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

.

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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.

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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

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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,

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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

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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

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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.

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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

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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

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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