Exploring the emissive properties of New Azacrown compounds bearing aryl, furyl, or thienyl moieties: A special case of chelation enhancement of fluorescence upon interaction with Ca2+, Cu2+, or Ni 2+

Article abstract of DOI:10.1021/ic101095y

Three new compounds bearing furyl, aryl, or thienyl moieties linked to an imidazo-crown ether system (1, 2, and 3) were synthesized and fully characterized by elemental analysis, infrared, UV-vis absorption, and emission spectroscopy, X-ray crystal diffra

Full text of DOI:10.1021/ic101095y

Inorg. Chem. 2010, 49, 10847–10857 10847
DOI: 10.1021/ic101095y
Exploring the Emissive Properties of New Azacrown Compounds Bearing Aryl,
Furyl, or Thienyl Moieties: A Special Case of Chelation Enhancement of
Fluorescence upon Interaction with Ca2þ, Cu2þ, or Ni2þ
Elisabete Oliveira,† Rosa M. F. Baptista,‡ Susana P. G. Costa,‡ M. Manuela M. Raposo,*,‡ and Carlos Lodeiro*,†,§
†REQUIMTE, Department of Chemistry, FCT-UNL, 2829-516 Monte de Caparica, Portugal, ‡CQ-UM,
Center of Chemistry, University of Minho, Campus Gualtar, 4710-057 Braga, Portugal, and §BIOSCOPE
Research Team, Faculty of Science, Physical-Chemistry Department, Campus Ourense, University of Vigo,
32004, Ourense, Spain.
Received May 31, 2010
Three new compounds bearing furyl, aryl, or thienyl moieties linked to an imidazo-crown ether system (1, 2, and 3) were
synthesized and fully characterized by elemental analysis, infrared, UV-vis absorption, and emission spectroscopy,
X-ray crystal diffraction, and MALDI-TOF-MS spectrometry. The interaction toward metal ions (Ca2þ, Cu2þ, Ni2þ, and
Hg2þ) and F- has been explored in solution by absorption and fluorescence spectroscopy. Mononuclear and binuclear
metal complexes using Cu2þ or Hg2þ as metal centers have been synthesized and characterized. Compounds 2 and 3
show a noticeable enhancement of the fluorescence intensity in the presence of Ca2þ and Cu2þ ions. Moreover
compound 3 presents a dual sensory detection way by modification of the fluorimetric and colorimetric properties in the
presence of Cu2þ or Hg2þ. EPR studies in frozen solution and in microcrystalline state of the dinuclear Cu(II)3 complex
revealed the presence of an unique Cu2þ type.
Introduction
increased notably the knowledge on crown-ether derivatives
as metal ion chemosensors.
Classically a fluorescent chemosensor is a molecular device
formed by an ionophore, a fluorophore, and a chemical
spacer in between.1 On the basis of this basic architectural
premise, the field of fluorescence chemosensors has grown
during recent years due to their importance in applications,
such as in material sciences, biomedical, analytical chemistry,
and environmental sciences.2
Pioneering studies by H.G. Lohr and F. Vogtle on the
properties of chromo and fluoroinophoric dyes,3 and the
studies by Okamoto on the chemiluminescent behaviors of
several crown-ether-modified Iophine peroxide ionophore,4
Incorporation of the imidazole group in abiotic systems
has been extensively explored since the initial work of Debus
in 18585 because of their interesting chemical and biochem-
ical properties. These compounds have important pharma-
cological properties and play an important role in many
biochemical processes, such as inhibitors of P38 MAP kinase,
fungicides or herbicides, and therapeutic agents.6
€
€
Besides their classical applications in medicinal chemistry,6
2,4,5-triaryl(heteroaryl)-imidazoles play also important roles
in materials science because of their optoelectronic properties.7
Recently, triaryl(heteroaryl)-imidazole-based chromophores
have received increasing attention because of their distinctive
linear and nonlinear optical properties and also because
of their excellent thermal stability in guest-host systems.
Therefore, they have found application as nonlinear optical
materials,7a-j fluorescent chemosensors,7k-m two-photon
absorbing molecules,7n and thermally stable luminescent
materials for several applications such as OLEDs.7
*Authors to whom correspondence should be addressed. E-mail: mfox@
quimica.uminho.pt (M.M.M.R.); clodeiro@uvigo.es (C.L.). Fax: þ351 253
604382 (M.M.M.R.); þ 34 988 387001 (C.L.).
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A. W. Acc. Chem. Res. 1998, 31, 201–207.
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Cavalli, S.; Albericio, F.; Kros, A. Chem. Soc. Rev. 2010, 39, 241–263. (c)
Sawada, T.; Takahashi, T.; Mihara, H. J. Am. Chem. Soc. 2009, 131(40), 14434–
14441. (d) Lodeiro, C.; Pina., F. Coord. Chem. Rev. 2009, 253, 1353–1383. (e)
Prodi, L; Bolleta, F.; Montalti, M.; Zaccheroni, N. Coord. Chem. Rev. 2000, 205,
Earlier studies on triaryl(heteroaryl)-imidazoles showed
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~
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r
2010 American Chemical Society
Published on Web 11/04/2010
pubs.acs.org/IC

10848 Inorganic Chemistry, Vol. 49, No. 23, 2010
Oliveira et al.
be tuned by substitution of the aryl group at the position 2
by a 5-membered heterocyclic ring such as thiophene or
thiazole.7n,o,q-r It is expected that the use of five-membered
heteroaromatics such as thiophenes and thiazoles in the
conjugation pathway should minimize the distortion of
conjugation between the imidazole ring and the aromatic
ring at the position 2, thus enhancing conjugation and
the charge transport properties along the oligomer back-
bone.7o,q,r Therefore, a comparative study of the fluorescence
properties for several 2,4,5-triaryl(heteroaryl)-imidazoles
showed that the substitution of the 2-phenyl ring in 2,4,5-
triphenyl-imidazole by a thiophene or a thiazole improved
the fluorescence quantum yields, from 0.48 to 0.86 in the case
of thiophene or to 0.57 in the case of the thiazole, due to a
more planar conformation of the heterocyclic imidazoles.7r
In addition the study of the effect of N-alkylation of the
imidazole ring on the fluorescent properties of 2,4,5-triaryl-
(hetero)aryl-imidazoles showed a significant fluorescence
reduction for the 1-substituted derivatives. However, the
fluorescence decrease is noticeably much smaller for imida-
zoles having thiophene or thiazoles in position 2 because of
the higher planarity of these conjugated systems.7r
earth metal ion, is normally recognized by the enhancement
in the fluorescence intensity (CHEF effect),11 while para-
magnetic transition metal ions or heavy metals, such as Cu2þ
,
Ni2þ, and Hg2þ, with unfilled d shells orbitals are usually
recognized by a chelation enhancement of the quenching
(CHEQ effect), via an electron- or an energy-transfer me-
chanism. Among these, Hg2þ as a diamagnetic d10 metal is an
exception, for which the quenching could also be caused by
the spin-orbit coupling, being the main route for the non-
radiative deactivation knr process.12
However, few examples are reported in the literature for
the recognition of Cu2þ, Ni2þ, or Hg2þ by fluorescence
enhancement,13 and so, the development of new sensors for
Cu2þ, Ni2þ, and Hg2þ by CHEF recognition is a key topic in
chemosensor research.
Following our current interests on colorimetric and fluori-
metric chemosensors for metal ion detection provided with
heterocyclic moieties bearing N, O, and S donor atoms,14 and
having in mind earlier studies concerning the optical proper-
ties of 2,4,5-tri(hetero)aryl-imizadole derivatives, we decided
to synthesize and characterize three new imidazo-crown ether
derivatives bearing a furyl (1), aryl (2), or thienyl (3) ring
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followed by fluorimetry.10 For example, Ca2þ, as an alkaline-
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Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10849
linked via the imidazo unit to the azacrown ether system, in
order to tune their photophysical properties and evaluate
their chemosensor ability.
Table 1. Crystal Data and Structure Refinement for Ligand 2
empirical formula
formula weight
temperature
C31 H35 N3 O4
513.62
293(2) K
0.71073 A
tetragonal
P4/ncc
The interaction with Ca2þ, Cu2þ, Ni2þ, and Hg2þ in solu-
tion and in solid state was explored using absorption and
fluorescence spectroscopy, electron paramagnetic resonance
(EPR), and MALDI-TOF-MS spectrometry. To explore the
acid-base behavior of these systems, interaction with Hþ
and a basic anion, F-, were also studied. The X-ray crystal-
lographic structure of compound 2 is also reported.
˚
wavelength
crystal system
space group
unit cell dimensions
˚
˚
a = 28.201(11) A, R = 90°
b = 28.201(11) A, β = 90°
˚
c = 15.453(12) A, γ = 90°
12290.5(12) A
3
˚
volume
Z
density (calculated)
absorption coefficient
F(000)
crystal size
θ range for data collection
index ranges
16
1.110 g/cm3
0.074 mm-1
4384
Experimental Section
0.50 ꢀ 0.46 ꢀ 0.30 mm3
Materials and Apparatus. Reaction progress was monitored
by thin layer chromatography (0.25 mm thick precoated silica
plates: Merck Fertigplatten Kieselgel 60 F254), while purifica-
tion was performed by silica gel column chromatography
(Merck Kieselgel 60; 230-400 mesh). NMR spectra of the
ligands were obtained on a Varian Unity Plus Spectrometer at
an operating frequency of 300 MHz for 1H NMR and 75.4 MHz
for 13C NMR or a Bruker Avance III 400 at an operating
2.04-24.99°
-33 e h e 32, -25 e k e 33,
-18 e l e 18
40683
reflections collected
independent reflections
completeness to θ
absorption correction
5300 [Rint = 0.0684]
97.7% (24.99°)
empirical (SADABS)
full-matrix least-squares on F2
5300/0/343
1.069
R1 = 0.0704, wR2 = 0.2291
R1 = 0.1441, wR2 = 0.2516
refinement method
1
frequency of 400 MHz for H NMR and 100.6 MHz for 13C
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I )]
R indices (all data)
NMR using the solvent peak as internal reference. The solvents
are indicated in parentheses before the chemical shift values
(δ relative to tetramethylsilane and given in ppm). Melting
points were determined on a Gallenkamp apparatus and are
uncorrected. Infrared spectra were recorded on a BOMEM MB
104 or a JASCO IR spectrophotometer. Mass spectrometry
analyses of the ligands were performed at the “C.A.C.T.I.;
Unidad de Espectrometria de Masas” at the University of Vigo,
Spain. Elemental analyses were carried out by the REQUIMTE
DQ, Universidade Nova de Lisboa Service on a Thermo Finnigan-
CE Flash-EA 1112-CHNS Instrument. Proton 1H NMR of
the complexes were recorded on a Bruker Avance III 400 at an
operating frequency of 400 MHz .The MALDI analysis has
been performed in a MALDI-TOF-MS model Voyager-DE
4700 Proteomics Analyzer, by positive reflector mode, at the
REQUIMTE, Chemistry Department, Universidade Nova de
Lisboa.
-3
˚
largest diff. peak and hole
0.341/-0.206 e A
GHz with a Bruker EMX spectrometer using a rectangular
cavity equipped with an Oxford continuous helium flow
cryostat. Modulation field: 5 Gpp. Modulation frequency: 100
kHz. Attenuation: 30 dB (200 microW).
X-ray Crystal Structure Determination. Single crystals of
ligand 2 was analyzed by X-ray diffraction and a summary of
the crystallographic data and the structure refinement param-
eters is reported in Table 1.
Crystallographic data were collected on a Bruker Smart 1000
CCD diffractometer at CACTI (Universidade de Vigo) at 20 °C
using graphite monochromated Mo KR radiation (λ = 0.71073 A)
and were corrected for Lorentz and polarization effects. The soft-
ware SMART16 was used for collecting frames of data, indexing
reflections, and the determination of lattice parameters, SAINT17
for integration of intensity of reflections and scaling, and
SADABS18 for empirical absorption correction. The structures
were solved by direct methods using the program SHELXS97.19
All non-hydrogen atoms were refined with anisotropic thermal
parameters by full-matrix least-squares calculations on F2 using the
program SHELXL97.20 Hydrogen atoms were inserted at calcu-
lated positions and constrained with isotropic thermal parameters.
The contribution of the disordered solvent to the diffraction pattern
that could not be rigorously included in the crystallographic
calculations, were subtracted by the SQUEEZE procedure imple-
mented in the PLATON software.21 Drawings were produced with
PLATON6 software (ellipsoids and ball and stick).
˚
Spectrophotometric and Spectrofluorimetric Measurements.
Absorption spectra were recorded on a Perkin-Elmer lambda
45 spectrophotometer and fluorescence on a Perkin-Elmer L55.
The linearity of the fluorescence vs concentration was checked in
the concentration used (10-4-10-6 M). A correction for the
absorbed light was performed when necessary. Stock solutions
of the compounds (∼10-3 M) were prepared in absolute ethanol
and acetonitrile for 1, and in absolute ethanol, acetonitrile,
and dichloromethane for 2 and 3. Titrations of ligands 1,
2, and 3 (10-5-10-6 M, prepared by dilution of the stock
solutions) were carried out by the addition of microliter
amounts of standard solutions of the ions in absolute ethanol
or acetonitrile. All the measurements were performed at 298 K.
The competition experiments were carried out on a JASCO 650
UV-vis spectrometer and on a Horiba-Jovin Ibon Fluoromax 4
spectrofluorimeter.
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre, CCDC 730341 for 2. Copies of
this information may be obtained free of charge from The
Luminescence quantum yields were measured using a solu-
tion of quinine sulfate in sulphuric acid (0.5M) as a standard15
[φ] = 0.54 and were corrected for different refraction indexes of
solvents,15b for compounds 2 and 3. For compound 1, the
relative quantum yield was measurement using an ethanol
solution of anthracene as standard [φ] = 0.27.15
(16) SMART: Instrument Control and Data Collection Software, version
5.054; Bruker Analytical X-ray Systems Inc.: Madison, WI, 1997.
(17) SAINT: Data Integration software package, version 6.01; Bruker
Analytical X-ray Systems Inc.: Madison, WI, 1997.
(18) Sheldrick, G. M. SADABS. A Computer Program for Absorption
€
€
EPR Measurements. EPR measurements were performed in
the REQUIMTE, Universidade NOVA de Lisboa at 70 K in
both finely powdered and ethanol dissolved samples at 9.65
Corrections; University of Gottingen: Gottingen, Germany, 1996.
(19) Sheldrick, G. M. SHELXS97. A Computer Program for the Solution
€
€
of Crystal Structures from X-ray Data; University of Gottingen: Gottingen,
Germany, 1997.
(20) Sheldrick, G. M. SHELXL97. A Computer Program for the Refine-
(15) (a) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules, 2nd ed.; Academic Press: New York, 1971. (b) Montalti, L.; Credi, A.;
Prodi, T.; Gandolfi, M. T.; Handbook of Photochemistry, 3rd ed.; Taylor and
€
ment of Crystal Structures from X-ray Data; University of Gottingen:
€
Gottingen, Germany, 1997.
(21) Spek, A. L. PLATON. A Multipurpose Crystallographic Tool; Utrecht
ꢀ
Francis Group: Boca Raton, FL, 2006.
University; Utrecht, The Netherlands, 2001.

10850 Inorganic Chemistry, Vol. 49, No. 23, 2010
Oliveira et al.
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0
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: þ44-1233-336033; e-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.ac.uk).
C5), 127.56 (2 ꢀ C4 ), 128.62 (2 ꢀ C2 and C6 ), 129.04 (2 ꢀ (C3 and
þ
0
C5 )), 147.79(C2), 148.91 (C1). MS (FAB) m/z (%): 514 ([M þ H] ,
100), 222 (8). HRMS m/z (FAB) for C31H35N3O4 calcd 514.26955;
found 514.27003.
Chemicals and Starting Materials. Na(BF4), Ca(CF3COO)2,
Cu(BF4)2, Ni(BF4)2, Hg(CF3COO)2 metal salts, F(NBu4), and
CH3SO3H have been purchased from Stream Chemicals, Sigma
Aldrich, or Solchemar. All solvents used were spectroscopic
grade from Chromasolv or PANREAC without any further
purification.
7-(4-(4,5-Di(thien-2-yl)-1H-imidazol-2-yl)phenyl)-1,4,10,13-
tetraoxa-7-azacyclopentadecane (3). The compound was iso-
lated as a yellow solid (20% method A, 47% method B). MP
(213.1-214.6) °C. UV-vis (EtOH): λexc = 325 nm, log ε320 nm
=
;
4.55. Emission (EtOH): λem = 440 nm, φ = 0.09. IR (cm-1
liquid film): v 3101, 1615, 1508, 1392, 1353, 1297, 1251, 1212,
1121, 988, 934, 819, 689. IR (cm-1; KBr pellet): v 3110, 1616,
1511-1495, 1122. 1H NMR (300 MHz, CDCl3): δ 3.56 (m, 16H,
8 ꢀ CH2), 3.76 (m, 4H, 2ꢀ CH2), 6.78 (d, 2H, J = 9.0 Hz, 2 and
6-H), 7.07 (m, 2H, 2 ꢀ 40-H), 7.30 (dd, 2H, J = 4 and 1.2 Hz, 2 ꢀ
30-H), 7.43 (br d, 2H, J = 4.8 Hz, 2 ꢀ 50-H), 7.91 (d, 2H, J = 9.0
Hz, 3 and 5-H). 13C NMR (75.4 MHz, acetone-d6): δ 53.20
(CH2) 69.27 (CH2), 70.65 (CH2), 70.89 (CH2), 71.87 (CH2),
Synthesis of Ligands. Synthesis of 4-(1,4,10,13-Tetraoxa-
7-azacyclopentadecan-7-yl)benzaldehyde IV. POCl3 (1.20 mmol)
was added to DMF (1.20 mmol) at 0 °C, and the mixture was
stirred for 15 min at 0 °C. 13-Phenyl-1,4,7,10-tetraoxa-13-aza-
cyclopentadecane (1.03 mmol) dissolved in DMF (1 mL) was
added dropwise with stirring. The reaction mixture was heated
for 2 h at 60 °C. The solution was then poured slowly into 5 mL
saturated sodium acetate aqueous solution and stirred during
30 min. The organic layer was diluted with ether, washed with
saturated NaHCO3 aqueous solution, and dried with anhy-
drous MgSO4. The organic extract was filtered and evapo-
rated under reduced pressure giving the 4-(1,4,10,13-tetraoxa-
7-azacyclopentadecan-7-yl)benzaldehyde IV22 as an yellow
solid in 90% yield.
0
79.17 (CH2) 112.18 (C2þC6), 118.37 (C4), 126.09 (2 ꢀ C3 and
0
0
2 ꢀ C5 ), 127.57 (C3 and C5), 127.96 (2 ꢀ C4 ), 147.97 (C2), 149.23
(C1). MS (FAB) m/z (%): 526 ([M þ H]þ, 100), 216 (7). HRMS
m/z (FAB) for C27H31N3O4S2 calcd 526.18255; found
526.18288.
Synthesis of Solid Complexes. General Method. The corre-
sponding metal salt (Cu(BF4)2 or Hg(CF3COO)2) (0.2 mmol)
was dissolved in abs. ethanol (5 mL) and added to a stirred
solution of the respective ligand 2 (0.2 mmol) or 3 (0.1 mmol) in
abs. ethanol. The resulting solution was stirred at reflux over-
night. The color of the solution changes from colorless to deep
red after the addition of the metal ion. The solvent was removed
under reduced pressure and the solid was precipitated with the
addition of diethyl ether. The solid was separated by centrifuga-
tion, washed several times with cold abs. ethanol and diethyl
ether and dried under vacuum.
1H NMR (300 MHz, CDCl3): δ 3.61 (m, 16H, 8 ꢀ CH2), 3.70
(m, 4H, 2 ꢀ CH2), 6.66 (d, 2H, J = 9 Hz, 2 and 6-H), 7.66 (d, 2H,
J = 9 Hz, 3 and 5-H), 9.67 (s, 1H, CHO).
General Procedure for the Synthesis of 2,4,5-Tri(hetero)aryl-
imidazo Crown Ether Ligands (1-3). A mixture of the formyl
azacrown ether IV (0.35 mmol), NH4OAc (10 equiv) and 1,
2-diones I-III (0.35 mmol) in glacial acetic acid (20 mL)
(Method A) or in ethanol (20 mL) (Method B) was stirred and
heated at reflux for 12 h. The mixture was then cooled to room
temperature and the product precipitated during neutralization
with NH4OH 5 M. The crude product was purified through
column chromatography on silica using chloroform/methanol
(9:1) as eluent.
[Cu2](BF4)2 2H2O (4). Color: Red. Yield: 70%. C37H51B2-
3
CuF8N3O6, FW = 870.9. Elemental analysis: found C, 47.2; H,
5.3; N, 4.9%. CHNS requires for C37H51B2CuF8N3O6: C, 47.5;
H, 5.5; N, 4.5. IR (cm-1; KBr pellet): v 3175, 1606, 1513-1495,
1124, 1183. UV-vis in ethanol (λ nm): Band at 336 nm, log ε ≈
4.59. Emission spectra in ethanol (λexc = 336 nm, λem = 410
nm), φethanol = 0.65. MALDI-TOF-MS calcd (found) = [2Cu]þ
576.7 (576.2), [(2)2Cu]þ 1089.0 (1089.5).
7-(4-(4,5-Di(furan-2-yl)-1H-imidazol-2-yl)phenyl)-1,4,10,13-
tetraoxa-7-azacyclopentadecane 1. Compound 1 was obtained as
a brown solid (Method B, 58%). MP: (94.2-95.8) °C. UV-vis
(EtOH): λexc = 328 nm, log ε328 nm= 4.69. Emission (EtOH):
λ
em=400 nm, φ = 0.87. IR (cm-1; liquid film): v 3125, 3006, 2870,
1614, 1495, 1392, 1355, 1295, 1205, 1123, 1008, 885, 820, 753. 1H
NMR (300 MHz, acetone-d6): δ 3.57 (m, 16H, 8 ꢀ CH2), 3.73
(m, 4H, 2ꢀ CH2), 6.55 (m, 2H, 2 ꢀ 40-H), 6.74 (d, 2H, J = 7.2 Hz,
2 and 6-H), 6.73 (dd, 2H, J = 4 and 0.8 Hz, 2 ꢀ 30-H), 7.60 (dd,
2H, J = 2 and 0.8 Hz, 2 ꢀ 50-H), 7.98 (d, 2H, J = 7.2, 3 and 5-H).
13C NMR (75.4 MHz, acetone-d6): δ 53.11 (CH2), 69.17 (CH2),
[Cu23](BF4)4] 4H2O (5). Color: Dark Red. Yield: 79%.
3
C25H35B4Cu2F16N3O8S2, FW = 1044. Elemental analysis:
found C, 30.2; H, 4.0; N, 4.0; S, 6.0%. CHNS requires for
C25H35B4Cu2F16N3O8S2: C, 30.2; H, 3.7; N, 3.9; S, 5.9. IR
(cm-1; KBr pellet): v 3087, 1604, 1500-1495, 1107, 1183.
UV-vis in ethanol (λ nm): Band at 331 nm, log ε ≈ 4.43.
Emission spectra in ethanol (λexc = 331 nm, λem = 422 nm),
φethanol = 0.20. MALDI-TOF-MS calcd (found) = [3 Cu]þ
588.6 (588.1), [(3)2Cu]þ 1113.8 (1113.4), [(3)2Cu2]þ 1177.4
(1177.3).
0
70.55 (CH2), 70.79 (CH2), 71.76 (CH2), 107.86 (2 ꢀ C3 ), 112.06
0
(C2 and C6), 112.22 (2 ꢀ C4 ), 118.06 (C4), 127.87 (C3 and C5),
148.35 (C2), 148.70 (C3a and C3b), 149.23 (C2). MS (FAB) m/z
(%): 494 ([M þ H]þ, 100), 234 (8). HRMS: m/z (FAB) for
C27H31N3O6 calcd 494.22785; found 494.22856.
[Hg3](CF3COO)2] 5H2O (6). Color: Dark red. Yield: 77%.
C31H41F6HgN3O13S2, FW=1043.17. Elemental analysis: found
C, 35.4; H, 4.0; N, 4.2; S, 6.5%. CHNS requires for C31H41F6-
HgN3O13S2: C, 35.7; H, 3.9; N, 4.0; S, 6.1. IR (cm-1; KBr pellet):
v 3097, 1604, 1517-1495, 1107, 1183. UV-vis in DMSO (λ nm):
7-(4-(4,5-Diphenyl-1H-imidazol-2-yl)phenyl)-1,4,10,13-tetraoxa-
7-azacyclopentadecane 2. Compound 2 was obtained as a yellow
solid (45% method A; 64% method B). MP (178.4-179.2) °C.
UV-vis (EtOH): λexc = 320 nm, log ε320 = 4.64. Emission
nm
(EtOH): λem = 420 nm, φ = 0.35. IR (cm-1; liquid film): v 3224,
3008, 2870, 1734, 1614, 1508, 1495, 1390, 1355, 1216, 1123, 821, 756,
Band at 342 nm, log ε ≈ 4.74. Emission spectra in DMSO (λexc
=
342 nm, λem=434 nm), j ethanol=0.01.
1
697. IR (cm-1; KBr pellet): v 3189, 1614, 1510-1495, 1122. H
NMR (300 MHz, acetone-d6): δ 3.59 (m, 16H, 8 ꢀ CH2), 3.75
(m, 4H, 2ꢀ CH2), 6.77 (d, 2H, J = 9 Hz, 2 and 6-H), 7.26 (m, 2H,
2ꢀ 40-H), 7.33 (m, 4H, 2 ꢀ (30 and 50-H), 7.58 (m, 4H, 2 ꢀ (20 and
60-H), 7.94 (d, 2H, J = 9 Hz, 3 and 5-H). 13C NMR (75.4 MHz,
acetone-d6): δ 53.16 (CH2), 69.27 (CH2), 70.62 (CH2), 70.84 (CH2),
71.84 (CH2), 112.13 (C2 and C6), 119.14 (C4), 127.41 (2 ꢀ (C3 and
Results and Discussion
Synthesis and Characterization of Organic Ligands. The
aldehyde precursor IV22 was synthesized in 90% yield
through Vilsmeier formylation of 13-phenyl-1,4,7,10-
tetraoxa-13-aza-cyclopentadecane. Heteroaromatic I and
III and aromatic II diones with furyl, thienyl, and aryl
groups were used as precursors of imidazo-crown ethers
1-3 to evaluate the effect of the electronic nature of the
(22) Qin, W.; Baruah, M.; Silwa, M.; Van der Auweraer, M.; De
Borggraeve, W. M.; Beljonne, D.; Averbeke, B. V.; Boens, N. J. Phys.
Chem. A 2008, 112, 6104–6114.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10851
Scheme 1. Synthesis of 2,4,5-Tri(hetero)aryl-imidazo-crown Ether Ligands 1-3
(hetero)aryl groups on the photophysical and sensory
properties of these compounds. Therefore, compounds
to the existence of lattice or coordinated water in the com-
plexes.24 A peak due to the BF4 counterions appears
1-3 with furyl, aryl, or thienyl moieties linked to the
imidazo-crown ether system were synthesized through the
condensation of commercially available diones I-III
with the formyl crown ether derivative IV and ammonium
acetate (see Scheme 1).7i,j,q,23 After the reaction was
performed using two different solvents (acetic acid: Meth-
od A; or ethanol: Method B) in refluxing conditions
(12 h), ethanol gave the highest yields (47-64%) for the
synthesis of compounds 1-3 compared to the classical
Radziszewski conditions7i,j,23 (20-45%). Application of
Radziszewski conditions to the synthesis of furyl deriva-
tive 1 gave a very complex mixture with decomposition
at 1183 cm-1
.
Photophysical Studies. The photophysical characteri-
zation of compound 1, 2, and 3 was performed in ace-
tonitrile, absolute ethanol and dichloromethane. Table 2
summarizes the optical data for all ligands in these protic
and aprotic solvents. The absorption and emission band
were centered at 328, 320, 325 and 400, 420-440,
440-445 nm, respectively, for 1, 2, and 3. The use of
protic and aprotic solvent apparently does not affect the
absorption spectra in all cases. However the steady-state
luminescence spectra is quenched in aprotic solvents more
strongly for compound 1 and 2, being unaffected for
compound 3. As can be seen in Table 2, compounds
1-3 in the same solvent, absolute ethanol, exhibit quan-
tum yields with values of 0.87 for 1, 0.35 for 2, and 0.09 for
3 being the thiophene derivative the less emissive system
because of the strong quenching caused by the sulfur
atom.26 It is also noteworthy that the substitution of the
furyl heterocycle at the 4 and 5 positions of the imidazo
system on compound 1 by two aryl rings give rise to a
dramatical reduction of the fluorescence probably due to
lesser planarity of the aryl-imidazo conjugated system 2.7r
The same experiment in an aprotic solvent like ace-
tonitrile, showed that all ligands are less emissive, being
the fluorescence quantum yields of 0.65 for 1, 0.07 for 2,
and 0.06 for 3. The highest value obtained in protic
solvents is probably due to protonation of the imidazole
nitrogen atom preventing photoinduced electron transfer
(PET) phenomena.27
1
(TLC and H NMR) in which it was not possible to
identify the target compound 1. This compound was only
synthesized through method B using mild reaction con-
ditions (Scheme 1).
Complexation reactions between ligands 2 and 3 with
the metal salts Cu(BF4)2 and Hg(CF3COO)2 in refluxing
ethanol and in a 1:2 L/M molar ratio for 3 and 1:1 for 2
were carried out to investigate the coordination capability
of both ligands in the solid state. Analytically pure
products were obtained and formulated as [Cu2](BF4)2
2H2O (4), [Cu23](BF4)4] 4H2O (5), and [Hg3](CF3CO-
3
3
O)2] 5H2O (6). All complexes were obtained in good yield
of 70% for 4, 79% for 5, and 77% for 6.
3
The MALDI-TOF mass spectra of the complexes
feature peaks corresponding to the free ligand [LH]þ,
and the fragments [ML]þ (L = 2 or 3), [M23]þ, and
[M232]þ. The IR spectra of the complexes were recorded
using KBr discs, and all show similar features. After
complexation to the metal ion, peaks attributable to the
presence of absorption bands from νNH imidazole
groups at ∼3175 cm-1, and the bands for νCdN groups
shift to lower wavenumbers.24 All spectra exhibit medium
to strong bands at ∼1600 and 1455 cm-1, as expected for
The absorption and emission spectra of compounds 1,
2, and 3 in acetonitrile solution are shown in Figure 1. The
Table 2. Optical Data of Compounds 1-3 in Protic and Aprotic Solvents
UV-vis
fluorescence
the two highest-energy benzene ring vibrations.25
A
λexc
Stokes’s
compounds solvents (nm) shift (cm-1) log ε
broad absorption band in the region 3450-3385 cm-1
present in the majority of the complexes is probably due
λem (nm)
φ
1
2
CH3CN
EtOH
CH3CN
CH2Cl2
EtOH
CH3CN
CH2Cl2
EtOH
328
328
320
320
320
325
325
325
5487
5487
8522
7440
7440
8791
8547
8042
4.69
4.69
4.64
4.64
4.64
4.55
4.55
4.55
400
400
440
420
420
455
450
440
0.65
0.87
0.07
0.09
0.35
0.06
0.07
0.09
(23) Davidson, D.; Weiss, M.; Jelling, M. J. J. Org. Chem. 1937, 2, 319–
327.
(24) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co-
ordination Compounds, 3rd ed.; Wiley-Interscience: New York, 1978.
(25) Peng, S. M.; Gordon, G. C.; Goedken, V. L. Inorg. Chem. 1978, 17,
119–126.
3

10852 Inorganic Chemistry, Vol. 49, No. 23, 2010
Oliveira et al.
insertion of the furan, thiophene, and benzene units in the
ligand structure poorly affect the absorption wavelength
band. On the other hand, the emission bands showed a
noticeable red shift from 400 (1) to 455 (3) nm, being the
highest for the thiophene derivative.
Table 3. Stability Constants with Compounds 1-3 by Hypspec Programa
1
2
3
F-
2.02 ( 0.01 (1:1)
4.75 ( 0.01 (1:1)
5.32 ( 0.01 (1:1)
4.29 ( 0.02 (1:1)
3.06 ( 0.01 (1:1)
4.16 ( 0.01 (1:1)
11.58 ( 0.01 (1:2)
3.91 ( 0.01 (1:1)
6.36 ( 0.01 (1:2)
8.42 ( 0.3 (1:1)
11.84 ( 0.3 (1:2
Ca2þ
Cu2þ
Ni2þ
4.23 ( 0.01 (1:1)
11.12 ( 0.01 (1:2)
Hg2þ
10.04 ( 0.01 (1:2)
Figure 1. Absorption (full line) and emission spectra (dotted line)
of compounds 1-3 in acetronitrile solution. ([1]=2.25 ꢀ 10-6 M, [2] =
9.05 ꢀ 10-6 M, [3]=8.55 ꢀ 10-6 M, λexc1=328 nm, λexc2=320 nm, λexc3
325 nm, T = 298K).
=
a (1:1) = LM. (1:2) = LM2.
Figure 2. Absorption (A) and emission (B) spectra of compound 1, with the addition of 0, 0.25, 0.5, 1, and 2 equivalents of methanesulfonic acid.
(T = 298 K, [1] = 2.23 ꢀ 10-6 M, [CH3SO3H] = 1.00 ꢀ 10-2 M, λexc = 328 nm.
Figure 3. Spectrofluorimetric titrations of compounds 1 (A), 2 (B), and 3 (C), in the presence of F-, in acetonitrile. The inset represents the emission for
1 (A) at 400 nm, for 2 (B) at 431 nm, and for 3 (C) at 455 and 490 nm.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10853
Figure 4. Absorption (A and C) and emission titrations (B and D) of compounds 2 and 3 with the addition of increased amount of Ca2þ in acetronitrile
(2) and absolute ethanol (3) solution. The inset represents the absorption at 326 (A) and 325 nm (C) and the emission at 403 (B) and 430 nm (D) as a function
of [Ca2þ]/[2] or [Ca2þ]/[3]. ([2] = 9.06 ꢀ 10-6 M, [3] = 6.34 ꢀ 10-6 M, [Ca(CF3COO)2] = 1.46 ꢀ 10-2 M, λexc2 = 320 nm nm, λexc3 = 325 nm, T = 298 K).
To explore the sensory ability of systems 1-3 in solution
toward Hþ, F-, Naþ, Ca2þ, Cu2þ, Ni2þ, and Hg2þ, several
UV-vis and fluorescence titrations were performed.
oxygen atoms, as reported previously for a macrocyclic
ligand based on a pseudocrown structure.29
With the addition of the fluoride anion, a small red shift
in the absorption spectra for all ligands was observed; at
the same time the emission spectra was quenched and red-
shifted. Figure 3 shows the fluorescence titrations of 1-3
with the addition of F-. Taking into account the results
observed previously for the protonation of the azacrown
nitrogen, deprotonation of this nitrogen should induce
a recovery of the fluorescence emission. However, the
quenching observed with the addition of a base can be
attributed to the deprotonation of the imidazole nitrogen,
inducing a PET process from the lone pair of electrons
located in this nitrogen atom to the excited chromo-
phore.30 This quenching is similar for imidazo-azacrown
derivatives 1-3. The interaction constants of ligands
2 and 3 with the various ions were calculated and are
summarized in Table 3.
The acid-base behavior of compounds 1-3 was stud-
ied with the increasing addition of protons using metha-
nesulfonic acid, CH3SO3H, and fluoride ion as basic
anion. The results obtained showed that protonation
and deprotonation of the nitrogen present in the aza-
crown-ether and also at the imidazole nitrogen atom
could modulate the fluorescence emission. As an exam-
ple, in Figure 2 shows the absorption and fluorescence
titration of compound 1 with the addition of increasing
amount of acid. The fluorescence band was slightly
red-shifted and quenched.28 Protonation induced a
similar behavior when the parent azacrown IV was
used, suggesting that the observed quenching in the
fluorescence emission is probably because of the for-
mation of a hydrogen-bond interaction between the
protonated nitrogen located in the azacrown and the
15-Crown-5 systems are usually used for the interaction
with Naþ,31 whereas 15-crown-5 monoazacrown ethers
(26) (a) Costa, S. P. G.; Oliveira, E.; Lodeiro, C.; Raposo, M. M. M.
Tetrahedron Lett. 2008, 49, 5258–5261. (b) Batista, R. M. F.; Oliveira, E.; Costa,
S. P. G.; Lodeiro, C.; Raposo, M. M. M. Tetrahedron Lett. 2008, 49, 6575–6578.
(27) Saleh, A.; Al-Soud, Y. A.; Nau, W. M. Spectrochim. Acta, Part A
2008, 71, 818–822.
(28) (a) Lodeiro, C.; Parola, A. J.; Pina, F.; Bazzicalupi, C.; Bencini, A.;
Bianchi, A.; Giorgi, C.; Masotti, A.; Valtancoli, B. Inorg. Chem. 2001, 40,
2968–2975. (b) Bazzicalupi, C.; Bencini, A.; Berni, E.; Bianchi, A.; Danesi, A.;
Giorgi, C.; Valtancoli, B.; Lodeiro, C.; Lima, J. C.; Pina, F.; Bernardo, M. A.
Inorg. Chem. 2004, 43, 5134–5146. (c) Pina, J.; Seixas de Melo, J.; Pina, F.;
Lodeiro, C.; Lima, J. C.; Parola, A. J.; Soriano, C.; Clares, M. P.; Albelda, M. T.;
(29) (a) NMR titration experiments were carried out in deuterated
DMSO, due to the low solubility of 1 in ethanol at NMR concentration,
and showed two protonation steps: a first step involving the azacrown
nitrogen and a second step involving the imidazole nitrogen. After proton-
ation, both set of signals were broadned and difficult to differentiate. In this
case, the signals attributed to the protonated azacrown nitrogen at 3.9-3.4
ppm were stabilized by increasing addition of protons before the signals
attributed to the protonated imidazole nitrogen. (b) Vicente, M.; Bastida, R.;
^
Lodeiro, C.; Macias, A.; Parola, A. J.; Valencia, L.; Spey, S. E. Inorg. Chem.
2003, 42, 6768–6779.
(30) (a) Yang, T.; Gao, M.; Qin, W.; Yang, J. Ind. J. Chem. A. 2010, 49,
45–48. (b) Li, R.-J.; Chang, I.-J. J. Chin. Chem. Soc. 2002, 49, 161–164.
(31) Gokel, G. W.; Leevy, W. M.; Weber, M. E. Chem. Rev. 2004, 104,
2723–2750.
~
Aucejo, R.; Garcia-Espana, E. Inorg. Chem. 2005, 44, 7449–7458. (d) Pedras, B.;
ꢀ
Oliveira, E.; Santos, H.; Rodríguez, L.; Crehuet, R.; Aviles, T.; Capelo, J. L.;
Lodeiro, C. Inorg. Chim. Acta 2009, 362, 2627–2635.

10854 Inorganic Chemistry, Vol. 49, No. 23, 2010
Oliveira et al.
Figure 5. Absorption (A and C) and emission titrations (B and D) of compounds 2 and 3 with the addition of increased amount of Ni2þ in acetronitrile.
The inset representsthe absorption at 320, 350 (A), and 325 nm(C) and the emission at423 (B) and 433 nm(D) as a functionof[Ni2þ]/[2] or [Ni2þ]/[3]. ([2] =
9.06 ꢀ 10-6 M, [3] = 8.55 ꢀ 10-6 M, [Ni(BF4)2] = 1.62 ꢀ 10-2 M, λexc2 = 320 nm nm, λexc3 = 325 nm, T = 298 K).
Figure 6. Spectrophotometric (A) and spectrofluorimetric (B) titration of 3 in the presence of Hg2þ, in an acetonitrile solution. The inset represents the
absorption (A) at 325, 350, and 510 nm and the emission (B) at 427 and 450 nm, as a function of [Hg2þ]/[3]. ([3] = 6.66 ꢀ 10-6 M, [Hg (CF3COO) 2] = 1.7 ꢀ
10-2 M, λexc3 = 325 nm, T = 298 K).
show better results for Ca2þ 32
.
In this case, compounds
Following our exploration of the different coordina-
tion sites present in ligands 1-3, the second site to be
explored will be the nitrogen atom located at the imid-
azole ring. This imidazole nitrogen atom can form a
chelate unit with the potential coordinative oxygen or
sulfur atoms present in the furan 1 or thiophene 3
heterocycles. To explore the interaction of transition
(Cu2þ and Ni2þ) and post-transition (Hg2þ) metal ions
with this coordination site, several metal titrations were
performed.
1-3 did not show any changes in the ground state
(absorption) and in the excited stated (emission) after
addition of Naþ. However, in the presence of Ca2þ
ligands 2 and 3 in acetonitrile or absolute ethanol,
respectively, showed a blue shift in the absorption spectra
and an enhancement of the fluorescence emission
(see Figure 4). The complexation constants with Ca2þ
were calculated using the program HYPSPEC33 and are
summarized in Table 3. In all cases, the constants suggest
the formation of a mononuclear complex with a value
between log β 4.75 ( 0.01 for 2 and log β 4.16 ( 0.01 for 3.
With the addition of Ni2þ, only ligands 2 and 3 showed
a red shift on the absorption spectra and an enhancement
on the fluorescence emission intensity (Figure 5). The
complexation constants fit to a mononuclear species
for ligand 2 with a value of log β 4.29 ( 0.02, and to
(32) Warmke, H.; Wiczk, W.; Ossowski, T. Talanta 2000, 52, 449–456.
(33) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739–1753.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10855
Figure 7. Absorption (A, C, and E) and emission titrations (B, D, and F) of compounds 1-3 with the addition of increased amount of Cu2þ in absolute
ethanol. The inset represents the absorption at 328 and 350 nm for A, 320 and 350 nm for C, and 325, 355, and 510 nm for (E), and the emission at 400, 430
(B), 420 (D), and 425 nm for (F) as a function of [Cu2þ]/[1], (Cu2þ]/[2], [Cu2þ]/[3] respectively. ([1] = 1.49 ꢀ 10-6 [2] = 1.62 ꢀ 10-6 M, [3] = 2.05 ꢀ 10-6 M,
[Cu(BF4)2] = 1.62 ꢀ 10-2 M, λexc1 = 328 nm, λexc2 = 320 nm, λexc3 = 325 nm, T = 298 K).
mononuclear and dinuclear species for compound 3 with
values of log β 3.91 ( 0.01 and 6.36 ( 0.01, respectively. It
is important to note that for compound 3 after formation
of the dinuclear species, a colored band centered at ∼550
nm was developed. This band could be attributable to a
tetracoordinated Ni2þ complexes.34
Taking into account the presence of two thiophene rings
in ligand 3, addition of a soft-metal ion35 could be used to
explore the involvement of the sulphur atoms in the
complex stabilization. Figure 6 represents the absorption
and emission titrations of compound 3 with addition of
Hg2þ. The absorption spectra showed a remarkable red
shift and the formation of band centered at ∼510 nm, with
the color of the final solution turning pale pink. At the
same time the fluorescence increased with the addition of
one metal ion equivalent, followed by an intense decrease
with further metal addition. This final quenching could be
attributed to a partial reabsorption of the emitted light by
the colored complex and by the heavy atom effect via the
enhancement of spin-orbit coupling.36 The complexation
constants are summarized in Table 3.
The most interesting results arise from the interaction
with Cu2þ. All ligands were explored in the presence of
Cu2þ in an absolute ethanol solution to prevent the self-
reduction to Cuþ sometimes observed in acetonitrile.37
For all cases, an enhancement of the fluorescence emis-
sion at 430 nm was observed (see Figure 7). The absorp-
tion spectrum showed a red shift with the formation of
well-defined isosbestic points at 333, 334, and 336 nm, for
(36) (a) McClure, D. S. J. Chem. Phys. 1952, 20, 682–686. (b) Burress,
C. N.; Bodine, M. I.; Elbjeirami, O.; Reibenspies, J. H.; Omary, M. A.; Gabbaie,
F. P. Inorg. Chem. 2007, 46, 1388–1395. (c) Mameli, M.; Lippolis, V.;
Caltagirone, C.; Capelo, J. L.; Nieto-Faza, O.; Lodeiro, C. Inorg. Chem. 2010,
49(18), 8276–828610.1021/ic1007439. (d) Tamayo, A.; Pedras, B.; Lodeiro, C.;
ꢀ
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Escriche, L.; Casabo, L.; Capelo, J. L.; Covelo, B.; Kivekas, R.; Sillanpaa, R.
Inorg. Chem. 2007, 46, 7818–7826.
(37) (a) Beloglazkina, E. Z.; Shimorsky, A. V.; Mazhuga, A. G.; Shilova,
O. V.; Tafeenko, V. A.; Zyk, N. V. Rus. J. Gen. Chem. 2009, 79, 1504–1508.
(b) Milne, J. B. Can. J. Chem. 1970, 48, 75–79.
(34) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier:
Amsterdam, 1968.
(35) Pearson, R. G. J. Am. Chem. Soc. 1963, 22, 3533–3539.

10856 Inorganic Chemistry, Vol. 49, No. 23, 2010
Oliveira et al.
Figure 8. (Right) Ethanol solutions of compound 3 in the presence of
one equivalent of Cu2þ and Ca2þ, and in the presence of two equivalents
of Cu2þ. (Left) Emission under irradiation at 365 nm of an ethanolic
solution of compound 3 in the presence of 2 equiv of Cu2þ
.
Figure 10. Normalizedfluorescenceofcompounds2(A) and3(B)inthe
presence of Cu2þ, Ca2þ, Ni2þ, and Hg2þ, in absolute ethanol.
NMR and EPR spectroscopy. The NMR spectra in
acetonitrile-d3 revealed a group of very broad signals,
suggesting the presence of Cu2þ paramagnetic centers.
The EPR solution spectra (see Figure 9) showed the
same behavior for all Cu2þ paramagnetic complexes.
Simulation of the solution spectrum yielded the EPR
parameters: g1= 2.435, g2= 2.097, g3= 2.074, A1= 110
G in all cases. These results suggest that the two metal
centers present in the complex with ligand 3 are in similar
environment coordination sites. It can be postulated that
one metal could be located in the azacrown unit, coordi-
nated to the nitrogen atom and at least four oxygen
atoms, and the second metal center could be located in
the imidazole ring, coordinated to the imidazole nitrogen
completing the coordination sphere with several water
molecules. The presence of these water molecules was
proved also by the infrared spectra in KBr pellets.
Figure 9. EPR spectra of compound [Cu23](BF4)4].4H2O (5) recorded
in a polycrystalline powdered sample (black) and dissolved in acetonitrile
(gray). Simulation of the solution spectrum yields the EPR parameters:
g1= 2.435, g2= 2.097, g3= 2.074, A1= 110 G. Simulation was performed
using Simfonia v1.25 (Bruker, Inc.).
1-3 respectively. This result indicates that the stoichiom-
etry of the reaction remains unchanged during the chem-
ical reaction and no secondary reactions occur during the
considered time range.
For compound 3 a visible band centered at 510 nm was
observed. Nevertheless, as previously discussed for the
Hg2þ complex, this fact did not influence the fluorescence
emission, and the final complex formed was highly emis-
sive with a fluorescence quantum yield of 0.48. The
complexation constants for all cases are summarized in
Table 3. The highest values were obtained for compounds
1 and 3, and agree with the formation of dinuclear species.
However due to the instability observed in the complex
with compound 1 bearing furyl substituent, further stud-
ies with this complex were prevented.
As is well-known, copper complexes with brown-red
color can be understood as complexes with Cuþ.38 Be-
cause of the deep dark red color observed in the complex
with ligand 3 (see Figure 8), additional studies concerning
the oxidation state of the metal centers was performed by
As ligand 3 can be coordinated by two metal ions, the
formation of heterodinuclear species by the addition of Ca2þ
followed by one Cu2þ equivalent was explored. The color of
the complex changes from deep dark red to orange.
The fluorescence quantum yield for the dinuclear Cu2þ
complex synthesized with 3 was 0.48; for comparison
purposes, the addition of 1 equiv of Cu2þ, followed by
the addition of Ca2þ lead to a more emissive species with
fluorescence quantum yield of 0.79. Taking into account
that Ca2þ will be coordinated to the azacrown, the Cu2þ
center must be now located at the imidazole ring.
To explore the effect of simultaneous coordination on
the fluorescence emission of ligands 2 and 3, several
competitive experiments were performed in absolute
ethanol. As was discussed previously, in ethanol the
imidazole nitrogen is partially or totally protonated pre-
venting the PET effect. Careful inspection of Figure 10A
shows that addition of Cu2þ, Ni2þ, and Ca2þ to ligand 2
(38) (a) Strange, A. F.; Klein, A.; Klinkhammer, K.-W.; Kaim, W. Inorg.
Chim. Acta 2001, 324, 336–341. (b) Rapenne, G.; Dietrich-Buchecker, C.;
Sauvage, J.-P. J. Am. Chem. Soc. 1999, 121, 994–1001.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10857
Table 4. Luminescence Quantum Yield of Compound 3 in the Presence of Ca2þ
adopt a nearly planar conformation, with the four oxygen
and the nitrogen atoms of the molecule describing a
slightly distorted plane. The dihedral angle between the
planes containing the phenyl and the imidazolyl units are
121°, while between the planes containing the phenyl and the
crown-ether units are 122°.
and Cu2þ
species
quantum yield, φ
3
0.07
0.48
0.30
0.79
3 þ Cu2þ
3 þ Ca2þ
3þ Cu2þ þ Ca2þ
Conclusions
A new family of emissive molecular probes 1-3, derived
from 15-crown-5 monoaza macrocyclic ligands bearing a
furyl, aryl or thienyl 4,5-disubstituted imidazole system have
been synthesized in good to excellent yields by a simple
reaction, and their photophysical properties have been eval-
uated in solution and in solid state by absorption and
fluorescence emission spectroscopy and by MALDI-TOF-
MS spectrometry in the gas phase.
Their capacity to act as potential sensors for divalent
metal ions Ca2þ, Cu2þ, Ni2þ and Hg2þ was explored in
solution and in solid state. Interesting results were found for
compound 3 bearing the thiophene rings being an example
of enhancement of fluorescent emission upon coordination to
Cu2þ, Ni2þ or Ca2þ. The complex (CuCa3)4þ in solu-
tion was the most emissive with a fluorescence quantum yield
of 0.79.
Figure 11. X-ray crystallographic structures of compound 2.
induced an one-fold increase of the fluorescence intensity
(observed previously in the metal titrations) and after
addition of pairs Cu2þ/Ca2þ and Cu2þ/Ni2þ a small
decrease of the intensity was observed. Finally, as ex-
pected, addition of Hg2þ and the pair Cu2þ/Hg2þ
quenched the fluorescence emission.
Acknowledgment. We are indebted to InOU Uvigo by
project K914 122P 64702 (Spain) and FCT-Portugal by
project PTDC/QUI/66250/2006 for financial support.
The NMR spectrometers are part of the National NMR
Network and were purchased in the framework of the
National Programme for Scientific Re-equipment, con-
tract REDE/1517/RMN/2005, with funds from POCI
2010 (FEDER) and FCT-Portugal. C.L. thanks Xunta
de Galicia, Spain, for the Isidro Parga Pondal Research
Program. E.O. and R.B. thank FC-MCTES (Portugal) by
their PhD grants SFRH/BD/35905/2007 and SFRH/BD/
36396/2007, respectively. We are gratefull to Dr. Cristina
In Figure 10B are shown the results of the same experi-
ments for ligand 3. An enhancement of the fluorescence
emission was observed after coordination by two equiva-
lent of Cu2þ with similar intensity as observed for the pair
Cu2þ/Ni2þ. This CHEF effect was also observed for the
addition of Ca2þ, being more intense when the pair Cu2þ
Ca2þ was used.
/
To use compound 3 as fluorescent chemosensor for
Cu2þ in solution, the detection (DL) and quantification
(QL) limits were determined in absolute ethanol.
The values obtained at 510 nm, where the visible band
was formed, was of 0.003 ( 0.001 (DL) and 0.007 ( 0.002
(QL). Taking into account these results, the minimal
amount of Cu2þ that could be determined in absolute
ethanol is 1.60 ppm.
~
ꢀ
Nunez and Dr. Pablo Gonzalez from the REQUIMTE,
Universidade NOVA de Lisboa, Portugal, for their im-
portant help with the crystallographic and EPR data,
ꢀ
respectively, and Dr. Jose Luis Capelo from the Univer-
sity of Vigo, Spain, for the help with the MALDI-TOF-
MS spectra.
Crystallography Data. Crystals of compound 2 (Figure 11)
suitable for X-ray diffraction were obtained by slow evapora-
tion of an ethanolic solution. Bond distances and angles are all
within the normal ranges. The modified azacrown ethers
Supporting Information Available: X-ray crystallographic
data of 2 in CIF format and Job’s plot for compound 3 in the
presence of Cu2þ. This material is available free of charge via the
Internet at http://pubs.acs.org.