University of New England CHEM210: Conjugated Dye Absorption Spectrum

This report presents an analysis of the electromagnetic radiation spectrum of conjugated dyes, focusing on their absorption characteristics in the visible and ultraviolet regions. The study investigates the relationship between the molecular structure of conjugated dyes and their interaction with electromagnetic radiation. The experiment involved the use of a UV/Vis spectrometer to obtain absorption spectra for six different conjugated dyes, with subsequent calculations of wavelength and absorbance. The report includes a comparison of experimental and theoretical data, employing the free-electron molecular orbital model and Kuhn's particle-in-a-box model theory to interpret the observed spectra. The findings highlight the impact of pi electrons on the absorption wavelength and the application of Kuhn's model in rationalizing the spectroscopy of complex systems, including a discussion of potential sources of error and the limitations of the theoretical model used. The report concludes with recommendations for future experiments, emphasizing the importance of minimizing experimental errors and utilizing quantum mechanics for more accurate results.

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 1
The Conjugated Dye`s Absorption Spectrum
[Author Name(s), First M. Last, Omit Titles and Degrees]
[Institutional Affiliation(s)]
Author Note
[Include any grant/funding information and a complete correspondence address.]

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 2
Abstract
The dye molecules of various compounds appear to be colored because they absorb
electromagnetic radiation in the visible region of about 400 to 700nm wavelength, the ultraviolet
spectra of about 180 to 400nm wavelength and the vacuum ultraviolet region. On absorption of
the electromagnetic radiation, the electrons in the molecules become excited and they “jump”
from the ground state which is at a lower orbital to the excited state at a higher orbital. The
interval spacing of the orbitals in the molecules are different and hence, the excitation leads to
production of different colors observed since each molecule absorbs at a different wavelength
depending on the spacing between the orbitals.
Keywords: visible region, wavelength, absorption, interorbital spacing, colors.

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 3
The Conjugated Dye`s Absorption Spectrum
Conjugated dyes are colored molecules as a result of their reaction with electromagnetic
radiation. The dye molecules absorb the electromagnetic radiation and their electrons get excited
and “jump” from the lower orbitals which is the ground state level to the excited state at higher
orbitals. Upon losing this energy the electrons fall back to the ground states emitting wavelengths
that are unique to each molecule and it is these transmitted wavelengths that are colored. The
wavelengths absorbed by these molecules are in the visible region that ranges for 400nm to
750nm while others absorb radiation with wavelengths in the ultraviolet region which ranges
from 150nm to 400nm. For this reason, the dyes appear to be colored as the transmitted
wavelengths after absorption of the radiation in the visible region, are colored. According to
Atkins and de Paula (2010), solution s that appear red absorb blue-green or cyan light in the
region between 400 and 600nm and transmit red wavelengths that are red in color of wavelength
between 600 and 700nm. The organic molecules that absorb the electromagnetic radiation at the
visible region indicate that unlike other organic molecules, their interorbital spacing is smaller
and hence they absorb the radiation at a wavelength below 400nm (Sturmer, 2000). For most
organic molecules however, their absorption wavelength is greater than 400nm and absorb at the
infrared region.
The six conjugated dye molecules studied in this experiment are heavily colored and
most of them exhibit the blue color. The dyes that show a blue color have been referred to as
cyanine dyes for many years. These dyes have a unique structure with alternating double bonds
appearing before the rings are joined (Garland, Shoemaker & Nibler, 2009). As a result of this
structure, the delocalized electrons of the molecules change the wavelength at which the
electromagnetic radiation is absorbed to the visible region that transmits the blue color seen.

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 4
According to Kuhn (1949), the pi electrons in a molecule are in motion in the orbital of the
molecules and they determine the characteristics of the visible region spectrum via the particle in
a box model theory. The electrons making the sigma bond are not considered as they are only
excited by the radiation in the vacuum ultraviolet region. The equation that is used to obtain the
wavelength of the absorption of radiation by the dyes is given below:
λ = 8mcl2 /h × (p+3)2/ p+ 4 Putting l = 1.39 Å = 0.139 nm
The objective of the experiment was to obtain the empirical parameter`s value and also to
find out the length of the carbon chains for the conjugated dyes using the UV/VIS spectrometer.
Procedure and Method
The absorption cell was filled with methanol and then a spectrum was run between 800 to
400nm in a spectrophotometer. Afterwards, the absorption cell was emptied and rinsed off with
the first dye and thereafter the first dye was added to it and its spectrum was also run between
800 to 400nm. The results were then recorded. The procedure was thereafter repeated for all the
other five dyes and their results were also recorded.

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 5
Results and Calculations.
0.32258606
0.470458984 0.467407227
0.286636353
0.419158936
0.515716553
0
0.1
0.2
0.3
0.4
0.5
0.6
400 450 500 550 600 650 700 750 800
Absorbance (AU)
Wavelength (nm)
Absorption Spectrum of Conjugated Dyes
1,1'-Diethyl-2,2'-cyanine Iodide
1,1'-Diethyl-2,2'-carbocyanine Chloride
1,1'-Diethyl-2,2'-dicarbocyanine Iodide
3,3'-Diethylthiacarbocyanine Iodide
3,3'-Diethylthiadicarbocyanine Iodide
3,3'-Diethylthiatricarbocyanine Iodide
1. Determine the wavelengths and absorbances manually by measuring directly off the
spectra.
The maximum peak for 1,1’-diethyl-2,2’-cyanine iodide is 524nm wavelength and
absorbance of 0.32258606.
The maximum peak for 3,3’-diethylthiacarbocyanine iodide is 558nm and absorbance
of 0.286636353.
The maximum peak for 1,1’-diethyl-2,2’-carbocyanine chloride is 604nm and
absorbance of 0.470458984.
The maximum peak for 3,3’-diethylthiadicarbocyanine iodide is 652nm and
absorbance of 0.419158936.

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 6
The maximum peak for 1,1’-diethyl-2,2’-dicarbocyanine iodide is 706nm and
absorbance of 0.467407227.
The maximum peak for 3,3’-diethylthiatricarbocyanine iodide is 756nm and
absorbance of 0.515716553.
2. Calculate the wavelength from the free electron model for each compound. Using λ =
8mcl2 /h × (p+3)2/ p+ 4 as the formula, the theoretical wavelength values for the
conjugated dyes become
1,1’-diethyl-2,2’-cyanine iodide
[8 × 9.1 × 10-31Kg × (1.3253 × 10-5 )2 × 3 × 108] ÷ [6.63 × 10-34] × [7 +3]/ [7+4]
= 526nm
1,1’-diethyl-2,2’-carbocyanine chloride
[8 × 9.1 × 10-31Kg × (1.416 × 10-5 )2 × 3 × 108] ÷ [6.63 × 10-34] × [9 +3]/ [9+4]
= 609.65nm
1,1’-diethyl-2,2’-dicarbocyanine iodide
[8 × 9.1 × 10-31Kg × (1.521 × 10-5 )2 × 3 × 108] ÷ [6.63 × 10-34] × [11 +3]/ [11+4]
= 711.01nm
3,3’-diethylthiacarbocyanine iodide
[8 × 9.1 × 10-31Kg × (1.3711 × 10-5 )2 × 3 × 108] ÷ [6.63 × 10-34] × [7 +3]/ [7+4]
=563nm
3,3’-diethylthiadicarbocyanine iodide
[8 × 9.1 × 10-31Kg × (1.48177 × 10-5 )2 × 3 × 108] ÷ [6.63 × 10-34] × [8+3]/ [8+4]
= 663nm
3,3’-diethylthiatricarbocyanine iodide

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 7
[8 × 9.1 × 10-31Kg × (1.583 × 10-5 )2 × 3 × 108] ÷ [6.63 × 10-34] × [9 +3]/ [9+4]
= 762nm
3. Calculate the oscillations and tabulate the data from the experimental and theoretical
data obtained from the experiment.
Name of the compounds Experimental
Data
Theoretical Data
1,1’-diethyl-2,2’-cyanine iodide C23H23N2I
1,1’-diethyl-2,2’-carbocyanine chloride
1,1’-diethyl-2,2’-dicarbocyanine iodide
3,3’-diethylthiacarbocyanine iodide
3,3’-diethylthiadicarbocyanine iodide
3,3’-diethylthiatricarbocyanine iodide
1.197 oscillations
1.636 oscillations
2.066 oscillations
1.197 oscillations
1.417 oscillations
1.636 oscillations
1.417 oscillations
1.852 oscillations
2.283 oscillations
1.417 oscillations
1.636 oscillations
1.852 oscillations
The oscillations experimentally and theoretically therefore become
Using the formula, F= 0.211 N2 (N +2)2 / (N+ 1)3
Therefore,
a) 0.211 × 62 (6 + 2)2 / (6+1)3
F= 1.417 oscillations
0.211 × 52 (5 + 2)2 / (5+1)3
F = 1.197 oscillations

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 8
b) 0.211 × 82 (8 + 2)2 / (8+1)3
F = 1.852 oscillations
0.211 × 72 (7 + 2)2 / (7+1)3
F = 1.636 oscillations
c) 0.211 × 102 (10 + 2)2 / (10+1)3
F= 2.283 oscillations
0.211 × 92 (9 + 2)2 / (9+1)3
F = 2.066 oscillations
d) 0.211 × 62 (6 + 2)2 / (6+1)3
F = 1.417 oscillations
0.211 × 52 (5 + 2)2 / (5+1)3
F = 1.197 oscillations
e) 0.211 × 72 (7 + 2)2 / (7+1)3
F= 1.636 oscillations
0.211 × 62 (6 + 2)2 / (6+1)3
F = 1.417 oscillations
f) 0.211 × 82 (8 + 2)2 / (8+1)3
F = 1.852 oscillations
0.211 × 72 (7 + 2)2 / (7+1)3
F = 1.636 oscillations
4. Plot the difference in the experimental and theoretical values as a function of
increasing p.

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 9
5 6 7 8 9 10 11 12 13 14 15
0
2
4
6
8
10
12
Plot of Experimental and Theoretical Value
differences against P.
p
Value Difference
5. Determine the alpha values of the experiment and briefly discuss the Kuhn model in
rationalizing the spectroscopy of the complex systems of the experiment.
Αlpha =( hγ ( p)
8 mc )1/2. Hence,
1,1’-diethyl-2,2’-cyanine iodide
(6.63 × 10-34 × 7 × 5.481 × 1017 / 8 × 9.31 × 10-31Kg × 3 × 108)1/2
= 1067ppm
1,1’-diethyl-2,2’-carbocyanine chloride
(6.63 × 10-34 × 9 × 6.3186 × 1017 / 8 × 9.31 × 10-31Kg × 3 × 108)1/2
=1299ppm
1,1’-diethyl-2,2’-dicarbocyanine iodide
(6.63 × 10-34 × 11 × 7.3701× 1017 / 8 × 9.31 × 10-31Kg × 3 × 108)1/2

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 10
= 1551ppm
3,3’-diethylthiacarbocyanine iodide
(6.63 × 10-34 × 7 × 9.626 × 1017 / 8 × 9.31 × 10-31Kg × 3 × 108)1/2
= 1414ppm
3,3’-diethylthiadicarbocyanine iodide
(6.63 × 10-34 × 8 × 1.4773 × 1017 / 8 × 9.31 × 10-31Kg × 3 × 108)1/2
= 1675ppm
3,3’-diethylthiatricarbocyanine iodide
(6.63 × 10-34 × 9 × 2.0056× 1017 / 8 × 9.31 × 10-31Kg × 3 × 108)1/2
= 2070ppm
Through Kuhn`s equations the wavelength and the alpha values were easily obtained
through calculations that enabled a suitable comparison between the compounds. It
was observed that these calculated values increased with a corresponding increase of
pi electrons in the compounds.
Discussion and Conclusion.
The spectra of the conjugated dyes were studies in order to obtain an analysis. Some
of the spectra contained well defined peaks whereas some of the spectra had low
resolution peaks as a result of errors present while carrying out the experiment. One
of the major sources of error that resulted in the low resolution was incorrect
calculation of the dilution factor. In addition, there might have been noise at those
wavelengths in the spectrometer that overlapped with the produced signal and hence
resulting to the poor resolution observed (Farrel, 1985). The conjugated dyes also

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 11
exhibited two peaks in their spectra and the greater peak was recorded for the
wavelength. The other peak that had formed for the dyes was as a result of presence
of suspected impurities in the dyes. The impurities did not include methanol as a
background scan was done. The values of the experimental wavelengths obtained
from the experiment were in the near range of the predicted theoretical values of the
dyes. The deviations of these values were attributed to the errors that had been
previously mentioned, that could have affected the measurements taken and hence
causing the deviations. The wavelengths obtained in the experiment were thereafter
used to calculate the box length and the results obtained showed that increasing the
number of the pi electrons of the compounds led to an increase in the box length. The
data from the experiment, Kuhl`s particle in a box model theory was not satisfying
the spectra of the cyanine dyes and hence was not a good method to use for the
spectral analysis of the conjugated dyes.
In conclusion, Kuhn`s model was deemed inappropriate for the experiment`s
analysis because it yielded big differences between the experimental and predicted
theoretical values. This consequently led to a big difference between the theoretical
box length established by scientists and the experimental box length from the
calculations obtained through Kuhl`s theory. According to Farrel (1985), in order to
be able to obtain accurate results in future experiments, the quantum mechanics
particle in a box model theory should be used in order to obtain the correct results.
Also, the experimental errors that emerged from the experiment should be minimized
for a higher precision and accuracy.

THE ELECTROMAGNETIC RADIATION SPECTRUM OF CONJUGATED DYES. 12
References.
Atkins, P.W., & de Paula, J. (2010). Physical Chemistry. Oxford, England: Oxford
University Press.
Kuhn, H. (1949). J. Chem. Phys. Available from,
http://www.chm.bris.ac.uk/webprojects2001/nix/Dye-Site/embedded/particlebox.html
Sturmer, D. (2000). In Kirk-Othmer Encyclopedia of Chemical Technology. John
Wiley and Sons Inc.
Garland, C., Shoemaker, D., & Nibler, J. (2009). Experiments in Physical
Chemistry. McGraw-Hill: New York. 393-398.
Farrel, J. (1985). The absorption spectra of a series of conjugated dyes:
Determination of the spectroscopic resonance integral. Journal of Chemical
Education. 64(4),351