Unusual specificity of a receptor for Nd3+ among other lanthanide ions for selective colorimetric recognition

Article abstract of DOI:10.1021/ic100236c

A rare example of a reversible recognition of Nd³⁺ by a newly synthesized molecular receptor (L₁), having a diazo group as the reporter functionality, is reported. Studies revealed that this receptor eventually forms a [Nd³⁺]₂L₁ type complex, through the formation of the intermediate complex [Nd³⁺]L ₁ following a two-step equilibrium process. Respective equilibrium constants for two successive processes were evaluated based on data obtained from the systematic fluorescence titration. Formation of [Nd³⁺] ₂L₁ caused a detectable change in color, and associated affinity constants were also evaluated from spectrophotometric titration data. A rather unusual binding mode of L₁ for Nd³⁺ ion is established by various spectroscopic studies. The remarkable specificity of L₁ for Nd³⁺, constitutes a rare example of a highly selective receptor for this ion in the presence of excess of all other lanthanide ions.

Full text of DOI:10.1021/ic100236c

Inorg. Chem. 2010, 49, 6909–6916 6909
DOI: 10.1021/ic100236c
Unusual Specificity of a Receptor for Nd3þ Among Other Lanthanide Ions for
Selective Colorimetric Recognition
Priyadip Das, Amrita Ghosh, and Amitava Das*
Central Salt and Marine Chemicals Research Institute (CSIR), G.B. Marg, Bhavnagar: 364002, Gujarat, India
Received February 5, 2010
A rare example of a reversible recognition of Nd3þ by a newly synthesized molecular receptor (L1), having a diazo
group as the reporter functionality, is reported. Studies revealed that this receptor eventually forms a [Nd3þ]2L1 type
complex, through the formation of the intermediate complex [Nd3þ] L1 following a two-step equilibrium process.
3
Respective equilibrium constants for two successive processes were evaluated based on data obtained from the
systematic fluorescence titration. Formation of [Nd3þ]2L1 caused a detectable change in color, and associated affinity
constants were also evaluated from spectrophotometric titration data. A rather unusual binding mode of L1 for Nd3þ ion
is established by various spectroscopic studies. The remarkable specificity of L1 for Nd3þ, constitutes a rare example
of a highly selective receptor for this ion in the presence of excess of all other lanthanide ions.
Introduction
emphasis is to design colorimetric sensors that can compete
with general fluorescence-based sensors in recognizing trace
metal ions, owing to their ability in detecting targeted metal
ion through visual detection. Among various cations, recog-
nition and sensing of certain alkali/alkaline earth/transition-
metal ions have been the focal point of the contemporary
research due to their role in different cellular functions or their
deleterious effects on biological systems and several other
environmental concerns.4 In this regard, reports on the sens-
ing and detection of different lanthanide ions are relatively
scarce despite of their well-known involvement in biologi-
cal/therapeutic/imaging processes and catalytic reactions.5,6
The lanthanides have an ionic radius similar to that of a
calcium ion, but by virtue of possessing a higher charge, they
Cation sensing has emerged as an important area of
research in the last four decades, owing to its application
potential in the area of biology, clinical, and environmental
studies.1,2 As a consequence, a large class of cation sensors,
based on metal-ligand coordination, has emerged.1,2 How-
ever, major efforts for developing such cation sensors were
focused in designing fluorescence-based sensor molecules due
to their demonstrated ability to detect targeted cationic
analyte present in trace/ultratrace quantities.3 In comparison,
development of colorimetric sensors has received much less
attention as a research area in the past. However, recent
*Corresponding author. Telephone: þ91 278 2567760 [672]. Fax: þ91 278
2567562/6970. Email: amitava@csmcri.org.
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Shin, I. J. Am. Chem. Soc. 2006, 128, 14150. (d) Chemosensors of Ion and
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r
2010 American Chemical Society
Published on Web 07/02/2010
pubs.acs.org/IC

6910 Inorganic Chemistry, Vol. 49, No. 15, 2010
Das et al.
Scheme 1. Methodologies Adopted for Synthesis of L and L1
generally have a higher affinity for Ca2þ binding sites present
in biological molecules.7 Influences that different lanthanide
ions (including Nd3þ) have in different biological processes in
living organism are reported by various researchers.8-10
Among different lanthanide ions, smaller trivalent metal ions
are reported to be the best T-channel antagonists, and the
potency for this varied inversely with ionic radii for the larger
M3þ ions.8a Lansman has shown that the strong interaction
of Ln3þs with biologically important proteins, including ion
channels and G protein-coupled receptors, occurs because
these agents share biologically important properties with the
divalent calcium (Ca2þ) cation. Their similarity to Ca2þ with
respect to ionic radii, coordination chemistry, and affinity for
the oxygen-donor groups underlies their strong interaction
with Ca2þ binding sites on a wide range of proteins.8b The
Ln3þ ions are also being used as biochemical probes to study
calcium transport in mitochondria and other organelles.10
Despite such biological and therapeutic importance, selective
sensing of lanthanide ions using chromogenic sensor mole-
cules has not grown as a research area as one would have
expected;11-13 whereas reports describing the lanthanide
recognition based on changes in the fluorescence as the
output signal are not so uncommon.11,12 Among various
lanthanide ions, Nd3þ ion is known for its biological sig-
nificance as well as its application potential in laser and
optoelectronics.14 Despite its vast importance, to date there is
no reference available in the literature on the colorimetric
detection of Nd3þ. Moreover, the close proximity of its ionic
radius with few other lanthanide ions has made the issue of
designing a selective receptor for Nd3þ further complicated
and presumably contributed to the fact that there is no report
in contemporary literature on specific recognition of the
Nd3þ ion.
In this article, we have reported a unique receptor molecule
(L1, Scheme 1) which could bind specifically to the Nd3þ ion
in the presence of all other lanthanide ions present in excess.
Studies revealed that on binding to the Nd3þ, an initial en-
hancement in the luminescence intensity for L1 in Nd3þL1
was observed due to the interrupted photoinduced electron
transfer(PET) process.15 Eventually, inthe presence of excess
Nd3þ, [Nd3þ]2L1 was formed with a red-shifted emission
maximum and a lower emission quantum yield. This was
attributed to an intramolecular charge transfer (ICT) transi-
tion in [Nd3þ]2L1. Further, a sharp change in color could be
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Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 6911
visually detected on formation of [Nd3þ]2L1, and this allows
Luminescence Titration. The standard solutions used for
spectrophotometric titrations were also used for luminescence
titration studies. For all measurements, λext = 508 nm was used
as an excitation wavelength. All titration experiments were per-
formed using 2.3 ꢀ 10-5 M solutions (air equilibrated) of L1 (in
predried and distilled acetonitrile) as the final and effective
concentration, while final concentrations for lanthanide ions
were varied between 0 to 12.02 ꢀ 10-3 M. The binding constant
for each consecutive processes was calculated from changes in
fluorescence intensity with varying [Nd3þ] using nonlinear least-
colorimetric detection of Nd3þ ion.
Experimental Section
Materials and Methods. Phthalic anhydride, bis(3-aminopro-
pyl)amine-1,40-sulfonyl chloride, 4-(dimethyl aminobenzene)azo
benzene, Eu(NO3)3 5H2O, Er(NO3)3 5H2O, Yb(NO3)3 5H2O,
3
3
3
La(OCOCH3)3 H2O, Pr(NO3)3 6H2O, Nd(NO3)3 6H2O, Tb-
(NO3)3 5H2O, Ce(NO3)3 6H2O, Sm(NO3)3 6H2O, and Dy-
(NO3)3 H2O were obtained from Sigma-Aldrich and were used
3
3
3
3
3
3
3
squares fitting (eqs 3 and 4 for 1:1 [Nd3þ] L1 and 1:2 [Nd3þ
] L1,
2
3
3
as received. All the other reagents used were procured from S. D
Fine Chemical, India. Acetonitrile was procured from Merck,
India. All solvents were dried and distilled prior to use following
standard procedure.
respectively, complex formation equilibrium, see also Support-
ing Information).17
!
ꢀ
ꢁ
Fc
F0
F
K1½Nd3þꢁ
¼ 1 þ
1 - 1
ð3Þ
Instrumentation. Electronic spectra were recorded using a
Shimadzu UV-3101 PC or a Cary 500 Scan UV-vis-NIR spectro-
meter, while steady-state luminescence spectral experiments
were carried out with a HORIBA JOBIN YVON spectrofluori-
1 þ K1½Nd3þꢁ
F0
K1 and K2 in eqs 3 and 4 correspond to respective binding
constants for 1:1 and 1:2 complex formations. For eq 3, F0 and
F1 are the fluorescence intensities at 550 nm for the free receptor
1
meter. H NMR spectra were recorded on a Bruker 200 MHz,
500 MHz FT NMR (model: Avance-DPX 200) using CD3CN as
the solvent and tetramethylsilane (TMS) as an internal standard.
The Fourier transform infrared spectroscopy (FTIR) spectra
were recorded using Perkin-Elmer Spectra GX 2000 spectro-
meter.
and the 1:1 ([Nd3þ] L1) complex, respectively. Fc is the fluores-
3
cence intensity for a specific [Nd3þ], and this value is restricted
for the concentration range of (0 - 1.01 ꢀ 10-3 M) for eq 3. The
concentration of the Nd3þ is denoted by [Nd3þ].
For eq 4, F00, F1, and F2 are fluorescence intensities of the
Spectrophotometric Studies. A 1.06 ꢀ 10-4 M solution of
compound L1 (Scheme 1) in acetonitrile was prepared and stored
in dark conditions. This solution was used for all spectroscopic
studies after appropriate dilution. A 0.1 M solution of nitrate salt
of the respective lanthanide was prepared in predried and distilled
acetonitrile and each solution was stored in an inert atmosphere.
For Lanthanum (La3+), acetate salt was used for studies. Solu-
tion of the compound L1 was further diluted for spectroscopic
titrations, and the effective final concentration was adjusted to
2.3ꢀ10-5 M, while the final analyte (lanthanide ion) concentra-
tion for titration was varied from 0 to 1.39ꢀ10-2 M.
The equilibrium constant for the complex formation between
L1 and Nd3þ was evaluated using a nonlinear least-squares
equation (see Supporting Information, eq 4) for the following
reaction:
free receptor L1 and the 1:1 ([Nd3þ] L1) and 1:2 ([Nd3þ
] L1)
2
3
3
complexes, respectively, while Fc represents the fluorescence
intensity for a specific concentration of added [Nd3þ] to the
1:1 complex, and this value is restricted within the concentration
range of (1.01 - 12.02) x10-3 M.
2
Fc
F0
1 þ F1=F0 K1½Nd3þꢁ þ F1=F0 K1K2½Nd3þꢁ
0
0
¼
ð4Þ
2
0
1 þ K1½Nd3þꢁ þ K1K2½Nd3þꢁ
The initial enhancement in emission intensity at 550 nm was
used for a tentative estimation of K1. Successive recalculations
of K1 were performed with eq 3 until a consistent fitting and a
true value for K1 was achieved. A similar procedure was adopted
using this value of K1 and eq 4 for the evaluation of K2.
Synthesis of Bis(phthalimidyl propyl)amine (L). Phthalic anhy-
dride (4.74 g, 32 mmol) was added to a solution of bis(3-
aminopropyl)amine (2.0 g, 15.24 mmol) in 150 mL of predried
and distilled acetonitrile. The reaction mixture was stirred under
reflux conditions for two days under an inert atmosphere. Then
solvent was evaporated off, and 200 mL of ethanol was added to
the solid residue. After stirring for 5 h, the white precipitates were
filtered, collected, and dried over P2O5 to give the desired com-
pound in pure form and was used for reaction without any further
L1 þ nNd3þ f ½Nd3þꢁnL1
while the stability constant for the formation of the compound
[Nd3þ]nL1 could be define as
n
Kn ¼ ½MnL1ꢁ=½L1ꢁ½Mꢁ
ð1Þ
where MnL1 and M stands for [Nd3þ]nL1 and Nd3þ, respectively.
L1 and Nd3þ do not have any absorption maxima above 440 nm,
so the new absorbance maxima that developed at around 508 nm
on addition of Nd3þ to a solution of L1 was attributed to the
formation of a coordination complex [Nd3þ]nL1. We have
selected absorbance values at these two wavelengths (A440 and
1
purification (Yield: 4.15 g; 70%). H NMR (200 MHz, CDCl3,
25 °C, TMS); δ (ppm): 7.82-7.78 (m, 4H, ArH), 7.71-7.66
(m, 4H, ArH), 3.77 (t, J = 6.6 Hz, 4H, -CH2), 2.78 (t, J = 7.0 Hz,
=
4H, -CH2), 1.99-1.92 (m, 4H, -CH2). IR (KBr): νmax/cm-1
3461, 3031, 2936, 2816, 1771, 1711, 1640, 1398, 1364, 1044, 719,
530. ESI-MS (Mþ/z): 392.16 [M þ H]þ (90%). Elemental analysis
C22H21N3O4 (391.15): calcd C, 61.51; H, 5.41; N, 10.74; found C,
61.6; H, 5.3; N, 10.7.
A508) for our further studies. For optical path length (l) of 1 cm,
as used for the present study, the ratio A440/A508 could be
expressed by eq 2 (Supporting Information).
n
A440=A508 ¼ ðKnε
ðKnε
½Mꢁ þ εfL g440Þ=
Synthesis of L1. Bis(phthalimidyl propyl) amine (L; 0.5 g,
1.278 mmol) and K2CO3 (0.265 g; 1.917 mmol) were added in
100 mL dry acetonitrile. The reaction mixture was refluxed for
2 h under inert atmosphere, and then acetonitrile solution of
4-(dimethylamino)phenyl)diazenyl)benzene-1-sulfonyl chloride
fMnL1g440
1
n
½Mꢁ þ εfL g508Þ
ð2Þ
fMnL1g508
1
where [M] = CM - n[MnL1] = (CM - nA440)/ε {M L } and CM
stands for total cooncentration of M.
n
1 440
The absorbance data were analyzed by using a nonlinear
least-squares method and a plot of A440/A508 vs [Nd3þ].16
Appropriate substitution and approximation was used for
evaluation of the stoichiometry and affinity constant (n and
Kn, respectively).
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J. Phys. Chem. 1996, 100, 7135. (d) Wagner, B. D.; McManus, G. J. Anal.
Biochem. 2003, 317, 233. (e) Wagner, B. D.; MacDonald, P. J. J. Photochem.
Photobiol., A 1998, 114, 151.
(16) Valeur, B.; Pouget, J.; Bouson, J. J. Phys. Chem. 1992, 96, 654.

6912 Inorganic Chemistry, Vol. 49, No. 15, 2010
Das et al.
(0.516 g; 1.6 mmol dissolved in about 40 mL of dry acetonitrile)
was added in a dropwise manner to the reaction mixture and
refluxed for 24 h. During the course of reaction, a significant
color change from red to orange was observed. Then the
reaction mixture was allowed to cool to room temperature
and was filtered. Solvent was removed under reduced pressure.
To the resulting residue, 30 mL of double distilled water was
added, and the desired compound was extracted thrice in
chloroform (3 ꢀ 40 mL) layer. The organic phase was collected
and dried over anhydrous sodium sulfate. A light-orange co-
lored oily residue was obtained on removal of the solvent under
vacuum. This was treated with n-hexane, and on constant
stirring, an orange solid was precipitated, which was filtered
and dried to give L1 (0.55 g; 63%) in pure form. 1H NMR (500
MHz, CD3CN, 25 °C, TMS); δ (ppm): 7.86 (d, J = 9.0 Hz, 2H;
ArH), 7.79 (m, 4H, ArH), 7.77-7.6 (m, 8H, ArH), 6.83 (d, J =
9.0 Hz, 2H, ArH), 3.60 (t, J = 7.0 Hz, 4H, -CH2), 3.23 (t, J =
7.2 Hz, 4H, -CH2), 3.09 (s, 6H, -CH3), 1.91-1.85 (m, 4H,
-CH2). IR (KBr): νmax/cm-1 = 3460, 3069, 2926, 1770, 1711,
1601, 1521, 1366, 1332, 1132, 712, 594. ESI-MS (Mþ/z): 679.09
(M þ H)þ (5%), 701.03 (M þ Na)þ (100%). Elemental analysis
C36H34N6O6S (678.23): calcd C, 63.70; H, 5.O5; N, 12.38; found
C, 63.8; H, 5.1; N, 12.3.
with the formulations proposed for these two ligands.
Thus, both L and L1 were isolated with desired purity and
were used for further studies.
Electronic Spectroscopy. Electronic spectrum for the
acetonitrile solution of L1 was recorded and found to have
a broad absorption band at 440 nm (Figure 1), which was
attributed to an intraligand charge-transfer transition band.
However, a new absorption band with λmax at 508 nm
appeared on addition of excess Nd3þ. No such spectral
shift was observed on addition of any other acetonitrile
solutions of lanthanide ions, like La3þ, Pr3þ, Yb3þ, Eu3þ
,
Er3þ, Sm3þ, Tb3þ, Ce3þ and Dy3þ (Figure 1). Further, inter-
ference studies revealed that absorbance intensity and
spectral pattern after the addition of 8.0 ꢀ 10-3 M of
Nd3þ ion remained unchanged even in presence of large
excess (0.4 M) of other lanthanide ions (Supporting
Information).
However, no detectable change in electronic spectra do
not completely rule out the possibility of a very weak or
insignificant binding of either of these lanthanide ions to
the L1, as such binding may not induce sufficient pertur-
bation in the energy of the frontier molecular orbitals to
affect the HOMO-LUMO energy gap and thereby the
electronic spectra.
Results and Discussion
Synthesis. Intermediate ligand L (Scheme 1) was synthe-
sized by reacting bis(3-aminopropyl)amine and phtha-
lic anhydride in appropriate ratio in a reasonably good
yield. Isolated compound L was further used for reaction
with 4-(dimethylamino)phenyl)diazenyl)benzene-1-sulfonyl
chloride synthesis for the desired compound L1. This was
characterized by using various analytical and spectros-
copic techniques, and data thus obtained matched well
A systematic spectrophotometric titration with varying
[Nd3þ] is shown in Figure 2a. Increase in intensity of the
absorbance band at 800, 736, and 581 nm with increasing
[Nd3þ] was solely due to the absorption of the Nd3þ ion
itself. Thus, these peaks were not observed when spectro-
photometric titrations were carried out after appropriate
correction for the [Nd3þ] for each individual spectra
recorded during the titration (Figure 2a inset). Further,
color of the solution of L1 turned intense red on addition
of excess Nd3þ, while no such color change could be
detected in naked eye when other lanthanide ions were
added (Figure 2b). A closer examination of the titration
shows that there was no change in absorption spectral
pattern until [Nd3þ] reaches 1.0 ꢀ 10-3 M (Figure 2a).
However, on further addition of increasing amount of
Nd3þ, a gradual spectral change was observed until
[Nd3þ] reached the limiting value of 1.2 ꢀ 10-2 M.
Beyond this [Nd3þ], no further change in spectral pattern
was observed. The absorbance data were analyzed by
using a nonlinear least-squares method (Figure 3) using
eq 2.13a,16
Figure 1. Absorption spectra of chemosensor ([L1] = 2.0 ꢀ 10-5 (M)) in
presence of different lanthanide ions ([Mnþ] = 8 ꢀ 10-3(M)) in CH3CN
medium.
The analysis provided the stoichiometry for the com-
plex formation between L1 and Nd3þ and was found to be
Figure 2. (a) Absorption spectra of L1 (2.3 ꢀ 10-5 M) in CH3CN in presence of varying concentration of Nd3þ ([Nd3þ] = 0-1.202 ꢀ 10-2 M). Inset:
absorption titration as shown in (a) after appropriate correction for respective [Nd3þ]. (b) Color changes observed for L1 (1.062 ꢀ 10-4 M) in the presence of
different lanthanide ions (from left to right: L1, Eu, Er, Nd, Pr, Sm, Yb, and Tb (tripositive charge of each metal ions is not shown for clarity), while [Mnþ] =
2.0 ꢀ 10-2 M (Mnþ stands for different lanthanide ions).

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 6913
signaling unit through a saturated methelene spacer, one
would expect that the photoinduced electron transfer
(PET) process was the dominant one for its luminescence
quenching and observed lower emission quantum yield.15
This PET process gets interrupted as the lone pair of
electrons on the tertiary Namine atom was involved in
coordination to the Nd3þ center. Appreciable increase in
luminescence intensity for such a receptor unit, on co-
ordination to the metal ion, without any change in band
maxima is generally the main feature of such interrupted
PET processes. Thus, the first set of changes, i.e., en-
hancement in luminescence intensity could account for
the binding of the Nd3þ ion to the N3 donor-receptor
fragment of L1 (A in Scheme 2).
Figure 3. Variation in the ratio of absorbances A508/A440 for a CH3CN
solution of L1 (2.3 ꢀ 10-5 M) as a function of the varying [Nd3þ]. The
solid line represents the best fit with eq 2.
However, one could argue in favor of the coordination to
the Nd3þ center through the OCdO atoms of the two
phthalimide moiety. A close comparison of the FTIR
spectra recorded for L1 and L1 in presence of added Nd3þ
ion only showed a nominal shift from 1711 to 1709 cm-1 for
the CdO stretching frequency (Supporting Information).
One would have expected a more pronounced stretching
frequency shift for CdO in the case of coordination
through one of the two carbonyl functionalities of each
phthalimide moiety, and further there was no evidence for
two different (coordinated and noncoordinated) CdO
stretching frequency bands in the FTIR spectra recorded
in the presence of added Nd3þ. Thus, such a possibility for
the coordination of L1 to Nd3þ ion through OCdO is not
considered. Further, few earlier studies reported metal ion
coordination involving the Nphthalimide atom.18 These led us
to propose a coordination through Namine(Nphthalimide)2 to
the Nd3þ ion for the first equilibrium process that lead to
the observed fluorescence enhancement at 550 nm, owing
to the effective interruption of the PET process (Scheme 2).
The increase in fluorescence intensity of L1 was observed
until [Nd3þ] was 1.01 ꢀ 10-3 M, however beyond this,
[Nd3þ] a small red-shifted emission band (549 nm in
presence of 1.01 ꢀ 10-3 M Nd3þ to 553 nm in the presence
of 12.02 ꢀ10-3 M Nd3þ) with concomitant decrease in
emission intensity was observed (Figure 4b). The depen-
dence of the fluorescence enhancement with an increase in
[Nd3þ] ((0-1.01) ꢀ 10-3 M) is shown in Figure 5.
1:2 with the association constant value of (8019 ( 425)
M
-1 (correlation coefficient of 0.997).
Luminescence Spectroscopy. On excitation either at
440 or 508 nm, a very weak emission band at 550 nm could
be observed in an air-equilibrated acetonitrile solution of
L1. This weak band could be assigned as the CT in nature,
while weak emission is understandable if one considers the
conformational flexibility of the molecule. To ensure that
any Nd3þ-based emission, which could arise due to the
energy transfer from L1 to Nd3þ, was not quenched due to
the presence of the coordinated water molecule, we have
recorded spectra of the Nd3þ ion after replacing the
coordinated H2O molecule with D2O. For this Nd3þ-salt
(Nd(NO3)3 5H2O) was dissolved in D2O and then evapo-
3
rated to dryness under an efficient vacuum system. This
process was repeated four times to ascertain complete
exchange of coordinated H2O molecule with D2O. This
was then dissolved in dry and distilled acetonitrile under
dinitrogen gas (to avoid any possibility of moisture getting
into the solution) to prepare the standard solution. Finally
emission spectra were recorded (in a cuvette fitted with a
dinitrogen gas inlet and outlet tube) in presence of added L1
(effective concentration: [L1] = 2.15 ꢀ 10-5 M and [Nd3þ] =
(0-8.0 ꢀ 10-3) to check whether any new emission band
around at 870 nm (characteristic of the Nd3þ ion) ap-
peared. However, no such new emission band could be
observed. This nullifies any possibility of the ligand to metal
energy transfer process and confirmed that the emission of
the Nd3þ ion, when bound to L1, was not quenched due the
presence of any coordinated water molecule.
Binding constant (K1 = 2978 ( 15 M-1) could be
evaluated following a nonlinear least-squares procedure
(eq 3),17 where F1/F0 (6 ( 0.03) was the fluorescence
response when no further increase in fluorescence inten-
sity was observed on a further increase in added [Nd3þ].
This also confirmed a 1:1 binding stoichiometry. Value
reported for K1 was the average value from five indepen-
dent trials. The proposed 1:1 complex formation was
further confirmed by plotting the double reciprocal plot
of [1/{(F/F0) - 1)}] vs {1/[ Nd3þ]}. Linearity of the plot
(shown as inset of Figure 5) and the goodness of the fit
Systematic studies revealed a gradual increase in lumi-
nescence intensity with a increase in [Nd3þ], until it
reached the value ∼1.01 ꢀ 10-3 M (Figure 4a). However,
as Nd3þ concentration was further enhanced from 1.01 to
12.02 mM, a steady decrease in emission intensity along
with a little red-shifted emission maximum was observed
(Figure 4b). Thus, the fluorescence titration tends to
suggest that the observed electronic spectral changes
shown in Figure 2a could be correlated to a binding
phenomenon that caused the decrease in fluorescence
(Figure 4b). This tends to suggest that two different
equilibrium and perhaps two different complexation
reactions were operational. However, this set off an
obvious question, what phenomena were responsible for
(r = 0.99) confirmed the 1:1 complex ([Nd3þ] L1) forma-
3
tion process (Scheme 2).
This decrease in luminescence intensity at 550 nm was
observed for the [Nd3þ] of 1.01 to 12.02 mM. Thus, this
specific response seems to be for the same binding process
which was responsible for the observed electronic spectral
the fluorescence enhancement of L1 on addition of Nd3þ
?
For L1, the amine functionality (with an unshared pair
of electrons) being covalently coupled to a photoactive
(18) (a) Lingappa, Y.; Rao, S.; Ravikumar, S. R. V. S. S. N.; Rao, P. A.
Radiat. Eff. Defects Solids 2007, 162, 11. (b) Narain, G.; Kla, P. Aust. J. Chem.
1967, 20, 227.

6914 Inorganic Chemistry, Vol. 49, No. 15, 2010
Das et al.
Figure 4. Changes in fluorescence spectral pattern of L1 (2.3ꢀ10-5 M) in presence for varying (a) [Nd3þ] = (0-1.01) ꢀ 10-3 M and (b) [Nd3þ] =
(1.01-12.02) ꢀ 10-3 M using λext = 508 nm.
Scheme 2. Probable Modes of Coordination for L1 to Nd3þ ion
Figure 5. Fluorescence response ratio, Fc/F0 for acetonitrile solution of
L1 as a function of varying [Nd3þ] ((0-1.01) ꢀ 10-3 M). The solid line
shows the fit of the data to eq 3 with K1 = 2978 M-1. The inset shows the
linear double reciprocal plot suggesting 1:1 complex formation.
changes (Figure 2a). Assuming a 1:2 complex formation
with respect to Nd3þ, i.e., [Nd3þ]2 L1, the binding constant
3
associated with this equilibrium process was further eval-
uated from the eq 4, where F1 is the fluorescence inten-
sity of the 1:1 receptor-analyte complex ([Nd3þ] L1) at
556 nm, at the beginning of the titration, and F2 is the
3
intensity of the 1:2 complex ([Nd3þ]2 L1) at 556 nm, res-
3
pectively. A good fit (r = 0.99) for the Fc/F00 vs [Nd3þ] plot
further confirmed 1:2 complex formation. This was also
confirmed by the nonlinear nature of the double reciprocal
plot shown in the inset of Figure 6. Association constant
(K2) for this process was calculated from the luminescence
titration data using eq 8 of the Supporting Information
and was found to be (9667 ( 740) M-1. This value was
Figure 6. A plot of the variation of the ratio Fc/F00 as a function of
close to the one that was evaluated from the spectrophoto-
metric titration (Ka = (8019 ( 425) M-1) data (vide infra).
Appearance of a new absorption band at 508 nm and the
red-shifted emission band on addition of Nd3þ tends to
suggest the formation of an intramolecular charge-transfer
varying [Nd3þ]. The solid line shows the fit of the data to eq 4 with K1 =
2978 and K2 = 9667M-1. The insetshows the nonlineardoublereciprocal
plot, indicating the formation of higher-order complexes.
phthalimide moieties, while according to the alternate
model (B in Scheme 2), the second Nd3þ ion is sandwiched
between two phthalimide moieties. Absence of any evi-
dence for two different CdO stretching frequencies for the
coordinated and noncordinated -CdO functionalities for
L1 in presence of large excess of Nd3þ led us to conclude
that the second model (B in Scheme 2) was the most
probable one. Possibility of the involvement of the sulpho-
nyl (SdO) functionality in the Nd3þ-OdSsulphonyl type
coordination could also be nullified based on the FTIR
studies. FTIR spectrum for L1, recorded in acetonitrile
solution, shows bands at 1335, 1366, and 1132 cm-1, which
are characteristic for the sulphonyl group. However, these
characteristic bands remained almost unchanged and ap-
peared at 1330, 1364, and 1133 cm-1 even after the
addition of 600 mol equiv of Nd3þ. Thus, one would rule
complex on binding tothe secondNd ,
3þ 3h,19 as thisprocess
is generally associated with spectral shift. Thus, the lumi-
nescence quenching process on binding to the second Nd3þ
could be accounted to the photoinduced ICT process
involving dimethylamino azobenzene as the donor func-
tionality and the coordinated phthalimide fragment as the
acceptor unit. Based on the above discussed observations,
two different models could be proposed to accommodate
two different equilibrium processes (Scheme 2). According
to the first model, a second Nd3þ is bound to Nd3þ L1 (C
3
in Scheme 2) involving one of the two OCdO atoms of each
(19) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; McCoy,
C. P.; Maxwell, P. R. S.; Rademacher, J. T.; Rice, T. E. Pure Appl. Chem.
1996, 68, 1443.

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 6915
Figure 7. (a) and (c)Partial1H NMR (inCD3CN) of L (5.1 mmol) and (b) and (d) partial1H NMR (inCD3CN) of L in presence ofadded Nd3þ (100equiv,
0.51 mol).
Scheme 3. Probable Binding Models for [Nd3þ
] L Formation
2
out any possibility of the involvement of sulphonyl moiety
in coordination with Nd3þ, as for such a situation, an
appreciable shift in the IR frequency for the sulphonyl
functionality was expected. The proposed binding stoichio-
metry of 2:1 for [Nd3þ]2L1 and the presence of the co-
ordinated H2O molecules, as shown in Scheme 2, are
predicted based on ES-MS spectra recorded for the
solution mixture containing L1 (2.3ꢀ10-5 M) and Nd3þ
(12.02 ꢀ 10-3 M) in acetonitrile (Supporting Information).
1HMR Studies. Further confirmation of the probable
binding mode for the 1:2 complex could be resolved with
the help of the 1H NMR spectra in absence and presence
of varying amounts of Nd3þ. Owing to the limited
solubility of the receptor (L1) in acetonitrile (the medium
in which binding studies were carried out), L was used as
the model receptor (Scheme 1). Partial 1H NMR spectra
for L (in CD3CN) in absence and presence of added Nd3þ
is shown in Figure 7. Figure 7a and c represent the partial
1H NMR spectra of compound L in CD3CN, while
Figure 7b and d are the partial spectra for L in presence
of 100 mol equiv of Nd3þ ion.
A closer examination reveals that there are some remark-
able differences between spectrum shown as Figure 7a and
b/7c and d. Signal for H(1) is found to be upfield shifted
(2.72-2.1 ppm), while an appreciable downfield shift in the
signal for -N(H) hydrogen (2.35-3.05 ppm) was ob-
served. No such detectable shift for the H(2) atom, except
broadening of the signal, was observed. Interestingly, a
downfield shift of the signal for H(3) (3.67-3.73 ppm)
could be observed for spectra recorded in the presence of
excess Nd3þ. Primarily, these changes in spectral pattern of
the free receptor (L) and the receptor bound to Nd3þ
signified complex formation between the receptor (L) and
Nd3þ. One would expect to see a downfield shift for the
H(1) on coordination of the Namine of L to the Nd3þ center.
However, the observed overall upfield shift could be
attributed to the well-known paramagnetic effect induced
by the bound Nd3þ ion.20 Little broadening with almost
negligible shift in the resonance signal for H(2) could also
be the result of these two opposing influences; downfield
shift due to the metal ion coordination and the influence
of the paramagnetic Nd3þ which generally induces the up-
field shift. Nphthalimide is known to be a weak-coordinating
atom for metal ion coordination, and thus, the longer
3
Nd3þ-N bond distances presumably lower the deshield-
ing effect, which accounts for the overall downfield shift
for H(3) signal. For aromatic H6/7 protons, a downfield
shift was observed (Figure 7c and d). The second Nd3þ
ion could be bound to L either through coordination of
the oxygen of the CdOphthalimide functionality (model 1,
Scheme 3) or the Nd3þ ion could form a sandwich-type
complex with two phthalimide moieties through the
interaction of a π-electron cloud (model 2, Scheme 3).
1H NMR signal for aromatic hydrogens (H6/7) in
[Nd3þ] L shows a multiplicity (Figure 7c), while these
3
signals get downfield shifted with little broadening on
binding to the second Nd3þ. Further, these become more
symmetrical in nature like a singlet (Figure 7d). All this
tends to suggest that a second Nd3þ forms a sandwich-
type complex between the two phthalimide rings of the
receptor molecule L. Any binding of Nd3þ, involving an
oxygen atom of the CdOphthalimide functionality in
[Nd3þ] L, would have made aromatic hydrogen atoms
3
in [Nd3þ]2 L even more dissymmetric in nature, a situa-
3
tion not observed experimentally. These observations
agree well with the results obtained from FTIR studies.
The balance between the up- and downfield shifts of the
protons in Nd3þ complexes is the consequence of change
in the geometry of the complexes upon coordination and
is often very difficult to ascertain. Thus, based on the
results of the electronic, fluorescence, and FTIR studies,
it can be concluded that the probable structure for
[Nd3þ] L1 and [Nd3þ]2 L1 could be best represented,
3
3
respectively, by A and B of Scheme 2. This result was
further consolidated by the results of the detailed H
NMR studies with the model receptor L.
1
Conclusions
Thus, in this article, we have described a unique molecular
receptor that could bind selectively only to Nd3þ in presence of
all other lanthanide ions. An unusual binding mode for the
(20) Ansari, A. A.; Irfanullah, M.; Iftikhar, K. Spectrochim. Acta, Part A
2007, 67, 1178.

6916 Inorganic Chemistry, Vol. 49, No. 15, 2010
Das et al.
receptor (L1) to the Nd3þ was established through detailed 1H
NMR studies of a related model receptor molecule L due to
the limited solubility of L1 in common organic solvents.
Further, studies revealed that this receptor could also be used
for the colorimetric recognition of Nd3þat the parts per million
(ppm) level of concentration. The only example of such a
specific colorimetric receptor is known for Yb3þ among all
other lanthanide ions,13b,d while only two other nonspecific
colorimetric sensors for lanthanides are known.13a,c
for a Senior Research Fellowship. Authors acknowledge
Prof. M.D. Ward, University of Sheffield, for his valuable
suggestions. A.D. thanks Dr. P.K. Ghosh, Director,
CSMCRI, for his encouragement.
Supporting Information Available: Interference study of L1
with Nd3þ ion in the presence of different lanthanides present in
excess; 1H NMR spectra of L1; 13C NMR spectra of L1.; mass
spectra of L1; mass spectra for the desired complex produced
between L1 and Nd3þ according to the probable binding model;
IR spectra of L1 in presence and absence various concentration
of Nd3þ. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. Authors thank DST and CSIR
(India) for supporting this research. A.G. thanks CSIR