 0.5 equivalents of different metal
ions. It is of interest to note that following addition of Fe2+ a strong
MLCT band originates at 575 nm with the evolution of a bright
violet color, in contrast to other metal ions. Whereas the cobalt
complex also shows much weaker absorption in the visible region
(l > 400 nm) due to metal-to-ligand charge-transfer absorption,
the complexes of other metals either absorb weakly in this area or
do not show any absorptions at wavelengths higher than 390 nm.
Interestingly, these events can be distinguished visually as shown
in the picture (Fig. 7). A predominant spectral red-shift from 340
nm to 575 nm in the case of Fe2+ over the other metal ions suggests
that the receptor can act as an effective chemosensor towards the
detection of Fe2+ in solution.
It is of interest to assess the colorimetric sensing ability of
the receptor towards different cationic guests. In order to do
so, UV-vis titration of tpy-HImzPh3 with different transition
metal cations was carried out in dimethylformamide-acetonitrile
(1 : 9). The absorption spectral changes were recorded between
250 and 700 nm for solutions in which the concentration of
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diminution of the intensity of the major peak at 341 nm with a
simultaneous increase in absorbance at 390 and 285 nm occurs.
In this case, successive absorption curves pass through well-
defined isosbestic points at 368 and 324 nm. The red-shift of
the absorption band in the Zn(II) complex could be attributed
to the Zn(II)-induced intraligand charge transfer (ILCT) process
from the triphenyl imidazole group to the tpy moiety, because
coordinated tpy is a better electron acceptor than the free tpy
moiety of the ligand.
For all metal ions, distinct changes in tpy absorption were
observed upon complexation and clear isosbestic points were
found (Fig. 9 and Fig. S4–S9, ESI†), indicating that only a single
equilibrium between two species, namely free and complexed tpy-
HImzPh3, occurs during the titration. Whereas the iron complex
shows intense absorption in the visible region due to metal-to-
ligand charge-transfer absorption, the complexes of other metal
ions only absorb very weakly in this area. Absorption titration
profiles of the free receptor with various metal ions (Fig. 9 and
Fig. S4–S9, ESI,† insets) show a linear increase and a sharp end
point at a metal/ligand ratio of 0.5 : 1, indicating the formation
of a [M(tpy-HImzPh3)2]2+ (1 : 2) complex. Due to the lack of
curvature in the titration curve, no binding constants could be
determined.33c,52
The sensitivity and selectivity of the bifunctional receptor for
different metal ions have also been studied. Spectrophotometric
measurements in the presence of competing cations have been
performed by treating a solution of tpy-HImzPh3 (c = 2.0 ¥
10-5 M) with 0.5 equivalents of Fe2+ in the presence of each of
the other metal ions viz. Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, and
Cd2+. In the competition experiments, we are still able to observe
Fig. 8 Changes in UV-vis spectra of the receptor in dimethylfor-
mamide-acetonitrile (1 : 9) upon the addition of different metal ions.
ligand was kept fixed (2 ¥ 10-5 M), while the concentrations of
metal perchlorates were increased progressively until the limiting
spectrum was reached, when no further changes in absorbance
occurred. Fig. 9(a) and 9(c) show the spectral changes that take
place upon the addition of increasing amounts of Fe(ClO4)2 and
Zn(ClO4)2 to the receptor, respectively. As may be noted, the new
band centered at 575 nm increases linearly with the incremental
addition of Fe2+ ion until the [Fe2+]/[tpy-HImzPh3] reaches 0.5.
Even higher concentrations of Fe2+ do not lead to any further
change. The titration profile based on absorbance at 575 nm and
the clear isosbestic points at 364, 331 and 273 nm imply the
single conversion of free tpy-HImzPh3 to [Fe(tpy-HImzPh3)2]2+.
In contrast to Fe2+, no color changes occurred for Fe3+, indicating
the lack of associative behavior of the receptor with trivalent iron.
On the other hand, with the progressive addition of Zn(ClO4)2,
Fig. 9 Changes in UV-vis (a, c) and photoluminescence (b, d) spectra of tpy-HImzPh3 in dimethylformamide-acetonitrile (1 : 9) upon addition of
Fe(ClO4)2 and Zn(ClO4)2. The inset shows the change of absorption and emission intensity as a function of the equivalents of Fe2+ and Zn2+ ions added.
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the band at 575 nm due to Fe2+ MLCT, though the intensity is a
little lower compared with [Fe(tpy-HImzPh3)2]2+ alone. The results
clearly indicate that the terpyridyl-imidazole receptor has a high
selectivity for Fe2+ over other metal ions. The selectivity of the
receptor for Fe2+ over other metal ions can also be judged by
comparing the epsilon (e) values of the Fe-complex with those
of the other metal complexes at a given wavelength. Table S3,
ESI† shows the e values of complexes of different metal ions
under investigation at three different wavelengths, assuming that
only two species (tpy-HImzPh3) and [M(tpy-HImzPh3)2]2+) exist in
solution, and there is no reason to believe that significant amounts
of intermediate complexes will exist other than the homoleptic
metal complexes at low metal concentrations. Much higher e values
of the Fe-complex over the others at a given wavelength suggests
that the receptor can act as an effective and selective sensor towards
the detection of Fe2+ over other metals in solution.
The electrochemical response of the sensor in the presence of
Fe2+ ion in the negative potential window 0 to -1.7 V has also
been observed with a glassy carbon working electrode. The cyclic
voltammogram of the free receptor shows an irreversible reduction
at -0.82 V with very low current intensity. Incremental addition of
Fe2+ ion to the solution of the receptor resulted in two successive
one-electron quasi-reversible reductions at -1.10 and -1.20 V
(Fig. 11).
Fluorescence titration of the receptor with different metal
cations was also investigated. Fig. 9(b) and 9(d) show the quench-
ing of emission intensity of the receptor on incremental addition of
Fe(ClO4)2 and Zn(ClO4)2, respectively. The insets of Fig. 9 indicate
that complete quenching occurs at 0.5 equiv. of both the metal ions.
It is interesting to note that coordination of the metal ions triggers
the emission quenching of the receptor in all cases except with
Zn2+. In the case of Zn2+, gradual quenching of the fluorescence
intensity at 448 nm accompanied by a distinct linear emission
enhancement at 620 nm occurs with the incremental addition of
Zn2+ ion (Fig. 9(d)). The emission at 620 nm probably arises from
the ILCT state of [Zn(tpy-HImzPh3)2]2+ complex.35
The electrochemical Fe2+ ion recognition and sensing of the
receptor in dimethylformamide-acetonitrile have been examined
using cyclic and square wave voltammetry. As shown in Fig.
10(a), with progressive addition of Fe2+ to the receptor, a new
redox couple is observed at 1.10 V (Pt working electrode) due
to the FeII/FeIII oxidation process, and gradually increases in
current height until the receptor : Fe2+ ratio reaches 2 : 1. The
change in current height for the redox couple with the variation of
concentration of added Fe2+ ion is shown in Fig. 10(b). On addition
of excess Fe2+ ions to the solution of the receptor, a new oxidation
peak is generated at 1.16 V (Fig. S10, ESI†) in the vicinity of the
original peak at 1.10 V. The evolution of a new oxidation couple
following further addition of Fe2+ may be associated with gradual
formation of mono-terpyridyl Fe(II) complexes.
Fig. 11 SWVs (a) and CVs (b) obtained upon incremental addition
of Fe(ClO4)2 to a dimethylformamide-acetonitrile (2 : 3) solution of the
receptor in the negative potential window. The changes in the current
intensities as a function of equivalents of Fe2+ ion added are shown in (b)
and (d), respectively.
It should be noted that the current intensity associated with
the new reduction peaks at -1.10 and -1.20 V due to Fe2+-
bound receptor gradually increases and reaches a limit when
approximately 0.5 equiv. of Fe2+ ion had been added to the
receptor. The observed redox behavior is in consonance with the
behavior of the isolated [Fe(tpy-HImzPh3)2]2+ compound (Fig.
S11, ESI†).
Anion sensing
Sensing of the anions by the receptor has been monitored by
observing the spectral changes that occur in dimethylformamide-
acetonitrile (1 : 9) solutions. As shown in Fig. 12(a), the peaks at
341 and 281 nm remain practically unchanged upon addition of
-
-
10 equiv. of Cl-, Br-, I-, AcO-, H2PO4 , and ClO4 ions to 2.0 ¥
10-5 M solutions of the receptor. On the other hand, following the
addition of excess F-, the band at 341 nm is red-shifted to 420 nm,
indicating that strong interactions occur between the receptor and
F- ion. Again, Fig. 12(b) shows that the emission intensity of the
band at 482 nm undergoes a nominal change with the addition of
all the anions except for F-. With a ten-fold addition of F- ion, the
emission intensity is completely quenched. Thus, the absorbance
and luminescence behavior of the receptor towards the anions
shows a close similarity. These observations are in consonance
with the visual changes already noted in Fig. 7. The red-shift of the
spectral band can be attributed to anion-induced deprotonation
of the imidazole NH proton of the receptor, which increases the
Fig. 10 CVs (a) obtained upon incremental addition of Fe(ClO4)2 to a
dimethylformamide-acetonitrile (2 : 3) solution of the receptor (1.0 ¥10-3
M). The change in the current intensities as a function of equivalents of
Fe2+ ion added is shown in (b).
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Fig. 12 Changes in UV-vis (a) and luminescence (b) spectra of tpy-HImzPh3 in dimethylformamide-acetonitrile (1 : 9) solution upon the addition of
different anions as TBA salts.
electron density at the imidazole fragment leading to a lowering
of the band energy.53
It is to be noted that the very high affinity of fluoride toward
imidazole N–H may be due to the very high stability of the
HF2- complex, whose formation allows N–H deprotonation.53 The
appearance of bright yellow colors following deprotonation of the
N–H proton does not prevent from the colorimetric determination
of the F- ion in solution by the terpyridyl-imidazole derivative,
which is indeed very efficient and suffers from the interference of
only one analyte, OH-.53
The electrochemical F- ion sensing of the receptor has also been
examined by cyclic and square wave voltammetry. It is interesting
to note that with progressive addition of F- to the receptor,
contrary to that of the free receptor, a fully reversible reduction
wave appeared at E1/2 = -0.77 V and the current height of this
couple grows gradually until 10 equiv. of F- ion is added (Fig. 14).
A similar electrochemical response is also observed upon addition
of OH- ion. In contrast to F- and OH-, no noticeable changes
occur in electrochemical responses for the receptor in the presence
of a large excess of other anions such as Cl- and AcO-. The results
indicate that the receptor could prove useful in the fabrication of
an electrochemical sensor for both Fe2+ and F- ions.
In order to obtain quantitative insight into the sensor–anion
interaction, spectrophotometric titration experiments were carried
out with tpy-HImzPh3 in dimethylformamide-acetonitrile (1 : 9)
and various anions. Fig. 13(a) presents the changes in the UV-vis
spectrum of tpy-HImzPh3 as a function of F- ion. As can be seen,
the intensity of the peak at 341 nm decreases gradually with a
simultaneous increase of the band at 420 nm, with the appearance
of two isosbestic points at 370 and 284 nm after the addition
of excess F-. Furthermore, the color of the solution was seen to
change from colorless to yellow. Standard curve-fitting procedures
were then used to derive a binding/equilibrium constant (1.9 ¥ 104
M-1). Spectrophotometric titration of the ligand was also carried
out with TBAOH. The spectral pattern obtained with OH- (Fig.
S12(a), ESI†) has a close resemblance to the spectra of the receptor
with F-. The result probably indicates that the imidazole NH is
deprotonated with a large excess of F- ions.
Photoluminescence titrations of the receptor with various
anions have been carried out in the same way as already described
for spectrophotometric measurements. In Fig. 13(b), the effect
of incremental addition of F- ions to the receptor is shown and
the inset shows that almost complete quenching of luminescence
intensity occurs at ca. 10 equivalents. The change in the emission
spectrum of tpy-HImzPh3 as a function of OH- is also shown in
Fig. S10(b), ESI.†
Sensing in the presence of competing ions
Since tpy-HImzPh3 contains two binding sites for the metal
ions and anion, we investigated the effect of competing F- ion
on the binding ability of tpy-HImzPh3 for metal ions and vice
Fig. 13 Changes in UV-vis (a) and luminescence (b) spectra of tpy-HImzPh3 in dimethylformamide-acetonitrile (1 : 9) upon the addition of F- ion. The
insets show the fit of the experimental absorbance and luminescence data to a 1 : 1 binding profile.
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deprotonation of the imidazole NH protons), rather resulting
in a gradual reduction of the intensity of the band at 575 nm
with concomitant change of color from violet to colorless with
up to 3 equivalents of F- ion. Further addition of F- beyond 3
equivalents leads to enhancement of the intensity of the band at
420 nm with the development of a bright yellow color, similar to
that observed upon the addition of F- ion to the free receptor.
Photoluminescence titrations of the tpy-HImzPh3·Fe2+ complex
with incremental addition of F- is shown in Fig. 15(e). As already
mentioned, the tpy-HImzPh3·Fe2+ complex is non-luminescent at
room temperature. Upon the addition of 3 equivalents of F-,
significant enhancement of emission at 482 nm (corresponding
to that of the free receptor) (Fig. 15(d)) occurs. Upon the addition
of an excess of F- (up to 13 equivalents), the emission of said
band is again completely quenched (Fig. 15(f)). These data clearly
indicate that addition of the first few equivalent of F- leads to
sequestering of the Fe2+ ion from the receptor to form a more
stable ion-pair or complex cation in solution, and anion binding
by the receptor is effectively inhibited by this process.24b,54,55 But in
the presence of excess F- ion, when sequestering of the metal ion
is complete, deprotonation of the imidazole NH proton of tpy-
HImzPh3 occurs. We repeated similar experiments with Zn2+ and
F- (shown in Fig. S14, ESI†), as Zn2+ salts are colorless and it is
possible to observe distinct changes due to metal coordination or
Fig. 14 CVs (a) obtained upon incremental addition of F- ion to a
dimethylformamide-acetonitrile (2 : 3) solution of the receptor in the
negative potential window. The change in the current intensities as a
function of equivalents of F- ion added is shown in (b).
versa. Two different strategies were adopted for carrying out
these experiments. First, we have titrated tpy-HImzPh3 with F-
in the presence of 0.5 equivalents of metal ions. Second, we have
titrated tpy-HImzPh3 with M2+ in the presence of 10 equivalents
of F- ion. Initially, Fe2+ and F- were used for the dual sensing
studies. Spectrophotometric measurements (Fig. 15) show that the
addition of F- to the tpy-HImzPh3·Fe2+ complex did not lead to
any significant red-shifts of the MLCT band (due to anion-induced
Fig. 15 Changes in UV-vis (a–c) and photoluminescence (d–f) spectra of tpy-HImzPh3·Fe2+ in dimethylformamide-acetonitrile (1 : 9) upon incremental
addition of F- ion.
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Fig. 16 Changes in UV-vis (a–c) and photoluminescence (d–f) spectra of tpy-HImzPh3·F- in dimethylformamide-acetonitrile (1 : 9) upon incremental
addition of Fe2+ ion.
decoordination without any interference from the color of the salts.
As can be seen, initial titration of tpy-HImzPh3 and 0.5 equivalents
of Zn2+ with F- results in a drastic reduction of the intensity of the
absorption band at 394 nm, suggesting sequestering of the metal
ion to form an ion pair with F- ion in solution.24b,54,55 However,
when sequestering of the metal ion is complete, further addition
of F- ion leads to enhancement of the intensity of the band at
420 nm. The emission spectral behavior of the tpy-HImzPh3·Zn2+
complex as function of F- is shown in Fig. S14(b), ESI.†
experiment by adding incremental amounts of Fe2+ to a solution
(DMF-MeCN) containing excess F- ion and we observed that the
species generated (ion-pair/complex species) absorb moderately in
the UV region. The sensing studies in the presence of competing
ions indicate that although tpy-HImzPh3 contains distinct binding
sites for both anion and cation, simultaneous binding of both the
species is not possible in this case, unless the total concentration
of the two species is much lower than that a particular binding site
can accommodate.
In the second category of experiment (shown in Fig. 16), which
is carried out in the presence of 10 equivalents of F-, addition of
either Fe2+ or Zn2+ ion leads to a gradual reduction of the intensity
of the absorption band at 420 nm, due to the sequestering of F-
ion. The presence of excess of F- in the solution engages the metal
ions in ion pair and/or complex formation in solution rather than
engaging them in complexation with the receptor. As shown in
Fig. 16, the enhancement of the intensity of the band at 583 nm
for the Fe2+ complex occurs only after addition of more than
3 equivalents of Fe2+ ion. It is to be noted that the absorption
spectral behavior in the UV region differs from that of the receptor
(either in the presence of excess F- or Fe2+). We performed a blank
Conclusions
In conclusion, we have developed a new sensor 4¢-[4-(4,5-diphenyl-
1H-imidazol-2-yl)-phenyl]-[2,2¢:6¢,2¢¢]terpyridine, where the ter-
pyridine moiety has been utilized as the cation binding site and the
imidazole motif as the anion binding site. The receptor can act as
colorimetric sensor for Fe2+ and F- ions in solution. The binding
properties have been confirmed by absorption, emission and
1H NMR spectroscopic techniques and also by electrochemical
studies. The binding site for the Fe2+ ion in the system has been
unambiguously established by single-crystal X-ray diffraction
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study of the Fe(II) complex of the receptor. Anion sensing
studies indicate that in the presence of excess F-, deprotonation
of the imidazole N–H proton of the receptor occurs with the
development of a bright yellow color. It is gratifying to note that
the terpyridylimidazole ligand behaves as a triple-channel sensor
for both Fe2+ and F- ions in solution. It is also interesting to note
that, as the emission color of the receptor was finely tuned from
violet to blue to green to finally yellow, the receptor can act as a
suitable solvatochromic probe.
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Acknowledgements
Financial assistance received from the Department of Science and
Technology, New Delhi [Grant No. SR/S1/IC-33/2010] and the
Council of Scientific and Industrial Research, New Delhi, India
[Grant No. 01(2084)/06/EMR-II], is gratefully acknowledged.
Thanks are due to the DST for providing a single crystal X-
ray diffractometer in FIST and a Time-Resolved Nanosecond
Spectrofluorimeter in the PURSE programme at the Department
of Chemistry of Jadavpur University. C.B., S.D. and D.M thank
CSIR for their fellowships.
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