Steady-state and time-resolved investigations of a crown thioether conjugated with methylacridinium and its complexes with metal ions

Article abstract of DOI:10.1039/c0cp01411g

The crown thioether 9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]phenyl- N-methylacridinium perchlorate (TCMA) was synthesized and characterized with the aim to verify its ability to interact selectively with metal ions and substantiate the possibility to detect easily the presence of heavy metals in fluid samples. The spectroscopic properties of TCMA, alone and in the presence of metal ions, were therefore studied in polar solvents (MeCN and H ₂O); in particular, steady-state UV-Vis spectrophotometric and fluorimetric techniques were used together with transient absorption spectroscopy with fs time resolution to investigate the spectral and dynamic properties of the lowest excited singlet state of TCMA and of TCMA/metal ion complexes. The absorption in the Vis region is characterized by a charge-transfer nature with the methylacridinium moiety acting as the electron-acceptor and the anilic group as the electron-donor. No emission from the S₁ was detected both in MeCN and H₂O, while a small S₂ → S₀ fluorescence emission (λ_max = 485 nm and _F = 0.0011) was detected in water. Time-resolved measurements with fs resolution of TCMA in MeCN have shown that the relaxed S₁ state is reached ～0.6 ps after the laser pulse, while the S₁ → S₀ time constant is 3.7 ps. Among the investigated metal ions, only Fe³⁺ (in MeCN) and Hg²⁺ (in MeCN and H₂O) were able to form stable complexes (association constant, K_ass = 1-11 × 10⁴ M^-1) with TCMA. The S₁ state of the TCMA/Mⁿ⁺ complexes emits with low quantum yield (_F = 0.0023-0.014) and decays with time constants much longer than TCMA itself, at least in the case of TCMA/Hg²⁺ in MeCN. This study showed that TCMA is a good candidate for colorimetric/ fluorimetric sensing of Hg²⁺ in aqueous media owing to its high selectivity towards metal ions. the Owner Societies.

Full text of DOI:10.1039/c0cp01411g

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Steady-state and time-resolved investigations of a crown thioether
conjugated with methylacridinium and its complexes with metal ions
Tiziana del Giacco, Benedetta Carlotti, Stefano De Solis, Arianna Barbafina and
Fausto Elisei*
Received 3rd August 2010, Accepted 1st November 2010
DOI: 10.1039/c0cp01411g
The crown thioether 9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]phenyl-N-methylacridinium
perchlorate (TCMA) was synthesized and characterized with the aim to verify its ability to interact
selectively with metal ions and substantiate the possibility to detect easily the presence of heavy
metals in fluid samples. The spectroscopic properties of TCMA, alone and in the presence of metal
ions, were therefore studied in polar solvents (MeCN and H2O); in particular, steady-state UV-Vis
spectrophotometric and fluorimetric techniques were used together with transient absorption
spectroscopy with fs time resolution to investigate the spectral and dynamic properties of the
lowest excited singlet state of TCMA and of TCMA/metal ion complexes. The absorption in the
Vis region is characterized by a charge-transfer nature with the methylacridinium moiety acting as
the electron-acceptor and the anilic group as the electron-donor. No emission from the S1 was
detected both in MeCN and H2O, while a small S2 - S0 fluorescence emission (lmax = 485 nm
and fF = 0.0011) was detected in water. Time-resolved measurements with fs resolution of
TCMA in MeCN have shown that the relaxed S1 state is reached B0.6 ps after the laser pulse,
while the S1 - S0 time constant is 3.7 ps. Among the investigated metal ions, only Fe3+
(in MeCN) and Hg2+ (in MeCN and H2O) were able to form stable complexes (association
constant, Kass = 1–11 ꢀ 104 Mꢁ1) with TCMA. The S1 state of the TCMA/Mn+ complexes emits
with low quantum yield (fF = 0.0023–0.014) and decays with time constants much longer than
TCMA itself, at least in the case of TCMA/Hg2+ in MeCN. This study showed that TCMA is a
good candidate for colorimetric/fluorimetric sensing of Hg2+ in aqueous media owing to its high
selectivity towards metal ions.
mercury has been linked with minamata, myocardial infarction
Introduction
and some kinds of autism and can lead to damages of the brain,
Chemosensors are compounds that modify significantly their
kidneys, central nervous system, immune system, and endo-
crine system;25,26 zinc is one of the most abundant transition
electrical, electronic, magnetic and/or optical properties by
interaction with specific guests. Due to the development of
metals in the brain whose excess release may occur in a variety
fluorescent and chromogenic chemosensors able to reveal
of pathological conditions which may contribute to a selective
neuronal cell death.27
quickly, selectively and sensitively polluting species, much
attention has been paid in the use of spectroscopic techniques
In spite of the wide scientific activity regarding the detection
to reveal the presence of metal ions in samples of various
of metal ions in various media, a detailed investigation of the
nature.1–4 In fact, most fluorescent sensors mainly based on
detection mechanism and of the photophysical and dynamic
photoinduced electron transfer (PET) and internal charge
properties of sensor/metal ion complexes is actually missing.
transfer (ICT) processes are used for in vitro and in vivo cellular
This work deals with the synthesis and characterization of a
imaging to make this approach more promising than other
crown thio-ether (9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]-
analytical methods. Thus, a number of fluorescent sensors
phenyl-N-methylacridinium perchlorate, TCMA) by steady-
have been successfully used to reveal the presence of
state absorption and emission techniques and transient
calcium,5–7 copper,8–11 mercury12–20 and zinc21,22 in solution
absorption spectroscopy with femtosecond time-resolution
and in cells. This interest is due to the bioactivity of these heavy
(Scheme 1). The ability of TCMA to interact with metal ions,
particularly stable and selective for Fe3+ and Hg2+, was
metals and/or their ability to induce diseases even in a short
period of time. In particular, disorders of calcium metabolism
investigated by recording the spectral and kinetic properties in
occur when the body has too little or too much calcium;23
the presence of numerous cations in MeCN and H2O.
copper can cause gastrointestinal disturbance (short-term) and
liver or kidney damage (long-term);24 bioaccumulation of
Experimental section
Starting materials
Chemistry Department, and Centro di Eccellenza sui Materiali
Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce
di Sotto 8, 06123 Perugia, Italy. E-mail: elisei@unipg.it;
Fax: +39 075-5855598; Tel: +39 075-5855588
All the reagents were obtained from commercial suppliers
(Aldrich, Fluka) and used as received. Hg2+, Fe3+, Na+,
c
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measured with a Fluorolog-2 (Spex, F112AI) spectrophotofluori-
meter. Fluorescence quantum yields (experimental error of ca.
7%) were determined from the emission spectra of the samples
recorded at an absorbance smaller than 0.1 at the excitation
wavelength, to avoid self-absorption effects, by use of anthra-
cene in ethanol as reference (fF = 0.22).34
The transient behavior on the ultrafast time scale was
investigated by use of femtosecond pulses generated by an
amplified titanium-sapphire laser system (Spectra Physics,
Mountain View, CA)35–37 that produces 1 W pulses centred
at 800 nm at a repetition rate of 1 kHz. Pump pulses centred
at 400 nm were obtained by second harmonic generation in a
500 mm b-barium-borate crystal. In the transient absorption
set-up (Helios, Ultrafast Systems) the pump pulses were passed
through a chopper which cut out every second pulse and
collimated to the sample in a 2 mm quartz cuvette. Probe
pulses for optical measurements were produced by passing a
small portion of 800 nm light through an optical delay line
(with a time window of 1600 ps) and focusing into a 2 mm thick
Ti:Sapphire window to generate a white-light continuum in the
430–800 nm window. The white light was focused onto the
sample and the chirp inside the sample cell was determined by
measuring the laser-induced Kerr signal of the solvent. The
temporal resolution of our time-resolved spectroscopic
technique is B150 fs while the spectral resolution is 1.5 nm.
All the measurements were carried out under magic angle in
order to avoid the rotational decay processes, in a 2 mm cell
and at an absorbance of about 0.5 at 400 nm. The solution was
stirred during the experiments to avoid photodegradation
processes. Transient Absorption Data were analyzed using
the Surface Xplorer PRO (Ultrafast Systems) software. It
was possible to visualize the 3D and 2D data surfaces, to
simultaneously display and save multiple spectra and kinetics,
to perform temporal chirp corrections, to make time zero
adjustments, to fit the kinetics by exponential(s) and perform
instrumental response function deconvolution, to perform
Singular Value Decomposition (SVD)38,39 of the 3D surface
into principal components (spectra and kinetics) and to
perform Global Analysis.
Scheme
1 9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]phenyl-N-
methylacridinium perchlorate (TCMA).
Li+ and Mg2+ were used as perchlorate salts, Ca2+
,
Ba2+,Co2+, Ni2+, Pb2+ and Cu2+ as nitrate salts, Cs2+ as
chloride and Fe2+ as ammonium sulfate. Dimethylformamide
(DMF, Aldrich) and tetrahydrofuran (THF, Aldrich) were
purified and dried by standard procedures.28 Acetonitrile
(MeCN, Fluka, for UV spectroscopy) was used as received.
Instrumental analysis
1H-NMR spectra were run on a Bruker AC 200 (200 MHz)
spectrometer from solutions in CDCl3 with TMS as internal
standard. GC-MS analyses were performed on a Hewlett
Packard 6890A gas-chromatograph (HP-Innovax capillary
column, 15 m) coupled with a MSD-HP 5973 mass selective
detector (70 eV). GC analyses were carried out on a HP 5890
gas-chromatograph using a HP-Innovax capillary column, 15 m.
HPLC analyses were performed with a liquid chromatograph
HP 1100.
Synthesis of 9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]
phenyl-N-methylacridinium perchlorate (TCMA)
TCMA was synthesized in five steps, starting from aniline
commercially available. Firstly, N,N-di(2-hydroxyethyl)aniline
was prepared by reaction of aniline with 2-chloroethanol in
water.29 The diol was converted by para-toluenesulfonyl
chloride in the corresponding tosylate in NaOH/H2O/THF,
N,N-bis[2-(para-toluenesulfonyl)ethyl]aniline:30 1H-NMR (CDCl3,
200 MHz): d 7.73 (d, 4H), 7.26 (d, 4H), 7.15 (t, 2H), 6.70
(t, 1H), 6.44 (d, 2H), 4.18 (t, 4H), 3.46 (t, 4H), 2.42 (s, 3H). This
product was added, together with 3,6-dithiaoctane-1,8-dithiol
(prepared as previously described)31 in a mixture of CsCO3 in
DMF to produce the receptor, 1-phenyl-4,7,10,13-tetrathia-
1-azacyclopentadecane.30 1H-NMR (CDCl3, 200 MHz): d 7.28
(dd, 2H), 6.73 (m, 3H), 3.57 (t, 4H), 2.73 (m, 16H). The
reaction of the receptor with N-methylacridinium iodide
(prepared as previously described)32 in the presence of I2 in
DMF, after elaborate workup of the crude, produced
TCMA.33 1H-NMR (CDCl3, 200 MHz) d 8.52 (d, 2H), 8.28
(d+t, 4H), 7.89 (t, 2H), 7.45 (d, 2H), 7.07 (d, 2H), 4.75 (s, 3H),
3.72 (t, 4H), 2.84 (m, 16H).
Transient kinetics on the time scale of several microseconds
were collected by a nanosecond laser flash photolysis setup
previously described (Nd:YAG Continuum, Surelite II, third
harmonics, lexc = 355 nm, pulse width ca. 7 ns and energy
r1 mJ per pulse).40,41 All measurements were carried out at
room temperature; the solutions were saturated by bubbling
with nitrogen.
Ground-state absorption spectra were recorded before and
after the transient absorption measurements to check for
photodegradation.
Quantum mechanical calculations
Quantum mechanical calculations were performed using the
CAChe 7.5 software. Transition energies and oscillator
strengths were obtained by INDO/1-CI calculations42 after
geometrical optimization with the PM3 semiempirical
model.43,44 Electron excitations (singly excited) from the
20 highest occupied to the 20 lowest virtual molecular
orbitals (MO) were used.
Photophysical measurements
Absorption spectra were recorded with a Perkin-Elmer Lambda
800 spectrophotometer. Fluorescence emission spectra were
c
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H2O for the Vis band can be ascribed to the stronger interaction
between the solvent and the anilic nitrogen of TCMA which
reduces the electron transfer efficiency from the anilic group to
the methylacridinium moiety and then destabilizes the lowest
excited singlet state.
Results and discussion
Absorption and emission properties of TCMA
Absorption. The S0 - Sn absorption spectra of TCMA in
H2O and MeCN are shown in Fig. 1. The bands recorded in H2O
are centred at about 350, 410 and 528 nm with relatively high
absorption coefficients (in the range of 3700–17 000 Mꢁ1 cmꢁ1
It has to be noted that the spectral behaviour of TCMA is
close to that already reported for a similar compound where
the crown thio-ether is replaced by a crown ether.48
,
Table 1). In MeCN, the hypsochromic bands were characterized
by the same absorption maxima and slightly smaller molar
absorption coefficients (B2700 and 15 000 Mꢁ1 cmꢁ1
The experimental data can be compared with the absorption
maxima and oscillator strengths obtained by quantum-
mechanical calculations using the INDO/1-CI method (see
Tables 1 and 2). Taking into account that no solvent effects
were included in the calculations, the predicted transition
energies are in good agreement with the experimental results.
Each absorption band is due to an allowed p - p* transition
(oscillator strength f = 0.067–0.20). The first transition is
mainly described by the HOMO - LUMO configuration
with charge transfer character, where HOMO and LUMO
are located in correspondence of the anilic (AN) and methy-
lacridinium (MA) moieties, respectively. Instead, the other two
transitions are characteristic of the methylacridinium moiety
(pMA - p*MA), as also shown by a comparison of the
absorption spectra of MA and TCMA shown in Fig. 1.
,
Table 1). Instead, the broad band in the Vis region shows
significant bathochromic and hyperchromic effects on going to
the aprotic solvent MeCN (Fig. 1). This is in agreement with the
charge-transfer nature of the first absorption band which is due
to the presence of a strong electron-donor substituent (such as
the anilic group N,N substituted with the crown thio-ether)
linked to the 9 position of the methylacridinium moiety.
Taking into account the electron-acceptor properties of
the excited acridinium moiety (E1red = 2.32 V)45 and the
electron-donor properties of N,N alkyl-substituted anilic group
(E1ox o 1 V),46 it is reasonable that the lowest excited singlet
state of TCMA has a charge-transfer nature. As a matter of fact,
the charge-transfer band associated with the N,N-diethylaniline/
N-methylacridinium complex is reported to be centred at
570 nm.47 Furthermore, the hypsochromic effect recorded in
Emission. The lowest excited singlet state of TCMA with CT
character does not show fluorescence in MeCN and H2O. In
fact, a fluorescence quantum yield fF smaller than 10ꢁ4 was
estimated upon excitation at 550 nm. Furthermore,
a
fluorescence signal characterized by an emission spectrum
centered at 485 nm (fF = 0.0011 at lexc = 358 nm) and an
excitation spectrum close to that of MA (Fig. 2) were detected
in H2O. The spectral characteristics of this emission suggest
that it is originated from the S2 state of methylacridinium
character; thus, in this solvent the radiative S2 - S0 process is
able to compete with the S2 - S1 internal conversion, also in
Table 2 Absorption maxima (lmax) and oscillator strengths (f) of the
lowest S0 - Sn transitions of TCMA calculated by INDO/1-CI
Compound
TCMA
Transition
l
max/nm
Nature
f
S0 - S1
S0 - S2
S0 - S3
S0 - S4
S0 - S5
513
367
335
317
302
p
p
p
p
p
AN - p*MA
MA - p*MA
MA - p*MA
AN - p*MA
AN - p*AN
0.067
0.20
0.19
0.007
0.087
Fig. 1 Absorption spectra of TCMA in MeCN (solid line) and H2O
(dotted line) and of MA in MeCN (dashed line) together with the
oscillator strengths (f) obtained by INDO/1-CI calculations (vertical
bars).
Table 1 Spectral properties of TCMA in the absence and in the presence of additive, together with the C50 parameter, the association equilibrium
constants and the fluorescence quantum yields
Solvent
MeCN
Additive
lambasx/nm
e/Mꢁ1 cmꢁ1
C50 (10ꢁ5 M)
Kass (104 Mꢁ1
)
lemmax/nm
fF
—
—
358
412
552
361
423
362
426
359
412
528
361
427
14 460
2890
9830
—
—
—
—
—
—
2.5
—
3.0
—
—
—
—
0.75
—
—
—
—
1.0
—
4.2
—
—
—
—
11
—
—
—
—
510
—
506
—
485
—
—
506
—
o0.0001
—
—
0.014
—
0.0023
—
0.0011
—
—
—
Fe3+
—
Hg2+
—
—
—
H2O
16 650
3730
7790
—
—
—
Hg2+
—
0.0040
—
—
c
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Fig. 2 Fluorescence excitation (lem = 480 nm) and emission spectra
(lexc = 358 nm) of TCMA (solid lines) and MA (dashed lines) in H2O.
agreement with the higher absorption coefficient recorded at
412 nm in H2O (Table 1). The lifetimes of the singlet excited
states of TCMA are very short and were characterized by fs
time-resolution absorption spectroscopy (see below).
Spectral properties in the presence of metal ions
The ability of TCMA to interact with metal ions was
investigated by recording steady-state absorption and
emission spectra in unbuffered water solutions and in
MeCN, in the presence of increasing amounts of salts of
alkali metals (Li+, Na+, and Cs+), alkaline earth metals
Fig. 3 Absorption spectra of TCMA in MeCN (3.0 ꢀ 10ꢁ5 M) in
the presence of (A) Fe(ClO4)3 (0–8.8 ꢀ 10ꢁ5 M) and (B) Hg(ClO4)2
(0–5.7 ꢀ 10ꢁ5 M) and (C) in H2O (8.0 ꢀ 10ꢁ6 M) in the presence of
Hg(ClO4)2 (0–2.7 ꢀ 10ꢁ5 M).
(Mg2+, Ca2+, and Ba2+), transition metals (Co2+, Ni2+
,
Fe2+, Fe3+, Cu2+, Zn2+ and Hg2+) and poor metals
(Pb2+). The experiments have shown that TCMA is able to
interact selectively with Hg2+ (in both the solvents) and Fe3+
(only in MeCN); in fact, the other metal ions were not able to
modify the absorption and emission spectra of TCMA even at
rather high concentrations (up to B3 ꢀ 10ꢁ4 M). This selective
behaviour of TCMA can be compared with those of analogous
crown ethers.48 In fact, the replacement of oxygen atoms (hard
type) with sulfur atoms (soft type) in the crown structure
decreases the electrostatic interactions between the thio-ether
and the metal ions, and then the affinity for metals, owing to
the smaller charge density in the S atoms.49
Table 1 shows the metal ion concentration (C50) able to halve
the absorbance of free TCMA.
Emission
The addition of Fe3+ and Hg2+ to TCMA also induced a
fluorescence enhancement: in general the spectral shape
remains always unstructured in the presence of salts but
there was an important variation in the fluorescence intensity
(Fig. 4). The effects of the metal ions are slightly different and
merit a comment. In fact, the fluorescence spectrum of the
TCMA–Fe3+ complex in MeCN is centred at 510 nm and its
shape does not change with the ion concentration (Fig. 4A).
Instead, the emission spectrum of the TCMA–Hg2+ complex
in MeCN is centred at B470 nm at low [Hg2+] and has a
bathochromic shift to B500 nm at high [Hg2+] (Fig. 4B); this
behaviour is in agreement with the absence of a well defined
isosbestic point in the absorption spectra and then with the
formation of a complex whose structure changes with the salt
concentration. Finally, the TCMA–Hg2+ complex in H2O
shows an emission centred at 506 nm, bathochromic shift of
B20 nm if compared with the emission of TCMA itself.
The values of the fluorescence quantum yields of TCMA in
Absorption. Upon addition of the perchlorate salts of Fe3+
(in MeCN) and Hg2+ (in MeCN and H2O) the absorption
spectra of TCMA show important changes in shape which
form isosbestic points in the 460–470 nm region (Fig. 3). The
absorption spectra were particularly affected above 450 nm
where the S0 - S1 band practically disappears at high ion
concentrations. On the contrary, the intensities of the
absorption bands due to the S0 - S2 and S0 - S3 increase
with the ion concentration while their position is practically
unchanged. These findings indicate that there is a TCMA-ion
interaction (likely 1 : 1) which affects mainly the lowest excited
singlet state with charge-transfer character.
the presence of the metal ions reported in Table 2 (fF
=
0.0023–0.014) were measured at a salt concentration where the
fluorescence intensity should have reached a plateau limit value
(Fig. 4, insets). The quantum yield values show the very low
efficiency of the emission process of the TCMA-ion complexes,
even if it takes place from a singlet state localized on the
methylacridinium moiety. This experiment allowed the
Linear trends of the ratio A0/A (where A0 and A represent
the absorbance in the absence and in the presence of the cation,
respectively) were recorded at 550 nm in MeCN and 530 nm in
H2O in the presence of increasing amounts of Fe3+ and Hg2+
.
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pairs of the nitrogen and sulfur atoms of the crown moiety.
Regarding Hg2+, it is quite surprising to have such a strong
association with TCMA, also taking into account that Ca2+
,
an ion with a similar diameter (2.0 A) and the same positive
charge, does not interact at all with TCMA. This can be due to
the different electronic structure of the cation and to the
different ability to interact with planar ligands.
Despite paramagnetic ions such as Fe3+ being efficient
fluorescent quenchers,51,52 in this study the presence of Fe3+
induces emission; it has to be noted that this behavior is quite
uncommon.53 This is likely due to the particular effect of the
cation on the nature of the emitting state. In fact, the forbidden
emitting state of TCMA (CT nature) is replaced by the allowed
p,p* state in the TCMA/Fe3+ complex.
The interference effect due to the concomitant presence of
different cations was not investigated in the present work. If
the absence of absorption and emission spectral changes
reflects the inability to form TCMA/cation complexes, the
interference effect should be absent, as shown in the
literature for other chemosensors.54
Even if an investigation of the pH effect was not carried out
in great detail, the ability of TCMA to interact selectively with
Hg2+ in aqueous solutions was still present at pH > 4 where
the N atom of the crown was not protonated (pKa E 2.9,
calculated by the pKa DB module of the program
ACDLABS 11.0).
Fig. 4 Fluorescence emission of 6.0 ꢀ 10ꢁ6 M TCMA in MeCN in
the presence of (A) Fe(ClO4)3 (0–8.8 ꢀ 10ꢁ4 M) and (B) Hg(ClO4)2
(0–4.6 ꢀ 10ꢁ5 M) and (C) in H2O in the presence of Hg(ClO4)2 (0–1.5 ꢀ
10ꢁ5 M) (lexc = 358 nm); the full lines represent the best fittings
obtained by use of eqn (1).
Femtosecond transient absorption
TCMA. The excited state dynamics of TCMA was investi-
gated in MeCN by pump–probe absorption spectroscopy upon
ultrafast excitation centred at 400 nm; the relatively low
solubility of TCMA prevented the same measurements to be
carried out in H2O, too.
detection limit of B10ꢁ5 M and B10ꢁ6 M to be estimated for
Fe3+ and Hg2+, respectively.
The emission intensities were analyzed by use of eqn (1)50 to
determine the Kass values (Fig. 4, insets):
Taking into account the absorption spectrum of TCMA and
the spectral broadening of the excitation pulses, hot vibrational
levels of the lowest excited singlet states (mainly S2) were
populated upon excitation. The time-resolved absorption
spectra of TCMA are shown in Fig. 5A while the decay
kinetics recorded at two noteworthy wavelengths are shown in
Fig. 6. The signals detected were negative, corresponding to
bleaching of the ground state, between 500 and 625 nm in the
region of the ground-state absorption and positive,
corresponding to transient absorption, in the regions below
500 nm and above 625 nm. In fact, the recorded spectra show
an hypsochromic shift in the position of the negative band in
time. The growth kinetic recorded for TCMA at 575 nm is the
mirror image of the decay kinetics recorded at 480 and 660 nm.
Very similar spectral and kinetic properties were detected in the
presence of increasing amounts (up to 0.001 M) of the metal ions
Na+, Co2+, and Pb2+ (Table 3).
IF;min þ IF;max ꢀ Kass ꢀ ½ionꢂ
IF
¼
ð1Þ
1 þ Kass ꢀ ½ionꢂ
where IF is the fluorescence intensity upon anion addition,
IF,min and IF,max are the fluorescence intensities of the free
substrate and of the complex, respectively, and [ion] is the
metal ion concentration in solution.
It has to be noted that the metal ion concentrations used in
the above experiments were always significantly larger than
those of TCMA and, therefore, the concentration of the
complex in solution was much smaller than that of the free ion.
The measured association constants (Table 1) are in the
10 000–110 000 Mꢁ1 range. The Kass values for the complexes
with Hg2+ were larger than those obtained with Fe3+ and the
values measured in H2O were larger than those obtained in
MeCN. This behaviour is in good agreement with the values of
C50 recorded by spectrophotometric measurements for the
same systems (Table 1), even if C50 obtained for the
TCMA–Fe3+ in MeCN is slightly low in comparison with
The kinetic traces were analyzed for each system at various
wavelengths (the best fittings were obtained by
a bi-
exponential function convoluted with the instrumental
response profile with a Gaussian shape). The global analysis
of the decay kinetics also revealed the presence of two
components (a short and a longer-lived one; S and L,
respectively). The values of tS and tL are in the 0.50–0.96 ps
and 3.3–3.9 ps ranges, respectively (Table 3). Furthermore, a
the corresponding Kass
.
It has to be noted that the ionic diameters of Fe3+ and Hg2+
are very different (1.24 A and 2.2 A, respectively). In fact, the
small diameter of Fe3+ determines a high charge density and,
subsequently, a strong electrostatic interaction with the lone
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Table 3 Time constants of TCMA and TCMA/Mn+ (0.001 M)
systems in MeCN obtained by Global Fit (lexc = 400 nm)a
S
L
l/nm
t/ps l/nm
t/ps
Metal
—
o460(ꢁ),570(+),660(ꢁ) 0.63 o460(+),570(ꢁ),700(+) 3.7
Na+ o470(ꢁ),560(+),700(ꢁ) 0.50 o470(+),565(ꢁ),720(+) 3.8
Co2+ o460(ꢁ),565(+),700(ꢁ) 0.96 o460(+),565(ꢁ),700(+) 3.3
Pb2+ o470(ꢁ),570(+),710(ꢁ) 0.90 o470(+),570(ꢁ),710(+) 3.9
Hg2+ o460(ꢁ),545(ꢁ),650(ꢁ) 3.7 o460(+),545(+),650(+) 45
a
The symbols (+) and (ꢁ) stand for positive and negative signals,
respectively.
these spectral properties are in agreement with
a
precursor–successor scheme which involves the S - L step.
Taking into account the data analysis and the time
constants, the transient L was assigned to the lowest singlet
state S1 with charge transfer character (also characterized by
the intense broad band centred at 552 nm in the ground-state
absorption spectrum), while S, which is precursor of L,
represents the quick solvent reorganization and S2,v>0
-
Fig. 5 (A) Pump–probe time-resolved absorption spectra of TCMA
in MeCN recorded 1, 3, 5.5, 8.5 and 15 ps after the laser pulse (lexc
400 nm). (B) Amplitudes of the decay components of TCMA in MeCN
S1,v=0 process after light absorption. The decay of L (S1) is
fast and it takes place by internal conversion (IC) followed by
vibrational relaxation (VR). These findings confirm the
absence of interactions between TCMA and Na+, Co2+, and
Pb2+; in fact, no spectral and kinetic changes were observed
even at a high concentration of the metal ions.
=
obtained by Global Analysis.
TCMA/Hg2+
.
In agreement with the steady-state
measurements (see above), addition of Hg2+ changed
significantly the spectral and kinetic properties of the lowest
excited singlet states, thus indicating a different photophysical
behaviour of the TCMA/Hg2+ complex. In fact, owing to the
disappearance of the ground-state absorption band centred at
552 nm, only positive signals with maxima at o460 nm,
545 nm and 650 nm were recorded in these experimental
conditions (see Fig. 7). Also in this case, the experimental
data treated by SVD and Global Analysis revealed the
presence of two components with spectral shapes shown in
Fig. 7C (negative amplitudes for S and positive amplitudes
for L) and lifetimes tS = 3.7 ps and tL = 45 ps (Table 3);
Fig. 6B shows, as an example, a decay kinetics recorded at 555
nm together with the best fitting obtained with a bi-exponential
function. Analogous to the other systems investigated, a very
small rest absorption replaced the signal of the L component
after its decay.
Considering the data analysis, the transient L was assigned
to the lowest singlet state S1 vibrationally relaxed, while S,
which is precursor of L, represents the excited singlet state
S1,v>0 produced upon direct excitation at 400 nm which is
quickly stabilized through solvent reorganization and
vibrational cooling. Comparing the tL values recorded in
these experiments, the decay of L (S1,v=0), which takes place
by fluorescence (F) and internal conversion (IC) followed by
vibrational relaxation (VR), is slowed down by about one
Fig. 6 Decay kinetics of (A) TCMA and (B) TCMA/Hg2+ (0.003 M)
in MeCN (lexc = 400 nm). The full lines represent the best fittings
obtained by bi-exponential functions.
very small and broad rest absorption replaced the longer lived
component signal after its decay. These signals were not
detected by ns laser flash photolysis measurements (see below).
The spectral shapes associated to S, L and to the rest
absorption were calculated by using the SVD method
followed by global analysis of the experimental data (see for
example Fig. 5B). The S component exhibited negative
amplitudes which are mirror images of the L component;
order of magnitude in the presence of Hg2+
.
The values of the radiative rate constant kF calculated by
the ratio fF/tF of TCMA and TCMA/Hg2+ are B1 ꢀ 108 and
0.5 ꢀ 108 sꢁ1, respectively, characteristic of allowed transitions.
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Scheme 2 State order of (left) TCMA and (right) TCMA/Mn+
complexes.
disappearing of the 550 nm band and the increase of the
absorption centred at 410 nm in the presence of Fe3+ or Hg2+
.
The order and nature of the lowest excited singlet states of
TCMA and their complexes with the metal ions are shown in
Scheme 2.
As a matter of fact, it has already been reported that Ag+ is
able to interact more efficiently with crown thio- and aza-ethers
rather than with crown ethers.49 This behaviour cannot be
understood by simply invoking electrostatic interactions
because the negative charge on the etheroatoms of the crown
decreases on going from oxygen to nitrogen and sulfur and then
also the electrostatic interaction should decrease, as observed
for alkali and alkaline earth metals. Taking also into account the
ability of Ag+ to form complexes with amines,55 it is then
probable that covalent bonds are involved in such complexes.
Analogous behaviour could explain the strong interactions
between Hg2+ and amines.52 In fact, X-ray photoemission
spectroscopy measurements have shown that the chemical
interactions between Hg2+ and the sulfur atoms of crown
thio-ethers in polymeric matrices involve the 4f electrons of
the mercury ion and the 2p lone pairs of the S atoms.56
Contrarily to the behaviour in MeCN, addition of Fe3+ in
H2O did not modify the spectral properties of TCMA. The
absence of interactions with TCMA can be ascribed to a
competition between the hydration and the complexation
processes of Fe3+ in the aqueous medium. Being this ion of
hard type, it is strongly solvated by water (more polar than
MeCN) and the interaction with TCMA becomes negligible.
The lowest excited singlet state of TCMA/Mn+ complexes
emits with low quantum yield (fF = 0.0023–0.014) and decays
with time constants much longer than TCMA itself, at least in
the case of TCMA/Hg2+ where it was possible to carry out
time-resolved experiments with fs resolution.
Fig. 7 Pump–probe time-resolved absorption spectra of TCMA/
Hg2+ (0.001 M) in MeCN recorded (A) 1, 2, 4.5 and 10 ps and (B)
10, 20, 30, 40, 50, 70 and 250 ps after the laser pulse (lexc = 400 nm).
(C) Amplitudes of the decay components of TCMA/Hg2+ (0.003 M) in
MeCN obtained by Global Analysis.
The high photochemical reactivity of the TCMA–Fe3+
system upon irradiation with fs laser pulses prevented reliable
kinetic and spectroscopic information to be obtained in MeCN.
Conclusions
The absorption spectrum of TCMA in a polar solvent is
formed by a broad Vis band, due to the S0 - S1 transition
responsible for the violet colour of the solution and two
structured bands due to the S0 - S2 and S0 - S3
transitions, characteristic of the methylacridinium moiety, at
shorter wavelengths. The absorption in the Vis region is
characterized by
a
charge-transfer nature with the
methylacridinium moiety acting as the electron-acceptor and
the anilic group as the electron-donor. No emission from the S1
was detected both in MeCN and H2O, while a small S2 - S0
fluorescence emission (lmax = 485 nm and fF = 0.0011) was
detected in the aqueous medium. Time-resolved measurements
with fs resolution of TCMA in MeCN have shown that the
relaxed S1 state is reached B0.6 ps after the laser pulse, while
the S1 - S0 time constant (associated to the internal
conversion process) is 3.7 ps.
Finally, this study showed the high selectivity of TCMA
towards metal ions, the property which makes this compound
a good candidate for colorimetric/fluorimetric sensing of Hg2+
(classified as soft) in aqueous media.
The formation of complexes between metal ions and
the crown thio-ether TCMA was shown both by
spectrophotometric and spectrofluorimetric measurements.
In particular, among the investigated metal ions, only Fe3+
and Hg2+ were able to form stable complexes with TCMA in a
polar solvent. This was also shown by the switching of the
solution colour from violet to yellow, owing to the
Acknowledgements
The authors gratefully acknowledge the financial support of
the Ministero per l0Universita e la Ricerca Scientifica e
Tecnologica (Rome, Italy) and the University of Perugia
[PRIN 2008, n. 20088NTBKR] and the Fondazione Cassa di
Risparmio di Perugia.
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28 P. D. Perrin, W. L. F. Amarego and D. R. Perrin, Purification of
Laboratory Chemicals, Pergamon Press, Elmsford, 1980.
29 S. Guang, S. Yin, H. Xu, W. Zhu, Y. Gao and Y. Song, Dyes
Pigm., 2007, 73, 285–291.
30 S. Yoon, A. E. Albers, A. P. Wong and C. J. Chang, J. Am. Chem.
Soc., 2005, 127, 16030–16031.
Notes and references
1 B. Valeur and I. Leray, Coord. Chem. Rev., 2000, 205, 3–40.
2 A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem.
Rev., 1997, 97, 1515–1566.
3 J. F. Callan, A. P. de Silva and D. C. Magri, Tetrahedron, 2005, 61,
8551–8588.
4 K. Tainaka, R. Sakaguchi, H. Hayashi, S. Nakano, F. F. Liew and
T. Morii, Sensors, 2010, 10, 1355–1376.
31 R. E. Wolf, J. R. Artman, J. M. E. Storey, B. M. Foxman and
S. R. Cooper, J. Am. Chem. Soc., 1987, 109, 4328–4335.
32 R. M. G. Roberts, D. Ostovic and M. M. Kreevoy, Faraday
Discuss. Chem. Soc., 1982, 74, 257–265.
5 A. Minta, J. P. Y. Kao and R. Y. Tsien, J. Biol. Chem., 1989, 264,
8171–8178.
33 S. A. Jonker, F. Ariese and J. W. Verhoeven, Recl. Trav. Chim.
Pays-Bas, 1989, 108, 109–115.
6 H. Komatsu, T. Miki, D. Citterio, T. Kubota, Y. Shindo,
Y. Kitamura, K. Oka and K. Suzuki, J. Am. Chem. Soc., 2005,
127, 10798–10799.
34 M. Montalti, A. Credi, L. Prodi and M. T. Gandolfi, Handbook of
Photochemistry, 3rd edn, (and references therein), CRC Press, Boca
Raton, FL, 2006.
7 X. Dong, Y. Yang, J. Sun, Z. Liu and B.-F. Liu, Chem. Commun.,
2009, 3883–3885.
35 A. Barbafina, M. Amelia, L. Latterini, G. G. Aloisi and F. Elisei,
J. Phys. Chem. A, 2009, 113, 14514–14520.
8 M. Royzen, Z. Dai and J. W. Canary, J. Am. Chem. Soc., 2005, 127,
1612–1613.
36 T. DelGiacco, B. Carlotti, S. DeSolis, A. Barbafina and F. Elisei,
Phys. Chem. Chem. Phys., 2010, 12, 8062–8070.
9 M. Yu, M. Shi, Z. Chen, F. Li, X. Li, Y. Gao, J. Xu, H. Yang,
Z. Zhou, T. Yi and C. Huang, Chem.–Eur. J., 2008, 14, 6892–6900.
10 M. Dong, T.-H. Ma, A.-J. Zhang, Y.-M. Dong, Y.-W. Wang and
Y. Peng, Dyes Pigm., 2010, 87, 164–172.
11 H. S. Jung, P. S. Know, J. W. Lee, J. I. Kim, C. S. Hong, J. W. Kim,
S. Yan, J. Y. Lee, J. H. Lee, T. Joo and J. S. Kim, J. Am. Chem.
Soc., 2009, 131, 2008–2012.
12 E. M. Nolan and S. J. Lippard, J. Am. Chem. Soc., 2003, 125,
14270–14271.
13 S.-K. Ko, Y.-K. Yang, J. Tae and I. Shin, J. Am. Chem. Soc., 2006,
128, 14150–14155.
37 A. Barbafina, L. Latterini, B. Carlotti and F. Elisei, J. Phys. Chem.
A, 2010, 114(19), 5980–5984.
38 G. H. Golub and C. F. V. Loan, Matrix Computations, 2nd edn,
Johns Hopkins University Press, Baltimore, 1989.
39 G. Strang, Introduction to linear algebra, Wellesley-Cambridge
Press, Wellesley, MA, USA, 1998.
40 H. Gorner, F. Elisei and G. G. Aloisi, J. Chem. Soc., Faraday
Trans., 1992, 88, 29–34.
41 A. Romani, F. Elisei, F. Masetti and G. Favaro, J. Chem. Soc.,
Faraday Trans., 1992, 88, 2147–2154.
42 M. C. Zerner, Semiempirical molecular orbital methods,in Reviews
in Computational Chemistry, ed. K. B. Lipkowitz and D. B. Boyd,
VCH, New York, 1991, Vol. 2, pp. 313–365.
14 S. Yoon, E. W. Miller, Q. He, P. H. Do and C. J. Chang, Angew.
Chem., Int. Ed., 2007, 46, 6658–6661.
15 X. Zhang, Y. Shiraishi and T. Hirai, Tetrahedron Lett., 2008, 49,
4178–4181.
16 P. D. Selid, H. Xu, E. M. Collins, M. S. Face-Collins and
J. X. Zhao, Sensors, 2009, 9, 5446–5459.
17 J. Fan, K. Guo, X. Peng, J. Du, J. Wang, S. Sun and H. Li, Sens.
Actuators, B, 2009, 142, 191–196.
18 E. M. Nolan, M. E. Racine and S. J. Lippard, Inorg. Chem., 2006,
45, 2742–2749.
19 S. Yoon, A. E. Alberts, A. P. Wong and C. J. Chang, J. Am. Chem.
Soc., 2005, 127, 16030–16031.
20 M. Tian and H. Ihmels, Chem. Commun., 2009, 3175–3177.
21 K. Kiyose, H. Kojima, Y. Urano and T. Nagano, J. Am. Chem.
Soc., 2006, 128, 6548–6549.
22 C. J. Chang, J. Jaworski, E. M. Nolan, M. Sheng and S. J. Lippard,
Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 1129–1134.
23 E. Murphy, J. H. Bassett and G. R. Williams, Practitioner, 2006,
250, 4–6.
24 P. G. Georgopoulos, A. Roy, M. J. Yonone–Lioy, R. E. Opiekun
and P. J. Lioy, J. Toxicol. Environ. Health, Part B, 2001, 4,
341–394.
43 J. J. P. Stewart, J. Comput. Chem., 1989, 10, 209–220.
44 J. J. P. Stewart, J. Comput. Chem., 1989, 10, 221–264.
45 S. Fukuzumi and T. Tanaka, in Photoinduced Electron Transfer, ed.
M. A. Fox and M. Chanon, Elsevier, Amsterdam, 1988, Part C, p. 578.
46 T. L. Macdonald, W. G. Guteim, R. B. Martin and
F. P. Guengerich, Biochemistry, 1989, 28, 2071–2077.
47 T. G. Beaumont and K. M. C. Davis, Nature, 1970, 225, 632.
48 S. A. Jonker, F. Ariese and J. W. Verhoeven, Recl. Trav. Chim.
Pays-Bas, 1989, 108, 109–115.
49 H. K. Frensdorff, J. Am. Chem. Soc., 1971, 93, 600–606.
50 R. D. Braun, Introduction to Chemical Analysis, McGraw-Hill,
1982.
51 J. H. Chang, Y. M. Choi and Y. K. Shin, Bull. Korean Chem. Soc.,
2001, 22, 527–530.
52 J. H. Chang, Y. Jeong and Y. K. Shin, Bull. Korean Chem. Soc.,
2003, 24, 119–122.
53 J. L. Bricks, A. Kovalchuk, C. Trieflinger, M. Nofz, M. Buschel,
A. I. Tolmachev, J. Daub and K. Rurack, J. Am. Chem. Soc., 2005,
127, 13522–13529.
54 Y. Xiang and A. Tong, Org. Lett., 2006, 8, 1549–1552.
55 T. H. Wirth and N. Davidson, J. Am. Chem. Soc., 1964, 86,
4314–4318.
56 A. J. Nelson, J. G. Reynolds, T. F. Baumann and G. A. Fox, Appl.
Surf. Sci., 2000, 167, 205–215.
25 R. von Burg, J. Appl. Toxicol., 1995, 15, 483–493.
26 T. W. Clarkson, L. Magos and G. J. Myers, N. Engl. J. Med., 2003,
349, 1731–1737.
27 J.-Y. Koh, Mol. Neurobiol., 2001, 24, 99–106.
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 2188–2195 2195