Influence of temperature on the lifetime of electronically excited uranyl ion: I. Liquid and supercooled H2SO4 solutions

Article abstract of DOI:10.1023/B:RACH.0000015749.25592.0e

The influence of temperature (varied in the range 77-300 K) on the lifetime (τ) of electronically excited uranyl (UO₂2⁺)* in 0.2-5.0 M aqueous solutions of H₂SO₄ was studied. The activation parameters of the temperature quenching of the uranyl photoluminescence (PL) were determined. The temperature dependences of the rate constant of (UO₂²⁺)* PL decay (k = 1/τ) in 0.5 and 5 M H₂SO₄ obey the Arrhenius equation only at T > 265 K and T > 280 K, respectively. The deviations at low temperatures are probably due to both non-exponential temperature dependence of the viscosity of these solutions and a change in the chemical composition of the solvent (H ₂SO₄ dissociation), which affect the uranyl speciation and hence the luminescence parameters.

Full text of DOI:10.1023/B:RACH.0000015749.25592.0e

Radiochemistry, Vol. 45, No. 6, 2003, pp. 550 554. Translated from Radiokhimiya, Vol. 45, No. 6, 2003, pp. 494 498.
Original Russian Text Copyright 2003 by Lotnik, Khamidullina, Kazakov.
Influence of Temperature on the Lifetime of Electronically
Excited Uranyl Ion: I. Liquid and Supercooled H2SO4 Solutions
S. V. Lotnik*, L. A. Khamidullina**, and V. P. Kazakov**
* Bashkir State Agricultural University, Ufa, Bashkortostan, Russia
** Institute of Organic Chemistry, Ufa Research Center, Russian Academy of Sciences, Ufa,
Bashkortostan, Russia
Received November 21, 2002
Abstract The influence of temperature (varied in the range 77 300 K) on the lifetime ( ) of electronically
excited uranyl (UO22+)* in 0.2 5.0 M aqueous solutions of H2SO4 was studied. The activation parameters of
the temperature quenching of the uranyl photoluminescence (PL) were determined. The temperature depen-
dences of the rate constant of (UO22+)* PL decay (k = 1/ ) in 0.5 and 5 M H2SO4 obey the Arrhenius equation
only at T > 265 K and T > 280 K, respectively. The deviations at low temperatures are probably due to both
non-exponential temperature dependence of the viscosity of these solutions and a change in the chemical com-
position of the solvent (H2SO4 dissociation), which affect the uranyl speciation and hence the luminescence
parameters.
The photochemical and photophysical properties of
detected by reaction with benzene in the course of
uranyl photoreduction with water [11].
2+
electronically excited uranyl (UO )* in aqueous solu-
2
tions depend on may factors including pH, uranyl
concentration, the nature and concentration of com-
plexing anions present in the solution, wavelength and
intensity of the exciting light, and isotopic composi-
tion and temperature of the solvent. The excited states
of uranyl in the solid phase and in solutions show
strong temperature quenching, with the quantum ef-
ficiency of photoluminescence (PL) and the lifetime
of electronically excited states in solutions varying
in a range two orders of magnitude wider than in the
solid phase [1 3]. A strong influence of the solvent
In this work we studied the temperature depen-
2+
dence of of (UO )* in the range 77 300 K in liquid
2
supercooled and glassy H SO solutions, as we sug-
2
4
gested that quenching of uranyl PL involves both
photophysical and photochemical processes. We used
2+
the H SO solutions since the (UO )* behavior in
2
4
2
them was studied to a lesser extent than that in HClO
solutions. We found that the temperature dependences
of the rate constant of PL decay of (UO )* (k = 1/ )
obey the Arrhenius equation only at relatively high
temperatures.
4
2+
2
2+
(H SO ) viscosity on the PL of (UO )* was deter-
2
4
2
mined for the first time by Vavilov and Levshin [1, 2].
They attributed the temperature quenching to a trans-
fer of the excitation energy of uranyl to the vibration
levels of the solvent molecules. Later, chemical reac-
tions of electron transfer and/or proton elimination
from a water molecule to form intermediate U(V)
compounds were suggested, along with physical proc-
esses, to account for quenching of uranyl PL [4 9].
Since this system was complex, discussion on the
EXPERIMENTAL
Sulfuric acid and water were distilled until the ab-
sorption at > 200 nm in a 5-cm cell disappeared.
3
1
The UO SO solutions (10 10 M) were prepared
2
4
by dissolution in H SO of UO prepared from UO
2
4
3
2
(NO ) 6H O of analytically pure grade [12]. The
3 2
2
2+
lifetime of (UO )* was measured on a -metric
2
unit consisting of an IGL-300/2 nitrogen laser (
=
exc
2+
8
mechanism of (UO )* deactivation was continued.
337.1 nm, pulse time 10 s); a -measuring block
giving digital value of time in which the luminescence
decreases by a factor of e after a single pulse of photo-
excitation (the error of a single measurement of
is less than 5%); and a cryostat maintaining sample
temperature in the range 77 300 K with an accuracy
of 1 K. A solution to be studied (0.2 0.5 ml) was
placed in a quartz test tube 5 mm in diameter, the
measuring junction of a thermocouple in a glass capil-
2
In particular, the first deactivation step by the reaction
(UO22+)* + H2O
UO2H+2 + OH,
postulated in [4], was not directly confirmed. Forma-
tion of U(V) and accumulation of U(IV) and H O in
2
2
large amounts during photolysis of sulfuric acid
solutions of uranyl were confirmed by the chemi-
luminescence procedure [10]. No OH radicals were
1066-3622/03/4506-0550$25.00 2003 MAIK Nauka/Interperiodica

INFLUENCE OF TEMPERATURE ON THE LIFETIME... : I.
lary was inserted in the solution, and the tube was
551
fixed in a quartz Dewar vessel purged with a vapor of
boiling nitrogen. Uranyl PL passed through crossed
light filters was registered at a right angle to the exci-
tation pulse with an FEU-119 detector.
The average of 25 30 measurements of , per-
formed within 5 6 s at a 5-Hz frequency of excitation
pulses, was calculated. So fast measurements allowed
determination of the temperature dependence of on
1
heating or cooling at a rate from 2 to 5 K min with-
out stabilization of the sample temperature, since it
changed only slightly during the measurement.
RESULTS AND DISCUSSION
2+
Fig. 1. (1) Lifetime of (UO )* and (2) rate constant of PL
2
It is believed [5, 8] that, in acidic aqueous solutions
(pH 1), the intensity of uranyl chemiluminescence
excited with a single pulse decreases exponentially
2+
3
decay of (UO )* in 10 M solution of UO SO in 0.5 M
H SO on cooling from 300 K as functions of temperature.
2
2
4
2
exc
4
= 337.1 nm.
and the temperature dependence of the rate constant of
1
PL decay k =
obeys the Arrhenius equation. In
1
0.2 M HClO , the lnk vs T dependence is linear
4
from the freezing point to the boiling point of the
solution [7]. In this work we studied in more detail
than in [5, 7, 8] the uranyl PL at temperatures below
the freezing point of uranyl solutions.
3
2+
When 10 M UO solutions in 0.2, 0.5, and
2
1.0 M H SO are cooled from 300 K to the tempera-
2
4
tures lower than their equilibrium solidification points,
monotonically increases until the supercooled liquid
spontaneously crystallizes. A typical temperature
2+
dependence of the lifetime of (UO )* is shown in
2
2+
Fig. 2. Temperature dependence of the lifetime of (UO )*
2
Fig. 1 (curve 1). It should be noted that this depen-
dence does not change when a 0.5 M H SO solution
becomes supercooled (the supercooled region is
marked by vertical dashed lines). The semilog plot of
k vs. T (Fig. 1, curve 2) is linear in the range 300
( ) in (1) 0.2, (2) 1.0, (3) 3.0, and (4) 5.0 M H SO , meas-
2
4
2
4
2+
2
ured on cooling from 300 K. [UO ] = 10 M.
2
which are lower by 10 15 K than those for 0.2
0.5 M H SO solutions. The temperature dependence
of is shown in Fig. 2. The Arrhenius plots of the
rate constant in 0.2, 1, 3, and 5 M H SO are shown
1
2
4
265 K. Minor deviation from the Arrhenius plot is
observed only at lower temperatures. On cooling
below 260 K, the solution supercooled by 10 K crys-
tallizes. As a result, 0.5 M H SO solution sharply
2
4
in Fig. 3 (curves 1 4, respectively).
2
4
The activation energies (E ) of temperature quench-
ing of uranyl PL are presented in the table. The pa-
rameters E and A were calculated by the least-squares
treatment of the linear sections of the lnk vs T
warms up to 270 K, and decreases from 110 to
57.5 s. The influence of phase transitions on the
a
2+
(UO )* deactivation will be considered in detail in
a
2
1
communication II.
dependence. The correlation coefficient given in the
right column shows how accurately the Arrhenius
equation reproduces the actual temperature depen-
dence of the rate constant of PL decay in the given
temperature range.
We failed to supercool relatively large volumes
3
1
2+
(V = 0.5 ml) of 10 10 M UO solutions in 0.2
2
1.0 M H SO below 257 K. Experiments at lower
2
4
temperatures were performed with 3 and 5 M H SO
2
4
solutions which can be superooled to 195 190 K.
We suggested that, in these cases, the temperature
dependence of k would also follow the Arrhenius law.
However, strong deviations from the Arrhenius plot
were observed at temperatures below 280 275 K,
The activation parameters of temperature quench-
2+
ing of (UO )* PL in 0.2 M H SO (E = 42.2
2
2
4
a
1
12
1
0.5 kJ mol , A = 2.8 10 s ) and 0.2 M HClO
4
1
12
1
(E = 41.2 0.7 kJ mol , A = 9 10
s
[5] and
a
RADIOCHEMISTRY Vol. 45 No. 6 2003

552
LOTNIK et al.
1
Activation energy (Ea) and preexponential factor (A, s ) of quenching of uranyl PL, determined on cooling UO2SO4
solutions in aqueous H2SO4 in the indicated temperature range
T
1
[UO2SO4], M
[H2SO4], M
Ea, kJ mol
logA
T, K
r
3
10
10
10
10
0.2
0.5
1.0
3.0
5.0
42.2 0.5
42.0 0.5
38.7 0.6
38.5 0.8
41.2 0.5
12.5 0.2
12.4 0.2
11.7 0.3
11.6 0.3
11.7 0.2
300 262
300 261
300 261
300 275
295 280
0.999
0.999
0.997
0.999
1.000
3
3
3
3
10
2
10
10
10
10
0.2
0.5
1.0
3.0
5.0
42.0 0.5
40.9 0.8
39.1 0.6
39.9 0.3
38.9 0.6
12.4 0.2
12.1 0.3
11.8 0.3
11.7 0.1
11.4 0.3
300 260
300 263
300 265
300 280
300 280
0.999
0.998
0.999
1.000
0.999
2
2
2
2
10
1
E
= 43.8 0.5 kJ mol [7]) are close. However,
0.2 to 1.0 M it slightly decreases. The activation
a
2+
when [HClO ] increases from 0.01 to 1.0 M [13], E
increases from 9.6 to 12.4 kcal mol (from 40.1 to
51.8 kJ mol ), whereas with increasing [H SO ] from
parameters do not change with increasing UO con-
4
a
2
1
3
2
centration from 10 to 10 M (see table).
1
2
4
Let us consider the temperature dependence of
2+
(UO )*, measured on cooling a solution in 5 M
2
4
1
H SO from 305 K at a rate of 2 3 K min (Fig. 4).
2
At 280 240 K, the lifetime of excited uranyl increases
almost linearly. The growth of decelerates at lower
temperatures. In the range from 190 to 200 K, the
decay of PL excited by a single pulse is not mono-
2+
exponential: two types of (UO )* with lifetimes
2
1
and
coexist at a given temperature (Fig. 4). The
2
first lifetime is registered in the first step of PL decay,
and , in a time sufficient for the species with to
2
1
have virtually no effect on the species with . Mov-
2
ing along the curve of uranyl PL decay is provided
by changing voltage on the photoelectron multiplier
1
Fig. 3. Arrhenius plot of the rate constant (k =
) of PL
(U
). If we want to find out whether the PL decay
PEM
2+
decay of (UO )*, measured on cooling of (1) 0.2, (2) 1.0,
(3) 3.0, and (4) 5.0 M H SO solutions. [UO ] = 10 M.
2
is monoexponential or not, i.e., single or several
2+
2
2
4
2
emitting species are present in the solution, U
should be changed by several hundreds of volts.
PEM
In aqueous solutions of sulfuric and perchloric
acids (pH
1), sulfate ligands in the coordination
sphere of uranyl are rapidly exchanged. As a result,
several sulfate complexes are formed within the life-
2+
time of (UO )*, and hence only averaged can be
2
measured [14]. When the solution is cooled and its
viscosity increases, the ligand exchange decelerates
and at definite temperature becomes comparable with
2+
the rate constant of PL decay of (UO )* (k = 1/ ). At
2
this temperature, the kinetics of PL decay ceases to be
monoexponential. Hence, the -metric procedure in
which uranyl is a luminescent probe can be used to
determine the diffusion mobility of molecules in
multicomponent frozen systems such as aqueous sul-
Fig. 4. Temperature dependence of of electronically ex-
2+
cited UO , measured on cooling of (1) 5.0 and (2) 3.0 M
2
H SO solutions. The sections of curves 1 marked with
2
4
1
and
(T < 195 K) were obtained, respectively, at 1600 and
2
1800 V voltage applied to an FEU-119 photoelectron
2+
3
multiplier. [UO ] = 10 M.
furic acid solutions. The difference
=
2
1
2
RADIOCHEMISTRY Vol. 45 No. 6 2003

INFLUENCE OF TEMPERATURE ON THE LIFETIME... : I.
553
remains virtually the same (Fig. 4, curve 1) at tem-
peratures below the glass transition point of 5 M
concentration from 0.05 to 5 M is almost the same.
This is likely due to H SO dissociation. Kanno [21]
2
4
H SO (T
160 K).
showed that even concentrated H SO is completely
2
4
g
2 4
ionized at temperatures below 100 C.
Let us consider why the temperature dependence
of the rate constant k does not obey the Arrhenius
law. First, Das et al. showed [15] that the tempera-
ture dependence of viscosity of aqueous H SO solu-
Thus, based on high sensitivity of uranyl PL to
various parameters of the environment, we showed
2+
that quenching of UO luminescence in liquid cooled
2
4
2
tions is better described by the power equation
(T/T 1) than by the Vogel Tammann Fulcher
= Aexp[B(T T )] [16] usually used to
=
and supercooled aqueous sulfuric acid solutions can-
not be attributed to a single activation process.
0
0
equation
describe the viscosity of glass-forming liquids. Since
parameters and are known only for solutions
with H SO concentration from 76.9 to 96.8%, we
0
ACKNOWLEDGMENTS
0
2
4
This work was financially supported by the Rus-
sian Foundation for Basic Research (project no.
02-03-32406) and by Leading Scientific Schools
(00-15-97323).
failed to correlate the temperature dependence of the
viscosity of the examined solutions and that of of
uranyl.
A study of dynamic quenching of luminescence of
excited uranyl with aliphatic alcohols, chloride anion,
and diethyl ether at low temperatures, when the vis-
cosity of the solution strongly changes, showed [17]
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2
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4
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2
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2
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2
2
4
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4
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2
4
RADIOCHEMISTRY Vol. 45 No. 6 2003

554
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RADIOCHEMISTRY Vol. 45 No. 6 2003