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Transcript of Kraus Lab 10
1
The Determination of the Rate Constant and Reaction Mechanism
for Ru(bpy)32+ Luminescence Quenching
Author: Megan Kraus
Lab Partners: Megan Moyer, Marley Pillion, Stefan Kozak
Submitted: March 22nd, 2012 (CHEM 457, Section 4)
Abstract
In this experiment, the basic principles of luminescence spectroscopy and quenching
were understood through the calculation of the rate constant of luminescence quenching of
ruthenium (II) trisbipyridine, also known as Ru(bpy)32+ by oxygen. Three Ru(bpy)3
2+ samples of
varying oxygen concentration (air-saturated, nitrogen –saturated, and oxygen-saturated) were
used in this experiment. The laser photolysis of the nitrogen-saturated sample was used to find
the fluorescence rate plus the radiationless rate constant, (kf+knr), which was calculated to be
1.557 ×1 06 ± 2525.919. The quenching rate constant was determined by plotting the concentrations of
oxygen with the ratio of the integration areas from emission data of each Ru(bpy)32+ sample and
applying the Stern-Volmer equation to find the value of the quenching rate constant, kq, divided
by the (kf+knr). The (kf+knr) was previously determined yielding a kq value of
2.971 ×1 09 ±2.18 × 107 .The collision rate constant, kd, of water was also calculated and found to
be 6.483 ×1 09 Lmol s
.
2
Introduction
The principles of luminescence spectroscopy are mainly due to light induced chemical
changes in photochemical reactions. The chemical change is due to the excitation of a
molecule’s electrons to a higher energy level from the molecules ground state. The ground state
of a molecule exists due to all the electrons residing in their lowest energy level and most stable
orbital. When the electrons are excited, they return to the more stable ground state in various
ways including luminescence, where light is emitted.
Ruthenium (II) trisbipyridine (Ru(bpy)32+), was the molecule used in this experiment.
The Ru(bpy)32+’s electrons absorbed UV light and became excited to a high energy state. The
excited electrons then lost their energy and returned to ground state via radiation or radiationless
mechanisms.1
Two radiation mechanisms are fluorescence and phosphorescence. These mechanisms
occur due to the quantum level interactions of the electrons in the molecule. Electrons in the
ground state are opposite spin paired (Pauli Exclusion Principle) and when one becomes excited
it can stay opposite and return to ground state (fluorescence) or flip and become parallel then go
through a transition excited state before returning to ground state (phosphorescence) shown in
the equations below for Ru(bpy)32+.1
Ru ¿¿ Fluorescence
Ru ¿¿ Phosphorescence
Where the hυ emitted is not the same hυ that was used to excite the electrons and is usually the a
longer wavelength.2
3
Radiationless or non-radiative mechanisms are where an electron in a molecule returns to
ground state without emitting a photon. One way in which this is done is through quenching.
The mechanism of quenching deactivates the excited state by reacting it with a quencher
molecule.3 In this experiment, Ru(bpy)32+ is quenched by oxygen.
Radiation and radiationless mechanisms have rate constants, k. The quantum yield, ϕ, is
the ratio of these rate constants where ϕ equals the rate of formation of the product divided by
the sum of all the rates.
In this experiment, the rate constants were determined by first finding the rate constants
of radiative decay, kf+knr, where the excited state decays in the absence of a quencher shown in
Eq. 1.
d ¿¿ (1)
The solution to Eq. 1 is shown in Eq. 2 below:
ln ¿¿ (2)
Using Eq. 2, a plot of the natural log of fluorescence intensity versus time will yield a graph with
a slope of –(kf + knr).
After the rate constant, (kf + knr) is found, the quenching rate constant can then be
determined. The quenching rate is proportional to the concentration of the oxygen present in the
sample, shown in Eq. 3 below.
d ¿¿ (3)
4
The quenching rate constant can be determined using quantum yields of Ru(bpy)3+2 in
the absence of a quencher as well as in the presence of a quencher. The quantum yield of
Ru(bpy)3+2 in the absence of a quencher is shown below in Eq. 4 and in the presence of a
quencher shown in Eq. 5 below.
ϕo=k f ¿¿ (4)
ϕ=k f ¿¿ (5)
Using these quantum yields, the Stern-Volmer equation can be used to find a relationship
between the kq and (kf+knr) by finding the ratio of Eq. 4 and Eq. 5 shown in Eq. 6.
ϕo
ϕ=1+
kq
k f +knr
[O 2] (6)
Eq. 6 will be used to calculate the quenching rate by utilizing fluorescence spectroscopy
data and concentrations of oxygen in the samples.
The collision rate constant will also be found for Ru(bpy)3+2 by using Eq. 7.
k d=8 RT3 η
(7)
where R is the universal gas constant, T is the temperature and η is the viscosity.
With these basic concepts and equations above, the basic principles of luminescence and
quenching will be investigated as well as the rate constants determined.
5
Experimental
This procedure was taken from chapter 10 of the CHEM 457 Experimental Physical
Chemistry, Spring 2012 Lab Packet.1
An absorption spectrum was taken for a sample of air saturated Ru(bpy)32+ using a Cary
4000 Varian UV-Vis spectrophotometer to find the excitation wavelength of the molecule.
Before the Ru(bpy)32+ was analyzed, an air blank was analyzed first. The scan for the cuvette
filled with the air saturated Ru(bpy)32+ sample was taken between the wavelengths 250 and 650
nm to yield an excitation wavelength of 450 nm. This excitation wavelength was used for the
emission spectra on the Horiba Scientific Fluorolog Spectrofluorimeter. The emission analysis
was conducted on air saturated, nitrogen saturated and oxygen saturated Ru(bpy)32+ samples
within the range of 500-850 nm. A graph of emission versus wavelength was found for each
sample and used to find integrate the area under the curve for data analysis.
The nitrogen saturated Ru(bpy)32+ sample was made by bubbling nitrogen through the air
saturated cuvette sample for 5 minutes to rid the sample of oxygen. The oxygen saturated
sample was made the same way except oxygen was bubbled through instead of nitrogen.
Additional analysis was performed on the nitrogen sample with a photolysis apparatus
(SRS NL-100). The sample was struck with UV light at a wavelength of 337.1 nm and the
fluorescence intensity of Ru(bpy)32+ was collected and sent to a Teletronix TDS 2022B Two-
Channel Digital Storage Oscilloscope which converted the data to a graph of intensity versus
time.
6
Results
The analysis from the air saturated Ru(bpy)32+ sample from the UV-Vis shows the
excitation wavelength at 450 nm. This wavelength was then used for the emission spectra. The
graph of the absorbance versus wavelength is shown in Fig. 1.
350 400 450 500 550 600 650 700-0.0999999999999991
9.15933995315754E-16
0.100000000000001
0.200000000000001
0.300000000000001
0.400000000000001
0.500000000000001
0.600000000000001
Absorbance vs. Wavelength
Wavelength (nm)
Abso
rban
ce
Fig. 1. The Plot of Absorbance vs. Wavelength to Find an Excitation Wavelength of 450 nm
After the excitation wavelength was found, the emission spectrum of the Ru(bpy)3+2
samples (air saturated, nitrogen saturated and oxygen saturated) were taken. The areas under the
curve of the emission vs wavelength for each sample of varying concentrations of oxygen were
then found using the integration tool. In order to apply the Stern-Volmer equation, a ratio of the
quantum yields is needed, therefore the integration areas were made into ratios with respect to a
standard area, Areao. Because the nitrogen saturated sample doesn’t have any oxygen present, its
integration area was the Areao. This area is representative of the total amount of radiative
emission because in the absence of oxygen, there are no excited molecules relaxing due to
quenching. The concentration of oxygen was also needed for the Stern-Volmer Equation and
7
calculated using Henry’s Law. Fig. 2 illustrates a table that shows the area, area ratio and
concentration of O2.
Sample [O2] (M) Area ratio AreaAir Saturated 2.68E-04 1.5297 6.01E+07
Nitrogen Saturated 0 1 9.19E+07Oxygen Saturated 0.00129 3.4678 2.65E+07
Fig. 2. Table of Integration Areas from Emission Curves, Ratio of areas used in Stern-
Volmer Equation and Concentration of Oxygen
Using the Stern-Volmer equation, a plot of (Areao/Area) versus the concentration of
oxygen yielded a straight line with a slope equal tokq
k f +knr =1908.41 ±13.66911 shown in Fig. 3.
0.00E+00 2.00E-04 4.00E-04 6.00E-04 8.00E-04 1.00E-03 1.20E-03 1.40E-030
0.51
1.52
2.53
3.54
f(x) = 1908.41049227308 x + 1.00806548434618R² = 0.999948700338734
Stern-Volmer Plot
Concentration of Oxygen (M)
area
o/ar
ea
Fig. 3. Stern-Volmer Plot of (Areao/Area) versus the concentration of oxygen
In order to find the quenching rate from the value obtained from the Stern-Volmer Plot,
the combined rate of radiative and nonradiative decay was needed to be found. Ru(bpy)3+2,
8
without the presence of a quencher (oxygen), decayed through radiative and nonradiative
mechanisms. The nitrogen saturated sample was used to find these combined rate constants
because there was no oxygen present. The plot of the natural log of intensity versus time with a
slope equal to -(kf + knr) is shown in Fig. 4.
0 0.0000005 0.000001 0.0000015 0.000002
-5-4.5
-4-3.5
-3-2.5
-2-1.5
-1-0.5
0
f(x) = − 1557349.97791148 x − 1.48792909138316R² = 0.995341839084038
ln (Intensity) vs. Time
Time (s)
Ln (I
nten
sity)
Fig. 4. Plot of the Natural Log of Intensity Versus Time in the Absence of a Quencher
(Nitrogen Sample)
With the (kf + knr) =1.557 ×1 06 ± 2525.919 now known, the quenching rate constant, kq,
was then calculated to be 2.971 ×1 09 ±2.18 × 107. The rate of collision was also calculated to be
6.483 ×1 09 Lmol s
using Eq. 7.
Discussion
Although I could not find any literature values for the rate constants, I feel that the results
obtained from this experiment seemed reasonable in relation to one another. The results show
that the quenching rate is much larger than the combined rate constants of radiative and
9
nonradiative decay. This comparison confirms that oxygen is a very effective quencher of
Ru(bpy)3+2 and therefore when oxygen is present in a Ru(bpy)3
+2 solution, it decays the excited
state molecules much faster. This also explains why the area below the curve for oxygen
saturated Ru(bpy)3+2 decreased greatly compared to the other samples.
The significance of the collision rate constant and quenching rate constant is further
explained in report question 1 in the appendix of this report.
Some sources of error that could have occurred during this lab was during the nitrogen
purge. The experiment advised that nitrogen was to bubble through the sample for 5 minutes.
This is assuming that five minutes is an adequate amount of time to completely rid the sample of
oxygen.
One way the lab could have been improved is with the nitrogen and oxygen purging. The
method used to purge the samples left much room for oxygen contamination from the
surrounding air. A way to improve this would be to use a septum to hold the sample and
puncture it with a vent needle. With this method, there is less exposure to the air because you
are not required to remove a cap to remove the vent needle.
Overall I felt that the lab was successful in confirming the chemistry and concepts behind
luminescence spectroscopy.
Conclusion
The Ru(bpy)3+2 solutions saturated with air, oxygen and nitrogen with varying oxygen
concentrations of 0.00027 M, 0.00129 M, and 0.0 M respectively were used to determine the rate
constants. With the nitrogen saturated sample, which was void of oxygen, the quencher, the sum
10
of the radiative and non radiative rate constants was found to be 1.557 ×1 06 ± 2525.919. The
fluorescence spectrometry was used to calculate the quantum yield for each of the samples by utilizing
the integration area under the curves of the emission scan data. The nitrogen purged sample was used for
the standard reference area in the Stern-Volmer plot which was a graph of (Areao/Area) versus
oxygen concentration. The slope of the linear fit was 1908.41 ±13.66911 which is also equal to
kq
(k f +knr ). From this, the quenching rate was determined to be 2.971 ×1 09 ±2.18 × 107. The
collision rate constant was also determined and calculated to be 6.483 ×1 09 Lmol s
. From this
collision rate constant, it was found that the average collision time was shorter than the average
quenching time of oxygen from the excited state of Ru(bpy)3+2 confirming that there was
successful quenching.
Acknowledgements
The author would like to acknowledge my lab partners, Marley Pillion, Stefan Kozak and
Megan Moyer, for their combined help in performing the procedure, Dr. Milosavljevic and the
TA’s, Ms. Shivangi Nangia and Mr. Stuart Friesen for their help in understanding the concepts
behind the experiment as well as the Pennsylvania State University for allowing us to use the
laboratory equipment.
11
References
1. Milosavljevic, B.H. Lab Packet for CHEM 457: Experimental Physical Chemistry,
Chapter 10. University Press: University Park, 2012.
2. Holler, F.J., Skoog, D.A., Crouch, S.R. Principles Of Instrumental Analysis. 2006
3. O’Reilly, J.E. Fluorescence Experiments With Quinine. Journal of Chemical Education.
52 (9). 1975
4. Birks, J.B. Excimers. Reports on Progress in Physics. Vol 38. 1975
5. Atkins, P., Paula, J., Friedman, R. Quanta, Matter and Change: A Molecular Approach to
Physical Change. W. H. Freeman and Company, New York. 2009
Appendix
Report Questions:
1. The calculated collision rate, kd, using Eq. 7, was found because of its
connection to the quenching rate constant. The reciprocal of kq can also be
known as the average time it takes for an excited molecule of Ru(bpy)3+2 to be
quenched by oxygen. With this being said, the reciprocal of kq is equal to
3.366 ×1 0−10 . The reciprocal of kd is equal to 1.54 × 10−10 which is the amount of
time it takes for two reactants in the solution of water to collide with each other.
From the two reciprocals, it is easy to see that the time of the collision between
Ru(bpy)3+2 and O2 is shorter than the time it takes for quenching collision to
occur.
2. One major source of error in the calculation of the quenching rate constant
was the atmospheric pressure reading used in Henry’s law to find the
concentration of oxygen. The atmospheric pressure was found using the
12
barometer in the organics lab that is an entire floor below where the samples
were being analyzed.
3. There are three common mechanisms for biomolecular quenching of an
excited state molecule. The Forster resonance energy transfer uses a
mechanism of dipole-dipole coupling of the excited state molecule and the
quenching molecule. The Exciplex mechanism is where the eximer is
quenched when the excited state and quencher form a dimer molecule.4 The
third mechanism is the Dexter electron transfer mechanism where the overlap
of the excited state molecule and donor orbital creates a shorter distance for
quenching to occur.5
Sample Calculations with Uncertainty:
A. [O2] (Henry’s Law)
Atmospheric Pressure = 745.5 mmHg=9.939 ×104 Pa
Percent O2 in the air = 20.95 %
Partial Pressure of O2 in atmosphere = (0.2095) (9.939 ×104 Pa) =2.082 ×104 Pa
Henry’s constant for O2: K H=4.259 ×104 atm=4.32 ×109 Pa
Air Sample:
K H=PO2
xO 2
xO2=
PO2
K H
=2.082×104 Pa4.32×109 Pa
=4.82×10−6 molO2
[O2 ]=(x¿¿O2)× ρH 2 O
M W H 2 O (1−xO2)¿
13
[O2 ]=(4.82 ×1 0−6 molO2)× 1
kgL
0.018kg
mol H 2O( 1−4.82×10−6 ) mol H 2O
=2.68 ×1 0−4 M ( molO2
L )
Oxygen Sample:
[O2 ]=[O 2 ]air
0.209 5=2.68× 10−4 M
0.2095=0. 00129 M
B. Calculation of Quenching Rate Constant, kq with uncertaintykq
(k f +knr )=1908.41± 13.66911
(k f +knr )=1.557 × 1 06± 2525.919
k q=1908.41 × ( k f +knr )=1908.41 ×(1.557 ×1 06)=2.971 ×109
C. Uncertainty:
∆ kq=kq ×√ (∆k q
(k f +knr ) )2
( k q
(k f +k nr ) )2 +
∆ (k f +knr )2
( k f +knr )2 =2.971 ×1 09×√ (13.67 )2
(1908.4 )2+
(2525.92 )2
(1.557 × 1 06 )2=2.18×1 07
k q=2.971 ×1 09 ±2.18 × 107
D. Kd Calculation
k d=8 RT3 η
=8(8.314
Pa m3
mol K ) (293 K )(1000 L)
3 (0.0010020 Pa s )(1m3)=6.483 ×1 09 L
mol s
14
Regression Analysis:
A. Stern-Volmer Plot:
Regression StatisticsMultiple R 0.999974R Square 0.999949Adjusted R Square 0.999897Standard Error 0.013159Observations 3
ANOVA
df SS MS FSignifican
ce F
Regression 1 3.3754443.37544
419492.3
1 0.00456
Residual 1 0.0001730.00017
3Total 2 3.375617
Coefficien
tsStandard
Error t Stat P-valueLower 95%
Upper 95%
Lower 95.0%
Upper 95.0%
Intercept 1.008065 0.01039896.9491
10.00656
6 0.8759481.14018
30.87594
81.14018
3
X Variable 1 1908.41 13.66911139.614
8 0.00456 1734.7282082.09
31734.72
82082.09
3
B. Natural Log of Intensity vs. time:
Regression StatisticsMultiple R 0.997668R Square 0.995342Adjusted R Square 0.995339Standard Error 0.054806Observations 1781
15
ANOVA
df SS MS FSignifican
ce F
Regression 1 1141.7811141.78
1380131.
4 0
Residual 1779 5.3434910.00300
4Total 1780 1147.125
Coefficien
tsStandard
Error t Stat P-valueLower 95%
Upper 95%
Lower 95.0%
Upper 95.0%
Intercept -1.48793 0.002638-
564.063 0 -1.4931
-1.4827
6 -1.4931
-1.4827
6
X Variable 1 -1557350 2525.919-
616.548 0 -1562304
-155239
6
-156230
4
-155239
6