Anthracene possessing amide functionality as a turn-on fluorescent probe for Cu2+ and Zn2+ ions

Article abstract of DOI:10.1080/00958972.2021.1878160

An anthracene appended PET chemosensor, anthracene-9-carboxylic acid (3,4-dimethoxy-phenyl)-amide (A₁), has been synthesized through condensation of corresponding acyl chloride and 3,4-dimethoxyaniline containing C = O and NH as receptors. The

Full text of DOI:10.1080/00958972.2021.1878160

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Anthracene possessing amide functionality as a
turn-on fluorescent probe for Cu2+ and Zn2+ ions
Navneet Kaur & Baljeet Kaur
To cite this article: Navneet Kaur & Baljeet Kaur (2021) Anthracene possessing amide
functionality as a turn-on fluorescent probe for Cu2+ and Zn2+ ions, Journal of Coordination
Chemistry, 74:4-6, 575-583, DOI: 10.1080/00958972.2021.1878160
To link to this article: https://doi.org/10.1080/00958972.2021.1878160
View supplementary material
Published online: 01 Feb 2021.
Submit your article to this journal
Article views: 37
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=gcoo20

JOURNAL OF COORDINATION CHEMISTRY
2021, VOL. 74, NOS. 4–6, 575–583
https://doi.org/10.1080/00958972.2021.1878160
Anthracene possessing amide functionality as a turn-on
fluorescent probe for Cu2þ and Zn2þ ions
Navneet Kaur and Baljeet Kaur
Department of Chemistry, Panjab University, Chandigarh, Punjab, India
ABSTRACT
ARTICLE HISTORY
Received 27 July 2020
Accepted 18 November 2020
An anthracene appended PET chemosensor, anthracene-9-carboxylic
acid (3,4-dimethoxy-phenyl)-amide (A1), has been synthesized
through condensation of corresponding acyl chloride and 3,4-dime-
thoxyaniline containing C ¼ O and NH as receptors. The sensor A1
exhibited selective fluorescence turn-on behavior towards Cu2þ and
Zn2þ ions in CH3CN. Cu2þ ions displayed 18-fold enhancement (U ¼
0.006!0.071) in the fluorescence spectrum of A1, which are other-
wise well known for fluorescence quenching phenomenon.
Moreover, A1 could easily discriminate between Zn2þ and Cd2þ ions,
the two metal ions of similar nature. The Job’s plots analysis deter-
mined 2:1 (A1:Cu2þ/Zn2þ) stoichiometry between the sensor A1 and
Cu2þ/Zn2þ ions. The LOD values were calculated to be 1.75 and 3.08
mM for Cu2þ and Zn2þ, respectively.
KEYWORDS
Chemosensor; turn on;
amide sensor; PET
1. Introduction
Metal ion recognition is an important area of research in supramolecular chemistry on
account of relevance in biological, environmental and clinical areas [1]. Of various
metal ions, Zn2þ ion is of immense importance for processes occurring in the human
body and is the second most abundant transition metal ion after iron [2]. It plays
CONTACT Navneet Kaur
neet_chem@yahoo.co.in; neet_chem@pu.ac.in
Department of Chemistry, Panjab
University, Chandigarh, Punjab, India
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2021.1878160.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

576
N. KAUR AND B. KAUR
significant roles in various pathological and physiological processes such as apoptosis,
catalytic co-factor in enzymatic reactions, regulation of gene expression and neuro-
logical signal transmission making it an essential trace element [3–6]. Abnormal levels
of Zn2þ ions in a human body is associated with disruption of the ongoing metabolic
processes responsible for various neurological disorders, Parkinson’s disease and
Alzheimer’s disease, hypoxia ischemia, epilepsy, etc. [7–11]. Numerous fluorescence
based chemosensors for Zn2þ ion have been reported. However, it is still a challenge
for detecting Zn2þ ions without interference of similar transition metal ions such as
Cd2þ [12, 13]. Similarly, Cu2þ is the third essential metal ion in the human body after
Zn2þ ions, playing roles of cofactor in various enzymes, cytochrome c oxidase, super-
oxide dismutase, tyrosinase, galactose oxidase and many more [14, 15]. Abnormal
amounts of Cu2þ disrupt the normal functioning of enzymes, affecting kidney, liver
and neurological disorders [16].
Anthracene fluorophore has been extensively used as a signaling unit in the fluor-
escence based chemosensors mainly for detection of metal ions and anions because
of its high emission properties. Absorbance and colorimetric based recognition in this
case are less, which might be because of difficult perturbations of the ground state of
the anthracene p-electron cloud [17–22].
In this work we have synthesized an amide (A1) incorporating anthracene fluoro-
phore with oxygen-rich dimethoxybenzyl moiety through semi-rigid amide functional
groups acting as a conjugating spacer. The sensor displayed efficient sensing ability in
detecting Cu2þ and Zn2þ ions. This is a rare example, where addition of paramagnetic
Cu2þ ions result in fluorescence enhancement by 18-fold.
2. Experimental
2.1. Materials and instrumentation
All chemicals and solvents were procured from Aldrich and used without purification.
Solvents were purchased from Avra synthesis and dry-distilled following the standard
purification methods. Stock solutions of metal perchlorates (Naþ, Kþ, Mg2þ, Al3þ
,
Mn2þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Cd2þ and Hg2þ) were prepared in double distilled
water. For carrying out UV-vis. and fluorescence experiments, distilled CH3CN was
used as solvent. All experiments were carried out at room temperature (298 K). UV-vis
spectra were recorded on a Shimadzu UV-240 spectrophotometer. Fluorescence spec-
tra were scanned on a Hitachi F–7000 equipped with 220–240 V Xe-lamp with a quartz
cuvette of 1.0 cm path length. FT-IR spectra were recorded on a NICOLET iS50 FT-IR
1
spectrophotometer from 4000-500 cmꢀ1. H NMR spectra were recorded on a Bruker
Advance II spectrometer operating at 400 MHz in CDCl3 using trimethylsilane as
internal reference and 13C NMR were collected at 100 MHz.
2.2. General procedure for spectroscopic measurements
Stock solutions of A1 (10ꢀ2 M) and metal ions (10ꢀ1 M) were prepared in DMSO and
de-ionized water, respectively. All metals were added as their perchlorate salts for the
UV-vis. and fluorescence experiments. The solutions were allowed to stand

JOURNAL OF COORDINATION CHEMISTRY
577
Scheme 1. Synthetic protocol for preparation of sensor A1.
undisturbed for two hours before carrying out optical studies. In fluorescence, the
excitation wavelength (kex) was 370 nm along with 5.0 nm of excitation and emission
slit widths. The association constants were calculated using the Benesi-Hildebrand
equation and LOD values were calculated using formula 3r/s, where r is standard
deviation and s is slope of titration curve between absorption/fluorescence intensity
and concentration of ion [23, 24].
2.3. Synthesis of anthracene-9-carboxylic acid (3,4-dimethoxy-phenyl)-amide (A1)
The procedure for synthesis of A1 is outlined in Scheme 1. Initially acyl chloride 1 was
synthesized by taking 9-anthracene carboxylic acid (0.8 gm, 3.8 mmol) and one drop of
N,N-dimethylformamide in 3 ml of thionyl chloride in a vacuum dried two neck round
bottom flask. After the reaction was complete (4-5 h), the in situ generated HCl was
removed with a gas trap apparatus containing KOH pallets and excess of N,N-dime-
thylformamide was removed by vacuum. This formed acyl chloride (1) was further
used without purification.
To a stirring solution of 1 (0.5 g, 2.07 mmol) in dry distilled CH3CN in a round bot-
tom flask, 3,4-dimethoxyaniline (0.317 g, 2.07 mmol) and K2CO3 (0.343 g, 2.48 mmol)
were added along with a catalytic amount of TBAHSO4 (tetrabutylammonium hydro-
gensulphate). The reaction was monitored via thin layer chromatography and took
approx. 6 h to complete. After completion of reaction, the contents of the flask were
cooled and filtered. The filtrate was then concentrated and left undisturbed. Light
brown crystals of pure A1 were obtained on slow evaporation.
2.4. Characterization of chemosensor A1
Light brown solid, yield 60%; m.p.: 214-216 ꢁC; FT-IR (cmꢀ1) 1656.22 (mC¼O), 3301.25
(mN-H); 1H NMR (CDCl3, 400 MHz) d (ppm): 8.44 (s, 1H, Ar-H) (e), 8.09 (d, 2H, Ar-H,
J ¼ 8.52 Hz) (c), 7.96 (d, 2H, Ar-H, J ¼ 7.64 Hz) (d), 7.6 (s, 1H, amide NH), 7.55 (s, 1H, Ar-

578
N. KAUR AND B. KAUR
Figure 1. Preliminary fluorescence spectral responses in sensor A1 (20 lM) in the presence of 100
equiv. of various metal ions in CH3CN on excitation at 360 and 350 nm, respectively.
H) (f), 7.48-7.40 (m, 4H, Ar-H) (a,b), 7.01 (dd, 1H, Ar-H, J1 ¼ 2.24 Hz, J2 ¼ 6.16 Hz) (g),
6.79 (d, 1H, Ar-H, J ¼ 8.61 Hz) (h), 3.87 (s, 3H, -OCH3), 3.82 (s, 3H, -OCH3); 13C NMR
(CDCl3, 100 MHz) d (ppm): 167.50, 149.27, 146.21, 131.76, 131.64, 131.10, 128.68,
128.63, 128.09, 127.01, 125.64, 124.97, 111.61, 111.46, 104.64, 56.21, 56.07; ESI-MS: m/z
1
(relative abundance (%), assignment) ¼ 358.32 [100, (M þ 1)þ]. H NMR, 13C NMR and
ESI-MS spectra of A1 are shown in Figures S1–S3.
3. Results and discussion
3.1. UV-vis spectral properties of sensor A1
The UV-vis. preliminary studies of A1 (20 mM) towards various metal ions such as Naþ,
Kþ, Mg2þ, Al3þ, Mn2þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Cd2þ and Hg2þ were performed in
CH3CN. The absorption spectrum of A1 exhibited three bands at 345 nm, 362 nm and
382 nm characteristics of anthracene moiety along with a shoulder band at 327 nm,
ꢂ
which may be ascribed to n-p transitions in the molecule [25]. The addition of 100
equiv. of various metal ions and anions did not show any noticeable changes in the
spectrum of A1, as shown in Figure S4. This behavior can be attributed to the negli-
gible electronic changes in the ground state of sensor A1.
3.2. Fluorescence spectral properties of sensor A1
The preliminary investigation of fluorescence properties of A1 (20 mM) towards metal
ions were performed in CH3CN on excitation at 360 nm. The sensor displayed a non-
fluorescent behavior (U ¼ 0.006) in CH3CN, probably due to effective PET between
anthracene and phenyl ring [26–28]. On addition of 100 equiv. of various metal ions,
only Cu2þ and Zn2þ ions exhibited significant enhancement of fluorescence intensity.
All other metal ions resulted in negligible changes in fluorescence spectrum of A1. In
the presence of 100 equiv. of Cu2þ and Zn2þ ions, the fluorescence intensity under-
went an increase of 18-fold (U ¼ 0.071) and 11-fold (U ¼ 0.047), respectively, with
emergence of three peaks at 390, 410 and 430 nm corresponding to anthracene

JOURNAL OF COORDINATION CHEMISTRY
579
Figure 2. Fluorescence spectra taken in the course of the titration of A1 (20 mM) in CH3CN with (a)
Cu2þ ions and (b) Zn2þ ions; Inset: plot of fluorescence intensity at 390, 410 and 430 nm vs. the
concentration of Zn2þ/Cu2þ ions (kex ¼ 360 nm).
Figure 3. Job’s plot of A1 at 410 nm with (a) Cu2þ and (b) Zn2þ (kex 360 nm).
emission spectrum (Figure 1) [29]. Here, addition of Cd2þ ions did not display any
changes in the spectrum, signifying the discriminating nature of A1 between Zn2þ and
Cd2þ ions, which otherwise share quite similar chemical properties and binding effi-
ciencies [12, 13].
The emission properties of sensor A1 were evaluated quantitatively by performing
its titration with Cu2þ and Zn2þ ions in CH3CN. With progressive addition of Cu2þ ions
(0-5.8 equiv.) to 20 mM solution of A1, the fluorescence intensity increased continu-
ously until the concentration of Cu2þ ions reached saturation at ꢃ5 equiv. (Figure 2a).
Likewise, the progressive addition of Zn2þ ions from 0 to 9.1 equiv. to A1 resulted in a
similar kind of fluorescence turn-on response (Figure 2b). The saturation in the titration
was attained at ꢃ8 equiv. of Zn2þ ions.
The Job’s plots revealed 2:1 stoichiometry between A1 and Cu2þ/Zn2þ ions as
depicted in Figure 3. Based on the Benesi-Hildebrand equation, the association con-
stants were calculated to be 6.32 ꢄ 105 and 4.90 ꢄ 105 Mꢀ2 for Cu2þ and Zn2þ ions,
respectively (Figures S5a and S6a). The limit of detection values were calculated to be
1.71 and 3.08 mM for Cu2þ and Zn2þ ions, respectively (Figures S5b and S6b).
3.3. Interference studies of sensor A1
To investigate the selective nature of sensor A1, the interference studies were per-
formed. For this purpose, an array of 10 ml solutions of A1 (20 lM) were prepared con-
taining 5/8 equiv. (or 100/160 lM) of Cu2þ/Zn2þ ions. To each solution, 50/80 equiv.

580
N. KAUR AND B. KAUR
Figure 4. Interference profile of various metal ions on the fluorescence intensity of (a) A1þCu2þ
and (b) A1þZn2þ (kex 360 nm) in CH3CN.
Figure 5. FT-IR spectra of A1 and its complexes, A11Cu2þ and A11Zn2þ
.
(or 1000/1600 lM) of each of the different metal ions were added to the solutions sep-
arately and kept undisturbed for 15 min before recording the fluorescence spectra.
Selective recognition of Cu2þ/Zn2þ by A1 was affected only by the presence of Fe3þ
ions, which caused ꢃ29% decrease in the fluorescence intensity of solution of
A1þCu2þ complex (Figure 4a). In the case of Zn2þ recognition, only Fe3þ and Cu2þ
ions perturbed the fluorescence intensity of A1þZn2þ solution by causing 36.9% of
decrease and 63% of increase in the fluorescence intensity, respectively, among the
various added metal ions, as shown in Figure 4b. The fluorescence quenching caused
by Fe3þ ions could be considered to result from paramagnetism of Fe3þ ions. Number
of literature reports points to the deactivation of the excited state by paramagnetic
metal ions via a non-radiative quenching process [30, 31].

JOURNAL OF COORDINATION CHEMISTRY
581
Scheme 2. Plausible sensing mechanism of Cu2þ/Zn2þ ions by sensor A1.
3.4. FT-IR studies
FT-IR spectra were recorded for A1 and its Cu2þ/Zn2þ complexes as depicted in
Figure 5. The C ¼ O and N-H stretching peaks of A1 initially present at 1656 and
3301 cmꢀ1 shifted to 1639 and 3038 cmꢀ1, respectively, in the presence of Cu2þ ions,
while in the presence of Zn2þ ions the corresponding peaks shifted to 1638 and
3238 cmꢀ1. These shifts clearly indicated binding of both ions occurred through inter-
action with C ¼ O group which also affected the strength of the N-H bond of A1.
Similar interaction can be expected between sensor A2 and Cu2þ owing to structural
similarities with A1.
3.5. Plausible sensing mechanism
The binding mechanism was investigated via fluorescence titration studies and observ-
ing the changes in FT-IR spectrum of A1 and its complexes with Cu2þ and Zn2þ ions
(Scheme 2). The PET process occurring from 3,4-dimethoxybenzyl group to the excited
state of anthracene core in the amide (A1) was responsible for the fluorescence turn-
off response. As confirmed from FT-IR data, the Cu2þ and Zn2þ ions bind through car-
bonyl O atoms of amide receptors A1 which might hamper the ongoing PET process
and thereby releasing the fluorescence, by which anthracene type fluorescence spec-
trum was observed in A1-Cu2þ and A1-Zn2þ complexes.
4. Conclusion
We have synthesized a new anthracene amide chemosensor A1 with high binding
affinities for Cu2þ/Zn2þ ions in CH3CN. The amide sensor was non-fluorescent
owing to PET from di-methoxybenzyl ring to anthracene acceptor. However,
enhancement in fluorescence intensity was observed after metal ion binding due

582
N. KAUR AND B. KAUR
to the inhibition of PET occurring in bare sensors. Here, 18-fold fluorescence
enhancement was observed with paramagnetic Cu2þ ions, which are known for
fluorescence quenching. Also, sensor A1 could discriminate two chemically similar
Zn2þ and Cd2þ metal ions.
Acknowledgements
The authors are thankful to the UGC – New Delhi (Grant no. 23/12/2012 (ii) EU-V) for the schol-
arship, DST PURSE-II (Grant # 48/RPC) New Delhi, India, for financial assistance and Sophisticated
Instrumentation Analytical Laboratory, Panjab University, Chandigarh, for providing the instru-
mentation facility.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
This study was funded by DST PURSE II; University Grants Commission.
References
[1] E.L. Que, D.W. Domaille, C.J. Chang. Chem. Rev. , 108, 1517 (2008).
[2] K.H. Falchuk. Mol. Cell. Biochem. , 188, 41 (1998).
[3] B.L. Vallee, K.H. Falchuk. Physiol. Rev. , 73, 79 (1993).
[4] J.M. Berg, Y. Shi. Science , 271, 1081 (1996).
[5] X. Xie, T.G. Smart. Nature, 349, 521 (1991).
[6] J.J.R.F. Da Silva, R.J.P. Williams. The Biological Chemistry of Elements: The Inorganic
Chemistry of Life, 2nd edn., Oxford University Press, New York 2001
[7] A. Takeda. Biometals , 14, 343 (2001).
[8] A.I. Bush. Curr. Opin. Chem. Biol. , 4, 184 (2000).
[9] M.P. Cuajungco, G.J. Lees. Brain Res. Brain Res. Rev. , 23, 219 (1997).
[10] A.I. Bush. Trends Neurosci, 26, 207 (2003).
[11] C.F. Walker, R.E. Black. Annu. Rev. Nutr. , 24, 255 (2004).
[12] P.S. Hariharan, S.P. Anthony. Anal. Chim. Acta. , 848, 74 (2014).
[13] J. Sun, B. Ye, G. Xia, H. Wang. Sens. Actuators B-Chem, 249, 386 (2017).
[14] H. Tapiero, D.M. Townsend, K.D. Tew. Biomed. Pharmacother. , 57, 386 (2003).
[15] J. Tkac, I. Vostiar, P. Gemeiner, E. Sturdik. Bioelectrochemistry , 56, 23 (2002).
[16] L.M. Gaetke, C.K. Chow. Toxicology, 189, 147 (2003).
[17] T. Gunnlaugsson, A.P. Davis, J.E. O’Brien, M. Glynn. Org. Biomol. Chem. , 3, 48 (2005).
[18] G.Y. Xie, L. Jiang, T.B. Lu. RSC Adv. , 2, 12728 (2012).
[19] K. Ghosh, A.R. Sarkar. Tetrahedron Lett, 50, 85 (2009).
[20] G. Zong, G.G. Lu. J. Phys. Chem. C. , 113, 2541 (2009).
[21] J. Prabhu, K. Velmurugan, R. Nandhakumar. J. Anal. Chem. , 70, 943 (2015).
[22] N. Kaur, G. Dhaka, J. Singh. New J. Chem. , 39, 6125 (2015).
[23] H.A. Benesi, J.H. Hildebrand. J. Am. Chem. Soc. , 71, 2703 (1949).
[24] J.D. Winefordner, G.L. Long. Anal. Chem. , 55, 712A (1983).
[25] A.E. Lee, M.R. Grace, A.G. Meyer, K.L. Tuck. Tetrahedron Lett, 51, 1161 (2010).
[26] K. Velmurugan, S. Suresh, S. Santhoshkumar, M. Saranya, R. Nandhakumar. Luminescence ,
31, 722 (2016).
[27] W. Gong, K. Hiratani. Tetrahedron Lett, 49, 5655 (2008).

JOURNAL OF COORDINATION CHEMISTRY
583
[28] Z.Q. Hu, C.L. Cui, H.Y. Lu, L. Ding, X.D. Yang. Sens. Actuators B-Chem, 141, 200 (2009).
[29] S. Mashraqui, T. Khan, M. Chandiramani, R. Betkar, K. Poonia. J. Incl. Phenom. Macrocycl.
Chem. , 67, 361 (2010).
[30] L. Fu, J. Mei, J.-T. Zhang, Y. Liu, F.-L. Jiang. Luminescence , 28, 602 (2013).
[31] K. Szaciłowski, W. Macyk, A. Drzewiecka-Matuszek, M. Brindell, G. Stochel. Chem. Rev. ,
105, 2647(2005).