Ethynylpyrene Linked Benzocrown Ethers as Fluorescent Sensors for Metal Ions

Article abstract of DOI:10.1111/php.13071

Substances containing ethynylpyrenes linked to either one or four benzocrown ethers were synthesized, and their absorption and fluorescence spectroscopic responses to metal ions were assessed. Addition of metal perchlorates to solutions of these substances promotes short wavelength shifts in their absorption and fluorescence maxima and increases in their fluorescence intensities. The magnitudes of the fluorescence intensity increases are dependent on the ring size and number of the crown ether and the nature of the metal cation. Association constants for complex formation were calculated using fluorescence intensity versus concentration data. Analysis using Job's plots showed that the substances containing one benzocrown ether moiety form 1:1 complexes with metal ions. Results of experiments employing repeated addition and removal of Mg(ClO₄)₂ demonstrate that the ON-OFF fluorescence response can be repeated at least three times. Results of molecular orbital calculations show that complexation with metal ions lowers the energies of both the π and π* levels of the ethynylpyrene moiety and that in some cases the vacant orbital on the metal becomes the LUMO of the complex. An explanation of the spectroscopic changes promoted by metal ions is proposed in terms of electrostatic repulsion and structural regulation.

Full text of DOI:10.1111/php.13071

DR. HAJIME MAEDA (Orcid ID : 0000-0001-8987-1790)
Article type : Research Article
Ethynylpyrene Linked Benzocrown Ethers as Fluorescent Sensors
for Metal Ions
Hajime Maeda*, Kazuhiro Tanaka, Mona Aratani, Masahito Segi
Division of Material Chemistry, Graduate School of Natural Science and Technology, Kanazawa
University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan
*Corresponding author e-mail: maeda-h@se.kanazawa-u.ac.jp (Hajime Maeda)
ABSTRACT
Substances containing ethynylpyrenes linked to either one or four benzocrown ethers were
synthesized and their absorption and fluorescence spectroscopic responses to metal ions were
assessed. Addition of metal perchlorates to solutions of these substances promotes short wavelength
shifts in their absorption and fluorescence maxima and increases in their fluorescence intensities. The
magnitudes of the fluorescence intensity increases are dependent on the ring size and number of the
crown ether and the nature of the metal cation. Association constants for complex formation were
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calculated using fluorescence intensity versus concentration data. Analysis using Job's plots showed
that the substances containing one benzocrown ether moiety form 1:1 complexes with metal ions.
Results of experiments employing repeated addition and removal of Mg(ClO4)2 demonstrate that the
ON-OFF fluorescence response can be repeated at least three times. Results of molecular orbital
calculations show that complexation with metal ions lowers the energies of both the  and * levels of
the ethynylpyrene moiety and that in some cases the vacant orbital on the metal becomes the LUMO
of the complex. An explanation of the spectroscopic changes promoted by metal ions is proposed in
terms of electrostatic repulsion and structural regulation.
INTRODUCTION
Crown ethers are known to incorporate metal ions selectively in a ring size dependent fashion (1-7).
Pyrene and its derivatives are often used as fluorescent materials owing to several advantageous
features including photochemical inertness, dual fluorescence (monomer/excimer) and high emission
quantum yield (8-14). Thus far, many substances containing linked pyrenes and crown ethers have
been designed to serve as fluorescent metal ion sensors (15-51). These types of sensors can be roughly
categorized into three groups based on their response to metal ions. The first two sensor groups are
those that undergo turn-ON (17-24) or turn-OFF responses (25-27) typically caused by the removal or
installation of photoinduced electron transfer (PET) or heavy atom effect promoted quenching
pathways. In the third group, sensing is based on monomer/excimer emission switching (28-43)
induced by changes in spatial distances between two pyrene moieties. Moreover, the pyrene and
crown ether groups in these types of sensors are normally linked by flexible chains, such as methylene
(17-21,28,29,44-46), ether (30,47,48), oligoethylene glycol (31-34), ester (22), amine (49), amide (25-
27,35-41,50), imine (23), triazole (51), sulfoxide (24), and DNA (42) units. As a consequence,
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decreases in fluorescence efficiencies by thermal (nonradiative) decay pathways arising from
molecular motion are inevitable.
In investigations carried out recently, we designed and synthesized a variety of functional
fluorescent materials that contain the pyrene fluorophore (52-63). In these studies, we observed that
alkynylpyrenes are highly emissive and, as a result, they can be utilized as fluorescent probes for the
detection of amino acids, proteins, and DNA (53). These findings led to the proposal that dyads
comprised of pyrene and crown ethers, linked by rigid acetylene linkages, would be highly effective
fluorescent sensors for metal ions. In the current study, we prepared the ethynylpyrene linked
benzocrown ethers 1-4 (Scheme 1) and explored their fluorescence and metal ion sensing. The results
of the effort showed that the presence of metal perchlorates in solutions of 1-4 promotes short
wavelength shifts in their absorption and fluorescence maxima. Importantly, we found that the metal
cations cause increases in the fluorescence intensities of these substances and the enhancement was
largest for 4, which contains four benzocrown ether moieties. In addition, the magnitudes of the
fluorescence intensity increases were highly dependent on the ring size of the crown ether and the
nature of the metal cation.
<Scheme 1>
MATERIALS AND METHODS
Solvents and materials. THF and Et2O were distilled from CaH2 and then from Na. CH2Cl2, CCl4,
benzene, and toluene were distilled from CaH2. 1-Bromopyrene (6) (64), 2-methyl-4-(pyren-1-yl)but-3-
yn-2-ol
(7)
(65,66),
1-ethynylpyrene
(8)
(10a)
(65,66),
(67),
12-iodo-2,3,5,6,8,9-
hexahydrobenzo[b][1,4,7,10]tetraoxacyclododecin
15-iodo-2,3,5,6,8,9,11,12-
octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin (10b) (67), 18-iodo-2,3,5,6,8,9,11,12,14,15-
decahydrobenzo[b][1,4,7,10,13,16]hexaoxacyclooctadecin (10c) (67), 1,3,6,8-tetrabromopyrene (11) (68)
1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (12) (53,61,69), and 1,3,6,8-tetraethynylpyrene (13) (61,69)
were prepared using literature procedures. Other chemicals were used as purchased.
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1
Equipment. Melting points were determined on a Gallenkamp MFB-595 melting point apparatus. H
13
and C NMR spectra were recorded using a JEOL JMN LA-400 (400 MHz and 100 MHz, respectively)
or a JEOL ECA-500 (500 MHz and 125 MHz, respectively) spectrometer with Me4Si as an internal
standard. IR spectra were determined by using a Shimadzu FTIR-8300 spectrometer. UV-vis spectra were
recorded using a Hitachi U-2900 spectrophotometer. Fluorescence spectra were recorded using a Hitachi
F-2500 or a Jasco FP-8000 spectrophotometer. Low- and high-resolution mass spectra were recorded on
JEOL JMS-AM50 and JMS-SM102A instruments, respectively. HPLC separations were performed using
recycling preparative HPLC instruments (Japan Analytical Industry Co. Ltd.) using LC-918 JAIGEL SIL
S-043-15 (silica) and LC-908 JAIGEL-H (GPC) columns. Column chromatography was conducted by
using Kanto-Chemical Co. Ltd., silica gel 60 N (spherical, neutral, 0.04-0.05 mm). Thin-layer
chromatography was performed using Merck Kiesel gel 60 F254 plates, and detection was performed by
using UV light and phosphomolybdic acid ethanol solution with heating. Hyflo Super-Cell(R) (nacalai
tesque, 08016-25) was used for removal of Pd catalysts by filtration. Theoretical calculations using the
B3LYP/3-21G basis set were performed by using Gaussian 09 software package.
Preparation of 12-(pyren-1-ylethynyl)-2,3,5,6,8,9-
hexahydrobenzo[b][1,4,7,10]tetraoxacyclododecin (1). A mixture of 1-ethynylpyrene (8
(65,66), 0.226 g, 1.00 mmol), 12-iodo-2,3,5,6,8,9-hexahydrobenzo[b][1,4,7,10]tetraoxacyclododecin (10a
(67), 0.420 g, 1.20 mmol), PdCl2(PPh3)2 (0.035 g, 0.05 mmol), CuI (0.009 g, 0.05 mmol), PPh3 (0.026 g,
0.10 mmol), THF (20 mL), and i-Pr2NH (20 mL) was stirred at 60 °C for 20 h under an argon atmosphere.
The solvents were removed in vacuo. The Pd catalyst was removed by filtration through Hyflo Super-
Cell(R) using THF as an eluant. The filtrate was concentrated in vacuo, giving a residue that was
subjected to silica gel column chromatography (CH2Cl2:hexane = 1:4, then AcOEt). The crude product
was crystallized from CH2Cl2:hexane = 1:4 to give 12-(pyren-1-ylethynyl)-2,3,5,6,8,9-
hexahydrobenzo[b][1,4,7,10]tetraoxacyclododecin (1, 0.117 g, 26% yield). Yellow solid; mp 135 °C; 1H
NMR (400 MHz, CDCl3) δ 3.81-3.84 (m, 4H), 3.88-3.92 (m, 4H), 4.26 (dt, J =13.0, 3.9 Hz, 4H), 7.02 (d, J
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= 7.8 Hz, 1H), 7.35 (s, 1H), 7.36 (d, J = 8.0 Hz, 1H), 8.01-8.25 (m, 8H), 8.65 (d, J = 9.3 Hz, 1H) ppm; 13C
NMR (125 MHz, CDCl3) δ 69.78, 69.80, 71.05, 71.23, 71.39, 72.10, 87.58, 94.84, 117.47, 117.60, 117.91,
121.55, 124.32, 124.48, 124.52, 125.51, 125.55, 126.21, 126.80, 127.24, 128.05, 128.24, 129.48, 131.06,
131.09, 131.24, 131.77, 150.27, 151.33 ppm; IR (KBr) 849, 1130, 1265, 1512, 2858, 2924 cm–1; MS (EI)
m/z (relative intensity, %) 180 (18), 250 (16), 276 (19), 304 (28), 360 (44), 448 (M+, 100); HRMS (EI)
calcd for C30H24O4: 448.1675, found: 448.1670; Anal. calcd for C30H24O4: C, 80.34; H, 5.39, found: C,
79.08; H, 5.35.
Preparation of 15-(pyren-1-ylethynyl)-2,3,5,6,8,9,11,12-
octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin (2). The same procedure used to
prepare 1, employing 15-iodo-2,3,5,6,8,9,11,12-octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin
(10b (67), 0.473 g, 1.20 mmol) instead of 10a, gave 15-(pyren-1-ylethynyl)-2,3,5,6,8,9,11,12-
octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin (2, 0.049 g, 10% yield). Yellow solid; mp 156.8
°C; 1H NMR (400 MHz, CDCl3) δ 3.75-3.84 (m, 8H), 3.94-3.98 (m, 4H), 4.19-4.26 (m, 4H), 6.91 (d, J =
8.3 Hz, 1H), 7.22 (d, J = 1.2 Hz, 1H), 7.31 (dd, J = 8.1, 1.7 Hz, 1H), 8.01-8.24 (m, 8H), 8.66 (d, J = 9.0
Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δ 68.79, 69.04, 69.41, 69.46, 70.38, 70.41, 71.10, 87.23,
95.23, 113.45, 116.01, 116.91, 118.04, 124.31, 124.47, 124.50, 125.46, 125.50, 125.52, 125.58, 126.17,
127.22, 127.97, 128.17, 129.45, 131.00, 131.05, 131.23, 131.69, 148.77, 149.80 ppm; IR (KBr) 841,
1134, 1227, 1253, 1516, 1597, 2194, 2866, 3448 cm–1; MS (EI) m/z (relative intensity, %) 202 (16), 303
(28), 360 (25), 492 (M+, 100); Anal. calcd for C32H28O5: C, 78.03; H, 5.73, found: C, 77.53; H, 5.80.
Preparation of 18-(pyren-1-ylethynyl)-2,3,5,6,8,9,11,12,14,15-
decahydrobenzo[b][1,4,7,10,13,16]hexaoxacyclooctadecin (3). The same procedure used to
prepare 1, employing 18-iodo-2,3,5,6,8,9,11,12,14,15-
decahydrobenzo[b][1,4,7,10,13,16]hexaoxacyclooctadecin (10c (67), 0. 526 g, 1.2 mmol) instead of 10a,
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gave 18-(pyren-1-ylethynyl)-2,3,5,6,8,9,11,12,14,15-
decahydrobenzo[b][1,4,7,10,13,16]hexaoxacyclooctadecin (3, 0.236 g, 44% yield). Yellow solid; mp 143
°C; 1H NMR (500 MHz, CDCl3) δ 3.71-3.73 (m, 4H), 3.74-3.77 (m, 4H), 3.80-3.83 (m, 4H), 3.96-4.00 (m,
4H), 4.25 (dt, J = 16.6, 4.3 Hz, 4H), 6.92 (d, J = 8.0 Hz, 1H), 7.23 (s, 1H), 7.31 (d, J = 8.6 Hz, 1H), 8.02-
8.25 (m, 8H), 8.66 (d, J = 9.2 Hz, 1H) ppm; 13C NMR (125 MHz, CDCl3) δ 68.99, 69.15, 69.51, 69.55,
70.71, 70.72, 70.75, 70.78, 70.91, 87.22, 95.23, 113.52, 116.02, 116.88, 118.07, 124.35, 124.51, 124.53,
125.48, 125.53, 125.61, 126.20, 127.25, 128.00, 128.20, 129.48, 131.03, 131.10, 131.26, 131.74, 148.61,
149.64 ppm; IR (KBr) 849, 1126, 1244, 1516, 2878 cm–1; MS (EI) m/z (relative intensity, %) 180 (14),
304 (12), 360 (26), 448 (5), 536 (M+, 100); HRMS (EI) calcd for C34H32O6: 536.2199, found: 536.2196;
Anal. calcd for C34H32O6: C, 76.10; H, 6.01, found: C, 75.26; H, 6.00.
Preparation of 1,3,6,8-tetrakis((2,3,5,6,8,9,11,12-
octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin-15-yl)ethynyl)pyrene (4). A
mixture of 1,3,6,8-tetraethynylpyrene (13 (61,69), 0.298 g, 1.00 mmol), 15-iodo-2,3,5,6,8,9,11,12-
octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin (10b (67), 1.77 g, 4.50 mmol), PdCl2(PPh3)2
(0.035 g, 0.05 mmol), CuI (0.010 g, 0.05 mmol), PPh3 (0.026 g, 0.10 mmol), THF (30 mL), and i-Pr2NH
(30 mL) was stirred at 80 °C for 19 h under an argon atmosphere. The Pd catalyst was removed by
filtration through Hyflo Super-Cell(R) with THF as the eluant. Concentration of the filtrate gave a solid
that was washed with hexane and ethanol to give 1,3,6,8-tetrakis[(2,3,5,6,8,9,11,12-
octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin-15-yl)ethynyl]pyrene (4, 0.101 g, 7% yield).
Orange solid; 1H NMR (400 MHz, CDCl3) δ 3.75-3.85 (m, 32H), 3.95-3.99 (m, 16H), 4.20-4.27 (m, 16H),
6.91 (d, J = 8.6 Hz, 4H), 7.21 (d, J = 1.8 Hz, 4H), 7.31 (dd, J = 8.0, 2.0 Hz, 4H), 8.40 (s, 2H), 8.73 (s, 4H)
ppm; IR (KBr) 1095, 1249, 1593, 1716 cm–1; MS (FAB) m/z (relative intensity, %) 77 (11), 107 (16), 136
(62), 154 (100), 307 (33), 391 (11).
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Preparation of 1,3,6,8-tetrakis(phenylethynyl)pyrene (14).
A
mixture of 1,3,6,8-
tetrabromopyrene (11 (68), 0.518 g, 1.00 mmol), phenylacetylene (0.439 g, 4.30 mmol), PdCl2(PPh3)2
(0.035 g, 0.050 mmol), CuI (0.010 g, 0.05 mmol), PPh3 (0.026 g, 0.100 mmol), THF (30 mL), and i-
Pr2NH (30 mL) was stirred at 80 °C for 19 h under an argon atmosphere. The Pd catalyst was removed by
filtration through Hyflo Super-Cell(R) with THF as eluant. Concentration of the filtrate gave a solid that
was washed with acetone and CH2Cl2 to give 1,3,6,8-tetrakis(phenylethynyl)pyrene (14 (70,71), 0.458 g,
76% yield). Orange solid; 1H NMR (500 MHz, THF-d8)  7.32-7.39 (m, 12H), 7.65-7.68 (m, 8H), 8.40 (s,
2H), 8.77 (s, 4H) ppm.
Titration method. In the experiments of UV-vis absorption and fluorescence spectroscopic changes in
Figures 1-3, 7, and S5-S7, solutions of each equiv of metal salts (5.0 × 10–3 M in CH3CN) were added to
the solutions of 1-3 (1.0 × 10–5 M in CH3CN). In the experiments in Figures 4 and S8, CH3CN:CHCl3 =
1:1 solutions were used instead of CH3CN because of low solubility of 4 and 14 in CH3CN. In Job's plots
in Figure 6, both CH3CN solutions (1.0 × 10–5 M each) were mixed in 10:0 to 0:10 ratios. Association
constants were calculated by using TitrationFit 2014-0630 based on changes in fluorescence intensities.
RESULTS AND DISCUSSION
Ethynylpyrene linked benzocrown ethers 1-4 were synthesized by using the routes shown in Scheme
2. The sequences used for the preparation of 1-3 began with mono-bromination of pyrene (5) using
Br2 that formed 1-bromopyrene (6) (64), which was subjected to Sonogashira coupling reaction with
2-methylbut-3-yn-2-ol to produce the propargylic alcohol 7 (65,66). Removal of the hydroxypropyl
group in 7 by using NaOH produced 1-ethynylpyrene (8) (65,66), which was coupled with the 4'-
iodobenzocrown ethers 10a-c (67) to give 1-3, respectively. In the route for the preparation of 4,
tetrabromopyrene (11) was generated by the reaction of pyrene 5 with 4.6 equiv of Br2 (68). The
reaction of 11 with trimethylsilylacetylene under Sonogashira coupling conditions formed 12
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(53,61,69), which was then subjected to silyl group removal using KOH to produce
tetraethynylpyrene 13 (61,69). The reaction of 13 with iodobenzocrown ether 10b generated the
tetrakis-benzocrown ether substituted pyrene 4. A reference compound, tetrakis(phenylethynyl)pyrene
14, was prepared using Sonogashira coupling reaction of 11 with phenylacetylene (70,71). The
structures of 1-4 and 14 were confirmed by using spectral and analytical methods (Figures S1-S4).
<Scheme 2>
The effects of metal cations on the absorption and fluorescence spectroscopic properties of
ethynylpyrene linked benzocrown ethers were assessed. First, the UV-vis absorption spectroscopic
changes caused by addition of perchlorate salts of alkali metal and selected alkaline earth metal
cations, Li+, Na+, K+, Rb+, Cs+, Mg2+, and Ba2+, to 1.0 × 10–5 M CH3CN solutions of 1-3 were
determined. Inspection of the spectra (Figures S5-S7) showed that the wavelength maxima and molar
absorptivities of these substances did not significantly change in the presence of these metal salts.
Changes in the fluorescence properties were evaluated by adding the metal perchlorates to 1.0 × 10–5
M CH3CN solutions of 1-3. As can be seen by viewing the spectra in Figures 1-3, the fluorescence
intensities of 1-3 increased and the emission wavelength maxima shifted to shorter wavelengths upon
addition of each metal salt. Moreover, the fluorescence responses were found to depend on the nature
of the metal ions and ring size of the crown ether. Specifically, much larger changes in the
fluorescence spectra took place in the following crown ether-metal ion combinations, 1-Li+, 1-Na+, 1-
Ba2+, 2-Mg2+, 2-Ba2+, and 3-Ba2+. The association constants (K) for the formation of complexes
between the crown ether and metal ions, given in Figures 1-3, were calculated using the fluorescence
intensity versus concentration data.
<Figure 1>
<Figure 2>
<Figure 3>
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We also determined the effects of metal cations on the absorption and emission properties of
the tetrakis-crown ether containing pyrene derivative 4. Unlike the cases of 1-3, the addition of metal
perchlorates to 1.0 × 10–5 M 1:1 CH3CN:CHCl3 solutions of 4 resulted in both shifts to shorter
wavelengths and increases in molar absorptivities of the UV-vis absorption bands (Figure 4).
Furthermore, in the presence of metal cations, the fluorescence band of 4 underwent a much larger
increase in intensity than those of 1-3 and a shift to shorter wavelength. Images showing light
emission from 1:1 CH3CN:CHCl3 solutions of 4 in the absence and presence of Mg(ClO4)2 under UV
(254 nm) irradiation are given in Figure 5. The change of colors from orange to green was clearly
observed by naked eyes.
<Figure 4>
<Figure 5>
For purpose of comparison, absorption and fluorescence spectra of the non-crown ether
containing model compound 14 in the absence and presence of metal ions were recorded (Figure S8).
Inspection of the spectra showed that only slight changes occurred upon the addition of Li+, Na+, and
Mg2+ ions. These results indicated that changes in the absorption and fluorescence properties of 4
caused by metal ions were a consequence of their complexation with the crown ether moieties.
In order to gain information about the stoichiometry of the complexes between the metal ions
and ethynylpyrene linked benzocrown ethers, Job’s plots were constructed from absorbance change at
abs = 254 nm versus metal ion concentrations. Analysis of the selected plots shown in Figure 6
demonstrated that 1:1 complexes were formed between 2 and 3 to Li+, K+, and Ba2+.
<Figure 6>
Association constants, K, for complex formation between 1-3 and metal ions were obtained
by analysis of fluorescence spectral changes promoted by addition of various concentrations of the
metal perchlorates (Table 1). Metal ions used in the complex formation study are displayed in the
order of Pauling's ionic radii (72) of the metal ions. All titration curves are shown in Figures S9-S29.
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The data showed that complexes 1-Li+, 1-Na+, 2-Mg2+, 2-Ba2+, 3-Na+, 3-K+, 3-Ba2+, and 3-Rb+ had
relatively larger K values and, consequently, that metal cation complexation depended on the ring size
of the crown ether and the ionic radius of the metal cation.
All association constants were obtained as they indicate the formation of 1:1 complexes for 1-
3 and metal ions in approximate calculations. Recent some papers describe that Job's plot sometimes
provides false results (73,74), however, the formation of 1:1 complexes could also be demonstrated
from fitting calculation by using the fluorescence spectral changes. By the way, detection of 1:1, 2:1,
and other metal complexes by mass spectrometry failed.
<Table 1>
With the aim of elucidating whether or not they can be utilized as fluorescent sensors for
detection of metal ions, the durability of the compound 2 to undergo repeated metal ion promoted
emission changes was evaluated using the strategy outlined in Figure 7(a). The fluorescence
intensities of a CH3CN solution of 2 (1.0 × 10–5 M) at em = 394 nm (ex = 388 nm) were determined
before (OFF1) and after addition of 500 equiv of Mg(ClO4)2 (ON1) (Figure 7(b)). Aqueous HIO4 and
CHCl3 were then added and the organic layer containing 2 was separated and concentrated in vacuo.
The fluorescence intensity of a 1.0 × 10–5 M CH3CN solution of the residue was measured (OFF2)
(Figure 7(b)). This series of operations was repeated three times. A plot of the fluorescence intensities
at em = 394 nm vs ON/OFF steps (Figure 7(c)) showed that 2 could be employed to detect Mg2+ ion
over at least three cycles.
<Figure 7>
Information about the molecular structures of 1, 2, and their metal complexes was gained
through calculations using the B3LYP/3-21G basis set. Selected molecular orbitals of the optimized
structures of these substances are given in Figure 8 and complete data are shown in Figures S30-S32.
In compound 1, the  and * orbitals of the pyrene moiety served as the respective HOMO and
LUMO (Figure 8(a)(b)). Likewise, the  and * orbitals of pyrene moiety were also the respective
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HOMO and LUMO of the 1-Li+ complex (Figure 8(c)(d)), and the vacant orbital on Li (M*) was the
LUMO+3 (Figure 8(e)). The results of calculations showed that the HOMO and LUMO of 2 (Figure
8(f)(g)) were similar to those of 1. However, in the case of the 2-Mg2+ complex,  and * orbitals of
pyrene moiety were the HOMO and LUMO+6, respectively, and M* became the LUMO. Energies of
, *, and M* orbitals of 1, 2, 1-M*, and 2-M* are displayed in Table 2. In complexes 1-Na+, 1-K+, 1-
Rb+, and 2-Li+,  and * orbitals were the HOMO and LUMO, respectively, and the M* orbitals were
the LUMO+1 or LUMO+3. On the other hand, in 1-Mg2+, 2-Na+, 2-K+, and 2-Rb+,  and M* orbitals
were the HOMO and LUMO, respectively, and * orbitals were the LUMO+1 or LUMO+6. These
findings showed that the energies of  and * orbitals of the pyrene moieties of 1 and 2 decreased
upon addition of metal ions. Furthermore, energies of M* orbitals decreased especially in the Mg ion
complexes. Thus, the probabilities of electron transitions from * orbitals of excited state of the
pyrene moiety to distant M* orbitals is likely to be low. Therefore, the changes in the absorption and
fluorescence spectroscopic properties promoted by metal cations are most likely a consequence of
effects on –* transitions of the pyrene moieties.
<Figure 8>
<Table 2>
The calculated dihedral angles of the O-C-C-O bonds in crown ethers of 1 and 2 before and
after metal ion complexation are shown in Table 2 and Figure S33. In both compounds, the presence
of complexed metal ions caused the dihedral angles to decrease and approach to 60°, which are stable
less-flexible all-gauche structures typically observed for crown ether-metal ion complexes.
Based on these results, it is possible to postulate that the mechanistic pathway displayed in
Figure 9 is responsible for the changes in fluorescence intensities of the ethynylpyrene linked
benzocrown ethers occurring upon complexation with metal cations. Firstly, complex formation
occurs between the ethynylpyrene linked benzocrown ethers and metal ions in a manner that depends
upon the nature of the metal ion and size of the crown ether ring as shown in association constants K
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values. Upon irradiation, electronic –* transition of the pyrene moiety takes place in either the
uncomplexed or metal ion complexed system to produce the excited singlet state of pyrene which
fluoresces (uncomplexed max = 440 nm and complexed max = 390-420 nm). The appreciable
decreases in fluorescence intensities of arene fluorophores, typically caused by PET quenching, was
not observed in this study. Therefore, single electron transfer (SET) from excited singlet state of
pyrene to captured metal ion, or that from captured metal ion to the excited singlet pyrene might be
negligible. This is a likely consequence of the fixed, large distance between the pyrene and crown
ether groups in the complexes, and the slight overlap of the  and * orbitals of the pyrene with the
vacant orbitals of the metal ions. It should be noted that the crown ethers serve as inefficient
quenchers of the pyrene singlet excited state as judged by the observation that benzo-15-crown-5
weakly quenches the fluorescence of 1-ethylnylpyrene (8) with a small quenching rate constant of 3 ×
10–5 s–1 (Figure S34).
<Figure 9>
Fluorescence of the metal ion free compounds is indigenously quenched by concentration
quenching and thermal nonradiative deactivation, both of which contribute to determining emission
intensities. However, the presence of complexed metal cations makes the compounds positively
charged, which results in intermolecular electrostatic repulsion and a corresponding reduction in
concentration quenching of pyrene fluorescence. Additionally, metal ion complexation also leads to
rigidification of the crown ether structure, which causes a reduction in thermal nonradiative
deactivation of the tethered pyrene excited state. The short wavelength shift of the absorption and
fluorescence maxima upon metal ion complexation can be attributed to a lowering of the HOMO level
and a decrease of Stokes shift arising from structural rigidification.
If sandwich complexes of 1-4:Mn+ = 2:1 for cations whose sizes are larger than the cavity of
the crown ether cycle are formed, emission of pyrene excimer may be observed (63). However, that
kind of emission was not observed due to the low concentration of 1-4 (1.0 × 10–5 M).
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Since spectral change of absorption and fluorescence of compound 14 which does not have
crown ether moiety was not observed upon addition of metal ions (Figure S8), effect of cation-
interaction between metal cation and electron-rich pyrene can be almost ignored. Therefore, spectral
change when metal ions were added to solutions of 1-4 is attributed to complexation of metal ions at
the crown ether moieties.
The titration experiments of 1-3 were done by using CH3CN as a solvent. However, because
of low solubilities of 4 and 14 in CH3CN, CHCl3 was added as a co-solvent in titration of 4 and 14.
Influence of the difference of solvents on the spectroscopic results might be slight.
Spectroscopic change of broad longer-wavelength fluorescence to sharp shorter-wavelength
fluorescence was observed by the addition of selected metal ions. In addition, fluorescence intensity
was dramatically enhanced in the case of the tetrasubstituted compound. Considering the wavelength
and low concentration, the spectral change is not attributed to the transfer from excimer to monomer,
but both monomer emissions. Such spectral change has not been observed in other pyrene-crown ether
linked molecules. As explained in Figure 9, this is the effect of rigid chains. The property that color is
changed and fluorescence intensity increases simultaneously is convenient for visibility as fluorescent
sensors.
Selectivity for detection of metal ions is medium level. The most responsible metal ion is
different depend on the size of crown ethers. In the case of 15-crown-5, response to Mg2+ ion is found
to be excellent. So far, many pyrene-crown ether based chemosensors have been developed and they
selectively detect Li+ (18), Na+ (18,35,38), K+ (17,22,28,33,37-39,41,42,50), Cs+ (38), Ca2+ (31,34),
Sr2+ (21,31), Ba2+ (28,43), Gd3+ (46), Mn2+ (19), Fe2+ (20), Cu2+ (40), Ag+ (25), Zn2+ (24,29), Hg2+
(20,26,27), Pb2+ (32,39), NH4+ (23), and methyl viologen (30,47,48) ions. However, to the best of our
knowledge, Mg2+-selective chemosensors based on pyrene and crown ethers are unprecedent.
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CONCLUSION
In the effort described above, we prepared the ethynylpyrene linked benzocrown ethers 1-4, and
explored changes in their absorption and fluorescence properties brought by complexation with metal
ions. The compounds showed the following metal ion selectivities: 1 (12-crown-4):Li+ and Na+, 2, 4
(15-crown-5):Mg2+, and 3 (18-crown-6):Ba2+. Addition of metal ions caused a short wavelength shift
of the absorption and fluorescence maxima of these substances along with a large increase in their
fluorescence intensities. Job's plots showed that 2 and 3 form 1:1 complexes with metal ions. These
substances were shown to be durable over repeated metal ion promoted fluorescence change cycles.
The combined observations showed that the ethynylpyrene linked benzocrown ethers can be used as
turn-ON type fluorescent sensors for the selective detection of metal ions.
ACKNOWLEDGEMENTS: This study was financially supported by a Grant-in-Aid for
Scientific Research (C) (20550049, 23550047, 26410040, 17K05777) and Cooperation for Innovative
Technology and Advanced Research in Evolutional Areas (CITY AREA) program from the Ministry
of Education, Culture, Sports, Science and Technology (MEXT) of Japan. H. M. is also grateful for
financial support from Mitsubishi Chemical Corporation Fund, The Mazda Foundation, The Murata
Science Foundation, Izumi Science and Technology Foundation, A-STEP (Adaptable and Seamless
Technology Transfer Program through target-driven R&D, JST), and Kanazawa University
SAKIGAKE Project. We are indebted to Professor Tsuyoshi Asakawa, Associate Professor Akio
Ohta, Professor Tadaaki Yamagishi, and Professor Tomoki Ogoshi at Kanazawa University for
permission to use UV-vis absorption and fluorescence spectrophotometers. We are also indebted to
Shigehisa Akine at Kanazawa University for donation of the TitrationFit 2014-0630 program.
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SUPPORTING INFORMATION
Supporting Information may be found in the online version of this article:
Figure S1. 1H NMR (400 MHz) spectrum of 1 in CDCl3.
Figure S2. 1H NMR (400 MHz) spectrum of 2 in CDCl3.
Figure S3. 1H NMR (400 MHz) spectrum of 3 in CDCl3.
Figure S4. 1H NMR (400 MHz) spectrum of 4 in CDCl3.
Figure S5. UV-vis absorption spectra of 1 (1.0 × 10–5 M in CH3CN) upon addition of (a) LiClO4, (b)
NaClO4, (c) KClO4, (d) RbClO4, (e) CsClO4, (f) Mg(ClO4)2, and (g) Ba(ClO4)2.
Figure S6. UV-vis absorption spectra of 2 (1.0 × 10–5 M in CH3CN) upon addition of (a) LiClO4, (b)
NaClO4, (c) KClO4, (d) RbClO4, (e) CsClO4, (f) Mg(ClO4)2, and (g) Ba(ClO4)2.
Figure S7. UV-vis absorption spectra of 3 (1.0 × 10–5 M in CH3CN) upon addition of (a) LiClO4, (b)
NaClO4, (c) KClO4, (d) RbClO4, (e) CsClO4, (f) Mg(ClO4)2, and (g) Ba(ClO4)2.
Figure S8. (a)-(c) UV-vis absorption and (d)-(f) fluorescence spectra of 14 (1.0 × 10–5 M in
CH3CN:CHCl3 = 1:1) upon addition of (a)(d) LiClO4, (b)(e) NaClO4, and (c)(f) Mg(ClO4)2. The
excitation wavelength was 464 nm.
Figure S9. Titration curves in fluorescence spectral change of 1 (1.0 × 10–5 M in CH3CN) upon
addition of LiClO4.
Figure S10. Titration curves in fluorescence spectral change of 1 (1.0 × 10–5 M in CH3CN) upon
addition of Mg(ClO4)2.
Figure S11. Titration curves in fluorescence spectral change of 1 (1.0 × 10–5 M in CH3CN) upon
addition of NaClO4.
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Figure S12. Titration curves in fluorescence spectral change of 1 (1.0 × 10–5 M in CH3CN) upon
addition of KClO4.
Figure S13. Titration curves in fluorescence spectral change of 1 (1.0 × 10–5 M in CH3CN) upon
addition of Ba(ClO4)2.
Figure S14. Titration curves in fluorescence spectral change of 1 (1.0 × 10–5 M in CH3CN) upon
addition of RbClO4.
Figure S15. Titration curves in fluorescence spectral change of 1 (1.0 × 10–5 M in CH3CN) upon
addition of CsClO4.
Figure S16. Titration curves in fluorescence spectral change of 2 (1.0 × 10–5 M in CH3CN) upon
addition of LiClO4.
Figure S17. Titration curves in fluorescence spectral change of 2 (1.0 × 10–5 M in CH3CN) upon
addition of Mg(ClO4)2.
Figure S18. Titration curves in fluorescence spectral change of 2 (1.0 × 10–5 M in CH3CN) upon
addition of NaClO4.
Figure S19. Titration curves in fluorescence spectral change of 2 (1.0 × 10–5 M in CH3CN) upon
addition of KClO4.
Figure S20. Titration curves in fluorescence spectral change of 2 (1.0 × 10–5 M in CH3CN) upon
addition of Ba(ClO4)2.
Figure S21. Titration curves in fluorescence spectral change of 2 (1.0 × 10–5 M in CH3CN) upon
addition of RbClO4.
Figure S22. Titration curves in fluorescence spectral change of 2 (1.0 × 10–5 M in CH3CN) upon
addition of CsClO4.
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Figure S23. Titration curves in fluorescence spectral change of 3 (1.0 × 10–5 M in CH3CN) upon
addition of LiClO4.
Figure S24. Titration curves in fluorescence spectral change of 3 (1.0 × 10–5 M in CH3CN) upon
addition of Mg(ClO4)2.
Figure S25. Titration curves in fluorescence spectral change of 3 (1.0 × 10–5 M in CH3CN) upon
addition of NaClO4.
Figure S26. Titration curves in fluorescence spectral change of 3 (1.0 × 10–5 M in CH3CN) upon
addition of KClO4.
Figure S27. Titration curves in fluorescence spectral change of 3 (1.0 × 10–5 M in CH3CN) upon
addition of Ba(ClO4)2.
Figure S28. Titration curves in fluorescence spectral change of 3 (1.0 × 10–5 M in CH3CN) upon
addition of RbClO4.
Figure S29. Titration curves in fluorescence spectral change of 3 (1.0 × 10–5 M in CH3CN) upon
addition of CsClO4.
Figure S30. Molecular orbitals of free and metal ion complexed 1, calculated by using B3LYP/3-
21G.
Figure S31. Molecular orbitals of free and metal ion complexed, calculated by using B3LYP/3-21G.
Figure S32. Energy levels of molecular orbitals of 1 and 2 calculated by using B3LYP/3-21G.
Figure S33. Optimized structures and dihedral angles of the ethylene moieties of free and metal ion
complexed 1 and 2.
Figure S34. Stern-Volmer plot for fluorescence quenching of 8 (1.0 × 10–5 M) in CH3CN upon
addition of 9b (0-500 equiv).
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72.
Pauling, L. (1960) The Nature of the Chemical Bond, 3rd Ed. Cornell University Press,
Ithaca, New York.
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FIGURE CAPTIONS
Scheme 1. Ethynylpyrene linked benzocrown ethers.
Scheme 2. Synthesis of ethynylpyrene linked benzocrown ethers 1-4 and reference compound 14.
Figure 1. Fluorescence spectra of 1 (1.0 × 10–5 M in CH3CN) upon addition of (a) LiClO4, (b)
NaClO4, (c) KClO4, (d) RbClO4, (e) CsClO4, (f) Mg(ClO4)2, and (g) Ba(ClO4)2. The excitation
wavelength was 386 nm.
Figure 2. Fluorescence spectra of 2 (1.0 × 10–5 M in CH3CN) upon addition of (a) LiClO4, (b)
NaClO4, (c) KClO4, (d) RbClO4, (e) CsClO4, (f) Mg(ClO4)2, and (g) Ba(ClO4)2. The excitation
wavelength was 388 nm.
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Figure 3. Fluorescence spectra of 3 (1.0 × 10–5 M in CH3CN) upon addition of (a) LiClO4, (b)
NaClO4, (c) KClO4, (d) RbClO4, (e) CsClO4, (f) Mg(ClO4)2, and (g) Ba(ClO4)2. The excitation
wavelength was 388 nm.
Figure 4. (a)-(c) UV-vis absorption and (d)-(f) fluorescence spectra of 4 (1.0 × 10–5 M in 1:1
CH3CN:CHCl3) upon addition of (a) and (d) LiClO4, (b) and (e) NaClO4, and (c) and (f) Mg(ClO4)2.
The excitation wavelength was 477 nm.
Figure 5. Images showing color changes of 4 (1.0 × 10–5 M in 1:1 CH3CN:CHCl3) containing (a) 0
equiv, (b) 10 equiv, (c) 20 equiv and (d) 30 equiv of Mg(ClO4)2. The solutions were irradiated at ex =
254 nm.
Figure 6. Job's plots for formation of complexes (a) 2-LiClO4, (b) 2-KClO4, (c) 2-Ba(ClO4)2, and
(d) 3-Ba(ClO4)2, obtained from UV absorption spectroscopic change at abs = 254 nm occurring upon
mix of both 1.0 × 10–5 M CH3CN solutions in ratios from 10:0 to 0:10.
Figure 7. (a) Experimental procedure for evaluating the durability to use 2 to detect Mg2+ ion, (b)
fluorescence spectra of 2 (1.0 × 10–5 M in CH3CN), ex = 388 nm, (c) fluorescence intensity at 394
nm, upon repeated addition and removal of Mg(ClO4)2.
Figure 8. Molecular orbitals of 1 and 2 free or complexed with Li+ and Mg2+, calculated by using
B3LYP/3-21G.
Figure 9. Possible mechanistic pathways for changes in the fluorescence of the ethynylpyrene
linked benzocrown ethers occurring upon complexation with metal ions.
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Table 1. Association constants for complex formation between
1-3 and metal ions.
metal ionic
K (M–1)b
ion
radius
a
1
2
3
(12-crown-
4)
(15-crown-
5)
(18-crown-6)
(pm)
60
Li+
11870
1764
29000
838
477
2815
405
Mg2+ 65
13710
904
Na+
K+
95
56720
18810
>105
133
135
148
169
704
Ba2+
Rb+
Cs+
3623
460
9918
136
20060
3773
848
111
a) Data from ref (72). b) Association constants calculated by
using TitrationFit 2014-0630 based on fluorescence intensity
changes.
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Table 2. Calculated energies of , * and M* orbitals and O-C-C-O dihedral angles of 1 and 2 free or complexed with metal ions.a
compound
ion
M*
Dihedral angles
(°)
*

(eV)
(eV)
(eV)
1 (12-crown-4)
1 (12-crown-4)
1 (12-crown-4)
1 (12-crown-4)
1 (12-crown-4)
1 (12-crown-4)
2 (15-crown-5)
2 (15-crown-5)
2 (15-crown-5)
2 (15-crown-5)
2 (15-crown-5)
2 (15-crown-5)
none
Li+
–5.12 (HOMO)
–7.08 (HOMO)
–7.02 (HOMO)
–6.94 (HOMO)
–6.42 (HOMO)
–8.90 (HOMO)
–4.95 (HOMO)
–6.75 (HOMO)
–6.79 (HOMO)
–6.67 (HOMO)
–6.77 (HOMO)
–8.76 (HOMO)
–1.80 (LUMO)
–3.97 (LUMO)
–3.91 (LUMO)
–3.78 (LUMO)
–4.01 (LUMO)
–5.13 (LUMO+6)
–1.66 (LUMO)
–3.54 (LUMO)
–3.34 (LUMO+1)
–3.40 (LUMO+1)
–3.54 (LUMO+1)
–4.68 (LUMO+6)
N/A
65, 81, 88
–2.90 (LUMO+3)
–3.49 (LUMO+1)
–3.68 (LUMO+1)
–3.62 (LUMO+1)
–8.22 (LUMO)
N/A
49, 55, 50
Na+
K+
56, 55, 64
50, 54, 70
Rb+
Mg2+
none
Li+
66, 56, 71
30, 44, 46
67, 69, 76, 71
66, 51, 53, 174
63, 57, 62, 177
66, 57, 55, 160
66, 57, 55, 168
45, 50, 46, 40
–2.79 (LUMO+1)
–3.56 (LUMO)
–2.80 (LUMO)
–3.62 (LUMO)
–6.69 (LUMO)
Na+
K+
Rb+
Mg2+
a) Calculated by using B3LYP/3-21G.
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