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)

ResultsAn 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.

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

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.