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

Abs

orba

nce

Wavelength (nm)

RESULTS

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

Abs

orba

nce

Wavelength (nm)

RESULTS

1,1'-diethyl-4,4'-dicarbocyanine iodideA

bsor

banc

e

Wavelength (nm)

RESULTS

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

Dye Π electrons n (HOMO) n (LUMO)

Cyanine 10 5 6

Carbocyanine 12 6 7

Dicarbocyanine 14 7 8

E=n2h2/8meL2

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

1. Clark, Roy W.; Howard, James C. Physical Chemistry Lab Manual, 4th ed., RonJon Publishing, Denton, TX, 1996.

2. Fleming, A.J.; Physical Chemistry Laboratory Notebook, 2007 p. 15-16.

3. Haskins, J.R.; Physical Chemistry Laboratory Notebook, 2007 p. 21-22.

4. Moore, L.G.; Physical Chemistry Laboratory Notebook, 2007 p. 16-17.

5. Petty, A..; Physical Chemistry Laboratory Notebook, 2007 p. 15-16.