1) Researchers used an oscilloscope to measure the fluorescence intensity of pyrene over time when exposed to varying concentrations of potassium iodide (KI) as a quencher. 2) The natural log of intensity was plotted against time for each concentration, and the slope of the best-fit line gave the observed relaxation rate (kobs) of excited pyrene. 3) A plot of kobs versus KI concentration yielded a quenching rate (kq) of 6.0×10^4 s-1 from the slope and a fluorescence rate (k0) of 6.0×10^6 s-1 from the intercept.

1. Time Resolved Pulsed Laser Photolysis Study of Pyrene
Fluorescence Quenching by I – Anion
Rashid Alsuwaidi, Chris Lieb, Chris Russell and Ralph Eachus
Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
Submitted: February 17, 2014 (CHEM 457, Section 2)
Results
An oscilloscope was used to collect the data, which was used to plot the fluorescence intensity vs
time in Excel. The intensity of the KI concentrations at 0, 10, 20, 30,40mM were plotted in an
overlay, as shown in Figure1.
Figure 1.
The natural log of the intensity against time was plotted for each concentration and the best-fit
equation and R2 of each concentration was determined. The kobs which was the slope of the
equation was recorded for each concentration, as shown in Table 1. The kobs is the observed
relaxation rate of the excited Pyrene.

2. Table 1.
[KI](mM) Kobs(s-1)
0 6.00E+06
10 7.00E+06
20 7.50E+06
30 8.00E+06
40 8.50E+06
The observed relaxation rate of the excited Pyrene (kobs) was plotted against the concentrations
of KI, as shown in Figure2.The best-fit line and R2 were determined. The quenching rate (kq)
was determined from the slope of the best-fit line and the fluorescence rate (k0) was determined
from the intercept. The kq and k0 were 6.0×10^ 4 ± 0.6×10^4 s-1 and 6.0×10^ 6 ± 0.1×10^6 s-1
respectively.
Figure 2.
Discussion
From Figure 1, we were able to show that the intensity decreases and that Pyrene decays faster at
higher KI concentrations. There seems to be some errors in the plot in Figure 2 at 10mM and
40mM of KI. This is probably due to the solution not being purged properly or that the solution
was contaminated with oxygen as we were disconnecting the vent needle. If oxygen is not

3. purged properly from the cell, it will quench Pyrene fluorescence and this will lead to errors in
your plot. In Figure 2, the observed relaxation rate (kobs) increases with increasing concentration
of KI. This is because there is more I- available to quench the excited Pyrene which causes it to
decay faster. When an excited Pyrene accepts an electron from the iodine ion, this is called an
induced electron transfer reaction.1 The results of the experiment from Figure 1 and 2, proves our
assumption that Pyrene decays faster with increasing amounts of KI concentrations. To make
sure that the values for kobs, kq and k0 are correct we could do 3 trials for each concentration and
take the average of the kobs to see if our results are consistent. Then plot them against
concentration to calculate the kq and k0 from the slope and intercept respectively from the best-fit
equation.
Acknowledgement
I would like to acknowledge Chris Lieb, Chris Russell and Ralph Eachus, who were the group
members that assisted in performing the experiment and data analysis. In addition; Dr.
Milosavljevic, teaching assistants Mr. Yuguang (Chris) Lee and Ms. Jennifer Tan.
Reference
1. Milosavljevic, B.H. Lab Packet for CHEM 457: Experimental Physical Chemistry, The
determination of thermodynamic functions of the reactions in commercial alkaline-manganese
dioxide galvanic cell. University Press: University Park, 2014.