New emissive dopamine derivatives as fluorescent chemosensors for metal ions: A CHEF effect for Al(III) interaction

Article abstract of DOI:10.1016/j.ica.2011.08.033

Three new Schiff-base ligands 1-3 derivatives from neurotransmitter dopamine, bearing an indazole (1), methylimidazole (2) or hydroxynaphthaldehyde (3) units were successfully synthesized and characterized by elemental analysis, ¹H NMR, ¹³

Full text of DOI:10.1016/j.ica.2011.08.033

Inorganica Chimica Acta 381 (2012) 203–211
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New emissive dopamine derivatives as fluorescent chemosensors for metal ions:
A CHEF effect for Al(III) interaction
Elisabete Oliveira a,b, , Hugo M. Santos a,c,d,e, José Luis Capelo a,e, Carlos Lodeiro a,e
⇑
a BIOSCOPE Group, Faculty of Science, Physical-Chemistry Department, Ourense Campus, University of Vigo, 32004 Ourense, Spain
b Veterinary Science Department, CECAV, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal
c Institute for Biotechnology and Biomedicine, University Autònoma of Barcelona, 08193 Bellaterra, Spain
d Institute for Biotechnology and Bioengineering, Centre of Genomics and Biotechnology, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal
e REQUIMTE/CQFB, Departamento de Química, Faculdade de Ciências e Tecnología, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 30 August 2011
Three new Schiff-base ligands 1–3 derivatives from neurotransmitter dopamine, bearing an indazole (1),
methylimidazole (2) or hydroxynaphthaldehyde (3) units were successfully synthesized and character-
ized by elemental analysis, 1H NMR, 13C NMR, IR (infrared), ESI-TOF (electrospray ionization-time of
flight-mass), UV–Vis absorption and emission spectroscopy. The interaction with the metal ions Li+,
Na+, Mg2+, Ca2+, Zn2+, Cd2+, Cu2+, Ni2+, Ag+, Fe3+ and Al3+ has been explored by absorption and fluorescence
emission spectroscopy. Compound 3 shows to be the most emissive one, with a quantum yield of 0.022,
followed by compound 2 (/ = 0.009) and 1 (/ = 0.001). Compound 1 reveals quite selective for Al3+, gen-
erating an emissive complex formed by three units of 1 per metal ion. On the other hand compound 3,
shows a ratiometric effect with Al3+ appearing as a new emission band centred at 530 nm. With the other
transition and post-transition metal ions, a quenching in the emission was observed, being 3 the most
sensitive one, being quenched by 0.34 ppm of Zn2+ and Cu2+, 0.48 ppm of Ni2+, 0.061 ppm of Fe3+ and
0.047 ppm of Al3+. In most cases a stoichiometry of three ligands by one metal ion was postulated. In
order to study the fluorescence properties in the solid state, the Al3+ metal complexes of 1 (4) and 3
(5) were synthesized as new emissive materials. In both cases a very strong emissive systems were
obtained, with a quantum yield of 0.01 and 0.04 for 4 and 5.
Fluorescence Spectroscopy: from Single
Chemosensors to Nanoparticles Science –
Special Issue
Keywords:
Neurotransmitter dopamine
Aluminium(III)
Chemosensors
Schiff-bases
Fluorescence
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
which often form adducts, like metal complexes. The metal interac-
tion of dopamine with Fe3+ is well reported in literature [5], which
Nowadays, the design of new efficient chemosensors capable of
selective recognition of guest species is an area of a huge research
in biological, analytical and environmental fields [1,2]. Selective
detection with an enhancement of the fluorescence emission of
metal ions like Al3+, is extremely important because of the well-
known neurotoxicity of aluminium(III) and also due to its possible
relationship in the appearance of Alzheimer’s disease [3]. Also,
compound derivatives from aluminium are widely used in water
treatment, in food additives and in medicine [3].
On the other hand, the catecholamines such as, dopamine, adren-
aline, noradrenaline and L-dopa are neurotransmitters that act in
the nervous system. The decrease of the dopamine neurotransmitter
in the brain also leads to neurodegenerative diseases, like Alzheimer,
Wilson and Parkinson [4]. Neurotransmitters are organic molecules,
describes the metal ion coordination in hydroxyl groups. Further-
more, other metal ions sensing lead us to the design of new efficient
chemosensors. Due to the presence of an electron donor imine and
two hydroxyl sites, Schiff-bases are really important for metal ions
complexation, resulting in highly stable complexes [6]. The elimina-
tion of pollutant metal ions in drinking water or waste water is nor-
mally realized via interaction with organic molecules such as, humic
substances, Schiff-bases or diazo compounds [7]. In the same way,
compounds containing indazole moieties are considered to have
pharmacology properties, exhibiting activity as HIV protease inhib-
itors, serotonin 5-HT1 , 5-HT2 and 5-HT3 receptor antagonists and
a
aldol reductase inhibitors [8]. Their properties could be improved
when metal ions coordinate to the organic compound [9].
Following our interest in fluorimetric chemosensors [10], here
we report the synthesis of three new Schiff-bases 1, 2, and 3, deriv-
atives from the neurotransmitter dopamine, bearing indazole,
methylimidazole and hydroxynaphthaldehyde, respectively (see
Scheme 1). All compounds were fully characterized and their sens-
⇑
Corresponding author at: BIOSCOPE Group, Faculty of Science, Physical-
Chemistry Department, Ourense Campus, University of Vigo, 32004 Ourense, Spain.
Fax: +34 988 387001.
ing ability with metal ions such as, Li+, Na+, Mg2+, Ca2+, Zn2+, Cd2+
,
E-mail address: e.oliveira@uvigo.es (E. Oliveira).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.08.033

204
E. Oliveira et al. / Inorganica Chimica Acta 381 (2012) 203–211
Scheme 1. Chemical structure of compounds 1–3.
Cu2+, Ni2+, Ag+, Fe3+ and Al3+. Moreover, new metal complexes of
aluminium(III) with compounds 1 and 3 were also synthesised
and characterized as new emissive materials.
and 2, and fluorescein in absolute ethanol [/] = 0.79 [12] for 3,
and were corrected for different refraction indexes of solvents.
Fluorescence spectra of solid samples were recorded using a fi-
bre optic system connected to the Horiba-Jovin Ybon Fluoromax 4
spectrofluorimetric exciting at appropriate k (nm) of the solid com-
pounds. All the measurements were performed at 298 K.
2. Experimental
2.1. Chemicals and starting materials
2.4. Synthesis of compounds 1–3
CH3SO3H (methanosulfonic acid), C16H37NO (tetrabutylammo-
nium hydroxide), LiBF4ꢀxH2O, NaBF4ꢀxH2O, Mg(CF3SO3)2ꢀxH2O,
Zn(BF4)2ꢀxH2O, Ca(CF3SO3)2ꢀxH2O, Cd(CF3SO3)2ꢀxH2O, Cu(BF4)2ꢀx-
H2O, Ni(BF4)2ꢀ6H2O, AgNO3ꢀxH2O, Al(NO3)3ꢀxH2O and Fe
(NO3)3ꢀxH2O, salts have been purchased from Strem Chemicals, Sig-
ma–Aldrich and Solchemar. Triethylamine, HCl Dopamine, 1H-
indazole-6-carboxaldehyde, 4-methyl-5-imidazole carboxalde-
hyde, indole-3-carboxaldehyde, thianaphthene-3-carboxaldehyde,
and 2-hydroxy-1-naphthaldehyde were from Sigma–Aldrich. All
were used without previous purification.
2.4.1. General synthesis
HCl. Dopamine (0.1045 g, 5.51 ꢂ 10ꢁ4 mol) was neutralized
with triethylamine (77
l
l, 5.51 ꢂ 10ꢁ4 mol) in absolute ethanol
for 30 min. The solution was filtered and the filtrate was added
to the corresponding aldehyde (5.51 ꢂ 10ꢁ4 mol), and the mixture
was gently refluxed with magnetic stirring for ca. 4 h. The solvent
was evaporated under reduced pressure and the residue was
washed several times with diethyl ether (10 mL) and dried under
vacuum.
2.4.1.1. 4-(2-(((1H-indazol-6-yl)methylene)amino)ethyl)benzene-1,2-
diol. Colour: Pale Yellow powder. Melting point: 113.9–117.1 °C.
Yield 0.14 g, (92%), FW = 317.3. Anal. Calc. for C16H15N3O2ꢀ2H2O:
C, 60.6; H, 5.9; N, 13.2. Found: C, 60.5; H, 6.1; N, 13.9%. IR (KBr
cmꢁ1): 3420 (–OH broad) 2980 (N–H bond), 1682 (–CH@N), 1512
(C@C). 1H NMR (DMSO, 400 MHz): dH = 2.94–2.96 (d, J = 4 Hz, 2H,
2.2. Physical measurements
Mass spectrometry analyses of the ligands were performed at
the ‘‘C.A.C.T.I. – Unidad de Espectrometria de Masas’’ at the Univer-
sity of Vigo, Spain (ESI-TOF). Elemental analyses were carried out
on a Fisons EA-1108 analyser in the University of Vigo Elemental
analyses Service.
NMR spectra of the ligands were obtained on a Bruker Spec-
trometer operating at frequency of 400 MHz for 1H NMR and 13C
NMR using the solvent peak as internal reference, in the facilities
of the University of Santiago de Compostela, Spain. Melting points
were determined on a Gallenkamp apparatus and are uncorrected.
Infrared spectra were recorded in KBr windows using JASCO FT/IR-
410 spectrophotometer.
b-CH2 dopamine), 3.20–3.21 (d, J = 2 Hz, 2H,
c CH2 dopamine),
5.45 (s, 1H, OH), 5.70 (s, 1H, OH), 6.59–6.60 (d, 2 Hz,1H, H5),
6.77–6.79 (d, J = 4 Hz, 1H, H6), 7.04 (s, 1H, H9), 7.68–7.70 (d,
J = 4 Hz, 1H, H60), 7.72–7.75 (m, 1H, H50), 7.88–7.90 (s, 1H, H30),
8.19 (s, 1H, HC@N), 8.36 (s, 1H, NH). 13C NMR (DMSO, 400 MHz):
dc = 25.78 (C3), 36.88 (C2), 54.25 (C8), 59.69 (C7), 110.97 (C9),
115.60 (C6), 118.90 (C5), 121.06 (C60), 121.84 (C50), 122.17 (C30),
133.78 (C80), 124.23 (C, HC@N). UV–Vis in absolute ethanol (k
nm): Bands at 282 nm (loge = 3.81). Emission spectra in absolute
ethanol (kexc = 282 nm, kemis = 318 nm). ESI-TOF calc.(found): [1]
H+ 282.3 (282.1).
2.3. Spectrophotometric and spectrofluorimetric measurements
Absorption spectra were recorded on a JASCO V-650 UV–Vis
spectrophotometer and fluorescence emission spectra on a spec-
trofluorimeter HORIBA JOBIN YVON Fluoromax-4. The linearity of
the fluorescence emission vs. concentration was checked in the
concentration used (10ꢁ4–10ꢁ6 M). A correction for the absorbed
light was performed when necessary. The spectrometric character-
izations and titrations were performed as follows: the stock solu-
tions of the compounds (ca. 10ꢁ3 M) were prepared by dissolving
an appropriate amount of the complex in a 10 ml volumetric flask
and diluting to the mark with absolute ethanol. The solutions were
2.4.1.2. 4-(2-(((4-methyl-1H-imidazol-5-yl)methylene) amino)ethyl)
benzene-1,2-diol. Colour: Pale yellow powder. Melting point:
182.1–185.0 °C. Yield 0.13 g, (98%), FW = 299.3. Anal. Calc. for
C
13H15N3O2ꢀ3H2O: C, 52.1; H, 7.0; N, 14.0. Found: C, 51.9; H, 7.0;
N, 14.5%. IR (KBr cmꢁ1): 3420 (–OH broad) 2980 (N–H bond),
1645 (–CH@N), 1506 (C@C). 1H NMR (DMSO, 400 MHz): dH = 2.45
(s, 3H, –CH3), 2.60–2.70 (d, J = 4 Hz, 2H, b-CH2 dopamine), 2.72–
2.90 (d, J = 2 Hz, 2H,
c CH2 dopamine), 5.40 (s, 1H, OH), 5.55 (s,
1H, OH), 6.15–6.17 (m, 1H, H5), 6.50–6.53 (m, 1H, H6), 6.60 (s,
1H, H9), 7.5 (s, 1H, H20), 7.70 (s, 1H, HC@N), 7.82 (s, 1H, NH). 13C
NMR (DMSO, 400 MHz): dc = 16.76 (–CH3), 37.88 (C3), 62.50 (C2),
116.01 (C6), 116.80 (C9), 122.10 (C6), 137.40 (C20), 144.01 (C7),
144.16 (C8), 138.01 (C, HC@N). UV–Vis in absolute ethanol (k
prepared by appropriate dilution of the stock solutions still 10ꢁ5
–
10ꢁ6 M. Titrations of the ligands 1–3 were carried out by the addi-
tion of microlitre amounts of standard solutions of the ions in
absolute ethanol. All the measurements were performed at
298 K. Luminescence quantum yields were measured using a solu-
tion of benzene in cyclohexane as a standard [/] = 0.05 [11] for 1
nm): Bands at 282 nm (loge = 3.81). Emission spectra in absolute
ethanol (kexc = 270 nm, kemis = 320 nm). ESI-TOF calc. (found): [2]
H+ 246.1 (246.1).

E. Oliveira et al. / Inorganica Chimica Acta 381 (2012) 203–211
205
2.4.1.3. 4-(2-(((2-hydroxynaphthalen-1-yl)methylene)amino)ethyl)
benzene-1,2-diol. Colour: Brown powder. Melting point: 106.4–
108.5 °C. Yield 0.14 g, (85%), FW = 361.3. Anal. Calc. for
peaks corresponding to [1H]+ 282.3 (282.1), [2H]+ 246.1 (246.1)
and [3H]+ 308.3 (308.1). The IR spectra of the three compounds
present a band at 1645–1682 cmꢁ1 corresponding to the imine
bond and a band at 2980 cmꢁ1 assignable to the N–H bond [13].
The 1H NMR spectra of compounds 1, 2 and 3 evidences that the
C
19H17NO3ꢀ3H2O: C, 63.1; H, 6.4; N, 3.9. Found: C, 62.9; H, 6.4; N,
4.1%. IR (KBr cmꢁ1): 3420 (–OH broad) 2980 (N–H bond), 1650
(–CH@N), 1540 (C@C). 1H NMR (DMSO, 400 MHz): dH = 2.74–2.76
signal corresponded to the imine HC@N, the bCH2 and cCH2 of the
(d, J = 4 Hz, 2H, b-CH2 dopamina), 2.99–3.01 (d, J = 4 Hz, 2H,
c
dopamine amino acid and the aromatic signals between 6.5 and
7.9 ppm, as well. In the 13C NMR spectra, the formation of the
amide linkage was also confirmed by the appearance of the signal
at about 125–140 ppm.
CH2 dopamine), 6.49–6.50 (d, J = 2 Hz, 1H, H5), 6.61–6.62 (d,
J = 2 Hz, 1H, H6), 7.12–7.14 (m, 1H, H60), 7.30–7.33 (m, 1H, H70),
7.55–7.57 (d, J = 4 Hz, H, H50), 7.58–7.59 (m, 1H, H80), 7.61 (s, 1H,
HC@N), 7.93–7.95 (d, J = 4 Hz, 1H, H30), 8.96–8.98 (d, J = 4 Hz, 1H,
H20), 13.91 (s, 1H, OH10). 13C NMR (DMSO, 400 MHz): dc = 36.34
(C2), 52.66 (C3), 116.68 (C6), 118.78 (C30), 119.88 (C5), 122.43
(C60), 126.13 (C70), 128.22 (C80), 129.49 (C50), 137.43 (HC@N),
159.22 (C20). UV–Vis in absolute ethanol (k nm): Bands at
3.2. Photophysical studies
Compounds 1, 2 and 3 were characterized in absolute ethanol
and the main photophysical data are gathered on Table 1. Com-
pounds 1, 2 and 3 show absorption bands at 282 nm, 270 nm,
420 nm, and emission bands at 318 nm, 320 nm and 480 nm,
respectively (see Fig. 1). Compound 3 is the most emissive com-
pound, with a quantum yield of 0.022, followed by compound 2
(/ = 0.009) and 1 (/ = 0.001) (see Table 1).
420 nm (loge = 3.54). Emission spectra in absolute ethanol
(kexc = 420 nm, kemis = 480 nm. ESI-TOF calc. (found): [5] H+ 308.3
(308.1).
2.5. Synthesis of the metal complexes
There are many signalling mechanisms involving the excited
state of the molecules, which can decrease or affect the emission
fluorescence signal of the chemosensors, such as, photoinduced
electron/energy transfer (PET) [14], metal ligand charge transfer
(MLCT) [15], intramolecular charge transfer (ICT) [16] and others.
In spite of the good properties of Schiff-bases, usually they pres-
ent very weak emission signal, due to the isomerization around the
imine linkage C@N [17], allowing a free rotation with a consequent
quenching of the emission intensity. It is well known that chelating
groups as C@N or C@O have a higher affinity for transition and
post-transition metal ions [17].
Taking into account this fact, several titrations of compounds 1,
2 and 3 with H+, OHꢁ, Li+, Na+, Mg2+, Ca2+, Zn2+, Cd2+, Cu2+, Ni2+, Ag+,
Fe3+ and Al3+ were performed.
The study of the acid–base behaviour of compounds 1–3 was
carried out with increasing amounts of protons using metanosulf-
onic acid, CH3SO3H, and fluoride ion as basic anion. Protonation in-
duces in compound 1 a decrease and a red-shift of the absorption
0.047 g (1.68 ꢂ 10ꢁ4 mol) of ligands 1 or 3 was dissolved in abso-
lute ethanol followed by the addition of 0.021 g (5.60 ꢂ 10ꢁ3) of the
Al3+ salt. The solution was stirred and refluxed for 2 h. The solvent
was evaporated under reduce pressure, yielding an oil. Diethyl ether
was added drop wise and then dried under vacuum, yielding a dark
green powder (1 Al3+ – (4)) and a brown powder (3 Al3+ – (5)).
2.5.1. [Al13](NO3)3. 6H2O (4)
Colour: Dark green. Yield: 90%. Anal. Calc. for C48H57AlN12O21
(MW: 1165.01): C, 49.50; H, 4.95; N, 14.43. Found: C, 49.80; H,
5.10; N, 14.20%. IR (KBr, cmꢁ1): 1621 [
m
(C@N)], 1570 [
(C–O st)], 1460–1452 ( 5), 1300 (
nitrate counterions], 1380 cmꢁ1
ionic nitrate].
UV–Vis in abs. ethanol (k nm): 290 nm, (
m
(C@C)],
1455 [
m
(–CH2 d)], 1336 [
m
m
m1) and
1028 (
m
2) cmꢁ1
[m
[m
e
ꢃ 4.16). Fluorescence
emission band in abs. ethanol (kexc = 290 nm); kemis = 320 nm.
2.5.2. [Al33] (NO3)3. 6H2O (5)
band from 282 to 293 nm (Dk = 11 nm) followed by an absorbance
Colour: Brown. Yield: 88%. Anal. Calc. for C57H63AlN6O24 (MW:
1243.11): C, 55.07; H, 5.11; N, 6.76. Found: C, 54.80; H, 4.90; N,
decrease of a shoulder at 330 nm (see Fig. 2A). On the other hand,
an intense increase of the emission fluorescence in 1 upon proton-
ation is also observed, wherever the fluorescence quantum yield
changes from 0.001 to 0.02 (20-fold) (see Fig. 2B). An inspection
of Fig. 2(B) indicates that 2 equivalents of proton are necessary
for stabilizing the system. Protonation takes place on the nitrogen
groups on the molecule, avoiding then the photoinduced electronic
transfer phenomena from the lone-pair of electrons to the excited
chromophore [18].
6.95%. IR (KBr, cmꢁ1): 1662 [
m
(C@N)], 1618 [
1) and 1028(
2) cmꢁ1
ionic nitrate].
UV–Vis in abs. ethanol (k nm): 420 nm, (
m
(C@C)], 1361 [
m(C–
O st)], 1460–1452 (
m
5), 1300(
m
m
[m
nitrate coun-
terions], 1380 cmꢁ1
[m
e
ꢃ 4.03). Fluorescence
emission band in abs. ethanol (kexc = 420 nm); kemis = 530 nm.
3. Results and discussion
In a basic environment, compound 1 is not affected, without
changes in the absorption and emission spectra.
3.1. Synthesis
Contrary to compound 1, compounds 2 and 3 are highly pH
dependent presenting spectral changes in acid and basic media
(see Fig. SN1). Upon addition of one equivalent of acid, compound
Dopamine. HCl was neutralized with triethylamine, followed by
the addition of an equimolar amount of the corresponding alde-
hyde compounds, 1H-indazole-6-carboxaldehyde (1), 4-methyl-5-
imidazolecarboxaldehyde (2) and 2-hydroxy-1-naphthaldehyde
(3), yielding compounds 1, 2 and 3, respectively. The solution
was stirred and refluxed for 2 h. After 2 h the solvent was evapo-
rated under reduced pressure and the product purified with diethyl
ether yielding a powder. All compounds synthesized were obtained
in an excellent yield, 92% (1), 96% (2) and 85% (3).
2
shows
a decrease and a red-shift from 270 to 294 nm
(Dk = 24 nm) on the absorption band, followed by a decrease of
about 40% in the emission fluorescence (see Fig. SN1). However,
with the addition of base compound 2 is practically not affected.
The same quenching behaviour on the absorption and emission
spectra in compound 2 was observed for compound 3, but the per-
centage of the emission quenching with the addition of one equiv-
alent of proton was higher (ca. 60%) (see Fig. SN1). This result
suggests that the observed quenching in the fluorescence emission
is probably due to the formation of hydrogen-bonds between the
protonated nitrogen located in the aromatic ring and the oxygen
atoms on the dopamine amino acid [19]. Furthermore in a basic
media, 3 shows a decrease in the emission intensity, probably
All ligands were characterized by elemental analysis, 1H NMR,
13C NMR, IR, ESI-TOF, absorption and fluorescence emission
spectroscopy.
A careful inspection of elemental analysis evidences that the
values fit perfectly with the expected chemical formula postulated.
This fact was also confirmed by ESI-TOF where the spectra show

206
E. Oliveira et al. / Inorganica Chimica Acta 381 (2012) 203–211
Table 1
UV–Vis and fluorescence data for 1, 2 and 3 in absolute ethanol.
Compounds
UV–Vis
Fluorescence
kexc (nm)
Log
e
kem (nm)
Stokes’shift (cmꢁ1
)
Quantum yield /
1
2
3
282
270
420
3.81
3.72
3.54
318
320
480
4014
5787
2976
0.001
0.009
0.022
ity will be higher. However, compounds 1–3 did not show any
change in the ground (absorption) and excited (emission) states
after the addition of alkali metals Li+, Na+ and alkali earth Mg2+
I norm. /a.u.
0.4
A
Abs. 1
Abs. 2
Abs. 3
and Ca2+
.
1
3.2.2. Transition and post-transition metal ions
0.3
0.2
0.1
0
0.8
0.6
0.4
0.2
0
Taking into account the previous results, we explored the inter-
action with the transition and post-transition metal ions, Zn2+
,
Emis. 1
Emis. 2
Emis. 3
Cd2+, Cu2+, Ni2+, Ag+, Fe3+ and Al3+
.
Compound 1 did not show spectral changes with the addition of
the metal ions aforementioned. However, compound 2 presented
spectral changes in the excited state (emission) in the presence
of Cu2+, Ni2+, Ag+ and Fe3+. In the ground state (absorption), were
observed changes only in the presence of Ag+ metal ion. Simulta-
neously, a quenching on the emission intensity was observed in
all cases. In Fig. 3(A and B) the emission titration of compound 2
in the presence of Cu2+ and Fe3+ is shown, as well as, the absorption
and emission titration of 2 in the presence of Ag+ (see Fig. 3C and
D).
300
400
500
600
700
As stated above, the addition of Cu2+ and Fe3+ to compound 2 in-
duces a quenching of the emission intensity being necessary less
than half equivalent of metal ion to quench the system. Based on
the high affinity of the Cu2+ and Fe3+ for oxygens [21], we believe
that the metal ion coordination occurs in the catechol’s presenting
in the dopamine amino acid [22]. The complexation constants with
both metals were calculated using the HYPSPEC [23] program, and are
summarized in Table 2. In both cases the complexation constants
suggest a complex of three ligands for one metal ion with an asso-
ciation constant of (L:M, 3:1), logKass. = 17.12 0.02 for Cu2+ and
logKass. = 13.47 0.02 for Fe3+. It is important to note that, com-
pound 2 gave a higher stability complex for Cu2+ than for Fe3+ me-
tal ion. Moreover, the interaction of 2 with Ag+ induces a small
quenching in the absorbance at 270 nm and an increase at
420 nm. This new band can be attributed to a metal to ligand
Wavelength/nm
Fig. 1. Absorption and emission spectra of compounds 1–3 in absolute ethanol
(T = 298 K, [1] = 5.9 ꢂ 10ꢁ5 M, [2] = 4.0 ꢂ 10ꢁ5 M, [3] = 3.25 ꢂ 10ꢁ5 M, kexc1 = 282 nm,
kexc2 = 270 nm, kexc3 = 420 nm).
due to the deprotonation of the oxygen and nitrogen atoms, allow-
ing a photoinduced electronic transfer phenomenon [18,20].
3.2.1. Alkali and alkali-earth metal ions
Although all compounds show two oxygen atoms in a catechol
unit specially designed for metal ion complexation, and according
to the Pearsons rules [21] are preferred for hard metal ions, it is ex-
pected that for alkali and alkali earth metal ions, the binding affin-
0.6
I norm. /a.u.
0.5
A
I
/a.u.
A
280 nm
330 nm
norm. 1
1
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0.8
0.6
0.4
0.2
0
0.8
316 nm
0.6
0.4
0.2
0 0.5 1 1.5 2 2.5 3
[H+]/[1]
0 0.5 1 1.5 2 2.5 3
[H+]/[1]
A
B
0
0 300
350
400
450
250
300
350
400
450
Wavelength/nm
Wavelength/nm
Fig. 2. Absorption (A) and emission (B) titration of compound 1, with the addition of methanesulfonic acid (CH3SO3H) (T = 298 K, [1] = 5.9 ꢂ 10ꢁ5 M, [CH3SO3H] = 1.00 ꢂ
10ꢁ2 M, kexc = 282 nm.

E. Oliveira et al. / Inorganica Chimica Acta 381 (2012) 203–211
207
I norm. /a.u.
I norm. /a.u.
I
norm. /a.u.
I
norm. /a.u.
1
1
1
1
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
320 nm
320 nm
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
0.5
1
1.5
2
2.5
3
0
0.5
1
1.5
2 2.5 3
[Cu2+]/[2]
[Fe3+]/[2]
A
B
0
300 320 340 360 380 400 420 440
Wavelength/nm
300 320 340 360 380 400 420 440
Wavelength/nm
I norm. /a.u.
0.3
I
/a.u.
0.25
A
0.05
A
A
norm.1
1
270 nm
420 nm
0.25
0.2
0.24
0.04
0.03
0.02
0.01
0.8
0.6
0.4
0.2
0
320 nm
0.8
0.6
0.4
0.2
0
0.23
0.22
0.21
0.15
0.1
0
5
10 15 20 25 30
[Ag+]/[2]
0
5 10 15 20 25 30
[Ag+]/[2]
0.05
C
D
0
300 320 340 360 380 400 420 440
Wavelength/nm
250
300
350
400
450
500
550
600
Wavelength/nm
Fig. 3. Spectrofluorimetric titrations of compound 2 with the addition of Cu2+ (A), Fe3+ (B) and Ag+ (D) in absolute ethanol. The inset shows the emission as function of [Cu2+]/
[2] (A), [Fe3+]/[2] (B) and [Ag+]/[2] (D) at 320 nm. Spectrophotometric titration (C) of compound 2 with the addition of Ag+ in absolute ethanol. The inset shows the absorption
as function of [Ag+]/[2] (C) at 320 nm ([2] = 4.1 ꢂ 10ꢁ5 M, T = 298 K).
Table 2
charge band [15]. A decrease in the emission intensity at 320 nm
Complexation constants of compounds 1–3 in the presence of metal ions, calculated
by Hypspec program (logKass.).
was observed, as well. The association constant suggests a mono-
nuclear complex formation with a small constant of (L:M, 1:1), log-
Kass. = 4.72 0.01 (see Table 2) when compared with Cu2+ or Fe3+
.
Compounds
Metal ion
(L:M) logKass.
1
2
Al3+
Ni2+
Cu2+
Al3+
Ag+
3:1–11.33 0.01
2:1–9.198 0.006
3:1–17.12 0.02
2:1–9.06 0.04
1:1–4.723 0.001
3:1–13.47 0.02
3:1–15.499 0.002
3:1–14.98 0.01
2:1–8.65 0.01
3:1–15.26 0.02
3:1–13.384 0.002
3:1–12.96 0.01
3:1–13.691 0.1
Compound 3 appears to be the most reactive system, presenting
similar spectral changes with Zn2+, Cd2+, Cu2+, Ni2+, Ag+ and Fe3+
metal ions. Here we will focus on Zn2+ and Fe3+ metal ions.
In Fig. 4, the absorption and emission spectra of compound 3
upon addition of Zn2+ metal ion is presented.
A similar behaviour is observed for all metals, wherever in the
absorption spectra (Fig. 4(A)) a decrease in the absorbance at
420 nm and an increase at 470 nm is shown. On the other hand,
a quenching in the emission intensity at 480 nm is also observed.
The amount of metal ion necessary to quench the system is less
than half equivalent.
Fe3+
Zn2+
Ni2+
Cd2+
Cu2+
Al3+
Ag+
3
Fe3+
In the same way upon addition of Fe3+ metal ions, a decrease in
the absorbance at 420 nm and an increase at 560 nm is observed in
the absorption spectra (see Fig. 5A). In emission spectra, a quench-
ing in the emission intensity is observed, as well.
The association constants for the transition and post-transition
metal ions mentioned above were determined by the program HYP-
SPEC [23], which revealed several complexes formed by three li-
gands and one metal ion, L:M; 3:1 for Zn2+, Cu2+, Ni2+, Ag+ and
Fe3+ and L:M; 2:1 for Cd2+ metal ion. The association constants
are presented in Table 2. A detailed analysis of these constants sug-
gests that a more stable complex is formed by Zn2+ (logKass.
=
=
15.50 0.01) > Cu2+
(logKass. = 15.26 0.02) > Ni2+
(logKass.
14.98 0.01) > Fe3+ (logKass. = 13.69 0.1) > Ag+ (logKass. = 12.96
0.01) > Cd2+ (logKass. = 4.72 0.01).
The most interesting results arise from the interaction with Al3+
using compounds 1 and 3, where an enhancement in the emission

208
E. Oliveira et al. / Inorganica Chimica Acta 381 (2012) 203–211
I norm. /a.u.
0.15
A
I norm. /a.u.
1
0.15
A
1
0.8
0.6
0.4
0.2
0
0.1
480 nm
420 nm
0.8
470 nm
0.05
0.1
0
0.6
0.4
0.2
0
0.2 0.4 0.6 0.8
1
0
0.2 0.4 0.6 0.8
1
[Zn2+]/[3]
[Zn2+]/[3]
0.05
A
B
0
0
280 320 360 400 440 480 520 560 600
440
480
520
560
600
640
680
Wavelength/nm
Wavelength/nm
Fig. 4. Spectrophotometric (A) and spectrofluorimetric (B) titrations of compound 3 with the addition of Zn2+ in absolute ethanol. The inset (A) shows the absorption as
function of [Zn2+]/[3] at 420 nm. The inset (B) shows the emission as function of [Zn2+]/[3]) at 480 nm ([3] = 3.25 ꢂ 10ꢁ5 M, T = 298 K).
I norm. /a.u.
0.4
I
norm. /a.u.
1
A
0.4
A
1
0.3
0.8
0.6
0.4
0.2
0
0.3
480 nm
0.8
0.2
0.1
0
420 nm
560 nm
0.6
0.4
0.2
0.2
0.1
0
0.2 0.4 0.6 0.8
1
0
0.2 0.4 0.6 0.8
1
[Fe3+]/[3]
[Fe3+]/[3]
A
B
0
0
300
400
500
600
700
800
440
480
520
560
600
640
680
Wavelength/nm
Wavelength/nm
Fig. 5. Spectrophotometric (A) and spectrofluorimetric (B) titrations of compound 3 with the addition of Fe3+ in absolute ethanol. The inset (A) shows the absorption as
function of [Fe3+]/[3] at 420 nm and 560 nm. The inset (B) shows the emission as function of [Fe3+]/[3] at 480 nm ([3] = 3.25 ꢂ 10ꢁ5 M, T = 298 K).
intensity was observed. In Fig. 6, the absorption and emission spec-
tra of compounds 1–3 with Al3+ is presented.
observed a decrease in the absorbance and a red-shift from 270
to 292 nm ( k = 22 nm) (see Fig. 6C and D). Similar to the other
metal ions studied for this compound, probably the metal com-
plexation takes place on the catecholate of dopamine.
D
In compound 1 upon addition of Al3,+ a decrease and a red-shift
in the absorption band from 282 to 293 nm (Dk = 11 nm) was ob-
served, followed by an absorption increase at 330 nm. The emis-
sion spectra show an enhancement in the intensity band at
318 nm, increasing the quantum yield of ca. 18-fold, from 0.001
to 0.018. Based on Upadhyay’s [24] work, we suggest that the com-
plexation probably involves the nitrogen atoms from the imine
bond, completing the coordination sphere with some of the nitro-
gens from the indazole, and the hydroxyl groups from the dopa-
mine. As stated above, usually Schiff-bases have low emission
due to the isomerization reaction around the imine linkage C@N
[17], being a non-planar molecule, but upon complexation by a
metal, this isomerization is inhibited and an emissive planar com-
plex is formed. This result was not observed for the other metal
ions reported.
Finally, Fig. 6(E and F) show the absorption and emission spec-
tra of compound 3 with Al3+. A decrease in the absorption at
410 nm is evidenced, and in the emission spectra, a fast quench
of emission at 480 nm (free ligand), was observed, followed by
the appearance of new intense emissive band at 530 nm relative
to the aluminium complex. These spectral changes suggest that
we are in the presence of a ratiometric fluorescent compound for
Al3+. Upon complexation, the quantum yield of 3 increases from
/ = 0.02 to / = 0.04 (see the complexation by the metal ion on
the imine bond and hydroxyl group picture 1). As explained for
compound 1, the enhancement of emission intensity arises from
the dopamine and hydroxynaphthalene, avoing then the C@N
isomerization (see Fig. 7).
It is interesting to note that relatively to compound 2, the oppo-
site effect was observed in the presence of Al3+. A quenching of 60%
in the emission intensity upon addition of one equivalent of metal
ion was observed. Moreover, in the absorption spectra was
The association constants for Al3+ interaction were determined
using the HYPSPEC program and complexes of three ligands for one
metal ion were found for compounds 1 and 3 with a log-
Kass. = 11.33 0.01 and logKass. = 13.38 0.01, respectively. For

E. Oliveira et al. / Inorganica Chimica Acta 381 (2012) 203–211
209
I norm. /a.u.
0.6
0.5
0.4
0.3
0.2
0.1
0.5
I
/a.u.
A
norm1.
0.8
0.6
0.4
0.2
0
1
A
0.4
0.3
0.2
0.1
0
0.8
318 nm
280 nm
330 nm
0.6
0.4
0.2
0 1 2 3 4 5 6 7 8
0
1
2
3
4
5
6
7
[Al3+]/[1]
[Al3+]/[1]
A
B
0
0
300
350
400
450
250
300
350
400
450
Wavelength/nm
Wavelength/nm
I norm. /a.u.
0.15
A
I
norm. /a.u.
0.15
A
1
1
0.8
0.6
0.4
0.2
0
0.1
480 nm
530 nm
0.8
0.6
0.4
0.2
0
420 nm
0.1
0.05
0
0
0.4 0.8 1.2 1.6
2
0
0.5
1
1.5
2
[Al3+]/[3]
[Al3+]/[3]
0.05
D
C
0
450
500
550
600
650
700
280 320 360 400 440 480 520 560 600
Wavelength/nm
Wavelength/nm
Fig. 6. Spectrophotometric (A, C) and spectrofluorimetric (B, D) titrations of compound 1 and 3 with the addition of Al3+ in absolute ethanol. The inset (A and C) shows the
absorption as function of [Al3+]/[1] (A) at 280 and 330 nm; and as function of [Al3+]/[3] (C) at 420 nm. The inset (B and D) shows the emission as function of [Al3+]/[1] (B) at
318 nm; and as function of [Al3+]/[3] (D) at 480 nm and 530 nm ([1] = 5.9 ꢂ 10ꢁ5 M, [3] = 3.25 ꢂ 10ꢁ5 M, kexc1 = 282 nm, kexc3 = 420 nm, T = 298 K).
OH HO
OH
O
O
Al3+
O
O
N
N
HO
N
N
O
HO
O
O
H
Free rotation around C=N disallowed
Fluorescent
Free rotation around C=N allowed
Not Fluorescent
Fig. 7. Proposed interaction mode of compound 3 with Al3+ and the fluorescence image with and without Al3+, under UV lamp kexc = 365 nm.
compound 2, a complex formation of two ligands for one metal
with a logKass. = 9.06 0.04 was postulated. The most stable alu-
minium complex was obtained with compound 3.
In order to study the potential analytical application of these
compounds, the detection (LOD) and quantification (LOQ) limits
were determined in absolute ethanol. For compound 1, the values
at 318 nm were of 1.016 0.026 (DL), 1.078 0.09 (QL); for com-
pound 2 at 320 nm were of 1.035 0.045 (DL), 1.140 0.150
(QL); and for compound 3 at 480 nm and 530 nm were of
1.086 0.13 (DL), 1.40 0.45 (QL) and 1.093 0.12 (DL),
1.379 0.41 (QL), respectively.
In conclusion, compound 1 shows to be quite selective for Al3+
,
with a strong enhancement of the emission intensity, (see Fig. 8)
while with the other metal ions did not show any spectral changes.

210
E. Oliveira et al. / Inorganica Chimica Acta 381 (2012) 203–211
The minimum amount for 1 of Al3+ that might be sensed in absolute
ethanol is ca. 4.73 ppm. Compound 2 is quite sensitive in the pres-
ence of transition and post-transition metal ions, generating non-
emissive quite stable complexes with Cu2+ and Fe3+. Moreover, this
4. Conclusions
Three new Schiff-base ligands 1–3 derivated from neurotrans-
mitter dopamine, bearing an indazole, methylimidazole or hydrox-
ynaphthaldehyde units were successfully synthesized and fully
characterized. Compound 3 was revealed as the most emissive
compound, with a quantum yield of 0.022, followed by compound
2 (/ = 0.009) and 1 (/ = 0.001). Compound 1 showed an enhance-
ment in the emission after protonation and Al3+ complexation,
being quite selective for this metal ion. After protonation, the
appearance of any PET (photoinduced electronic transfer) phenom-
ena was prevented, and after complexation by the metal which
inhibits the C@N isomerization. The minimum amount of Al3+ that
might be sensed by 1 in absolute ethanol is ca. 4.73 ppm.
compound could sense in absolute ethanol 0.5 ppm of Cu2+
,
0.39 ppm of Fe3+ and 5.00 ppm of Al3+. On the other hand, the com-
pound 3 is the most reactive compound presenting changes with all
the transition and post-transition metal ions studied. Very stable
contents were obtained with Zn2+, Cu2+, Ni2+, Fe3+ and Al3+ metal
ions. The minimal amount of these metal ions that can be quantified
is of 0.34 ppm for Zn2+ and Cu2+, 0.48 ppm for Ni2+, 0.061 ppm for
Fe3+ and 0.047 ppm for Al3+
.
3.3. Aluminium(III) metal complexes
Compound 2 is quite sensitive in the presence of the transition
and post-transition metal ions studied, forming non-emissive quite
stable complexes with Cu2+ and Fe3+. This compound could sense
In order to study the emission properties in the solid state, the
solid aluminium(III) complexes of 1 and 3 were obtained. The syn-
thesis was carried out in absolute ethanol by direct reactions, lead-
ing to the formation of complexes with 3:1 metal-to-ligand ratio.
The metal synthesis yields a green powder for (1 Al3+ – (4)) and
a red-brown powder for (3 Al3+ – (5)) with excellent yields of
90% and 88%. Both the metal complexes were characterized by ele-
mental analysis, IR, 1H NMR, UV–Vis and fluorescence emission.
The aluminium metal complexes gave the formula: [Al13](NO3)3.
6H2O (4) and [Al33] (NO3)3. 6H2O (5).
in absolute ethanol 0.5 ppm of Cu2+ 0.39 ppm of Fe3+ and
,
5.00 ppm of Al3+. On the other hand while the selectivity is lost,
compound 3 is the most reactive compound presenting changes
with all the transition and post-transition metal ions studied. The
presence of these metal ions, induces in 3 a quenching in the emis-
sion, forming complexes of three ligands for one metal ion, as well.
In these both cases, 2 and 3, we postulated that the complexation
probably takes place on the catecholate of the neurotransmitter
dopamine, activating also the C@N isomerization. In compound
3, the addition of Al3+ generates a very emissive and ratiometric
complex with an emission band at 530 nm, with a stoichiometry
of L:M; 3:1. The minimal amount sense with 3 of these metal ions
The IR spectra were recorded in KBr discs, and show bands
attributed to the imine linkage C@N at 1621–1618 cmꢁ1
.
The absorptions of the nitrate counterions, at ca. 1460–1452
(m5), 1300(m1) and 1028(m
2) cmꢁ1, suggest the presence of bidentate
can be quantified as: 0.34 ppm for Zn2+ and Cu2+, 0.48 ppm for Ni2+
0.061 ppm for Fe3+ and 0.047 ppm for Al3+
As a conclusion, very promising results were obtained for Al3+
where a CHEF effect was observed in compounds 1 and 3. Other-
wise compound 3 could be explored in the future for analytical
purposes since it has a very low quantification limit, as well as,
ratiometric properties.
,
nitrate groups; an intense band at ca. 1380 cmꢁ1 attributable to io-
nic nitrate, is also present [25–27].
.
The appearance of a broad band at around 3400 cmꢁ1 in all
complexes suggests the presence of coordinated and/or hydrated
water molecules [13].
,
The absorption and emission spectra of the metal complexes 4
and 5 were performed in absolute ethanol. Complexes 4 and 5
show bands at 290 and 420 nm in absorption and at 320 and
530 nm in emission spectra, respectively (see Supporting informa-
tion). As, shown previously in the aluminium metal titration for 1
and 3 in solution, very strong emissive systems were also obtained
for the synthesized aluminium metal complexes, with a quantum
yield of 0.01 and 0.04 for 4 and 5. However, due to their dark col-
our, dark green for 4 and brown for 5, the complexes were not
emissive in solid state.
Acknowledgements
We are indebted to Xunta de Galiza (Spain) for Projects
09CSA043383PR and 10CSA383009PR (Biomedicine) and the Sci-
entific Association ProteoMass for financial support. E.O. and
H.M.S. thank the FCT-MCTES (Portugal) for the Postdoctoral Grants
SFRH/BPD/72557/2010 and SFRH/BPD/73997/2010, respectively.
J.L.C and C.L. thank Xunta de Galicia (Spain) for the Isidro Parga
Pongal research contracts.
I norm. /a.u.
1.4
318 nm
Appendix A. Supplementary material
1.2
Cd2+
Zn2+
1Al3+
Ni2+ Ag+
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.08.033.
Fe3+
Cu2+
1
0.8
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0.6
0.4
0.2
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