Ligand sensitized luminescence of uranyl by benzoic acid in acetonitrile medium: A new luminescent uranyl benzoate specie

Article abstract of DOI:10.1016/j.saa.2014.11.033

Benzoic acid (BA) is shown to sensitize and enhance the luminescence of uranyl ion in acetonitrile medium. Luminescence spectra and especially UV-Vis spectroscopy studies reveal the formation of tri benzoate complex of uranyl i.e. [UO₂(C₆H₅COO)₃]^- which is highly luminescent. In particular, three sharp bands at 431, 443, 461 nm of absorption spectra provides evidence for tri benzoate specie of uranyl in acetonitrile medium. The luminescence lifetime of uranyl in this complex is 68 μs which is much more compared to the lifetime of uncomplexed uranyl (20 μs) in acetonitrile medium. In contrary to aqueous medium where uranyl benzoate forms 1:1 and 1:2 species, spectroscopic data reveal formation of 1:3 complex in acetonitrile medium. Addition of water to acetonitrile results in decrease of luminescence intensity of this specie and the luminescence features implode at 20% (v/v) of water content. For the first time, to the best of our knowledge, the existence of [UO₂(C₆H₅COO)₃]^- specie in acetonitrile is reported. Mechanism of luminescence enhancement is discussed.

Full text of DOI:10.1016/j.saa.2014.11.033

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Ligand sensitized luminescence of uranyl by benzoic acid in acetonitrile
medium: A new luminescent uranyl benzoate specie
Satendra Kumar a, S. Maji a, M. Joseph b, K. Sankaran a,
⇑
a Materials Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
b Fuel Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
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
ꢀ
ꢀ
Uranyl luminescence is sensitized by
benzoic acid in acetonitrile medium.
UV–Vis and luminescence
spectroscopy is used to characterize
uranyl benzoate at different uranyl to
benzoate ratio.
Uranyl luminescence
UV-Vis spectra of uranyl in acetonitrile
Quenching in aqueous medium
Without BA
With BA
0.4
With BA
ꢀ
ꢀ
The specie formed is
450
475
500
525
550
575
[UO2(C6H5COO)3]À, which is highly
luminescent.
Sensitization in acetonitrile
0.2
Without BA
With BA
Acetonitrile plays an important role
in the sensitized luminescence by
forming tris complex of uranyl.
Without BA
450
475
500
525
550
575
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 May 2014
Received in revised form 18 November 2014
Accepted 20 November 2014
Available online 5 December 2014
Benzoic acid (BA) is shown to sensitize and enhance the luminescence of uranyl ion in acetonitrile
medium. Luminescence spectra and especially UV–Vis spectroscopy studies reveal the formation of tri
benzoate complex of uranyl i.e. [UO2(C6H5COO)3]À which is highly luminescent. In particular, three sharp
bands at 431, 443, 461 nm of absorption spectra provides evidence for tri benzoate specie of uranyl in
acetonitrile medium. The luminescence lifetime of uranyl in this complex is 68
ls which is much more
compared to the lifetime of uncomplexed uranyl (20 s) in acetonitrile medium. In contrary to aqueous
l
Keywords:
medium where uranyl benzoate forms 1:1 and 1:2 species, spectroscopic data reveal formation of 1:3
complex in acetonitrile medium. Addition of water to acetonitrile results in decrease of luminescence
intensity of this specie and the luminescence features implode at 20% (v/v) of water content. For the first
time, to the best of our knowledge, the existence of [UO2(C6H5COO)3]À specie in acetonitrile is reported.
Mechanism of luminescence enhancement is discussed.
Luminescence
Sensitization
Benzoic acid
Uranyl
Acetonitrile
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
luminescence, the molecule/specie must have high absorption
coefficient and quantum yields. In electronic spectroscopy, after
In luminescence spectroscopy, absorption coefficient and quan-
tum yield are the two important factors which decide the lumines-
cence behavior of any molecule/specie. In order to have a strong
the excitation, the molecule will release the excess energy, photo
physically, in the form of two decay channels: radiative and non-
radiative. Solvent plays an important role in the decay of excited
state of the molecules [1,2].
Lanthanides and actinides are known to be weak luminescent
elements in aqueous medium because of their low molar
⇑
Corresponding author.
E-mail address: ksran@igcar.gov.in (K. Sankaran).
http://dx.doi.org/10.1016/j.saa.2014.11.033
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

510
S. Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
absorption coefficient and poor quantum yields [3,4]. The low
absorption coefficient arises from the forbidden d–d or f–f transi-
tions. In aqueous medium, the luminescence of the lanthanide
and actinide ions is highly quenched by the water molecules. It is
reported that in the case of lanthanides, the O–H oscillators of
water molecules take the excess energy and cause the molecule
to de-excite through non-radiative processes [3,5,6], where as in
the case of uranyl ion the electron transfer mechanism is responsi-
ble for quenching in aqueous medium [7]. Methods to enhance the
luminescence of lanthanides and actinides in aqueous solutions are
therefore required. In the case of lanthanides, ligand sensitized
luminescence has been widely used to enhance the luminescence
intensity and hence the determination of lanthanides in trace level
have been reported [8–15]. In ligand sensitized luminescence,
ligand absorbs the light and then transfers the energy to the metal
ions which results in enhancement in luminescence intensities.
According to the theory of ‘‘Antenna Effect’’, the luminescence
intensity of complexes of metal ions is decided by the efficiency
of the energy transfer from the ligand to the coordinated metal
ion, which in turn dependent on the energy level matching
between the triplet state of the ligand and the lowest excited state
of metal ion [5,6].
Experimental details
Instrumentation
All luminescence spectra were recorded using Edinburgh spec-
trofluorimeter, model FLS920, with a 450 W xenon lamp as the
excitation source. Fused silica cuvette of path length 2 mm was
used as a sample cell for recording the luminescence spectra. The
band pass of 3 nm was set for both the excitation and emission
monochromators. A long-wavelength pass filter, (UV – 39, Shima-
dzu) with a maximum and uniform transmittance (>85%) above
400 nm, was placed in front of the emission monochromator, to
reduce the scatter of the incident beam into the emission mono-
chromator. Spectra were recorded at room temperature with a
90° collection geometry. All spectra were blank subtracted; a blank
spectrum was recorded using identical experimental conditions
without the uranyl ion in the solution. All spectra were also cor-
rected for instrument response.
Time resolved spectra are recorded using a
ls-Xe flash lamp.
Luminescence life times were determined by fitting the observed
time resolved luminescence signals to an exponential decay func-
tion. A single or double exponential fit was found to be adequate
for the decay processes observed in this study. The v2 values of
all the fits ranged between 0.9 and 1.1. Since the temporal profile
of the pulsed source was around 1.5 ls, lifetimes that were of this
order of magnitude were obtained after correcting the instrument
response function before fitting. However for systems which dis-
played lifetimes of the order of 20 ls or longer, the lifetimes were
extracted through a tail-fit, where the data points in the decay pro-
file extending to long temporal regions were used for the fitting.
The relative standard deviation of the lifetime values was less than
5%.
UV–Vis absorption spectra were recorded using Avantes fiber
optic spectrophotometer, model AvaSpec-3648-USB2 with 300
lines per mm grating. An integration time of 6 ms was used and
20 spectra were averaged to improve the signal to noise ratio.
In order to enhance the luminescence of uranyl ion in aqueous
medium, luminescence enhancing reagents such as H3PO4, H2SO4,
HClO4 have been widely used [16–18]. These agents make complex
with uranyl ion, thereby eliminating water molecules from the pri-
mary coordination sphere of uranyl ion and consequently reducing
the quenching effects due to water and hence results in enhance-
ment of uranyl ion luminescence. It has also been observed that
the luminescence lifetime of uranyl ion increases from 2
ls
(uncomplexed) to 10–230 s (complexed) with the above men-
l
tioned agents [16–18]. Luminescence measurements of lantha-
nides and actinides at low temperature (Cryo-TRLFS) have also
been reported in literature [19–21]. As a consequence of reducing
quenching effects at low temperature, an increase in luminescence
life time of uranyl has been observed at low temperatures [20,21].
The other method to enhance the uranyl luminescence is by ligand
sensitized luminescence, a method well established for lantha-
nides. Although there are plenty of ligands which enhance the
luminescence of lanthanides, only a few such as 2–6, pyridine
dicarboxylic acid and trimesic acid were found to enhance the
luminescence of uranyl ion [22–23].
Recently uranyl luminescence has been studied in non-aqueous
medium [24–26]. These studies have reported the formation of dif-
ferent species, their structure and their spectroscopic properties in
acetonitrile and ionic liquid medium. The aim of the present work
is to examine the possibility of using non-aqueous medium for
enhancing the luminescence intensity of uranyl ion for trace level
detection. In our earlier work we have reported large enhancement
of lanthanide luminescence intensity in acetonitrile compared to
aqueous medium [27]. In this work, luminescence and ligand (ben-
zoic acid) sensitized luminescence of uranyl ion has been studied
in acetonitrile medium. While earlier works involved luminescence
of different uranyl species in acetonitrile medium [24–26], no work
has been reported on the ligand sensitized luminescence. It should
be noted that in aqueous medium, benzoic acid does not enhance
the uranyl luminescence although it forms 1:1 and 1:2 complexes
with uranyl ion [28,29]. The luminescence of uranyl ion is found to
be enhanced by benzoate in acetonitrile medium and the enhance-
ment is due to sensitization of uranyl by benzoate ions. UV–Vis
spectroscopy has been utilized to characterize the uranyl–BA
specie in acetonitrile. Mechanism for uranyl luminescence
enhancement in acetonitrile is discussed. To the best of our knowl-
edge, this is the first report on ligand sensitized luminescence of
uranyl in acetonitrile medium and characterization of the complex
of uranyl–BA system.
Reagents
Uranyl perchlorate solution was prepared from UO2 powder
(Nuclear Fuel Complex, India). Towards this, first uranium dioxide
was dissolved in nitric acid and the solution was evaporated to
dryness. Subsequently, the uranyl nitrate residue was then dis-
solved in perchloric acid and evaporated to dryness until the white
fumes of perchloric acid disappear and finally yellow residue of
uranyl perchlorate was obtained. This residue was then dissolved
in acetonitrile or water to get a stock solution of 10À1 M uranyl.
The aqueous solution was acidified with a few drops of 1 M per-
chloric acid. Stock solution of benzoic acid (Fluka make, AR grade)
was prepared by dissolving the required amount in water. To
ensure complete dissolution of the acid, small amount of sodium
hydroxide was added. The pH of the solutions was adjusted by
the addition of sodium hydroxide (AR grade)/perchloric acid
(Sigma make). Ionic strength of the solution was adjusted using
sodium perchlorate (99.99%, Sigma make). Acetonitrile used in
our study was of Merck HPLC grade (purity > 99.8%). All chemicals
were used as purchased from the supplier. De-ionized water
(18 MX) obtained with a Milli-Q (Millipore) system was used for
preparing the solutions.
Preparation of uranyl and uranyl–benzoate solution in acetonitrile
Aqueous uranyl solutions of different concentrations
(4 Â 10À3 M to 8 Â 10À5 M) were prepared from the 10À1 M uranyl
(aqueous) stock solution. The ionic strength of these solutions was

S. Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
511
520
544
499
fixed to 0.1 M and pH was varied from 2.4 to 5.0. From this solu-
tion, 5 L was taken into 0.5 mL of acetonitrile to get the working
l
÷ 20
pH 5.0
solution for recording the luminescence spectrum.
Similarly uranyl–BA complexes at different pH were prepared
by mixing the required amount of benzoic acid and 10À1 M uranyl
(aqueous) stock solution. The ionic strength was adjusted to 0.1 M.
5 lL of this solution was then mixed with 0.5 mL of acetonitrile
and the luminescence spectrum recorded.
However to record the absorption spectra, the working solution
was prepared as follows. At first, benzoate solutions of concentra-
tions at 4 Â 10À1 M, 6 Â 10À1 M and 8 Â 10À1 M were prepared.
The pH of the solutions was adjusted at 5.5 and the ionic strength
pH 4.5
pH 4.0
÷ 14
÷ 9
of the solution was kept constant at 0.1 M. 20
lL of each of these
solutions along with 40
l
L of 10À1 M uranyl (acetonitrile) stock
solution was taken in a 2 mL vial and the volume was made up
with acetonitrile.
÷ 4
pH 3.5
pH 3.0
All the working solutions prepared were mixed thoroughly by
shaking manually. In all our experiments carried out in acetonitrile
medium, about 1% water was present to begin with.
÷ 2.5
Results and discussion
In the following text the term ‘pH in acetonitrile medium’ is
used. It must be noted that this term implies the pH in aqueous
solution of uranyl (not the pH of acetonitrile), from which 5 lL
pH 2.4
was taken and dissolved in 0.5 mL of acetonitrile. Also the term
uncomplexed uranyl and complexed uranyl refer to uranyl specie
in acetonitrile/water medium without and with benzoate ion,
respectively. The term free uranyl refers to hydrated uranyl ion.
425 450 475 500 525 550 575 600
Wavelength (nm)
Fig. 1. Emission spectra of uranyl at different pH in water (dashed line); [UO22+] = 4 -
 10À4 M and in acetonitrile (solid line); [UO22+] = 4  10À5 M. kex = 250 nm. Ionic
strength = 0.1 M.
Uranyl luminescence and decay profile in acetonitrile medium
Since uranyl–BA complexes in acetonitrile medium were pre-
pared from aqueous solution of uranyl–BA at different pH, initially
we have recorded the luminescence spectra of uncomplexed ura-
nyl ion as a function of pH in acetonitrile medium. Fig. 1 compares
the luminescence spectra of 4 Â 10À5 M uranyl ion solution as a
function of pH from 2.4 to 5.0 in acetonitrile medium. For compar-
ison, uranyl ion luminescence in aqueous medium is also shown as
dotted lines in the same figure. The concentration of the uranyl ion
used in the aqueous medium is 4 Â 10À4 M.
Table 1 compares the lifetimes of uranyl ion at different pH in
acetonitrile and aqueous medium. At pH 2.4 the life time showed
single decay of 20 ls in acetonitrile medium which indicates the
luminescence life time of free uranyl ion. Since 1% water is present
in acetonitrile to begin with and being a weak coordinating sol-
vent, acetonitrile will not bind to the first coordination sphere of
uranyl ion which is similar to a situation seen in the luminescence
of lanthanides in non-aqueous solvents [34]. Therefore small
At pH 2.4, uranyl showed five sharp peaks at 470, 488, 509, 532
and 558 nm in acetonitrile medium which is similar to what is
observed in aqueous medium (dotted lines) and hence these fea-
tures are assigned to free uranyl luminescence in acetonitrile. From
pH 3.0 onwards the behavior of uranyl luminescence in acetonitrile
is different from that in aqueous medium. At pH 3.0, new lumines-
cence peaks appeared at 499, 520 and 545 nm along with the peaks
of uranyl ion in the acetonitrile medium. These new features in
acetonitrile medium could be due to hydrolysis of uranyl ion as
small amount of water is always present in the solution. Similar
features were observed in literature for different uranium-hydroxo
complexes in aqueous medium [30,31]. Above pH 3.0, only these
new peaks were observed while the peaks due to free uranyl ion
start disappearing. In aqueous medium, the peak positions of ura-
nyl ion do change with pH and it is well reported [30–33]. This
implies that only one type of hydroxide is formed and strongly sol-
vated by acetonitrile molecules which help in preventing uranyl to
polymerize into other higher species. Appearance of these peaks at
pH 3.0 suggests that this specie might be uranyl hydroxide (UO2-
OH+) and this hydroxide specie dominates even at this low pH in
acetonitrile medium, which is present in negligible amount in
water at the same pH. The luminescence intensity of this specie
is found to increase with pH.
increase in lifetime from 1.5 to 20 ls when we go from aqueous
Table 1
Lifetimes of uncomplexed uranyl species at different pH in water and acetonitrile
medium.
pH
2.4
3.0
In aqueous medium
In acetonitrile medium
Life timesa
(l
s)
Refs.
Life timesa
(l
s)
Refs.
1.5
1.9
1.3
1.5
This work
[28]
[29]
This work
[29]
[28]
This work
[28]
This work
[28]
This work
[29]
[28]
20
This work
20, 58
1.3
1.8, 32.8
1.4, 9.0
1.9, 32.8, 8.8
1.8, 11.0
1.6, 32.8, 9.9
1.8, 10
2.2, 6.8
1.6, 9.3
14
3.5
4.0
4.5
21, 65
62
60
5.0
This work
[28]
70
12
a
5% deviation in present study, ionic strength of all the aqueous solutions was
0.1 M.

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S. Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
to acetonitrile medium suggests that the removal of water mole-
cules from the secondary sphere of uranyl ions by acetonitrile
and not due to the removal of inner sphere water molecules by
acetonitrile. As pH increases the lifetime decays bi exponentially
800000
600000
400000
200000
0
with life times around 20 and 60 ls, indicating that two different
species are present in the solution, first one due to free uranyl
ion and other one due to hydroxide specie of uranyl. Above pH
4.0, only the hydroxide specie is predominant, showing a single
decay lifetime of about 60
ls. The disappearance of first life time
20 s at pH 4.0 and above is due to the fact that hydroxide forma-
l
tion takes place at higher pH and there may be negligible amount
of free uranyl ion to be present. Interestingly, from pH 3.0 to 5.0, no
changes in the peak positions of the luminescence spectra as well
as in second decay time which is around 60 ls observed, indicating
one type of hydroxide specie of uranyl dominates in acetonitrile
medium.
3.0
3.5
4.0
4.5
5.0
5.5
6.0
pH
Uranyl–BA luminescence and decay profile in acetonitrile medium
Fig. 3. Variation of luminescence intensity of uranyl–BA complex in acetonitrile
with pH of the uranyl solution; [UO22+] = 8 Â 10À7 M, [BA] = 1 Â 10À4 M.
Uranyl ion was complexed with benzoic acid and uranyl lumi-
nescence was measured as a function of benzoic acid concentration
and pH of the solution. Fig. 2 shows the variation of luminescence
intensity of uranyl–BA complex, measured at 481 nm as a function
of benzoic acid concentration at pH 4.5. The optimum acid concen-
tration was determined to be 1 Â 10À4 M and hence this concentra-
tion was used in all our subsequent experiments. Fig. 3 shows the
variation of luminescence intensity of uranyl–BA complex at
481 nm as a function of pH. For the pH range 3.5–5.0 the lumines-
cence intensity was almost constant for a given ligand concentra-
tion. Therefore in all our experiments pH of solutions was
maintained at 4.5. Fig. 4 compares the excitation spectra of uncom-
plexed uranyl ion and uranyl–BA complex in acetonitrile medium.
The excitation spectrum of the uncomplexed uranyl was recorded
by monitoring the luminescence at 499 nm, while the spectrum of
complexed uranyl was recorded by monitoring the luminescence
at 481 nm. The excitation spectra for uranyl–BA complex display
a maxima at 243 nm (Fig. 4a), which is different from uncomplexed
uranyl (250 nm), indicating the complexation of uranyl with ben-
zoate ion.
at 499, 520 and 545 nm. However, emission spectrum of uranyl–
BA complex (Fig. 5a), shows five well resolved sharp peaks at
462, 481, 501, 522 and 546 nm. Although the luminescence inten-
sities shown in Fig. 5a and b for the complexed and uncomplexed
uranyl ion are comparable, the concentrations of uranyl ion used to
record the spectra are different. Concentration of uranyl ion used
to record the spectrum in its complex in acetonitrile medium
was 8 Â 10À7 M. At this concentration, uncomplexed uranyl ion
showed no measurable luminescence, and hence to record the
spectrum of uncomplexed uranyl ion,
a concentration of
4 Â 10À5 M was used. For comparison, emission spectrum of ura-
nyl–BA in aqueous medium is also shown as dotted line (Fig. 5c),
the intensity of which is much lower than the above two spectra.
The enhancement in luminescence is thus obvious and clearly indi-
cates the role of benzoate in sensitizing the uranyl luminescence in
acetonitrile. Complexation with benzoate yields more than two
orders of enhancement in the luminescence of uranyl ion in aceto-
nitrile medium.
Fig. 5a and b shows the uranyl luminescence spectra with and
without benzoic acid in acetonitrile medium at pH 4.5. The emis-
sion spectrum of uncomplexed uranyl ion (Fig. 5b) shows bands
The calculated average spacing between the spectral features in
the emission spectrum of uranyl–BA complex in acetonitrile is
832 cmÀ1, which is close to the value of 855 cmÀ1 reported for
vibrational spacing in the ground electronic state of free uranyl
ion [31]. While the spacing between the vibronic features in our
spectrum (Fig. 5a), agrees with that of the reported value for other
uranyl complexes in aqueous [22–23], each of the features is blue-
shifted by 8 nm in acetonitrile medium, compared with that
observed for free uranyl ion (Fig. 1, pH 2.4). We believe that
blue-shift in the case of acetonitrile medium stabilizes the ground
state more than the excited state without perturbing the shape of
the ground state potential, as indicated by the constancy in the vib-
ronic spacing. Similar blue shifting has been observed in the lumi-
nescence spectra of uranyl–trimesic acid complex in aqueous
medium [23].
550000
500000
450000
400000
350000
300000
250000
The uranyl–BA system shows a single exponential decay with a
lifetime of 68
specie. Complexation of BA to uranyl ion increases its lifetime from
20 to 68 s in acetonitrile medium. These life time results suggest
ls, which indicates presence of single uranyl–BA
l
that upon complexation with benzoate ion, water molecules are
displaced from the first coordination sphere of the central uranyl
ion and hence the non-radiative decay, induced by collisions with
water molecules are suppressed further. Similar enhancements in
lifetime were observed in earlier studies on the luminescence of
uranyl complexed with 2-6-pyridinedicarboxylic acid (PDA) [22],
0.0
1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4
[BA] in M
Fig. 2. Variation of luminescence intensity of uranyl–BA complex in acetonitrile
with the concentration of BA; pH 4.5, [UO22+] = 8 Â 10À7 M.

S. Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
513
700000
[UO22+] = 8x10-7
M
a
600000
500000
400000
300000
200000
100000
0
pH 4.5
[BA] = 1x10-4 M
λems = 481 nm
b
a
[UO22+] = 4x10-5 M
λems = 499 nm
b
220
240
260
280
300
320
340
Wavelength (nm)
Fig. 4. Excitaion spectra of uranyl in acetonitrile with (a) and without (b) benzoic acid.
[UO22+] - 8x10-7
M
[UO22+] - 4x10-4 M (in water)
600000
400000
200000
With BA
a
c
0
1000000
Without BA
[UO22+] - 4x10-5 M
500000
b
0
420 440 460 480 500 520 540 560 580 600
wavelength (nm)
Fig. 5. Emission spectra of uranyl in aceonitrile with (a) and without (b) benzoic acid (1 Â 10À4 M) at pH 4.5. Uranyl–BA spectrum in water (c) at pH 4.5 (dashed line).
trimesic acid (TMA) [23], malonic acid [35], glycine [36] in aqueous
medium.
tonitrile, we have studied the effect of water on uranyl–BA lumi-
nescence in acetonitrile medium. For these studies, steady state
and lifetime experiments were performed after deliberate addition
of water to the uranyl–BA in acetonitrile. It must be noted that in
all the experiments in acetonitrile medium, 1% water was initially
present to begin with. Fig. 6 illustrates the variation of lumines-
cence intensity of the uranyl–BA in acetonitrile with addition of
water at pH 4.5. It is clear from the figure that luminescence inten-
sity decreases with the increase in water content in acetonitrile
medium. Both, luminescence intensity and life time decreases
exponentially with increase in water content in acetonitrile
(Fig. 7a and b). This decrease in luminescence intensity as well as
life time is due to the removal of acetonitrile by water molecules
from the bulk of the complex, as water forms stronger complex
compared to acetonitrile. From this it can also be concluded that
even though luminescence intensity of the complex decreases, its
structure (peak positions) remains intact up to 20% of water rela-
tive to acetonitrile. Above 20% of water, the sharp uranyl lumines-
cence features become broader [Fig. 6 in inset]. Therefore it can be
Recently Lutke et al. has reported that uranyl forms 1:1 and 1:2
complexes in aqueous medium with benzoic acid [29] and they
found that after complexation with benzoic acid, luminescence
intensity of uranyl ion decreased with benzoic acid concentration.
Furthermore, it was deduced that uranyl benzoate specie shows no
luminescence. In contrary to aqueous medium, uranyl benzoate
specie formed in acetonitrile medium exhibited strong lumines-
cence through intramolecular energy transfer from benzoate moi-
ety. These observations clearly indicate that luminescence specie
of uranyl benzoate formed in acetonitrile is different from that is
formed in aqueous medium. The structure of the uranyl–BA specie
and the luminescence mechanism will be discussed later.
Effect of water on luminescence of uranyl–BA in acetonitrile
Since benzoic acid did not enhance the luminescence of uranyl
in aqueous medium, to examine the tolerance limit of water in ace-

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S. Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
800000
700000
600000
500000
400000
300000
200000
100000
0
spectrum is broad and no sensitized luminescence of uranyl
observed, a situation similar to the one seen in pure aqueous
medium.
30000
20000
10000
0
a
i
Structure of the complex
UV–Vis spectroscopy was used to see the possibility of charac-
terizing the uranyl–BA specie formed in the acetonitrile medium
which is responsible for luminescence. Fig. 8a shows the UV–Vis
spectrum of uranyl in acetonitrile. The spectrum exhibited three
absorption bands which resembles the absorption of hydrated ura-
nyl specie [37]. This suggests that being the less coordinating sol-
vent acetonitrile cannot replace the water, which is present in
acetonitrile in trace level, from the inner coordination sphere of
uranyl. The UV–Vis spectra of solutions containing uranyl with dif-
ferent benzoate concentration in acetonitrile are also shown in
Fig. 8b–d. In acetonitrile medium we observed an increase in the
baseline of these spectra and we believe that it could be caused
by particles which are not visible to naked eye. The particle forma-
tion could be due to limited solubility of complex in acetonitrile
medium. When uranyl to ligand ratio is 1:3 (Fig. 8c), UV–Vis spec-
tra displayed three intense sharp peaks at 431, 443 and 461 nm.
Similar sharp peaks were first observed in the spectrum of CsUO3
(NO3)3, reported by Dieke and Duncan and were named as ‘mag-
netic series’ [38]. The absorption bands at this region were also
reported in literature for uranyl tris nitrato, uranyl tris acetato
complexes in ionic liquid medium [26]. The observation of these
peaks in our experiment, which is typical for uranyl complexes
with D3h symmetry [26], is a clear indication of formation of tris
complex of uranyl. Hence we believe that the specie formed in ace-
tonitrile medium could be that of [UO2(C6H5COO)3]À. It is noted
that in aqueous medium two benzoate ligands are coordinated to
uranyl ion [29]. A similar observation has been found [25] when
uranyl betaine complex dissolved in water showed spectrum
similar to hydrated uranyl indicating betaine ligands are no longer
440
480
520
560
600
420 440 460 480 500 520 540 560 580 600
wavelength (nm)
Fig. 6. Luminescence spectra of uranyl–BA complex in acetonitrile as a function of
water (a) 1%; (b) 2%; (c) 4%; (d) 6%; (e) 8%; (f) 10%; (g) 15%; (h) 20%; (i) 25% content
relative to acetonitrile. [UO22+] – 8 Â 10À7 M, [BA] – 1 Â 10À4 M. For clarity the
spectra with 25% water is again shown in inset.
600000
500000
a
400000
300000
200000
100000
0
0.5
0
5
10
15
20
Water content in acetonitrile (%)
d-1:4
0.4
70
60
50
40
30
20
10
c-1:3
b
d
b-1:2
0.3
c
b
0.2
a
0.1
0
5
10
15
20
Water content in acetonitrile (%)
350
400
450
500
550
600
Fig. 7. Plots showing the uranyl–BA (a) intensity and (b) life time as a function of
water content relative to acetonitrile; [UO22+] – 8 Â 10À7 M, [BA] – 1 Â 10À4 M.
Wavelength (nm)
Fig. 8. UV–Vis absorption spectra of (a) uranyl (1 Â 10À2 M) and with different
uranyl (2 Â 10À3 M) to benzoate ratio (b) 1:2; (c) 1:3; (d) 1:4 in acetonitrile. The
three peaks shown by arrow are the characteristic of uranyl tris complex. The plots
c and d are offset to Y axis by 0.05 unit for clarity.
concluded that above 20% of water in acetonitrile, luminescence
from hydroxide specie of uranyl dominates and hence the

S. Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
515
while the uranyl emitting level energy is about 22,000 cmÀ1 [40],
one can expect intramolecular energy transfer from the triplet
level of benzoic acid to the excited electronic level of uranyl ion
to take place. Therefore like lanthanides [11], uranyl is also
expected to show enhanced luminescence in aqueous medium
but this is not the case. In fact luminescence intensity is sup-
pressed after the addition of benzoic acid in aqueous uranyl solu-
tion [29]. Although benzoic acid might be transferring energy
from its triplet level to uranyl ion, it appears that excited uranyl
ions are decaying non-radiatively and water molecules which are
present in the coordination sphere of uranyl as well as in bulk
are acting as sink in accepting this excess energy by electron trans-
fer mechanism and thus results in ineffective ligand sensitized
energy transfer processes. Unlike lanthanide luminescence where
coordinating water molecules are only quenchers, in the case of
uranyl luminescence, the bulk water molecules in the secondary
sphere also act as major quencher. Therefore when solvent is chan-
ged from water to acetonitrile, there is reduction of non-radiative
decay channels and hence luminescence lifetime is increased from
e
a
420 440 460 480 500 520 540 560 580 600
Wavelength (nm)
Fig. 9. Luminescence spectra of uranyl (4 Â 10À5 M) with different uranyl to
benzoate ratio in acetonitrile (a) 1:1; (b) 1:2; (c) 1:3; (d) 1:4; (e) 1:5. kex = 243 nm.
1.5 to 20 ls. It should be noted that in the case of lanthanides,
quenching mechanism involves the energy transfer to higher
energy vibrational level of water molecules and thereby replacing
one or more water molecule helped in reduction of non-radiative
decay. Therefore it appears that electron transfer quenching mech-
anism is more detrimental than energy transfer quenching and
therefore just by removing one or two water molecules from the
inner coordination sphere of uranyl as it does in uranyl–BA com-
plexes in aqueous medium, ligand sensitized luminescence could
not be achieved. Thus replacing bulk water molecules by acetoni-
trile and removal of inner coordinated water molecules by three
benzoate molecules in a bidentate fashion helped in reducing the
non-radiative path ways of excited [UO2(C6H5COO)3]À and finally
makes it to be luminescent. These results also corroborate that in
ligand sensitized luminescence of uranyl, even though energy
transfer is the key factor, decay of excited states, which generally
depends on solvent molecules, decides the fate of molecule/specie
whether to be luminescent or not, eventually.
coordinated to uranyl ion, and the same complex exhibited sharp,
intense peaks as carboxylate groups of three betaine ligands
remain co-ordinated after dissolution in acetonitrile.
Luminescence spectra recorded at different uranyl to benzoate
ratio also supports the formation of this specie. Fig. 9a–e shows
the luminescence spectra of uranyl–BA at different uranyl to ben-
zoate ratio (1:1–1:5) in acetonitrile medium. The concentration of
uranyl was 4 Â 10À5 M and concentration of benzoate was varied
from 4 Â 10À5 M to 2 Â 10À4 M. It can be seen from Fig. 9c that
sharp features of uranyl–BA luminescence originate when the ura-
nyl to benzoate ratio reaches 1:3. This observation is found to be
consistent with other concentrations of uranyl (2 Â 10À5 M,
2 Â 10À4 M and 2 Â 10À3 M) also. For uranyl to ligand ratio of 1:4
or above, no change in luminescence intensity was observed, thus
concentration of [UO2(C6H5COO)3]À reached maximum when ura-
nyl to ligand ratio is 1:4. Nockmen et al. [26] also prepared tris nit-
rato compound of uranyl by mixing 1:4 ratio of uranyl to nitrate
and they also observed that above 1:4 there was no further change
in complex structure. The schematic structure of the complex
[UO2(C6H5COO)3]À is shown in Fig. 10.
Uranyl–BA luminescence in other solvents
Apart from acetonitrile we have also studied the luminescence
of uranyl ion in other medium such as dimethyl sulphoxide, diox-
ane, tetrahydrofuran, N–N dimethyl sulphoxide, methanol, etha-
nol, cyclohexane and ether. In none of the above solvents we
could get any signature of ligand sensitized luminescence of ura-
nyl. In cyclohexane and ether the complex was not soluble. Other
solvents are strong coordinating solvents and competition of sol-
vent molecules with benzoate towards uranyl ion might be the rea-
son for the absence of enhanced luminescence. Thus being polar
and less coordinating solvent, acetonitrile seems to be an ideal
choice for studying the luminescence of uranyl–BA system.
Mechanism of ligand sensitization in acetonitrile
Since uranyl forms complex with benzoic acid in aqueous med-
ium and the triplet energy of benzoic acid is 25,641 cmÀ1 [39],
O
O
O
Analytical application of this study
O
O
O
U
This present study of uranyl luminescence in acetonitrile med-
ium finds an analytical application for uranyl detection. The large
enhancement of uranyl luminescence of uranyl–BA complex in
acetonitrile medium can be used for trace level detection of uranyl
ion. Linearity in the luminescence intensity is seen over the uranyl
concentration range of 6.7 Â 10À8 to 6.7 Â 10À6 M and the detec-
O
O
tion limit calculated using the criterion of 3
r
is ꢁ4.2 Â 10À9 M.
The detection limit obtained here is only for pure uranium. How-
ever in order to apply this method to environmental samples, lumi-
nescence quenching studies of uranyl in presence of other foreign
ions have to be carried out.
Fig. 10. Schematic diagram for the [UO2(C6H5COO)3]À structure having D3h
symmetry. Benzoate moieties are in the plane of the paper.

516
S. Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 509–516
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Ligand sensitized luminescence has been demonstrated for ura-
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the absence of BA, only one type of uranyl hydroxide i.e. UO2OH+
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spectrum and lifetime. When BA is present, formation of [UO2(C6-
H5COO)3]À has been reported for the first time for uranyl, which
shows a well resolved and enhanced emission spectrum in acetoni-
trile medium. Electron transfer quenching which caused the
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