Ultra-sensitive and selective Hg2+ chemosensors derived from substituted 8-hydroxyquinoline analogues

Article abstract of DOI:10.1039/c3nj01308a

Novel analogues of 8-hydroxyquinoline with phosphinate or thiophosphinate functions and styryl fluorophores in the para position to the nitrogen atom were prepared via multi-step syntheses, using phosphorylation and Wittig coupling reactions. A strong affinity between the quinoline analogues and heavy metal ions such as Pb²⁺, Cd²⁺ and Hg²⁺ was highlighted. The interaction of the metal ions with the nitrogen of the styrylquinoline leads to a large red shift of the absorption and emission spectra in agreement with an increase of the photoinduced charge transfer character of the styryl fluorophore. In the presence of metal ions the appearance of a green fluorescence emission is also observed upon excitation at 420 nm or 840 nm, thanks to a significant increase of the two-photon response. Under optimal conditions, a mercury concentration of 15 ppt in a partially aqueous medium can be detected using the thiophosphinate derivative without interference from other metal ions.

Full text of DOI:10.1039/c3nj01308a

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Ultra-sensitive and selective Hg2+ chemosensors
derived from substituted 8-hydroxyquinoline
analogues†
Cite this: New J. Chem., 2014,
38, 1072
Jeremy Bell,a Issa Samb,b Patrick Y. Toullec,b Olivier Mongin,‡c
´ ´
Mireille Blanchard-Desce,*cd Veronique Michelet*b and Isabelle Leray*a
´
Novel analogues of 8-hydroxyquinoline with phosphinate or thiophosphinate functions and styryl
fluorophores in the para position to the nitrogen atom were prepared via multi-step syntheses, using
phosphorylation and Wittig coupling reactions. A strong affinity between the quinoline analogues and
heavy metal ions such as Pb2+, Cd2+ and Hg2+ was highlighted. The interaction of the metal ions with
the nitrogen of the styrylquinoline leads to a large red shift of the absorption and emission spectra in
agreement with an increase of the photoinduced charge transfer character of the styryl fluorophore. In
the presence of metal ions the appearance of a green fluorescence emission is also observed upon
excitation at 420 nm or 840 nm, thanks to a significant increase of the two-photon response. Under
optimal conditions, a mercury concentration of 15 ppt in a partially aqueous medium can be detected
using the thiophosphinate derivative without interference from other metal ions.
Received (in Montpellier, France)
22nd October 2013,
Accepted 12th December 2013
DOI: 10.1039/c3nj01308a
www.rsc.org/njc
sensitivity and selectivity it offers.3 Some examples described
the use of chemodosimeters which have some drawbacks
Introduction
The design of sensors for the detection of heavy-metal ions has because of their irreversibility.4 However, among fluorescent
been a field of continuing research since the control of water molecular sensors currently available, only a few are competitive
resources and natural environments has become a major in terms of sensitivity and selectivity, while direct measurements
challenge worldwide.1 Among heavy metal ions, lead, nickel, in aqueous medium remain a crucial issue.5 The development of
cadmium and mercury salts specifically need to be detected,2 the sensitive and selective fluorescent sensors is thus a subject of
last two being classified among ‘‘priority hazardous substances’’, constant interest. In the course of our recent ongoing program
toxic, persistent and bioaccumulative chemicals. The Hg2+ ion is directed toward the synthesis of novel fluorophores6 and on the
considered to be highly toxic causing environmental and health basis of recent work on 8-hydroxyquinoline (8-HQ) analogues,7
problems.1,2 The level of this ion is therefore subjected to strict we designed novel sensors for the detection of Hg2+ and wish to
regulations and should presently not exceed 1 mg LÀ1 2
. Apart describe herein their synthesis, photophysical and complexing
from analytical techniques (atomic absorption or emission properties.
spectroscopy and inductively coupled plasma spectroscopy),
the methodology based on fluorescent molecular sensors has
Results and discussion
Synthesis
attracted considerable interest due to the superior sensing
a
´
PPSM, ENS Cachan, CNRS, 61, avenue du President Wilson 94230 Cachan,
We chose the 8-hydroxyquinoline moiety as one of the most
important chelators of metal ions8 and anticipated that the
introduction of a phosphane oxide or thiophosphinate moiety
would enhance the selectivity towards mercury ions. We there-
fore engaged in the synthesis of thiophosphinates 8-HQ-4-St-PS
4 and 8-HQ-4-St-PO 3 (Scheme 1), the arylvinylidene side chain
being selected in order to shift the fluorescence emission in the
visible region (as well as excitation in the near UV region) for
easier implementation in on-field devices.
France. E-mail: isabelle.leray@ppsm.ens-cachan.fr; Fax: +33 147402454
b Institut de Recherche de Chimie Paris, UMR CNRS 8247 - Chimie ParisTech,
11 rue Pierre et Marie Curie, 75005 Paris, France.
E-mail: veronique.michelet@chimie-paristech.fr; Fax: +33 144071062
c
´
Chimie et Photonique Moleculaires, CNRS (UMR 6510), Univ. Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France
d
´
Univ. Bordeaux, ISM, (UMR 5255), 351 cours de la Liberation, 33405 Talence,
France. E-mail: mireille.blanchard-desce@u-bordeaux1.fr; Fax: +33 540006735
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj01308a
The synthesis of model 8HQ-PO 2 was straightforward
starting from 8HQ in the presence of sodium hydride and
‡ Institut des Sciences Chimiques de Rennes, CNRS (UMR 6226), Univ. Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France.
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Scheme 1 Hydroxyquinoline fluorescent sensors.
Scheme 2 Synthesis of chemosensors 8-HQ-4-St-PS 3 and 8-HQ-4-St-
PO 4.
diphenylphosphanoyl chloride at 0 1C in THF.9 The introduc-
tion of the donating vinylidene groups required a multistep
synthesis starting from the known aldehyde-substituted
8-hydroxyquinoline 5 (Scheme 2).10 The aldehyde 5 was phos-
phorylated with diphenylthiophosphanyl chloride11 in good
yield. The corresponding phosphorous derivative 6 was then
engaged in a Wittig reaction in the presence of the phospho-
nium salt 8.12 The Z isomer13 8-HQ-4-St-PS 3 was isolated in
64% yield after an isomerization reaction in the presence of
iodine. A similar synthesis was accomplished for the prepara-
tion of 8-HQ-4-St-PO 4 and led to the desired sensor in 38%
yield over two steps.
Fig. 1 (a) Absorption of quinoline derivatives in CH3CN. (b) Corrected
normalised emission spectra of the quinoline derivatives in CH3CN.
carbonyl derivatives,7a the proximity of the phosphoryl oxygen
(sulfur) lone pair to the acceptor nitrogen of the fluorophores
induces
a radiationless deactivation process through a
3(n–p)* state.
Cation-induced photophysical changes
The complexation of the phosphane oxide and the phos-
phane sulfide derivatives was investigated in the presence
of heavy metal ions in acetonitrile. The complexation of
mercury salts with the phosphane oxide derivative 8HQ-PO
2 (Fig. 2) was studied in acetonitrile with a ligand concen-
tration of 22 mM upon addition of increasing aliquots of
mercury perchlorates. As expected, a red shift of the absorp-
tion band from 332 nm to 380 nm occurred, confirming the
role of the quinoline nitrogen atom in the coordination of
the metal ion.
Photophysical properties of the ligands
The absorption and fluorescence properties of the quinoline
derivatives are collected in Table 1, and their absorption and
emission spectra are shown in Fig. 1(a) and (b), respectively.
The fluorophores show an absorption band in the UV region.
The phosphane oxide derivative and the carbonyl compound
have comparable extinction coefficients. As expected the intro-
duction of a styryl derivative in the para position of the quino-
line derivative induces a noticeable red-shift of the absorption
and the emission spectra due to the intramolecular charge
transfer character of the transition. As previously described for the
Therefore, when using an excitation wavelength of 337 nm,
an emission band appears at 490 nm and a decrease of the
fluorescence is observed for the higher concentrations. The
decrease of the fluorescence intensity for larger concentration
of mercury can be explained by stronger interaction between
Table 1 Photophysical properties of quinoline derivatives in CH3CN.
Maxima of the one-photon absorption labs [nm] and of steady-state
emission lem (nm), the molar absorption coefficient e (MÀ1 cmÀ1), and the fluorophore and the mercury and by a possible electron
the fluorescence quantum yield FF
transfer process. The analysis of the evolution of the spectra
using SPECFIT software revealed that different complexes are
formed with the following different stoichiometries 1 : 1 and
1 : 2 (ML and ML2) (Fig. 2).
The stability constants derived for the phosphane oxide
derivatives and the phosphane sulfide derivatives are given in
Table 2. For the 8HQ-4St-PO 3 (Fig. 3), the addition of mercury
induces a larger photophysical effect in relation to an efficient
max
max
Product
l
abs
(nm) emax (MÀ1 cmÀ1
)
l
(nm) FF
em
8HQ
308
275
290
2400
4700
5000
7600
7600
393
385
396
461
459
3.8 Â 10À3
8HQ-COa
8HQ-PO
7.6 Â 10À4
2.7 Â 10À3
2.6 Â 10À3
2.6 Â 10À3
8HQ-4St-PO 332
8HQ-4St-PS
342
a
Ref. 7a.
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Fig. 3 (a) Absorption and (b) corrected emission spectra of 8HQ-4St-PO
3 in the presence of increasing concentrations of Hg2+ in CH3CN (CL
22 mM, lexc = 420 nm).
=
Fig. 2 (a) Absorption and (b) corrected emission spectra of 8HQ-PO 2 in
the presence of increasing concentrations of Hg2+ in CH3CN (CL = 22 mM,
lexc = 335 nm).
that for compounds 3 and 4 only the 1 : 1 stoichiometry complex is
observed because of the steric hindrance due to the incorporation
of the styryl conjugated group.
Table 2 Stability constants of quinoline derivatives with Pb2+, Cd2+ and
Hg2+ in CH3CN
8HQ-COa 8HQ-PO
8HQ-4St-PO 8HQ-4St-PS
Two-photon absorption
log bML log bML log bML2 log bML
log bML log bML2
In addition to standard one-photon excitation, we also investigated
the potential use of two-photon excitation (TPE) for selective
excitation of the probe. There has been a lot of efforts in designing
bright biphotonic fluorescent probes14,15 in the last decade mainly
driven by the advantages TPE provides for biological imaging.16 In
the field of fluorescent probes, TPE could offer an additional
Hg2+ 4.5 Æ 0.7 7.0 Æ 0.2 12.6 Æ 0.2 5.8 Æ 0.5
7.5 Æ 0.5 13.5 Æ 0.7
4.7 Æ 0.1 11.2 Æ 0.3
5.2 Æ 0.2 —
Pb2+ 1.2 Æ 0.2 6.7 Æ 0.2 12.0 Æ 0.3 6.3 Æ 0.2
Cd2+
—
—
—
6.6 Æ 0.2
a
Ref. 7a.
photoinduced intramolecular charge transfer phenomenon. benefit by allowing selective excitation in the presence of fluor-
The absorption band shifts from 345 nm to 410 nm in the escent contaminants. We have investigated the TPE response of
presence of mercury. Direct emission of the complex at 420 nm the thiophosphinate 8-HQ-4-St-PS 4 for mercury detection, expect-
induces the formation of a non-structured band at 560 nm ing that only the complex would show a significant TPE response
(Fig. 3). For the phosphane sulfide derivative 8HQ-4St-PS 4 in the particular spectral range of interest. This indeed proved to
(Fig. 4), similar photophysical effects have been observed upon be the case (Fig. 5a), as pure 8-HQ-4-St-PS 4 did not show
complexation of heavy metal ions. In comparison to the results detectable fluorescence when two-photon excited in the same
obtained by Jiang’s group,7a the higher stability constants range. Taking advantage of the large contrast between the TPE
obtained here confirm that the introduction of a phosphane oxide response of 8-HQ-4-St-PS 4 and of its mercury complex and of the
or a phosphane sulfide function increases the ability of the probe to high sensitivity and affinity for mercury of the 8-HQ-4-St-PS 4, we
coordinate heavy metal ions. Because of the high affinity of the further demonstrated that monitoring fluorescence by two-photon
phosphane sulfide derivative for mercury, the highest complexation excitation at an optimum wavelength (typically in the 800–900 nm
constant was observed for the 8HQ-4St-PS 4. It should be noticed range) indeed allowed monitoring Hg2+ concentration (Fig. 5b).
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Scheme 3 Equilibrium involved upon complexation with Hg2+
.
than the mercury complex 4ÁHg2+, we optimized the sensing
experimental conditions (pH = 3) to exploit the replacement of a
proton by mercury to ensure larger fluorescence variations, and
consequently improved sensitivity (Scheme 3).18
As previously observed, the addition of mercury induced a
bathochromic shift of the absorption band, and quenching of the
fluorescence was observed in relation to the replacement of the
proton by mercury (Fig. 6). Scheme 3 outlines the equilibrium
involved upon complexation with mercury under these operating
conditions. Taking advantage of this highly efficient quenching
process, a calibration curve can be established. The measured
detection limit was found to be 0.5 nanomol per liter, and
the theoretical detection limit, calculated to be three times the
standard deviation of the background/noise ratio, was found to be
0.075 nanomol per liter (15 ppt).
Fig. 4 (a) Absorption and (b) corrected emission spectra of 8HQ-4St-PS 4
in the presence of Hg2+ in CH3CN (CL = 15 mM, lexc = 350 nm).
The selectivity of the probe was also tackled. As shown in
Fig. 7, no significant change in the fluorescence was observed
upon addition of high concentrations of interfering cations such
as Li+, Na+, Mg2+, K+, Ca2+, Mn2+, Fe2+, Co2+, Ni+, Cu2+, Zn2+, Ag+,
Cd2+ and Pb2+ at 1 or 0.1 mM. Moreover, after addition of 1
equiv. of Hg2+, the fluorescence was still quenched immediately,
indicating that the presence of other cations does not affect the
sensing process and its sensitivity.
Fig. 5 (a) Two-photon absorption of 8-HQ-4-St-PS 4 in the presence of
an excess of Hg2+ in CH3CN (CL = 10À4 M, CHg(ClO = 10À3 M). No TPEF
)
4 2
signal is observed in the absence of Hg2+. (b) Fluorescence signal of
8-HQ-4St-PS 4 in the presence of increasing concentrations of Hg2+ in
CH3CN (two-photon excitation at 840 nm) CL = 10À4 M.
Practical determination of mercury in water
In view of practical applications, complexation studies were
then carried out in a partially aqueous medium of 40/60
CH3CN–H2O. Titration experiments show that 8-HQ-4St-PS 4
was able to coordinate mercury in aqueous medium (see ESI†).
Analysis of the spectra revealed that under these conditions a
1 : 1 complex is formed with mercury with log K11 = 3.8 Æ 0.1. In
CH3CN : H2O (40 : 60 v/v),17 the pKa of 8-HQ-4St-PS 4 was
measured to be 3.23 Æ 0.02, which is lower than the complexa-
tion constant of 8-HQ-4St-PS 4 with mercury. In addition, the
quantum yield of the protonated form 4ÁH+ being 10 fold higher
Fig. 6 Corrected emission spectra of 8-HQ-4-St-PS 4 in the presence of
increasing concentrations of Hg2+ in 40 : 60 v/v CH3CN–H2O (CL = 13 mM,
lexc = 420 nm).
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Syntheses of 8-hydroxyquinoline analogues
Thiophosphinate 6. To a solution of 4-formyl-8-hydroxy-
quinoline 5 (300 mg, 1.7 mmol) in tetrahydrofuran (10 mL) at
0 1C was added 60% w/w NaH in mineral oil (69 mg, 1.7 mmol).
The resulting mixture was stirred for 15 minutes at 0 1C, then
diphenylthiophosphinic chloride (429 mg, 1.7 mmol) was
added dropwise. The mixture was stirred for 3 h at room
temperature. The mixture was quenched with water and
extracted with dichloromethane (2 Â 20 mL). The organic phase
was dried over MgSO4 and the volatiles removed under vacuum.
The residue was purified by flash silica gel chromatography
(cyclohexane–ethyl acetate 80/20) to give 450 mg (71%) of the
Fig. 7 (light gray) Response IF/I0 of 8-HQ-4-St-PS 4 (13 mM) in CH3CN–
H2O (40 : 60 v/v) at pH = 3 plus interfering ions Li+, Mg2+, Ca2+, Co2+, Zn2+
and Cd2+ at 1 mM, and Na+, K+, Mn2+, Fe2+, Ni+, Cu2+, Ag+ and Pb2+ at
0.1 mM (lexc = 420 nm, lem = 565 nm). (dark gray) idem in the presence of
Hg2+ (13 mM).
1
product. H NMR (300 MHz, CDCl3): d = 10.47 (s, 1H), 9.18 (d,
1H, J = 4.2 Hz), 8.76 (d, 1H, J = 7.3 Hz), 8.22–8.14 (m, 4H), 7.79
(d, 1H, J = 4.2 Hz), 7.65–7.56 (m, 1H), 7.51–7.46 (m, 7H) 13C
NMR (75.5 MHz, CDCl3): d = 192.6, 150.0, 147.3, 136.5, 135.2,
133.7, 132.1, 131.4 (d, J = 11.7 Hz), 129.0, 128.4 (d, J = 13.8 Hz),
126.2, 125.2, 121.1 (d, J = 5.7 Hz), 120.8. 31P (121.5 MHz, CDCl3):
d = 84.2. HRMS (ESI+): m/z: calcd for C90H94O4P: 1269.6884;
found: 1269.6937 [M + H]+.
Conclusion
In conclusion, we have synthesized a series of fluorescent
chemosensors based on hydroxyquinoline bearing chalcogenic
phosphanes such as phosphane oxide and sulfide derivatives.
These original derivatives showed an intense absorption band
in the near-UV region and a fluorescence emission in the visible
region. The large Stokes shift value and the unresolved vibronic
structure of the fluorescence spectra strongly support the
intramolecular charge transfer (ICT) character of the low-
energy transition, which is thus expected to be highly sensitive
to complexation by a cationic species. Indeed, addition of a
heavy metal cation such as Hg2+ induced a bathochromic shift
of the absorption and emission spectra thanks to the increase of
the electron-withdrawing character of the complexed quinoline
moiety. In view of practical application, the cation complexation
was studied in a CH3CN–H2O mixture under optimized pH
conditions (pH = 3) and the presence of mercury cations induced
a major fluorescence quenching which allowed reaching a
detection limit in the picomolar range (75 pM–15 ppt). This
outstandingly low detection limit does therefore meet the
requirements defined by the World Health Organization for
drinking water. Further efforts will be dedicated to insert the
fluorescent sensor in a microfluidic chip.
Thiophosphinate 4. To a solution of thiophosphinate 6 (100 mg,
0.25 mmol) and phosphonium salt 8 (154 mg, 0275 mmol) in
tetrahydrofuran (1 mL) at 0 1C were added 60% w/w NaH in
mineral oil (20 mg, 0.50 mmol) and 18-crown-6 (13.2 mg,
0.05 mmol). The resulting mixture was stirred at room temperature
for 2 hours, then quenched with water and extracted with dichloro-
methane (2 Â 10 mL). The organic phase was dried over MgSO4
and the volatiles removed under vacuum. The crude product was
redissolved in dichloromethane (5 mL) and a catalytic amount of
iodine was added. The solution was stirred at room temperature for
3 h under light exposure then washed with saturated aqueous
Na2S2O3 and dried over MgSO4. The residue was purified by flash
silica gel chromatography (cyclohexane–ethyl acetate 80/20 then
1
70/30) to give 94 mg (64%) of the product. H NMR (300 MHz,
CDCl3): d = 8.79 (d, 1H, J = 3 Hz), 8.20–8.27 (m, 4H), 7.80 (d, 1H,
J = 6.2 Hz), 7.64 (d, 1H, J = 6.2 Hz), 7.40–7.55 (m, 7H), 7.35–7.27
(m, 3H), 6.91 (d, 1H, J = 9 Hz), 6.85 (d, 2H, J = 12.4 Hz), 6.76 (d, 2H,
J = 12.4 Hz), 6.60 (d, 1H, J = 9 Hz), 3.85 (t, 2H, J = 6.6 Hz) 1.65–1.72
(m, 2H), 1.25–1.43 (m, 10H), 0.87 (t, 3H, J = 6.7 Hz). 13C NMR
(75 MHz, CDCl3): d = 159.8, 149.9, 147.4, 144.4, 135.5, 133.8, 131.8
(d, J = 11.4 Hz), 130.4, 128.3 (d, J = 13.9 Hz), 128.0, 125.6, 123.2,
121.4, 120.1, 114.1, 67.9, 31.8, 29.3, 29.2, 26.0, 22.6, 14.1. 31P (121.5
MHz, CDCl3): d = 83.4. HRMS (ESI+): m/z: calcd for C39H37NO2PS:
592.2439; found: 592.2449 [M + H]+.
Experimental
General information: all reactions were performed under a dry
Phosphinate 7. To a solution of 4-formyl-8-hydroxyquinoline
argon atmosphere by using standard Schlenk techniques. Col- 5 (300 mg, 1.7 mmol) in tetrahydrofuran (10 mL) at 0 1C was
umn chromatography was performed with E. Merck 0.040– added 60% w/w NaH in mineral oil (69 mg, 1.7 mmol). The
1
0.063 mm Geduran silica gel. H NMR, 13C NMR and 31P NMR resulting mixture was stirred for 15 minutes at 0 1C, then
were recorded on Bruker AV 300 and 400 instruments. All signals diphenylphosphinic chloride (402 g, 1.7 mmol) was added
were expressed as ppm (d) and internally referenced to residual dropwise. The mixture was stirred for 3 h at room temperature.
protio solvent signals. Coupling constants ( J) are reported in Hz The mixture was quenched with water and extracted with dichloro-
and refer to apparent peak multiplicities. Mass spectrometry methane (2 Â 20 mL). The organic phase was dried over MgSO4
analyses were performed at IMAGIF/ICSN and at the University and the volatiles removed under vacuum. The residue was purified
P. et M. Curie. Compounds 510 and 811 were prepared according by flash silica gel chromatography (cyclohexane–ethyl acetate 80/20)
1
to published procedures. The preparation of diphenylthiopho- to give 450 mg (71%) of the product. H NMR (300 MHz, CDCl3):
sphanyl chloride was realized according to the literature.11
d = 10.49 (s, 1H), 9.27 (d, 1H, J = 4.1 Hz), 8.72 (d, 1H, J = 8.7 Hz),
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8.06–8.14 (m, 4H), 7.92 (dt, 1H, J = 7.7 Hz, J = 1.3 Hz), 7.82 (d, 1H, beam is focused by a microscope objective (10Â, NA 0.25,
J = 4.1 Hz), 7.57 (t, 1H, J = 8.5 Hz), 7.53–7.38 (m, 6H). 13C NMR Olympus, Japan) into a standard 1 cm absorption cuvette
(75 MHz, CDCl3): d = 192.6, 150.3, 132.5, 132.1 (d, J = 10.6 Hz), containing the sample. The applied average laser power arriving at
129.4, 128.5 (d, J = 13.5 Hz), 126.1, 120.6. 31P (121.5 MHz, CDCl3): the sample was between 0.5 and 15 mW, leading to a time-averaged
d = 32.1. HRMS (ESI+): m/z: calcd for C90H94O4P: 1269.6884; found: light flux in the focal volume on the order of 0.1–1 mW mmÀ2. The
1269.6937 [M + H]+.
generated fluorescence is collected in epi-fluorescence mode,
Phosphinate 3. To a solution of phosphinate 7 (100 mg, through the microscope objective, and reflected by a dichroic mirror
0.26 mmol) and phosphonium salt 8 (160 mg, 0286 mmol) in (675dcxru, Chroma Technology Corporation, USA). Residual excita-
tetrahydrofuran (1 mL) at 0 1C were added 60% w/w NaH in tion light is removed using a barrier filter (e650-2p, Chroma) and
mineral oil (21 mg, 0.52 mmol) and 18-crown-6 (13.7 mg, the fluorescence is coupled to a 600 mm multimode fiber by an
0.052 mmol). The resulting mixture was stirred at room tem- achromatic doublet. The fiber is connected to a compact CCD-based
perature for 2 hours, then quenched with water and extracted spectrometer (BTC112-E, B&WTek, USA), which measures the
with dichloromethane (2 Â 10 mL). The organic phase was two-photon excited emission spectrum. The emission spectra are
dried over MgSO4 and the volatiles removed under vacuum. The corrected for the wavelength-dependence of the detection
crude product was redissolved in dichloromethane (5 mL) and efficiency using correction factors established through the
a catalytic amount of iodine was added. The solution was measurement of reference compounds having known fluorescence
stirred at room temperature for 3 h under light exposure then emission spectra. Briefly, the set-up allows for the recording of
washed with saturated aqueous Na2S2O3 and dried over MgSO4. corrected fluorescence emission spectra under multiphoton excita-
The residue was purified by flash silica gel chromatography tion at variable excitation power and wavelength.
(cyclohexane–ethyl acetate 80/20 then 70/30) to give 80 mg
Absolute values for the two-photon excitation action cross
(53%) of the product. 1H NMR (300 MHz, CDCl3): d = 8.88 sections s2F were obtained according to the method described
(d, 1H, J = 3 Hz), 8.11–8.19 (m, 4H), 7.83 (d, 1H, J = 7.7 Hz), 7.77 by Xu & Webb, using 10À4 M fluorescein in 0.01 M NaOH(aq.) as
(d, 1H, J = 8.3 Hz), 7.40–7.65 (m, 8H), 6.89 (d, 2H, J = 8.6 Hz), a reference.21 4 was dissolved in CH3CN at a concentration of
6.84 (d, 1H, J = 12.0 Hz), 6.74 (d, 1H, J = 12.0 Hz), 6.59 (d, 2H, J = 3.8 Â 10À5 M. The complexation studies were performed by
8.7 Hz), 3.85 (t, 2H, J = 6.6 Hz) 1.69–1.76 (m, 2H), 1.25–1.44 (m, using different concentrations of mercury.
10H), 0.87 (t, 3H, J = 6.8 Hz). 13C NMR (75 MHz, CDCl3): d =
Solvents and salts. Acetonitrile from Aldrich (spectrometric grade)
158.8, 149.9, 144.7, 133.9, 132.2 (d, J = 10.6 Hz), 130.4, 128.4 and millipore filtered water (conductivity o6 Â 108 OÀ1 cmÀ1 at
(d, J = 13.5 Hz), 127.9, 126.2, 123.1, 121.4 (d, J = 14.9 Hz), 119.9, 20 1C) were employed as solvents for absorption and fluorescence
114.1, 67.9, 31.8, 29.3, 29.2, 26.0, 22.6, 14.1. 31P (121.5 MHz, measurements. Sodium thiocyanate, potassium thiocyanate,
CDCl3): d = 31.8. HRMS (ESI+): m/z: calcd for C25H30NO2: lithium perchlorate, magnesium perchlorate, calcium perchlorate,
376.2277; found: 376.2272 [M-C12H8OP]+. (ESI–): m/z: calcd for manganese perchlorate, iron perchlorate, cobalt perchlorate, nickel
C
12H10O2P: 217.0418; found: 217.0423 [M-C25H28NO]À.
Spectroscopic measurements. UV/Vis absorption spectra cadmium perchlorate and lead(II) thiocyanate, from Aldrich or Alfa
perchlorate, copper perchlorate, zinc perchlorate, silver perchlorate,
were recorded on a Varian Cary5000 spectrophotometer and Aesar, were of the highest quality available and vacuum dried over
corrected emission spectra were obtained on a Jobin–Yvon Spex P2O5 prior to use.
Fluorolog-3 spectrofluorimeter. The fluorescence quantum
yields were determined by using quinine sulfate dihydrate in
sulfuric acid (0.5 N; FF = 0.54619) as standards. For the emission
Acknowledgements
measurements, the absorbance at the excitation wavelengths was
below 0.1 and so the concentrations were below 10À5 mol LÀ1
.
This work was supported by the National Research Agency
The complexation constants were determined by global analysis program CP2D ‘‘FLUOSENSIL’’. I.S. and J.B. are grateful to
of the evolution of all absorption and/or emission spectra by the National Research Agency for their grants (2009–2011)
using the Specfit Global Analysis System V3.0 for 32-bit Windows and (2009–2012), respectively.
system. This software uses singular value decomposition and
non-linear regression modelling by the Levenberg–Marquardt
method.20
Notes and references
Two-photon excited fluorescence spectroscopy. Two-photon
excited fluorescence spectroscopy was performed using a mode
locked Ti:sapphire laser generating 150 fs wide pulses at a
76 MHz rate, with a time-averaged power of several hundreds
of mW (Coherent Mira 900 pumped by a 5 W Verdi). The laser
light is attenuated using a combination of half-wave plates and
a Glan-laser polariser and the excitation power is further
controlled using neutral density filters of varying optical
density mounted in a computer-controlled filter wheel. After
five-fold expansion through two achromatic doublets, the laser
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1078 | New J. Chem., 2014, 38, 1072--1078
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