Detection of environmental pollutants heavy metal ions based on the complexation with fluorescent dyes: Reaction of 2-(2?-hydroxyphenyl)-5-amino-benzotriazole with the Sn2+, Hg2+, and Pb2+ ions

Article abstract of DOI:10.1016/j.inoche.2020.108408

Providing a clean environment free of pollutants, toxic substances, and hazardous materials is a pressing worldwide concern. Fluorescent dyes are a promising means to achieve this goal, and their application extends to medicine, academia, and industry. This work aims to provide basic data on a dye that can be used for the detection, quantification, and extraction of heavy metal ions based on its complexation with these environmental pollutants. A hydroxyphenylbenzotriazole derivative, 2-(2?-hydroxyphenyl)-5-amino-benzotriazole (LH), was synthesized, characterized, and chelated with Sn²⁺, Hg²⁺, and Pb²⁺ metal ions. The synthesized LH dye and its metal complexes were characterized using a range of physicochemical and spectroscopic techniques. Binding the dye to the investigated metal ions resulted in colored complexes with 1:2 (metal: dye) stoichiometry and the general composition of [ML₂]·xH₂O. Larger spectral shifts were observed in the absorption and emission spectra of dye molecule after complexation with the Sn²⁺ ion compared with the other two metal ions.

Full text of DOI:10.1016/j.inoche.2020.108408

Inorganic Chemistry Communications 124 (2021) 108408
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Detection of environmental pollutants heavy metal ions based on the
complexation with fluorescent dyes: Reaction of
2
+ 2+
2
-(2ʹ-hydroxyphenyl)-5-amino-benzotriazole with the Sn , Hg , and
2
+
Pb ions
a
,
b
,
*
b
b
b
Abdel Majid A. Adam
, Tariq A. Altalhi , Samy M. El-Megharbel , Hosam A. Saad ,
b
c
a
Moamen S. Refat , Ivo Grabchev , R.A. Althobaiti
a
High Altitude Research Center, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
b
c
Department of Chemistry, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
Chemistry and Biochemistry, Physiology and Pathophysiology, Faculty of Medicine, University of Sofia, 1407 Sofia, Bulgaria
A R T I C L E I N F O
A B S T R A C T
Keywords:
Providing a clean environment free of pollutants, toxic substances, and hazardous materials is a pressing
worldwide concern. Fluorescent dyes are a promising means to achieve this goal, and their application extends to
medicine, academia, and industry. This work aims to provide basic data on a dye that can be used for the
detection, quantification, and extraction of heavy metal ions based on its complexation with these environmental
pollutants. A hydroxyphenylbenzotriazole derivative, 2-(2ʹ-hydroxyphenyl)-5-amino-benzotriazole (LH), was syn-
thesized, characterized, and chelated with Sn2 , Hg , and Pb metal ions. The synthesized LH dye and its
metal complexes were characterized using a range of physicochemical and spectroscopic techniques. Binding the
dye to the investigated metal ions resulted in colored complexes with 1:2 (metal: dye) stoichiometry and the
Fluorescent dye
Metal complex
Photophysical properties
Environmental remediation
+
2+
2+
general composition of [ML
2
]⋅xH
2
O. Larger spectral shifts were observed in the absorption and emission spectra
2
+
of dye molecule after complexation with the Sn ion compared with the other two metal ions.
1
. Introduction
Industrial progress and technological development have greatly
and/or skilled professionals [5,6].
Several novel methods are being developed to overcome these issues.
Using novel fluorescent dyes to capture metal ions is one of the more
promising modern remediation methods. This technique is useful,
powerful, and popular for detecting, monitoring, and removing metal
ions physiologically and environmentally for the following reasons
[7–13]:
improved human quality of life but also have increased the amount of
pollution around the globe, which represents a serious threat to the
health and wellbeing of humans and the planet. Every day, factories, oil
refineries, raw material extraction plants, and the various means of
transportation release a huge mass of chemical pollutants into the soil,
water, air, and other environments. One such toxic chemical pollutant,
heavy metal ions, is well established as harmful for humans, plants, and
animals [1–4]. Managing the heavy metal ions in the environment is one
of the most pressing areas of concern and extensive research efforts are
dedicated to developing methods for their detection, quantification,
elimination. Ion exchange chromatography, spectrophotometry, and
electrochemical methods are examples of traditional approaches applied
for such purposes, which are all highly sensitive and selective, but also
time-consuming, often expensive, and require specialized equipment
)i. Low cost
)ii. Operational simplicity
)iii. Easy handling
)iv. Direct visual perception
)v. Rapid response
)vi. Intercellular detection
)vii. High selectivity and sensitivity
Several fluorescent dye-based sensors have been used to selectively
*
Corresponding author.
E-mail addresses: majidadam@tu.edu.sa, majidadam@yahoo.com (A.M.A. Adam).
https://doi.org/10.1016/j.inoche.2020.108408
Received 1 November 2020; Received in revised form 11 December 2020; Accepted 13 December 2020
Available online 19 December 2020
1
387-7003/© 2020 Elsevier B.V. All rights reserved.

A.M.A. Adam et al.
Inorganic Chemistry Communications 124 (2021) 108408
2
+
3+
3+
+
detect, quantify, and image metal ions (e.g., Ni , Ga , In , Ag ,
solution containing 34 g of sodium acetate was added gradually. The pH
of the resulting solution was brought to 8 using 17 mL of aqueous
ammonium hydroxide (25%). At this pH value, a dark-red precipitate
formed (azo product) that was then filtered, washed, and dissolved in
160 mL of methanol. This methanolic azo solution was refluxed for 2 h
with aqueous solutions of ammonium hydroxide (25%, 120 mL) and
cuprous sulfate (30 g in 70 mL). At the end of the refluxing process, the
suspension was filtered and the remaining residue was stirred for 1 h
with hydrochloric acid (5 N, 100 mL). The pH of the resulting acidic
solution was brought to 8 using aqueous ammonium hydroxide (25%),
and, at this pH, a white precipitate formed (dye product). The white
precipitate was filtered, washed, and recrystallized three times using a
1:1 ethanol: water solution, which resulted in a colorless product. This
colorless dye was labeled LH and characterized according to its element
2
+
3+
2+
3+
2+
2+
2+
Hg , Al , Cd , Fe , Pb , Cu , and Zn ) in aqueous media, living
cells, and other environmental conditions [14–28]. Fluorescent dyes are
chemical compounds that have become a part of everyday life and
modern culture. Their uses range from industrial (e.g., liquid crystals,
potential fluorescence sensors, polymer brightening, solar energy col-
lectors, daylight fluorescent pigments, and dispersing dyes for textile
materials), to academic (e.g., membrane studies, active units, fluores-
cent markers, and cell imaging), to clinical (e.g., antitumor medications
and disease monitoring) [29–37].
Hydroxyphenylbenzotriazoles are an important class of fluorescent
dyes as they act as photodegradation stabilizers and ideal UV absorbers.
These dyes are of great interest due to their high photostabilizing effi-
ciency and, therefore, have many applications as light stabilizers in
polymeric materials to protect the substrate from sunlight-induced
photo-oxidation [38–40]. The present work studied the ability of a
hydroxyphenylbenzotriazole derivative, 2-(2ʹ-hydroxyphenyl)-5-amino-
benzotriazole (Fig. 1), to form stable metal complexes with environ-
1
content, melting point, IR, and H NMR spectra.
2.2.2. Complex synthesis
Three 150-mL beakers were labeled as A, B, and C and 1 mmol of
mentally hazardous heavy metal ions (e.g., Sn2 , Hg , Pb ) aiming to
broaden the scope of hydroxyphenylbenzotriazoles through coordina-
tion chemistry and provide basic data that will facilitate the detection,
quantification, and elimination of heavy metal ions from the environ-
ment based on complexation with hydroxyphenylbenzotriazoles.
+
2+
2+
2 2 2
SnCl , HgCl , and PbCl dissolved in 25 mL Milli-Q purified water was
added to each beaker, respectively. The chloride solutions were stirred
◦
for a few minutes on heat-controlled magnetic stir plates at 65 C. A 2
mmol methanolic solution of LH was added gradually to each beaker.
The stirring continued for an additional few minutes, during which no
precipitation was observed. Colored precipitates were observed when
the pH of each solution was brought to 8 by adding a few drops of
ammonium hydroxide (25%). Generally, the metal ions formed brown-
colored precipitates when bound to LH but with different degrees of
2
. Experimentation
2
.1. Chemicals and analytical instruments
2
+
2+
brown color: light brown for the Sn ion, orange-brown for the Hg
2
+
All metal chlorides, solvents, and starting materials for the prepa-
ion, and yellow–brown for the Pb ion. The beakers were stirred for 20
min at the same temperature and then left to cool at room temperature
to ensure complete precipitation and harvest the products by slow
evaporation. The products were collected using filter paper, thoroughly
washed, and dried. The products were characterized according to
elemental, spectral, and thermal data.
ration of the fluorescent dye were analytical grade, at the highest purity
available from BDH (UK) and Sigma-Aldrich (USA). Milli-Q purified
water was used in the preparation. Thermal, molar conductivity and
elemental measurements of the synthesized complexes were performed
using the Shimadzu TGA–50H Thermal Analyzer for scanned the TG
◦
curves from 25 to 800 C under constant airflow, the Jenway 4010
conductivity meter, and the Perkin-Elmer 2400CHN elemental analyzer
to determine the relative content (%) of nitrogen, hydrogen, and carbon.
The content (%) of water and metal in the synthesized complexes were
3. Results and discussion
3.1. Dye characterizations
1
determined gravimetrically. Fluorescence, UV ꢀ Visible, FT ꢀ IR, and H
NMR spectroscopies were used to collect the fluorescence, electronic,
and molecular spectra of the synthesized complexes using the Perkin ꢀ
Elmer LS-55 Fluorescence, Perkin ꢀ Elmer Lambda 25 UV/Vis, Shi-
madzu FT ꢀ IR, and Bruker DRX-250 spectrophotometers.
The fluorescent dye used in this work, 2-(2ʹ-hydroxyphenyl)-5-amino-
benzotriazole, was synthesized according to a published protocol [40]
and referred to as LH. Using this protocol resulted in a 75% yield of a
colorless, long rod, high-purity dye. The dye was characterized ac-
1
cording to its element composition, melting point, IR, H NMR, and XRD
spectra; these characterization data agree well with those described by
Bojinov and Grabchev [40]. A sample of the synthesized dye was
analyzed by CHN elemental analyzer and the observed values for the C
2
2
.2. Syntheses
.2.1. Dye synthesis
%
, H%, and N% were 63.46%, 4.60%, and 24.55%, respectively. These
data agree well with the theoretically calculated values from the mo-
lecular formula of the dye (C12 O; 226 g/mol), (calculated values C
63.72, H% 4.42, and N% 24.78). The melting point of the dye was
The fluorescent dye used in this work is 2-(2ʹ-hydroxyphenyl)-5-
amino-benzotriazole. The IUPAC name of this dye is 2-(2ʹ-hydroxyphenyl)-
-amino-2H-benzo[d] [1,2,3]triazole or 2-(5ʹ-amino-2H-benzo[d] [1,2,3]
10 4
H N
5
%
triazol-2-yl)phenol. The target dye was synthesized at high yield ac-
cording to a published protocol [41]. Briefly, an aqueous solution (25
mL) containing 5.45 g of 2-amino-4-chlorophenol was mixed with 9 mL
◦
1
from 212 to 215 C. Fig. 1S presents the FTꢀ IR and H NMR spectra of
the dye. The frequencies of the characteristic FTꢀ IR bands for the dye
ꢀ
–
–
1
(
cm ) were 3480
ν
(O
–
–
–
H), 3377
C), 1339 δdef(C
as(C N), 1161
H), 842 δwag(N H), and 788 δrock(C
six functional groups: O
stretching and bending vibrations of the phenolic OH group were
ν
(N H), 1665 δdef(N H), 1592
– –
◦
of concentrated hydrochloric acid and then diazotized at 0 C with an
ν
(C
N), 1514
ν
(C
–
H), 1280 δ(O H) in-plane
O), 1093 (C N), 977
H). The dye molecule has
–
aqueous solution containing 3.5 g of sodium nitrite and 0.2 g of cuprous
sulfate. To the resultant diazonium chloride solution, an aqueous solu-
bending, 1248
ν
–
ν
(C
–
ν
–
s
δrock(N
–
–
–
◦
tion (100 mL) containing 5.4 g of m-phenylenediamine at 5 C was
–
H, N
–
H, C
–
N, C
– –O, and C–
C, C N. The
◦
added. The mixture was stirred for 2 h at 5 C, during which an aqueous
ꢀ 1
apparent in the IR spectrum. The sharp, strong band at 3480 cm was
ꢀ 1
attributed to O
–
H stretching vibrations, the medium band at 1280 cm
H bending vibrations, and the weak, broad
band at 650 cm was associated with out-of-plane O-H deformation
vibrations. The amino group (ꢀ NH ) displayed four vibrations at 3377,
665, 977, and 842 cm due to the (N H), δrock(N H),
H), δdef(N
and δwag(N H) modes, respectively. The two first vibrations appeared
was assigned to in-plane O
–
ꢀ 1
2
ꢀ 1
1
ν
–
–
–
Fig. 1. Chemical structure of the synthesized dye (LH).
–
2

A.M.A. Adam et al.
Inorganic Chemistry Communications 124 (2021) 108408
ꢀ
3
as very strong, sharp bands, while the latter two bands were much
1.0 × 10 M. The molar conductance of the dye molecule alone was 21
ꢀ 1
weaker. The group of bands at 3088–2990 and 2948 cm were assigned
to the as(C H) and (C H) modes. The bands observed at 788 and
339 cm were assigned to the C H rocking and deformation vibra-
tions, respectively. The ring vibrations of the dye molecule contain the
stretching modes of C–
μ
S. After complexation with the metal ions, no considerable change was
2
+
ν
–
ν
s
–
observed in conductivity. The values for the complexes containing Sn
,
ꢀ 1
2+
2+
1
–
Hg , and Pb ions were 17, 26, and 19 S, respectively. These values
μ
reflect the non-electrolytic behavior of the complexes [41].
–
–
N, and C–N, and these were observed at
1
–
C, C
ꢀ
1
1
592, 1514, and 1248–1093 cm , respectively. A band resonated at
3.3. IR and 1H NMR spectral results
ꢀ
1
248 cm for the
ν
as(C
–
N) asymmetric stretching mode and at 1093
ꢀ
1
cm for the
ν
s
(C
–
N) symmetric stretching mode. The C
–
O stretching
Table 1 lists the frequencies of the characteristic IR bands (cm 1) for
ꢀ
ꢀ
1
vibrations resonated at 1161 cm in the form of a medium intensity,
complexes containing Sn2 , Hg , and Pb ions, and their IR spectra
are presented in Fig. 2. Most of the characteristic vibration frequencies
of the dye molecule were affected in position and intensity by
complexation with the metal ions. The frequency associated with the
stretching vibrations of the phenolic OH group appeared as a sharp,
+
2+
2+
broad band. The LH dye’s structure and atom number are presented in
1
Fig. 2S. The H NMR chemical shifts in (ppm) collected in DMSO‑d
6
at
), 6.82 (d, 1H,
H, C4‘H), 7.33 (d, 1H, J = 1.8 Hz,
12H, C12‘H), 7.55 (s, 2H, C10H, C10‘H), 7.80 (dd, 1H, C H, C5‘H), 7.91 (d,
H, C3‘H), 9.68 (s,
6
00 MHz at room temperature were: δ = 4.86 (s, 2H, NH
2
J = 9.5 Hz, C H,C6‘H), 6.92 (dd, 1H, C
6
4
C
5
ꢀ 1
strong band at 3480 cm in the dye alone and disappeared in the
1
1
8
H, J = 8.8 Hz, C13H, C13‘H), 8.00 (d, 1H, J = 9.3 Hz, C
3
complexes. This reflects the deprotonation of the dye’s phenolic OH
H, OH). The seven aromatic protons of the dye resonated in the 6.82 to
.00 ppm range. The proton of the phenolic OH group gave a single
group. Also, the C O stretching vibration band was affected, as its
–
ꢀ 1
ꢀ 1
,
frequency decreased from 1161 cm in dye alone to 1128–1120 cm
suggesting that the deprotonated phenolic OH group was involved in the
signal at 9.68 ppm, where the two protons of the amino group (ꢀ NH
2
)
resonated as a single signal at 4.86 ppm.
coordination reaction. All of the frequencies due to the amino group
vibrations:
ν
(N ꢀ H), δdef(N
–
H), δrock(N
–
H), and δwag(N
–
H) present in
3
.2. Elemental and conductivity results
–
–
the dye shifted upon complexation. The C
N band shifted from 1592
ꢀ 1
cm in the dye alone to a lower frequency in the range of 1556–1545
cm in the complexes, implicating the C
The reactions of the synthesized dye with the Sn2 , Hg , and Pb
+
2+
2+
ꢀ 1
–
–
N group in the coordination
◦
ions were carried out via 1:2 (metal: dye) at 65 C in weakly basic media
reaction. Two groups of bands not present in the dye were observed in
(
pH ~ 8) using a mixed 1:1 MeOH: H
2
O solvent. In weakly basic media,
the IR spectra of the complexes. The first group resonated in the 588 to
ꢀ
the dye (LH;
C
12
H
10
N
4
O) deprotonated to form an anion (L ;
ꢀ 1
5
21 cm region and may have resulted from the
ν
(Mꢀ O) stretching
ꢀ
ꢀ
C
12
H
9
N
4
O ). In its deprotonated form (L ), the dye chelated with the
ꢀ 1
vibrations, and the second group around 476 to 462 cm may have
metal ions to form brown-colored products with the general composition
of [ML O. A CHN elemental analyzer generated the following re-
]⋅xH
sults for each synthesized complex:
resulted from the
ν
(Mꢀ N) stretching vibrations. These observations
2
2
ꢀ
suggest that the deprotonated dye (L ) coordinated to the metal ions as a
bidentate ligand ionically through the oxygen atom of the phenolic OH
2
+
Sn
SnL
]⋅3H
complex: Gross formula, C24
H
24
N
8
O
ꢀ
5
Sn; General formula,
–
–
group and covalently with the nitrogen atom of the C N group.
1
[
2
2
O; Molecular weight, 622.71 g mol ; Yield 70%. Observed
To confirm the deprotonation of the dye, one of the synthesized
complexes was examined by proton NMR analysis to observe the exis-
tence or disappearance of the signal corresponding to the proton of the
(
%) for C 46.47, H 4.11, N 17.82, Sn 18.85; Calculated (%) for C 46.25, H
.85, N 17.99, Sn 19.06.
3
2
+
Hg
complex: Gross formula, C24
H
26
N
8
O
ꢀ
6
1
Hg; General formula,
1
phenolic OH group. Fig. 3S presents the H NMR spectrum of the com-
[
HgL
2
]⋅4H O; Molecular weight, 722.59 g mol ; Yield 69%. Observed
2
2+
plex containing Sn
ions. This spectrum contained the following
), 6.83 (d, 2H, J = 8.3 Hz,
H, C4‘H), 7.34 (d, 2H, J = 1.6 Hz, C12H,
12‘H), 7.64 (s, 2H, C10H, C10‘H), 7.83 (dd, 2H, C H, C5‘H), 8.01 (d, 2H, J
H, C3‘H). Indeed,
(
%) for C 39.62, H 3.83, N 15.70, Hg 27.57; Calculated (%) for C 39.86,
chemical shifts in (ppm): δ = 4.86 (s, 4H, 2NH
2
H 3.60, N 15.50, Hg 27.76.
C
C
6 4
H,C6‘H), 6.95 (dd, 2H, C
2
+
Pb
PbL
]⋅3H
%) for C 40.23, H 3.15, N 15.93, Pb 29.30; Calculated (%) for C 40.49,
complex: Gross formula, C24
24 8 5
H N O Pb; General formula,
5
ꢀ
1
[
2
2
O; Molecular weight, 711.20 g mol ; Yield 73%. Observed
=
6.3 Hz, C13H, C13‘H), and 8.33 (d, 2H, J = 7.1 Hz, C
3
(
the signal from the proton of the phenolic OH group observed at 9.68
ppm for the dye molecule alone was no longer observed in the spectrum
H 3.37, N 15.75, Pb 29.13.
The elemental results confirmed that the reaction between the
deprotonated dye and all of the metal ions proceeded with 1:2 stoichi-
ometry (metal: dye) in the synthesis process. The conductivity of the dye
molecule alone and its metal complexes was measured at room tem-
perature for each compound resuspended in DMSO at a concentration of
Table 1
ꢀ 1
Frequencies of the characteristic IR bands (cm ) for the synthesized complexes.
Free dye
Complexes
Assignments
2
+
2+
2+
Pb
Sn
Hg
3
3
3
480
377
088
–
–
–
ν
ν
ν
(O
–
H)
H)
3195
3003
2939
3198
3042
2964
3190
3007
2950
(N
–
as(C
–
H) +
s
ν (C H)
–
2
2
990
948
1
1
1
1
1
1
1
1
1
9
8
7
6
–
665
592
514
339
280
248
161
093
005
77
1610
1550
1467
1340
–
1265
1122
1080
1015
946
–
1605
1556
1465
1368
–
1264
1128
1086
–
958
870
805
638
521
462
1608
1545
1466
1340
–
1267
1120
1082
–
945
905
752
662
588
475
δ
def(N
–
(C N)
H)
–
ν
ν
–
–
(C
def(C
H) in-plane bending
N)
O)
N)
C)
–
C) (in-ring), aromatic
δ
δ(O
–
H)
–
ν
ν
ν
as(C
(C
(C
–
–
–
s
δ(C
–
δ
δ
δ
rock(N
–
H)
H)
H)
42
wag(N
–
88
751
663
560
476
rock(C
–
50
δ(O
–
H); δ(C
–
N) out-of-plane bending
ν
(M
–
N)
O)
–
–
ν(M
Fig. 2. IR spectra of the synthesized complexes.
3

A.M.A. Adam et al.
Inorganic Chemistry Communications 124 (2021) 108408
Fig. 3. Proposed structure of the synthesized complexes (M: metal ion; x = 4
2
+
2+
2+
for Hg and × = 3 for Sn and Pb ).
of the complex. This finding supports those obtained by IR analysis and
confirmed the deprotonation of the dye. The protons of the amino group
(
ꢀ NH
2
) still appeared at the same position in the dye molecule alone
(
4.86 ppm). Also, the aromatic protons still resonated at the same po-
sition in the dye molecule alone, though most of these protons under-
went slight down-field shifts (6.83–8.33 ppm range). These down-field
shifts were caused by the positive metal ion attracting the electrons
around the carbon atoms and thus decreased the shielding effect of these
atoms on the protons. The positive ion effect particularly influenced the
protons of carbons C13 and C13‘, which are much closer to the nitrogen
atom coordinated to the metal ion. These protons exhibited a down-field
shift of nearly 0.1 ppm (from 7.91 ppm for the dye molecule alone to
8
.01 ppm for the complex).
1
Taken together, the elemental, conductivity, IR, and HNMR results
ꢀ
support that the deprotonated dye (L ) coordinated to each metal ion as
a bidentate ligand by an ionic bond through the oxygen atom of the
phenolic OH group and covalent bond with the nitrogen atom of the
–
–
-
C
N group. Each complex contains two L anions, one metal ion, and
several uncoordinated water molecules. The complexes were composed
2
+
2+
of [ML
2
]⋅xH O (M: metal ion; x = 4 for Hg and × = 3 for Sn and
2
2
+
Pb ) and had square planar stereochemistry. The proposed structure of
the complexes is given in Fig. 3.
3
.4. Photophysics
To generate a complete comparative picture of the influence of metal
ions on the photophysical properties of LH, the dye molecule alone and
its complexes with Sn2 , Hg , and Pb ion were examined using ab-
+
2+
2+
sorption and fluorescence spectroscopies performed at room tempera-
ꢀ
3
ture. The electronic absorption spectra of dye molecule alone (1 × 10
ꢀ 4
2+
2+
2+
M) and in the presence of 1 × 10 M Sn , Hg , and Pb ions were
scanned between 200 and 500 nm in DMSO (Fig. 4). The fluorescence
ꢀ 3
spectra of dye molecule alone (1 × 10 M) and in the presence of 1 ×
ꢀ 4 2+ 2+ 2+
1
0
M Sn , Hg , and Pb ions were scanned between 400 and 700
nm in DMSO (Fig. 5). The electronic absorption and fluorescence
emission spectra of each complex, presented in Figs. 4 and 5, respec-
tively, were plotted individually along with that of the dye molecule
alone as a mirror image to best illustrate the differences.
Fig. 4. UV ꢀ Visible spectra of the synthesized complexes.
3
.4.1. Absorption
The dye molecule alone showed high absorption in the whole UV
2+
from 267 to 285 nm (~18 nm). Complexation with Sn ions increased
range. Its UV ꢀ visible spectrum was dominated by a very strong and
broad longwave that covered the whole UV range and extended to the
beginning of the visible range. The broad band ranging from 214 to 430
nm had two maxima around 250 and 265 nm and eight headbands at
the intensity of the dye’s absorption from 300 to 430 nm only. Generally,
2+
2+
the Hg and Pb complexes exhibited long-absorption bands with the
2+
same extinction as the dye molecule alone. After binding to the Hg
ion, generally, the heads that characterized the absorption band of dye
molecule blue-shifted. The heads at 220 and 240 nm were still observed
λ
max 220, 240, 250, 265, 328, 350, 380, and 405 nm. The near-UV heads
were narrower than those observed in the far UV region. After binding to
2+
in the absorption spectrum of the Hg complex but at new positions
2
+
the Sn ion, the resulting complex absorbed in the range of 245 to 430
nm. The heads at 220 and 240 nm were no longer observed in the ab-
sorption spectrum of the Sn2 complex. The head at 250 nm red-shifted
to 258 nm, while the head at 265 nm red-shifted to 278 nm and became
broader, up to twice as wide. The width of the head at 265 nm in the dye
(
217 and 231 nm, respectively). No observable change in intensity was
observed for this complex. Some of the heads that characterized the
absorption band of the dye molecule blue-shifted after complexation
+
2+
with the Pb ion. Specifically, the heads observed at 265 nm shifted to
2
62 nm, 380 nm shifted to 374 nm, and 405 nm shifted to 400 nm. The
molecule ranged from 262 to 270 nm (~8 nm) and in the Sn2 complex,
+
4

A.M.A. Adam et al.
Inorganic Chemistry Communications 124 (2021) 108408
Complexes containing Hg2 and Pb ions displayed similar fluores-
cence spectra with some intensity differences. These two complexes
behaved similarly, the fluorescence intensity of the head around 528 nm
decreased. The fluorescence intensity of the head around 446 nm
increased, but the maximum of this head underwent a blue-shift to 430
+
2+
nm for the Hg2 complex and 440 nm for the Pb complex. An obvious
+
2+
2
+
change was observed when the dye bound to Sn ions. The resulting
complex emitted a fluorescent signal with a maximal wavelength of 435
nm, and the intensity of the emission band began to decrease rapidly
after 500 nm; so, the head observed at 528 nm for the dye molecule
alone disappeared. While this head was still observed in the emission
spectra of the Hg2 and Pb
emission profiles distinguish the Sn2 complex from the other two
+
2+
complexes, the distinct fluorescence
+
complexes based on the fluorescence emission around 528 nm. In the
2
+
complex, the intensity of the head around 446 nm increased
Sn
greatly, but the maximum of this head blue-shifted to 435 nm.
The dye molecule alone displayed longwave absorption and emission
bands. The absorption maxima of the dye molecule were observed in the
UV region (i.e., 250 and 265 nm), while the fluorescence emission
maxima were in the visible region (i.e., 446 and 528 nm). All of the eight
headbands that characterized its absorption spectrum were still
2
+
2+
observed after binding with the Hg and Pb ions, and no remarkable
change in intensity was observed after complexation. Interestingly,
when the dye bound to the Sn2 ion, the intensity of the dye absorption
from 300 to 430 nm increased considerably, and the first two heads were
no longer observed in the spectrum. Correspondingly, the dye molecule
alone had two emission headbands at λmax 446 and 528 nm. The metal
ions increased the intensity of dye’s fluorescence emission around 446
nm. The fluorescence intensity around 528 nm was measurable for the
+
2
+
2+
2+
Hg and Pb complexes and negligible for the Sn complex. The
decrease in absorption intensity from 220 to 240 nm and emission in-
tensity around 528 nm associated with the Sn2 complex can be used to
+
distinguish it from the other two complexes.
3
.5. Thermal profiles
The dye molecule and its metal complexes were subjected to TG
analysis to elucidate their thermal stabilities. Fig. 4S plots the weight
◦
◦
lost in mg against temperature ( C) from 25 to 800 C for the dye
molecule and its complexes. In the case of the decomposing dye mole-
cule, the TG profile indicated that it is thermally stable in air up to
◦
◦
1
70 C. It starts to thermally decompose after 170 C and was completely
◦
decomposed by ~320 C in one degradation step. The total weight lost
corresponding with this step was 94.69%, which indicated that some
carbon atoms did not combust and remained as the final residue at
◦
2+
2+
8
00 C. The Sn and Pb complexes behaved similarly, staring their
◦
thermal decomposition at 60 C and decomposing in two stages by
◦
~
545 C. The first and second degradation steps occurred from 60 to
◦ ◦
-
3
80 C and 380 to 545 C, respectively. In the first degradation step, the
first L molecule was released and pyrolyzed along with three lattice
-
water molecules. The second L molecule started to split off in the second
degradation step, but did not completely combust due to a lack of oxy-
2
+
Fig. 5. Fluorescence emission spectra of the synthesized complexes.
gen. In the Sn complex, around 50% of the carbon atoms in the second
-
◦
L molecule remained as the final residue at 800 C with the metal oxide
2
+
head at 220 nm was still present in the absorption spectrum of the Pb2
+
(SnO). The final products in the decomposition of the Sn complex
were SnO + 6C, which represented an observed weight loss of approx-
complex but it red-shifted to 230 nm (~10-nm shift), while the head at
2
+
complex,
imately 33.59% (calculated to be 33.20%). For the Pb
2
40 nm was still at the same position as for the dye molecule.
-
around 33% of the carbon atoms from the second L molecule remained
◦
at 800 C with the metal oxide (PbO). The final decomposition products
3
.4.2. Fluorescence
resulting from the combustion of the Pb2 complex were PbO + 4C,
+
The dye molecule alone emitted a strong fluorescent signal with a
reflected in an observed weight loss of approximately 37.55% (calcu-
maximal emission wavelength of 446 nm. Its fluorescence emission
spectrum was less complicated than its electronic absorption spectrum.
The dye molecule displayed a very broad, longwave band with high-
intensity emission ranging from 400 to 600 nm. This long band had
two heads at λmax 446 nm (high-emission) and 528 nm (low-emission).
lated to be 38.13%). Interestingly, binding Hg2 ions highly increased
+
the thermal stability of the dye molecule. While the dye molecule alone
◦
2+
is thermally stable in air up to 170 C, after binding Hg ions, its sta-
◦
bility increased to ~320 C, which is a remarkable shift. In contrast, the
2
+
2+
Sn and Pb complexes were much less stable. The thermoanalytical
5

A.M.A. Adam et al.
Inorganic Chemistry Communications 124 (2021) 108408
behavior of the Hg2 complex suggests that after losing the lattice water
+
Appendix A. Supplementary material
◦
◦
molecules around 100 C, it remained thermally stable until ~320
C
◦
and, from there, decomposed in one step by ~600 C. The total weight
loss in this one step was close to 100.0%, which indicated that the
complex underwent complete combustion without any remaining car-
bons or metal oxide. The mercury metal in the complex was most likely
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.inoche.2020.108408.
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