Synthesis and evaluation of arylfuryl-bis(indolyl)methanes as selective chromogenic and fluorogenic ratiometric receptors for mercury ion in aqueous solution

Article abstract of DOI:10.1016/j.dyepig.2013.11.008

A series of arylfuryl-bis(indolyl)methane derivatives were prepared in good yields by electrophilic substitution of indole with furyl aldehydes through a simple and mild hydrogensulfate-catalysed reaction and studied as chemosensors for transition metal cations by performing spectrophotometric and spectrofluorimetric titrations. Selective recognition of Hg²⁺ was achieved in organic aqueous mixture (CH₃CN/H₂O, 7:3) for the various receptors, with an easily detectable colour change from colourless to purple and also through a fluorescence quenching, making these compounds suitable for dual chromo- and fluorogenic ratiometric sensing of Hg ²⁺. The binding stoichiometry between the receptors and Hg ²⁺ was found to be 1:1. The binding process was also followed by ¹H NMR titrations which corroborated the previous findings.

Full text of DOI:10.1016/j.dyepig.2013.11.008

Dyes and Pigments 102 (2014) 293e300
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Dyes and Pigments
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Synthesis and evaluation of arylfuryl-bis(indolyl)methanes as selective
chromogenic and fluorogenic ratiometric receptors for mercury ion in
aqueous solution
Rosa M.F. Batista, Susana P.G. Costa, Regina M.P. Silva, Nuno E.M. Lima,
*
M.Manuela M. Raposo
Centre of Chemistry, University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2013
Received in revised form
5 November 2013
Accepted 6 November 2013
Available online 14 November 2013
A series of arylfuryl-bis(indolyl)methane derivatives were prepared in good yields by electrophilic
substitution of indole with furyl aldehydes through a simple and mild hydrogensulfate-catalysed reac-
tion and studied as chemosensors for transition metal cations by performing spectrophotometric and
spectrofluorimetric titrations. Selective recognition of Hg2þ was achieved in organic aqueous mixture
(CH3CN/H2O, 7:3) for the various receptors, with an easily detectable colour change from colourless to
purple and also through a fluorescence quenching, making these compounds suitable for dual chromo-
and fluorogenic ratiometric sensing of Hg2þ. The binding stoichiometry between the receptors and Hg2þ
was found to be 1:1. The binding process was also followed by 1H NMR titrations which corroborated the
previous findings.
Keywords:
Bis(indolyl)methanes
Arylfuran
Ó 2013 Elsevier Ltd. All rights reserved.
Ratiometric fluorogenic chemosensors
Hg2þ
Direct visual detection
Aqueous solution
1. Introduction
are not suitable for on-site assays [2]. Therefore, an easy and
straightforward method that detects and quantifies Hg2þ is ad-
Chemosensors and chemodosimeters that can monitor heavy
metal ions with high sensitivity and selectivity are especially
important owing to the great concern with environment and
health. Among transition and heavy metals, mercury, from a variety
of natural and anthropogenic sources and widely distributed in
water, soil and air, is considered to be one of the most toxic because
both ionic and elemental mercury can be converted by bacteria in
the environment to methyl mercury, which subsequently bio-
accumulates through the food chain. Mercury causes serious
neurotoxic, genotoxic, and immunotoxic effects and thus poses
severe risk for human beings and other organisms. Therefore, se-
lective and sensitive detection of Hg2þ has been an important topic
of investigation [1]. Presently, several methods such as atomic ab-
sorption/emission spectroscopy, inductively coupled plasma-mass
spectrometry, voltammetry, etc. can be used in order to detect
Hg2þ. However, all these methods have drawbacks such as expen-
sive instrumentation, only measure the total metal ion content and
vantageous for instantaneous monitoring of environmental, bio-
logical, and industrial samples which occur in aqueous solution.
Consequently, much attention has been paid to developing low-
cost and easily-prepared colorimetric and/or fluorimetric mercury
sensors that work in the aqueous medium [1a,b,def,hem]. Hg2þ is
usually associated with a fluorescence quenching by different
mechanisms, so a ratiometric approach for its detection (simulta-
neous monitoring of fluorescence intensity at two different wave-
lengths) would be preferable, since it minimizes interferences from
other species present in the media [1b,i,l].
Although indoles are predominant in natural compounds and
widely applied in the synthesis of pharmaceutical products, they
were only recently recognized as useful building blocks for the
assembly of synthetic anionic receptors [3]. Among the several
indole-based systems, triaryl- and triheteroarylmethanes have
found wide range applications in different areas of chemistry, due
to their structure (presence of a pyrrolic NH group) and photo-
physical properties, such as chemical sensors [3e5] and as pre-
cursors in the synthesis of heterocyclic systems for molecular
electronics [6]. Therefore, there is a great deal of interest in the
synthesis of this class of compounds and while several indole-
* Corresponding author. Tel.: þ351 253 604381; fax: þ351 253 604382.
E-mail address: mfox@quimica.uminho.pt (M.M.M. Raposo).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.11.008

294
R.M.F. Batista et al. / Dyes and Pigments 102 (2014) 293e300
based receptors have been developed for the selective sensing and
detection of anions [3e5], only a few examples can be found in the
literature for cation receptors bearing an indole group [7].
Recently, the synthesis and evaluation of new bis-indolylme-
thenes containing thienyl and bithienyl moieties has been reported
were collected and evaporated under vacuum to afford product 1b
as an orange oil (82%). IR (liquid film):
¼ 1667, 1590, 1500, 1213,
n
1176,1112,1066,1020, 965, 830, 767 cmꢂ1. 1H NMR (CDCl3):
d
¼ 3.86
(s, 3H, OCH3), 6.96 (dd, 2H, J ¼ 9.0 and 2.1 Hz, H30 and H50), 7.29 (d,
1H, J ¼ 3.9 Hz, H4), 7.62 (dd, 2H, J ¼ 9.0 and 2.1 Hz, H20 and H60), 7.71
(d, 1H, J ¼ 3.9 Hz, H3), 9.86 (s, 1H, CHO) ppm.
by us in order to evaluate the effect of the length of the p-conjugated
bridge and also the electronic nature of the heterocycles on the
sensory properties of these chromophores [8]. Keeping in mind our
interests in the synthesis of colorimetric/fluorimetric chemosensors
for selective detection of cations and anions, we proceeded to syn-
thesize new non-oxidized bis-indolylmethane derivatives contain-
ing functionalized arylfuryl moieties and evaluate their
chemosensory ability. The main difference resides in the replace-
2.3. General procedure for the synthesis of bis(indolyl)methanes
2aed
KHSO4 (1.20 mmol) was added to
a mixture of indole
(2.40 mmol) and the corresponding aldehyde 1aed (1.20 mmol) in
dry methanol (10 mL), and the reaction was stirred at room tem-
perature for 7 h. Then water (10 mL) was added to quench the re-
action, and the aqueous phase was extracted with CHCl3 (3 ꢁ 20 mL).
The organic phase was dried with anhydrous MgSO4, and the crude
compounds 2 were purified by recrystallization from CHCl3.
ment of the
p-conjugated bi(thiophene) spacer by an arylfuryl
system (instead of the more commonly used aryl group), allowing a
more sensitive fluorimetric detection due to the higher fluorescent
character of the heteroaromatic furan when compared to thiophene
[9]. On the other hand, the different electronic nature of the sub-
stituents at the arylfuryl group was used to tune the photophysical
properties of the resulting compounds. These non-conjugated bis-
indolylmethane derivatives with an sp3 carbon at meso position may
even promote more significant colour changes when compared
with the previous examples, probably because the interaction with
cations can occur through a variation in carbon hybridization with
concomitant modulation of the internal charge transfer (ICT) state.
2.3.1. 3-((1H-Indol-3-yl)(50-phenylfuran-20-yl)methyl)-1H-indole
(2a)
Pink solid (50%). Mp: 192.0e193.0 ꢀC. IR (Nujol)
n
¼ 3412, 3055,
2955, 2925, 1610, 1594, 1544, 1419, 1336, 1214, 1203, 1090, 1018, 789,
763, 745 cmꢂ1. 1H NMR (acetone-d6)
d
¼ 6.09 (s, 1H, CH), 6.23 (d, 1H,
J ¼ 3.3 Hz, H30), 6.79 (d, 1H, J ¼ 3.3 Hz, H40), 6.98 (dt, 2H, J ¼ 7.6 and
1.4 Hz, 2 ꢁ H5), 7.11 (dt, 2H, J ¼ 7.6 and 1.4 Hz, 2 ꢁ H6), 7.17 (d, 2H,
J ¼ 2.1 Hz, 2 ꢁ H2), 7.24 (m,1H, H400), 7.38e7.42 (m, 2H, H300 and H500),
7.44 (dt, 2H, J ¼ 7.5 and 1.4 Hz, 2 ꢁ H7), 7.59 (d, 2H, J ¼ 7.5 Hz, 2 ꢁ H4),
7.68 (d, 2H, J ¼ 8.3 Hz, H200 and H600), 10.11 (s, 2H, 2 ꢁ NH). 13C NMR
2. Experimental
2.1. Synthesis general
(acetone-d6)
d
¼ 35.2 (CH), 106.7 (C40), 109.2 (C30), 112.2 (2 ꢁ C7),
117.2 (2 ꢁ C3),119.4 (2 ꢁ C5),120.2 (2 ꢁ C4),122.1 (2 ꢁ C6),124.0 (C200
and C600), 124.2 (2 ꢁ C2), 127.6 (C400), 127.8 (2 ꢁ C3a), 129.5 (C300 and
C500), 132.2 (C100), 137.9 (2 ꢁ C7a), 153.0 (C50), 158.8 (C20). MS (EI) m/z
(%): 388 (Mþ, 42), 283 (100), 273 (23), 168 (20), 117 (21), 77 (13).
HRMS: (EI) m/z (%) for C27H20N2O; calcd 388.1576; found 388.1577.
Reaction progress was monitored by thin layer chromatography
(0.25 mm thick precoated silica plates: Merck Fertigplatten Kie-
selgel 60 F254), while purification was effected by silica gel column
chromatography (Merck Kieselgel 60; 230e400 mesh). NMR
spectra were obtained on a Varian Unity Plus Spectrometer at an
operating frequency of 300 MHz for 1H and 75.4 MHz for 13C or a
Bruker Avance III 400 at an operating frequency of 400 MHz for 1H
and 100.6 MHz for 13C using the solvent peak as internal reference.
The solvents are indicated in parenthesis before the chemical shift
2.3.2. 3-((1H-Indol-3-yl)(50-(400-methoxyphenyl)furan-20-yl)
methyl)-1H-indole (2b)
Purple solid (40%). Mp: 115.0e116.0 ꢀC. IR (Nujol)
n
¼ 3411, 2920,
1616, 1598, 1547, 1498, 1420, 1295, 1249, 1176, 1094, 1021, 967, 832,
782, 743 cmꢂ1. 1H NMR (acetone-d6)
values (
d
relative to TMS and given in ppm). Mps were determined
d
¼ 3.82 (s, 3H, OCH3), 6.06 (s,
on a Gallenkamp apparatus. Infrared spectra were recorded on a
BOMEM MB 104 spectrophotometer. Mass spectrometry analyses
were performed at the “C.A.C.T.I. -Unidad de Espectrometria de
Masas” at the University of Vigo, Spain. Fluorescence spectra were
collected using a FluoroMax-4 spectrofluorometer. UVevisible ab-
sorption spectra (200e700 nm) were obtained using a Shimadzu
UV/2501PC spectrophotometer. Luminescence quantum yields
were measured using quinine sulphate in 0.5 M sulphuric acid
solution as standard (FF ¼ 0.54) [10]. All commercially available
reagents were used as received.
1H, CH), 6.18 (d, 1H, J ¼ 3.3 Hz, H30), 6.61 (d, 1H, J ¼ 3.3 Hz, H40),
6.94e7.00 (m, 4H, H300, H500 and 2 ꢁ H5), 7.10 (dt, 2H, J ¼ 7.8 Hz,
2 ꢁ H6), 7.15 (d, 2H, J ¼ 2.4 Hz, 2 ꢁ H2), 7.42 (d, 2H, J ¼ 7.8 Hz,
2 ꢁ H7), 7.57e7.63 (m, 4H, H200, H600 and 2 ꢁ H4), 10.10 (s, 2H,
2 ꢁ NH). 13C NMR (acetone-d6)
d
¼ 35.2 (CH), 55.5 (OCH3), 104.9
(C40), 109.0 (C30), 112.2 (2 ꢁ C7), 114.9 (C300 and C500), 117.4 (2 ꢁ C3),
119.4 (2 ꢁ C5), 120.2 (2 ꢁ C4), 122.0 (2 ꢁ C6), 124.2 (2 ꢁ C2), 125.2
(C100), 125.5 (C200 and C600), 127.8 (2 ꢁ C3a), 137.9 (2 ꢁ C7a), 153.2
(C50),157.9 (C20), 159.8 (C400). MS (EI) m/z (%): 418 (Mþ, 37), 303 (27),
283 (100), 168 (13), 135 (18), 117 (21). HRMS: (EI) m/z (%) for
C
28H22N2O2; calcd 418.1681; found 418.1682.
2.2. Synthesis of 5-(40-methoxyphenyl)furan-2-carbaldehyde (1b)
2.3.3. 3-((1H-indol-3-yl)(50-(400-Bromophenyl)furan-20-yl)methyl)-
5-Bromofuran-2-carbaldehyde (1.5 mmol) was coupled with 4-
methoxyphenylboronic acid (1.9 mmol) in a mixture of DME
(15 mL), aqueous 2 M Na2CO3 (1 mL) and Pd(PPh3)4 (6 mol%) at
80 ꢀC under argon. The reaction was monitored by TLC and after
cooling, the mixture was filtered. Ethyl acetate and a saturated
solution of NaCl were added and the phases were separated. The
organic phase was washed with water (3 ꢁ 50 mL) and with a so-
lution of NaOH (10%) (1 ꢁ 50 mL). The organic phase obtained was
dried (MgSO4), filtered and the solvent removed to give a crude
mixture. The crude residue was submitted to silica gel column
chromatography using mixtures of hexane and chloroform of
increasing polarity. The fractions containing the purified product
1H-indole (2c)
Pale pink solid (54%). Mp: 116.0e117.0 ꢀC. IR (Nujol)
n
¼ 3409,
2919, 1538, 1477, 1418, 1338, 1242, 1205, 1094, 1073, 1008, 967, 826,
783, 743 cmꢂ1. 1H NMR (acetone-d6)
d
¼ 6.08 (s, 1H, CH), 6.25 (d, 1H,
J ¼ 3.2 Hz, H30), 6.85 (d, 1H, J ¼ 3.2 Hz, H40), 6.97 (t, 2H, J ¼ 7.2 Hz,
2 ꢁ H5), 7.10 (t, 2H, J ¼ 7.2 Hz, 2 ꢁ H6), 7.16 (d, 2H, J ¼ 1.6 Hz,
2 ꢁ H2), 7.42 (d, 2H, J ¼ 8.4 Hz, 2 ꢁ H7), 7.54e7.63 (m, 6H, H200, H300,
H500, H600 and 2 ꢁ H4), 10.10 (s, 2H, 2 ꢁ NH). 13C NMR (CDCl3)
d
¼ 34.3 (CH), 106.4 (C40), 109.0 (C30), 111.1 (2 ꢁ C7), 116.9 (2 ꢁ C3),
119.4 (2 ꢁ C5), 119.7 (2 ꢁ C4), 120.4 (C400), 121.9 (2 ꢁ C6), 123.1
(2 ꢁ C2), 124.9 (C200 and C600), 126.7 (2 ꢁ C3a), 130.1 (C100), 131.6
(C300and C500), 136.5 (2 ꢁ C7a), 151.5 (C5), 157.3 (C20). MS (ESI) m/z

R.M.F. Batista et al. / Dyes and Pigments 102 (2014) 293e300
295
(%): 467 (Mþ 81Br, 35), 465 (Mþ 79Br, 35), 431 (28), 367 (28), 345
(66), 289 (14), 231 (49), 227 (100), 187 (30), 159 (16). HRMS: (ESI)
m/z (%) for C27H1881BrN2O calcd 467.05779; found 467.05753.
Table 1
Yields, IR, UVevis absorption and emission data for arylfuryl-bis-(indolyl)methanes
2aed, in MeCN/H2O (7:3) solution.
C
27H1879BrN2O; calcd 465.05970; found 465.05900.
Cpd
R
Yield dH
(%)
IR
n
UVevis Fluorescence
b
(ppm)a (cmꢂ1
)
labs
lem
Stokes’ shift FF
(nm)
(nm) (cmꢂ1
)
2.3.4. 3-((1H-Indol-3-yl)(50-(400-nitrophenyl)furan-20-yl)methyl)-
2a
2b
2c
2d
H
50
10.11
10.10
10.10
10.15
3412
3411
3409
3409
288
289
287
370
425
358
387
412
10,953
0.21
0.58
0.01
0.001
1H-indole (2d)
Yellow solid (65%). Mp: 119.0e120.0 ꢀC. IR (Nujol)
n
¼ 3409,
2925, 1602, 1536, 1507, 1335, 1109, 1096, 1057, 1022, 968, 852, 783,
742, 722 cmꢂ1. 1H NMR (acetone-d6)
OMe 40
Br 54
NO2 65
6549
11,613
2755
d
¼ 6.14 (s, 1H, CH), 6.36 (d, 1H,
a
For the NH proton (in acetone-d6).
For the NH stretching band recorded in Nujol.
J ¼ 4.4 Hz, H30), 6.99 (dt, 2H, J ¼ 7.2 and 0.8 Hz, 2 ꢁ H5), 7.09e7.15
(m, 3H, 2 ꢁ H6 and H40), 7.19 (d, 2H, J ¼ 2.4 Hz, 2 ꢁ H2), 7.44 (d, 2H,
J ¼ 8.5 Hz, 2 ꢁ H7), 7.20 (d, 2H, J ¼ 8.5 Hz, 2 ꢁ H4), 7.91 (dd, 2H,
J ¼ 7.2 and 2.0 Hz, H200 and H600), 8.25 (dd, 2H, J ¼ 7.2 and 2.0 Hz, H300
b
with substituents of different electronic nature in dry methanol in
the presence of potassium hydrogensulfate [11] (Scheme 1). Formyl-
arylfuran precursors 1a and 1ced were commercially available and
5-(40-methoxyphenyl)furan-2-carbaldehyde 1b [12] was synthe-
sized by us through a Suzuki cross-coupling reaction [13].
and H500), 10.15 (s, 2H, 2 ꢁ NH). 13C NMR (acetone-d6)
¼ 35.3 (CH),
d
110.3 (C30), 111.5 (C40), 112.3 (2 ꢁ C7), 116.7 (2 ꢁ C3), 119.5 (2 ꢁ C5),
120.1 (2 ꢁ C4), 122.2 (2 ꢁ C6), 124.3 (2 ꢁ C2), 124.4 (C200 and C600),
125.1 (C300and C500), 127.7 (2 ꢁ C3a), 137.7 (C100), 137.9 (2 ꢁ C7a),
146.9 (C400), 151.1 (C50), 161.5 (C20). MS (EI) m/z (%): 456 (Mþ, 19),
432 (58), 359 (25), 313 (42), 245 (100), 218 (72), 179 (23). HRMS:
(EI) m/z (%) for C27H19N3NaO3; calcd 456.13186; found 456.13041.
These new systems consisting of aryl-furan
p-conjugated
bridges displayed an increased intramolecular charge transfer due
to the high eletronegativity of the oxygen at the heteroaromatic
ring. As a consequence, the acidity of the indolyl NH was increased,
with a bathochromic shift in absorption and emission bands and
quite pronounced colour changes. In compound 2b the electron
donor methoxy group induced a certain decrease in the acidity of
the NH proton (
bis(indolyl)methane functionalized with a nitro group (compound
2d,
370 nm was observed for compound 2d, when compared with
compound 2b, being a direct consequence of the electronic nature
of the substituent at the arylfuran moiety (Fig. 1). The high fluo-
rescence of the furan ring also gave these new systems the
advantage of using a more sensitive technique to detect metal
cations. The relative fluorescence quantum yields were determined
2.4. Spectrophotometric and spectrofluorimetric titrations of
compounds 2aed
d
¼ 10.10 ppm) when compared to the arylfuryl-
Solutions of bis-(indolyl)methane derivatives 2aed (ca.
1.0 ꢁ 10ꢂ5 to 1.0 ꢁ 10ꢂ6 M) and of the cations under study (ca.
1.0 ꢁ 10ꢂ1 to 1.0 ꢁ 10ꢂ3 M) were prepared in MeCN/H2O (7:3) (in the
d
¼ 10.15 ppm). A dramatic bathochromic shift from 289 nm to
form of hexahidratated tetrafluorborate salts for Cu2þ, Co2þ, Ni2þ
,
,
Pd2þ, and perchlorate salts for Cd2þ, Ca2þ, Naþ, Cr3þ, Zn2þ, Hg2þ
Fe2þ and Fe3þ). Titration of the compounds was performed by the
sequential addition of a metal cation to the bis-(indolyl)methane
derivative solution, in a 10 mm path length quartz cuvette and
emission spectra were measured by excitation at the wavelength of
maximum absorption for each compound, indicated in Table 1. The
binding stoichiometry of the bis-(indolyl)methane derivatives with
the metal cations was determined by using Job’s plots, by varying
the molar fraction of the cation while maintaining constant the total
bis-(indolyl)methane derivative and metal cation concentration.
The association constants were obtained from Hyperquad Software.
Fig. 1. Solid samples and MeCN/H2O (7:3) solutions (10ꢂ4 M) of compounds 2aed.
3. Results and discussion
using a 10ꢂ6 M solution of quinine sulfate in 0.5 M H2SO4 as stan-
dard (FF ¼ 0.54) [10]. The absorption and emission spectra of
arylfuryl-bis(indolyl)methanes 2 were measured in MeCN/H2O
(7:3) solution (10ꢂ6 to 10ꢂ5 M solution) (Table 1). Arylfuryl-bis(in-
dolyl)methanes 2 exhibited different fluorescence quantum yields
according to the functionalization at the furan ring: higher FF for
the hydrogen and methoxy groups (0.21 and 0.58, respectively for
compounds 2aeb), and low fluorescence in the case of compounds
3.1. Synthesis and characterization
A new series of arylfuryl-bis(indolyl)methanes 2 was synthesized
with substituents such as alkoxy, bromo or nitro, in order to evaluate
the influence of the electron donatingor withdrawing strength of the
substituent groups on the optical and sensing properties of the re-
ceptors. These compounds were obtained in moderate to good yields
(40e65%) bycondensation of indole with several formyl precursors 1
Scheme 1. Synthesis of arylfuryl-bis(indolyl)methanes 2.

296
R.M.F. Batista et al. / Dyes and Pigments 102 (2014) 293e300
2ced with bromine and nitro groups (FF 0.01 and 0.001, respec-
tively). Considering these photophysical properties, derivatives 2ae
b would be the more interesting candidates as chemosensors due to
the higher fluorescence quantum yields, important for maximiza-
tion of the response in the analysis of very dilute samples.
Ca2þ, Cr3þ, Zn2þ, Hg2þ, Fe2þ and Fe3þ) in MeCN/H2O (7:3) so-
lutions. The hexahydratated tetrafluorborate salts of Cu2þ
,
,
Co2þ, Ni2þ and Pd2þ and perchlorate salts of Cd2þ, Ca2þ, Cr3þ
Zn2þ, Hg2þ, Fe2þ and Fe3þ were added to solutions of 2ae
d (10ꢂ4 M), in order to evaluate their chemosensory ability. A
preliminary study with 100 equiv of the cations revealed that
compounds 2aed responded selectively to the presence of
Hg2þ with a distinct colour change from colourless to purple
(Fig. 2).
3.2. Spectrophotometric/spectrofluorimetric titrations and
chemosensing studies of 2aed with metallic cations
Regarding the fluorimetric response, selectivity towards Hg2þ
was also achieved with a clearly visible quenching in the fluo-
Compounds 2aed were evaluated as chemosensors in the
presence of several metal cations (Cu2þ, Co2þ, Ni2þ, Pd2þ, Cd2þ
,
Fig. 2. Colour changes of compound 2b (10ꢂ4 M in MeCN/H2O (7:3)) in the presence of 100 equiv. of Ca2þ, Cd2þ, Co2þ, Cr3þ, Cu2þ, Fe2þ, Fe3þ, Hg2þ, Pd2þ, Ni2þ and Zn2þ (in the form
of tetrafluorborate or perchlorate salts).
I/I0
1
rescence intensity, (Fig. 3), that can be attributed to electron
and/or energy transfer processes due to empty d shell of Hg2þ
[7c].
Spectrophotometric titration of compounds 2aeb, d in MeCN/
H2O (7:3) (10ꢂ5e10ꢂ6 M) with Hg2þ revealed a trend in the UVe
vis spectra: the intensity of the longest wavelength absorption
band (between 290 and 377 nm) decreased progressively upon
addition of the metal cation, with the simultaneous growth of a
new red-shifted absorption band located between 505 and
0.8
0.6
0.4
0.2
0
2a Ca2+ Cd2+ Co2+ Cr3+ Cu2+ Fe2+ Fe3+ Hg2+ Pd2+ Ni2+ Zn2+
Fig. 3. Relative fluorimetric response (I/I0) of compound 2a in the presence of
100 equiv of Ca2þ, Cd2þ, Co2þ, Cr3þ, Cu2þ, Fe2þ, Fe3þ, Hg2þ, Pd2þ, Ni2þ and Zn2þ, in
MeCN/H2O (7:3, v/v) solution.
Fig. 4. Spectrophotometric titrations of 2a (A), 2b (B), 2c (C) and 2d (D) with addition of increasing amounts of Hg2þ in MeCN/H2O (7:3). The inset represents the normalized
absorption at: 290 and 516 nm (A); 290 and 543 nm (B); 267 and 518 nm (C) and 377 and 505 nm (D) ([2aed] ¼ 2.5 ꢁ 10ꢂ5 M, T ¼ 298 K).

R.M.F. Batista et al. / Dyes and Pigments 102 (2014) 293e300
297
Table 2
The stoichiometry of the complexes was obtained by the
changes in the colorimetric response of compounds 2aed in the
presence of varying concentrations of Hg2þ (Fig. 5). The results
indicated an empirical 1:1 ratio (L:M), which is also in agree-
ment with the stoichiometry suggested from Hyperquad
Software.
As for the spectrofluorimetric titrations in MeCN/H2O (7:3),
only compounds 2aeb were studied since the other compounds
were not fluorescent. The response to the presence of Hg2þ was
seen by a variation in the fluorescence intensity of the emission
UVevis absorption and limit of detection (LOD) and limit of quantification (LOQ)
data upon titration of arylfuryl-bis-(indolyl)methanes 2aed with Hg2þ, in MeCN/
H2O (7:3) solution (L, ligand; L-Hg2þ, complex ligand-metal).
Cpd.
UVevis
Isosbestic
points
(nm)
log Kass
LOD
(ppm)
LOQ
(ppm)
L
L-Hg2þ
labs (nm)
labs (nm)
2a
2b
2c
2d
290
290
270
377
516
543
518
505
274, 307
271, 299
e
2.01 ꢃ 0.03
2.90 ꢃ 0.01
1.90 ꢃ 0.03
1.76 ꢃ 0.02
20.2
10.6
24.7
31.7
67.5
35.6
82.2
344, 420
105.7
Fig. 5. Job’s plot for the complexation of compounds 2aed with Hg2þ, indicating the formation of 1:1 complexes. The total [2aed] þ [Hg2þ] ¼ 4.0 ꢁ 10ꢂ5 M.
543 nm. On the other hand, for compound 2c there was a very
slight increase of the corresponding absorption band at 267 nm
together with the appearance of a new band at 518 nm of low
intensity (Fig. 4). The number of necessary metal equivalents to
band (Fig. 6), after excitation at the maximum wavelength of
absorption of the ligand and of the complex between the metal
and the ligand. The results are similar for both compounds, with
a quenching of fluorescence being visible for the ligand emission
band, whereas the opposite effect occurred with an increase of
the fluorescence intensity for the complex emission band
(Fig. 6), illustrating the ratiometric response of receptors 2aeb
to different Hg2þ concentrations. The number of necessary metal
equivalents to achieve a plateau was at about 30 equivalents for
compounds 2aeb.
achieve
a plateau was between 30 and 40 equivalents for
compounds 2aec and at about 100 equivalents for compound
2d.
In order to gain insight into the interaction mechanism
between the cation and the receptors, similar titrations with
Hg2þ were also conducted in MeCN/H2O at acidic pH (pH
2.05). In this case there was no formation of the red-shifted
band upon addition of the metal, revealing that no interac-
tion took place due to the fact that the indolyl NH was
protonated.
From the results of the spectrofluorimetric titrations in
MeCN/H2O (7:3) for compounds 2aeb, it was possible to calcu-
late association constants (Kass), the detection (LOD) and quan-
tification (LOQ) limits, which are between
8 and 11 ppm
The association constants (Kass), the detection (LOD) and
quantification (LOQ) limits were also obtained from the results of
the spectrophotometric titrations in MeCN/H2O (7:3) (Table 2).
The highest association constant was found for compound 2b
bearing the electron donor methoxy group linked to the arylfuryl
moiety.
(Table 3). Recently, several receptors have been reported for the
detection and quantification of Hg2þ in organic and aqueous
media, with limits of detection and quantification in the ppme
ppb range [1b,i].
As can be seen by the LOD and LOQ data presented in
Tables 2 and 3, the values obtained by spectrophotometric and

298
R.M.F. Batista et al. / Dyes and Pigments 102 (2014) 293e300
Fig. 6. Spectrofluorimetric titrations of 2a (A, B) and 2b (C, D) with the addition of increasing amounts of Hg2þ in MeCN/H2O (7:3) at pH 7.0 aqueous solution. The inset represents
the normalized fluorescence intensity at 425 nm (A), 585 nm (B), 358 nm (C) and 625 nm (D) (T ¼ 298 K; [2a] ¼ [2b] ¼ 2.5 ꢁ 10ꢂ5 M, lexc(A) ¼ 290 nm, lexc(B) ¼ 516 nm.
Hg2þ in acetone-d6 at room temperature (representative example
for compound 2b in Fig. 7).
Table 3
Fluorescence data and limit of detection (LOD) and limit of quantification (LOQ) data
upon titration of arylfuryl-bis-(indolyl)methanes 2aed with Hg2þ, in MeCN/H2O
The signal of the indolyl NH appearing at about 10 ppm was
further shifted downfield (Dd w 0.8 ppm) upon addition of up to 6
equivalents of Hg2þ, thus suggesting that the interaction with the
metal cation is occurring at this site. The chemical shifts of the
remaining protons were unaffected by the interaction with the
metal.
(7:3) solution (L, ligand; L-Hg2þ, complex ligand-metal).
Cpd
Fluorescence
log Kass
LOD
(ppm)
LOQ
(ppm)
L
Stokes
L-Hg2þ
lemis
Stokes
shift
lemis
shift
(nm)
(cmꢂ1
)
(nm)
(cmꢂ1
)
2a
2b
425
358
10,953
6549
585
625
2285
2416
3.801 ꢃ 0.003
3.691 ꢃ 0.005
8.5
10.8
28.3
36.1
4. Conclusions
The selective determination of Hg2þ in MeCN/H2O (7:3) solution
among various transition metal cations was possible with novel
arylfuryl-bis(indolyl)methane derivatives bearing different electron
donor and acceptor substituents. All the compounds 2aed exhibited
a selective and significant colour change from colourless to purple,
whereas a marked quenching of the fluorescence was additionally
observed for compounds 2aeb, making these compounds suitable
for dual chromo- and fluorimetric sensing of Hg2þ in aqueous
mixtures. The binding stoichiometry between the receptors and
Hg2þ was found to be 1:1, and the results obtained through UVevis
and 1H NMR titrations were found to be in agreement, suggesting
spectrofluorimetric titrations are in agreement, especially for
compound 2b.
3.3. 1H NMR titrations
The sensory behaviour observed by the spectrophotometric/
spectrofluorimetric titrations was also confirmed by performing 1H
NMR titrations but due to the limited solubility of compounds 2ae
d in deuterated acetonitrile, the titrations were carried out with

R.M.F. Batista et al. / Dyes and Pigments 102 (2014) 293e300
299
Fig. 7. Partial 1H NMR spectra of 2b (1.4 ꢁ 10ꢂ2 M) in acetone-d6 in (a) the absence and (b) the presence of 1.0, (c) 2.0, (d) 4.0 and (e) 6.0 equiv of Hg2þ
.
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Acknowledgements
Thanks are due to the Foundation for Science and Technology
(FCT, Portugal) for financial support to the NMR Portuguese
network (PTNMR, Bruker Avance III 400-Univ. Minho), FCT and
FEDER (European Fund for Regional Development)-COMPETE-
QREN-EU for financial support to the Research Centre, CQ/UM
[PEst-C/QUI/UI0686/2011 (FCOMP-01-0124-FEDER-022716)] and a
post-doctoral grant to R.M.F. Batista (SFRH/BPD/79333/2011).
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