A selective and sensitive phenanthroimidazole-based "reactive" ratiometric sensor for recognition of Hg2+ ions in aqueous solution

Article abstract of DOI:10.1002/cplu.201402266

Based on specific reactivity of the vinyloxy group to Hg²⁺ ions, a new ratiometric phenanthroimidazole-based fluorescent sensor, 2-(2-(vinyloxy)phenyl)-1H-phenanthro[9,10-d]imidazole, is prepared. The selectivity of the probe toward Hg²⁺

Full text of DOI:10.1002/cplu.201402266

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DOI: 10.1002/cplu.201402266
A Selective and Sensitive Phenanthroimidazole-Based
2
+
“
Reactive” Ratiometric Sensor for Recognition of Hg Ions
in Aqueous Solution
[
a]
Yuanyuan Yan, Rui Zhang, Xiang Yu, and Hui Xu*
2
+
Based on specific reactivity of the vinyloxy group to Hg ions,
new ratiometric phenanthroimidazole-based fluorescent
sensor, 2-(2-(vinyloxy)phenyl)-1H-phenanthro[9,10-d]imidazole,
a
2
+
is prepared. The selectivity of the probe toward Hg ions is
very high, and shows minimal interference from other com-
monly coexistent metal ions. The reactive stoichiometry of the
2
+
probe with Hg ion is 2:1. Its detection limit is evaluated and
2
+
Hg ions can be detected down to a lower limit of 2.5ꢀ
À8
2+
10
m. Moreover, the Hg -promoted selective hydrolysis of
1
a vinyloxy group of the probe is further confirmed by H NMR
spectroscopy and mass spectrometry.
Scheme 1. Synthetic approach for sensor 3. DMSO=dimethyl sulfoxide.
Mercury is one of the most dangerous and toxic environmental
pollutants to human beings. Even at a low concentration the
benzaldehyde (1) in a 74% yield. Then in the presence of
tBuOK, 2-vinyloxybenzaldehyde (2) was obtained in a 38%
yield. Finally, compound 2 was treated with 9,10-phenanthre-
2
+
Hg
ion can bioaccumulated through the food chain and
1
cause damage to the central nervous system and other organs
nequinone and afforded 3, which was characterized by H and
[
1,2]
13
by effecting the blood–brain barrier.
Accordingly, develop-
C NMR spectroscopy, and mass spectrometry. As shown in
ment of selective and sensitive chemosensors for the detection
Figure 1, the UV/Vis spectrum of 3 showed a strong band at
388 nm (excitation=310 nm). When Hg was added to a solu-
2
+
2+
of Hg ions over other heavy- and transition-metal (HTM) ions
[
3]
has received much attention. To overcome the incomplete
tion of 3, interestingly, although the fluorescence intensity at
388 nm decreased, an increase in the fluorescence intensity
having a red-shift at 445 nm was observed. Whereas other
2
+
selectivity for Hg ions over other competing metal ions, de-
velopment of more accurate and sensitive ratiometric fluores-
2
+
3+
2+
2+
2+
2+
2+
cent probes based on the specific recognition of Hg ion is
highly desirable. Many attractive sensors for the detection of
metal ions tested, such as Fe , Ca , Ni , Cu , Ba , Zn ,
2
+
2+
2+
2+
2+
+
+
+
Cd , Co , Pb , Mn , Mg , K , Cs , and Ag , showed no
clear enhancement or decrease in the fluorescence intensity.
2
+
Hg ions have been reported that depend on a chemical re-
[
4]
action specific to the mercury species. Recently, a few phe-
nanthroimidazole-based fluorescent probes were used for the
2
+ [5a]
2+ [5b]
À [5c]
recognition of Mg
,
Cu
,
ClO , cysteine and homocys-
[
5d]
2+
2+ [5e]
[5f]
teine, Cu and Cd
,
and H S.
As part of our ongoing
2
research program on developing selective and sensitive che-
2
+
[6]
mosensors for the detection of Hg ions, herein we have de-
signed and prepared a new ratiometric phenanthroimidazole-
based fluorescent sensor 3 (Scheme 1) by introduction of the
vinyl group into the fluorophore. Hereby the specific reactivity
2
+
of Hg ions with the vinyloxy unit is exploited.
As shown in the Scheme 1, 2-hydroxybenzaldehyde was
treated with 1,2-dibromoethane and gave 2-(2-bromoethoxy)-
+
+
[
[
a] Dr. Y. Yan, R. Zhang, X. Yu, Prof. Dr. H. Xu
Research Institute of Pesticidal Design & Synthesis
College of Plant Protection/Science
Northwest A&F University, Yangling 712100 (P. R. China)
Fax: (+86)029-87091952
E-mail: orgxuhui@nwsuaf.edu.cn
À6
Figure 1. Variation of the fluorescence intensity of 3 (5ꢀ10 m) in CH
3
CN-
+
] These authors contributed equally to this work
HEPES (1/1, v/v; 0.02m, pH 7.0) at 293 K in the presence of 10.0 equivalents
2
+
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402266.
of the respective metal ions; the excitation wavelength for Hg was
330 nm, and excitation wavelength for other ions was 310 nm.
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1
Figure 2. Partial HNMR spectra of 3 (a), 4 (b) and 4’ (c) in [D
6
]DMSO.
We envisaged that the above results may be due to selective
the signal for these protons disappear while a new signal cor-
2
+
reactivity of the vinyloxy group of 3 in the presence of Hg
ions. To testify this idea, the product 4 was separated from
responding to the OH proton of 4 appears at d=13.72 ppm
1
(Figure 2b). Moreover, the H NMR spectrum of 4 is identical to
2
+
a mixture of 3 and Hg ions (see the Supporting Information).
Solid evidence for the proposed reaction came from compar-
ing the ESI-MS spectra of 3 and 4. As shown in Scheme 2, the
that of the control 4’ (Figure 2c). Therefore, 4 and 4’ are the
same compound. Based on the above results, a proposed sens-
ing mechanism is described in Scheme 4.
+
signal at m/z 337.32 corresponds to 3 ([M+H] ), whereas the
The ratios of fluorescent intensity at 445 and 388 nm (I /
445
+
2+
2+
2+
2+
3+
signal at m/z 311.28 corresponds to 4 ([M+H] ). Meanwhile,
I388) of sensor 3 in the presence of Hg , Ca , Ni , Cu , Fe
2
+
2+
2+
2+
2+
2+
2+
+
+
+
we directly prepared 4’ as a control by treatment of 9,10-phe-
, Ba , Zn , Cd , Co , Pb , Mn , Mg , K , Cs , and Ag
nanthrenequinone with 2-hrdroxybenzaldehyde (Scheme 3).
in a solution of CH CN-HEPES (1/1 v/v; 0.02m, pH 7.0) are
3
shown in Figure 3. Notably, a drastic increase of I445/I388 value
2
+
from 0.11 to 4.21 was observed only when Hg was added to
the solution of 3. This increase in intensity was accompanied
by a resonance color change from purple/blue to bright blue
Scheme 2. The selective cleavage of probe 3 to form 4 in the presence of
2
+
Hg ions.
2
+
Evidence for the Hg -promoted hydrolysis reaction was fur-
1
ther supported by comparing H NMR spectra of 3, 4, and 4’.
1
As shown in the partial H NMR spectrum of Figure 2a, the
chemical shifts corresponding to the -CH=CH protons of 3
2
2
+
appear at d=4.77 and 5.15 ppm; upon addition of Hg ions,
À6
Figure 3. Fluorescent intensity ratio (I445/I388) of 3 (5ꢀ10 m) in CH
3
CN-
HEPES (1/1, v/v; 0.02m, pH 7.0) at 293 K in the presence of 10.0 equivalents
2
+
of different metal ions; the excitation wavelength for Hg was 330 nm, and
excitation wavelength for other ions was 310 nm. Inset: Photograph of 3 in
À6
À6
the presence of various metal ions. A: 3 (5ꢀ10 m); Q: 4 (5ꢀ10 m); B–P:
2
+
+
2+
2+
2+
2+
+
3+
3
3
+Hg , 3+Ag , 3+Ba , 3+Ca , 3+Cd , 3+Co , 3+Cs , 3+Fe ,
+ +
+Cu , 3+K , 3+Mg , 3+Mn , 3+Ni , 3+Pb , and 3+Zn
2
2+
2+
2+
2+
2+
À5
Scheme 3. Preparation of 4’ as the control.
(5ꢀ10 m).
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To investigate the utility of 3
as an ion-selective fluorescent
2
+
sensor for Hg ions, cross-con-
tamination tests were conducted
2
+
in the presence of Hg at a con-
À6
centration of 2.5ꢀ10 m mixed
with other metal ions such as
2
+
2+
2+
+
3+
Ni
,
Ca
,
Ba
,
Cs , Fe
,
2
+
2+
2+
2+
+
Mn , Cd , Zn , Mg , K ,
+
2+
2+
2+
Ag , Hg , Co , and Pb
at
À5
5
ꢀ10 m, respectively (Figure 5).
This experiment demonstrated
that although the concentration
of these competing metal ions
was 20-fold greater than that of
2
+
Hg , the selectivity of 4 to-
2
+
wards Hg was essentially un-
changed in the presence of com-
petitive ions.
4 2.
Scheme 4. The possible reaction mechanism of 3 for selective recognition of Hg(ClO )
under illumination with UV light at 365 nm. In contrast other
metal ions had negligible response. The I445/I388 value of 4’ was
equal to 4.26, and was used as the control. The reaction time
À6
2+
À6
profile of 3 (5ꢀ10 m) towards Hg (2.5ꢀ10 m) in aqueous
solution was examined (see Figure S1 in the Supporting Infor-
mation), and the I445/I388 value reached a maximum and re-
mained steady after 20 min.
2
+
The fluorescent intensity of 3+Hg remained almost con-
stant when the pH value was maintained between 4.5 and 12
(
Figure S2). The titration reaction curve of probe 3 toward
2
+
Hg ions is shown in Figure 4. Upon excitation at 330 nm, the
free 3 displays a double emission band centered at 388 nm.
2
+
On addition of Hg ions, the fluorescent intensity of 3 at
88 nm significantly decreases with the simultaneous appear-
3
À6
Figure 5. Results of the competition experiments for 3 (5ꢀ10 m) between
ance of a new red-shift emission band at around 445 nm. The
2
+
Hg and other metal ions in CH
3
2+
CN-HEPES (1/1, v/v; 0.02m, pH 7.0) at
93 K; the concentration of Hg ions was 2.5ꢀ10 m, and that of each
fluorescent intensity ratio at 445 and 388 nm (I445/I338) of 3 in-
À6
2
À6
2+
creases linearly at 1–2ꢀ10 m of Hg ions. Furthermore, the
À5
competing metal ion was 5ꢀ10 m.
titration reaction curve shows a steady increase until a plateau
2
+
is reached when 0.5 equivalents of Hg ions are added.
2
+
Finally, to examine the sensitivity of 3 towards Hg ions,
the detection limit was evaluated. As shown in Figure 6, the
À5
2+
fluorescence titration profile of 3 (1ꢀ10 m) with Hg ions in-
2
+
À8
dicates that Hg ions can be detected down to 2.5ꢀ10 m,
and the I4 /I338 value of 3 increases linearly with the concentra-
45
2
+
À7
2
tion of Hg ion (0–5.0)ꢀ10 m (R =0.9944).
In summary, we have developed a phenanthroimidazole-
based “reactive” ratiometric fluorescent probe for selective and
2
+
sensitive detection of Hg ions in aqueous solution. The reac-
2
+
tive stoichiometry of the probe with Hg ion was 2:1. It was
shown that the probe also responded to organomercury spe-
2
+
cies. Its selectivity toward Hg ions was very high, and little
interference was detected for other commonly coexistent
2
+
metal ions. Moreover, the Hg -promoted selective hydrolysis
1
of a vinyloxy group of 3 was further confirmed by H NMR
spectroscopy and MS spectrometery. The present probe could
À6
Figure 4. Fluorescence titration of 3 (5ꢀ10 m) in CH
3
CN-HEPES (1/1, v/v;
2
+
2
+
be used to determine Hg ion concentrations in aqueous en-
vironments having a wide range of pH values.
0
.02m, pH 7.0) at 293 K with the concentration of Hg ions ranging from 0
to 3ꢀ10 m; the excitation wavelength was 330 nm.
À6
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1
3
31.40, 127.5, 125.6, 124.2, 121.6, 117.4, 96.7 ppm; ESI-MS: m/z (%):
37.32 (100) [M+H] .
+
Acknowledgements
The present research was supported by the Special Funds of Cen-
tral Colleges Basic Scientific Research Operating Expenses
(
YQ2013008), Northwest A&F University.
Keywords: fluorescence · mercury · phenanthroimidazoles ·
reactive probes · sensors
À5
Figure 6. Fluorescence titration of 3 (1ꢀ10 m) with the concentration of
[
[
[
1] a) H. N. Kim, W. X. Ren, J. S. Kim, J. Yoon, Chem. Soc. Rev. 2012, 41, 3210–
244; b) H. N. Kim, M. H. Lee, H. J. Kim, J. S. Kim, J. Yoon, Chem. Soc. Rev.
008, 37, 1465–1472.
2] a) I. Onyido, A. R. Norris, E. Buncel, Chem. Rev. 2004, 104, 5911–5929;
b) W. F. Fitzgerald, C. H. Lamborg, C. R. Hammerschmidt, Chem. Rev. 2007,
2
+
À7
3
Hg ranging from 0 to 5ꢀ10 m in CH CN-HEPES (1/1, v/v, 0.02m, pH 7.0)
at 293 K; the excitation wavelength was 330 nm.
3
2
1
07, 641–662.
3] For selected examples, see: a) A. B. Descalzo, R. Martinez-Manez, R. Rade-
glia, K. Rurack, J. Soto, J. Am. Chem. Soc. 2003, 125, 3418–3419; b) D. Li,
L. D. Campbell, B. A. Austin, K. Koide, ChemPlusChem 2012, 77, 281–283;
c) P. Scharf, J. Mꢂller, ChemPlusChem 2013, 78, 20–34; d) M. C. Aragoni,
M. Arca, F. Demartin, F. A. Devillanova, F. Isaia, A. Garau, V. Lippolis, F.
Jalali, U. Papke, M. Shamsipur, L. Tei, A. Yari, G. Verani, Inorg. Chem. 2002,
41, 6623–6632; e) J. P. Xie, Y. G. Zheng, J. Y. Ying, Chem. Commun. 2010,
46, 961–963; f) Y. Zhou, C. Y. Zhu, X. S. Gao, X. Y. You, C. Yao, Org. Lett.
2010, 12, 2566–2569; g) B. L. Ma, F. Zeng, F. Y. Zheng, S. Z. Wu, Chem.
Eur. J. 2011, 17, 14844–14850; h) G. Pelossof, R. Tel-Vered, X. Q. Liu, I.
Willner, Chem. Eur. J. 2011, 17, 8904–8912; i) T. Kang, S. M. Yoo, I. Yoon, S.
Lee, J. Choo, S. Y. Lee, B. Kim, Chem. Eur. J. 2011, 17, 2211–2214; j) B.
Tang, B. Y. Ding, K. H. Xu, L. L. Tong, Chem. Eur. J. 2009, 15, 3147–3151;
k) P. S. Salini, A. P. Thomas, R. Sabarinathan, S. Ramakrishnan, K. C. G.
Sreedevi, M. L. P. Reddy, A. Srinivasan, Chem. Eur. J. 2011, 17, 6598–6601;
l) L. Liu, G. X. Zhang, J. F. Xiang, D. Q. Zhang, D. B. Zhu, Org. Lett. 2008,
Experimental Section
Synthesis of 2-(2-bromoethoxy)benzaldehyde (1): To a stirred so-
lution of 2-hydroxybenzaldehyde (1220 mg, 10.0 mmol) in dry
CH CN (30 mL), 1,2-dibromoethane (9400 mg, 50.0 mmol) and an-
hydrous K CO (2070 mg, 15.0 mmol) were added. The reaction
mixture was heated at reflux for 15 h then cooled to RT. The mix-
ture was filtered, and the filtrate was concentrated in vacuo and
purified by column chromatograph on silica gel y using petroleum
ether/ethyl acetate (8/1, v/v) as the eluent to afford 1 (1.694 g,
4% yield) as a white solid. M.p. 53–548C; H NMR (400 MHz,
3
2
3
1
7
CDCl ): d=10.54 (s, 1H), 7.87–7.84 (m, 1H), 7.57–7.53 (m, 1H),
.09–7.05 (m, 1H), 6.97 (d, J=8.4 Hz, 1H), 4.44 (t, J=6.0 Hz, 2H),
.73 ppm (t, J=6.0 Hz, 2H); ESI-MS: m/z (%): 229.02 (37) [M+H] ,
3
7
3
2
+
+
31.04 (33) [M+H] .
1
1
0, 4581–4584; m) X. F. Guo, X. H. Qian, L. H. Jia, J. Am. Chem. Soc. 2004,
26, 2272–2273; n) Z. Guo, W. Zhu, M. Zhu, X. Wu, H. Tian, Chem. Eur. J.
Synthesis of 2-vinyloxybenzaldehyde (2): To a solution of com-
pound 1 (1145 mg, 5.0 mmol) in DMSO (20 mL) was added tBuOK
673 mg,6.0 mmol). The reaction mixture was stirred at RT for
5 min. Then the mixture was diluted with ethyl acetate (30 mL),
and washed with ice water (3ꢀ20 mL) and brine (3ꢀ20 mL). The
organic layer was dried over anhydrous Na SO , concentrated in
vacuo and purified by column chromatography on silica gel using
petroleum ether/ethyl acetate (10/1, v/v) as the eluent to give 2
281 mg, 38% yield) as a colorless oil. H NMR (400 MHz, CDCl ):
d=10.45 (s, 1H), 7.89–7.86 (m, 1H), 7.59–7.55 (m, 1H), 7.19–7.16
2
010, 16, 14424–14432; o) H. N. Kim, S. Nam, K. M. K. Swamy, Y. Jin, X.
Chen, Y. Kim, S. Kim, S. Park, J. Yoon, Analyst 2011, 136, 1339–1343;
p) R. M. Souto, Y. Gonzalez-Garcia, D. Battistel, S. Daniele, Chem. Eur. J.
2012, 18, 230–236; q) P. J. J. Huang, J. W. Liu, Chem. Eur. J. 2011, 17,
5004–5010; r) X. Chen, S. W. Nam, M. J. Jou, Y. Kim, S. J. Kim, S. Park, J.
Yoon, Org. Lett. 2008, 10, 5235–5238; s) N. M. M. Moura, C. Nunez, S. M.
Santos, M. A. F. Faustino, J. A. S. Cavaleiro, M. G. P. M. S. Neves, J. L.
Capelo, C. Lodeiro, ChemPlusChem 2013, 78, 1230–1243.
(
1
2
4
1
[4] For selected examples, see: a) M. Y. Chae, A. W. Czarnik, J. Am. Chem. Soc.
992, 114, 9704–9705; b) M. H. Lee, S. W. Lee, S. H. Kim, C. Kang, J. S.
(
3
1
Kim, Org. Lett. 2009, 11, 2101–2104; c) M. H. Lee, B.-K. Cho, J. Yoon, J. S.
Kim, Org. Lett. 2007, 9, 4515–4518; d) J. S. Wu, I.-C. Hwang, K. S. Kim, J. S.
Kim, Org. Lett. 2007, 9, 907–910; e) M. G. Choi, Y. H. Kim, J. E. Namgoong,
S. K. Chang, Chem. Commun. 2009, 3560–3562; f) W. Jiang, W. Wang,
Chem. Commun. 2009, 3913–3915; g) G. X. Zhang, D. Q. Zhang, S. W. Yin,
X. D. Yang, Z. G. Shuai, D. B. Zhu, Chem. Commun. 2005, 2161–2163;
h) B. Liu, H. Tian, Chem. Commun. 2005, 3156–3158; i) X. Zhang, Y. Xiao,
X. Qian, Angew. Chem. Int. Ed. 2008, 47, 8025–8029; Angew. Chem. 2008,
120, 8145–8149; j) J. Ros-Lis, M. D. Marcos, R. Martinez-Manez, K. Rurack,
J. Soto, Angew. Chem. Int. Ed. 2005, 44, 4405–4407; Angew. Chem. 2005,
117, 4479–4482; k) F. Wang, S. Nam, Z. Guo, S. Park, J. Yoon, Sens. Actua-
tors B 2012, 161, 948–953; l) X. Cheng, S. Li, H. Jia, A. Zhong, C. Zhong,
J. Feng, J. Qin, Z. Li, Chem. Eur. J. 2012, 18, 1691–1699; m) J. Jiang, W.
Liu, J. Cheng, L. Yang, H. Jiang, D. Bai, W. Liu, Chem. Commun. 2012, 48,
(
(
m, 1H), 7.09 (d, J=8.4 Hz, 1H), 6.73 (dd, J=13.6, 6.0 Hz, 1H), 4.88
dd, J=13.6, 2.0 Hz, 1H), 4.61 ppm (dd, J=6.0, 2.0 Hz, 1H); ESI-MS:
+
m/z (%): 149.03 (22) [M+H] .
Synthesis of 2-(2-(vinyloxy)phenyl)-1H-phenanthro[9,10-d]imida-
zole (3):
.0 mmol),
A
mixture of 9, 10-phenanthroquinone (208 mg,
2 (148 mg, 1.0 mmol), and ammonium acetate
1
(
1540 mg, 20 mmol) in glacial HOAc (10 mL) was heated at 908C
for 4 h. The mixture was cooled to RT and filtered to collect the
crude white solid, which was purified by preparative thin-layer
chromatography using petroleum ether/ethyl acetate (5/1, v/v) as
the eluent to afford 3 (141 mg, 42% yield) as a white solid. M.p.
1
1
98–2008C; H NMR (400 MHz, [D ]DMSO): d=13.15 (s, 1H), 8.88 (t,
6
8371–8373.
J=8.4 Hz, 2H), 8.59 (d, J=0.8 Hz, 1H), 8.57 (d, J=0.8 Hz, 1H), 8.11
dd, J=8.0, 2.0 Hz, 1H), 7.74 (t, J=7.2 Hz, 2H), 7.65 (t, J=7.6 Hz,
[
5] a) K. C. Song, M. G. Choi, D. H. Ryu, K. N. Kim, S. K. Chang, Tetrahedron
Lett. 2007, 48, 5397–5400; b) W. Lin, L. Yuan, W. Tan, J. Feng, L. Long,
Chem. Eur. J. 2009, 15, 1030–1035; c) W. Lin, L. Long, B. Chen, W. Tan,
Chem. Eur. J. 2009, 15, 2305–2309; d) W. Lin, L. Long, L. Yuan, Z. Cao, B.
Chen, W. Tan, Org. Lett. 2008, 10, 5577–5580; e) J. Wang, W. Lin, W. Li,
(
2
1
1
H), 7.53–7.57 (m, 1H), 7.30–7.36 (m, 2H), 6.96 (dd, J=13.6, 6.0 Hz,
H), 4.86 (dd, J=13.6, 1.6 Hz, 1H), 4.56 ppm (dd, J=6.0, 1.6 Hz,
1
3
H); C NMR (100 MHz, [D ]DMSO): d=153.9, 148.8, 146.6, 131.43,
6
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Chem. Eur. J. 2012, 18, 13629–13632; f) K. Zheng, W. Lin, L. Tan, Org.
Biomol. Chem. 2012, 10, 9683–9688.
628–631; e) H. L. Dai, Y. Y. Yan, Y. Guo, L. L. Fan, Z. P. Che, H. Xu, Chem.
Eur. J. 2012, 18, 11188–11191.
[6] a) H. L. Dai, H. Xu, Bioorg. Med. Chem. Lett. 2011, 21, 5141–5144; b) Y.
Yan, Z. Che, X. Yu, X. Zhi, J, Wang, H. Xu, Bioorg. Med. Chem. 2013, 21,
508–513; c) Y. Guo, Y. Yan, X. Zhi, C. Yang, H. Xu, Bioorg. Med. Chem. Lett.
Received: August 17, 2014
2
013, 23, 3382–3384; d) Y. Yan, Y. Zhang, H. Xu, ChemPlusChem 2013, 78,
Published online on October 31, 2014
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