Experiment 11

EXPERIMENT 11: THE ABSORPTION SPECTRA OF CONJUGATED DYES AND THE FEMO THEORY OF CONJUGATED PI ELECTRONS

Andrew Fleming, Jordan Haskins, Gino Moore, Amanda Petty

INTRODUCTION In our experiment, we measured the absorbencies and wavelengths of 3 different conjugated dyes:   

1,1' diethyl-4,4'-cyanine iodide 1,1' diethyl-4,4'-carbocyanine 1,1' diethyl-4,4'-dicarbocyanine iodide

This experiment was done in order to test the FEMO (Free Electron Molecular Orbital) theory and can be thought of as a particle-in-a-box problem because of the long, conjugated chains in the molecules.

MATERIALS AND METHODS Spectronic 20 spectrophotometer  Methanol  3 disposable pipettes  A waste beaker  3 dyes in solid form  4 Cuvettes (used in spectrophotometer) 

MATERIALS AND METHODS A

single crystal of 1,1' diethyl4,4'-cyanine iodide was placed in the cuvette and diluted with methanol.  The absorption was read, following directions located with the spectrophotometer.  A graph of the absorbance versus the wavelength was displayed on the spectrophotometer screen.  Initially, our absorbance was much higher than the recommended value of 2.

MATERIALS AND METHODS A small amount of solution was taken out using the pipette and the mixture was further diluted with methanol.  Dilution was continued until the absorbance was less than 2.  At this point, we printed the graph from the spectrophotometer.  These steps were repeated for the other 2 dyes. 

RESULTS

Absorbance

1,1'-diethyl-4,4'-cyanine iodide

Wavelength (nm)

RESULTS

Absorbance

1,1'-diethyl-4,4'-carbocyanine iodide

Wavelength (nm)

RESULTS

Absorbance

1,1'-diethyl-4,4'-dicarbocyanine iodide

Wavelength (nm)

RESULTS Dye

Π electrons

n (HOMO)

n (LUMO)

Cyanine

10

5

6

Carbocyanine

12

6

7

Dicarbocyanine

14

7

8

E=n2h2/8meL2 Dye

λmax (m), calc

λmax (m), exp

Percent Error

Cyanine

5.88E-07

5.90E-07

0.36

Carbocyanine

7.16E-07

7.08E-07

1.16

Dicarbocyanine

8.45E-07

8.14E-07

3.67

DISCUSSION Our experimental wavelengths of maximum absorbance were proportional to the lengths of the dye molecules. Put simply, as the dye molecule length increased, the wavelength of maximum absorbance also increased. This was predicted by the FEMO theory.

CONCLUSION When compared to the FEMO predictions, our data for the actual λmax seem to be accurate. Since the values for the experimental and calculated wavelengths of maximum absorption were similar, this experiment was a good illustration of the FEMO theory and the “particle-in-a-box” concept.

Citations •

• • • •

Clark, Roy W.; Howard, James C. Physical Chemistry Lab Manual, 4th ed., RonJon Publishing, Denton, TX, 1996. Fleming, A.J.; Physical Chemistry Laboratory Notebook, 2007 p. 15-16. Haskins, J.R.; Physical Chemistry Laboratory Notebook, 2007 p. 21-22. Moore, L.G.; Physical Chemistry Laboratory Notebook, 2007 p. 16-17. Petty, A..; Physical Chemistry Laboratory Notebook, 2007 p. 15-16.