Pyrimidine based highly sensitive fluorescent receptor for Al3+ showing dual signalling mechanism

Article abstract of DOI:10.1039/c0ob00171f

A new fluorescent probe (5-[(4-diethylamino-2-hydroxy-benzylidene)-amino]- 1H-pyrimidine-2, 4-dione) (Receptor1) has been synthesized by the Schiff base condensation of 5-aminouracil with 4-(diethylamino)salicylaldehyde. The receptor 1 exhibits high selectively for Al³⁺ in DMSO as well as in aqueous solution even in the presence of biologically relevant cations such as Na ⁺, K⁺, Ca²⁺, Mg²⁺, Pb²⁺ and several transition metal ions. The lowest detection limit for the receptor 1 was found to be 1.62 × 10^-10 M with its linear response towards Al³⁺ in the concentration range of 1.75 × 10 ^-9 to 3.3 × 10^-8 M in DMSO. Receptor 1 is the first ever example where a single molecular probe is able to show imine (CN) isomerization inhibition along with twisted intramolecular charge transfer (TICT) in combinatorial fashion.

Full text of DOI:10.1039/c0ob00171f

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Pyrimidine based highly sensitive fluorescent receptor for Al3+ showing dual
signalling mechanism†
K. K. Upadhyay* and Ajit Kumar
Received 25th May 2010, Accepted 29th July 2010
DOI: 10.1039/c0ob00171f
A new fluorescent probe (5-[(4-diethylamino-2-hydroxy-benzylidene)-amino]-1H-pyrimidine-2, 4-dione)
(Receptor 1) has been synthesized by the Schiff base condensation of 5-aminouracil with 4-(diethyl-
amino)salicylaldehyde. The receptor 1 exhibits high selectively for Al3+ in DMSO as well as in aqueous
solution even in the presence of biologically relevant cations such as Na+, K+, Ca2+, Mg2+, Pb2+ and
several transition metal ions. The lowest detection limit for the receptor 1 was found to be 1.62 ¥ 10-10
M with its linear response towards Al3+ in the concentration range of 1.75 ¥ 10-9 to 3.3 ¥ 10-8 M in
DMSO. Receptor 1 is the first ever example where a single molecular probe is able to show imine
(C N) isomerization inhibition along with twisted intramolecular charge transfer (TICT) in
combinatorial fashion.
1 with high sensitivity towards Al3+ in aqueous solution with a
Introduction
lowest detection limit of the order of 10-10 M has its own worth.
Besides detecting the Al3+ selectively in aqueous solution
through receptor 1, present study also deals with the mech-
anistic aspect of the same. The inhibition of imine (C N)
isomerization15 in association with twisted intramolecular charge
transfer (TICT)16 seem to be operating under present study. Hence,
receptor 1 is the first ever example where a single molecular probe
is able to show imine isomerization inhibition along with TICT in
combinatorial fashion.
Furthermore, this is the only second report for Al3+ enhanced
fluorescence of a chemoreceptor showing dual emission bands.17
Most of the fluororeceptors exhibit only single emission band on
their interaction with a particular analyte. Out of single and dual
emissions it is always the second one which is preferred one due to
its high accuracy and precision in terms of analyte sensing.18 The
solvent effect is always an important effect in most of the studies
so is here also. The fluorescence measurement in aqueous medium
showed single emission band. The mechanistic details have been
explored in a systematic way by performing the Al3+ enhanced
fluorescent studies with two controlled receptors besides receptor
1. Present study is a part of our current research interest to evaluate
the analyte sensing ability of receptors synthesized through simple
reaction protocol.19
Aluminium is widely used for industrial and domestic purposes.
The toxicity of aluminium towards a variety of living beings
including the human beings also is well discussed in the lit-
erature time to time.1 The bone and joint diseases along with
neuronal disorder leading to dementia, myopathy and Alzheimer’s
disease2 are a few serious diseases caused by the toxicity of
Al3+. Out of various methods available for the detection of this
ion3 the spectrofluorometry is widely used one due to its high
sensitivity. For this purpose several fluorescent probes such as
hydrazones,4 Schiff bases,5 coumarin,6 pyrollidine,7 calixarene,8
boron dipyrromethene,9 hydroxyflavone,10 8-hydroxyquinoline,11
oxazoline and imidazoline12 derivatives have been synthesized and
used time to time. All the fluorescent receptors reported for Al3+
till now have a few serious lacunae like interferences by Fe3+ and
Cu2+ most commonly.13 At the same time their synthetic protocols
are a bit tedious one.
Through this communication we are reporting a Schiff base
(Receptor 1) derived from the condensation of 5-aminouracil with
4-(diethylamino)salicylaldehyde as a fluorescent probe for the
selective determination of Al3+ in DMSO as well as in aqueous
solution with almost non-interference by Fe3+ although Cu2+
did interfere. The lowest detection limit for Al3+ with a recently
reported fluorescent probe14 is of the order of 10-9 M while the
same for the receptor 1 was found to be 1.62 ¥ 10-10 M and 4.03 ¥
10-9 M in DMSO and aqueous solution respectively.
To the best of our knowledge, this is for the first time that any
uracil derivative has been exploited as the fluorescent receptor for
any ion. Furthermore, the fluorescent probes for Al3+ detection in
aqueous media is still rare. Hence a fluororeceptor like receptor
Experimental
Synthesis of (5-[(4-diethylamino-2-hydroxy-benzylidene)-amino]-
1H-pyrimidine-2,4-dione) (Receptor 1)
An ethanolic solution of 4-(diethylamino)salicylaldehyde
(4 mmol) was added dropwise, to a solution of 5-aminouracil
(4 mmol) in 30 mL ethanol at room temperature (Scheme 1).
The reaction mixture was further stirred for ~30 min at room
temperature and then refluxed for 6 h on a boiling water bath
leading to formation of a yellow colored product, which was
filtered and dried under vacuum. Other probes 2 and 3 were
synthesized through similar procedure.
Department of Chemistry, Faculty of Science, Banaras Hindu University,
Varanasi, Uttar Pradesh 221005, India. E-mail: drkaushalbhu@yahoo.co.in;
Tel: +91-542-6702488
† Electronic supplementary information (ESI) available: General methods,
1H & 13C NMR, IR and Mass spectra of synthesized receptors; ESI-
mass spectra of receptor 1-Al3+ complex; UV-visible spectral titrations of
receptor 1 with various analytes. See DOI: 10.1039/c0ob00171f
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Scheme 1 Synthesis of fluorescent probes.
Important spectroscopic data for receptor 1
isosbestic point at 410 nm indicated chemical interaction of Al3+
with receptor 1. At this stage the color of solution was olive green,
(see ESI;† Fig. 2) clearly observable to naked eye.
Yield 88%; mp >300 ◦C; MS [M+H] = 303.4 Calc. for
C15H18N4O3 = 302.33; dH (400 MHz, DMSO-d6, Me4Si): 13.22
(1H, –OH), 11.31 (1H, –N3H, Uracil), 11.02 (1H, –N1H, Uracil),
8.99 (1H, –CH N–, Imine), 7.59 (1H, Ar–H), 7.19 (1H, Ar–H),
6.27 (1H, –C6H, Uracil), 6.03 (1H, Ar–H), 3.37 (4H, –CH2), 1.23
(6H, –CH3); dC (400 MHz, DMSO–d6, Me4Si): 162.47, 161.27,
160.67, 150.88, 150.06, 133.41, 132.84, 121.34, 108.51, 103.43,
96.91, 43.72, 12.44; nmax (KBr)/cm-1: 3147, 3064, 2971, 2905, 2816,
2743, 1703, 1574, 1518, 1422, 1352, 1297, 1243, 1213, 1126, 1071,
1006, 851, 815, 598, 550. lmax (DMSO) nm: 387 nm.
Results and Discussion
UV-visible studies
The metal recognition properties of receptor 1 was initially evalu-
ated by UV-visible spectral analysis. Fig. 1 shows the absorption
spectra of receptor 1 in the absence and presence of different
amounts of Al3+ (as its chloride salts). The UV-visible titrations
were carried out in 10 mM DMSO solution of receptor 1. Free
receptor 1 displayed an absorption band with a maximum at
387 nm, which was red shifted (by 62 nm) to 449 nm (Fig. 1)
upon addition of increasing concentrations of Al3+ ions. A clear
Fig. 2 Fluorescence titration curve of 0.5 mM DMSO solution of receptor
1 upon concomitant additions of Al3+ ion (0–3 equivalents).
A series of biologically important transition metal ions such as
Co2+, Ni2+, Cu2+ and Zn2+ (as its chloride salts) also perturbed the
UV-visible spectral pattern of the receptor 1 (see ESI;† Fig. 1).
Addition of Cu2+ to the receptor 1 solution produce absorption
bands at 431 and 441 nm while Zn2+ and Co2+ produced at 430
nm and 436 nm respectively on their concomitant additions to
the receptor solution separately. The spectral change for Ni2+ was
almost similar to that of Al3+ and a new absorption band was
observed at 449 nm with lesser intensity than Al3+. In all these
cases the color of the solution turned light yellow from colorless
(see ESI;† Fig. 2). The alkali, alkaline and Cd2+, Hg2+ or Pb2+
were not able to produce either spectral or visible color change
(see ESI;† Fig. 2) upon their respective additions to the DMSO
solution of receptor 1. Thus the colorimetric detection of Al3+ in
DMSO solution of receptor 1 lacked selectivity.
The binding stoichiometry between Al3+ and receptor 1 was
confirmed through Job’s plot (see ESI;† Fig. 3) which clearly
showed a 1 : 1 stoichiometry between guest (Al3+) and host
(receptor 1). The 1 : 1 stoichiometry for host–guest interaction was
further supplemented by ESI-MS of receptor 1-Al3+ complex as
its chloride salt with receptor 1 (see ESI;† Fig. 4) showing a peak
Fig. 1 UV-visible spectral changes of 10 mM DMSO solution of receptor
1 upon concomitant additions of Al3+ (0–5 equivalents).
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Fig. 3 (a) Proposed binding mode for interaction of Al3+ with receptor 1 (b) Non-fluorescent nature of receptor 2 even after binding with Al3+
.
fluorescent but it became fluorescent when 1 equivalent of Al3+ as
its chloride salt was added to 0.5 mM DMSO solution of receptor
1 (Fig. 2).
In order to have an idea about the binding characteristics of
the receptor 1 towards Al3+ the fluorescent titrations (Fig. 2) were
carried out in 0.5 mM DMSO as well as in aqueous medium by
the concomitant addition of Al3+. The absence of any noticeable
emission in receptor 1 itself on excitation at 449 nm may be
attributed to the cis-trans isomerization of the receptor 1 across
imine (C N) bond.15
Fig. 4 Schematic representation of the formation of PICT (planer
intramolecular charge transfer) and TICT (twisted intramolecular charge
transfer) states of receptor 1.
The addition of Al3+ (1 equiv.) elicits a drastic increase in the
emission intensity of receptor 1 in the form of dual emission bands
at 472 nm and 500 nm in DMSO (Fig. 2). The binding of Al3+ with
the receptor 1 seems to be responsible for the fluorescent emission
of the same. The Al3+ binding with receptor 1 may lead to the
inhibition of its cis-trans interconversion (Fig. 3a) and making
it fluorescent active according to recently reported Wang et.al.,
mechanism.15a At the same time an obviously blue–green emission
(see Graphical abstract) of receptor 1 solution upon addition of
Al3+ can also be easily observed by the naked eye.
at m/z 435.4 (Calc. for C15H18N4O3AlCl3 = 435.67) corresponding
to the 1 : 1 complex. The same mass spectrum showed a relatively
more intense peak at 453.6 (Calc. for C15H18N4O3AlCl3·H2O =
453.68) which may be due to monohydrated form of the receptor
1-Al3+ complex. The binding constant for the complex between
receptor 1 and Al3+ was determined by the non-linear fitting of
the corresponding UV-visible titration data (see ESI;† Fig. 5) in
1 : 1 binding equation20 as (3.02 0.18)¥105 M-1 with satisfactory
correlation coefficient value (R = 0.9983).
Our above mechanistic proposal was further supported by
the non-fluorescent and fluorescent behaviours of the control
compounds 2 and 3 respectively in the presence of Al3+. The only
structural difference between them is of phenolic –OH. Due to
absence of phenolic –OH in compound 2 it is not able to have three
way coordination with Al3+ (Fig. 3b) on the similar line of receptor
1 (Fig. 3a) hence the free rotation around C N is not expected to
Fluorescence studies
Since receptor 1 lacked the specificity towards Al3+ in colorimetric
measurement hence the fluorescence emission was further investi-
gated to test the specificity of receptor 1 towards Al3+ through
excitation at 449 nm (Fig. 2). The receptor 1 itself was not
Fig. 5 (a) Proposed binding mode for interaction of Al3+ with receptor 3 (b) Fluorescence titration curve of 0.5 mM DMSO solution of receptor 3 upon
concomitant additions of Al3+ ion (0–3 equivalents).
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Table 1 Linear response of receptors and value of detection limit with R2
be quenched in this case which was observed in its non-fluorescent
nature. On the other hand the receptor 3 possesses phenolic –OH
hence it is able to have the three way coordination with Al3+ (Fig.
5a) on the similar pattern of receptor 1. Hence the free rotation
around C N is expected to be quenched here also and this was
reflected in the form of its fluorescent behaviour. A number of
research papers in the recent past describing the specific detection
of metal ions of various types through Schiff base receptors have
resorted on the above C N isomerization mechanism.15 In all
these papers the three way coordination of the concerned metal
ion with corresponding receptors has been a common feature for
the locking of the receptors into their trans form and hence making
them able to fluoresce.
Furthermore, the presence of dual fluorescence (at 472 and
500 nm) in DMSO solution of receptor 1 with Al3+ may be a con-
sequence of presence of the normal planar intramolecular charge
transfer (PICT) state due to quenching of imine isomerization
and twisted intramolecular charge transfer (TICT) state due to
presence of N,N-diethylamino group (DEA) respectively. In the
excited state, the donor part (DEA group) of the initially planar
molecule rotates around the amino-phenyl bond which may be
accompanied by the development of a charge separation between
donor and acceptor moieties (Fig. 4). Here it is worth to mention
that in the present study even in the presence of quenching of imine
(C N) isomerization signalling mechanism the TICT process is
observable in the system while in the previous reports15a,b the TICT
process was inhibited.
Detection
Limit/M
Receptor
Linear Range/M
R2
1 (in DMSO)
1 (in Water)
3 (in DMSO)
1.75 ¥ 10-9 to 3.30 ¥ 10-8
8.00 ¥ 10-7 to 2.80 ¥ 10-6
1.00 ¥ 10-8 to 1.00 ¥ 10-7
1.62 ¥ 10-10
4.03 ¥ 10-9
2.70 ¥ 10-9
0.99948
0.99966
0.99963
The detection limit was calculated (see ESI;† Fig. 7) using
fluorescence titration data according to the IUPAC definition.21
The corresponding values of detection limits for the respective
receptors have been given in Table 1. A perusal of the same clearly
established an edge of receptor 1 over 3 in terms of detection
limit. Moreover receptor 1 is certainly more effective in DMSO as
compared to the aqueous medium. At the same time receptor 1 has
also an edge over the previously reported fluorescent probes4–12,14,22
for the Al3+ in terms of its detection limit.
To further support our above explanation of dual fluorescence
of receptor 1 in DMSO, we synthesized another controlled
compound as receptor 3 (Fig. 5a) where N,N-diethylamino group
(DEA) was absent. Our strategy really worked when we observed
only one emission band at 482 nm (Fig. 5b) upon binding of
receptor 3 with Al3+.
Fig. 7 Fluorescence spectra (excitation at 449 nm) of receptor 1 (0.2 mM)
in aqueous solution in the presence of 1 equivalent of Al3+, Na+, K+, Mg2+
The absorption spectra of receptor 3-Al3+ complex absorbed
at 400 nm (see ESI;† Fig. 6). When 0.5 mM DMSO solution of
receptor 3 having one equivalent of Al3+ was excited at 400 nm a
single emission band at 482 nm was observed (Fig. 5b). Thus the
receptor 3 did not show the dual fluorescence phenomena as it was
observed with receptor 1 and this proved our above explanation
regarding the dual emission of receptor 1 in its DMSO solution
with Al3+.
Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+
.
Moreover, when the fluorescence titration of receptor 1 was
performed in aqueous medium (0.5 mM) with concomitant
additions of Al3+ only one emission band at 472 nm (PICT) while
the another band at 500 nm (TICT) was not observed (Fig. 6).
The non-occurrence of TICT band in aqueous solution23 may be
attributed to extremely high polarity of water and its involvement
in hydrogen bonding with N-atom of DEA group of the receptor
1. The twisting of DEA group along amino-phenyl bond is thus
inhibited due to involvement of N atom in hydrogen bonding
interaction with water through its lone pair. Thus the fluorescent
detection of Al3+ by the receptor 1 is possible in aqueous medium
also however the detection limit calculated for Al3+ in water with
receptor 1 was of the order of 10-9 M (Table 1).
Competition experiment of receptor 1 towards Al3+
In order to establish the selectivity of the receptor 1 towards
Al3+ we performed the competition experiment by adding one
equivalent each of Na+, K+, Mg2+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+,
Cu2+, Ni2+, Zn2+, Cd2+, Hg2+, Pb2+ and Al3+ in 0.2 mM aqueous
solution of the receptor 1 separately. Fluorescence spectra were
obtained in each case by exciting receptor 1 at 449 nm. As
shown in Fig. 7, receptor 1 exhibited a selective fluorescence
enhancement only with Al3+ with emission band at 472 nm.
Fig. 6 Fluorescence titration curve of 0.5 m M aqueous solution of
receptor 1 in upon concomitant additions of Al3+ ion (0–18 equivalents).
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Fig. 8 Partial 1H NMR titration experiment of receptor 1 with Al3+ ion; from bottom to top: receptor 1; 1+ 1 equiv. Al3+; 1+2 equiv. of Al3+
.
In contrast, other metal ions did not induce any significant
fluorescence enhancement of receptor 1. This interesting feature
reveals that 1 can serve as a selective fluorescent chemosensor for
Al3+.
In another set of competition experiment the mixture of one
equivalent of receptor 1 and Al3+ in aqueous solution was added
to the solutions of above metal ions separately having 1 equivalent
of each ions. This time our observation was a bit different. The
presence of either Mn2+ or Cu2+ inhibited the fluorescence emission
of the receptor 1 even in the presence of Al3+.
up the new possibility of highly sensitive chemosensors for cations
based on pyrimidine bases.
Acknowledgements
Authors are thankful to CSIR [01(2258)/08/EMR-II], New Delhi
for financial assistance and Dr P. K. Roychowdhury, Associate di-
rector, Chembiotek Research International, Kolkata for enabling
us the NMR and mass spectrometer facility.
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