Selective fluoride chemosensors based on coumarin semicarbazones

Article abstract of DOI:10.1016/j.jphotochem.2021.113168

Twelve (five new) colorimetric anion sensors based on coumarin semicarbazone were studied for selective sensing of fluoride anion. The distinct colour change (by 44–125 nm) under ambient light observed in the Vis region makes the present sensors suitable for naked eye detection of fluoride anion, with detection limit (LOD) 6.6–8.6 ppm for the most sensitive ones. Whereas significant colorimetric changes are observed in the presence of fluoride anion in Me₂SO, the commonly competing AcO^?, Cl^?, Br^?, HSO₄^?, NO₂^? or NO₃^? ions induce none or relatively negligible changes. Deprotonation of the semicarbazone NH group is responsible for the sensing event, albeit the mechanism depends on electron density distribution in the coumarin moiety. Quantum chemical calculations suggest several possible geometries for the semicarbazones; the geometry with a hydrogen bond between the hydrazone nitrogen and the NH hydrogen of all of the aniline, cyclohexylamine and aminopyridine fragments, however, results in the most stable structures.

Full text of DOI:10.1016/j.jphotochem.2021.113168

Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
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Journal of Photochemistry & Photobiology, A: Chemistry
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Selective fluoride chemosensors based on coumarin semicarbazones
´
ˇ
´
ˇ
´
Hana Janekova, Jan Gaspar, Anton Gaplovský, Henrieta Stankovicova*
ˇ
Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Ilkovicova 6, SK-842 15, Bratislava, Slovakia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Twelve (five new) colorimetric anion sensors based on coumarin semicarbazone were studied for selective
sensing of fluoride anion. The distinct colour change (by 44–125 nm) under ambient light observed in the Vis
region makes the present sensors suitable for naked eye detection of fluoride anion, with detection limit (LOD)
6.6–8.6 ppm for the most sensitive ones. Whereas significant colorimetric changes are observed in the presence of
fluoride anion in Me2SO, the commonly competing AcOꢀ , Clꢀ , Brꢀ , HSO4ꢀ , NOꢀ2 or NO3ꢀ ions induce none or
relatively negligible changes. Deprotonation of the semicarbazone NH group is responsible for the sensing event,
albeit the mechanism depends on electron density distribution in the coumarin moiety. Quantum chemical
calculations suggest several possible geometries for the semicarbazones; the geometry with a hydrogen bond
between the hydrazone nitrogen and the NH hydrogen of all of the aniline, cyclohexylamine and aminopyridine
fragments, however, results in the most stable structures.
Coumarin semicarbazone
Colorimetric sensor
Fluoride anion
Acetate anion
1. Introduction
applied even with low concentration levels of the analyte.
Selective anion sensors containing neutral hydrogen bonding re-
Fluoride anion recognition is an area of intensive current research
due to the duplicitous nature of fluoride anions. Although several ways
of fluoride detection have been elaborated so far, the synthesis and
design of new chemosensors and the development of new sensing
mechanisms with high selectivity are still intriguing challenges for
research in this area [1–7]. With the development of anthropogenic
activities, particularly industrial manufacturing of aluminium, semi-
conductors, pesticides, and food products, the environmental contami-
nation with fluorides becomes a serious problem [8–12]. On the other
hand, fluorides are often used in human medicine: low concentrations of
fluorides possess significant potential perhaps most notably in the pre-
vention of dental caries or in the treatment of osteoporosis; but fluorides
are also contained in anaesthetics and psychiatric drugs. Fluorides are
highly soluble in water and therefore interactions with living systems
pose a threat especially for water organisms. Nonetheless, fluoride
accumulation in living systems has been accused of several pathologies,
such as changes in the structure of DNA and interference with metabolic
pathways (in the liver and kidneys) or e.g. a loss of mobility in humans
and lowering of IQ in children [10–16].
ceptor moiety (e.g. urea/thiourea) covalently linked to a chromogenic
fragment are of considerable interest due to the ease of property tuning
in the binding site [17–20]. H-bond induced π-electron delocalization or
NH deprotonation are believed to be responsible for signalling the
binding event in the (thio)ureas [21–23]. The variations in the physical
properties of anions, such as size, shape, charge distribution, the sol-
vation in polar and protic solvents and pH sensitivity are key parameters
that govern their selective and sensitive sensing and its mechanism
[24–26].
The present study is a part of our current research focused on che-
mosensors based on coumarin derivatives [27,28]. In our continuing
effort to develop novel chromophores/fluorophores leading to sensitive
chemosensors we linked coumarin (as a signalling unit) and semi-
carbazone (as a responding unit) moieties together into one molecule.
Coumarin is a common chromophore/fluorophore known for its
ability of significant intramolecular charge transfer (ICT) depending on
substitution and surrounding environment. The responding – semi-
carbazone – fragment as an easily accessible functional group seems to
be a proper alternative to urea/thiourea fluoride-recognizing scaffold
offering a high degree of tunability of the NH protons acidity by adjacent
substituents.
Naked eye (colorimetric) detection of anions is becoming increas-
ingly appreciated because visual detection can – generally quickly –
provide both qualitative and quantitative information. Colorimetry does
not require any specific equipment and due to its low cost can be widely
Several coumarin semicarbazones (Fig. 1) were rationally designed
and synthesized. Compared to the conventional coumarin dyes, both
* Corresponding author.
ˇ
´
E-mail address: henrieta.stankovicova@uniba.sk (H. Stankovicova).
https://doi.org/10.1016/j.jphotochem.2021.113168
Received 9 December 2020; Received in revised form 19 January 2021; Accepted 23 January 2021
Available online 30 January 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

´
H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
Fig. 1. Structure of substituted coumarin semicarbazones.
dimethylamino and imine units in the compounds 2 – 10 and 12 extend
the conjugated system of the chromophore. Although some of the
investigated semicarbazones are not new compounds [28–31], they
have not been previously used as fluoride colorimetric sensors.
Earlier, we discussed the fluorescence of the compounds 2, 5, 6, and
9 [29]. It should be only mentioned here that, independently on solvent
polarity, all EWG (on aniline ring) substituted coumarin semicarbazones
exhibit high fluorescent quantum yields (ΦF = 0.5–0.93). In comparison,
EDG (on aniline ring) substituted derivatives show significantly lower
ΦF values (about 0.14ꢀ 0.28) [28,29,31]. We reported their use as
fluorescent sensors for traces of water in organic solvents in the presence
of fluoride anions [28]. Dimethylamino derivatives 2, 5, 6, and 9 behave
as turn off fluorometric sensors for fluoride anion, the fluorescence of
semicarbazones decreases/quenches upon interaction with fluoride
anion. However, no more than air humidity diffusion as well as low-level
water addition (< 1 wt%) leads to fluorescence recovery [28].
Despite the fluorescent sensing being highly accurate it requires so-
phisticated instrumentation. For studied derivatives 1 and 11 unsub-
stituted on coumarin moiety, the fluorescent sensing is not applicable
because they exhibit very low fluorescent quantum yields (e.g for 1 ΦF <
0.001 in methanol [31]). Even though the possibility of the applicability
of the aforementioned compounds as sensors led us to the study of
practical colorimetric detection of interaction of the substances 1 – 12
with an analyte. Substitution effect as well as interaction with different
ions were studied to delineate the complex picture of analyte – receptor
interaction. It was revealed that only the fluoride anion could be
detected without interference from other anions. Derivatives 1 and 11
that do not have suitable fluorescence properties have proven to be ones
with the most sensitive colorimetric abilities among the studied
chemosensors.
2. Materials and methods
2.1. Synthesis
2.1.1. General
Melting points were measured on a Kofler hot stage and are uncor-
rected. The 1H and 13C NMR spectra were recorded at 300 MHz and at
75 MHz, respectively, in a 5 mm NMR tube on a Varian VNMRS 300 MHz
spectrometer (Agilent, Santa Clara, CA, USA) in CDCl3 or Me2SO-d6 with
tetramethylsilane as internal standard. Chemical shift values are recor-
ded in δ units (ppm) and coupling constants (J) are expressed in Hertz
(Hz). IR spectra were recorded on a Nicolet FT-IR-ATR 6700 spectrom-
eter (Thermo Fisher Scientific, Waltham, MA, USA) using the ATR
technique and
ν
are given in wavenumbers (cmꢀ 1). Elemental analyses
were performed on a Carlo Erba Strumentacione 1106 apparatus. HRMS
spectra were recorded on a Thermo Scientific Orbitrap Velos Pro Mass
Spectrometer (ThermoFisher Scientific, Waltham, MA, USA), HESI
(heated electrospray), MeOH, analyzer: FTMS, resolution 120000,
capillary temperature 275 ◦C, direct injection to MS. Chemicals and
solvents were purchased from the major chemical suppliers (Merck,
Darmstadt, Germany; Acros Organics, Geel, Belgium) as the highest
purity grade and all solvents were dried by standard methods and
distilled prior to use. 2-Oxo-2H-chromene-3-carbaldehyde (13) [32],
7-(N,N-dimethylamino)-2-oxo-2H-chromene-3-carbaldehyde (14) [33],
semicarbazides (15-18) [29, ESI S12], semicarbazones 1-3, 5, 8, 9 [29],
6 [28] were prepared according to literature procedures. The structures
of the prepared compounds were proven by 1H NMR spectra.
2.1.2. General procedure for the synthesis of substituted (E)-1-[(2-oxo-2H-
chromen-3-yl)methylidene]semicarbazones
A solution of semicarbazide (1 eq) in hot absolute ethanol (10 mL)
was added to a solution of aldehyde (1 eq) in hot absolute ethanol
Scheme 1. Reaction scheme of the synthesis of semicarbazones 1 – 12.
2

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H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
(10 mL). The reaction mixture was refluxed for 15 min and the desired
product precipitated. The precipitate was filtered off, washed with cold
ethanol, dried and recrystallized from ethanol, crystalline solid products
obtained in 62–98 % yields. (Scheme 1)
153.09, 159.61. MS: m/z [M+H]+ 309.09836 (theoretical value:
309.0943). Anal. calcd for C16H12N4O3 (308.29): C, 62.33; H, 3.92; N,
18.17; found C, 62.35; H, 3.84; N, 17.99.
(E)-1-{[7-(N,N-Dimethylamino)-2-oxo-2H-chromen-3-yl]methylidene}-
4-(pyridine-2-yl)semicarbazone (12): Obtained from 50 mg (0.23 mmol)
of aldehyde 14 and 35 mg (0.23 mmol) of 4-(pyridine-2-yl)semi-
carbazide (18) in 81 % yield (65 mg), orange powder: mp (ethanol)
(E)-1-{[7-(N,N-Dimethylamino)-2-oxo-2H-chromen-3-yl]methylidene}-
4-(4-methylphenyl)semicarbazone
(4):
Obtained
from
100 mg
(0.46 mmol) of aldehyde 14 and 76 mg (0.46 mmol) of 4-(4-methyl-
phenyl)semicarbazide (15) in 93 % yield (155 mg), orange powder: mp
(ethanol) 262ꢀ 265 ◦C (decomp.); UV–Vis (Me2SO): 440 nm; IR (ATR)
230ꢀ 233 ◦C (decomp.); UV–Vis (Me2SO): 440 nm; IR (ATR)
ν
/cmꢀ 1
:
28310 (s, br) and 1606 (m) amine, 1704 (s) and 1141 (s) ester, 1621 (m)
imine; 1H-NMR (Me2SO-d6): δ (ppm) 3.07 (s, 6H, N(CH3)2), 6.61 (d, 1J
=2.3 Hz, 1H, H-8), 6.80 (dd, 2J =8.4 Hz, 1J =2.3 Hz, 1H, H-6), 7.08 (dd,
3J = 7.5 Hz, 4J =5.4 Hz, 1H, Hpy-5), 7.65 (d, 5J =9 Hz, 1H, Hpy-3), 7.78
(ddd, 5J =8.9 Hz, 3J = 7.2 Hz, 6J =1.8 Hz, 1H, Hpy-4), 7.98 (d, 2J
=8.4 Hz, 1H, H-5), 8.05 (s, 1H, H-4), 8.30 (dd, 4J =5.5 Hz, 6J =1.9 Hz,
1H, Hpy-6), 8.55 (s, 1H, HC = N), 9.00 (brs, 1H, NH), 11.11 (brs, 1H,
NH); 13C-NMR (Me2SO-d6): δ (ppm) 39.72, 96.96, 108.39, 109.93,
112.52, 113.00, 118.56, 130.17, 136.72, 138.08, 138.73, 147.65,
151.71, 152.02, 153.31, 155.81, 160.46. MS: m/z [M+H]+ 352.14021
(theoretical value: 352.1365). Anal. calcd for C18H17N5O3 (351.13): C,
61.53; H, 4.88; N, 19.93; found C, 61.22; H, 4.81; N, 19.59.
ν
/cmꢀ 1: 3340 (s, br), 2956 (s, br) and 1522 (m) amine, 1686 (s) and
1137 (s) ester, 1601 (m) imine; 1H-NMR (Me2SO-d6): δ (ppm) 2.26 (s,
3H, CH3), 3.07 (s, 6H, N(CH3)2), 6.61 (d, 1J =2.4 Hz, 1H, H-8), 6.81 (dd,
2J =8.9 Hz, 1J =2.4 Hz, 1H, H-6), 7.11 (d, 3J =8.3 Hz, 2H, H-2’, H-6’),
7.53 (d, 2J =9 Hz, 1H, H-5), 7.54 (d, 3J =8.3 Hz, H-3’, H-5’), 7.98 (s, 1H,
H-4), 8.68 (s, 1H, HC = N), 8.81 (brs, 1H, NH), 10.80 (brs, 1H, NH); 13C-
NMR (Me2SO-d6): δ (ppm) 20.37, 38.67, 97.15, 108.48, 110.02, 113.48,
119.91, 128.84, 129.80, 131.33, 134.80, 136.41, 138.37, 152.85,
153.20, 155.77, 160.71. MS: m/z [M+H]+ 365.16077 (theoretical value:
365.1569). Anal. calcd for C20H20N4O3 (364.40): C, 65.92; H, 5.53; N,
15.38; found C, 65.94; H, 5.51; N, 15.40.
(E)-1-{[7-(N,N-Dimethylamino)-2-oxo-2H-chromen-3-yl]methylidene}-
4-(2-nitrophenyl)semicarbazone (7): Obtained from 100 mg (0.46 mmol)
of aldehyde 14 and 90 mg (0.46 mmol) of 4-(2-nitrophenyl)semi-
carbazide (16) in 91 % yield (163 mg), orange powder: mp (ethanol)
2.2. Spectroscopic measurements
UV–Vis absorption spectra were recorded on a HP 8452A (Hewlett
Packard, USA) diode array spectrophotometer. All measurements were
performed in a 1 cm cuvette.
269ꢀ 271 ◦C (decomp.); UV–Vis (Me2SO): 446 nm; IR (ATR)
ν
/cmꢀ 1
:
3270 (s, br), 1712 (s) and 1139 (s) ester, 1621 (m) imine, 1607 (m)
amine, 1505 (s) and 1344 (s) nitro; 1H-NMR (Me2SO-d6): δ (ppm) 3.08
(s, 6H, N(CH3)2), 6.63 (d, 1J =2.2 Hz, 1H, H-8), 6.83 (dd, 2J =8.9 Hz, 1J
=2.1 Hz, 1H, H-6), 7.24 (ddd, 3J =8.6 Hz, 4J = 7.5 Hz, 5J =1.6 Hz, 1H,
H-4’), 7.47 (d, 2J =9 Hz, 1H, H-5), 7.73–7.79 (m, 2H, H-4, H-5’), 8.06
(brs, 1H, NH), 8.21 (dd, 6J =8.3 Hz, 5J =1.6 Hz, 1H, H-6’), 8.41 (s, 1H,
CH = N), 8.59 (d, 3J =8.6 Hz, 1H, H-3’), 11.34 (brs, 1H, NH); 13C-NMR
(Me2SO-d6): due to low solubility in Me2SO-d6 ndt. MS: m/z [M+H]+
396.13006 (theoretical value: 396.1263). Anal. calcd for C19H17N5O5
(395.37): C, 57.72; H, 4.33; N, 17.71; found C, 57.71; H, 4.34; N, 17.70.
(E)-4-Cyclohexyl-1-{[7-(N,N-dimethylamino)-2-oxo-2H-chromen-3-yl]
methylidene}semicarbazone (10): Obtained from 150 mg (0.69 mmol) of
aldehyde 14 and 108 mg (0.69 mmol) of 4-cyclohexylsemicarbazide
Both anions in the titration experiments were added in the form of
tetrabutylammonium salts purchased from Sigma–Aldrich (USA), and
used without further purification. The anhydrous dimethyl sulfoxide
(Me2SO) solvent was of UV-spectroscopy grade (Uvasol®, Merck,
Darmstadt, Germany).
All titration experiments were carried out at 298.16 K. The receptor
solutions were titrated with the corresponding anion solutions (Fꢀ ,
AcOꢀ ) to obtain receptor concentration of the resultant solution as
indicated in ESI Figs. S1–S12. The titration process in the anion con-
centration range of 1.43 × 10-5 mol/L to 1 × 10-2 mol/L was monitored
by UV–Vis spectroscopy (in a 1 cm cuvette).
(17) in 87
% yield (210 mg), yellow powder: mp (ethanol)
2.3. Association constant determination
256ꢀ 258 ◦C; UV–Vis (Me2SO): 439 nm; IR (ATR)
ν
/cmꢀ 1: 3410 (s, br),
2846 (s, br) and 1526 (m) amine, 1714 (s) and 1138 (s) ester, 1667 (m)
imine, 1440 (m) alkane (cyclohexyl); 1H-NMR (CDCl3): δ (ppm)
1.16–1.48 (m, 4H, CH2-CH2-CH2-CH2-CH2), 1.61–1.81 (m, 4H, CH2-
CH2-CH2-CH2-CH2), 2.01–2.06 (m, 2H, CH2-CH2-CH2-CH2-CH2), 3.11 (s,
6H, N(CH3)2), 3.70–3.78 (m, 1H, CH2-CH-CH2), 5.91–5.95 (m, 1H, NH),
6.51 (d, 1J =2.5 Hz, 1H, H-8), 6.65 (dd, 2J =9 Hz, 1J =2.5 Hz, 1H, H-6),
7.39 (d, 2J =8.9 Hz, 1H, H-5), 7.70 (brs, 1H, NH), 7.81 (s, 1H, H-4), 8.08
(s, 1H, CH = N); 13C-NMR (CDCl3): δ (ppm) 25.07, 25.61, 33.78, 40.24,
48.74, 97.75, 108.91, 109.73, 113.97, 129.62, 135.15, 137.97, 153.34,
154.39, 156.27, 161.46. MS: m/z [M+H]+ 357.19202 (theoretical value:
357.1882). Anal. calcd for C19H24N4O3 (356.42): C, 64.03; H, 6.79; N,
15.72; found C, 64.06; H, 6.80; N, 15.75.
Association constants Kass for apparent coumarin semicarbazone:
anion 1:1 complex formation were determined by the acknowledged
formula describing complex anion concentration [34]:
[
]
[
]
1/2
Alim ꢀ A0
2c0
ꢀ
ꢀ
ꢀ
A = A0 +
c0 + cA + 1/Kass ꢀ (c0 + cA + 1/Kass)2-4c0cA
(1)
where: A0 is the absorbance of free coumarin semicarbazone, A is the
coumarin semicarbazone absorbance measured after anion addition,
A
lim is the coumarin semicarbazone absorbance measured with excess of
the particular anion, c0 is the overall concentration of coumarin semi-
carbazone and cAꢀ is the overall concentration of the added anion Aꢀ .
Eq. (1) was rewritten to the following form for nonlinear fit in Ori-
ginPro8.1 software:
(E)-1-[(2-Oxo-2H-chromen-3-yl)methylidene]-4-(pyridine-2-yl)semi-
carbazone (11): Obtained from 50 mg (0.287 mmol) of aldehyde 13 and
43 mg (0.287 mmol) of 4-(pyridine-2-yl)semicarbazide (18) in 62 %
yield (55 mg), light yellow powder: mp (ethanol) 215 ◦C (decomp.);
(
)
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
ν
/cmꢀ 1: 3351 (s, br), 2948 (s, br)
A = A0 + c1 ∗ (P1-A0) ∗ c0 + x + 1/P2- (c0 + x + 1/P2)ˆ2-4 ∗ c0 ∗ x
UV–Vis (Me2SO): 362 nm; IR (ATR)
and 1576 (m) amine, 1699 (s) and 1149 (s) ester, 1620 (m) imine; 1H-
NMR (Me2SO-d6): δ (ppm) 7.09 (ddd, 1J = 7.5 Hz, 2J =5.0 Hz, 3J
=0.7 Hz, 1H, Hpy-5), 7.40–7.46 (m, 2H, Hpy-4, H-8), 7.65 (dd,
4J = 7.1 Hz, 5J =1.4 Hz, 1H, H-7), 7.79 (ddd, 6J =8.4 Hz, 4J = 7.0 Hz, 7J
=1.8 Hz, 1H, H-6), 7.89 (dd, 1J = 7.6 Hz, 5J =1.3 Hz, 1H, Hpy-3), 7.98 (d,
6J =8.4 Hz, 1H, H-5), 8.10 (s, 1H, H-4), 8.32 (d, 2J =4.9 Hz, 1H, Hpy-6),
8.77 (s, 1H, HC = N), 9.13 (brs, 1H, NH), 11.35 (brs, 1H, NH); 13C-NMR
(Me2SO-d6): δ (ppm) 112.76, 116.11, 118.74, 119.08, 120.97, 124.88,
129.22, 132.29, 135.44, 137.74, 138.14, 147.67, 151.64, 152.02,
(2)
where: c0 = 1×10ꢀ 4, c1 = 1/2c0 = 5×103, parameter P1 = Alim, param-
eter P2 = Kass and x = cF-. The A0 value was fixed to the absorbance A
value for x = 0.
2.4. Theoretical calculations
The relative stability of the coumarin semicarbazones was
3

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H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
Table 1
Absorption maxima, extinction coefficients and equilibrium constant KW (of protonation of formed anion after interaction with fluoride) for the semicarbazones 1 – 12
in Me2SO.
Sensor
R1
R2
without Fꢀ ions
with Fꢀ ions
KW
#
λ1max[nm]/log(
ε)
λ2max[nm]/log(
ε)
(λ2max- λ1max)[nm]
molꢀ 1
1
C6H5
H
368/4.25
441/4.45
442/4.55
440/4.50
442/4.47
439/4.52
446/4.50
444/4.74
443/4.68
437/4.49
362/4.30
440/4.30
493/4.37
493/4.43
491/4.50
495/4.40
487/4.44
496/4.47
469/overlap
507/4.83
486/4.65
504/4.45
478/4.43
478/4.43
125
53
49
53
44
60
22
66
44
67
116
38
4.01
2.58
2.58
2.17
1.73
1.64
2.20
1.53
1.63
2.73
2.50
2.51
2
4-F-C6H4
4-Br-C6H4
4-CH3-C6H4
4-CF3-C6H4
4-OCH3-C6H4
2-NO2-C6H4
4-NO2-C6H4
4-CN-C6H4
Cyclohexyl
2-Py
N(CH3)2
N(CH3)2
N(CH3)2
N(CH3)2
N(CH3)2
N(CH3)2
N(CH3)2
N(CH3)2
N(CH3)2
H
3
4
5
6
7
8
9
10
11
12
2-Py
N(CH3)2
Fig. 2. Charge distribution in the 7-N,N-dimethylaminocoumarin fragment of the compounds 2 – 10, 12.
investigated using quantum chemical calculations. Structural geome-
tries were optimized at the semiempirical PM6 level. Stationary points
were characterized as minima by computations of harmonic vibration
frequencies at the same theoretical level as the geometric optimization.
Single point energies were calculated at the M062X/6-311++G(d,p)
level. All calculations were performed by the Gaussian 09 program
package [35].
spectra of all studied derivatives exhibit one intense long-wavelength
absorption band without vibrational structure. Electronic transitions
corresponding to these bands are proposed to be n-π* transitions and are
related to the charge transfer within the coumarin moiety [36].
Measured and calculated data suggest that electron donating substituent
at C-7 of the coumarin skeleton has crucial effect on the position of the
absorption maxima. In the substances containing the dimethylamino
group (2 – 10 and 12), compared to the ones with unsubstituted
coumarin skeleton (1, 11), the long wave absorption maximum is
bathochromically shifted by approx. 80 nm indicating significant influ-
ence of the substitution on excitation.
3. Results and discussion
3.1. Spectral characteristics and optimal geometry
The dimethylamino group as an electron donating group (EDG)
contributes to the high degree of ICT in the coumarin fragment. During
3.1.1. Spectral characteristics
–
Absorption spectral characteristics of the studied semicarbazones 1 –
the charge transfer either enolization of the coumarin carboxyl C O
–
12 in dimethyl sulfoxide, such as the absorption maxima (λ1max) and the
group can occur or an enamine-like structure can be formed (Fig. 2). The
molar extinction coefficients (
ε
) are presented in Table 1. The absorption
degree of enolization/“enamine” formation depends on the electron
Fig. 3. Correlation of the absorption maxima of the compounds 2 – 6, 8, 9 with
σp in Me2SO.
Fig. 4. Effect of solvent polarity on the position of the absorption maxima of
the methyl substituted derivative 4, c = 1 × 10ꢀ 5 M.
4

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Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
Table 2
Calculated relative energies of selected isomeric forms of the compound 1.
donating power of substituent in the C-7 position – in accordance with
1H NMR spectra of the compounds. The chemical shift (δ) of imine H
–
(HC N) shifts upfield with the introduction of the EDG, e.g. for the
–
compound 11 HCN δ is 8.77 ppm and for 12 HCN δ is 8.54 ppm.
Resultantly, the contribution of the enamine-like structure to the charge
distribution is non-negligible for the EDG substituted derivatives.
The influence of aromatic, aliphatic and heterocyclic substitution on
the semicarbazone fragment on absorption spectra is demonstrated in
the differences in the absorption maxima. All derivatives with the same
coumarin substitution (2 – 9, 11, 12) exhibit absorption maxima at
441 ± 5 nm. Thus the aniline ring of the semicarbazone moiety together
with its substitution have minor effect on S0-S1 transition as the sub-
stituents do not affect the position of the absorption maxima signifi-
cantly (Table 1). The insignificance is supposedly caused by cross
conjugation of this fragment with the coumarin chromophore through
–
–
the HN
C O bridge.
–
Fig. 5. Interaction of compound 11 with AcOꢀ and Fꢀ in Me2SO.
Nevertheless, a linear correlation of the absorption maxima (recip-
rocal) with the Hammett σp constants can be observed within the series
5

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H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
of the compounds 2 – 6, 8, 9 (Fig. 3). The correlation indicates a
anion-solvent interactions and also because of the low solubility of
semicarbazones (in particular nitroderivatives) in other solvents. We
found out that increasing concentration of Fꢀ anions in a solution of
semicarbazone resulted in a decrease of absorbance at the absorption
maximum at around 440 nm and simultaneously the formation of a new
absorption band with an absorption maximum at around 500 nm
occurred (Table 1). Correspondingly, the absorption red-shift of
53–125 nm resulted in the solution distinct colour change, which can be
directly observed by naked eyes. As an example, UV-Vis titration of
derivatives 6 and 8 in Me2SO with TBAF is displayed in the Figs. 6 and 7.
Similar changes in the UV–Vis spectra in the presence of Fꢀ anions were
observed in all studied semicarbazones (Table 1, ESI Figs. S1–S12).
The detection and quantification limits were calculated on the basis
of absorption titrations (see ESI S26-S30). We compared the estimated
detection limits to other fluoride sensors with coumarin moiety with
same sensing mechanism, specifically H-bond induced deprotonation, as
well as with few recently reported sensors containing neutral hydrogen
bonding receptor moiety (Tables 3 and S1) operating primarily in
Me2SO. As resulted from Tables 3 and S1, the studied semicarbazones
exhibit good (semicarbazones 1, 3, 9, 11) to moderate (other semi-
carbazones investigated) colorimetric fluoride anion sensing abilities.
It is well known, that coumarins are compounds with interesting
fluorescent properties. Recently, a variety of fluorescent turn off fluoride
anion sensors with coumarin moiety have been developed [4], but ma-
jority of them involves a EDG group substituted coumarin skeleton
which has suitable fluorescence properties and their detection limits are
considerable interconnection between the two π-systems (the coumarin
moiety and the aniline ring), presumably a result of variation of the
strength of intramolecular H-bond.
Solvent polarity affects the position of the absorption maxima only
slightly (Fig. 4). This behaviour comes to an agreement with the nature
of the corresponding aldehydes [30,31]. Furthermore, in respect to the
effect of aniline (or substituted aniline) ring addition on UV-Vis spectra –
compared with the UV-Vis spectra of the corresponding aldehydes – it is
the coumarin subunit which is the one responsible for the long wave
absorption maximum (chromogenic unit). This is also supported by the
fact that coumarin substituents significantly affect the S0-S1 electronic
transition of the semicarbazones 1 – 12.
3.1.2. Optimal geometry
To confirm that the UV-Vis spectra were in accordance with the
proposed charge distribution in the semicarbazones 1 – 12, we per-
formed quantum chemical calculations of the relative energies for the
most probable isomeric forms of the compounds. As an illustration, the
selected isomeric forms of the compound 1 are shown in the Table 2 (for
the corresponding cartesian coordinates see ESI S17-S25). The intra-
molecular hydrogen bond between the hydrazone nitrogen and the NH
hydrogen of the aniline fragment together with small electronic repul-
sion contribute to the stabilization of the structure C with the lowest
energy amongst all studied isomers. The (E)-geometric isomer C5
(Table 2) is approximately 14 kJ/mol more stable than the corre-
sponding (Z)-isomer C4. It is a relatively small difference in order to
photochemically prepare single isomer, or to work with a single isomer
in solution at room temperature.
as well as double bonds configuration
T–autome–rism
– – –
– whether s-cis or s-trans) are another important fac-
(
C
C
C
N
tors –in isom–ers’ stability. As the Table 2 shows, enolization of the sem-
icarbazone carbonyl leads to less stable structures (C2, C3, C6, C8). Even
if a structure can be stabilized by an intramolecular H-bond, its contri-
bution to the over-all energy cannot compete with the stability of the
–
C
O bond.
–
3.2. Effect of anions on UV-Vis spectra of the substituted coumarin
semicarbazones
3.2.1. Effect of fluoride anion
Despite the small effect of solvent or substitution pattern on the
aniline ring on the UV-Vis spectra of the studied semicarbazones, the
presence of anions in Me2SO solution affects the spectra considerably.
Ions AcOꢀ , Clꢀ , Brꢀ , HSO4ꢀ , NO2ꢀ and NOꢀ3 induce none or relatively
negligible changes, whereas substantial naked eye colorimetric changes
are almost immediately observed (within few seconds) in the presence of
fluoride anion.
Fig. 6. Effect of Fꢀ anions on UV–Vis spectra of the methoxy derivative 6 in
Me2SO, c = 2.9 × 10-5 M.
The most significant colour change after the addition of (Bu4N)F
(TBAF) to Me2SO solutions of semicarbazones was detected for the de-
rivatives 1 and 11 – substances unsubstituted at the coumarin C-7 po-
sition. The magnitude of these changes was dependent on the
concentration of anions in the solution. Fluoride anion produces an
appreciable visual colour change upon interaction with the semi-
carbarbazone 11 leading to an immediate orange colour (Fig. 5). For
dimethylamino derivatives, we did not observe such a distinct colour
change as their solutions are yellow to orange and, after adding fluo-
rides, their colour only deepens so the difference between the colour
before and after the addition of anions is not so intense.
To check the anion sensing capability of semicarbazones 1 – 12
against fluoride, UV–Vis spectrophotometric titrations were carried out
at 298.16 K. Solutions of compounds (concentrations are indicated in
ESI, Figs. S1–S12) in Me2SO were prepared followed by the addition of
TBAF in different concentrations (for all samples from 1.43 × 10ꢀ 6 mol/
L and its folds to 1 × 10-2 mol/L). Aprotic, polar and water miscible
Me2SO was chosen as the solvent for anion titrations in order to avoid
Fig. 7. Effect of Fꢀ anions on UV–Vis spectra of the nitro derivative 8 in Me2SO,
c = 2.9 × 10-5 M.
6

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H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
Table 3
The analytical performance of studied semicarbazones compared with similar type of fluoride sensors*,**
.
#
Sensor
Medium
λ [nm]
LOD [
μmol/L]
Approach
Ref.
1
Me2SO
493
8.59
colorimetric
this work
3
9
Me2SO
Me2SO
490
486
6.61
7.39
colorimetric
colorimetric
this work
this work
11
A
Me2SO
476
448
7.13
9.2
colorimetric
this work
[37]
Me2SO/CH3CN
colorimetric/fluorescent
B
C
THF
504
458
0.21
colorimetric/fluorescent
colorimetric/fluorescent
[38]
[39]
Me2SO
not specified
D
E
CH3CN (0.08% Me2SO)
CH3CN
455
620
not specified
not specified
colorimetric/fluorescent
[40]
[41]
colorimetric
F
Me2SO
422
490
not specified
0.95
colorimetric/fluorescent
colorimetric
[42]
[43]
G
Me2SO:H2O 1:1
H
I
CH3CN
Me2SO
494
485
0.72
15
colorimetric/fluorescent
[44]
[45]
colorimetric
J
Me2SO
391
5
colorimetric
[46]
*
Coumarin derivatives with different Fꢀ sensing mechanism are not listed (e.g. silyl capped coumarins [4]).
**
For comparison with other types of fluoride sensors see review article [4].
lower than for colorimetric ones as fluorometry is more sensitive
together with the distinct colour change under ambient light are highly
compared to colorimetry. However, many of the currently known fluo-
ride sensors show less preferable turn-off fluorescence response, con-
trary to the turn-on fluorescence response. Indeed, the turn-off probes
may exhibit an undesirable false positive response with high probability
because number of other factors instead of the target analyte alone can
be responsible for fluorescence quenching of the sensor molecule [47,
48]. Considerably less attention has been paid to the development of
colorimetric sensors with coumarin moiety capable of operating without
undesirable false positive responses in many applications. Semi-
carbazones listed in Table 3 with the major shift of absorption maxima
(116–125 nm) found in the absorption spectra in the presence of Fꢀ
suitable for naked eye detection of Fꢀ with good detection limit 7.13
μM
for derivative 11 resp. 8.59 μM for semicarbazone 1. These detection
limits are below the detection limit determined by fluorescent approach
for coumarin derivative A with similar structural motive [37].
3.2.2. Effect of commonly competing anions
According to visual inspection of the solutions of the semicarbazones
(1 × 10-4 M in Me2SO) before and after the addition of commonly
competing anions, the addition of either AcOꢀ , Clꢀ , Brꢀ , HSOꢀ4 , NO2ꢀ or
NOꢀ3 does not result in appreciable changes in colour. Additionally,
binding properties of the semicarbazones were investigated by 1H NMR
7

´
H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
related either to the ability to form H-bonds or to the possibility of
deprotonation of the probe by the anion.
We carried out detailed studies to gain further insight into the anion
sensing capability of the coumarin semicarbazones for Fꢀ and AcOꢀ , as
well as to check the possibility of the detection of the fluoride anion in
the presence of acetate.
As expected from the visual inspection, the UV-Vis spectra show
remarkable difference between interaction of the compounds 1 – 12
with Fꢀ and AcOꢀ (Figs. 5, 8 , shown for the cyano compound 9). An
addition of more than 20 equivalents of TBAOAc to the solutions of
semicarbazones results in a slight increase of band intensity at 440 nm
instead of the bathochromic shift of the long wave absorption maximum
observed for fluoride interaction with the chemosensors (as shown in
Figs. 5, 6). Furthermore, an appearance of a new band at around 350 nm
can be used for the distinction of Fꢀ in the presence of AcOꢀ .
Contrary to the UV-Vis spectra, in our 1H NMR spectral study we
observe the complete disappearance of the signal of the most acidic
hydrogen from the spectra after the addition of the excess amount of Fꢀ
as well as AcOꢀ to the Me2SO solutions of the semicarbazones (the cyano
derivative 9 displayed in Fig. 9 as an example).
Fig. 8. Effect of CH3COOꢀ TBA+ (TBAOAc) on UV-Vis spectra of the cyano
derivative 9 in Me2SO, c = 7 × 10-4 M.
These changes in spectra (UV-Vis, 1H NMR) can be explained by the
proposed structures of the formed species (Fig. 10). As Fꢀ is a stronger
base than AcOꢀ in Me2SO (HF pKa(Me2SO) = 15, AcOH pKa(-
Me2SO) = 12.3 [49]; and in particular the HFꢀ2 anion formed in an excess
amount of Fꢀ [21]), it is more likely for deprotonation – accompanied by
an elongation of the conjugated system resulting in the colour change –
to occur in comparison to acetate with weaker basic properties. Besides,
acetate can also act as a bifurcate H-bond acceptor capable of interacting
with the two acidic hydrogens of the semicarbazone substructures
forming a cyclic bimolecular structure, e.g. 9A. This interaction can
explain the disappearance of the signal of the more acidic hydrogen from
1
1
the H NMR spectra, the different shift of the aromatic protons in H
Fig. 9. 1H NMR spectra of the cyano derivative 9 in Me2SO 1) without TBAF, 2)
with 20 eq of TBAF, 3) with 20 eq of TBAOAc.
and UV-Vis spectroscopy in Me2SO using tetra-n-buthylammonium salts
of the anions (TBA-X: X = AcOꢀ , Clꢀ , Brꢀ , HSO4ꢀ , NO2ꢀ , NOꢀ3 ). Absorp-
tion spectra were recorded after 5 min upon addition of 1 eq of each of
these anions. Only one of the studied analytes, AcOꢀ , did affect the
spectra. Another one – OHꢀ – might interfered with the analysis.
Nevertheless, detailed studies with this anion could not be carried out –
in required concentrations the strong basic/nucleophilic properties of
hydroxide anions led to fast destruction of the semicarbazones.
As highly basic anions, though different in shape, fluoride and ace-
tate generally engage in hydrogen bond interactions with a variety of H-
bond donors. The selectivity of AcOꢀ and Fꢀ chemosensors is typically
Fig. 11. Effect of CH3COOꢀ TBA+ (TBAOAc) on UV–Vis spectra of 1 in Me2SO,
c = 7 × 10-4 M.
Fig. 10. Proposed structures: coordination of the cyano derivative 9 with TBAOAc (9A) and the deprotonated cyano derivative 9 (9B) with TBAF.
8

´
H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
and thus we expect an interaction of a different kind. For instance, the
deprotonation may partially occur from the aniline NH group instead of
the hydrazone NH group (being more acidic in the other derivatives;
similar to 9B in Fig. 10).
For further understanding of the sensing mechanism, after the
deprotonation of the semicarbazones with Fꢀ in Me2SO we performed a
study of protonation of the formed anion by a spectrophotometric
titration with water in anhydrous Me2SO. For demonstration we chose
titration of the derivative 12 (Fig. 13).
According to the equilibrium showed in Scheme 2, we calculated
equilibrium constants Kw of this reaction for all compounds (Table 1). In
all cases we observed a higher rate constant for the forward reaction (Kw
was always higher than 1).
The most plausible explanation of the Kw trend is that the anions are
of low stability (high energy) because the negative charge cannot be
distributed throughout the whole molecule and is localized in a 3 atom
region (the hydrazide part of the semicarbazone moieties). Because of
this the compounds 1 – 12 are not acidic, which is confirmed by that the
interaction with mildly basic acetate anion is based more on
coordination/H-bonding than on deprotonation.
Fig. 12. Correlation of the absorption maxima of the compounds 2 – 6, 8, 9
with σp in Me2SO in the presence of F¡.
NMR and also the differences in the UV-Vis spectra.
Similar behaviour in the presence of AcOꢀ is observed in all studied
substances except the unsubstituted derivative 1 (Fig. 11). After the
addition of acetate, a slight bathochromic shift of the long wave ab-
sorption maximum (by 11 nm) can be observed in the UV-Vis spectrum
in this case. The aforementioned indicates an effect of the electron
density distribution in the coumarin scaffold on the character of inter-
action because the hydrogens on the semicarbazone substructure
become less acidic with the presence of EDG on the coumarin skeleton.
Correlation of the absorption maxima (reciprocal) with Hammett σp
constants of the series of the compounds 2 – 6, 8 and 9 is observed after
the addition of fluoride as well as without fluorides (Fig. 12). It shows
that even though the effect of substitution on the absorption maxima is
small, it has a remarkable impact on the stabilization and the spectral
4. Conclusions
Colorimetric naked eye detection without the need for any sophis-
ticated instrumentation as the semicarbazones presented in this article
offer is attractive especially for applied fields of chemistry such as
environmental analysis or waste water treatment. For these fields it is
highly important to detect fluoride anions, as Fꢀ can cause severe
damage to water organisms. As such, the chemosensors based on
coumarin semicarbazone moiety designed by our research group may be
potentially utilizable for the aforementioned applications.
Our results offer a complex view of coumarin semicarbazones with
aryl, hetaryl and alkyl substituents on the semicarbazone scaffold as
potential colorimetric sensors for fluoride anions. The interaction of Fꢀ
with the sensors is, according to the reported results, based on depro-
tonation, however the exact mechanism depends on the electron density
distribution in the coumarin moiety.
properties of the species formed by deprotonation, as the two
are interconnected.
π-systems
The nitro derivative 8 does not fit in the correlation in the Fig. 12,
The described sensing of fluorides is highly selective with to 10 μM
detection limits, with short response time even in the presence of acetate
anions, which is confirmed by the appearance of a red shifted high en-
ergy absorption band only upon interaction with fluoride. This consid-
erable competitor – acetate – exhibits another type of interaction with
the semicarbazones. The pKa of conjugated acid is not as different as it is
for some of the other anions (HF pKa(Me2SO) = 15; AcOH pKa(-
Me2SO) = 12.3; HCl pKa(Me2SO) = 1.8; HBr pKa(Me2SO) = 0.9), never-
theless these two anions (Fꢀ and AcOꢀ ) are the only studied ones
affecting UV-Vis and 1H NMR spectra of the compounds. We assume this
interaction is selective for fluorides in the presence of majority of anions.
The colour change observed for the dimethylamino derivatives in the
Vis region (by approx. 50 nm) is sufficient for naked eye detection. The
highest shift in the UV-Vis spectra for the sensor – analyte interaction is
observed for the derivatives unsubstituted on the coumarin scaffold. In
these cases the colour of the solution changes from colourless (absorp-
tion in the “invisible” UV region) to yellow (in the Vis region), thus these
sensors can be used for detection without sophisticated instrumentation
as the colour difference is significant.
Fig. 13. Effect of H2O addition on UV-Vis spectra of the pyridine derivative 12,
c = 5 × 10ꢀ 5 M, in excess of TBAF in anhydrous Me2SO.
Scheme 2. Reaction of 12 with H2O and Fꢀ respectively.
9

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H. Janekova et al.
Journal of Photochemistry & Photobiology, A: Chemistry 410 (2021) 113168
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Hana Janekova: Methodology, Validation, Formal analysis, Inves-
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Health 18 (2019) 1–17, https://doi.org/10.1186/s12940-019-0551-x.
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Gaspar: Methodology, Validation, Investigation. Anton Gaplovský:
Conceptualization, Methodology, Software, Data curation, Writing -
original draft, Visualization, Supervision, Funding acquisition. Henrieta
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Declaration of Competing Interest
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[20] V. Blazek Bregovic, N. Basaric, K. Mlinaric-Majerski, Anion binding with urea and
thiourea derivatives, Coord. Chem. Rev. 295 (2015) 80–124, https://doi.org/
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The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgement
This contribution is the result of the project of the University Sci-
entific Park of the Comenius University in Bratislava (ITMS
26240220086) supported by the OPRaD funded by the ERDF.
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2021.
113168.
[26] J. Zhao, D. Yang, X.J. Yang, B. Wu, Anion coordination chemistry: from recognition
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