PARTICLE IN A ONE-DIMENSIONAL BOX

EXPERIMENTAL PROCEDURE (4/17/03) INTRODUCTION A fiber optic spectrometer will be used to determine absorption spectra for three dyes. The electronic transition energies within the dyes’ conjugated π systems will be determined from the spectra. Theory and more extensive background information for this experiment can be obtained from the 6th edition of Experiments in Physical Chemistry, by David P. Shoemaker, pp.378-381. A copy of this reference can be obtained from the lab supervisor. PRE LABORATORY EXERCISE 1. Draw structures of each of the following dyes and include them in your pre lab report.

a. cryptocyanine b. 1,1’-diethyl-2,2’-cyanine iodide c. 1,1’-diethyl-2,2’-dicarbocyanine iodide

2. Which of the dyes do you predict will have the largest wavelength at the maximum absorption?

Explain. 3. Prepare a pre laboratory report with an introduction, theory, procedure, and safety sections in

the format described on the lab’s Web pages. LABORATORY EXCERCISE A series of cyanine dyes (listed in the pre lab exercise) will be studied using the lab’s UV-VIS fiber optic spectrometer. The three dyes are available as 10-3 M stock solutions in methanol. Follow the spectrometer operating instructions (below) and obtain a spectrum for each of the dyes. If necessary, dilute the initial solutions (with methanol) until a spectrum is obtained that has an absorbance of approximately 0.2 at the peak absorption wavelength(s). The band shape may be different at higher concentrations, since many dyes form dimers. Spectrometer Operating Instructions A. Initial Set-up/Zeroing of the Instrument

1. Turn on the computer and log on (Username: “PChem”, Password: “<enter>”. Open OO1BV32. The program should open up in the scope mode. This is indicated by a highlighted “S” box above the spectrum window. Under the “File”

menu (uppermost toolbar), go to the Autoincrement Filename option and make sure the “enabled” option is not checked.

2. Place the measurement probe into the reference solvent (methanol). Check for air bubbles present in the probe. NOTE: To prevent the formation of bubbles, place the probe into solutions slowly. To remove an air bubble, quickly tap the probe several times against the test tube used to hold the solution (not too vigorously).

3. In the scope mode, turn the light source on (box at base of fiber optic cable). Allow the spectrum to stabilize for a few seconds, then select the camera icon just above the spectrum window to create a “snapshot” of the reference (methanol) spectrum. Turn off the light source and select the lighted light bulb icon. With the light source turned off and the snapshot mode unselected, allow the spectrum to stabilize (a flat baseline should be visible at this point). Select the dark light bulb icon. This will save the spectrum as the dark spectrum. Turn off the spectrometer, switch to absorbance mode ("A" button on toolbar), rinse probe with distilled water, and dry with KimWipe. The unit is now ready for data collection.

B. Data Collection

1. When ready to begin collecting data, place pobe in the solution you wish to measure and then turn the light source on.

2. Once a spectrum is observed and is reasonably stable, select the camera icon to preserve the observed spectrum as a “snapshot” of the spectrum. Turn off the light source. Save the data onto a disk. Under the “File” menu (uppermost toolbar), go to the Autoincrement Filename option and unselect the “enabled” option. The scan is saved by choosing “PROCESSED” as the spectrum you wish to export. The data will be saved as a *.Absorbance file.

3. Collect spectra for the remaining dyes in the same manner, performing dilutions and re-measuring as necessary. The instrument should be re-zeroed when the baseline absorbance is >0 and/or between each of the dyes (as described in steps 2-3).

C. Opening the data using M.S. Excel����

1. Select “Open” from the File menu i. Under “Files List” click “All Files” ii. Under “File Name”, open the file name of the saved spectrum from above. iii. Select “OK”

2. Step 1 of the Text Import Wizard Screen should now be open. i. Under “Original Data Type”, select “Delimited”, then “Next” ii. Under “Delimiters”, select “Tab” and “Space”, then “Next” iii. Under Column Data Format”, select “General”, then “Finish”.

3. The data should now be visible as an MS Excel� Worksheet CALCULATIONS Include a spectrum (A vs. λ) for each of the dyes at the final concentration(s) in your final report. Determine λMAX for each of the dyes by locating the wavelength where the maximum absorption occurs. Compare this to a calculated λ using the free-electron model for each dye (equation 6, Shoemaker). Choose α to give the best fit for this series of dyes. Alternatively, fix α by fitting any one of the dyes and use this value for the others. Include a table comparing your experimental and theoretical values of λMAX. REFERENCE David P. Shoemaker, Experiments in Physical Chemistry, 6th Ed., McGraw-Hill, 1996, pp. 378-383.