Reusable colorimetric and fluorescent chemosensors based on 1,8-naphthalimide derivatives for fluoride ion detection

Article abstract of DOI:10.1016/j.saa.2020.118395

Two 1, 8-naphthalimide derivatives, 2-(2-ethylhexyl)-6-(2-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzylidene)hydrazinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (named as NAOZ) and 2-(2-ethylhexyl)-6-(2-(4-(5-phenyl-1,3,4-thiadiazol-2-yl)benzylidene)hydrazinyl)

Full text of DOI:10.1016/j.saa.2020.118395

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118395
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Short Communication
Reusable colorimetric and fluorescent chemosensors based on 1,8-
naphthalimide derivatives for fluoride ion detection
b
a
c
Liang Zhang a, , Feng Zhang , Liang Ding , Junkuo Gao
⁎
a
School of Material Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, Jiangsu, China
Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, Jiangsu, China
Institute of Fiber Based New Energy Materials, School of Materials Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Two 1, 8-naphthalimide derivatives, 2-(2-ethylhexyl)-6-(2-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzylidene)hy-
drazinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (named as NAOZ) and 2-(2-ethylhexyl)-6-(2-(4-(5-phe-
nyl-1,3,4-thiadiazol-2-yl)benzylidene)hydrazinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (named as NATZ),
containing hydrazone and thiadiazole/oxadiazole, were successfully synthesized. Compounds NAOZ and NATZ
were further served as efficient reversible colorimetric and fluorescent chemosensors for fluoride ion over a
wide range of other anions (Cl−, Br−, I−, NO3−, ClO−4 , HSO−4 , BF−4 , and PF−6 as their (C4H9)4N+ salts) because fluo-
ride ion can deprotonate the hydrazone moiety, leading to the observed long-wavelength color change. Interest-
ingly, CO2 can recover the charge transfer (CT) absorption and photoluminescence intensities of compound
NAOZ/NATZ + F−, because CO2 can react with a small amount of water to form acids, which can provide protons.
© 2020 Published by Elsevier B.V.
Received 9 January 2020
Received in revised form 13 April 2020
Accepted 19 April 2020
Available online 22 April 2020
Keywords:
Fluorescence
Naphthalimide
Fluoride ion
Chemosensor
1. Introduction
1,8-Naphthalimide derivatives have been widely investigated be-
cause of their interesting and excellent electro-optical properties
The development of chemical sensors for the recognition and sens-
ing of anions has become important in supramolecular chemistry be-
cause anions play vital roles in a wide range of biological and
environmental processes [1–14]. Among all anions, the recognition
and detection of fluoride ion has become a fast growing field of research
because they are important in human physiology and environment
[15–19]. Therefore, measuring the exact concentration of fluoride ions
is important. A popular design approach for chemical sensors used to
recognize and sense fluoride ion involves the synthesis of chromo-
phores/fluorophores furnished with one or more charged (such as thio-
ureas and imidazoles with NH group) or charged-neutral (such as silyl
ether materials) recognition moieties [20–26]. However, the develop-
ment of fluoride ion sensors with a large hypsochromic shift or
bathochromic shift has been challenging. Large hypsochromic or
bathochromic shifts are crucial to the development of excellent fluoride
ion sensors, which are utilized as naked eye colorimetric sensors. Nu-
merous excellent colorimetric and fluorescent chemosensors for fluo-
ride ion have been synthesized [27–31], but the development of
reusable colorimetric and fluorescent chemosensors for fluoride ion
has been rarely explored.
[32–36]. In general, the backbone of 1,8-naphthalimide can be easily
functionalized along two directions, namely, 1,8 and 4,5 positions.
Thus, 1,8-naphthalimide derivatives have been applied to recognize
and sense fluoride ion [37–43]. As a 1,8-naphthalimide derivative, 1,8-
naphthalimide hydrazones are conjugated organic compounds with a
planar structure because of their amino conjugation effect [44,45]. We
are interested in the synthesis of 1,8-naphthalimide furnished with
one hydrazine and thiadiazole/oxadiazole to develop naked eye colori-
metric sensors for the recognition and sensing of fluoride ions.
Herein, two 1,8-naphthalimide derivatives, 2-(2-ethylhexyl)-6-(2-
(4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzylidene)hydrazinyl)-1H-benzo
[de]isoquinoline-1,3(2H)-dione (named as NAOZ) and 2-(2-
ethylhexyl)-6-(2-(4-(5-phenyl-1,3,4-thiadiazol-2-yl)benzylidene)hy-
drazinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (named as NATZ),
containing hydrazone and thiadiazole/oxadiazole, were successfully
synthesized. Compounds NAOZ and NATZ were further used as reusable
colorimetric and fluorescent chemosensors for fluoride ion. The strate-
gies for the design of compounds NAOZ and NATZ were investigated
in terms of three aspects. (1) 1,8-Naphthalimide and thiadiazole/
oxadiazole moieties act as fluorophoresy. (2) Fluoride ion can
deprotonate the hydrazone moiety because fluoride ion can serve as a
sufficient base, leading to the observed long-wavelength color change.
(3) The changes in the color of compounds NAOZ and NATZ in the pres-
ence of fluoride ion can be reversed under atmospheric carbon dioxide
(CO2).
⁎
Corresponding author.
E-mail address: zhangliang@ycit.edu.cn (L. Zhang).
https://doi.org/10.1016/j.saa.2020.118395
1386-1425/© 2020 Published by Elsevier B.V.

2
L. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118395
2. Results and discussion
solution of compound NAOZ is light blue in the presence of fluoride
ion, suggesting that compound NAOZ can be used as a colorimetric
chemosensor for fluoride ion. Fig. 1b displays the UV–vis absorption
spectra of the DMSO solution of compound NAOZ in the absence
Scheme 1 depicts the synthetic procedure for the preparation of
compounds NAOZ and NATZ. Compounds 3, 8, and 11 were synthesized
in accordance with previously described methods [46,47]. Compounds
NAOZ and NATZ were produced by reacting one equivalent of com-
pound 3 in 2-methoxyethanol at 100 °C with compound 8 or 11. Subse-
quently, compounds NAOZ and NATZ were obtained as orange powders
with 65% and 61% yields through recrystallization from chloroform/
ethanol.
(blank) and presence of nine anions (F−, Cl−, Br−, I−, NO−3 , ClO−4
,
HSO4−, BF−4 , and PF−6 as their (C4H9)4N+ salts (20 equiv). The CT absorp-
tion intensity at λmax 462 nm dramatically decreases upon the addition
of F−, and two red-shifted bands at λmax 615 and 652 nm appear. The
bathochromic shift is as high as 190 nm. The large bathochromic shift
is necessary to develop excellent fluoride ion sensors, which are used
as naked eye colorimetric sensors. Under the same conditions, no evi-
dent change occurs upon the addition of eight other anions, suggesting
that compound NAOZ exhibits the highest selectivity for the fluoride ion
among the eight other anions. The two bands at λmax 615 and 652 nm
are attributed to the variations in CT because of the occurrence of a
strong interaction between fluoride ion and hydrazone moiety. The ti-
tration absorption spectra of the DMSO solution of compound NAOZ
with the addition of different amounts of fluoride ion are examined to
verify the interaction between fluoride ion and hydrazone moiety.
Fig. 1c/d and Fig. S11 shows that the CT absorption intensity at λmax
462 nm decreases and becomes slightly red shifted to 464 nm as the
fluoride ion concentration increases. The absorption intensities at λmax
615 and 652 nm steadily increase. As shown in Fig. S12–S14, the change
in color can be recovered a little due to absorption CO2 from the air, sug-
gesting that such sensor may be reused by adding CO2 atmosphere. As
shown in Fig. S17, the absorption behavior of the DMSO solution of com-
pound NATZ in the absence (blank) and presence of nine anions is sim-
ilar to that of the DMSO solution of compound NAOZ. Fig. S18 and S19
Compounds NAOZ and NATZ can take advantage of the charge trans-
fer (CT) character of fluorophore. As shown in Fig. S7-S10, the UV/vis
absorption and emission wavelengths of compounds NAOZ and NATZ
are red-shifted with the increase of the polarity of the solvent from tol-
uene, to chloroform, to dimethyl sulfoxide (DMSO). Thus, the CT state of
compounds NAOZ and NATZ may be modulated by fluoride ion through
the strong deprotonation of or hydrogen bonding to the electron-
donating amino moiety, thereby altering the photophysical properties
of compounds NAOZ and NATZ in the presence of fluoride ion. We ini-
tially investigated these possibilities by observing the changes in the
UV/vis spectra of compounds NAOZ and NATZ upon the addition of
F−, Cl−, Br−, I−, NO3−, ClO4−, HSO−4 , BF−4 , and PF6− as their (C4H9)4N+
salts. In Fig. 1a, the change in the color of the DMSO solution of com-
pound NAOZ upon the addition of F−, Cl−, Br−, I−, NO−3 , ClO4−, HSO−4
,
BF−4 , and PF6− as their (C4H9)4N+ salts (20 equiv) can be clearly ob-
served by the naked eyes. The color of the DMSO solution of compound
NAOZ is green in the absence (blank) and presence of eight anions (Cl−,
Br−, I−, NO−3 , ClO4−, HSO4−, BF4−, and PF−6 ). The color of the DMSO
Scheme 1. Synthesis of compounds NAOZ and NATZ: (a) 2-ethylhexylamine, ethanol; (b) 85% hydrazine hydrate, 2-methoxyethanol; (c) 4-methylbenzoyl chloride, anhydrous
triethylamine and tetrahydrofuran; (d) phosphorus oxychloride; (e) N-bromosuccinimide, benzoyl peroxide, anhydrous tetrachloromethane; (f) hexamethylenetetramine, chloroform;
(g) glacial acetic acid; (h) Lawesson's Reagent, toluene; (i) 2-methoxyethanol.

L. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118395
3
The fluorescence emission behavior of the DMSO solutions of
compound NAOZ in the absence (blank) and presence of- nine an-
ions (F−, Cl−, Br−, I−, NO3−, ClO4−, HSO4−, BF4−, and PF−6 as their
(C4H9)4N+ salts (20 equiv)) is also investigated to confirm that com-
pound NAOZ can be used as a fluorescent chemosensor for fluoride
ion. In Fig. 2a/b, the photoluminescence (PL) intensity (λex
=
462 nm) of the DMSO solution of compound NAOZ is dramatically
quenched in the presence of fluoride ion (20 equiv), whereas no ev-
ident fluorescence change occurs in the presence of the eight other
anions, indicating that compound NAOZ can be used as a fluorescent
chemosensor for fluoride ion. Fig. 2c/d shows the titration emission
spectra of the DMSO solution of compound NAOZ with the addition
of different amounts of fluoride ion. The PL intensity of the DMSO so-
lution of compound NAOZ at λmax 533 nm is almost quenched in the
presence of 50 equiv. fluoride ion. As shown in Fig. S23 and S24, the
emission behavior of the DMSO solution of compound NATZ in the
absence (blank) and presence of fluoride ion is similar to that of
the DMSO solution of compound NAOZ.
The interaction between compound NAOZ and F− is investigated by
the titration of the proton NMR spectra of NAOZ upon the addition of
different amounts of F− as its (C4H9)4N+ salt. Fig. 3 and Fig. S27 show
that the proton signal of N\\H proton at 11.81 disappeared upon addi-
tion of 1 equiv. F− as its (C4H9)4N+ salt. Subsequently, a new proton sig-
nal at 16.59 (triplet) appears upon addition of excess F− as its (C4H9)
4N+ salt. Such results suggest that compound NAOZ interacts with F−
through hydrogen bonding at lower concentration of F−. When excess
F− leads to the deprotonation of the N\\H proton (Scheme S1). To get
better understanding of interaction between compound NAOZ and F−,
the hybrid density functional theory (B3LYP) with 6-31G* basis set
using the Gaussian 03 program package was performed. As shown in
Fig. 4, the highest occupied molecular orbital (HOMO) of NAOZ is a π or-
bital and the electron density is distributed on the whole molecular
backbone except the terminated benzene ring, while the lowest unoc-
cupied molecular orbital (LUMO) is of π* character and the electron
density is the similar to that of the HOMO. However, for NAOZ + F−
,
the electron density distributions change significantly between HOMO
and LUMO, suggesting strong charge transfer nature, which is consistent
with the experimental results.
Although numerous colorimetric and fluorescent chemosensors for
fluoride ion have been developed, the development of reusable colori-
metric and fluorescent chemosensors for fluoride ion has been rarely in-
vestigated. When CO2 is passed to the DMSO solution of compound
NAOZ + F−, the change in color and fluorescence can almost be recov-
ered because CO2 can react with a small amount of water to form acids,
which can provide protons. To confirm this hypothesis, small amount
(0.5%, v/v) of hydrochloric acid (pH = 1.5) was used to instead of CO2.
As shown in Fig. S41 and S42, the change in color and fluorescence
can be easily recovered by adding small amount (0.5%, v/v) of hydro-
chloric acid (pH = 1.5). In Fig. 5a, the CT absorption intensity at λmax
462 nm can be recovered, and the absorption intensities at λmax 615
and 652 nm for the DMSO solution of compound NAOZ + F− dramati-
cally decrease. This phenomenon is confirmed by the fluorescence spec-
tra (Fig. 5b). The PL intensity of the DMSO solution of compound
NAOZ + F− is almost recovered by passing CO2 to the DMSO solution
of compound NAOZ + F−, suggesting that compound NAOZ can be
used as a recyclable colorimetric and fluorescent chemosensor for fluo-
ride ion.
Fig. 1. (a) Change in the color of the DMSO solutions of compound NAOZ observed upon
the addition of F−, Cl−, Br−, I−, NO3−, ClO−4 , HSO−4 , BF−4 , and PF6− as their (C4H9)4N+
salts (20 equiv) ([NAOZ] 10−5 mol L−1). (b) UV/Vis absorption spectra of the DMSO
solution of compound NAOZ upon the addition of F−, Cl−, Br−, I−, NO−3 , ClO4−, HSO−4
,
BF−4 , and PF−6 as their (C4H9)4N+ salts (20 equiv) ([NAOZ] 10−5 mol L−1). (c) Change in
the color of the DMSO solutions of compound NAOZ observed upon the addition of
different amounts of F− as its (C4H9)4N+ salt ([NAOZ] 10−5 mol L−1). (d) UV/Vis
absorption spectra of DMSO solutions of compound NAOZ with different amounts of F−
as its (C4H9)4N+ salt ([NAOZ] 10−5 mol L−1).
exhibit that the CT absorption intensity of the DMSO solution of com-
pound NATZ at λmax 462 nm dramatically decreases upon the addition
of F−, and two red-shifted bands at λmax 618 and 658 nm appear. Ob-
serving with the naked eyes is better when the bathochromic shift is
larger. Although the CT absorption of compound NAOZ is the same as
that of NATZ, the absorption of compound NATZ + F− is slightly red
shifted because of the lower ionization potential of the heavier chalco-
gen atoms compared with that of compound NAOZ + F−.
3. Conclusions
In summary, two reusable colorimetric and fluorescent
chemosensors (compounds NAOZ and NATZ) for fluoride ion are suc-
cessfully synthesized. Compounds NAOZ and NATZ can act as an effi-
cient anion sensor for fluoride ion over a wide range of other anions
(Cl−, Br−, I−, NO3−, ClO4−, HSO4−, BF4−, and PF−6 ) because fluoride ion
can deprotonate the hydrazone moiety, leading to the observed

4
L. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118395
Fig. 3. The titration of the proton NMR spectra of NAOZ upon the addition of different
amounts of F− as its (C4H9)4N+ salt (a. 1 equiv. of F−; b. 4 equiv. of F−; c. pure
compound NAOZ) in DMF-d7.
results suggest that the introduction of hydrazone and thiadiazole/
oxadiazole groups to the 1,8-naphthalimide backbone can improve
the performance in detecting fluoride ion. We will introduce water-
soluble groups such as poly(ethylene glycol) methyl ether into
start materials to improve the solubility of target compounds in
water in our future work.
CRediT authorship contribution statement
Liang Zhang: Conceptualization, Methodology, Investigation, Data
curation, Writing - original draft, Writing - review & editing, Supervi-
sion. Feng Zhang: Writing - review & editing. Liang Ding: Writing - re-
view & editing. Junkuo Gao: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
This work was supported by the Natural Science Foundation of
Jiangsu Province (no. BK20181477), Jiangsu Six Talent Peaks Program
(XCL-087, JNHB-068) and the National Natural Science Foundation of
China (no. 21774107).
Fig. 2. (a) Photographs of DMSO solutions of compound NAOZ observed upon the addition
of F−, Cl−, Br−, I−, NO−3 , ClO4−, HSO4−, BF4−, and PF6− as their (C4H9)4N+ salts (20 equiv)
under UV light at 365 nm ([NAOZ] 10−5 mol L−1). (b) Emission spectra of the DMSO
solutions of compound NAOZ observed upon the addition of F−, Cl−, Br−, I−, NO3−
,
ClO4− HSO4− BF4− and PF−6 as their (C4H9)4N+ salts (20 equiv). Excitation
, , ,
wavelength = 462 nm ([NAOZ] 10−5 mol L−1). (c) Photographs of the DMSO solutions
of compound NAOZ with different amounts of F− as its (C4H9)4N+ salt ([NAOZ]
10−5 mol L−1). (d) Emission spectra of DMSO solutions of compound NAOZ with
different amounts of F− as its (C4H9)4N+ salt. Excitation wavelength = 462 nm ([NAOZ]
10−5 mol L−1).
long-wavelength color change. The CT absorption peak of the DMSO
solution of compound NATZ at λmax 462 nm is dramatically red
shifted at 197 nm upon the addition of F−. Observing with the
naked eyes is better when the bathochromic shift is larger. Interest-
ingly, CO2 can recover the CT absorption and PL intensities of com-
pound NAOZ/NATZ + F− because CO2 can react with a small
amount of water to form acids, which can provide protons. Our
Fig. 4. Calculated spatial distributions of the HOMO and LUMO levels of compound NAOZ
and NAOZ + F−
.

L. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118395
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change in the color of the absence (blank) and presence of F− as its (C4H9)4N+ salts (10
equiv) and CO2 atmosphere. (b) Emission spectra of the DMSO solutions of compound
NAOZ in the absence (blank, i) and presence of F− as its (C4H9)4N+ salts (10 equiv., ii)
and CO2 atmosphere (iii, [NAOZ] 10−5 mol L−1). Excitation wavelength = 462 nm.
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Appendix A. Supplementary data
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