A 2,5-diaryl-1,3,4-oxadiazole-based fluorescent probe for rapid and highly selective recognition of hydrogen sulfide with a large Stokes shift through switching on ESIPT

Article abstract of DOI:10.1016/j.tetlet.2016.02.056

A new 2,5-diaryl-1,3,4-oxadiazole derived ratiometric fluorescent probe (OXDNP) for hydrogen sulfide recognition has been developed. Probe OXDNP displays highly selective and sensitive detection to HS^? over other anions and thiol-containing amino acids in DMSO solution with fast response and a large Stokes shift. Through HS^? induced thiolysis of the dinitrophenyl ether, the excited state intramolecular proton transfer (ESIPT) featured precursor was released, which led to dual fluorescence emission ‘turn on’ and ratiometric emission behavior of the sensing system. The pseudo-first-order reaction rate constant was calculated to be 1.234 s^?1. The HS^? recognition mechanism was proved by HPLC–MS and ¹H NMR comparison investigations.

Full text of DOI:10.1016/j.tetlet.2016.02.056

Accepted Manuscript
A 2,5-diaryl-1,3,4-oxadiazole-based fluorescent probe for rapid and highly se-
lective recognition of hydrogen sulfide with a large Stokes shift through switch-
ing on ESIPT
Lijun Tang, Zhuxuan Zheng, Keli Zhong, Yanjiang Bian
PII:
S0040-4039(16)30165-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.02.056
TETL 47331
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
11 December 2015
3 February 2016
15 February 2016
Please cite this article as: Tang, L., Zheng, Z., Zhong, K., Bian, Y., A 2,5-diaryl-1,3,4-oxadiazole-based fluorescent
probe for rapid and highly selective recognition of hydrogen sulfide with a large Stokes shift through switching on
ESIPT, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.02.056
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A 2,5-diaryl-1,3,4-oxadiazole-based fluorescent probe for rapid and highly
selective recognition of hydrogen sulfide with a large Stokes shift through
switching on ESIPT
Lijun Tang, Zhuxuan Zheng, Keli Zhong, Yanjiang Bian
Department of Chemistry, Bohai University, Jinzhou 121013, China
Graphical abstract
A new 2,5-diaryl-1,3,4-oxadiazole-based ratiometric fluorescent probe (OXDNP)
for rapid and highly selective recognition of hydrogen sulfide has been developed.
 Corresponding author. Tel.: +86-416-3400302; E-mail address: ljtang@bhu.edu.cn
1

A 2,5-diaryl-1,3,4-oxadiazole-based fluorescent probe for rapid and highly
selective recognition of hydrogen sulfide with a large Stokes shift through
switching on ESIPT
Lijun Tang, Zhuxuan Zheng, Keli Zhong, Yanjiang Bian
Department of Chemistry, Bohai University, Jinzhou 121013, China
Abstract: A new 2,5-diaryl-1,3,4-oxadiazole derived ratiometric fluorescent probe
(OXDNP) for hydrogen sulfide recognition has been developed. Probe OXDNP
displays highly selective and sensitive detection to HS- over other anions and
thiol-containing amino acids in DMSO solution with fast response and a large Stokes
shift. Through HS- induced thiolysis of the dinitrophenyl ether, the excited state
intramolecular proton transfer (ESIPT) featured precursor was released, which led to
dual fluorescence emission “turn on” and ratiometric emission behavior of the sensing
system. The pseudo-first-order reaction rate constant was calculated to be 1.234 s-1.
1
The HS- recognition mechanism was proved by HPLC-MS and H NMR comparison
investigations.
Keywords: Hydrogen sulfide; Fluorescence probe; Ratiometric; ESIPT;
1,3,4-Oxadiazole
1. Introduction
Hydrogen sulfide (H2S) is a inflammable colorless gas with a distinctive odor of
rotten eggs, exposure to H2S can elicit irritation of the eyes and infuriation in the
respiratory tract.1 Abnormal level in H2S has also been recognized to be related with
 Corresponding author. Tel.: +86-416-3400302; E-mail address: ljtang@bhu.edu.cn
2

several diseases including Alzheimer’s disease, Down’s syndrome, diabetes, and liver
cirrhosis.2 On the other hand, H2S plays paramount roles in a wide range of
physiological processes,3 such as vasodilation, neuromodulation, apoptosis,
anti-inflammation, anti-oxidation, inhibition of insulin signaling.4 Therefore, easy and
convenient methods for H2S detection are of importance for rapid assessment of this
biologically and environmentally important species.
To date, various H2S detection methods including polarography, electrochemical
analysis, gas chromatography, colorimetry method as well as semiconductor-based
transistors/resistors have been developed.5 Among the methods, fluorescence
detection of H2S continuously attracted the attention of researchers due to the
operational simplicity, real-time response and high sensitivity of fluorescence
technique. Recently, several types of reaction-based fluorescent probes for H2S
detection have been reported, mainly including H2S-triggered azide reduction,6
nucleophilic addition,7 H2S induced metal ion displacement of Cu2+ or Zn2+
complexes,8 and H2S-induced cleavage of 2,4-dinitrophenyl ether.9 Superior to the
probes that depend on single emission intensity alteration, ratiometric fluorescent
probes could monitor the sensing process at two emission wavelength and possess a
distinct advantage of built-in correction, which can avoid various environmental
effects and favorable to more accurate detection. Although a few ratiometric
fluorescent probes for H2S detection have been documented,7d, 10 most of the probes
suffered from a delayed response time (more than 20 min) except the works reported
by Guo,7c Tang,7d Guo and He,7e and Goswami.11 Therefore, development of new
3

fluorescent probes for H2S recognition with ratiometric emission output and rapid
response is still highly desirable.
Recent studies manifest that fluorophores with ESIPT characteristics are ideal
candidates for constructing ratiometric fluorescent probes due to their dual emission
bands and large Stokes shift. Besides the commonly used ESIPT featured
fluorophores
including
2-(2′-hydroxypehnyl)benzoxazole,
2-(2′-hydroxypehnyl)benzimidazole, and
2-(2′-hydroxypehnyl)benzothiazole,
3-hydroxyflavone, the ESIPT behavior of some 2,5-diaryl-1,3,4-oxadiazole
compounds has also been investigated.12 However, most of the reports on
2,5-diaryl-1,3,4-oxadiazole derivatives are mainly focused on the synthetic methods13
and their application as ligands for organic light-emitting diode (OLED) materials,14
taking the advantage of ESIPT characteristic of these fluorogens for metal ion or
anion recognition are still rare,15 partly due to the short emission wavelength of these
fluorogens. For instance, 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol (1) (Scheme 1),
which displays ESIPT behavior, generally emits at a wavelength less than 500 nm.15a,
16
A commonly employed method to obtain a long wavelength emission fluorogen is
to expand the conjugation system of a certain fluorophore. Thus, it is reasonable to
envision that further expand the conjugation effect of 1 may result in
longer-wavelength emission while retain the ESIPT property. Our preliminary
examination showed that compound 2, which possesses a longer conjugation system
than that of 1, indeed exhibits dual emission in DMSO solution with a
shorter-wavelength emission at 418 nm and a longer-wavelength emission at 556 nm.
4

Along with this line, we herein designed and synthesized a new
2,5-diaryl-1,3,4-oxadiazole derived fluorescent probe OXDNP by masking the
phenolic hydroxyl group of 2 with 2,4-dinitrophenyl (DNP) group (Scheme 1). The
incorporated H2S reactive DNP ether moiety acts not only as a fluorescence
quencher,17 but also as a ESIPT inhibition group. We speculated that the H2S induced
nucleophilic cleavage of the ether bond in OXDNP can release free 2 and thereby
restore the dual emission and large Stokes shift behavior of 2, thus realizing a
selectively fluorescence “turn on” recognition of H2S. Subsequent investigations
demonstrate that OXDNP indeed behaves highly selective and sensitive ratiometric
recognition to H2S with a fast response and a large Stokes shift in DMSO solution.
Scheme 1. The structure of model compound 1 and synthesis of probe OXDNP.
2. Results and discussion
Probe OXDNP was readily prepared by reaction of 218 with
2,4-dinitrofluorobenzene in DMF as illustrated in Scheme 1, which was structurally
1
13
characterized by H NMR, C NMR and HRMS analysis (see supplementary data).
The fluorescence responses of OXDNP to different anions, thiol-containing amino
acids were firstly evaluated (Fig. 1). As expected, probe OXDNP (10 μM) exhibits a
weak emission band centered at 413 nm in DMSO (λex = 334 nm), which is attributed
to the photo-induced electron transfer (PET) process resulted from the DNP moiety.
5

Upon addition of HS- (NaHS was used as H2S source), dramatic emission
enhancements of two bands at 418 nm and 556 nm were observed. The major
emission band centered at 418 nm was attributed to the enol emission from 2, and the
emission band centered at 556 nm came from the H-transferred Keto tautomer
emission, which results in a Stokes shift as large as 222 nm. Whereas, on individual
3-
2-
addition of various anions including AcO-, F-, Br-, Cl-, CN-, I-, SCN-, PO4 , CO3 ,
-
-
2-
-
2-
4-
-
-
2-
HCO3 , H2PO4 , HPO4 , NO3 , SO4 , P2O7 , HSO4 , HSO3 , S2O3 and
thiol-containing amino acids (Cys, Hcy, GSH) to OXDNP solution, there was no
significant fluorescence intensity variations was observed except that the fluorescence
intensity was weakened in the presence of GSH. Similarly, UV-Vis spectra response
of OXDNP to the tested analytes also revealed a selective response to HS-. Addition
of HS- to OXDNP solution elicited a significant increase in absorption intensity at
473 nm (Fig. S1) accompanied with a color change from colorless to yellow. Other
examined analytes only promoted minor absorption intensity alterations. The HS-
elicited color changes under UV light and daylight are naked eye observable (Fig. S2).
These results demonstrate that probe OXDNP exhibits excellent selectivity to HS-
through fluorescence and colorimetry dual-channel responses.
6

Figure 1. Fluorescence spectra changes of OXDNP in DMSO upon individual
addition of 10.0 equiv. of different anions and thiols (Cys, Hcy, GSH). λex = 334 nm.
Inset: Fluorescence color changes of OXDNP solution before and after addition of
HS- under illumination at 365 nm with a portable UV lamp.
To obtain better insight into the sensing behavior of OXDNP to HS-, fluorescence
titration experiments were then conducted. As shown in figure 2, upon incremental
addition of HS- (0 to 10.0 equiv.) to OXDNP solution, gradual increase in
fluorescence intensities at 418 nm and 556 nm were observed, this emission spectra
change was terminated when 10 equiv. of HS- was employed. Similarly, the UV-Vis
spectra of OXDNP showed gradual increase in absorbance at 473 nm upon
incremental addition of HS- (Fig. S3). The emission intensity ratio of I556 nm/I418 nm
showed a good linearity between concentration ranges of HS- from 2 μM to 20 μM
(Fig. 2, inset). Based on the fluorescence titration profile, the detection limit of
OXDNP for HS- was estimated to be 2.0 μM (Fig. S4),19 indicating that OXDNP is
sensitive enough to detect HS- concentration in DMSO system. Time-dependent
fluorescence spectra changes reveal that 10 equiv. of HS- is sufficient to complete the
sensing reaction within a few seconds under the experimental conditions. Therefore,
7

the reaction kinetics of OXDNP in the presence of 10 equiv. of HS- according to a
pseudo-first-kinetic was investigated (Fig. 3) with the equation It=Imax+A×exp(k×t).20
In the equation, It represents the fluorescence intensity (418 nm) of the test solution at
time t, Imax represents the maximum fluorescence intensity (418 nm) of the test
solution. The pseudo-first-order rate constant k was calculated to be 1.234 s-1,
indicative of the remarkable high sensitivity of OXDNP for H2S detection.
Figure 2. Fluorescence spectra changes of OXDNP (10 μM) in DMSO solution upon
incremental addition of HS- (0 to 10.0 equiv.). λex = 334 nm. Inset: fluorescence
intensity ratio of F556/F418 as a function of HS- amount.
Figure 3. The kinetic study of the response of OXDNP (10 μM) to HS- (10.0 equiv.)
under pseudo-first-order conditions in DMSO.
8

To examine the interfering effects of the analytes as mentioned above on HS-
recognition, competition assays were subsequently carried out (Fig. 4). The results
show that co-existence of the same concentration of other background analytes
promoted no significant influence on HS- detection, demonstrating the excellent
anti-interference ability of OXDNP for HS- recognition.
Figure 4. Fluorescence intensity (418 nm) changes of OXDNP (10 μM) in DMSO
solution upon sequential addition of various background analytes (10.0 equiv.) and
HS- (10.0 equiv.). λex = 334 nm.
It is noteworthy that the fluorescence spectrum of OXDNP in the presence of HS-
exhibits an almost identical emission pattern as that of free 2 (Fig. S5), suggesting that
the speculated HS--triggered thiolysis of dinitrophenyl ether happened and free 2 was
released. To further corroborate this reaction process, HPLC-MS chromatograms of
compound 2, probe OXDNP, and the reaction mixture of OXDNP with HS- were
compared (Fig. S6). Probe OXDNP showed a single peak at retention time of 16.882
min with m/z = 445.0 ([M+1]+) (Fig. S6a). Compound 2 and the reaction product of
OXDNP with HS- exhibited retention time of 26.171 min and 26.315 min respectively
9

with the same m/z value (289.0 ([M+1]+) (Fig. S6b and 6c). These results reveal that
HS- can completely cleave the dinitrophenyl ether moiety and release 2. Another solid
1
evidence for this sensing mechanism comes from the H NMR comparison studies
(Fig. 5). Since the HS- anion used in the experiments is the corresponding water
solution, a mixed solvent of D2O-DMSO-d6 (5:95, v/v) was used when comparing the
1H NMR spectra of OXDNP, 2 and OXDNP+HS-. On addition of HS- to OXDNP
solution, a resemblant spectrum of 2 was observed from that of the mixture (Fig. 5c),
unambiguously demonstrating that 2 was indeed reoccurred on treatment of OXDNP
with HS-. Based on the above investigation results, a reaction-based HS- detection
mechanism was illustrated in Scheme 2.
1
Figure 5. Partial H NMR spectra of OXDNP (a), 2 (b) and OXDNP+HS- (c) in
DMSO-d6/D2O (95:5, v/v).
10

Scheme 2. The sensing mechanism of OXDNP for HS-.
3. Conclusions
In summary, we have synthesized a new 2,5-diaryl-1,3,4-oxadiazole-based
fluorescent probe OXDNP for hydrogen sulfide recognition. In DMSO solution,
probe OXDNP displays highly selective and sensitive recognition toward HS- over
other examined anions as well as thiol-containing amino acids. The HS- sensing
mechanism, namely, HS- triggered thiolysis reaction, was proved by HPLC-MS and
1H NMR investigations. The HS- recognition event is fast and can complete within a
few seconds, which leads to releasing of the ESIPT featured precursor 2 and results in
dual “turn on” ratiometric emission and a large Stokes shift. The pseudo-first-order
reaction rate constant was estimated to be 1.234 S-1. Therefore, through this
“protection-deprotection” strategy, an effective fluorescent recognition of HS- was
achieved. The results of this work are believed to be instructive for the design of
novel ESIPT featured 2,5-diaryl-1,3,4-oxadiazole-based fluorescent probes.
Acknowledgments
The project was supported by the National Natural Science Foundation of China
(Nos. 21176029, 21476029), Liaoning BaiQianWan Talents Program (No.
2012921057), and the Program for Liaoning Excellent Talents in University (No.
11

LR2015001).
Supplementary data
Instrumental details, synthesis and characterization of OXDNP, and supplementary
figures (Figure S1-S9).
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