Highly sensitive and selective chemosensors for Cu2+ and Al3+ based on photoinduced electron transfer (PET) mechanism

Article abstract of DOI:10.1039/c5ra00123d

Two novel acridine based chemosensors 7a and 7b were synthesized and configured as "fluorophore-spacer-receptor" systems based on photoinduced electron transfer. The probes 7a and 7b exhibited high selectivity and sensitivity for the detection of Cu²

Full text of DOI:10.1039/c5ra00123d

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Research Highlights
1) Two new fluorescent PET chemosensors were designed and synthesised
from acridine core.
2) Sensors can be used to monitor Cu2+ and Al3+ in CH3CN.
3) The detection limits for 7a-Cu2+ and 7b-Al3+ were calculated to be 2.8 x
10-7 M and 5.8 x 10-7 M, respectively.
4) Binding study of metals to the probes showed that the nature of receptor
plays very important role in the selective detection of metal ions.

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A highly sensitive and selective chemosensors for Cu2+ and Al3+ based on
photoinduced electron transfer (PET) mechanism
Santosh Chemate, Nagaiyan Sekar*
Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical
Technology, Mumbaiꢀ 400 019 (India)
Email: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com Tel: + 91 22 3361 2707.
Abstract: Two novel acridine based chemosensors 7a and 7b were synthesized and configured as
“fluorophoreꢀspacerꢀreceptor” systems based on photoinduced electron transfer. The probes 7a and
7b exhibited high selectivity and sensitivity for the detection of Cu2+ and Al3+ respectively over
commonly coexistent metal ions in CH3CN. The binding association constants (Ka) of 7aꢀCu2+ and
7bꢀAl3+ were obtained to be 4.0 x 104 Mꢀ1 and 1.7 x 104 Mꢀ1 in CH3CN, and the corresponding
detection limits were calculated to be 2.8 x 10ꢀ7 M and 5.8 x 10ꢀ7 M, respectively. The fluorescence
response of 7aꢀCu2+ and 7b-Al3+ with respect to pH change was studied and the result demonstrated
fluorescence enhancement was observed in the pH range of 7.0 ꢀ 9.0. The chromophores were
characterized by FTꢀIR, 1HꢀNMR, 13CꢀNMR and HR Mass spectral analysis.
Keywords: Metal sensors, acridine, emission, copper, aluminum, quantum yield.
1. Introduction
The designing and synthesis of fluorescent chemosensors for transition metal ions in various
biological systems are of growing interest because of their fundamental role in chemical and
biological processes1–6. Copper is the third most abundant transition metal ion (after Fe2+ and Zn2+)
in human body and is involved in various metabolic processes such as copper combines with proteins
to produce enzymes involved in oxygen processing and also acts as a catalyst in body functions7–9. It
is an essential element at very low concentration for all the organisms but in high concentration it is
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hazardous to living organisms. Wherever excess concentration of Cu2+ in human body causes severe
neurological diseases such as Alzheimer’s disease, Menkes, Wilson’s disease and Parkinson’s
disease10–12. Recently copper ions in prolonged exposure to human body have been suspected to
cause children’s liver and kidneys damage12. Therefore it is important to detect copper at very low
concentration in the biological systems. It is also observed that Cu2+ deficiency leads to severe fatal
problems in animals13,14 thus the deficiency or excessive intake of Cu2+ is very harmful and this
demands development of molecular probe for selective detection of Cu2+ ion.
Aluminum plays very imperative role in human life and happens to be the third most abundant
element and the most abundant metal in the earth's crust (approx. 8% by mass). Because of high
chemical reactivity of aluminum metal it gets easily converted to Al3+ due to acid rain and human
activities15,16. Aluminum compounds are also frequently utilized as pharmaceutical drugs in human
and veterinary medicine17,18. Among them, buffered aspirin containing aluminum glycinate is
commonly used as an analgesic19 as well as in antacid20,21. Accumulation of excessive amounts of
Aluminum metal damages the kidney22,23, central nervous system causing Alzheimer’s disease24,25, it
reduces total bone and matrix causing osteoporosis, osteomalacia26,27 and kills fish in acidic water28.
The incremental increase of Al3+ concentrations in the environment disturbs the growing plants29,30
.
Recently C. Exley and coꢀworkers reported that excess intake of aluminumꢀbased antiperspirants
causes breast cancer in woman17. Thus the development of fluorescent chemosensor for detection of
hazardous metal ion is having more importance for biological and environmental aspect.
Several methods are in practice for the detection of metal ions like atomic absorption
spectrometry31, liquidꢀphase chromatography32, solidꢀphase extraction33, Xꢀray fluorescence34,
inductively coupled plasma mass spectrometry35 and voltammetry36. Most of these techniques are
expensive, time consuming as well less susceptible for biological systems and in environmental
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media. This demands for the development of chemosensor that can work for all samples in
physiological conditions37.
A variety of fluorescent chemosensor for Cu2+ are reported based on fluorescence resonance energy
transfer (FRET)38, intramolecular charge transfer (ICT)39, photoinduced electron transfer (PET)40,
eximers based41 and chelation enhanced fluorescence (CHEF)42. Compared to the transition metals,
the detection of Al3+ has always been challenging and problematic due to the poor coordination
ability43 but few sensors for Al3+ are reported44–49. However, there is still scope for improvement in
the design of such sensors as they often suffer some problems such as narrow pH range, less
selectivity, less response time, unfavourable absorption emission, stability, synthesis cost and low
fluorescence quantum yield. From the available sensor most of the fluorescent sensors are based on
the PET mechanism, in which enhancement or quenching in fluorescence intensity is observed. A
typical fluorescent sensor contains a receptor (the recognition site) linked to a fluorophore (the signal
source) which translates the recognition event into the fluorescence signal50. Thus the receptor unit
must have strong binding affinity to the relevant target. Photoinduced electron transfer based sensor
(PET) usually have three components such as fluorophore–spacer–receptor in which the ionic or
molecular input at receptor site modulate the emission such as fluorescence quantum yield, lifetime
and leads to OnꢀOff or OffꢀOn sensing mechanism51. The choice of the receptor totally depends on
the target analytes. In the present paper, we have presented PET based sensors for Cu2+ and Al3+ in
presence of other metal ions in CH3CN. The binding study of metals to the probe shows that nature
of receptor plays very important role in selective detection of metal ions. The studied chromophores
shows fluorescence turn on response to Cu2+ and Al3+, based on classical photoinduced electron
transfer (PET) principle developed by de Silva52,53 and are stable for the pH range 7.0ꢀ9.0.
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2. Results and Discussion
2.1.Chemistry
The acridine derivatives 7a and 7b were synthesized by substitution, formylation and addition type of
reactions. The intermediate 1 was prepared from aniline and methyl bromoacetate in basic media by
nucleophilic substitution reaction which was further converted to the compound 2 by formylation
reaction54. The intermediate 4 was prepared by formylation of the compound 3 in DMF/POCl3 at 80
OC. The intermediate 2 and 4 were reacted with dimedone undergoes classical Knovengel
condensation leads to the formation of the tetraketone 5a-b, which on cyclization in presence of
ammonium acetate in methanol leads to formation of the acridine esters 6a-b. Hydrolysis of acridine
esters in 10% NaOH leads to formation of 7a-b. The structures of the dyes were confirmed by FTꢀIR,
1H NMR, 13C NMR and mass spectral analysis. The synthetic details are presented in Scheme 1.
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Scheme 1: Synthesis of acridine sensors 7a and 7b
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2.2.Absorption and emission properties
2.2.1. Metal sensing study
The binding activities of the sensors 7a and 7b with various metal ions such as Zn2+, Hg2+, Cd2+, Fe2+,
Ba2+, Mn2+, Ni2+, Mg2+, Co2+, Cu2+, Cu+, Ag+, Na+, K+ and Al3+ were investigated by UVꢀvisible
absorption and fluorescence spectroscopy. The free probes 7a (20 µM) and 7b (20 µM) exhibited two
absorption peaks at 305 and 365 nm in CH3CN. With the addition of Cu2+ (5 equiv.) to the probe 7a
(20 µM) in CH3CN, the absorption peak at 305 nm disappeared. In contrast, no significant change was
observed for the absorption band at 365 nm (Fig. S1, ESI†). The addition of Al3+ (5 equiv.) to the
probe 7b (20 µM) leads to decrease in the absorbance at 365 nm while the absorption peak at 305 nm
disappeared (Fig. S2, ESI†). However, no remarkable change in absorption spectra was observed after
addition of other metal ions to probe 7a and 7b. Upon the addition of increasing concentration of Cu2+
(0 to 5 equiv.) to the probe 7a (10 µM) slight increase in the absorbance was observed at 365 nm while
the absorption peak at 305 nm gradually disappeared (Fig. S3, ESI†). A similar trend was observed for
the addition of Al3+ (0 to 5 equiv.) to 7b (10 µM) in CH3CN (Fig. S4, ESI†). In addition, the
fluorescence spectrum was obtained by excitation at 365 nm. In the absence of metal ions the probes
7a-b showed very weak fluorescence signal in the range from 420 to 440 nm in CH3CN. Upon the
addition of Cu2+ (5 equiv.) to 7a (20 µM) and Al3+ (5 equiv.) to 7b (10 µM) significant enhancement in
the fluorescence intensity was observed at 420ꢀ440 nm (Fig. 1 ꢀ 2). Whereas no considerable change in
the fluorescence intensity was observed for other metal ions.
Please Insert Fig. 1 and Fig. 2
To investigate the interaction of Cu2+ with probe 7a and Al3+ with probe 7b, the emission spectra with
varying Cu2+ and Al3+ concentrations in CH3CN were recorded separately. Fluorescence intensity of
the probe 7a (10 µM) reaches a maximum when 5 equivalent of Cu2+ was added (Fig. 3). After
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addition of 5 equivalent of Cu2+ to probe 7a (10 µM), a blue shifted emission from 429 nm to 420 nm
(~9 nm) was observed. However, the fluorescence intensity of 7b (10 µM) increases gradually at 425
nm upon addition of increasing concentration of Al3+ (0 to 5 equiv.) in CH3CN (Fig. 4) while no shift
in maximum emission was observed. It is, thus, expected that there is appreciable interaction between
the probe 7a and 7b with Cu2+ and Al3+ respectively.
Please Insert Fig. 3 and Fig. 4
From the photophysical properties it is observed that the binding of the sensors 7a-b to Cu2+ and Al3+
ion takes place by electrostatic interactions between ꢀCOOH and nitrogen atom present in the receptor
part of the molecule (Fig. 5). By conjugating a receptor into the metal ion, the binding would perturb
the energy levels of the excited states, which results in a change of emission wavelength. The sensing
ability depends on size of receptor, spacer and availability of binding units which forms coordination
complex with metal ions. It also depends upon the geometry of the coordination complex formed after
ligand to metal binding. The Cu2+ and Al3+ have different chemical coordination characteristics. Thus,
the sensor 7a is connected to the receptor by a short spacer which shows selective fluorescence
enhancement to Cu2+ ion while for sensor 7b, the receptor is connected to fluorophore by long spacer
which shows strong selectivity and sensitivity to Al3+ ion.
Please Insert Fig. 5
The selectivity of the probe 7a (10 µM) was studied in the presence of other competitive metal ions.
The competition experiments revealed that the Cu2+ induced fluorescence enhancement was unaffected
in 5 equivalent of environmentally relevant alkali or alkalineꢀearth metals, such as Na+, K+ and Mg2+
as well as the other transition metal ions Zn2+, Ni2+, Cd2+,Co2+, Mn2+, Fe2+ (Fig. 6). Obviously, all of
these results confirmed that our proposed chemosensor 7a (10 µM) has remarkably high selectivity
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toward Cu2+ ions over the other competitive metal ions in CH3CN. The similar observations are
observed for the sensor 7b in presences of other metal ions (Fig. 7).
Please insert Fig. 6 and Fig. 7
The quantum yields of free sensors as well as metalꢀsensor complex were evaluated by relative
method.
The fluorescence quantum yields of 7a without and with Cu2+ were 0.083 and 0.85, respectively. Thus
10 fold increase in fluorescence quantum yield of the sensor 7a was observed after binding with Cu2+
in CH3CN. In the case of sensor 7b, 15 fold increase in quantum yield was observed for 7b-Al3+
complex (0.81) as compared to free sensor (0.054). The change in fluorescence intensity of the probe
7a (10 µM) in 5 eq. of Cu2+ in CH3CN/H2O mixture was studied at pH 9.0. Notably, we observed that
the fluorescence intensity remains unaltered up to 60% CH3CN and 40% H2O mixture. However,
further increase in H2O concentration leads to slight decrease in the fluorescence intensity (Fig. S5,
ESI†).
The effect of pH on 7a, 7b and their complexes 7aꢀCu2+, 7bꢀAl3+ were studied from pH range 3ꢀ12.
The fluorescence intensity of the sensors 7a and 7b decreases from pH 3 to 7 and after it remains
constant till pH = 12. The fluorescence intensity of 7a-Cu2+ complex increases slightly from pH 3 to
pH 7 and after that remains constant up to pH 12. In case of 7b-Al3+ complex fluorescence intensity
slightly increases from pH 3 to pH 7 and it remains constant from pH 7 to pH 9. After pH 9 the
fluorescence intensity of 7bꢀAl3+ complex decreases suddenly because of hydrolysis of Al3+ in basic
condition (Fig. S6, ESI†). At lower pH both the probes 7a and 7b showed high fluorescence intensity.
In acidic pH conditions the protonation of aromatic amine nitrogen takes place which supress
photoinduced electron transfer mechanism.
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The binding association constant (Ka) for 7aꢀCu2+ and 7bꢀAl3+ were determined from Beneseiꢀ
Hildebrand equation. The binding association constants of 7a with Cu2+ and 7b with Al3+ were
observed to be 4.0 x 104 Mꢀ1 and 1.7 x 104 Mꢀ1, respectively. The estimated limit of detection (LOD)
came out to be 2.8 x 10ꢀ7 M for 7aꢀCu2+ and 5.8 x 10ꢀ7 M for 7bꢀAl3+ complex in CH3CN (Fig. S7, ESI
†) and compared with some recently reported Cu2+ and Al3+ sensors (Table 1).
Please insert Table 1
The binding stoichiometry of the 7aꢀCu2+ complex was determined using the continuous variations
(Job's method) method (Fig. S8 a, ESI†). When the molar fraction of the sensor was 0.5, the
absorbance value approached a maximum, which demonstrated the formation of a 1:1 complex
between the receptor sensor and Cu2+. The similar trend observed for 7bꢀAl3+ complex (Fig. S8 b, ESI
†). The reaction media was investigated to obtain a suitable reaction system. The fluorescence
response of 7a (20 µM) with Cu2+ (5 equiv.) was studied in different solvents such as CH3CN, MeOH,
EtOH and DMF. It was observed that in CH3CN the fluorescence intensity of 7aꢀCu2+ was slightly
higher than the MeOH and EtOH. However, noticeable decrease in the fluorescence intensity was
observed for DMF (Fig. S9, ESI†). So, CH3CN was chosen as the solvents for the analyses. The
binding of Cu2+ with 7a and Al3+ with 7b was further supported by 1H NMR study in DMSOꢀd6 (Fig.
S14 and S15, ESI†). The interaction of Cu2+ or Al3+ with the probes takes place through carboxylic
oxygen and nitrogen loneꢀpairs present in the receptor part of the molecule. Other part of the molecule
is not involved in the complex formation with Cu2+ and Al3+. Upon the addition of metal to 7a and 7b
the shift in the signals for receptor protons adjacent to carboxylic acid group was not observed. In
contrast, before addition of metal ions the probes 7a and 7b showed signal at δ = 12.67 ppm and δ =
12.17 ppm respectively accounting for two acid protons. But after addition of one equivalent Cu2+ to
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probe 7a and Al3+ to the probe 7b, the signal at δ = 12.67 ppm and δ = 12.17 ppm disappeared which
supports the binding of Cu2+ and Al3+ to the probe 7a and 7b through ꢀCOOH group.
3. Experimental:
3.1.Methods and materials
All the reagents were purchased from S. D. Fine Chemical Limited (India) of commercial grade and
used without further purification. All the common chemicals were of analytical grade. The solvents
were purified by standard procedures. All the reactions were monitored by TLC (thinꢀlayer
chromatography) with detection by UV light. The absorption spectra of the chromophores were
recorded on PerkinꢀElmer spectrophotometer, Lambda 25. The emission spectra were recorded on
Varian Cary Eclipse fluorescence spectrophotometer. The freshly prepared solutions in solvents of
different polarities at a concentration of 1 x 10ꢀ6 mol Lꢀ1 solution were used in 1 cm optical path
length quartz cuvette. The photophysical properties were investigated using solvatochromic and
solvatofluoric behaviours of the chromophores. The excitation wavelength used for fluorescence
measurements as absorption maxima of the compounds in respective solvents. The FTꢀIR spectra
were recorded on a PerkinꢀElmer Spectrum 100 FTꢀIR Spectrometer. 1H NMR spectra were recorded
on Varian 500 MHz instrument using TMS as an internal standard. Mass spectra were recorded on
Finnigan mass spectrometer.
3.2.Preparation of salt solution
The commercially available salts such as ZnCl2.6H2O, HgCl2, CdCl2, FeCl2.4H2O, BaCl2.2H2O,
MnCl2.4H2O, NiCl2.6H2O, MgCl2.6H2O, CoCl2.6H2O, CuCl2.2H2O, CuCl, NaCl, KCl, AgNO3 and
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Al(NO3)3.9H2O were used to prepare the stock solution of metal ions in double distilled deionise
water at room temperature.
Quantum yields determinations
The relative quantum yields of synthesized compounds in different solvents were calculated by using
equation 1. The refractive indices of the solvents have been taken from literature55. The quantum
yields of the sensors 7a-b were evaluated in free as well as metal bound form in acetonitrile at room
temperature by using quinine sulfate in 0.1 N H2SO4 (Φf = 0.51) as a standard55,56
.
2
Gradx ηx
φx = Φst ×
×
2
Gradst
ηst
………………….
(1)
Where:
Φx = Quantum yield of compound, Φst = Quantum yield of standard sample
Gradx = Gradient of compound, Gradst = Gradient of standard sample
ηx
= Refractive index of solvent used for synthesized compound
ηst = Refractive index of solvent used for standard sample
3.3.Synthesis
3.3.1. Synthesis of intermediate 4
Phosphorous oxychloride (POCl3) (2.75 ml, 0.03 mol) was slowly added to dimethyl formamide
O
(DMF) (3.65 mL, 0.05 mol) at 5–10 C under constant stirring. To this cooled reagent N-substituted
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amino benzene (0.01 mmol) was added by dissolving it in DMF (6 mL) under constant stirring and
O
the resulting mixture was heated at 75 C for 4 h. The reaction mixture was cooled to room
temperature and then poured into ice cold water (60 mL). The reaction mass was neutralized with
sodium carbonate and extracted by ethyl acetate. The organic layer was dried by sodium sulphate and
evaporated on rotary evaporator offered brown colored liquid which was used for the further reaction.
1
Yield: 85 %. FTꢀIR (cmꢀ1): 1730 (ester), 1705 (aldehyde), 1650, 1600 (ꢀC=Cꢀ, aromatic). H NMR
(500 MHz, CDCl3, ppm, Me4Si): δ = 2.62 (t, 4H, ꢀCH2), 3.68 (s, 6H, ꢀCH3), 3.75 (t, 4H, ꢀCH2), 6.71
13
(d, J = 9 Hz, 2H), 7.72 (d, J = 9 Hz, 2H), 9.72 (s, 1H). C NMR (125.6 MHz, CDCl3, ppm, Me4Si):
32.0, 46.7, 51.9, 111.2, 126.0, 132.2, 151.3, 171.9, 190.2.
3.3.2. Synthesis of Intermediate 5a-b
Dimedone (2.0 mmol) and substituted aromatic aldehydes (1.0 mmol) were stirred in ethylene glycol
O
at 80 C for 6 h. The progress of the reaction was monitored by TLC. After completion of reaction,
the reaction mixture was poured in water the obtained solid was filtered and dried. The crude product
was recrystallized from 95 % ethanol.
O
5a: Yield = 95%, Melting point: 190 C. FTꢀIR (cmꢀ1): 2955 (ꢀCH), 1770, 1740 (ester), 1665
(ketone), 1530 (ꢀC=Cꢀ, aromatic). 1H NMR (500 MHz, CDCl3, ppm, Me4Si): δ = 1.09 (s, 6H, –CH3),
1.21 (s, 6H, –CH3), 2.35 (m, 10 H), 3.79 (s, 6H), 4.11 (s, 4H), 5.43 (s, 1H), 6.52 (d, J = 9 Hz, 2H),
13
6.92 (d, J = 9 Hz, 2H). C NMR (125.6 MHz, CDCl3, ppm, Me4Si): 27.3, 29.7, 31.3, 46.4, 47.1,
52.1, 53.3, 112.4, 115.7, 127.8, 145.8, 171.5, 190.2. Mass: m/z 527.4 [M]+.
O
5b: Yield = 93%, Melting point: 195 C. FTꢀIR (cmꢀ1): 2954 (ꢀCH), 1766, 1741 (ester), 1662
(ketone), 1519 (ꢀC=Cꢀ, aromatic). 1H NMR (500 MHz, CDCl3, ppm, Me4Si): δ = 1.09 (s, 6H, –CH3),
1.22 (s, 6H, –CH3), 2.38 (broad, 10 H), 2.57 (t ,4H), 3.61 (t, 4H), 3.66 (s, 6H, ꢀCH3), 5.44 (s, 1H),
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6.61 (d, J = 9 Hz, 2H), 6.93 (d, J = 9 Hz, 2H). C NMR (125.6 MHz, CDCl3, ppm, Me4Si): 27.1,
29.7, 31.3, 31.8, 32.3, 46.4, 46.9, 51.7, 112.5, 115.8, 127.9, 144.7, 172.5, 190.3. Mass: m/z 556.9
[M+H]+.
3.3.3. Synthesis of Intermediate 6a-b
The tetraketone intermediates 5a-b (1.0 mmol) were taken in aq. ethanol and ammonium acetate (3.0
mmol) was added and the reaction mixture heated for 6 h. The progress of the reaction was monitored
by TLC. Ethanol was evaporated and diluted by water. The obtained solid was filtered and dried. The
crude product was recrystallized from 95% ethanol.
6a: Yield = 91%, Melting point: 210 OC. FTꢀIR (cmꢀ1): 2960 (ꢀCH), 1727 (ester), 1661 (ketone), 1608,
1
1514 (ꢀC=Cꢀ, aromatic). H NMR (500 MHz, DMSOꢀd6, ppm, Me4Si): δ = 0.87 (s, 6H, –CH3), 0.98
(s, 6H, –CH3), 1.98 ꢀ 2.12 (2d, J = 16 Hz, 4H –CH2), 2.35 (s, 4H), 3.79 (s, 6H), 3.98 (s, 4H), 4.77 (s,
1H), 6.26 (d, J = 9 Hz, 2H), 6.91 (d, J = 9 Hz, 2H). 13C NMR (125.6 MHz, DMSOꢀd6, ppm, Me4Si):
27.3, 29.4, 32.0, 32.6, 46.4, 50.8, 53.4, 111.1, 112.3, 128.6, 136.5, 145.9, 149.3, 173.1, 194.8. Mass:
m/z 479.51 [MꢀH]+.
6b: Yield = 92%, Melting point: 221 OC. FTꢀIR (cmꢀ1): 2963 (ꢀCH), 1729 (ester), 1666 (ketone), 1618,
1519 (ꢀC=Cꢀ, aromatic). 1H NMR (500 MHz, DMSOꢀd6, ppm, Me4Si): δ = 0.99 (s, 6H,–CH3), 1.07 (s,
6H,–CH3), 1.98 ꢀ 2.12 (2d, J = 16 Hz, 4H, ꢀCH2), 2.32 (m, 4H), 2.52 (t, 4H), 3.55 (t, 4H), 3.64 (s, 6H),
13
4.97 (s, 1H), 6.62 (d, J = 9 Hz, 2H), 7.16 (d, J = 9 Hz, 2H). C NMR (125.6 MHz, DMSOꢀd6, ppm,
Me4Si): 27.4, 29.3, 32.4, 31.7, 41.3, 47.0, 50.8, 51.6, 112.3, 114.1, 129.0, 135.7, 144.9, 146.9, 172.7,
195.3. Mass: m/z 537.5 [M+H]+.
Synthesis of sensors 7a-b
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The ester intermediates 6a-b (1.0 mmol) were taken in aq. ethanol and 10% sodium hydroxide (3.0
mmol) was added and the reaction mixture refluxed for 2 h. The progress of the reaction was
monitored by TLC. Ethanol was evaporated and water (10 ml) was added in the residue. The resultant
reaction mass acidified by dil. HCl to pH 6. The obtained solid was filtered, dried and recrystallized
using by 95% ethanol.
7a: Yield = 91%, Melting point: 227 OC. FTꢀIR (cmꢀ1): 2963 (ꢀCH), 1729 (acid), 1666 (ketone), 1618,
1
1519 (ꢀC=Cꢀ, aromatic). H NMR (500 MHz, DMSOꢀd6, ppm, Me4Si): δ = 0.87 (s, 6H, –CH3), 0.98
(s, 6H, –CH3), 1.98 ꢀ 2.12 (2d, J = 16 Hz, 4H –CH2), 2.35 (s, 4H), 3.98 (s, 4H), 4.67 (s, 1H), 6.26 (d, J
= 9 Hz, 2H), 6.91 (d, J = 9 Hz, 2H), 9.15 (s, 1H, NH), 12.67 (s, 2H). 13C NMR (125.6 MHz, DMSOꢀ
d6, ppm, Me4Si): 27.3, 29.4, 32.0, 32.6, 50.8, 53.4, 111.1, 112.3, 128.6, 136.5, 145.9, 149.3, 173.1,
194.9. HRMS m/z [M+Na]+ Calcd for C27H32N2O6Na: 503.2158. Found: 503.2159. Mass: m/z 503.59
[M+Na]+.
O
7b: Yield = 92%, Melting Point: 221 C. FTꢀIR (cmꢀ1): 2963 (ꢀCH), 1732 (acid), 1636 (ketone),
1
1603, 1519 (ꢀC=Cꢀ, aromatic). H NMR (500 MHz, DMSOꢀd6, ppm, Me4Si): δ = 0.88 (s, 6H, –
CH3), 0.98 (s, 6H, –CH3), 1.98 ꢀ 2.12 (2d, J = 16 Hz, 4H, ꢀCH2), 2.32 (s, 4H), 2.48 (t, 4H), 3.44 (t,
4H), 4.67 (s, 1H), 6.45 (d, J = 9 Hz, 2H), 6.93 (d, J = 9 Hz, 2H), 9.15 (s, 1H, NH), 12.17 (s, 2H).
13C NMR (125.6 MHz, DMSOꢀd6, ppm, Me4Si): 27.3, 29.4, 31.8, 32.5, 32.6, 46.7, 50.8, 111.8,
112.3, 128.8, 135.8, 145.1, 149.3, 173.7, 194.8. HRMS m/z [M+Na]+ Calcd for C29H36N2O6Na:
531.2471. Found: 531.2474. Mass: m/z 531.62 [M+Na]+.
4. Conclusion
In summary, we have demonstrated that the two chemosensors 7a-b showed ideal photoinduced
electron transfer behavior for the detection of Cu2+ and Al3+ respectively. The absorption spectra do
14

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not changes significantly upon binding with various metal ion. In contrast, the emission spectra change
dramatically after addition of Cu2+ to probe 7a and Al3+ to probe 7b even in presence of other metal
ions. Upon the addition of Cu2+ to the probe 7a in CH3CN, 10 fold enhancement in the fluorescence
intensity was observed. However, probe 7b showed 15 fold enhancement in the fluorescence intensity
with Al3+ in CH3CN. The binding of Cu2+ and Al3+ to the sensors resulted in maximum fluorescence
enhancement in the pH range of 7.0 ꢀ 9.0. The limit of detection (LOD) came out to be 2.8 x 10ꢀ7 M
for 7aꢀCu2+ and 5.8 x 10ꢀ7 M for 7bꢀAl3+ complex in CH3CN which makes 7a and 7b suitable
candidate for the development of a potential probe for biological applications.
5. Acknowledgements
Santosh Chemate is thankful for JRF and SRF fellowship from the Principal Scientific Adviser
(PSA), Government of India.
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………………Table……………
Table 1. Performance comparison of various chemosensors reported for Cu2+ and Al3+
detection.
Association Limit of
Metal
Ion
Sr. No.
Receptor
Constant
(Ka)(M-1)
Detection Reference
(M)
1
Cu2+
8.55 x 105 2.55 x 10-6
[57]
2
3
Cu2+
1.5 x 104
1.15 x 10-6
5.0 x 10-7
[58]
Al3+
8.84 x 103
[59]
4
5
Cu2+
Cu2+
NA
NA
1.0 x 10-4
1.0 x 10-4
[60]
[60]
6
7
Al3+
8.32 x 106 3.28 x 10-6
[61]
Cu2+
4.0 x 104
1.7 x 104
2.8 x 10-7 This work
HOOC
COOH
N
Al3+
5.8 x 10-7 This work
O
O
8
N
H

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………………Figures all…………………
1000
Cu2+
800
600
400
200
0
Al3+
7a, Zn2+, Hg2+, Cd2+, Fe2+,
Ba2+, Mn2+, Ni2+, Mg2+,
Co2+, Cu+, Ag+, Na+, K+
380
410
440
470
500
530
560
Wavelength (nm)
Fig. 1 Fluorescence emission spectra of 7a (20 ꢀM) upon addition of various metal ions (5 equiv.)
in CH3CN. λex = 365 nm (Slit widths: 5 nm/5 nm).
1

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1000
800
600
400
200
0
Al3+
Cu2+
7b, Zn2+, Hg2+, Cd2+, Fe2+,
Ba2+, Mn2+, Ni2+, Mg2+,
Co2+, Cu+, Ag+, Na+, K+
380
410
440
470
500
530
560
Wavelength (nm)
Fig. 2 Fluorescent emission spectra of 7b (20 ꢀM) upon addition of various metal ions (5 equiv.)
in CH3CN. λex = 365 nm (Slit widths: 5 nm/5 nm).
1000
5 eq. Cu2+
20
15
800
10
5
0
600
400
200
0
0 eq
0
1
2
3
4
5
Cu2+/ ꢀM
380
410
440
470
500
530
560
Wavelength (nm)
Fig. 3 Changes in the fluorescence emission spectra of probe 7a (10 ꢀM) in CH3CN upon titration
with 0 to 5.0 equiv. of Cu2+. Inset: The relative fluorescence intensity (R.F.I) at 420 nm as a
function of Cu2+ ion concentration. λex = 365 nm (Slit widths: 5 nm/5 nm).
2

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1000
800
600
400
200
0
120
80
40
0
5 eq. Al3+
0 eq
0
1
2
3
4
5
Al3+/µM
380
430
480
530
580
Wavelength (nm)
Fig. 4 Changes in the fluorescence emission spectra of probe 7b (10 ꢀM) in CH3CN upon titration
with 0 to 5.0 equiv. of Al3+. Inset: The relative fluorescence intensity (R.F.I) at 427 nm as a
function of Al3+ ion concentration. λex = 365 nm (Slit widths: 5 nm/5 nm).
Fig. 5 Proposed binding mechanism of Cu2+ and Al3+ with sensors 7a and 7b.
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10
5
0
Fig. 6 Relative fluorescent intensity (R.F.I) of probe 7a (10.0 ꢀM) in presence of 2.0 equiv. other
metal ions (blue bars) in CH3CN. The red bars represent the change of the emission that occurs
upon the subsequent addition of 2.0 equiv. of. Cu2+ to above solution (25 OC), λex = 365 nm (Slit
widths: 5 nm/5 nm).
12
8
4
0
Fig. 7 Relative fluorescent intensity (R.F.I) of probe 7b (10.0 ꢀM) in presence of 2.0 equiv. other
metal ions (blue bars) in CH3CN. The red bars represent the change of the emission that occurs
O
upon the subsequent addition of 2.0 equiv. of. Al3+ to above solution (25 C). λex = 365 nm (Slit
widths: 5 nm/5 nm).
.
4