Spectroscopic, photophysical, solution thermodynamics and computational study of europium and terbium complexes with a flexible quinolinol-based symmetric tripodal chelator

Article abstract of DOI:10.1016/j.saa.2020.119124

Coordination behaviour and photonic properties of a flexible tripodal multidentate ligand, 5,5′,5′'-(Cyclohexane-1,3,5-triyltris(oxy))tris(methylene)triquinolin-8-ol (CYTOM5OX), with Eu and Tb ions were explored through potentiometric, UV-visible and fluorescence spectrophotometric methods. The interaction of tripod towards the lanthanides has been examined for thermodynamic equilibrium, photophysical behavior, and time-resolved luminescence measurements. Besides, a theoretically empirical method followed by quantum mechanical treatment of these chelates has also been summarized. A noticeable change in the electronic spectra of these complexes disclose CYTOM5OX as a pH-sensitive optical sensor for lanthanides detection; also, a strong green fluorescence allows simultaneous sensing in the visible region in a competitive medium. The intense fluorescence intermittently gets quenched under acidic and basic conditions due to photoinduced intramolecular electron transfer from the excited 8-hydroxyquinoline (8-HQ) moiety to the metal ion. The thermodynamically stable Ln³⁺ complexes in aqueous solution show stability constants in the range log β₁₁₀ = 33–35 and pLn³⁺ in the range of 21–22. The hydration number that was predicted by the coordination scan was substantiated by lifetime measurements reveal presence of coordinated water molecules in Eu(CYTOM5OX) and Tb(CYTOM5OX) complexes. Theoretical spectra were also calculated by ZINDO/s methodology at single excitations (CIS) level on PM7 as sparkle energy-minimized geometries.

Full text of DOI:10.1016/j.saa.2020.119124

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic, photophysical, solution thermodynamics and
computational study of europium and terbium complexes with a flexible
quinolinol-based symmetric tripodal chelator
a
b
a,
⇑
Rifat Akbar , Minati Baral , B.K. Kanungo
a
Department of Chemistry, Sant Longowal Institute of Engineering & Technology, Longowal, Punjab 148106, India
Department of Chemistry, National Institute of Technology Kurukshetra, Haryana 136119, India
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
Synthesis and studies on a new
tripodal multidentate quinolinate
ligand is described.
The photophysical and solution
thermodynamics of Eu and Tb
complexes are enumerated.
The ligand can act as ideal fluorescent
probe for detection of lanthanide
ions.
The theoretical tools are used for the
electronic investigations of the
molecules.
a r t i c l e i n f o
a b s t r a c t
0
0
Article history:
Coordination behaviour and photonic properties of a flexible tripodal multidentate ligand, 5,5 ,5 ’-(Cycl
ohexane-1,3,5-triyltris(oxy))tris(methylene)triquinolin-8-ol (CYTOM5OX), with Eu and Tb ions were
explored through potentiometric, UV-visible and fluorescence spectrophotometric methods. The interac-
tion of tripod towards the lanthanides has been examined for thermodynamic equilibrium, photophysical
behavior, and time-resolved luminescence measurements. Besides, a theoretically empirical method fol-
lowed by quantum mechanical treatment of these chelates has also been summarized. A noticeable
change in the electronic spectra of these complexes disclose CYTOM5OX as a pH-sensitive optical sensor
for lanthanides detection; also, a strong green fluorescence allows simultaneous sensing in the visible
region in a competitive medium. The intense fluorescence intermittently gets quenched under acidic
and basic conditions due to photoinduced intramolecular electron transfer from the excited 8-hydrox-
Received 15 September 2020
Received in revised form 14 October 2020
Accepted 16 October 2020
Available online 27 October 2020
Keywords:
Photoluminescence
Lifetime measurement
Thermodynamics
Photonic
Coordination scan
ZINDO/s
3
+
yquinoline (8-HQ) moiety to the metal ion. The thermodynamically stable Ln complexes in aqueous
solution show stability constants in the range log b110 = 33–35 and pLn3 in the range of 21–22. The
hydration number that was predicted by the coordination scan was substantiated by lifetime measure-
ments reveal presence of coordinated water molecules in Eu(CYTOM5OX) and Tb(CYTOM5OX) com-
plexes. Theoretical spectra were also calculated by ZINDO/s methodology at single excitations (CIS)
level on PM7 as sparkle energy-minimized geometries.
+
Ó 2020 Published by Elsevier B.V.
1
. Introduction
Lanthanide coordination complexes with organic ligands have
long attracted fundamental and applied research interest owing
to the possibility of sensitizing rare-earth radiative transitions at
⇑
Corresponding author at: Department of Chemistry, Sant Longowal Institute of
Engineering & Technology, Longowal 148106, Punjab, India.
E-mail address: b.kanungo@gmail.com (B.K. Kanungo).
https://doi.org/10.1016/j.saa.2020.119124
1
386-1425/Ó 2020 Published by Elsevier B.V.

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
visible and at near-infrared wavelengths. The promise is to achieve
sensitization with high efficiency by resorting to advantageous
light-harvesting properties of the organic part [1], that is, by
designing ligands to absorb light with high efficiency in selected
spectral regions. The unique properties of lanthanide ions have
been exploited in diverse applications such as contrast agents in
medical imaging [2], as a source of energy, and as luminescent
complexes [3]. Lanthanide complexes of the trivalent ions of euro-
pium, gadolinium, and terbium display a wide range of applica-
tions, with newer ones appearing in increasing numbers [4].
More specifically, europium and terbium complexes exhibit lumi-
nescence and are thus mostly used as luminescent or phosphores-
cent sensors or probes [5]. Despite the tremendous experimental
research on luminescent lanthanide complexes, electronic struc-
ture and thermodynamic relationships are not yet clearly under-
stood, and therefore, a fundamental concept for the development
and design of new luminescent lanthanide complexes is still miss-
ing. Similarly, the typical coordination number for lanthanides is
eight or nine, and due to the spatial distribution of the 4f and 5f
orbitals, the resultant coordination environment of these elements
is often dominated by the steric restraints of the ligand [6]. As a
consequence, the specific design of ligands for particular f-
elements remains a challenging task. A better understanding of
the photophysical properties of lanthanide luminescent complexes
requires, at first, an accurate description of their coordination
behavior and electronic structures of the Ln-ligand systems. The
coordination scan, energy levels, and ligand absorption bands
obtained using different computational schemes appear as essen-
tial characteristics for the prediction of the ligand-lanthanide(III)
charge transfer and energy transitions.
As a contribution to this field, we report the development of
europium and terbium chelates of a symmetrical tripodal ligand
explicitly designed to bind lanthanides containing the well-
documented 8-hydroxyquinoline chelating motif with a more flex-
ible cyclohexane central framework. In this paper, an attempt has
been made to gain some insight into the aqueous chemistry of a
0
0
new synthesized polydentate tripodal ligand 5,5 ,5 ’-(Cyclohex
ane-1,3,5-triyltris(oxy))tris(methylene)triquinolin-8-ol (CYTO-
M5OX), (Fig. 2) with the above said lanthanides. In a further effort,
the thermodynamic equilibria, detailed investigations of photo-
physical properties by potentiometric, UV–visible and fluorescence
spectrophotometry methods are reported. The Quantum mechani-
cal semiempirical-sparkle model is used to reinvestigate the exper-
imental results.
2
. Experimental
2.1. Materials and measurements
The chemicals procured from Sigma-Aldrich were of high purity
and were used as such with no extra purification. A standardized
.1 M HCl was used to prepare hydrochloride salt of the ligand to
0
make it water-soluble, and the investigation on potentiometric,
spectrophotometric, and luminescence studies were made in aque-
ous medium at an ionic strength of 0.1 M KCl. Millipore grade
deionized water was used in making the solutions. KOH (0.1 M)
was standardized against a primary standard 0.1 M solutions of
potassium hydrogen phthalate (KHP), and then the accurate
strength of HCl was obtained by using the standardized KOH solu-
tion. Solutions of lanthanides (Ln = Eu and Tb; n = 5) were prepared
8
-hydroxyquinoline (8HQ) and its metalloquinolates possess
in water from Ln(NO
3
)
3 2
ꢁnH O, and experiments were accomplished
superb ability to chelate many metal ions and have been proved
to be a suitable complexing agent. 8HQ and derived ligands are ideal
candidates for the design of electroluminescent devices [7], and are
at room temperature (25.0 ± 0.2 °C). The titration procedures for
potentiometric and spectrophotometric measurements are fol-
lowed as described earlier [11–13]. It may be noted that, for the
pH-metric method, it is possible to reproduce the results and reuse
the solutions by a back titration; however, the change in concen-
tration of the solutions should be taken into account for calcula-
tion. The equilibrium constants of the species formed in solution
during complexation were evaluated by using HYPERQUAD [14],
and the distribution of species was plotted with the program HYSS
promising light-harvesting chromophores for sensitization of visi-
ble and NIR luminescence from Ln3+ ions [8]. However, 8-
hydroxyquinolines being bidentate monoanionic ligands and hence
cannot saturate the coordination sphere of a lanthanide ion upon
the formation of a charge-neutral tris-complex [9]. The lack of coor-
dination control around the lanthanide ion imposes a challenge to
the formation of lanthanide supramolecular architectures. Mazzanti
et. al. [10] have reported few polydentate 8-hydroxyquinoline
based chelators (Fig. 1), suitable for lanthanides coordination.
2
009 [14].
Spectrophotometric titrations for the ligand and its Ln+3 metal
complexes (1:1 stoichiometry) were monitored as a function of
Fig. 1. Tetrapodal hydroxyquinoline ligands for the coordination of rare-earth ions.
2

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
green precipitate after cooling at room temperature was poured
into ice cold (70 mL) water and then filtered. The solid product
was collected, washed several times with diethyl-ether and dried
in vaccuo to afford CYTOM5OX 3 as a off-white colour light
1
weighed powder (1.7 g, 14.7 mmol, 72%). m.p. 167 °C; HNMR
(
2
3 2
DMSOd6, 25 °C, TMS): d = 0.89 (t, 3H, CH ); 1.2 (q, 3H, CH );
3
2 2
.04 (s, 3H, CH ); 4.64 (s, 6H, CH ); 8.85 (d, J H,H = 6.9 Hz, 3H,
3
H,H = 8.3 Hz, 3H, Ar–H); 6.93 (d, 3J H,H = 7.8 Hz,
Ar–H); 8.43 (d,
3
ppm. CNMR (DMSOd
(
J
3
H, Ar–H); 7.52 (dd,
J
H,H = 8.5 Hz, 3H, Ar–H); 9.65 (s, 3H, OH)
): d = 37.06 (CH ), 55.61 (CH ), 60.32
CH), 111.05, 113.20, 117.56, 121.07, 123.91, 128.70, 135.90,
1
3
6
2
2
1
1
38.42, 146.66(C = N), 153.58 (C–O). IR (ATR)
623, 1580, 1474, 1419, 1371, 1239, 1227, 1172, 1154, 1088, 828
: 71.63C;
.51H; 6.96 N; found: 71.98C; 5.34H; 6.99. MS (ES + ), m/Z (I,%)
m(cm-1) = 3372,
33 3 6
and 570. Elemental analysis Calcd (%) for C36H N O
5
5
87.4 [M + ](50).
0
0
Fig. 2. Structure of 5,5 ,5 ’-(Cyclohexane-1,3,5-triyltris(oxy))tris(methylene)
triquinolin-8-ol (CYTOM5OX).
2.3. Theoretical calculations
pH over the range ~ 1.8–10.2 by titrating with a standardized
NaOH solution. The equilibrium and the formation constants from
the spectral data were calculated by using a non-linear least-
square fitting program, HYPSPEC [15]. The formation constants of
the complexes were also evaluated through competitive batch
titrations using strong chelator EDTA in a pH range between ~ 10.0–
The spectral behavior, electronic and binding properties were
evaluated computationally by performing geometry optimization,
conformational analysis, and coordination scan of the lcompounds
[16]. The initial optimized structures were computed by sparkle/
PM7 calculations, which has scope for better depiction of elec-
tronic dispersion, H-bonds, and non-covalent bonding by permit-
ting UHF partial open shells [17].
2
.5.
Fluorescence emission measurements were recorded at excita-
tion wavelength 390 nm with excitation and emission slit widths
.0 nm. The luminescence titrations of CYTOM5OX and Ln
2
.3.1. Co-ordination scan
The coordination number in complexes can be predicted by
5
+3
+3
(
Ln = Eu , Tb ) of 1:1 stoichiometry in water solution established
adopting the coordination scan technique. The technique utilized
in these studies is the calculation of the complex strain energy as
a function of varying coordination numbers by covalently binding
the appropriate number of water molecules to the lanthanide
metal ion [18]. The preferred coordination number 9 was observed
for Ln[CYTOM5OX] complexes calculated by the coordination scan
technique with the stepwise addition of three or more water mole-
cules and calculating strain energy of the molecules using SYBYL. It
calculates water as having strain energy of 0.00 kcal/mol; thus, the
waters added to the complex add no energy other than steric inter-
actions with the ligand. For investigating coordination ability, a
complex with a generic metal was chosen and was scanned by
altering its ionic radius. The molecular energy corresponding to
each ionic radius was calculated; the best fit ionic size resulted
the formation of complexes. The stability constants of the metal
complexes were evaluated from the fluorescence spectral data by
using HYPSPEC [15], and the quantum yields were determined
[
13].
Horiba Jobin Yvon Fluorolog FL3-22 (1576C-4012-FL) spec-
trofluorimeter was used for lifetime measurements of the com-
pounds. The averages of two independent decay measured at the
maxima (slit widths 0.1 nm), produced mono-exponential decays
showing a delay of 0.05–0.4.
0
0
2
.2. Synthesis of 5,5 ,5 ’-(cyclohexane-1,3,5-triyltris(oxy))tris
(
methylene)triquinolin-8-ol (CYTOM5OX)
3
+
The synthesis of the ligand, CYTOM5OX, is shown in Scheme 1.
To a suspension of 5-chloromethyl-8-hydroxyquinoline,1a, (2.9 g,
in minimum strain energy. The ionic radius of Ln was varied by
systematically altering the Ln-N and Ln-O equilibrium bond
lengths through the relationship: equilibrium bond length = Ln
1
0
2
.44 mmol) was added 1,3,5 tris(hydroxy)cyclohexane 2 (0.52 g,
.48 mmol) in an acetone–water mixture and K CO (1.56 g,
.3 mmol) was used as an acid acceptor. The mixture was refluxed
2
3
ionic radius + x (x = 1.7 Å for Ln-N, 1.4 Å for Ln-O) at a fixed force
ꢂ1 ꢂ1
constant of 500 kcal mol
Å . The complex was then optimized
with stirring on magnetic stirrer for 4 hrs. The suspension gets dis-
solved upon reflux. The colour changes from yellow to green with
the progress of reaction. The resulting mixture which contained
to obtain the least energy. After then, the ionic radius was
increased by 0.1 Å unit, and the complex was again minimized
with the modified equilibrium bond length. This process was
Scheme 1. Synthesis of 1, 3, 5-Tris[(8-hydroxyquinoline)-5-oxymethyl]cyclohexane (CYTMOM5OX). Reagents and conditions; (i) HCHO, 37% HCl, 70 °C, 7 hrs, dry HCl gas; (ii)
CO , acetone: water, reflux, 3 hrs.
K
2
3
3

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
repeated until the Ln ionic radius has reached 1.5 Å. The range of
waters; the stoichiometry is proposed to be [M(TAME5OX)
0
.3–1.5 Å is large enough to accommodate the radii of all possible
2 3
(H O) ]. The molecular modeling parabola (Fig. 3) clearly shows
3+
coordination states [18]. The same procedure was continued by
considering one water molecule in the coordination sphere of the
complex, which was followed by the successive addition of more
waters until the best possible coordination atmosphere is explored.
The minimized energy of the complexes calculated is then plotted
versus a metal ionic radius for each coordination number.
that when water molecules are added or removed to the Ln com-
plex of the said ligand, the stability gets distorted. At nine coordi-
nation, the stability (as measured in strain energy, -E) is well
elevated than the 8 and 10 coordination state. The lifetime mea-
surement further confirms this fact.
The formation of lanthanide CYTOM5OX chelates was studied in
3+
aqueous solution for two trivalent lanthanide ions viz., Eu and
3
+
Tb by potentiometric, UV–visible, and fluorescence spectropho-
tometric method. CYTOM5OX can be considered a hexaprotic acid,
and its six pKa’s were determined directly by pH-metry and elec-
tronic and luminescence spectrophotometry under similar experi-
mental conditions. The calculated values of the protonation
2.3.2. Ground state geometries
Prediction of ground state geometries of Eu and Tb complexes
viz., [EuCYTOM5OX(H
by semiempirical MOPAC [19] with the sparkle model using PM7
17] parameters. The structures were confirmed for the existence
of any imaginary frequency.
2 3 2 3
O) ] and [TbCYTOM5OX(H O) ] were made
[
constants are: pKa1
= 10.31, pKa2 = 9.10, pKa3 = 8.05(2),
pKa4 = 6.32(5), pKa5 = 4.14(3) and pKa6 = 2.81(4). The potentiomet-
3
+
ric titration curves of 1:1 solution of each Ln and CYTOM5OX in
the pH range of ~ 0.3–11.5, are presented in Fig. 4. The deviation in
the metal–ligand titration curves from the free ligand titration
curve implies the formation of metal complexes. Complexes due
to Eu were formed at a relatively higher pH compared to Tb.
At ~ 10.3 pH, turbidity appeared for all metal ions; data beyond this
were excluded from the calculation. Keeping in view these prelim-
inary observations, many sets of possible stoichiometries were
tested in the minimization program. The best fit model was
2
.3.3. Excited states energies and absorption spectra
The sparkle optimized geometries of the complexes were
implemented for the prediction of the electronic spectroscopic
properties [20]. The Zerner’s intermediate neglect of differential
overlap/spectroscopic (ZINDO/S) procedure [21], was implemented
for calculation of singlet and triplet excited states using the config-
uration interaction with single excitations (CIS). The metal ions
were replaced by the +3e point charge for the trivalent lanthanide
ions in the optimized structures. The calculated data of energy and
oscillator strength were fitted for a Lorentzian line shape with a
half-height bandwidth of 25 nm to match the simulated spectrum
with the experimental one.
4
+
3+
obtained when the formation of protonated Ln(H
4
L) , Ln(H
3
L) ,
2
+
+
-
2
Ln(H L) , Ln(HL) and hydroxyl Ln(LOH) species were considered
in the model. This enabled us to determine the global constants
defined by Equations 1–6.
Lnaq þ H
n
LꢀLnðHnꢂiLÞ þ iHþ
ð1Þ
3
. Results and discussion
Â
þÃ
i
3
.1. Synthesis and characterization of CYTOM5OX
½LnðHnꢂiLÞꢃ: H
logK11n
¼
¼
Â
Ã
ð2Þ
½
H
n
Lꢃ: Lnaq
The ligand, CYTOM5OX, was obtained according to the syn-
thetic pathway depicted in Scheme 1. The precursor 8-
hydroxyquinoline (1) is a commercial product and was easily
transformed into
½
LnðHnꢂiLÞꢃ
logb11n
Â
Ã
ð3Þ
½
H
n
Lꢃ: Lnaq
a
5-chloromethyle derivative (1a) by
chloromethylation of 8-hydroxyquinoline at 5-position previously
described [22]. In the next step, the commercially available 1,3,5-
tris(hydroxy)cyclohexane, (2), was reacted with three equivalents
of 5-methyl-8-hydroxyquinoline hydrochloride. The reaction was
performed via a Mannich reaction in the presence of paraformalde-
hyde in 37% HCl at 70 °C to give the desired tripodal ligand CYTO-
M5OX. The compound was isolated as an air-stable off-white solid,
washed with super-dry ethanol followed by dry ether, as pure
compound with 72% yield and was characterized [22] by CHN anal-
ysis, FTIR, 1H, and 13C NMR and mass spectrometry.
3.2. Complexation: Design and solution thermodynamics
The interaction of Eu3+ and Tb3+ ions with new synthesized hex-
adentate CYTOM5OX were studied in solution by the use of a com-
bination of pH-metric, electronic spectral, and fluorescence
titrations. Not much difference in the stability of the resulting
podates is expected as these metal ions are present in the middle
of lanthanide series [23].
Though the lanthanide ions vary its coordination number from
6
3 6
to 12, the multidentate N O chelator CYTOM5OX is expected to
satisfy only six-coordination due to the unfavourable positioning
of donor atoms. The two above mentioned Ln3 complexes of
CYTOM5OX examined with the coordination scan were found to
have a hydration number of 3. The geometry of the CYTOM5OX
is such that it facilitates 6-coordination of a square-face tricapped
trigonal prism geometry of Ln3 with bonding six sites of three oxi-
nate groups, leaving three coordination sites open for binding
+
+
Fig. 3. Coordination scan plots for the ligand (CYTOM5OX) with the varied
coordination number for Ln complexes.
3+
4

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
pH ~ 1.8 to pH 10.2 under the same concentrations of ligand and
metal ions. The spectra obtained during the spectrophotometric
titrations of Eu and Tb complexes of CYTOM5OX are shown in
Fig. 5. It may be noted that, for the refinement, the data obtained
in two different pH ranges (acidic or basic) were fitted discretely
in the refinement program. The absorbance data of each set for
both complexes refined by Hypspec provided global stability con-
stants, are returned in Table 1. The best fit of absorbance data
between pH ~ 1.6 and ~ 7.5 corroborates the existence of four com-
3
+
3+
plex species in the case of Tb whereas for Eu complex system,
two species formed. The combination of these data with that
obtained from the pH titrations permitted us to consider four pro-
tonated species for each metal ion.
Analysis of the spectrophotometric data above pH ~ 7.5 resulted
3
+
3+
presence of two more species for Eu , and three for Tb . Their for-
mation is assumed by considering successive deprotonation from
+
Ln(HL) to form LnL. Unlike Eu, the formation of a low percentage
2
of LnL was detected for Tb ions in the same pH range. The exis-
3
+
-5
Fig. 4. Potentiometric titration curves: (i) [CYTOM5OX]/[Eu ] = 1/1, 5 ꢄ 10 M, and
tence of hydroxo species for all the complexes was also observed
at pH higher than 10.2 in each case. All these complex species,
along with their stability constants, are listed in Table 1.
(
ii) [CYTOM5OX]/[Tb3+] = 1/1, 5 ꢄ 10 M. Solvent H
-5
2
O, I = 0.1 M (KCl), T = 25(2) C,
and ‘a’ is the moles of base added per mole of Ln[CYTOM5OX] present. Symbols and
solid lines represent the experimental and calculate data, respectively.
°
The various lanthanide species and the corresponding forma-
tion constants obtained above were also authenticated by the
luminescence method. These titrations were executed for 1:1 com-
plex solutions of europium and terbium ions with the chelator
from ~ 1.8–10.2 pH range. The experimental data for the lumines-
cence titrations of these complexes are presented in Figure 6. For
the Eu(CYTOM5OX) system, the optimum fit data returned five
þ
LnðHLÞ þ nH
2
OꢀLnLðOHÞ þ nH
ð4Þ
ð5Þ
n
Â
à  þÃ
n
LnLðOHÞ : H
n
K
LnLðOHÞn
¼
½
LnðHLÞꢃ
3
+
2+
+
Â
Ã
3 2
complexes: three protonated [Eu(H L)] , [Eu(H L)] [Eu(HL)] ,
LnLðOHÞ
n
-
logb
¼
ð6Þ
one neutral (EuL), and one hydrolyzed-complex [EuL(OH)] . For
LnLðOHÞn
n
½
LnðHLÞꢃ:½H
2
Oꢃ
the Tb complex, the most acceptable refinement fluorescence-pH
4
+
3+
2+,
data resulted in M(H
4
L) , M(H
3
L) , M(H
2
L) and M(HL) proto-
The log b110 could not be determined from the potentiometric
data. In order to determine the missing formation constants, the
UV–visible spectrophotometric titration was carried out from
nated species, two neutral ML and ML
2
species, and only one
hydroxo species. The stability constants determined by the above
Fig. 5. Electronic spectra of 1:1 solution of Ln and CYTOM5OX as a function of pH]: (a) p[H] = 1.8–7.4 (b) p[H] = 7.6–10. 2, for [CYTOM5OX]/[Eu3+] = 1/1; and (c) p[H] = 1.81–
3
+
3
+
3+
-5
7
.4 (d) p[H] = 7.5–10.2 for [CYTOM5OX]/[Tb ] = 1/1; [CYTOM5OX] = [Ln
]
tot = 5 ꢄ 10 M, ionic strength = 0.1 M(KCl), Temp = 25.0(2)°C.
5

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
-5
Fig. 6. Fluorescence spectra of Ln: CYTOM5OX (1:1, 5.0 ꢄ 10 M in water) as a function of pH at Temp. = 25.0 °C, slit widths = 5.0 nm, and kexc = 390 nm for each system (a) Eu
CYTOM5OX], (1.88–10.13 pH) and (b) Tb[CYTOM5OX] (1.87–10.27 pH).
[
Table 1
Equilibrium constants (log b11n), absorption, emission characteristics of Eu3+ and Tb3+ complexes of CYTOM5OX and corresponding pLn values.
a
Log b11n Log b11nc Log b11n Average
b
d
k
max (nm)
e(M cm
ꢂ1
ꢂ1
)
k
em(nm) / PL
e
pLn3+ f
Complexes
Species
L = CYTOM5OX
Log b11n
(
EuL)
[Eu
53.35(4) 52.85(5)
48.43(4) 47.94(2)
_
_
53.10
48.18
245, 270, 335
120 000, 22 000, 21 400
423
0.007 21.9
(
[
(
[
(
[
H
Eu
H
Eu
4
L)]4+
L)]3+
L)]2+
245, 260, 290, 390
245, 265, 290, 390
245, 275, 291, 392
265, 278, 310, 400
114 000, 50 300, 41 000, 17 000
112 000, 64 000, 45 000, 18 900
102 000, 71 300, 59 000, 23 000
101 000, 65 300, 57 000, 23 000
423,
475
424,
500
423,
0.008,
0.014
0.009,
0.031
0.009,
0.047
0.008,
0.041
3
_
41.32(4) 41.26(3) 41.29
37.06(3) 37.19(4) 37.21(5) 37.15
33.95(3) 33.99(1) 33.97
H
2
+
Eu(HL)]
5
05
422,
18
[
[
(EuL)]
–
5
EuL(OH)]- 10.34(2) 10.43(4) 10.54(3) 10.43
53.45(5) 53.43(3) 53.44
265, 274, 312, 400
247, 270, 345
99 800, 68 000, 73 000, 24 100
178 300, 67 000, 65 000
420
425
0.012
0.008 22.4
(
TbL)
[Tb
_
(
[
(
[
(
[
H
Tb
H
Tb
4
L)]4+
L)]3+
L)]2+
_
45.44(1) 45.25(5) 45.34
38.99(6) 38.78(5) 38.71(6) 38.82
248, 268, 285, 342, 400 155 000, 72 600, 49 800, 51 820, 21 000 425,
0.009,
0.013
0.010,
0.023
0.010,
0.045
0.039
0.021
0.011
3
475
250, 265, 290, 340, 400 141 000, 78 000, 62 000, 46 200, 36 500 425,
500
H
2
+
Tb(HL)]
36.33(7) 36.39
63)
–
–
36.46(1) 36.39
250, 262, 295, 400
123 000, 95 000, 71 500, 46 000
425,
505
(
[
[
[
(TbL)]
(TbL )]
2
TbL(OH)] 9.51(6)
34.41(3) 34.73(7) 34.57
30.17(6) 30.21(6) 30.19
260, 274, 315, 345, 405 130 000, 100 100, 70 000, 50 300, 42 000 512
262, 275, 310, 345, 400 101 600, 127 000, 81 200, 42 000, 49 000 425
-
9.62(5)
9.48(4)
9.53
276, 305, 345, 395
14 800, 7 300, 33 000, 38 200
420
a
b
c
Numbers in parentheses represent the standard deviation in the last significant digit. In water (I = 0.1 M KCl, T = 25.0 °C).
Potentiometric method.
UV–visible spectrophotometric method.
Luminescence spectrophotometric method.
d
e
as standard (/ = 0.546), pLn = ꢂlog [Ln3+] calculated for [Ln]tot = 10 M, [L]tot = 10 M, and p[H] = 7.4.
f
3+
ꢂ5
ꢂ5
0
2 4
.1 M solution of quinine sulphate in 0.5 M H SO
three methods, defined by equations 1ꢂ6, are reported in Table 1,
along with the average log bs.
Formation constants determined for theLn3+ complexes of CYTO-
M5OX are compared to those obtained for its acyclic counterparts
reported earlier [25]. The stability of the present ligand with the
metal ions was found to be higher. Replacement of the central scaf-
fold from the acyclic C pivot unit to a more flexible cyclic unit is
expected to cause more stability for the chelates. It is misleading
when compare the relative affinities of the metal ions studied for
a given ligand in terms of log K since the competition with hydrogen
ions for the ligand are not considered. The pM values give an
impression of the relative sequestering ability of the ligands under
a standard set of conditions. Therefore, to compare the ML stability,
the pLn for CYTOM5OX chelates were calculated for both lanthanide
ions, which turn out between ~21–22 (Table 1). The pLn vs. pH plot
in the pH range of 2–9 are presented in Fig. 7. This effect is mani-
fested here where [Ln(CYTOM5OX)] complexes are 1–3 orders of
magnitude more stable than the analogous [Ln(TAME5OX)] and
The stability constants were also estimated by spectrophoto-
metric batch titrations using a potential competitor EDTA [24] in
an opposite pH range 10.2–2.1. The equilibrium constants can be
calculated from the given Eq. (7) where Y designates tetra anion
of EDTA, and LnL signifies Eu and Tb complexes of the ligand CYTO-
M5OX. For more clarity, charges are not shown deliberately.
LnY þ LꢀLnL þ Y
ð7Þ
LnL
½
½
LnLꢃ½Yꢃ
LnYꢃ½Lꢃ
b
b
1
10
LnY
10
K ¼
¼
1
Putting the known values of equilibrium constants correspond-
ing to Ln-EDTA [24] and K EDTA in the above Eq. (7), log b110 val-
a
ues were obtained to be 33.28 and 34.19 respectively for Eu and Tb
complexes, which are consonant with the results derived from the
other methods.
[
Ln(TMOM5OX)] complexes. It has been observed that sequestering
ability increases with the atomic number of metal ions.
6

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
donation from the ligand that serves to destabilize the metal-
centered HOMO and raise its energy. Besides the appearance of
these red-shifted bands at about 263, 300, and 400 nm, the parent
peaks were shifted hypochromically. This concurrent downslope at
2
45, 330 nm, and upslope at 263, 300, and 400 nm, give four to five
isosbestic pints in the absorption spectra of complexes, which
reveal that upon complexation, different protonated complexes
are formed. The electronic absorption maxima and extinction coef-
ficients of these complexes are listed in Table 1, together with the
related species.
Above physiological pH, the behavior of [Eu(CYTOM5OX)] was
found less significant as compared to Tb complex. The spectral pat-
tern of two complexes shows two distinct series of lines, which on
analysis correspond to two neutral and one hydroxo-species, as
discussed in section 3.1. For Eu, two isosbestic points at 263 and
2
68 nm were observed, whereas the spectra in the visible region
were overlayed on each other. The bands at about 390–400 nm
+
3
are assigned to charge transfer by O ? Ln . It should be noted that
the bands due to Npyr ? Ln disappear in this pH range; the change
in the absorption spectrum can be interpreted by a change in the
coordination sphere of the lanthanide ion. In contrary to the acidic
to neutral pH range, the concurrent downslope of LMCT bands of
Tb complexes was observed over the pH range of ~7.5–10.2.
The formation of lanthanide complexes was further examined
by theoretical methods. The electronic transitions and electronic
structures of different complex species formed in the solution were
calculated by using sparkle/ PM7 parameter. The computed optical
data of all species are listed in Table 2. The ground state optimized
structures of considered Ln complexes of CYTOM5OX are shown in
Fig. 9. The theoretical absorption spectra of the species assessed by
ZINDOS/CIS, are in congruence with the experimental ones with
respect to contour, the number of bands, and relative intensities,
shown in Fig. 10. From the calculated UV–Vis spectra and the
HOMO-LUMO energy levels computed for different species, the
alterations that occur in transitions energies and absorption band
could be assessed.
Fig. 7. Plot of pLn versus p[H] for CYTOM5OX, pLn = -log [Ln3+], calculated for
3+
ꢂ5
ꢂ5
3+
3+
[
Ln ] = 10 M and [L] = 10 M, for Ln = Eu , and Tb .
The solution species distribution curves for metal ion complex-
ation are shown in Fig. 8. The aqueous-phase free energies for both
the metal–ligand systems were calculated through the sparkle/
PM7 parameter and are compared with the calculated experimen-
tal log Ks.
3
3
.3. Photophysical properties
.3.1. Electronic spectral studies
Electronic absorption spectra of the complexes of CYTOM5OX in
acidic- neutral pH range (shown in Fig. 5) display mainly two sets
of absorption bands, one from the intra-ligand * transition at
p
ꢂ
p
ca. 245 nm and the other at ca. 330 nm due to ligand-centered
transition of quinoline. A shoulder at around 262 nm is also visible
on the high-energy side of the broad band of [LnL]. A progressive
red-shift in the lowest energy absorption band from 257 to
The first band was due to the 72% contribution of
HOMO
? LUMO transition. The band with a maximum
at ~ 370 nm is attributed to HOMO ? LUMO + 1 (78%), HUMO-
3
00 nm and from 375 to 425 nm is observed as the pH is varied
1
3
? LUMO (93%), HOMO ? LUMO + 2 (70%), HOMO ? LUMO +
(71%, 51%), HUMO-2 ? LUMO (85%) and HUMO-1 ? LUMO + 1
from acidic to neutral range. The appearance of long-
a
wavelength band is typically associated with a metal–ligand
charge transfer (MLCT) state in which an electron is promoted from
(
2
60%) transitions for EuL, TbL, and TbL , respectively. As the HOMO
and LUMO are localized on the quinoline orbitals, the ligand–li-
gand charge transfer transitions (LLCT) are associated with these.
a metal orbital to the
p*-orbital of the most electronegative ligand.
The bathochromic shift is most likely due to increased electron
Fig. 8. Species distribution curves of (a) Eu[CYTOM5OX]and (b) Tb[CYTOM5OX], containing species, computed from the formation constants given in Table. Calculated for
CYTOM5OX]tot = [Ln3+
]tot = 10 M, Solvent : H O, I = 0.1 M (KCl), T = 25.0(2)°C.
-5
[
2
7

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
Table 2
Optical properties computed for Eu[CYTOM5OX] and Tb[CYTOM5OX]: Absorption-S0 and Emission-S1; Wavelengths, Oscillator Strengths (fcalc), Electronic Transitions and
Energies Calculated at the CIS-ZINDO/S Method, Using Sparkle/PM7 Ground State and Excited State Geometry of the Complexes.
S0
S1
Complexes
EuL
k
nm (abs)
f
calc
Nature
Transition
Excitation energy)
k
nm(em)
f
calc
Nature
Transition
(Emission energy)
(
265.89
0.1420
0.0321
0.0472
0.0209
0.1952
0.1436
0.0352
0.0209
0.0279
0.1132
0.0134
0.0351
0.0208
0.0198
HOMO-3 ? LUMO + 2
HOMO-2 ? LUMO + 1
HOMO ? LUMO
HOMO-3 ? LUMO + 1
HOMO-3 ? LUMO + 2
HOMO-2 ? LUMO + 2
HOMO-2 ? LUMO + 1
HOMO-3 ? LUMO + 1
HOMO ? LUMO
HOMO-1 ? LUMO + 2
HOMO ? LUMO + 1
HOMO-1 ? LUMO + 1
HOMO-2 ? LUMO + 1
HOMO-2 ? LUMO + 1
116 ? 122 (1.154 eV)
117 ? 121 (0.952 eV)
119 ? 120 (0. 312 eV)
116 ? 121 (0.023 eV)
116 ? 122 (1.242 eV)
117 ? 122 (0.923 eV)
117 ? 121 (0.432 eV)
116 ? 121 (0.162 eV)
119 ? 120 (0.135 eV)
118 ? 122 (0.979 eV)
119 ? 121 (0.731 eV)
118 ? 121 (0.306 eV)
118 ? 121 (0.217 eV)
117 ? 121 (0.221 eV)
500.32
1.9503
1.1708
HOMO-1 ? LUMO + 1
HOMO ? LUMO + 1
118 ? 121 (5.365 eV)
119 ? 121 1. 016 eV)
3
3
4
13.05
72.45
79.47
TbL
247.31
513.97
426.12
HOMO-1 ? LUMO + 1
HOMO ? LUMO
HOMO-1 ? LUMO
118 ? 121 (4.490 eV)
119 ? 120 (1.207 eV)
118 ? 120 (1.591 eV)
3
3
4
4
16.63
65.13
00.19
67.78
TbL
2
283.14
0.2439
HOMO-1 ? LUMO + 1
HOMO ? LUMO
118 ? 121 (2.634 eV)
119 ? 120 (0.926 eV)
3
3
4
4
31.08
79.83
25.37
82.56
Fig. 9. Molecular Structures of (a) Eu[CYTOM5OX] and (b) Tb[CYTOM5OX] at ground level, optimized by Sparkle/PM7.
Fig. 10. Simulated CIS-ZNDO/s absorption spectra of neutral (LnL, Ln = Eu and Tb) complexes of CYTOM5OX for ground-state geometries of complexes.
The higher energy bands at 400, 465 nm have been attributed to
the metal–ligand charge transfer transitions (MLCT). The intense
band at 245 and 292 nm are attributed to LLCT with MLCT transi-
tions (HOMO-7 ? LUMO (63%), HUMO-12 ? LUMO (37%), and
HUMO-1 ? LUMO + 5(70%)).
3.3.2. Luminescence studies
With the fluorogenic ligands, a simple way of investigating and
depicting the influence of pH on the efficiency of complexation is
to study the variation of the fluorescence emitted by chelates as
a function of pH. Upon excitation in the visible domain about
8

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
Fig. 11. Simulated CIS-ZNDO/s emission spectra of neutral (LnL, Ln = Eu and Tb) complexes of CYTOM5OX for first excited state geometries of complexes.
Fig. 12. Luminescence decay curves for complexes (a) Eu(CYTOM5OX) in H2O and (b) Eu(CYTOM5OX) in D2O, (c) Tb(CYTOM5OX) in H2O and (d)Tb(CYTOM5OX) in D2O.
Excited at 390 nm in the solution state. The scattering points are experimental data, and the solid lines are the fitting results.
3
90 nm, these lanthanide-ligand systems give moderate fluores-
to ~ 7.4. A decrease of solution acidity leads to gradual quenching
of the same wavelength emission band up to ~ 8.95 pH with no
apparent changes are observed in shapes. On the other hand, the
addition of the more alkali also results in the notable blue shift
of this green emission at the original position (about 430 nm).
The fluorescence enhancement up to 3-fold for both Eu and Tb
complexes is more intense than at 8.85 pH at about 420 nm, which
get again quenched under more basic conditions at pH ~ 10.2, in a
similar manner, as observed under acidic conditions. The high effi-
ciency of emission for Tb(CYTOM5OX) validates the stability of
cence with maximum emission at 425 nm at acidic pH, which gets
red-shifted with efficient growth in intensity while moving
towards the physiological medium. Fig. 6 shows the fluorescence
emission spectra of Ln(CYTOM5OX) at variable pH ranging
from ~ 1.8–10.2. Although the fluorescence peaks show remarkable
changes with different pH, the intensity of the emission band at
about 510 nm changes hugely. At pH ~ 2.6, a broad emission about
4
50–500 nm is observed, and the intensity of emission at a partic-
ular wavelength is enhanced greatly until the pH value reaches
9

R. Akbar, M. Baral and B.K. Kanungo
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 247 (2021) 119124
Table 3
are described. The ligand leads soluble and thermodynamically
stable complexes in aqueous solution with high stability constants
as compared to its acyclic counterparts. The terbium complex of
CYTOM5OX displays a longer excited-state lifetime than the corre-
sponding europium complex. Moreover, the increased coordinated
water molecules reduce the luminescence lifetimes. The computed
electronic absorption and emission spectra of all the possible spe-
cies, studied by single excitations (CIS) method based using ZINDO/
S, were in excellent agreement with experimentally observed data.
The observed emissions of the complexes could be attributed pre-
dominantly to a ligand centred (LC) emission with mixed contribu-
Ligand-centered lifetimes (s) for Eu and Tb in the Ln(CYTOM5OX) podates in water
and D O solution and number of water molecules bound in the inner coordination
2
sphere (q).
Ln[a]
k
ex nm
s
[ms] H
51.52(9)
65.32(13)
O
s
[ms] D
O
q[b]
2
2
Eu
Tb
390
390
0.78(5)
1.63(4)
1.8
2.9
[
a]
-5
[
Ln(CYTOM5OX)] = 5 ꢄ 10 M (Ln = Eu, Tb).
[b]
From equation (8).
3
+
tion from Ln ions. The nonacoordinated Ln complexes containing
hydroxyquinolinate based ligand studied in this work show a lumi-
nescent response toward metal ions that were modulated in the
wide range of pH medium.
2
TbL species, which is in harmony with its absorption behavior in
this pH range. Similarly, for Eu(CYTOM5OX), the less significant
absorption behavior at alkaline pH was also attested from its pH-
dependent fluorescence with the absence of any significant peak
at 430–440 nm range (cf. ‘a’ of Fig. 6).
The vertical excitation spectra were computed for all species
using a model including the different number of explicit water
molecules to complete nine coordination sphere for mono-
capped chelates and water-free molecules for bicapped chelates
at the ZINDO/s level on first excited optimized ground state struc-
tures obtained at the Sparkle/PM7 level. All transitions and corre-
sponding oscillator strengths computed for each species in this
spectral range are reported in Table 2, and the spectra correspond-
ing at each isolated neutral species are reported in Fig. 11. It is
worth noting that these transitions are well separated from the
others, thus allowing us to envisage the selective population of
the S1 state.
CRediT authorship contribution statement
Rifat Akbar: Investigation, experimental work, draft prepara-
tion. Minati Baral: Reviewing, correction of draft and editing,
B K Kanungo: Supervision of work, reviewing and editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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3
.3.2.1. Lifetime measurements. The radiative decay constants of the
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(
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3
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3+
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O are presented in Table 3.
When a coordinated water molecule is present in the complex,
2
O and
[
[
[
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due to the difference in the energy levels between the radiative
and ground states.
3+
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[
[
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mined by the method developed by Horrocks et. al.. [26] The
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s) of the lanthanide excited-state
[
15] P. Gans, Protonic Software (2006) ‘‘HypSpec” Leeds.
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2
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in Eq. (8).
2
O) with the hydration number (q) is shown
[
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[
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q ¼ ALn
ð
D
k
obs
Þ
ð8Þ
3+
[19] J. J. P Stewart, MOPAC2012, Version 12.239W, S. C. Chemistry (Eds.) Colorado
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[20] M. O. Rodrigues, F. A. Almeida Paz, R. O. Freire, G. F. de S a´ , A. Galembeck, M. C.
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Tb3+, kobs
¼
1
s
; and
Dkobs ¼ kobsðH
OÞ ꢂ kobsðD OÞ. In the present case,
2
2
B. S. M. Montenegro, N. A. J. Alberto, S. Alves, Jr., J. Phys. Chem. B 2009, 113,
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[
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1
4
. Conclusion
(
Synthesis, characterization and interaction of a flexible cyclic
[26] W.D.W. Horrocks Jr., D.R. Sudnick, J. Am. Chem. Soc. 101 (1979) 334–340.
tripodal ligand, CYTOM5OX, with trivalent europium and terbium
10