Rhodamine-based molecular clips for highly selective recognition of Al 3+ ions: Synthesis, crystal structure and spectroscopic properties

Article abstract of DOI:10.1039/c3nj01447a

A novel fluorescent chemosensor based on a rhodamine derivative (L) was designed, synthesized, and used as a selective Al⁺³ ion sensor. Upon addition of Al³⁺ to an aqueous-acetonitrile solution of L, the development of a strong fluor

Full text of DOI:10.1039/c3nj01447a

NJC
View Article Online
View Journal | View Issue
PAPER
Rhodamine-based molecular clips for highly
selective recognition of Al3+ ions: synthesis,
crystal structure and spectroscopic properties†
Cite this: New J. Chem., 2014,
38, 1627
Anamika Dhara,a Atanu Jana,*a Nikhil Guchhait,*a Prasanta Ghoshb and
Susanta K. Kar*a
A novel fluorescent chemosensor based on a rhodamine derivative (L) was designed, synthesized, and
used as a selective Al+3 ion sensor. Upon addition of Al3+ to an aqueous-acetonitrile solution of L, the
development of a strong fluorescence signal by a chelation-enhanced fluorescence (CHEF) process was
observed with an attractive glowing orange emission. This sensor shows high selectivity towards Al3+
ions in the presence of other competing metal ions. The fluorescence quantum yield of L–Al3+ (Ff =
0.30) was found to be very high compared to the bare ligand. The limit of detection (LOD) of Al3+ ions
was calculated to be 2 ꢀ 10ꢁ8 M by fluorescence titration. The 1 : 1 binding stoichiometry of the metal
complex was established by combined UV-vis, fluorescence and TOF-MS spectroscopy.
Received (in Victoria, Australia)
20th November 2013,
Accepted 30th January 2014
DOI: 10.1039/c3nj01447a
www.rsc.org/njc
toxicity of aluminum causes damage to the central nervous
system, is suspected to be involved in neurodegenerative dis-
Introduction
The recognition and sensing of biologically and environmen- eases such as Alzheimer’s and Parkinson’s and is responsible
tally important species have emerged as a significant goal in the for intoxication in hemodialysis patients.4 In the biosphere, the
field of chemical sensors in recent years, since they allow detection and estimation of Al3+ levels have significant impor-
nondestructive and prompt detection of the species (cations tance for human health. Recently, the design and construction
or anions) by a simple fluorescence enhancement (turn-on) or of chemosensors with high selectivity and sensitivity towards
quenching (turn-off) response.1 Aluminum is the third most Al3+ have become the focus in numerous studies in the field of
abundant metallic element in the earth’s crust, which is found supramolecular chemistry. The poor coordination ability,
in water and most biological tissues in its ionic form Al3+. The strong hydration ability, and the lack of spectroscopic charac-
amount of free Al3+ in the surface water is increased by leaching teristics of Al3+ have hindered development of a suitable
from soil due to acid rain. It is toxic to plants and kills fish fluorescence sensor compared to other metal ions.5 Practically,
in acidified water.2 The World Health Organization (WHO) Al3+ is a hard-acid; it is found that Al3+ prefers a coordination
recommended the average daily human intake of Al3+ of around sphere containing N and O as hard-base donor sites.6 As a
3–10 mg and the weekly tolerable dietary intake of 7 mg kgꢁ1 result, most of the reported Al3+ sensors contain mixed nitrogen
body weight.3 The widespread use of aluminum around us in and oxygen donor sites. So, the design and synthesis of highly
the modern society are in water treatment, food additives, sensitive and selective fluorescent chemosensors for Al3+ is still
medicines, and of course, in the production of cooking utensils, in great demand.
aluminum foil, etc. Al3+ toxicity causes microcytic hypochromic
The rhodamine moiety has been used widely in the field
anemia, Al-related bone disease (ARBD), encephalopathy, neuronal of chemosensors, especially as a chemodosimeter, given its
disorder leading to dementia, myopathy, and also affects the fluorescence OFF–ON behavior resulting from its unique struc-
absorption of iron in blood, causing anemia. In addition, the tural architecture and properties. In general, the spirolactam
form of rhodamine derivatives is found to be nonfluorescent,
a Department of Chemistry, University College of Science, University of Calcutta, 92,
A.P.C. Road, Kolkata, 700 009, India. E-mail: skkar_cu@yahoo.co.in;
Fax: +91 33 23519755; Tel: +91 33 24322936
whereas its ring opened amide system gives rise to a strong
fluorescence emission.7 Furthermore, the rhodamine fluoro-
phore exhibits a longer wavelength emission (over 550 nm),
often serving as a sensor for the analyte to avoid the influence
of background fluorescence (below 500 nm).8 Rhodamine spiro-
lactam based chemosensors are attractive because of their
excellent photophysical properties, such as long absorption
b Department of Chemistry, R. K. Mission Residential College, Narendrapur,
Kolkata-103, India
† Electronic supplementary information (ESI) available. CCDC 973400. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3nj01447a
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1627--1634 | 1627

View Article Online
Paper
NJC
and emission wavelengths elongated to the visible region,
high fluorescence quantum yield, and a large absorption
coefficient.9
Here, we report a novel rhodamine-based spirolactam deriva-
tive (L) as a chemosensor for Al3+ 6,10 where the binding pheno-
mena could be probed through binding induced changes in the
electronic spectral pattern via the chelation enhanced fluores-
cence (CHEF)6,10d effect in the presence of Al3+ ions. Interestingly,
binding of these metal ions to L causes color changes, which
could also be detected by the ‘‘naked eye’’. Most interestingly, the
fluorescence emission at 581 nm for rhodamine is relatively
unaffected in the pH range between 4.8 and 9.2. Therefore, we
have speculated that the introduction of the dihydroxybenz-
aldehyde receptor to a rhodamine-based probe would (1) increase
its affinity towards Al3+ ions in aqueous–acetonitrile media, (2)
quickly induce the fluorescent and color responses, that is,
Fig. 1 Crystal structure of L (different color balls indicate different atoms;
blue = N, red = O, black = C and grey = H).
realize the real-time detection, (3) improve the selectivity, and CH3CN–aqueous (8 : 2, v/v) buffered with HEPES, pH = 7.2. A
(4) recognize reversible binding to Al3+ and hence can be useful as significant change in the UV-vis spectral pattern was observed only
a potential chemosensor material. To the best of our knowledge, in the presence of Al3+ among all the other metal ions used (Fig. S7,
there are very few reports on aluminum sensors based on ESI†). Upon gradual addition of Al3+ ions (0–100 mM) to chemo-
rhodamine dyes through the spirocyclic ring-opening mecha- sensor L (10 mM) in CH3CN–H2O (8 : 2, v/v), a concomitant red shift
nism.11 In this work we report a new molecule for selective in the spectral position at 554 nm was observed along with an
detection of Al3+ ions fluorogenically as well as colorimetrically. increase in the absorption intensity. The emergence of the absorp-
tion band at 554 nm was due to the opening of the spirolactam ring
of the rhodamine moiety along with a color change from colorless
to deep magenta (Fig. 2). As depicted in the inset of Fig. 2, the
Results and discussion
absorbance at the 554 nm band as a function of Al3+ concentration
Rhodamine B hydrazide was synthesized following a literature predicted a 1 : 1 stoichiometry for the complex between L and Al3+
method12 and characterized by H NMR spectra, mass data, and ions. The association constant13 between L and the Al3+ ion was
1
FT-IR. It was then condensed with 2,3-dihydroxybenzaldehyde in calculated from the absorption titration result and was found to be
methanol to form L in 71% yield (Scheme 1). The structure of 2.11 ꢀ 103 Mꢁ1 at 25 1C (Fig. 3). Under the same conditions,
compound L was confirmed by its spectroscopic, analytical data addition of other metal ions did not cause any discernible changes.
(1H NMR, 13C NMR, ESI-MS, FT-IR, and Fig. S1–S4, ESI†) and X-ray This unique selectivity of L towards Al3+ can be explained in terms of
crystal structure analysis (Fig. 1). Single crystals of L were obtained the absolute hardness (Z), defined as Z = (I + A)/2, where I and A are
by slow evaporation of the methanol solution (Tables 1 and 2).
the ionization potential and proton affinity, respectively. Namely, it
We have carried out UV-vis titration experiments to understand was reported by Paar and Pearson that Al3+ is the hardest acid
the nature of binding of L to Al3+. The chemosensor L (10 mM) in among all the cations considered in this study14 (Scheme 2). The
CH3CN–H2O (8 : 2, v/v) buffered with 2-[4-(2-hydroxyethyl)piperazin- color of the solution was significantly changed from deep magenta
1-yl]ethanesulfonic acid (HEPES), pH = 7.2 showed an absorption to bright orange when illuminated with a hand-held UV lamp
maximum at 347 nm, suggesting that the spirolactam ring of the (Fig. 4). So it could easily be detected by the ‘‘naked-eye’’ without
rhodamine B unit preferred its ring-closed state under these the need of any other instrumental assistance.
conditions. The selectivity of L has been checked with different
The fluorescence spectrum of L (10 mM) in CH3CN–H2O
biologically important metal ions e.g., Na+, Mg2+, Hg2+, Cu2+, (8 : 2, v/v) buffered with HEPES, pH = 7.2, exhibited an emission
Pb2+, Zn2+, Mn2+, Fe3+, Co2+, Ni2+, Cd2+, Cr3+, In3+, Ga3+ and Al3+ in band at 542 nm when excited at 554 nm. Upon interaction with
Scheme 1 Synthesis of chemosensor L.
1628 | New J. Chem., 2014, 38, 1627--1634
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
Table 1 Crystallographic data of L
Identification code
L
Empirical formula
Formula weight
C35H36N4O4
576.68
Temperature, K
Wavelength, Å
293(2)
0.710730
Crystal system
Space group
Monoclinic
P21
a, Å
b, Å
c, Å
a, 1
9.3382(4)
27.1749(12)
12.0639(5)
90.00
b, 1
102.556(2)
90.00
2988.2(2)
4
1.282
0.085
g, 1
Volume, Å3
Z
Density (calc), Mg mꢁ3
Abs. coefficient, mmꢁ1
F(000)
Fig. 3 Benesi–Hildebrand plot of chemosensor L (10 mM) in CH3CN–H2O
(8 : 2, v/v) with HEPES buffer, pH = 7.2 at 25 1C, absorbance at 554 nm
assuming a 1 : 1 stoichiometry between L and Al3+
.
1224
Crystal size, mm3
y Range (1) for data collection
0.05 ꢀ 0.03 ꢀ 0.02
1.5–28.3
Independent reflections (Rint
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
R1, wR2 [I 4 2s(I)]
)
38 950, 13 485, 0.045
MULTI-SCAN
Full-matrix least squares on F2
13485/1/755
1.121
0.0775, 0.2308
0.5(15)
Flack’s parameter
Largest difference peak
ꢁ0.56, 0.70
and hole (e Åꢁ3
CCDC number
)
973400
Scheme 2 Proposed binding mode of chemosensor L with Al3+
.
Table 2 Selected bond distances (Å) and angles (1) in L
various metal ions e.g., Al3+, Cr3+, In3+, Ga3+, Na+, Mg2+, Pb2+, Hg2+,
Fe3+, Cu2+, Ni2+, Co2+, Mn2+, Cd2+ and Zn2+ a much weaker spectral
response was observed relative to Al3+ at the same concentration
(Fig. 5a). During sequential titration (0–100 mM of Al3+), the
emission band peaked up at 581 nm which was attributed to
delocalization in the xanthene moiety of the rhodamine and
hence emission intensity was increased significantly (Fig. 6).
The observed fluorescence occurred due to the metal binding
event induced an electronic rearrangement within the dye that
opened the spirolactam and yielded a fully conjugated rhodamine
dye. The solution showed an intense orange fluorescence, with an
approximately 15-fold enhancement in the fluorescence intensity
at 581 nm. The inset in Fig. 6 shows the dependence of the
emission intensity at 581 nm on increase of Al3+ ion concen-
tration. This fact means that L could be used as an ‘off–on’
fluorescent chemosensor for Al3+. Relative fluorescence enhance-
ment of L in the absence and presence of various other metal ions
and thereby its selectivity for Al3+ are shown in Fig. 5b.
Selected bonds
Value (Å)
O4–C92
1.219(5)
1.366(5)
1.388(7)
1.336(6)
N13–N14
C122–C123
O16–C123
Selected angles
Value (1)
O4–C92–N13
N13–N14–C121
C121–C122–C123
125.2(4)
122.5(4)
122.5(4)
To investigate the recognition ability between L and the Al3+
ion, both Job’s plot15 and Benesi–Hildebrand plot13 experi-
ments were carried out to determine the binding stoichiometry
of the L–Al3+ complex. The absorption intensity varies through
a maximum at a molar fraction of about 0.5 of Al3+, indicating a
1 : 1 stoichiometry and this is the most possible case of binding
of Al3+ with L (Fig. 7). To confirm further the stoichiometry
between L and the Al3+ ion, TOF-MS analysis was conducted
(Fig. S5, ESI†). A mass peak at m/z 729.09 corresponding to
LꢂAl(SO4)(OMe) is indicative of the formation of a 1 : 1 complex.
Fig. 2 UV-vis spectra of chemosensor L (10 mM) in CH3CN–H2O (8 : 2, v/v)
with HEPES buffer, pH = 7.2 at 25 1C, in the presence of different amounts
of Al3+ ions (0–100 mM). Inset: absorbance at 554 nm as a function of Al3+
concentration, indicating a 1 : 1 metal–ligand ratio.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1627--1634 | 1629

View Article Online
Paper
NJC
Fig. 4 Photographs of chemosensor L (10 mM) in CH3CN–H2O (8 : 2, v/v) with HEPES buffer, pH = 7.2 at 25 1C, in the presence of different metal ions
(100 mM) under (a) visible light and (b) UV light.
Fig. 5 Fluorescence spectra (excitation at 554 nm) of L (10 mM) in CH3CN–H2O (8 : 2, v/v) with HEPES buffer, pH = 7.2 at 25 1C, with addition of
biologically important metal ions e.g., Al3+, Cr3+, In3+, Ga3+, Na+, Mg2+, Pb2+, Hg2+, Fe3+, Cu2+, Ni2+, Co2+, Mn2+, Cd2+ and Zn2+ (100 mM).
Fig. 6 Emission spectra of chemosensor L (10 mM) in the presence of
Fig. 7 Absorbance at 554 nm of L (10 mM) and Al3+ with a total con-
centration of 100 mM in CH3CN–H2O (8 : 2, v/v) with HEPES buffer, pH =
increasing concentrations of Al3+ (0, 15, 25, 30, 35, 40, 45, 50, 55, 60, 65,
70, 75, 80, and 100 mM) in CH3CN–H2O (8 : 2, v/v) with HEPES buffer, pH =
7.2 at 25 1C. Excitation was performed at 554 nm. Inset: fluorescence
intensity at 581 nm as a function of Al3+ concentration.
7.2 at 25 1C, indicating a 1 : 1 metal–ligand ratio.
different metal ions (Cr3+, In3+, Ga3+, Na+, Mg2+, Pb2+, Hg2+,
Fe3+, Cu2+, Ni2+, Co2+, Mn2+, Cd2+ and Zn2+). The fluorescence
From the emission intensity data, the association constant (K)13 response of the L–Al3+ system remains the same by comparison
of L with Al3+ was observed to be 2.56 ꢀ 103 Mꢁ1, indicating with or without the other metal ions (Fig. 9). These findings
strong binding of L with the Al3+ ion with 1 : 1 stoichiometry confirmed the selectivity and effective interaction of probe L
(Fig. 8). Values thus evaluated using two different spectroscopic with Al3+. For practical applications, the detection limit of L was
techniques were in good agreement.
also estimated. The fluorescence titration profile of L (10 mM)
To check the practical ability of chemosensor L as a selective with Al3+ demonstrated that the detection limit16 of Al3+ is
Al3+ ion fluorescent chemosensor, we have carried out competitive 2 ꢀ 10ꢁ8 M which is far below the WHO acceptable limit
experiments in the presence of Al3+ ions mixed with other (0.05 mg Lꢁ1 or 1.85 mM of Al3+) in drinking water (Fig. 10).
1630 | New J. Chem., 2014, 38, 1627--1634
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
rhodamine B. All other protons of L viz. c, d, e, f, g, and h have
been shifted downfield after interaction with Al3+. Further, IR
spectra of chemosensor L with Al3+ ions also confirm the
proposed mechanism (Fig. S6, ESI†). Upon addition of 1 equiv.
of Al3+, the characteristic carbonyl amide stretching frequency
shifts from 1690 cmꢁ1 in L to 1662 cmꢁ1 in the complex,
indicating coordination of carbonyl oxygen with Al3+. These
findings clearly support the ring-opening mechanism.
Reversible binding of Al3+ to L was also established through
spectral studies in the presence of Na2EDTA in CH3CN–H2O
solution. Upon addition of EDTA (excess) to the mixture of L
(10 mM) and Al3+ in CH3CN–H2O (8 : 2, v/v), the color of the
solution disappeared instantly. If Al3+ was added to the system
again, the signal is almost completely recovered with solution
changing from colorless to pink (Fig. S8, ESI†). These findings
Fig. 8 Benesi–Hildebrand plot ([F ꢁ F0/Fx ꢁ F0] vs. 1/[Al3+]) for complexa-
tion between chemosensor L and Al3+ derived from emission spectral data.
indicate that L was a reversible fluorescent probe for Al3+
.
Fluorescence quantum yield (Ffs)17 of L in the free and
Al3+-bound state was found to be 0.02 and 0.30 respectively.
The enhancement of fluorescence of L upon binding with Al3+
has been supported by the results obtained from fluorescence
decay measurements, using the Time Correlated Single Photon
Counting (TCSPC) technique (Fig. 12). The fluorescence decay of L
was fitted with triexponential function with time constant of (t1)
1.2 ns (15.5%), (t2) 4.38 ns (4.5%) and (t3) 0.49 ns (80%). Upon
binding with Al3+ ions the same fluorescence decay was observed at
(t1) 3.05 ns (20%), (t2) 1.63 ns (3%) and (t3) 0.04 ns (76%) (w2 = 1.08
and 1.30) in CH3CN–H2O (8 : 2 v/v) using HEPES buffer, pH = 7.2 at
25 1C. The average fluorescence lifetime of the bare molecules
(2.66 ns) was decreased in the complex (2.07 ns); this was due to the
opening of the spirolactum ring (Table S1, ESI†). Upon addition of
Al3+ to the chemosensor (L) formation of a tight binding complex
occurs (as observed from the higher binding constant) along with
the opening of the spirolactam ring to convert the free (hanging)
rhodamine B part of the complex, thereby increasing non-radiative
decay channels and as a result the average lifetime decreased.
To study the practical applicability, the effect of pH on the
fluorescence response of the new chemosensor L towards Al3+
has also been investigated. Experimental results show that
for free L, under acidic conditions (pH o 3), an obvious
fluorescence off–on (Fig. S9, ESI†) situation appears due to
the formation of the open-ring state because of the strong
protonation, and it shows a fluorescence quenching effect upon
addition of Al3+. In the pH range from 4.8 to 9.2, neither the
color nor the fluorescence (excited at 554 nm) characteristics
of rhodamine could be observed for L, suggesting that the
spirocyclic form was still preferred in this range.
Fig. 9 Fluorescence responses of L (10 mM) to various cations in CH3CN–
H2O (8 : 2, v/v) using HEPES buffer, pH = 7.2 at 25 1C. The blue bars
represent the emission intensities of L in the presence of cations of interest
(100 mM). The red bars represent the change of the emission that occurs
upon the subsequent addition of Al3+ to the above solution. The intensities
were recorded at 581 nm.
Fig. 10 Determination of the detection limit of Al3+ by L (10 mM) in
CH3CN–H2O (8 : 2, v/v) using HEPES buffer, pH = 7.2 at 25 1C, (lex
554 nm and lem = 581 nm).
=
Conclusion
In order to gain insight into the sensing mechanism,
1H NMR titration has been performed by 1 equiv. addition of In summary, we have developed a novel turn-on fluorescent che-
Al3+ to the DMSO-d6 solution of L (Fig. 11). Upon addition of mosensor based on a rhodamine–dihydroxybenzaldehyde conju-
1 equiv. of Al3+, the i proton of L has gradually shifted down- gate. The sensor L displays an excellent selectivity and high
field from 8.85 (free L) to 9.03 ppm. Downfield shifts of both a sensitivity toward the detection of Al3+ in CH3CN–H2O over a wide
and b in the N,N-diethyl group of L clearly indicate that Al3+ range of tested metal ions with remarkably enhanced fluorescent
induces the formation of a delocalised xanthene moiety of intensity and also shows a clear color change from colorless to deep
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1627--1634 | 1631

View Article Online
Paper
NJC
Fig. 11 1H NMR spectra of L and Al3+ ions (DMSO-d6). (b) L only (a) L with 1 equiv. of Al3+
.
Fig. 12 (a) Time-resolved fluorescence decay of L (red) in the absence of any metal ion with the excitation source of lext = 340 nm while monitored
at lem = 581 nm in CH3CN–H2O (8 : 2, v/v) media. (b) Emission decay for L (blue) in the presence of the aluminum ion with the excitation source of
lext = 450 nm while monitored at lem = 581 nm in CH3CN–H2O (8 : 2, v/v) media.
magenta. A comparison of the present probe with other existing Al3+ The solvents were distilled prior to use. The rhodamine B
sensitive ‘‘turn-on’’ fluorescent probes reveals that it is very compe- hydrazide was prepared by literature methods. Elemental ana-
titive and somewhat better than others in respect of all the para- lyses (carbon, hydrogen and nitrogen) were done using a
meters. The present probe is relatively cheap as it involves a facile Perkin-Elmer CHN analyzer 2400. Melting points were deter-
1
two step reaction with the commercially available much cheaper mined using a Buchi 530 melting apparatus. H and 13C NMR
chemicals. Under UV light illumination, one can visually detect even spectra were recorded on Bruker Avance II 500 MHz and Bruker
2 ꢀ 10ꢁ8 M Al3+ in aqueous–acetonitrile buffer solution using this Avance 300 MHz spectrometers in DMSO-d6 and CDCl3. Mass
sensor without the aid of any sophisticated instruments. Thus the spectra were recorded in methanol solvent on a Qtof Micro
chemosensor L is able to serve as a ‘naked eye’ chemosensor for Al3+ YA263. The electronic spectra were recorded in CH3CN–H2O
ions. We believe that L can be used for many practical applications solution on a Hitachi model U-3501 spectrophotometer. IR
in chemical, environmental and biological systems.
spectra (KBr pellet, 400–4000 cmꢁ1) were recorded on a
Perkin-Elmer model 883 infrared spectrophotometer. Emission
spectra were measured using a Perkin Elmer (Model LS-50B)
fluorimeter.
Experimental section
General information
Time resolved spectral measurements
The solvents and reagents were purchased from Sigma-Aldrich Fluorescence lifetimes were measured by the method of Time
commercial sources and were used without any further purification. Correlated Single-Photon counting (TCSPC) using a HORIBA
1632 | New J. Chem., 2014, 38, 1627--1634
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
Jobin Yvon Fluorocube-01-NL fluorescence lifetime spectro- Here DF = Fx ꢁ F0 and DFmax = F ꢁ F0, where F0, Fx, and F are the
meter. The sample was excited using a nanosecond laser diode emission intensities of L considered in the absence of Al3+, at
at 340 nm and 450 nm and the signals were collected at the an intermediate Al3+ concentration, and at a concentration of
magic angle of 54.71.18 The typical time resolution of our complete interaction, respectively, at 581 nm and where K is the
experimental set-up is B800 ps. The decays were deconvoluted binding constant and [C] the Al3+ concentration. The plot of
using DAS-6 decay analysis software. The acceptability of the (F ꢁ F0)/(Fx ꢁ F0) against [C]ꢁ1 and the association constant (K)
fits was judged by w2 criteria and visual inspection of the was obtained using the intercept/slope ratio.
residuals of the fitted function to the data. Mean (average)
Calculation of the detection limit
The detection limit (DL) of L for Al3+ was determined using the
fluorescence lifetimes were calculated using the following
eqn (1):
following equation:
P
P
aiti2
ai ti
tav
¼
(1)
DL = K ꢀ Sb/S
(5)
where K = 2 or 3 (we take 3 in this case), Sb is the standard
deviation of the blank solution and S is the slope of the
calibration curve.
in which ai is the pre-exponential factor corresponding to the
ith decay time constant, ti.
Fluorimetric analysis
General procedures of spectra detection
Stock solutions (2 ꢀ 10ꢁ3 M) of different metal ions such as,
Al3+, Cr3+, In3+, Ga3+, Na+, Mg2+, Pb2+, Hg2+, Fe3+, Cu2+, Ni2+,
Co2+, Mn2+, Cd2+ and Zn2+ were prepared. High concentration
of the stock solutions chemosensor L (10 mM) was prepared in
acetonitrile. Before spectroscopic measurements, the solution
of L was freshly prepared by diluting the high concentration
stock solution to the concentration of desirable solution. The
suspension solutions of L were prepared by dispersing the fine
powder in water. Each time a 2 mL solution of L was filled in a
quartz cell of 1 cm optical path length, and different stock
solutions of cations were added into the quartz cell gradually by
using a micro-pipette. The volume of cationic stock solution
added was less than 100 L with the purpose of keeping the total
volume of testing solution without obvious change. All the
spectroscopic measurements were performed at least in tripli-
cate and averaged.
For measurement of the quantum yields (F) of L and its
complex, we recorded the absorbance of the compounds in
CH3CN–H2O solution. The emission spectra were recorded
using the maximal excitation wavelengths, and the integrated
areas of the fluorescence-corrected spectra were measured. The
fluorescence quantum yield was measured using rhodamine 6G
(F = 0.95 in ethanol) as the reference19 according to the
following equation:20
FX = FS ꢀ (IX/IS) ꢀ (AS/AX) ꢀ (ZX/ZS)2
(2)
where X and S indicate the unknown and standard solution,
respectively, F is the quantum yield, I is the integrated area
under the fluorescence spectra, A is the absorbance and Z is the
refractive index of the solvent.
Association constant
(i) Calculation of the association constant using spectro-
photometric titration data. The association constant for the
formation of the complex, [Al3+–L] was evaluated using the
Benesi–Hildebrand (B–H) plot
Synthesis of chemosensor L
Rhodamine B hydrazide was synthesized following a litera-
ture procedure.12 To a solution of 2,3-dihydroxybenzaldehyde
(0.151 gm, 1.095 mmol) in methanol (10 mL) was added
rhodamine B hydrazide (0.5 gm, 1.095 mmol) in methanol
(10 mL). The resulting solution was heated to reflux at 100 1C
in an oil bath using a fused CaCl2 guard tube for 15 h. A reddish
brown crystalline solid which was precipitated out after 30 min
of stirring was isolated by filtration. The isolated product was
washed with cold methanol several times and dried under
vacuum. A reddish brown crystalline solid was obtained. Yield:
0.45 g, 71%. M.P. ꢁ232 1C (decomp.). FT-IR (KBr, cmꢁ1): n =
2971, 1690, 1661, 1617, 1546, 1516, 1467, 1428, 1373, 1269,
1220, 1119, 1020, 867, 815, 782, 700. 1H-NMR (500 MHz, DMSO-
d6): d (ppm), 1.09 (t, 12H, NCH2CH3, J = 5 Hz), 3.35 (q, 8H,
NCH2CH3), 6.37–6.35 (q, 2H, xanthene-H, J = 5 Hz), 6.46–6.43
(q, 4H, xanthene-H), 6.67 (d, 1H, phen-H, J = 8 Hz), 6.79 (d, 1H,
Ar–H J = 8 Hz), 7.12 (d, 1H, benzo-dioxole-H), 6.79 (d, J = 8 Hz,
1H), 7.12 (d, J = 7.5 Hz, 1H), 7.63–7.59 (q, 2H), 7.93 (d, J = 7.5 Hz,
1H), 9.06 (s, 1H, NQC–H), 9.19 (s, 1H), 10.29 (s, 1H). 13C-NMR
(300 MHz, CDCl3 + DMSO-d6) d (ppm): 12.87, 44.12, 65.94,
1/(A ꢁ A0) = 1/{K(Amax ꢁ A0)C} + 1/(Amax ꢁ A0)
(3)
where A0 is the absorbance of L at absorbance maxima
(l = 554 nm), A is the observed absorbance at that particular
wavelength in the presence of a certain concentration of the
metal ion (C), Amax is the maximum absorbance value that was
obtained at l = 554 nm (for Al3+) during titration with varying
[C], K is the association constant (Mꢁ1) and was determined
from the slope of the linear plot, and [C] is the concentration of
the Al3+ ion added during titration studies. The goodness of the
linear fit of the B–H plot of 1/(A ꢁ A0) vs. 1/[Al3+] for 1 : 1
complex formation confirms the binding stoichiometry
between L and Al3+
.
(ii) Calculation of the association constant using emission
titration data. Association constant was calculated using the
Benesi–Hildebrand equation:
1/DF = 1/DFmax + (1/K[C])(1/DFmax
)
(4)
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1627--1634 | 1633

View Article Online
Paper
NJC
W. J. Walker and P. R. Bloom, Nature, 1986, 324, 140;
(d) G. Berthon, Coord. Chem. Rev., 2002, 228, 319; (e) D. R.
Burwen, S. M. Olsen, L. A. Bland, M. J. Arduino, M. H. Reid
and W. R. Jarvis, Kidney Int., 1995, 48, 469.
5 K. Soroka, R. S. Vithanage, D. A. Phillips, B. Walker and
P. K. Dasgupta, Anal. Chem., 1987, 59, 629.
6 (a) D. Maity and T. Govindaraju, Inorg. Chem., 2010, 49, 7229;
(b) D. Maity and T. Govindaraju, Chem. Commun., 2010,
46, 4499.
7 B. Valeur, Molecular Fluorescence: Principles and Applications,
Wiley-VCH Verlag GmbH, New York, 2001, ch. 10.
8 (a) G. Grynkiewicz, M. Poenie and R. Y. Tsien, J. Biol. Chem.,
1985, 260, 3440; (b) A. Minta and R. Y. Tsien, J. Biol. Chem.,
1989, 264, 19449; (c) E. Kimura and T. Koike, Chem. Soc.
Rev., 1998, 27, 179.
97.84, 105.17, 108.70, 118.23, 119.26, 119.72, 120.74, 123.51,
124.28, 128.14, 129.34, 134.44, 146.00, 146.41, 149.07, 151.65,
153.14, 164.03; ESI-MS: m/z calculated for C35H37N4O4 [M+H]+
(m/z): 577.28, found 577.6. Anal. calc. for C35H36N4O4: C, 73.02;
H, 6.36; N, 9.76; found: C, 72.90; H, 6.29; N, 9.72.
Synthesis of an L–Al3+ complex
A 5 mL methanolic solution of Al2(SO4)3ꢂ16H2O (0.054 g,
0.0867 mmol) was added dropwise to a magnetically stirred
solution (5 mL) of L (0.05 g, 0.0867 mmol) in methanol. The
color of the ligand solution changed from almost colorless to
deep magenta upon addition of Al2(SO4)3ꢂ16H2O. After two
hours of stirring at room temperature, the solution was dried
using a rotary evaporator which yielded a magenta L–Al3+
complex. The complex was characterized by mass spectral
1
studies, FT-IR and H NMR studies.
9 H. Kim, M. Lee, H. Kim, J. Kim and J. Yoon, Chem. Soc. Rev.,
2008, 37, 1465.
Crystallographic measurements
10 (a) T. Li, R. Fang, B. Wang, Y. Shao, J. Liu, S. Zhang and
Z. Yang, Dalton Trans., 2014, 43, 2741; (b) C.-H. Chen, D.-J.
Liao, C.-F. Wan and A.-T. Wu, Analyst, 2013, 138, 2527;
(c) S. H. Kim, H. S. Choi, J. Kim, S. J. Lee, D. T. Quang and
J. S. Kim, Org. Lett., 2010, 12, 560; (d) K. K. Upadhyay and
A. Kumar, Org. Biomol. Chem., 2010, 8, 4892; (e) Z.-C. Liao,
Z.-Y. Yang, Y. Li, B.-D. Wang and Q.-X. Zhou, Dyes Pigm.,
2013, 97, 124.
11 (a) S. B. Maity and P. K. Bharadwaj, Inorg. Chem., 2013,
52, 1161; (b) A. Dhara, A. Jana, S. Konar, S. K. Ghatak, S. Ray,
K. Das, A. Bandyopadhyay, N. Guchhait and S. K. Kar,
Tetrahedron Lett., 2013, 54, 3630; (c) C.-Y. Li, Y. Zhou, Y.-F.
Li, C.-X. Zou and X.-F. Kong, Sens. Actuators, B, 2013,
186, 360.
12 (a) U. Anthoni, C. Christophersen, P. Nielsen, A. Puschl and
K. Schaumburg, Struct. Chem., 1995, 3, 161; (b) Y. Xiang,
A. Tong, P. Jin and Y. Ju, Org. Lett., 2006, 8, 2863.
13 K. A. Connors, Binding Constants, Wiley, New York, 1987.
14 R. G. Paar and R. G. Pearson, J. Am. Chem. Soc., 1983, 105, 7512.
15 W. C. Vosburgh and G. R. Copper, J. Am. Chem. Soc., 1941,
63, 437.
Measurements were done on a Bruker SMART APEX II CCD area
detector equipped with graphite monochromated Mo Ka radia-
tion (k = 0.71073 Å) source in o scan mode at 293 K. The
structures of the complexes were solved using the SHELXS-97
package of programs and refined by the full-matrix least square
technique based on F2 in SHELXL-97.21 All non-hydrogen atoms
were refined anisotropically. Positions of hydrogen atoms attached
to carbon atoms were fixed at their ideal position.
Acknowledgements
A. D. thanks CSIR, New Delhi, India, for financial support by
awarding a senior research fellowship (Sanc. No. 01(2401)/
10/EMR-II, dated 05.01. 2011).
References
1 For books and reviews: (a) A. W. Czarnik, Fluorescent
Chemosensors for Ion and Molecule Recognition, American
Chemical Society, Washington DC, 1993; (b) A. P. de Silva,
H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P.
McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997,
97, 1515; (c) B. Valeur and I. Leray, Coord. Chem. Rev., 2000,
16 G. L. Long and J. D. Winefordner, Anal. Chem., 1983,
55, 712A.
17 (a) K. Teuchner, A. Pfarrherr, H. Stiel, W. Freyera and
D. Leupold, Photochem. Photobiol., 1993, 57, 465;
(b) W. Freyer, S. Dahne, L. Q. Minh and K. Teuchner,
Z. Chem., 1986, 26, 334; (c) H. Stiel, K. Teuchner, A. Paul,
W. Freyer and D. Leupold, J. Photochem. Photobiol., A, 1994,
80, 289.
´
´˜
´
205, 3; (d) R. Martınez-Manez and F. Sancenon, Chem. Rev.,
2003, 103, 4419; (e) J. F. Callan, A. P. de Silva and D. C.
Magri, Tetrahedron, 2005, 61, 8551; ( f ) L. Basabe-Desmonts,
D. N. Reinhoudt and M. Crego-Calama, Chem. Soc. Rev.,
2007, 36, 993.
18 (a) J. R. Lakowicz, Principles of Fluorescence Spectroscopy,
Plenum, New York, 1999; (b) A. Jana, P. K. Sukul, S. K.
Mandal, S. Konar, S. Ray, K. Das, J. A. Golen, A. L. Rheingold,
S. Mondal, T. K. Mondal, A. R. K. Bukhsh and S. K. Kar,
Analyst, 2014, 139, 495.
2 (a) M. A. M. Rogers and D. G. Simon, Age Ageing, 1999,
28, 205; (b) T. P. Flaten and M. Ødegard, Food Chem.
Toxicol., 1988, 26, 959; (c) J. Ren and H. Tian, Sensors,
2007, 7, 3166; (d) R. A. Yokel, Neurotoxicology, 2000, 21, 813.
3 (a) J. Barcelo and C. Poschenrieder, Environ. Exp. Bot., 2002,
48, 75; (b) Z. Krejpcio and R. W. Pol Wojciak, J. Environ. 19 R. F. Kubin and A. N. Fletcher, J. Lumin., 1952, 455, 27.
Stud., 2002, 11, 251.
20 B. Bhattacharya, S. Nakka, L. Guruprasad and A. Samanta,
4 (a) G. D. Fasman, Coord. Chem. Rev., 1996, 149, 125;
J. Phys. Chem. B, 2009, 113, 2143.
(b) P. Nayak, Environ. Res., 2002, 89, 111; (c) C. S. Cronan, 21 G. M. Sheldrick, Acta Crystallogr. Sect. A, 2008, 64, 112.
1634 | New J. Chem., 2014, 38, 1627--1634
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014