Fluorescence ratiometric selective recognition of Cu2+ ions by dansyl-naphthalimide dyads

Article abstract of DOI:10.1021/jo901164w

(Chemical Equation Presented) Chimeric dyads 1a and 1b based on dansyl and naphthalimide units linked through the polymethylene group were synthesized, and their photophysical and interactions with various metal ions were investigated under different conditions. These dyads showed dual emission centered at around 375 and 525 nm, respectively, due to the locally excited state of the naphthalimide chromophore and energy-transfer-mediated emission (FRET) from the dansylmoiety. When titrated with various metal ions, these systems exhibited unusual selectivity for Cu²⁺ ions as compared to Na⁺, Li⁺, K⁺, Zn²⁺, Pb²⁺, Hg ²⁺, Co²⁺, Fe²⁺, Cd²⁺, Mg ²⁺, and Ba²⁺ ions and signaled the binding event through inhibition of FRET mediated emission at 525 nm with concurrent enhancement in the emission intensity of the naphthalimide chromophore at 375 nm. The uniqueness of these dyads is that they form stable 2:1 stoichiometric complexes involving sulfonamide functionality and act as visual fluorescence ratiometric probes for the selective recognition of Cu²⁺ ions.

Full text of DOI:10.1021/jo901164w

pubs.acs.org/joc
Fluorescence Ratiometric Selective Recognition of Cu2þ Ions by
Dansyl-Naphthalimide Dyads
Vadakkancheril S. Jisha, Anu J. Thomas, and Danaboyina Ramaiah*
Photosciences and Photonics, Chemical Sciences and Technology Division National Institute for
Interdisciplinary Science and Technology (NIIST) CSIR, Trivandrum 695 019, India
rama@csrrltrd.ren.nic.in; d_ramaiah@rediffmail.com
Received June 4, 2009
Chimeric dyads 1a and 1b based on dansyl and naphthalimide units linked through the polymethylene
group were synthesized, and their photophysical and interactions with various metal ions were
investigated under different conditions. These dyads showed dual emission centered at around 375 and
525 nm, respectively, due to the locally excited state of the naphthalimide chromophore and energy-
transfer-mediated emission (FRET) from the dansyl moiety. When titrated with various metal ions, these
systems exhibited unusual selectivity for Cu2þ ions as compared to Naþ, Liþ, Kþ, Zn2þ, Pb2þ, Hg2þ
,
Co2þ, Fe2þ, Cd2þ, Mg2þ, and Ba2þ ions and signaled the binding event through inhibition of FRET
mediated emission at 525 nm with concurrent enhancement in the emission intensity of the naphthalimide
chromophore at 375 nm. The uniqueness of these dyads is that they form stable 2:1 stoichiometric
complexes involving sulfonamide functionality and act as visual fluorescence ratiometric probes for the
selective recognition of Cu2þ ions.
Introduction
ions has received considerable attention due to their impor-
tance in several biological processes.3 At higher concentra-
tions, Cu2þ ions can be highly toxic to the organisms, since they
can displace other metal ions that act as cofactors in enzyme-
catalyzed reactions.4 Also, the unregulated Cu2þ ions can
cause oxidative stress, and their concentration in neuronal
cytoplasm may contribute to the etiology of Alzheimer’s or
Parkinson’s disease.5 In this context, the design of functional
Development of highly sensitive and specific probes for
various metal ions has been a subject of intense interest due
to their potential applications in clinical biochemistry and the
environment.1,2 Of all metal ions, design of probes for Cu2þ
*To whom correspondence should be addressed. Tel: þ91 471 2515362.
Fax: þ91 471 2490186.
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1515. (b) Fluorescent Chemosensors for Ion and Molecule Recognition;
Desvergne, J. P., Czarnik, A. W., Eds.; Kluwer Academic Publishers: Dordrecht,
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DOI: 10.1021/jo901164w
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Published on Web 07/29/2009
J. Org. Chem. 2009, 74, 6667–6673 6667
2009 American Chemical Society

Jisha et al.
JOCArticle
FIGURE 1. Structures of the dyads 1a,b and the model compounds 2 and 3 under investigation.
molecules that selectively bind to Cu2þ ions and signal the
event through sensitive and easily detectable outputs is highly
important.6 Of the various techniques, the optoelectronic
detection has several advantages, and the fluorescence-based
techniques, in particular, offer high sensitivity.1,7
wherein enhancement in the fluorescence intensity has been
observed upon complexation with Cu2þ ions.11 The low
sensitivity and the high order of interference by chemically
closely related metal ions has thus necessitated the design of
highly selective probes for Cu2þ ions.10g,12 In this context, we
synthesized two chimeric dyads 1a and 1b based on dansyl
and naphthalimide chromophores and the model com-
pounds 2 and 3 for comparison (Figure 1) and investigated
their photophysical properties on interactions with various
monovalent and divalent metal ions. We designed these
dyads because of the fact that their individual units have
been investigated as chemosensors13 and fluorescent labels14
and can, in principle, undergo intramolecular fluorescence
resonance energy transfer (FRET) and photoinduced elec-
tron transfer (PET) reactions.15 Our results demonstrate that
these dyads can interact selectively with Cu2þ ions as com-
pared to other metal ions and signal the binding event
through inhibition of FRET-mediated emission, thereby
indicating their potential use as sensitive fluorescence ratio-
metric probes for the selective recognition of Cu2þ ions.
Recently, the development of fluorescence ratiometric
probes for metal ions has attracted much attention since
they allow the measurement of emission intensities at two
different wavelengths.8 This method provides a built-in
correction for environmental effects (i.e., artifacts as a result
of probe concentration variations) as well as increases the
dynamic range of emission measurements.9 Since the sensi-
tivity and dynamic range of a ratiometric probe are con-
trolled by the ratio of emission intensities, the design of
probes that selectively interact with metal ions and show high
ratiometric signals has been challenging. In particular, ratio-
metric probes for Cu2þ ions is a challenge due to its inherent
paramagnetic nature, and hence, the complexation generally
results in quenching of the fluorescence intensity of the
probe.10 However, there were also a few examples reported
Results and Discussion
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ω-alkylnaphthalimide with dansylchloride (Scheme 1), whereas
the model compounds 2 (90%) and 3 (50%) were synthesized
as per the reported procedure16,17 by the reaction of dansyl
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FIGURE 2. Projection view of the dyad 1a with 50% probability ellipsoids.
SCHEME 1. Synthesis of the Dyads 1a and 1b
chloride with butylamine and p-tert-butylphenol, respectively.
These products were purified through recrystallization and
were characterized on the basis of analytical and spectral data
(Figures S1-S4, Supporting Information). In addition, the
structure of the dyad 1a has been confirmed through X-ray
crystallographic analysis (Figure 2 and Tables S1-S7, Support-
ing Information). This structure shows a distance of 11.6 A
between the naphthalimide and dansyl chromophores, which is
well below the critical distance, required for an effective energy
transfer between the donor and acceptor dyads.
Figure 3 shows the absorption spectra of the dyads 1a and
1b in acetonitrile. These dyads showed absorption maximum
at 332 nm with a shoulder at 339 nm. Similar observations
have been made in other solvents like methanol, ethanol, and
toluene (Figure S5, Supporting Information). In all these
solvents, the absorption spectrum of these dyads is found to
be the sum of the individual units; indicating thereby that no
significant interactions exist between the naphthalimide and
dansyl chromophores in the ground state. Inset of Figure 3
shows the emission spectra of the dyads 1a and 1b. The
fluorescence spectra of these dyads exhibited two emission
maxima at 375 and 525 nm, when excited at 339 nm, where
most of the photons are absorbed by naphthalimide chro-
mophore. Based on the excitation spectral analysis and
literature reports,15a the bands at 375 and 525 nm, respec-
tively, could be assigned to the locally excited state of the
naphthalimide chromophore and fluorescence resonance
energy transfer (FRET) mediated emission from the dansyl
moiety. When compared to the dansyl-based model com-
pound 2, the fluorescence intensity of the dyads 1a and 1b is
significantly quenched. This is because of the fact that the
emission observed at 525 nm in the case of these dyads is due
to the existence of FRET from the excited state of the
naphthalimide chromophore to the dansyl unit and the
FIGURE 3. Absorption and fluorescence emission (inset) spectra
of the dyads 1a and 1b (3 μM) in acetonitrile. Path length of the cell,
1 cm, and the excitation wavelength, 339 nm.
photoinduced electron transfer (PET) reaction from the
dansyl chromophore to the naphthalimide moiety. The dyad
1b with longer spacer length (octamethylene unit) showed
efficient FRET-mediated emission with a fluorescence in-
tensity ratio I525/I375 of 1.2 when compared to the ratio of ca.
0.5 observed with the dyad 1a having a shorter spacer group
(hexamethylene unit). This can be attributed to increased
energy transfer from the naphthalimide chromophore to the
dansyl moiety with the increase in spacer length as observed
in the case of the donor-acceptor systems.18
Metal Ion Binding Properties of the Dyads. As the naptha-
limide- and dansyl-based dyads 1a and 1b interestingly
exhibited intramolecular dual emission, it was our objective
to evaluate their potential use as ratiometric sensors and
identify the ideal conditions for the detection of metal ions in
the aqueous medium. In this context, we have investigated
the interactions of these dyads with various metal ions under
different conditions, including the micellar medium. Of all
the conditions examined, it has been observed that a solvent
system consisting of a mixture (4:1) of water and acetonitrile
containing neutral micelles triton X-100 (TX-100; 2 mM) has
been found to be very effective with respect to the stability of
dyads as well as the selectivity and sensitivity of the metal ion
binding event. For example, Figure 4 shows the changes in
the absorption spectrum of the dyad 1b with the addition of
ꢀ
˚
(18) (a) Joseph, J.; Eldho, N. V.; Ramaiah, D. J. Phys. Chem. B. 2003,
107, 4444. (b) Joseph, J.; Eldho, N. V.; Ramaiah, D. Chem.;Eur. J. 2003, 9,
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21997.
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FIGURE 6. Job’s plot for the complexation of the dyad 1b with
Cu2þ ions in 20% acetonitrile containing neutral micelles TX-100
(2 mM) with increase in addition of Cu2þ ions. The variation of the
emission at 525 nm was plotted as a function of the mole fraction of
1b. Inset shows the Benesi-Hildebrand analysis of the emission
changes of the dyad 1b under similar conditions.
FIGURE 4. Changes in the absorption spectrum of the dyad 1b
(3 μM) in 20% acetonitrile containing micelles TX-100 (2 mM) with
increase in addition of Cu2þ ions. [Cu2þ] (a) 0 and (j) 20 μM. Path
length of the cell, 1 cm, and excitation wavelength, 339 nm.
with the dyads involving the sulfonamide group of the dansyl
moiety.19 As a consequence, the dansyl moiety becomes
incapable of quenching the excited state of the naphthali-
mide chromophore resulting in the revival of blue naphtha-
limide emission, thereby facilitating the visual detection of
Cu2þ ions.
To understand the stability of the complex formed, the
observed fluorescence changes of the dyad 1b in the presence
of Cu2þ ions were analyzed through Job’s20 and Benesi-
Hildebrand plots21(Figure 6). These analyses gave a 2:1
stoichiometry for the complex formed between the dyad 1b
and Cu2þ ions with an association constant of (Kassoc) of 5.2 (
0.1 ꢀ 1010 M-2. The association constant was further calcu-
lated using the curve-fitting method,8e and the binding con-
stant obtained by this method is in agreement with the value
obtained from theBenesi-Hildebrand method. TheMALDI-
TOF mass spectral analysis of this complex showed a molec-
ular mass of 1141.96 (Figure S6, Supporting Information),
which is in agreement with the calculated molecular mass
corresponding to a 2:1 stoichiometric complex between the
dyad 1b and Cu2þ ions. Similar observations have been made
with the dyad 1a (Figures S7-S8, Supporting Information),
albeit withlessersensitivityas comparedtothe dyad1b. In this
case, we observed ca. 6-fold increase in the fluorescence ratio
of I375/I525 in the presence of Cu2þ ions with an association
FIGURE 5. Changes in the fluorescence spectrum of the dyad 1b
(3 μM) in 20% acetonitrile containing neutral micelles TX-100 (2 mM)
with increase in addition of Cu2þ ions. [Cu2þ] (a) 0 and (j) 20 μM.
Path length of the cell, 1 cm, and excitation wavelength, 339 nm.
copper perchlorate in micellar medium. With the increase in
concentration of Cu2þ ions, we observed a decrease in the
absorption band at 340 nm with the concomitant increase in
the absorbance at 284 nm with isosbestic points at 325 and
360 nm. Interestingly, in the fluorescence spectrum of the
dyad 1b, we observed a regular decrease in the intensity of
FRET-mediated emission from the dansyl moiety at 525 nm
with the increase in concentration of Cu2þ ions. Correspond-
ingly, we observed a concomitant increase in the emission
intensity of the naphthalimide chromophore at 375 nm with
an isoemissive point at 450 nm. Further additions of 20 μM
of Cu2þ ions resulted in the complete quenching of the
FRET-mediated emission with ca. 12-fold increase in the
fluorescence intensity ratio of I375/I525. The significant “turn
on” intensity with a blue shift of ca. 150 nm led to the visual
fluorescence ratiometric detection of Cu2þ ions by 1b (insets
of Figure 5). Similar observations have been made with the
dyad 1a having a shorter spacer group. In this case, we
observed ca. 6-fold increase in the fluorescence intensity
ratio of I375/I525 with the addition of 20 μM of Cu2þ ions.
Of the two systems investigated, the dyad 1b was found to be
very sensitive for the selective recognition of Cu2þ ions as
compared to 1a because of the efficient FRET-mediated
emission observed in the former case. The selective quench-
ing of FRET-mediated emission at 525 nm alone upon
addition of Cu2þ ions indicate that these ions form complex
constant of Kassoc = 4 ( 0.1ꢀ 1010 M -2
.
Nature of Metal Ion Complexation. To understand the
nature of the complex formed as well as the functional
groups involved in the coordination, we have analyzed the
1H NMR and Fourier transform infrared spectra (FTIR) of
the dyads 1a and 1b in the presence and absence of Cu2þ ions
and compared with the model compounds 2 and 3. For
example, with the addition of Cu2þ ions, we observed a
downfield shift of Δδ=0.02 for the N-H proton of the dyad
1b, while the peaks corresponding to the aromatic pro-
tons showed significant broadening as well as considerable
downfield shifts in the range of Δδ=0.03-0.07 (Figure S9,
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FIGURE 8. Changes in the absorption and fluorescence (inset)
spectrum of 3 (100 μM) in acetonitrile with increase in addition of
Cu2þ ions. [Cu2þ] (a) 0 and (f) 500 μM. Path length of the cell, 1 cm,
and the excitation wavelength, 339 nm.
FIGURE 7. Changes in absorption and fluorescence (inset) spec-
trum of the model compound 2 (100 μM) in acetonitrile with
increase in addition of Cu2þ ions. [Cu2þ] (a) 0 and (j) 500 μM. Path
length of the cell, 1 cm, and the excitation wavelength, 339 nm.
Supporting Information). In contrast, negligible changes were
observed in the chemical shift values of N-methyl protons of
the dansyl unit and methylene protons of the spacer group. In
the FTIR spectrum of the dyad 1b, the characteristic sulfon-
amide and carbonyl groups stretching frequencies were ob-
served at 1323, 1654, and 1695 cm-1, while the NH stretching
frequency was observed at3284 cm-1 (Figure S10, Supporting
Information). Upon interaction with Cu2þ ions, we observed
significantly decreased stretching frequencies of both NH and
sulfonyl groups, confirming thereby the involvement of these
functional groups in the complexation with Cu2þ ions.
To confirm the role of sulfonamide group of the dyads 1a
and 1b on their selective complexation with Cu2þ ions, we
have investigated the interactions with the model com-
pounds 2 and 3. Of these two model derivatives, the com-
pound 2 possess both dialkylamino and sulfonamide groups
as in the case of 1a and 1b. In contrast, the derivative 3, in
addition to the dialkylamino group, been substituted with
a sulfonate group instead of the sulfonamide functionality.
With an increase in the addition of Cu2þ ions to a solution
of the model compound 2 (100 μM), we observed a regu-
lar decrease in absorbance at 260 and 339 nm, with the
concomitant increase in absorbance at 286 nm with isosbes-
tic points at 265 and 318 nm (Figure 7). At 500 μM of Cu2þ
ions, we observed ca. 50% hypochromicity in the absorp-
tion spectrum of the model compound 2. In the emission
spectrum of the compound 2, we observed regular and
significant fluorescence quenching at 525 nm with the
addition of Cu2þ ions (inset of Figure 7). In contrast, the
model compound 3 showed negligible changes in the absorp-
tion and fluorescence spectra with an increase in concentra-
tion of Cu2þ ions under identical conditions (Figure 8).
These results further demonstrate that the complexation
between Cu2þ ions and the dyads 1a and 1b and the model
compound 2 occur due to the involvement of the sulfonam-
ide group of the dansyl chromophore and rule out the
possibility of participation of the dialkylamino group in
the binding event.
FIGURE 9. Relative changes in the fluorescence intensity of the
dyad 1b (3 μM) in the presence of various metal ions. Inset shows
visual observation of fluorescence changes: (a) 1b alone; (b-d) 1b in
the presence of Cu2þ, Liþ, Hg2þ; (e) equivalent mixture of various
metal ions without Cu2þ ions; (f) equivalent mixture of various
metal ions with Cu2þ ions.
negligible changes in the fluorescence intensity of the dyad
1b (Figure S11, Supporting Information). Similar observa-
tions have been made with the dyad 1a in the presence of
various metal ions (Figure S12, Supporting Information).
The selectivity of the dyad 1b toward Cu2þ ions can be
observed visually since the green fluorescence intensity of
the dyad 1b remained unchanged upon addition of these
metal ions, while with Cu2þ ions, we observed the enhance-
ment in blue fluorescence intensity at 375 nm (inset Figure 9).
Interestingly, the presence of equimolar concentrations of all
other metal ions used for the present studies showed negli-
gible influence on the sensitivity of the detection of Cu2þ ions
by the dyads 1a and 1b. Moreover, the complexation of the
dyads 1a and 1b with Cu2þ ions was found to be reversible as
confirmed by the addition of EDTA under identical condi-
tions.
The novel dyads 1a and 1b having naphthalimide chro-
mophore as the donor and dansyl group as the acceptor
moiety showed negligible interactions in the ground state
but exhibited efficient FRET reaction in the excited state.
The evidence for such a reaction was obtained through
excitation spectral analysis, observation of the spacer length
dependent energy transfer, as well as on the basis of literature
Selectivity of the Metal Ion Complexation. To demonstrate
the selectivity of the dyads 1a and 1b for Cu2þ ions,
we have investigated their interactions with other impor-
tant monovalent and divalent metal ions such as Naþ, Liþ,
Kþ, Zn2þ, Pb2þ, Hg2þ, Co2þ, Fe2þ, Cd2þ, Mg2þ, and Ba2þ
ions under identical conditions (Figure 9). As can be
seen from Figure 9, the addition of these metal ions caused
reports.15a The presence of a favorable distance of 11.6 A
˚
between the donor and acceptor units in the case of the dyad
1a as characterized through X-ray crystal analysis further
J. Org. Chem. Vol. 74, No. 17, 2009 6671

Jisha et al.
JOCArticle
FIGURE 10. Schematic representation of the complexation between the dyad 1b and Cu2þ ions.
indicates that such a reaction is quite possible in these dyads.
In accordance with the above hypothesis, the emission
spectrum of the dyads consisted of two emission maxima,
one at 375 nm and the other at 525 nm, where the latter band
is due to the FRET-mediated emission from the dansyl
moiety. The observation of significant FRET-mediated
emission in the case of the dyad 1b as compared to 1a could
be attributed to the presence of a spacer with an appropriate
length for an effective overlap between the donor and
acceptor groups.
corresponding N-(ω-aminoalkyl)-1,8-naphthalimide (4.2 mmol)
and triethylamine (5.4 mmol) in dry chloroform (15 mL). The
mixture was refluxed for 10 h. After the reaction mixture was
cooled to room temperature (25 °C), the precipitate obtained was
filtered off. The clear filtrate solution was evaporated to dryness
under vacuum to give a pale yellow residue. The product mixture
was chromatographed over silica gel. Elution with a mixture (1:9)
of ethyl acetate and hexane gave the dyads 1a (60%) and 1b
(65%), which were further purified through recrystallization
from acetonitrile.
Dyad 1a (60%): mp 121-122 °C; IR (KBr) νmax 3248, 2927,
1
Investigation of the interactions with various metal ions
indicates that the dyads 1a and 1b undergo selective interac-
tions with Cu2þ ions as compared to other monovalent and
divalent metal ions. Interestingly, such selective interactions
with Cu2þ ions results in the formation of 2:1 stoichiometric
complexes with significant association constants and the
formation as confirmed through MALDI-TOF mass spec-
tral analysis (Figure 10). The driving force for the selective
complexation with Cu2þ ions is due to the presence of
sulfonyl and NH groups in these dyads. The involvement
of these groups in the complexation was evidenced through
1693, 1651 cm-1; H NMR (500 MHz, CD3CN) δ (ppm) 1.1-
1.12 (m, 4H), 1.12-1.13 (m, 4H), 1.254 (d, J = 4 Hz, 2H), 2.80 (s,
6H), 3.93 (t, J = 7.5 Hz, 2H), 5.7 (t, J = 5.5 Hz,1H), 7.20 (d, J=
7.5 Hz, 1H), 7.53-7.59 (m, 2H), 7.78-7.81(m, 2H), 8.14-8.15
(dd,1H), 8.25 (d, J = 8.5 Hz, 1H), 8.31-8.33 (dd, 2H), 8.47-8.51
(m, 3H); 13C NMR (125.77 MHz, CD3CN) δ (ppm) 25.3, 25.7,
27.1, 28.5, 29.0, 39.4, 42.40, 44.3, 114. 8, 118.7, 122.5, 123.0, 126.7,
127.6, 127.7, 128.7, 129.1, 129.3, 129.6, 130.3, 131.4, 133.7, 135.4,
151.7, 163.6; HRMS (FAB) calcd for C30H31N3O4S 531.21,
found 531.23. Anal. Calcd for C30H31N3O4S: C, 68.03; H, 5.90;
N, 7.93. Found: C, 67.90; H, 6.03; N, 7.89.
Dyad 1b (65%): mp 108-109 °C; IR (KBr) νmax 3284,
1
2933,1697, 1654 cm-1; H NMR (500 MHz, CD3CN) δ (ppm)
1
absorption, fluorescence, and H NMR and FTIR spectral
1.05-1.08 (m, 8H), 1.18-1.21 (m, 4H), 1.53-1.59 (m, 2H),
2.80 (s, 6H), 4.01 (t, J=7.5 Hz, 2H), 5.75 (t, J=6 Hz 1H), 7.21
(d, J=7.5 Hz 1H), 7.52-7.59 (m, 2H), 7.76-7.80 (m, 2H), 8.14-
8.16 (dd,1H), 8.25 (d, J = 8.5 Hz, 1H), 8.29-8.31(m, 2H), 8.47-
8.49 (d, J=8.5 Hz, 3H); 13C NMR (125.77 MHz, CD3CN) δ
(ppm) 27.2, 27.9, 28.9, 29.7, 30.0, 30.1, 41.2, 44.0, 46.0, 116.4,
120.3, 124.1, 124.6, 128.3, 129.3, 130.4, 130.7, 130.9, 131.2, 131.9,
132.9, 135.3, 137.0, 153.3, 165.2; HRMS (FAB) calcd for C32H35-
N3O4S 559.23, found 559.11. Anal. Calcd for C32H35N3O4S: C,
68.92; H, 6.33; N, 7.53. Found: C, 68.66; H, 6.51; N, 7.56.
Calculation of Association Constants . The association be-
tween the dyads and Cu2þ ions was analyzed using the fluores-
cence data. The association constants were calculated employ-
ing Benesi-Hildebrand method6c,21 using eqs 1 and 2
analysis and further confirmed by studies with the appro-
priately substituted model compounds 2 and 3. Uniquely, the
complexation of the dyads with Cu2þ ions involving sulfon-
amide group alters the interaction between the naphthali-
mide and dansyl chromophores leading to the disruption of
FRET, thereby enabling the visual fluorescence ratiometric
detection of Cu2þ ions.
Conclusion
In conclusion, we have developed novel donor and acceptor
dyad systems 1a and 1b having naphthalimide and dansyl
units, which exhibit spacer length dependent intramolecular
energy transfer mediated emission from the dansyl moiety in
addition to the emission from the naphthalimide chromo-
phore. These dyads showed selective interactions with Cu2þ
ions as compared to other metal ions and signal the event
through inhibition of energy transfer mediated emission
intensity. The uniqueness of these dyads is that they form
stable complexes with Cu2þ ions and act as visual fluorescence
ratiometric probes for the specific detection of Cu2þ ions.
1
1
1
¼
þ
ð1Þ
KðI -IfcÞ½Cu2þꢁ
ðI -I0Þ ðI -IfcÞ
1
1
1
¼
þ
ð2Þ
1=2
KðI -IfcÞ½Cu2þꢁ
ðI -I0Þ ðI -IfcÞ
where K is the association constant, I is the fluorescence intensity
of the free dyad, I0 is the observed fluorescence intensity of the
dyad-Cu2þ complex, and Ifc is the fluorescence intensity at the
saturation. The plot of 1/(I - I0) vs 1/[Cu2þ 1/2
gave a linear fitting,
]
Experimental Section
indicating a 2:1 stoichiometry between the dyad and Cu2þ ions.
Alternatively, the association constants were also calcula-
ted using the fluorescence titration data using a reported
General Procedure for the Synthesis of the Dyads 1a and 1b.
Dansyl chloride (5.0 mmol) was added to a stirred solution of the
6672 J. Org. Chem. Vol. 74, No. 17, 2009

Jisha et al.
JOCArticle
method for a 1:2 metal-ligand binding mode7b,8e,22 and accor-
ding to eq 3
Acknowledgment. We thank the Council of Scientific and
Industrial Research (NWP-0023) and Department of Science
and Technology, Government of India, for financial support
and SAIF, IIT, Madras, for X-ray structure analysis. This is
contribution No. NIIST-PPD-277 from the National Insti-
tute for Interdisciplinary Science and Technology (NIIST),
Trivandrum.
R2
ð1 -RÞ 2KCF½Mꢁ
1
¼
ð3Þ
where CF denotes the total concentration of ligand in the system
and Ris defined as the ratio between the free dyad concentration [C]
and the total concentration of ligand CF.R was evaluated using eq 4
Supporting Information Available: General experimental
methods and starting material used for the synthesis. Figures
S1-S4 showing the 1H and 13C NMR spectra of the dyads 1a,b
and the model compounds 2 and 3, Figures S5-S12 showing the
changes in absorption and fluorescence spectra of the dyads 1a
and 1b in the presence of various metal ions, and Tables S1-S7
showing the data of X-ray crystallographic structure analysis of
the dyad 1a. This material is available free of charge via the
Internet at http://pubs.acs.org/.
½F -F0ꢁ
R ¼
ð4Þ
½F1 -F0ꢁ
where F1 and F0 are the limiting emission values for R = 1 (in the
absence of metal) and R = 0 (probe 1 is completely complexed with
the metal ion), respectively.
(22) Yang, R.; Li, K.; Wang, K.; Zhao, F.; Li, N.; Liu, F. Anal. Chem.
2003, 75, 612.
J. Org. Chem. Vol. 74, No. 17, 2009 6673