Synthesis and photophysical studies of two luminescent chemosensors based on catechol and 8-Hydroxyquinoline chromophores, and their complexes with group 13 metal ions

Article abstract of DOI:10.1016/j.inoche.2011.02.023

Two ligands, L¹ and L², derived from 2,3-dihydroxybenzaldehyde and 8-hydroxyquinoline-2-carbaldehyde using 4,4′-methylene-dianiline as spacer have been used to study their interaction with the group 13 metals, Al³⁺, Ga³⁺ and In³⁺. In all cases, emissive dinuclear metal complexes were synthesized and characterized. The complexation reactions with the diiminic ligands were obtained by direct reactions between ligand and metal ions. The complexes have been characterized by elemental analyses, mass spectrometry, IR, UV-vis and fluorescence spectroscopy. The interaction towards metal ions has been explored in solution by absorption and fluorescence emission spectroscopy, obtaining results that support the solid-state helicate-type complexes.

Full text of DOI:10.1016/j.inoche.2011.02.023

Inorganic Chemistry Communications 14 (2011) 831–835
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and photophysical studies of two luminescent chemosensors based on
catechol and 8-Hydroxyquinoline chromophores, and their complexes with group 13
metal ions
a
b
a,c
Cristina Nuñez a,b, , Javier Fernandez-Lodeiro , Mário Dinis , Miguel Larguinho
,
⁎
José Luis Capelo a,b, Carlos Lodeiro a,b,⁎⁎
a
BIOSCOPE Group, Faculty of Science, Physical-Chemistry Department, Campus Ourense, University of Vigo, 32004, Ourense, Spain
REQUIMTE, Department of Chemistry, FCT-UNL, 2829–516 Monte de Caparica, Portugal
CIGMH, Department of Life Sciences, FCT-UNL, 2829–516 Monte de Caparica, Portugal
b
c
a r t i c l e i n f o
a b s t r a c t
Two ligands, L1 and L2, derived from 2,3-dihydroxybenzaldehyde and 8-hydroxyquinoline-2-carbaldehyde
using 4,4′-methylene-dianiline as spacer have been used to study their interaction with the group 13 metals,
Al3+, Ga3+ and In3+. In all cases, emissive dinuclear metal complexes were synthesized and characterized.
The complexation reactions with the diiminic ligands were obtained by direct reactions between ligand and
metal ions. The complexes have been characterized by elemental analyses, mass spectrometry, IR, UV–vis and
fluorescence spectroscopy. The interaction towards metal ions has been explored in solution by absorption
and fluorescence emission spectroscopy, obtaining results that support the solid-state helicate-type
complexes.
Article history:
Received 11 January 2011
Accepted 26 February 2011
Available online 12 March 2011
Keywords:
Aluminum
Galium
Indium
© 2011 Elsevier B.V. All rights reserved.
Complexes Schiff-base helicates
Much attention has been focused on the development of
chemosensors for the selective and efficient detection of chemically
and biologically important metal ions [1]. A fluorescent chemosensor
for metal ions consists of a molecule incorporating fluorophores
linked to complexing units [2]. There are three main classes of
fluorescent molecular sensors for cation recognition which differ
by the nature of the cation-controlled photoinduced processes:
a) sensors based on control of photoinduced electron transfer (PET),
b) those based on cation control of photoinduced charge transfer
(PCT), and c) systems based on cation control of excimer formation or
disappearance [2].
Since the discovery of the double-stranded helical structure of
DNA by Watson and Crick [3], chemists have been searching for
simple linear molecules that could be able to form artificial double
helices through noncovalent interactions, such as π-stacking, electro-
static interactions, hydrogen bonding, or metal coordination [4]. In
artificial analogs of DNA, the hydrogen bonding interaction between
the two strands can be substituted by metal coordination. There are
numerous reports on coordination polymers that exhibit double-
helical motifs in the crystal [5]. A special class of well-defined
molecular double-stranded helical metal complexes was introduced
in 1987 by J.-M. Lehn and was termed double-stranded helicates [6].
Of the factors integral to helicate formation, the geometric
preferences of the metal and the group that separates and links the
donor atoms of the ligand are particularly influential. Chirality is
introduced upon wrapping of the helicating ligands around the metal
centers. In the case of the meso-helicates [7–12] (“side-by-side
complexes” [13] or “mesocates” [14]) an achiral supermolecule is
obtained that bears two oppositely configured chiral units. Different
factors can be responsible for the stereocontrol in the formation of the
helicate-type complexes [15,16]. Templating effects might influence
the diastereoselective formation of helicates [17,18]. Chiral units in
the ligand spacer can control the stereochemistry at the metal
complex units [19–21], or steric constraints enforce one of the two
possible diastereomeric forms of coordination compounds [22,23].
Ligands with an odd number of methylene units in the spacer
possess a “horizontal” mirror plane as the most influential symmetry
element of the idealized C2v symmetry, which mirrors the two
attached chiral metal-complex moieties onto each other. This
symmetry transformation leads to an opposite configuration at the
complex units, and thus the ligand is predisposed to form the achiral
dinuclear meso-helicate [24]. However, to preserve this symmetry in a
⁎
Correspondence to: C. Nuñez, REQUIMTE, Department of Chemistry, FCT-UNL,
2829–516 Monte de Caparica, Portugal. Tel.: +351 21 2948300; fax: +351 21 2948550.
⁎⁎ Correspondence to: C. Lodeiro, BIOSCOPE Group, Faculty of Science, Physical-
Chemistry Department, Campus Ourense, University of Vigo, 32004, Ourense, Spain.
Tel.: +34 988 368894; fax: +34 988 387001.
E-mail addresses: cristina.nunez@dq.fct.unl.pt (C. Nuñez), clodeiro@uvigo.es
(C. Lodeiro).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.02.023

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C. Nuñez et al. / Inorganic Chemistry Communications 14 (2011) 831–835
Scheme 1. Schematic synthetic route for ligands L1 and L2 in absolute ethanol.
dinuclear complex with short spacers, the ligand has to adopt an
unfavorable conformation for complex formation. If long spacers are
present, this conformation should not be “unfavorable” (vide infra).
Following our ongoing research project in fluorescent materials and
emissive compounds and complexes [25], in the present work we have
reported the synthesis of a new helicate-type ligand L2 derived from
4,4′-methylene-dianiline and 8-hydroxyquinoline-2-carbaldehyde and
the derived aluminum(III), gallium(III) and indium(III) complexes. For
comparative purposes the corresponding cathecol ligand L1 was also
synthesized following the method described previously by M. Albrecht
The complexes were characterized by elemental analysis, IR, and ESI
MS spectra [29]. Electro-spray Ionization Mass Spectrometry (ESI-MS)
in negative ion mode of the metal complexes displays peaks that
confirm the formation of the metal complexes. The IR spectra of the
0.35
A
1
I
Abs
norm / a.u.
0.3
[26] and complexed with the same metal ions, Al3+, Ga3+ and In3+
.
Excitation
Emission
0.8
Compounds L1 and L2 were synthesized following a one-pot
reaction, by direct condensation of 4,4′-methylene-dianiline and the
commercial carbonyl precursors 2,3-dihydroxybenzaldehyde and
8-hydroxyquinoline-2-carbaldehyde, respectively [27]. The reactions
were performed by a conventional method, heating an ethanolic
solution during 4 h. Both compounds were isolated as air-stable yellow
solids, with ca. 63 and 53% yields, respectively. The reaction pathways
are shown in Scheme 1.
0.25
0.2
0.6
0.4
0.2
0
0.15
0.1
A
The elemental analysis of the Schiff-bases L1 and L2 isolated as a
dry orange and yellow powders, respectively, confirmed the purity of
our sample. The infrared spectrum (in KBr) in both cases shows bands
at 1623 and 1627 cm−1, respectively, corresponding to the imine
bond ν(C=N)imine and no peaks attributable to unreacted amine or
carbonyl groups were present. The ESI-MS spectra of both sensors
show parent peaks at 439.1 and 531.2 m/z, corresponding to the
0.05
300
400
500
600
700
800
Wavelength / nm
A
Abs
1
0.6
Inorm / a.u.
Excitation
Emission
protonated imine forms of the ligands [L1 +H]+ and [Na+L2 +H]+
,
0.8
0.6
0.4
0.2
0
respectively. The 1H NMR spectra of compounds L1 and L2 were
recorded using DMSO-d6 as a solvent. As it could be expected for a
Schiff-base, the 1H NMR spectra show peaks corresponding to the
imine protons ca. 9–8 ppm.
0.4
0.2
0
The presence of six potential donor atoms (N2O4) in the ligand
structure of L1 and (N4O2) in the ligand structure of L2, gives strong
recognition ability towards metal ions. For the preparation of the
complexes Na6[(L1)3M2], the ligand L1-H4 (3 equiv.), [MO(acac)2]
(2 equiv.) (M=Al3+, Ga3+ and In3+) and NaHCO3 (2 equiv.) were
mixed in ethanol. For the preparation of the complexes [(L2)3M2], the
ligand L1-H2 (3 equiv.), [MO(acac)2] (2 equiv.) (M=Al3+, Ga3+ and
In3+) and NaHCO3 (2 equiv.) were mixed in the same solvent. After
4 h the resulting solutions were concentrated in a rotary evaporator
and diethyl ether were slowly infused into the solution producing
powdery precipitates that were separated by centrifugation and dried
under vacuum [28].
B
240 320 400 480 560 640 720 800
Wavelength / nm
Fig. 1. Absorption (full line), emission (broken line) and excitation (dotted line) spectra
of compound L1 after the addition of 5 equivalents of (Bu4N)OH (λexc=425 nm;
λem=542 nm, [L1]=1.07 10−5 M) in CH3CN at room temperature (A). Spectra of
compound L2 after the addition of 20 equivalents of (Bu4N)OH (λexc=470 nm;
λem=710 nm, [L2]=1.14 10−5 M) in CH2Cl2 at room temperature (B).

C. Nuñez et al. / Inorganic Chemistry Communications 14 (2011) 831–835
833
complexes were recorded as KBr disks. In the case of metal complexes
derived from ligand L1, the band due to the imine bond [ν(C=N)imine
After addition of tetrabutylammonium hydroxide, the spectro-
photometric characterization of receptors L1 and L2 are reported in
Fig. 1. The absorption, emission and excitation spectra of this
deprotonated species were studied in CH3CN at 298 K [31]. The
absorption spectrum of ligand L1 shows two bands with the maxima
centered at 325 and 425 nm, coincident with the excitation spectrum.
In the case of ligand L2 this bands appeared at 270, 340 and 470 nm.
Only after deprotonation, a very low emission was observed; the
luminescence of ligands L1 and L2 appears centered at 542 nm and
710 nm, respectively.
]
and the [ν(C=C)ar] stretching modes of aromatic rings are shifted to
higher wavenumbers when compared to its position in the spectrum of
the free ligand. In the case of metal complexes derived from ligand L2,
the band due to the imine bond [ν(C=N)imine] is shifted to lower
wavenumbers and the [ν(C=C)ar and ν(C=N)py] stretching modes are
shifted to higher wavenumbers when compared to its position in the
spectrum of the free ligand. Both effects suggest that in solid state the
N
iminic atom could be involved in the coordination to the metal ion, and
in the case of metal complexes derived from ligand L2 the Npy are
involving in the coordination in solid state [30].
Titration of chemosensors L1 with tetrabutylammonium hydroxide
in CH3CN solution at 298 K (Fig. 2), can be followed by the formation
of a new band centered at ca. 425 nm assigned to a charge transfer
(CT) process and a decrease in the band assigned to the π–π*
transition of the chromophores centered at 325 nm. In the case of
chemosensor L2 this new band appeared at ca. 470 nm and a decrease
in the bands centered at 270 and 340 nm was observed.
0.6
0.5
A
A
320 nm
425 nm
The weak luminescence of compounds L1 and L2 could be
attributed to the competition between the intramolecular photoin-
duced proton transfer (PPT) from the hydroxyl groups present in both
compounds, and the photo-induced electron transfer (PET) from the
imines [32]. After addition of tetrabutylammonium hydroxide, the
deprotonation effect prevents the PPT process, resulting in a short
enhancement of the fluorescence signal.
0.4
0.5
0.4
0.3
0.2
0.1
0
0.3
0.2
0.1
0
0
5
10 15 20
[OH-] / [L1]
The sensing behavior of L1 and L2 toward Al3+, Ga3+ and In3+ has
been studied by UV–vis spectroscopy in CH3CN and CH2Cl2 as the
solvent for compounds L1 and L2, respectively.
As it can be seen in Figs. 3A, 4A and B, addition of increasing
amounts of anhydrous aluminum, gallium and indium nitrates to an
A
250
300
350
400
450
500
Wavelength / nm
1
0.8
0.6
0.4
0.2
0
1
I
norm / a.u.
0.6
A
0.5
0.4
A
0.3
0.8
0.6
0.4
0.2
0
542 nm
10 15 20
[OH-] / [L1]
320 nm
425 nm
0.2
0.1
0
0
5
0.4
0.3
0.2
0.1
0
0
0.2 0.4 0.6 0.8
[Al3+] / [L1]
1
B
450
500
550
600
650
700
A
250
Wavelength / nm
300
350
400
450
500
0.6
A
Wavelength / nm
0.6
A
0.5
0.5
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
1
0.8
0.6
0.4
0.2
1
270 nm
470 nm
542 nm
I / a.u.
0.8
0.6
0.4
0.2
0
0
5
10 15 20 25
[OH-] / [L2]
0
0
0.20.40.60.8
[Al3+] / [L1]
1
C
B
250 300 350 400 450 500 550
Wavelength / nm
450
500
550
600
650
700
Fig. 2. Spectrophometric (A) and spectrofluorimetric (B) titrations of ligand L1 (yellow
line) in CH3CN as a function of added (Bu4N)OH (orange line). The insets show the
absorption at 320 and 425 nm; and the normalized fluorescence intensity at 542 nm
([L1]=1.07*10−5 M; λexc=425 nm). Spectrophometric titration (C) of ligand L2
(yellow line) in CH3CN as a function of added (Bu4N)OH (orange line). The insets show
the absorption at 270 and 470 nm ([L2]=1.14 10−5 M).
Wavelength / nm
Fig. 3. Spectrophometric (A) and spectrofluorimetric (B) titrations of ligand L1 (yellow
line) in CH3CN as a function of added Al3+, after the addition of 5 equivalents of (Bu4N)
OH (orange line). The insets show the absorption at 320 and 425 nm; and the
normalized fluorescence intensity at 542 nm ([L1]=1.07 10−5 M).

834
C. Nuñez et al. / Inorganic Chemistry Communications 14 (2011) 831–835
acetonitrile solution of the deprotonated ligand L1 (1.07 10−5 M), at
298 K, led to an increase in the absorption bands centered at 320 nm,
and an decrease in the bands centered at 425 nm. Addition of
increasing amounts of the same metal ions to a dichloromethane
solution of the deprotonated ligand L2 (1.14 10− M), at 298 K, led to
an increase in the absorption bands centered at 270 nm, and a
decrease in the bands centered at 470 nm (Fig. 4C and D).
Table 1
Stability constants for compounds L1 and L2 in the presence of OH−, Al3+, Ga3+ and In3+
in acetonitrile.
Ligand
L1
Interaction
Σ log β (absorption)
Σ log β (emission)
OH− (1:1)
OH− (1:2)
OH− (1:3)
OH− (1:4)
Al3 +(1:1)
Al3+(1:2)
Ga3+(1:1)
Ga3+(1:2)
In3+(1:1)
In3+(1:2)
OH- (1:1)
–
3.91 2.41.10−2
–
7.88 1.28.10−2
12.29 1.56.10−2
16.65 1.76.10−2
–
–
–
Simultaneously, upon addition of increasing amounts of anhy-
drous aluminum, gallium and indium nitrates to the deprotonated
ligands L1 (1.07 10−5 M) and L2 (1.14 10−5 M), at 298 K, respec-
tively, the emission disappears (Fig. 3B). This quenching can be
attributed to the coordination effect towards the hydroxyl groups.
The stability constants for the interaction of ligands L1 and L2 with
OH−, Al3+, Ga3+ and In3+ followed by absorption and fluorescence
titrations were calculated using HypSpec software and are summa-
rized in Table 1. Taking into account these values, the sequence of the
strongest interaction expected for compound L1, in decreasing order is
L1
L1
L1
L2
L2
L2
L2
4.23 4.31.10−3
6.98 6.93.10−2
–
6.36 4.93.10−2
–
7.09 5.08.10−3
–
–
–
–
–
–
–
5.56 3.36.10−2
4.56 4.68.10−3
8.38 4.53.10−3
4.96 1.41.10−3
9.55 1.44.10−3
3.57 6.98.10−3
6.79 7.04.10−3
4.06 8.64.10−3
7.28 8.47.10−3
OH− (1:2)
Al3+(1:1)
Al3+(1:2)
Ga3+(1:1)
Ga3+(1:2)
In3+(1:1)
In3+(1:2)
–
–
–
–
Al3+ N Ga3+ N In3+; and the decreasing order for system L2 is Al3+
In3+ N Ga3+
N
.
In conclusion, chemosensor L1 and the new ligand (L2) containing
two 8-HQ units have been synthesized and fully characterized. The
deprotonation behavior and sensing capability of these ligands
toward trivalent Al3+, Ga3+ and In3+ metal ions have been studied
by UV–vis and fluorescent emission spectroscopy. The fluorescence of
chemosensors L1 and L2 is weak but an Enhancement of the
Fluorescence (EF) effect was observed in both cases after addition of
tetrabutylammonium hydroxide. Titration of ligands L1 and L2 with
trivalent Al3+, Ga3+ and In3+ metal ions led to a decrease in the
fluorescence emission. This effect in solution is due to the coordina-
tion of the oxygen atoms and the uncoordinated nitrogen donor
atoms present in the ligands, activating the quenching through the
PET phenomena. All of these results support our hypothesis of the
formation of helicate-type structures.
Acknowledgements
Authors thank University of Vigo INOU-ViCou K914 and K915
(Spain) and Scientific PROTEOMASS Association (Ourense-Spain) for
financial support. J.F.L. thanks Xunta de Galicia for his research
contract in the aim of the Project 09CSA043383PR. C.N. and M.L.
thanks Fundação para a Ciência e a Tecnologia/FEDER (Portugal/EU)
for the postdoctoral contract SFRH/BPD/65367/2009 and the doctoral
grant SFRH/BD/64026/2009 respectively. J.L. and C.L. thank Xunta de
Galicia for the Isidro Parga Pondal Research program.
0.6
A
0.6
A
0.4
A
0.3
0.4
A
320 nm
425 nm
0.3
0.5
0.5
320 nm
0.2
0.1
0
425 nm
0.2
0.4
0.4
0.3
0.2
0.1
0
0.1
0
0.2 0.4 0.6 0.8
[In3+] / [L1]
1
0
0.3
0
0.2 0.4 0.6 0.8
[Ga3+] / [L1]
1
0.2
0.1
0
(
A
250
B
250
300
350
400
450
500
300
350
400
450
500
Wavelength / nm
Wavelength / nm
0.7
A
0.7
A
0.6
0.6
A
0.6
A
0.5
0.5
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
270 nm
470 nm
0.4
0.3
0.2
0.1
0
270 nm
470 nm
0.5
0.4
0.3
0.2
0.1
0
0
0.2 0.4 0.6 0.8
[In3+] / [L2]
1
0
0.2 0.4 0.6 0.8
[Ga3+] / [L2]
1
D
C
250 300 350 400 450 500 550
250 300 350 400 450 500 550
Wavelength / nm
Wavelength / nm
Fig. 4. Spectrophometric titrations of ligand L1 and L2 (yellow lines) in CH3CN as a function of added Ga3+ (A and C) and In3+ (B and D), after the addition of 5 equivalents of (Bu4N)
OH (orange lines). The insets show the absorption at 320 and 425 nm ([L1]=1.07 10−5 M) and the absorption at 270 and 470 nm ([L2]=1.14 10−5 M).

C. Nuñez et al. / Inorganic Chemistry Communications 14 (2011) 831–835
835
(30 mL). The resulting solution was gently refluxed with magnetic stirring for ca.
4 h. The colour changed from yellow to orange. The solution was then allowed to
cool and an orange solid appeared. The solid was filtered off, washed with diethyl
ether and methanol and dried under vacuum. This compound was characterized
as L2.L1. Anal. Calcd for C27H22N2O4: C, 73.96; H, 5.06; N, 6.39. Found: C, 73.78; H,
5.24; N, 6.33%. Yield: 63%. IR (KBr, cm-1): 1623 [ν(C=N)imine]; 1583 [ν(C=C)].
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infused into the solution producing powdery precipitates. The products were
separated by centrifugation and dried under vacuum. Na6[Al2(L1)3]. Anal. Calcd for
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Angew. Chem. Int. Ed Engl. 34 (1995) 996.
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Angew. Chem. Int. Ed. 37 (1998) 932.
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[25] (a) C. Lodeiro, F. Pina, Coord. Chem. Rev. 253 (2009) 1353;
(b) M.R. Bermejo, M. Vazquez, M. Sanmartin, J. Garcia-Deibe, M. Fondo, C.
Lodeiro, New J. Chem. 26 (2002) 1365;
C
81H66Al2N6Na6O12: C, 64.54; H, 4.41; N, 5.58. Found: C, 64.74; H, 4.15; N, 4.94%.
Yield: 53%. IR (KBr, cm-1): 1635 [ν(C=N)imine]; 1596 [ν(C=C)ar]. MS (ESI-, m/z):
461 [AlL1–4H]-, 923 [Al2(L1)2–7H]-, 945 [Al2(L1)2Na-8H]-, 1405 [Al2(L1)3Na2–9H]-,
1427 [Al2(L1)3Na3–10H]-. Na6[Ga2(L1)3]. Anal. Calcd for C81H66Ga2N6Na6O12: C,
61.08; H, 4.18; N, 5.28. Found: C, 61.66; H, 3.34; N, 5.55%. Yield: 53%. IR (KBr, cm-1):
1633 [ν(C=N)imine]; 1595 [ν(C=C)ar]. MS (ESI-, m/z): 437 [L1–1H]+, 459
[L1Na-2H]+, 503 [Ga(L1)-4H]+, 1009 [Ga2(L1)2–5H]+, 1031 [Ga2(L1)2Na-6H]+
.
Na6[In2(L1)3]. Anal. Calcd for C81H66In2N6Na6O12: C, 57.81; H, 3.95; N, 4.99.
Found: C, 41.80; H, 3.15; N, 3.26%. Yield: 53%. IR (KBr, cm-1): 1632 [ν(C=N)imine];
1592 [ν(C=C)ar]. MS (ESI-, m/z): 549 [In(L1)-4H]+, 1121 [In2(L1)2Na-8H]+. [Al2
(L2)3]. Anal. Calcd for C99H88Al2N12O14: C, 68.98; H, 5.15; N, 9.75. Found: C, 68.69; H,
4.15; N, 8.89%. Yield: 53%. IR (KBr, cm-1): 1612 [ν(C=N)imine]; 1567, 1460
[ν(C=C)ar and ν(C=N)py]. MS (ESI-, m/z): 1039 [Al(L2)2–4H]+. [Ga2(L2)3]. Anal.
Calcd for C98H82Ga2N12O11: C, 67.74; H, 4.71; N, 9.58. Found: C, 67.24; H, 4.21; N,
8.84%. Yield: 53%. IR (KBr, cm-1): 1615 [ν(C=N)imine]; 1563, 1455 [ν(C=C)ar and
ν(C=N)py]. MS (ESI-, m/z): 1081 [Ga(L2)2–4H]+
99H82In2N12O11: C, 64.43; H, 4.48; N, 9.11. Found: C, 64.11; H, 4.26; N, 9.09%.
Yield: 53%. IR (KBr, cm-1): 1623 [ν(C=N)imine]; 1594, 1447 [ν(C=C)ar and
ν(C=N)py]. MS (ESI-, m/z): 1127 [In(L2)2–4H]+
.
[In2(L2)3]. Anal. Calcd for
C
.
[29] Elemental analyses were carried out at the REQUIMTE DQ Service (Universidade
Nova de Lisboa), on a Thermo Finnigan-CE Flash-EA 1112- CHNS instrument.
Infrared spectra were recorded as KBr discs using Bio-Rad FTS 175-C spectropho-
tometer. Proton NMR spectra were recorded using a Bruker WM-400 spectrometer.
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(1995) 5825.
[31] Absorption spectra were recorded on a Shimadzu UV-2501PC or in a Perkin Elmer
lambda 35 spectrophotometer. Fluorescence emission spectra were recorded on a
Horiba–Jobin–Yvon SPEX Fluorolog 3.22 or a Perkin Elmer LS45 spectrofluori-
meters. The linearity of the fluorescence emission versus concentration was
checked in the concentration range used (10–4 to 10–6 M). A correction for the
absorbed light was performed when necessary. All spectrofluorimetric titrations
were performed as follows: the stock solutions of the ligand (ca. 1.10–3 M) were
prepared by dissolving an appropriate amount of the ligand in a 50 mL volumetric
flask and diluting to the mark with acetonitrile (ligand L1) and dichloromethane
(ligand L2) UVA-sol. All measurements were performed at 298 K. The titration
solutions (ca. [L]=1.0.10–5 M) were prepared by appropriate dilution of the stock
solutions. Titrations of the ligand were carried out by addition of microliter
amounts of standard solutions of the ions in acetonitrile or dichloromethane.
[32] R.T. Bronson, M. Montalti, L. Prodi, N. Zaccheroni, R.D. Lamb, N.K. Dalley, R.M. Izatt,
J.S. Bradshaw, P.B. Savage, Tetrahedron 60 (2004) 11139.
(c) C. Bazzicalipi, A. Bencini, E. Berni, A. Bianchi, A. Danesi, C. Giorgi, B. Valtancoli,
C. Lodeiro, J.C. Lima, F. Pina, M.A. Bernardo, Inorg. Chem. 43 (2004) 5134;
(d) A. Tamayo, B. Pedras, C. Lodeiro, L. Escriche, J. Casabo, J.L. Capelo, B. Covelo, R.
Kivekas, R. Sillanpaa, Inorg. Chem. 46 (2007) 7818;
(e) M.J. Mayoral, P. Ovejero, J.A. Campo, J.V. Heras, E. Pinilla, M.R. Torres, C.
Lodeiro, M. Cano, Dalton Trans. (2008) 6912.
[26] M. Albrecht, I. Janser, H. Houjou, R. Fröhlich, Chem. Eur. J. 10 (2004) 2839.
[27] Synthesis of L1 and L2. The synthesis of L1 was carried out through the slow
addition of 4,4′-methylene-dianiline to an ethanolic solution of 2,3-dihydrox-
ybenzaldehyde using the methodology reported previously Albrecht et al. [26]
The synthesis of L2 was attempted through the slow addition of 4,4′-methylene-
dianiline (0.11419 g, 0.57 mmol) in absolute ethanol (20 mL) to a solution of 8-
hydroxyquinoline-2-carbaldehyde (0.1766 g, 1.02 mmol) in the same solvent