Steady-state and time-resolved investigations on pyrene-based chemosensors

Article abstract of DOI:10.1021/ic301365y

Two novel fluorescent probes bearing a single (P) and two (a podand-like structure, L) pyrene units derived from 1,5-bis(2-aminophenoxy)-3-oxopentane have been synthesized and investigated in dioxane using UV-vis absorption, and steady-state and time-resolved (in a picosecond time scale) emission spectroscopy; in the gas phase, matrix-assisted laser desorption ionization mass spectrometry was employed. In dioxane, the absorption and emission spectra of P present a unique band with maxima at 361 and 392 nm, which have been associated with the monomer absorption and emission bands, respectively. In dioxane, for compound L, an additional band with a maximum at ～525 nm is observed; upon the addition of water, an emissive band (with maxima varying from 405 to 490 nm) appears in both P and L spectra; this is discussed in terms of the emission of a species with charge character. Upon metal addition (Cu²⁺, Zn ²⁺, and Ag⁺) to P, a gradual quenching effect of the monomer emission is observed and found to be more pronounced with Cu ²⁺. In the case of L, upon the addition of metal cations, the long emission band (～550 nm) decreases and the monomer emission band increases (with an isoemissive point at ～450 nm) and no evidence for the intermediate band (at ～405-490 nm) now exists. Time-resolved data in dioxane/water mixtures showed that for P and L these two fit double- and triple-exponential decay laws, respectively. With P, this has been attributed to a two-state system, which involves the monomer and a charged species, with its emission maxima varying with the polarity of the media (here mirrored by its dielectric constant), which can potentially be addressed to an exciplex-like species, whereas with L, it has been attributed to a three-state system involving, in addition to these two species, an excimer. From absorption and fluorescence excitation and time-resolved data, evidence is given for the presence of intramolecular dimer formation in the ground state.

Full text of DOI:10.1021/ic301365y

Article
pubs.acs.org/IC
Steady-State and Time-Resolved Investigations on Pyrene-Based
Chemosensors
Javier Fernandez-Lodeiro,†,‡ Cristina Nunez,‡ Catherine S. de Castro,§ Emilia Bertolo,⊥
́
́
́
̃
J. Sergio Seixas de Melo,*,§ Jose Luis Capelo,†,‡ and Carlos Lodeiro*,†,‡
́
́
†Grupo BIOSCOPE, Departamento de Química, Facultade de Ciencias, Campus de Ourense, Universidade de Vigo, 32004 Vigo,
Spain
‡REQUIMTE/CQFB, Departamento de Química, Faculdade de Cien
̂
cias e Tecnologia, Universidade Nova de Lisboa, 2829-516
Monte de Caparica, Portugal
§Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal
⊥Ecology Research Group, Department of Geographical and Life Sciences, Canterbury Christ Church University, Canterbury, Kent
CT1 1QU, United Kingdom
S
* Supporting Information
ABSTRACT: Two novel fluorescent probes bearing a single
(P) and two (a podand-like structure, L) pyrene units derived
from 1,5-bis(2-aminophenoxy)-3-oxopentane have been syn-
thesized and investigated in dioxane using UV−vis absorption,
and steady-state and time-resolved (in a picosecond time scale)
emission spectroscopy; in the gas phase, matrix-assisted laser
desorption ionization mass spectrometry was employed. In
dioxane, the absorption and emission spectra of P present a
unique band with maxima at 361 and 392 nm, which have been
associated with the monomer absorption and emission bands, respectively. In dioxane, for compound L, an additional band with
a maximum at ∼525 nm is observed; upon the addition of water, an emissive band (with maxima varying from 405 to 490 nm)
appears in both P and L spectra; this is discussed in terms of the emission of a species with charge character. Upon metal addition
(Cu2+, Zn2+, and Ag+) to P, a gradual quenching effect of the monomer emission is observed and found to be more pronounced
with Cu2+. In the case of L, upon the addition of metal cations, the long emission band (∼550 nm) decreases and the monomer
emission band increases (with an isoemissive point at ∼450 nm) and no evidence for the intermediate band (at ∼405−490 nm)
now exists. Time-resolved data in dioxane/water mixtures showed that for P and L these two fit double- and triple-exponential
decay laws, respectively. With P, this has been attributed to a two-state system, which involves the monomer and a charged
species, with its emission maxima varying with the polarity of the media (here mirrored by its dielectric constant), which can
potentially be addressed to an exciplex-like species, whereas with L, it has been attributed to a three-state system involving, in
addition to these two species, an excimer. From absorption and fluorescence excitation and time-resolved data, evidence is given
for the presence of intramolecular dimer formation in the ground state.
performed.8 Chemosensors based on the ion-induced changes
INTRODUCTION
■
in fluorescence have been actively investigated because of their
Selective detection of transition-metal ions using self-organized
simplicity and high detection limit.9 The sensing ability of a
Schiff-base host molecules is an area of considerable interest in
the field of supramolecular chemistry.1,2 Several conjugate
fluorescence chemosensor is dependent upon the interaction
macrocyclic and acyclic Schiff-base chemosensors based on
polyoxa and/or oxaaza,3 polyaza,4 and azathia5 receptor units
have been reported. Our research group has studied a number
of polyaza, polyoxaaza, and polythioaza systems as potential
chemosensors for the recognition of metal ions.6
The synthesis of highly sensitive and selective chemosensors
that can detect heavy- and transition-metal ions such as Hg2+,
Pb2+, Cd2+, and Cu2+ is attracting considerable attention.7 One
possible way of detecting cations is by using fluorescence
spectroscopy as the sensing tool of choice; fluorescence
spectroscopy is a very sensitive technique and is inexpensive,
versatile, and nondestructive, and the experiments are easily
between the ionophore (the recognizing unit responsible for
selectively binding the ions) and the fluorophore (the
fluorogenic unit for signal transduction) by either enhancement
or inhibition of the fluorescence signal.10 Signal-conducting
mechanisms such as photoinduced electron transfer, intra-
molecular charge transfer, and Forster resonance energy
̈
transfer provide the connection between the binding event
and reported signal.11
Received: June 26, 2012
Published: December 11, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Molecular Probes P and L
the BIOSCOPE Group. The spectra represent accumulations of 5 ×
100 laser shots. The reflectron mode was used. The ion source and
flight tube pressures were less than 1.80 × 10−7 and 5.60 × 10−7 Torr,
respectively. The MALDI-MS spectra of the soluble samples (1 or 2
mg/mL), such as the ligand and metal complexes, were recorded using
the conventional sample preparation method for MALDI-MS.
Chemicals and Starting Materials. 1,5-Bis(2-aminophenoxy)-3-
oxopentane was synthesized following the method previously
published;20 1-pyrenecarboxaldehyde and hydrated tetrafluoroborate
metal salts were commercial products from Alfa and Aldrich and were
used without further purifications.
Spectrophotometric and Spectrofluorimetric Measure-
ments. UV−vis absorption spectra were performed using a Jasco
650 or a Shimadzu 2450 UV−vis spectrophotometer; fluorescence
spectra were recorded on a Horiba Jobin-Yvon Fluoromax 4
spectrometer. All of the fluorescence spectra were corrected for the
wavelength response of the system. The linearity of the fluorescence
emission versus concentration was checked in the concentration range
used (10−5−10−6 M). A correction for the absorbed light was
performed when necessary. The fluorescence decays of the
compounds were obtained with picosecond resolution with equipment
described elsewhere17 and were analyzed using the method of
modulating functions implemented by Striker et al.18 The
experimental excitation pulse [full width at half-height (fwhm) = 21
ps] was measured using a LUDOX scattering solution in water. After
deconvolution of the experimental signal, the time resolution of the
apparatus was ca. 2 ps.
All spectrofluorimetric titrations were performed as follows: a stock
solution of the ligand was prepared by dissolving an appropriate
amount of the ligand in dioxane and diluted to the desired
concentration ([P] = [L] = 5.00 × 10−6 M). Titrations were carried
out by the addition of microliter amounts of standard solutions of the
ions dissolved in the same solvent. All of the measurements were
performed at 298 K.
Synthesis of Organic Ligands. Synthesis of P. A solution of 2-
methoxyaniline (0.124 g, 1.006 mmol) in absolute ethanol (20 mL)
was added dropwise to a refluxing solution of 1-pyrenecarboxaldehyde
(0.231 g, 1.006 mmol) in the same solvent (50 mL). The resulting
The pyrene moiety is one of the most useful fluorophores for
the construction of fluorescence chemosensors.12 Two pyrene
moieties situated close enough can yield face-to-face
interaction, which can be sandwich-like or deviated from the
planarity orientation for the excimer emission.13 Moreover, the
intramolecular interaction of the pyrene units can be made in
the ground (dimer) or excited state, leading to changes in the
spectra (absorption, emission, and excitation) and photo-
physical behavior. The presence of the absorption of an
intramolecular dimer will also lead to a contribution of direct
excitation of this species to the long emission band character-
istic of the pyrene excimer. Indeed, upon coordination with a
specific guest ion, the resulting compound could be fine-tuned
to yield monomer and/or excimer/dimer emissions depending
on the orientation of the two pyrene moieties.14 On the basis of
the pyrene ratio of monomer versus excimer emission, several
“on−off” recognition systems for heavy- and transition-metal
ions have been successfully devised.15
In this work, we report the synthesis and photophysical
characterization of two new fluorescence probes, P and L.
Chemosensor L contains two emissive pyrene units as
chromophores, linked by a flexible polyoxaethylene bridge
and chemosensor P a single pyrene unit.
EXPERIMENTAL SECTION
■
Physical Measurements. Elemental analyses were performed in a
Fisons EA-1108 analyzer at the CACTI, University of Vigo Elemental
Analyses Service. IR spectra were recorded as KBr disks on a Bio-Rad
1
FTS 175-C spectrophotometer. H, 13C, COSY, DEPT, and HMQC
NMR spectra were recorded on a Bruker AMX-500 spectrometer, and
dimethyl sulfoxide (DMSO) was used as the solvent in all cases.
Matrix-assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF-MS) analysis was performed in a
MALDI-TOF-MS model Bruker Ultraflex II workstation equipped
with a nitrogen laser radiating at 337 nm from Bruker (Germany) at
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solution was gently refluxed with magnetic stirring for ca. 4 h. The
yellow powder precipitate formed was filtered off, washed several times
with cold absolute ethanol, dried under vacuum, and characterized as
compound P. The compound was found to be soluble in acetonitrile,
decane, dichloromethane, dioxane, DMSO, toluene, and water and
insoluble in absolute ethanol, methanol, and diethyl ether.
Synthesis of L. A solution of 1,5-bis(2-aminophenoxy)-3-
oxopentane (0.153 g, 0.531 mmol) in absolute ethanol (50 mL) was
added dropwise to a refluxing solution of 1-pyrenecarboxaldehyde
(0.247 g, 1.062 mmol) in the same solvent (75 mL). The resulting
solution was gently refluxed with magnetic stirring for ca. 4 h. Solvent
removal produced a red oil, which was washed three times with a
mixture of cold cyclohexane/diethyl ether. A red powder precipitate
formed and was filtered off and dried under vacuum. The compound
was characterized as chemosensor L. Compound L was soluble in
acetonitrile, decane, dichloromethane, dioxane, DMSO, toluene, and
water and insoluble in absolute ethanol, methanol, and diethyl ether.
P. Anal. Calcd for C24H17NO.H2O: C, 81.5; H, 5.4; N, 3.9. Found:
C, 81.6; H, 5.4; N, 3.9. Yield: 89%. IR (KBr): 1649 [ν(CN)
]
imine
1
cm−1. MALDI-TOF-MS: m/z 336.21 ([P + H]+). Color: yellow. H
NMR (400 MHz, DMSO): δ 8.87 (s, 1H), 8.46−6.68 (m, 9H), 3.85
(s, 3H).
L. Anal. Calcd for C50H38N2O3·H2O: C, 81.3; H, 5.5; N, 3.8. Found:
C, 81.6; H, 5.7; N, 3.8. Yield: 81%. IR (KBr): 1647 [ν(CN)
]
imine
cm−1. MALDI-TOF-MS: m/z 713.11 ([L + H]+). Color: red. 1H
NMR (400 MHz, DMSO): δ 9.58 (s, 2H), 8.71−7.98 (m, 18H),
7.32−6.21 (m, 8H), 4.26−4.00 (m, 2H), 3.92−3.77 (m, 2H).
RESULTS AND DISCUSSION
■
Synthesis. Probes P and L were synthesized, following a
one-pot reaction, by direct condensation between the
commercial carbonyl precursor 1-pyrenecarboxaldehyde and
2-methoxyaniline (P) or 1,5-bis(2-aminophenoxy)-3-oxopen-
tane (L).19 The final products P and L have been characterized
by microanalysis, melting points, IR, 1H and 13C NMR, UV−vis
and fluorescence emission spectroscopy, and MALDI-TOF-MS.
All techniques confirm the integrity of these species in solution
and in the gas phase (see the Supporting Information, SI). The
reaction pathways for the synthesis of compounds P and L are
shown in Scheme 1.
MALDI-TOF-MS Titrations. In order to explore the
potential application of chemosensor L as a molecular probe
for metal ions in the gas phase, several MALDI-TOF-MS
titrations were performed. Compound L was dissolved in
acetonitrile, without any additional external MALDI matrix. L
was titrated with Ag+, Cu2+, Zn2+, Cd2+, and Hg2+ in 1:1 and 1:2
(ligand/metal) molar ratios. To perform the metal titrations,
two different strategies were used, a dried droplet solution and
a layer-by-layer deposition sample preparation. On the first
method, two solutions containing compound L (1 μL) and the
metal salt (1 μL) were first mixed and then applied in the
MALDI-TOF-MS sample holder. On the second method, a
solution of L was spotted in the MALDI-TOF-MS plate and
then dried under vacuum; subsequently, 1 μL of the solution
containing the metal salt was deposited on the sample holder
and dried. The plate was then inserted into the ion source. On
this second method, the complexation reaction between the
ligand and metal salts occurred in the holder, and the complex
species were produced directly in the gas phase.
Figure 1. From top to bottom, MALDI-TOF-MS spectra of
compound L before and after titration with Cu(BF4)2·6H2O and
Ag(BF4)2 (1 equiv of a metal ion). Note that the Na+ ion on the top
spectrum is from the glassware.
to the mononuclear species [LCu]+. In the titration with Ag+,
upon the addition of 1 equiv of Ag+, an equivalent peak
corresponding to the mononuclear species [LAg]+ appears at
m/z 821.24 (100% relative intensity). No peaks attributable to
dinuclear species were detected in either case. For Zn2+, Cd2+,
and Hg2+, the MALDI-TOF-MS titration did not show any
peaks attributable to the metal complexes. These results suggest
that L can be used to sense selectively Cu2+ and Ag+ by
MALDI-TOF-MS. Similar results were observed for an
analogous system described previously.20
The peak attributable to the protonated species [LH]+
appears always at m/z 713.11. The titrations for Cu2+ and
Ag+ are shown in Figure 1. Irrespective of the sample
preparation method, upon the addition of 1 equiv of Cu2+,
the peak attributable to the ligand disappears, and a new peak
with 100% relative intensity appears at m/z 775.25, attributable
Spectrophotometric Studies. Absorption, emission, and
excitation spectra of P and L were obtained in dioxane at
298 K, and the results are presented in Figure 2 and Table 1.
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Figure 2. Absorption, emission, and excitation spectra of compounds P (left panel) and L (right panel) in dioxane. λexc = 360 nm, λem(P) = 420 nm,
λem1(L) = 420 nm, λem2(L) = 500 nm, and [P] = [L] = 5 × 10−6 M.
In the case of compound L in dioxane, an absorption band in
the 400−500 nm region is now visible together with an
emission band with maxima at ∼500 nm. Two potential reasons
can be appointed to rationalize the origin of this band in L: (i)
a small amount of water that persists from the synthetic and
purification procedure, even if this is dried to exhaustion; (ii)
the formation (resulting from the interaction between the two
pyrene units in L) of an intramolecular dimer.
In dioxane, the presence of the 400−500 nm absorption
band (seen as a tail) in L and its absence in P suggests that (ii)
mirrors a more realistic situation. Along the text, the reference
to this dimer should be viewed as resulting from an
intramolecular interaction between the two pyrene units in L.
In the case of L, it is also worth noting that the excitation
spectra collected in the monomer emission region mirrors the
absorption spectra of the monomer (see Figure 2), whereas
when collected in the long emission wavelength band (∼550
nm), it displays a band in 400−550 nm (Figure 2, right panel),
which strongly suggests that the great majority of the latter
Table 1. Absorption, Emission, and Excitation Wavelength
Maxima (λmaxA, λmaxem, and λmaxexc) and Molar Extinction
Coefficients (ε) for Chemosensors P and L in Dioxane at
298 K
A
em
exc
compound
λmax (nm)
361
348
log ε
λmax (nm)
392
386
λmax (nm)*
361
349
P
L
4.65
4.74
*
Obtained from the excitation spectra with emission collected at 420
nm (see Figure 2).
For P, both the absorption and emission spectra show a
single band, which is attributed to the monomer (absorption
and emission) of this compound by comparison with pyrene.21
However, it should be noticed that in contrast with pyrene,
where the first electronic transition has a forbidden character, in
the case of P, the 0,0 transition is now allowed, as can be seen
from Figure 2.
Figure 3. Absorption (left panels) and emission (right panels) spectra of P (top panels) and L (bottom panels) in dioxane at room temperature after
the addition of water (λexc = 360 nm and [P] = [L] = 5.00 × 10−6 M). The insets show variation with the mole fraction of water of the absorption at
390 nm and the fluorescence intensities at 390 and 420 nm for P and 390, 420, and 500 nm for L. The bold dark line is for xH O = 0, whereas the
2
bold red line is for xH O = 0.29.
2
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Figure 4. Absorption and fluorescence emission and excitation spectra (λem1 = 420 nm for P and L and λem2 = 500 nm for L) for P (left panel) and L
(right panel) in a dioxane/water mixture with xH O = 0.29.
2
band is originated through a static mechanism (from direct
excitation of preformed intramolecular dimers).
P and L upon the Addition of Water. Upon the addition of
water, a new band, in addition to the monomer (390 nm) band,
appears for both P and L with a maximum that varies between
405 and 495 nm; see Figure 3.
By collection of the excitation spectra with emission in this
band (i.e., at values in the extremities of this band: 420 and 500
nm), a vibronically resolved band with a maximum at 360 nm is
observed for the two compounds (see Figure 4). This seems to
attest further the identical nature of this band in the two
compounds. Moreover, it is worth noticing the fact that with P
upon collection at 420 and 500 nm, i.e., along this new band
that only appears by the addition of water, the excitation
spectra match the absorption spectra. The same is, however,
not valid for L, where although the excitation bands when
collected at 420 and 500 nm are identical, a partial departure
from full overlap with the absorption spectra is now obtained.
This suggests that in the case of L the long emission band is the
result of more than one species; i.e., one is identical with that
observed with P (not absorbing in the ground state, as attested
by the left top panel in Figure 3), and the additional band (in
L) is likely to be due to the excimer/dimer, which is also
present in the ground state.
As can be seen from Figure 3, water strongly influences the
absorption and emission spectra of L. To exclude the possibility
of reaction between L and water, an independent experiment
was carried out involving the run of the absorption spectra
upon the (i) addition of water to L and (ii) subsequent removal
of water from L. A solution of L in a dioxane/water mixture
Figure 5. Dependence of the wavelength maxima (λmax) with the mole
fraction of water (xH O) and the dielectric constant (ε) taken from the
2
emission spectra of P (closed symbols) and L (open symbols) for λexc
= 360 nm and [P] = [L] = 5 × 10−6 M.
similar compounds have associated the shift of this emergent
band to an exciplex.23 However, further and convincing
arguments on the nature of this band come from time-resolved
data (see below).
Time-Resolved Data for P and L in Dioxane/Water
Mixtures. The fluorescence decays for the two compounds
(P and L) were obtained with excitation at 381 nm (monomer
absorption band; see Figure 2) and were collected at 420 and
500 nm for P and 420 and 550 nm for L. With both P and L,
the longest emission wavelength was specifically chosen away
from its maximum in order to avoid emission contribution from
the monomer species. Upon the addition of water to P, the
fluorescence decays are now biexponential (with values ranging
from 40 to 70 ps and from 190 to 700 ps), whereas with L,
these are only properly fit to a triexponential decay law; see
Figure 6.
With P, the two decay times increase with the addition of
water, but clearly the longer decay time increases deeply.
Moreover, whereas at the monomer (420 nm) emission
wavelength the preexponential factors are kept rather constant
(ranging from 0.8 to 0.9 and therefore associated with the
highly quenched monomer decay time), when collected at 500
nm, the preexponential factor associated with the longer
component increases at the expense of the shorter component,
which indicates the longer component to be associated with the
(xH O = 0.80) was dried with the aim of complete removal of
2
the solvent mixture (water included). The dried compound (in
a film form) was again dissolved in dioxane; the two obtained
spectra (before and after the removal of water) were found to
be identical (Figure S4 in the SI).
In order to have further knowledge on the nature of the
emissive species at long emission in both P and L, the emission
wavelength maximum of this emission band was plotted as a
function of the water fraction and dielectric constant22 of the
media for T = 293 K; see Figure 5. It can be seen that this band
red-shifts with an incremental amount of water and that the
values are basically identical with the two compounds in the
same dioxane/water mixtures. The pronounced red shift of this
band with the increase in the dielectric constant of the media
strongly suggests the presence of a charged species. Others with
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Figure 7. Absorption (left panels) and fluorescence emission (right
panels) titrations of chemosensor P as a function of the increasing
amounts of (A and B) Cu(BF4)2, (C and D) Zn(BF4)2, and (E and F)
Ag(BF4) in dioxane. The insets show the absorption at 360 and 390
nm and the fluorescence intensity at 420 nm. [P] = 5.00 × 10−6 M,
and λexc = 360 nm. The bold dark line is for [Cu2+]/[P] = [Zn2+]/[P]
= [Ag+]/[P] = 0, whereas the bold red line is [Cu2+]/[P] = 0.6,
[Zn2+]/[P] = 1.6, and [Ag+]/[P] = 4, respectively.
Figure 6. From left to right: Dependence of the decay times (τi) and
preexponential factors obtained from the fluorescence decays collected
at the monomer emission wavelength (420 nm, A1,i) and at 500 nm for
P and 550 nm for L (A2,i) in dioxane/water mixtures (here expressed
other metals are needed, this suggests a peculiar selectivity of P
toward Cu2+.
It is relevant to point out that in the presence of metal
cations, for both P and L, the emission band of the exciplex is
absent.
in mole fraction of water, xH O). P: top panels. L: bottom panels.
2
exciplex formed species. Furthermore, the reduced contribution
of the longer decay time (preexponential factors varying from
0.1 to 0.2) mirrors the fact that the exciplex is also emitting at
this emission wavelength. At 500 nm, the preexponential factor
associated with the exciplex is dominant and increases with the
water amount, in agreement with the steady-state spectra.
In the case of compound L, we now observed three decay
times in which besides the two previously indicated τ1 and τ2,
an additional longer decay time (with a value of ∼1 ns) is
present but only with a significant contribution at longer
emission wavelengths. In addition, the absence of a rising
component at this emission wavelength gives a clear indication
that the majority of this species is already present in the ground
state.
Interaction of P and L with Metal Cations. In order to
further explore the behavior of compounds P and L as potential
chemosensors toward Cu2+, Zn2+, and Ag+, additional studies
involving titrations with these metal ions were performed.
The addition of incremental amounts of the metal cations to
a dioxane solution of P leads to a decrease in the absorption
maxima with, however, the same shape in the spectra; see
Figure 7. The same is valid for the emission spectrum, which
shows that the decrease of the fluorescence intensity is likely
due to a quenching process that involves metal-ligand
interaction in the ground state. It is worth noting that the I0/
I ratio for the same [metal ion]/[P] ratio is 5 times higher with
Cu2+ than it is with Ag+ and Zn2+; although further studies with
Figure 8 depicts the absorption and emission spectra of L in
dioxane and in the presence of increasing amounts of Cu2+ (A
and B), Zn2+ (C and D), and Ag+ (E and F). Upon the addition
of metal cations, the long wavelength absorption tail becomes
well-defined with an isoabsorptive wavelength (425 nm for
Cu2+, 430 nm for Zn2+, and 410 nm for Ag+), which indicates a
ground-state equilibrium between the monomer and dimer
species. In contrast with this behavior, in the emission spectra,
the intensity of the band collected at the monomer region (390
nm) increases and that for the excimer/dimer decreases upon
metal addition with well-defined crossing points in the titration
curve (see insets in Figure 7). This shows that, upon the
addition of the metal cation, the pyrene units in L split, and
there is an increment of the monomer emission with a decrease
of the excimer/dimer emission.
The formation of a 1:1 (metal/ligand) stoichiometry after
titration of L with Cu2+, Zn2+, and Ag+ was confirmed
unambiguously by the Job’s plot method.24 The new low-
energy bands (ca. 121 nm red shift) were responsible for the
change of color, perceptible to the naked eye, which is shown in
Figure 9. In dioxane, the color of the solution changes from
yellow to dark red (Cu2+), intense red orange (Zn2+), and
intense gold yellow (Ag+) after the addition of 1 equiv of the
appropriate metal ion to a solution of L in the same solvent.
In the case of probe P, the color of the dioxane solution
changes from pale yellow to intense yellow (Cu2+ and Zn2+)
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Table 2. Stability Constants for Chemosensor L in the
Presence of Cu2+, Zn2+, and Ag+ in Dioxane for an
Interaction 1:1 (Metal/Ligand)
interaction
(M/L) in
dioxane
compound
∑log β(absorption)
∑log β(emission)
L
Cu2+ (1:1)
Zn2+ (1:1)
Ag+ (1:1)
3.67 2.59 × 10−3
3.99 1.81 × 10−3
10.69 1.68 × 10−3
3.10 3.13 × 10−2
4.08 8.01 × 10−3
10.82 1.50 × 10−2
Ag+. These values are in agreement with the MALDI-TOF-MS
results previously discussed.
In the case of P, we were unable to obtain with HypSpec the
stability constants for the interaction with Ag+, Cu2+, and Zn2+.
In this case (P), a great number of equivalents of each metal ion
were necessary to achieve a plateau (equilibrium). This result
suggested L as the more favorable system over P for interaction
with the explored metal ions. This could be due to the fact that
a better stabilization of each metal ion was obtained with the
bischromophoric system L.
Interaction between L and Metal Cations: Time-Resolved
Data. Time-resolved data provide further and relevant
information regarding the excimer-forming processes in the
absence and presence of metal cations. In pure dioxane, the
results presented so far have shown that the band at 390 nm
can be attributed to a monomer emission, i.e., to an excited
pyrene with no other pyrene in its vicinity. In the case of L, the
longer emission wavelength can be attributed to the emission of
an excimer/dimer. In order to avoid contribution/"contami-
nation" of the monomer emission in the excimer region (and
vice versa), we collected the emission decays in the extremities
of each emission band: 420 nm (monomer) and 550 nm
(excimer).
It is worth remembering that, from steady-state data, the
exciplex emission is absent for L in the presence of metal
cations. It is also worth noting the extraordinary quenching
suffered by the decay times of these pyrene derivatives in
comparison with other known and investigated pyrene system
derivatives, i.e., from nano-26 to picosecond (in our work with P
and L) time regime. This shows that the imine groups are
strong quenchers of pyrene fluorescence.
Figure 8. Absorption (left panels) and fluorescence emission (right
panels) titrations of chemosensor L as a function of the increasing
amounts of (A and B) Cu(BF4)2, (C and D) Zn(BF4)2, and (E and F)
Ag(BF4) in dioxane. The insets show the absorption at 360 and 390
nm and the fluorescence intensity at 420 nm ([L] = 5.00 × 10−6 M,
and λexc = 360 nm). The bold dark line is for [Cu2+]/[L] = [Zn2+]/[L]
= [Ag+]/[L] = 0, whereas the bold red line is for [Cu2+]/[L] =
[Zn2+]/[L] = [Ag+]/[L] = 30.
and colorless (Ag+) after the addition of 1 equiv of the
appropriate metal ion.
From Figure 9, it can be seen that the colors observed in all
cases are the result of the metal-to-ligand interaction and not
just to the mixture of two (colored) solutions; e.g., in the case
of Cu2+ when the (pale) blue solution of Cu2+ is added to the
yellow solution of L, it should be green with just the result of
color overlap and not dark red, as was obtained. These results
suggest the use of L for colorimetric detection of the explored
metal ions.
Figure 10 depicts the emission decays in dioxane in the
absence (A) and presence (B) of Ag+. Global analysis of the
decays shows that these are again fitted with the sums of three
exponentials. It is also worth noting that the decay times are
generally kept constant upon the addition of Ag+. However, the
same does not happen with the preexponential factors.
The stability constants for the interaction of L with Cu2+,
Zn2+, and Ag+ were calculated using HypSpec software and are
summarized in Table 2.25 Taking into account the values
obtained, the strongest interaction expected for sensor L is with
Figure 9. Color changes observed after the interaction of L (6.80 × 10−4 M) and P (1.00 × 10−4 M) with Cu2+ (1 equiv), Zn2+ (1 equiv), and Ag+ (1
equiv) in dioxane. Initial colors of L, P, and metallic solutions.
127
dx.doi.org/10.1021/ic301365y | Inorg. Chem. 2013, 52, 121−129

Inorganic Chemistry
Article
rearrangement of the pyrene groups in the (instantaneously
formed) dimer to a more stable (dimer) conformation.
CONCLUSIONS
■
In this work, the synthesis and characterization of two new
compounds, P and L, containing pyrene units was undertaken.
The investigation of these two chemosensors has shown that
these are sensitive to water and metal cations. In the case of
water sensing, the two ligands showed a new band that was
attributed to an exciplex species and whose contribution
increases with the water amount. The interaction of L toward
Cu2+, Zn2+, Ag+, Cd2+, and Hg2+ metal ions was also explored in
the gas phase using MALDI-TOF-MS.
Several metal-ion titrations of L with Cu2+, Ag+, and Zn2+
followed by absorption and emission studies were performed in
dioxane. In all cases, the stability constant values suggest a
stoichiometry of 1:1 ligand/metal. The strongest interaction
expected for sensor L was with Ag+ in all of the solvents tested,
with the highest value being observed in the noncoordinative
solvent dioxane. Compound P was found to be more selective
to Cu2+.
Further detailed information could be obtained from time-
resolved experiments: (1) Extraordinary quenching is suffered
by these pyrene derivatives, mirrored from the decrease from
nanoseconds, the usual range of pyrene derivative lifetimes, to
picoseconds, which is likely due to the imine groups that are
strong quenchers of pyrene fluorescence. (2) Global analysis of
the emission decays in dioxane in the absence and presence of
Ag+ shows that the decays are only fitted with the sums of three
exponentials, which were associated with the monomer, and the
two additional components with the kinetics of a dimer
formation.
Figure 10. Fluorescence decays of L in dioxane (T = 293 K) at λexc
=
381 nm in (A) absence and (B) presence of Ag+ (ratio [Ag+]/[L] =
2:1). The instrument profile curve, decay times (τi), preexponential
factors (Ai,j), χ2 values, weighted residuals, and autocorrelation
functions (A.C.) are shown as the insets.
From Figure 10, it can be seen that, upon the addition of Ag+
at λem = 420 nm, there is basically no change in the
preexponential value associated with the shorter component
(the decay associated with the monomer ∼30 ps). However,
the same is not valid with the A2,1 preexponential at λem = 550
nm, where an increase is observed upon the addition of Ag+.
Indeed, the addition of Ag+ leads to an increase (basically
doubling the values) of the A2,1 and A2,3 preexponential factors,
which means that upon complex formation the monomer
contribution (at the excimer/dimer emission) increases and the
excimer/dimer emission decreases as a result of complexation
with the imine moieties and a consequent split of the pyrene
chromophores.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic details, excitation spectra of P and L with the addition
of metals (Cu2+, Zn2+, and Ag+) and water collected at λem
=
420 and 500 nm, and absorption and emission spectra of L in
pure dioxane before and after the removal of water. This
material is available free of charge via the Internet at http://
pubs.acs.org.
In addition, it can also be seen that a substantial decrease
occurs, upon the addition of Ag+, with the preexponential factor
associated with the 140−150 ps decay time (related to the
decay of the excimer/dimer species) at λem = 420 nm. This
means that the addition of Ag+ leads to a decrease in the
excimer-to-monomer reversibility.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sseixas@ci.uc.pt (J.S.S.d.M.), clodeiro@uvigo.es or
cle@fct.unl.pt (C.L.). Tel: +351 212948300 (C.L.). Fax: +34
988 387001 or +351 212948550 (C.L.).
■
Notes
With these facts in mind and also with the previous
knowledge in polymer systems bearing pyrene pendant
chromophores where the process of dimer formation has
been shown to occur with tens of picoseconds,27 we propose
the following interpretation. The shorter decay component
(∼30 ps) should be attributed to the monomer decay that gives
rise to the excimer/dimer. This is the major component at 420
nm with a preexponential factor (concentration of the species
at time zero) of ∼0.8−0.9 in both the absence and presence of
Ag+. This short component is kinetically linked to the ∼0.14 ns
component (which is associated with the decay of the
instantaneously formed dimer/excimer). This is likely to be
kinetically linked to the other species (the other long
component with ∼1.7−1.9 ns), which should be associated
with the emission of a relaxed dimer.25 This mirrors the quick
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to Xunta de Galicia (Spain) for Project
10CSA383009PR (Biomedicine) and to the Scientific Associ-
ation Proteomass for financial support. J.F.-L. thanks Xunta de
Galicia (Spain) for a research contract by Project
09CSA043383PR in Biomedicine. C.N. and C.S.d.C. thanks
the Fundaca̧ o para a Ciencia e a Tecnologia/FEDER
̂
̃
(Portugal/EU) programme postdoctoral Contracts SFRH/
BPD/65367/2009 and SFRH/BD/75134/2010, respectively.
J.L.C. and C.L. thank Xunta de Galicia for the Isidro Parga
Pondal Research program. E.B. thanks the CCCU Research
Development and Enhancement Fund.
128
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Inorganic Chemistry
Article
(11) Valeur, B. Molecular Fluorescence Principles and Applications;
Wiley-VCH: New York, 2002.
REFERENCES
■
(1) (a) Liu, D. W.; Wang, Z.; Jiang, X. Nanoscale 2011, 3, 1421.
(b) Manandhar, E.; Wallace, K. J. Inorg. Chim. Acta 2012, 381, 15.
(2) (a) Linton, B.; Hamilton, A. D. Chem. Rev. 1997, 97, 1669.
(b) Godman, M. S.; Weiss, J.; Hamilton, A. D.; Jubian, V.; Linton, B.;
Hamilton, A. D. J. Am. Chem. Soc. 1995, 117, 11610. (c) Al-Sayah, M.;
Branda, N. R. Angew. Chem., Int. Ed. 2000, 39, 945. (d) Moriuchi, T.;
Nishiyama, M.; Hirao, T. Eur. J. Inorg. Chem. 2002, 447. (e) Jiao, T. F.;
Liu, M. H. J. Phys. Chem. B 2005, 109, 2532. (f) Ramstrom, O.; Lehn,
J. M. Nat. Rev. Drug Discovery 2002, 1, 26.
(3) Ma, Z.; Yang, R. D.; Yan, L. J. Chem. Res., Synop. 1999, 12, 712.
(4) (a) Radecka-Paryzek, W.; Patroniak, V.; Lisowski, J. Coord. Chem.
Rev. 2005, 249, 2156. (b) Herrera, A. M.; Kalayda, G. V.; Disch, J. S.;
Wilstrom, R. P.; Korendovych, I. V.; Staples, R. J.; Campana, C. F.;
Nazarenko, Q. Y.; Haas, T. E.; Rybak-Akimova, E. V. Dalton Trans.
2003, 23, 4482. (c) Menon, S. C.; Panda, A.; Singh, H. B.; Butcher, R.
J. Chem. Commun. 2000, 2, 143.
(5) (a) Chaviara, A. T.; Cox, P. J.; Repana, K. H.; Pantazaki, A. A.;
Papazisis, K. t.; Kortsaris, A. H.; Kyriakidis, D. A.; St. Nikolov, G.;
Bolos, C. A. J. Inorg. Biochem. 2005, 99, 467. (b) Won, D. H.; Lee, C.
H. Tetrahedron Lett. 2001, 42, 1969. (c) Adams, H.; Elsegood, M. R. J.;
Fenton, D. E.; Heath, S. L.; Ryan, S. J. Inorg. Chem. Commun. 1999, 2,
139. (d) Adams, H.; Fenton, D. E.; Ryan, S. J. Inorg. Chem. Commun.
1999, 2, 52. (e) Adams, H.; Bailey, N. A.; Collinson, S. R.; Fenton, D.
E.; Hawley, J. C.; Kitchen, S. J. J. Organomet. Chem. 1998, 550 (8), 7.
(12) (a) Winnik, F. M. Chem. Rev. 1993, 93, 587. (b) Nishizawa, S.;
Kato, A.; Teramae, N. J. Am. Chem. Soc. 1999, 121, 9463. (c) Sahoo,
D.; Narayanaswami, V.; Kay, C. M.; Ryan, R. O. Biochemistry 2000, 39,
6595. (d) Kim, S. K.; Yoon, J. J. Org. Chem. 2003, 68, 597. (e) Yuasa,
H.; Miyagawa, N.; Izumi, T.; Nakatani, M.; Izumi, M.; Hashimoto, H.
Org. Lett. 2004, 6, 1489.
(13) (a) Zachariasse, K. A.; Duveneck, G.; Kuhnle, W. Chem. Phys.
Lett. 1985, 113, 337. (b) Seixas de Melo, J.; Costa, T.; Francisco, A.;
Maca
2007, 9, 1370.
̧ nita, A. L.; Gago, S.; Gonca̧ lves, I. S. Phys. Chem. Chem. Phys.
(14) (a) Shiraishi, Y.; Tokitoh, Y.; Hirai, T. Org. Lett. 2006, 8, 3841.
(b) Kim, H.; Hong, J.; Hong, A.; Ham, S.; Lee, J.; Kim, J. S. Org. Lett.
2008, 10, 1963. (c) Jung, H.; Park, M.; Han, D.; Kim, E.; Lee, C.;
Ham, S.; Kim, J. S. Org. Lett. 2009, 11, 3378.
(15) (a) Moon, S. Y.; Youn, N. J.; Park, S. M.; Chang, S. K. J. Org.
Chem. 2005, 70, 2394. (b) Kim, J. S.; Choi, M. G.; Song, K. C.; No, K.
T.; Ahn, S.; Chang, S. K. Org. Lett. 2007, 9, 1129. (c) Park, S. M.; Kim,
M. H.; Choe, J. I.; No, K. T.; Chang, S. K. J. Org. Chem. 2007, 72,
3550. (d) Xu, Z.; Yoon, J.; Spring, D. R. Chem. Commun. 2010, 46,
2563.
(16) (a) Xie, J.; Menand, M.; Maisonneuve, S.; Metivier, R. J. Org.
Chem. 2007, 72, 5980. (b) Zhang, H.; Han, L. F.; Zachariasse, K. A.;
Jiang, Y. B. Org. Lett. 2005, 7, 4217. (c) Yang, J. S.; Lin, C. S.; Hwang,
C. Y. Org. Lett. 2001, 3, 889. (d) Hung, H. C.; Cheng, C. W.; Ho, I. T.;
Chung, W. S. Tetrahedron Lett. 2009, 50, 302. (e) Hung, H.-C.; Cheng,
C.-W.; Wang, Y.-Y.; Chen, Y.-J.; Chung, W.-S. Eur. J. Org. Chem. 2009,
6360.
(f) Dias, P.; Domenech, A.; García-Espana, E.; Soriano, C. Polyhedron
2002, 21, 1523.
(6) (a) Lodeiro, C.; Capelo, J. L.; Mejuto, J. C.; Oliveira, E.; Santos,
H. M.; Pedras, B.; Nunez, C. Chem. Soc. Rev. 2010, 39, 2948.
̃
́
̃
(17) Pina, J.; Seixas de Melo, J. S.; Burrows, H. D.; Maca̧ nita, A. L.;
(b) Lodeiro, C.; Pina, F. Coord. Chem. Rev. 2009, 253, 1353.
Galbrecht, F.; Bunnagel, T.; Scherf, U. Macromolecules 2009, 42, 1710.
(18) Striker, G.; Subramaniam, V.; Seidel, C. A. M.; Volkmer, A. J.
Phys. Chem. B 1999, 103, 8612.
(c) Nunez, C.; Bastida, R.; Macías, A.; Valencia, L.; Neuman, N. I.;
́
̃
Rizzi, A. C.; Brondino, C. D.; Gonzal
́
ez, P. J.; Capelo, J. L.; Lodeiro, C.
Dalton Trans. 2010, 39, 11654. (d) Nunez, C.; Bastida, R.; Macías, A.;
Valencia, L.; Ribas, J.; Capelo, J. L.; Lodeiro, C. Dalton Trans. 2010,
39, 7673.
́
̃
(19) (a) Vicente, M.; Lodeiro, C.; Adams, H.; Bastida, R.; de Blas, A.;
Fenton, D. E.; Macías, A.; Rodríguez, A.; Rodríguez-Blas, T. Eur. J.
Inorg. Chem. 2000, 6, 1015. (b) Lodeiro, C.; Bastida, R.; de Blas, A.;
Fenton, D. E.; Macías, A.; Rodríguez, A.; Rodríguez-Blas, T. Inorg.
Chim. Acta 1998, 267, 55.
(7) (a) Jiang, P. G.; Chen, L. Z.; Lin, J.; Liu, Q.; Ding, J.; Gao, X.;
Guo, Z. Chem. Commun. 2002, 1424. (b) Zhang, H.; Han, L.-F.;
Zachariasse, K. A.; Jiang, Y. B. Org. Lett. 2005, 7, 4217. (c) He, Q.;
Miller, E. W.; Wong, A. P.; Chang, C. J. J. Am. Chem. Soc. 2006, 128,
9316. (d) Martínez, R.; Zapata, F.; Caballero, A.; Espinosa, A.;
Tairraga, A.; Molina, P. Org. Lett. 2006, 8, 3235. (e) Avirah, R. R.;
Jyothish, K.; Ramaiah, D. Org. Lett. 2007, 9, 121. (f) Yoon, S.; Miller,
E. W.; He, Q.; Do, P. H.; Chang, C. J. Angew. Chem., Int. Ed. 2007, 46,
6658. (g) Shiraishi, Y.; Maehara, H.; Ishizumi, K.; Hirai, T. Org. Lett.
2007, 9, 3125. (h) Mameli, M.; Lippollis, V.; Caltagirone, C.; Capelo,
J. L.; Nieto-Faza, O.; Lodeiro, C. Inorg. Chem. 2010, 49, 8276.
(i) Oliveira, E.; Baptista, R. M. F.; Costa, S. P. G.; Raposo, M. M. M.;
Lodeiro, C. Inorg. Chem. 2010, 49, 10847.
(8) (a) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Coord.
Chem. Rev. 2000, 205, 59. (b) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.;
Rice, T. E. Chem. Rev. 1997, 97, 1515. (c) Fabbrizzi, L.; Poggi, A.
Chem. Soc. Rev. 1995, 24, 197. (d) Valeur, B.; Leray, I. Coord. Chem.
Rev. 2000, 205, 3.
(9) (a) Czarnik, A. W. Acc. Chem. Res. 1994, 27, 302. (b) de Silva, A.
P.; Gunaratne, H. Q. N.; Gunnlaugsson, T. A.; Huxley, T. M.; McCoy,
C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515.
(c) Gunnlaugsson, T.; Glynn, M.; Tocci, G. M.; Kruger, P. E.; Pfeffer,
F. M. Coord. Chem. Rev. 2006, 250, 3094. (d) Kim, J. S.; Quang, D. T.
Chem. Rev. 2007, 107, 3780. (e) Xu, Z.; Chen, X.; Kim, H. N.; Yoon, J.
Chem. Soc. Rev. 2010, 39, 127. (f) Chen, X.; Zhou, Y.; Peng, X.; Yoon,
J. Chem. Soc. Rev. 2010, 39, 2120. (g) Xu, Z.; Kim, S. K.; Yoon, J.
Chem. Soc. Rev. 2010, 39, 1457. (h) Kim, H. N.; Guo, Z.; Zhu, W.;
Yoon, J.; Tian, H. Chem. Soc. Rev. 2011, 40, 79. (i) Chen, X.; Kang, S.;
Kim, M. J.; Kim, J.; Kim, Y. S.; Kim, H.; Chi, B.; Kim, S.-J.; Lee, J. Y.;
Yoon, J. Angew. Chem., Int. Ed. 2010, 49, 1422. (j) Xu, Z.; Baek, K.-H.;
Kim, H. N.; Cui, J.; Qian, X.; Spring, D. R.; Shin, I.; Yoon, J. J. Am.
Chem. Soc. 2010, 132, 601.
(20) Fernan
́
dez-Lodeiro, J.; Nunez, C.; Carreira, R.; Santos, H. M.;
́
̃
Silva Lopez, C.; Mejuto, J. C.; Capelo, J. L.; Lodeiro, C. Tetrahedron
́
2011, 67, 326.
(21) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London,
1970.
(22) Åkerlof, G. O.; Short, A. J. Am. Chem. Soc. 1936, 58 (7), 1241−
̈
1243.
(23) Bichenkova, E. V.; Sardarian, A. R.; Wilton, A. N.; Bonnet, P.;
Bryce, R. A.; Douglas, K. T. Org. Biomol. Chem. 2006, 4, 367.
(24) MacCarthy, P. Anal. Chem. 1978, 50, 2165.
(25) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739.
(26) (a) Lodeiro, C.; Lima, J. C.; Parola, A. J.; Seixas de Melo, J. S.;
Capelo, J. L.; Covelo, B.; Tamayo, A.; Pedras, B. Sens. Actuators, B
2006, 115, 276. (b) Seixas de Melo, J. S.; Costa, T.; Francisco, A.;
Maca
2007, 9, 1370.
(27) Costa, T.; Seixas de Melo, J. S.; Castro, C. S.; Gago, S.; Pillinger,
M.; Goncalves, I. S. J. Phys. Chem. B 2010, 114, 12439.
̧ nita, A. L.; Gago, S.; Gonca̧ lves, I. S. Phys. Chem. Chem. Phys.
̧
(10) Kim, J. S.; Quang, D. T. Chem. Rev. 2007, 107, 3780.
129
dx.doi.org/10.1021/ic301365y | Inorg. Chem. 2013, 52, 121−129