UV-Vis of Conjugated Dyes
Cindy Spangler
Westminster College
Spartan 4.0 was used to determine maximum absorption wavelength of three dye
molecules; 1,1’-diethyl-4,4’-cyanine iodide, -carbocyanine iodide, and –
dicarbocyanine iodide. The data obtained in Spartan was compared to the
observed data collected using absorption spectroscopy. The maximum absorption
wavelength obtained by Spartan for 1,1’-diethyl-4,4’-cyanine iodide, carbocyanine iodide, and –dicarbocyanine iodide are 201, 225, and 241 nm,
respectively. The observed values were 589, 706, and 813 nm, respectively for
the dyes. This differ because the semi-empirical data is flawed but shows a
relationship between the ΔE, wavelength, and box length.
Introduction
Absorption spectroscopy is based on the absorption of light by a chemical and the promotion of
electrons from one energy level to the next. When the appropriate wavelength is absorbed by the
chemical, an electronic transition occurs from the highest occupied molecular orbital (HOMO) to
the lowest unoccupied molecular orbital (LUMO). Typically, ultraviolet and visible light are
energetic enough to promote the electrons. The absorption of the solution is measured by a
detector and analyzed on a computer. The absorption spectrum provides data on the maximum
absorption wavelength and the emission spectrum. By knowing the maximum absorption
wavelength, the difference in available energy levels can be determined using:
E photon 
hc

eq. (1)
where c is the speed of light in a vacuum and  is the absorbed wavelength. By using absorption
spectroscopy with the dyes, the maximum wavelength absorbance can be experimentally
determined and compared with particle-in-a-box model.
The examination of absorption spectra of 4,4 series of carbocyanine dyes will provide observed
data for the electronic structure of three dye molecules. The maximum absorption wavelength of
these dyes will be interpreted by using particle-in-a-box model. This provides a model of a
particle with one dimensional motion in a three dimensional space with zero potential along the
length of movement. The particle is confined to the length, L, of the box because potential is
infinite at the limits of the box. In the case of a dye molecule, the electrons of the  bonds
remain on the length of the carbon chain between the two terminal nitrogen atoms. The potential
is also assumed to be zero along the chain, therefore the potential along the length is set to zero.
Since the dye molecules concur with the particle-in-a-box model, it is known that energy is:
En 
n2h2
8mL2
eq. (2)
And the wave function is:
2
L
 n ( x)   
1/ 2
sin
nx
L
eq. (3)
where n is a positive integer from 1 to ∞, m is the mass of the particle, L is the length of the
chain, and h is Planck’s constant.
To determine the electronic transitions for the three dyes, the spectra will be analyzed by using
two quantum mechanical models. The particle-in-a-box model explained earlier and semiempirical molecular orbital calculation. This is done by solving the Schrodinger equation for the
entire molecule by making approximations. By using computation methods, the structure and
the reactivity of molecules can be predicted from developed computer software based on
experimental data. Using these two methods, 1, 1’-diethyl-4,4’-cyanine iodide, 1, 1’-diethyl4,4’-carbocyanine iodide, and 1, 1’-diethyl-4,4’-dicarbocyanine iodide dye. Figure 1 shows the
structure of 1, 1’-diethyl-4,4’-carbocyanine iodide:
N
N+
Figure 1.
A comparison of these methods will be completed and interpreted.
Experimental
Experimental Determination
A Varian Cary 100 Bio UV-Vis spectrometer was used to obtain the absorption spectra and max
for 1,1’-diethyl-4,4’-cyanine iodide, -carbocyanine iodide, and –dicarbocyanine iodide dyes.
Each sample was prepared by dissolving 1 mg of dye in 10 mL of methanol and scanned in the
visible range from 400 to 800 nm. The dye solutions were diluted to reduce the dimer peak of
dyes during successive scans.
Computational Chemistry
Spartan 4.0 was used to build the three dye molecules. Semi-empirical geometry optimization
calculation was performed on the molecules to obtain the accurate structure. Equilibrium
geometry at gound state with semi-emperical method and AM1 parameterization set was
submitted for calculation. After completion of the calculations, the differences in the HUMO
and LUMO energies were determined for calculation of the light wavelength for the excitation of
an electron.
The distances between the nitrogen atoms were measured using Spartan by two separate
methods. The first method followed the distance between individual bonds of atoms separating
the nitrogen atoms. The second method determined the shortest linear distance between the two
nitrogen atoms.
Results and Discussion
Figure 1 shows the absorption spectra for the dyes. Each dye had a maximum absorption
wavelength in different regions of the electromagnetic spectrum corresponding to the visible
color observed by the human eye. The peaks vary in height due to the concentrations used for
the individual dyes. The maximum absorption was determined by using Excel®.
Absorbance v. Wavelength
0.9
Absorbance
0.8
0.7
0.6
1,1’-diethyl-4,4’carbocyanine iodide
0.5
1,1’-diethyl-4,4’dicarbocyanine iodide
0.4
1,1’-diethyl-4,4’-cyanine
iodide
0.3
0.2
0.1
0
400
500
600
700
Wavelength (nm)
800
900
Figure 1. Absorption Spectra for three dyes.
The maximum absorption wavelength was used to calculate the change in energy between
HOMO and LUMO, according to equation 1. Box length was calculated by solving equation 2
for L.
Summary of Experimental Data
1,1’-diethyl-4,4’cyanine iodide
1,1’-diethyl-4,4’carbocyanine iodide
1,1’-diethyl-4,4’dicarbocyanine iodide
Observed Wavelength (nm)
598
706
813
E (eV)
2.07
1.77
1.53
Box Length, L (Å)
4.33
4.52
4.68
Table 1. Summary of Experimental Data collected from absorption spectroscopy and calculations.
As the box length L increases, the ΔE decreases. This means that the increase in the carbon
linker decreases the difference in HOMO and LUMO. The increase in wavelength is inversely
proportional to energy, but proportional to L. This is stated in equation 1.
Computational data obtained from Spartan 4.0 is given in Table 2. Values obtained were used in
equations 1 and 2. The predicted max was calculated by solving equation 1 for the wavelength
and using the difference in HOMO and LUMO. Box length L, straight was a straight line
between the terminal nitrogen atoms. The box length, bonds is the sum of all bond lengths of the
carbon linkers. Calculations were based off of straight measurements of the terminal atoms.
Summary of Computational Data
1,1’-diethyl-4,4’cyanine iodide
1,1’-diethyl-4,4’carbocyanine iodide
1,1’-diethyl-4,4’dicarbocyanine iodide
HOMO (eV)
-10.62
-10.11
-9.69
LUMO (eV)
-4.62
-4.60
-4.54
E (eV)
6.00
5.51
5.15
Predicted max
207
225
241
Box length, L (Å) Straight
7.38
8.01
Box length, L (Å) Bonds
11.36
13.89
Table 2. Summary of Computational Data obtained from Spartan.
8.6
16.93
The same relationships between the values exist; as ΔE decreases, bond length increases.
However, comparison between the data obtained differently provides inconsistencies. Energy
differences and box length are much higher for Spartan than observed, but tend to have the same
upward or downward trend. Wavelength also differs; the experimental data has higher values
than computational data shows. In order to determine whether the computational data has any
meaning, ratios of the values were taken and are shown in table 3.
Data
Comparison
1,1’-diethyl-4,4’1,1’-diethyl-4,4’1,1’-diethyl-4,4’cyanine iodide
carbocyanine iodide
dicarbocyanine iodide
2.9
3.1
3.4
E (eV)
Box Length, L (Å)
1.7
1.7
1.8
Wavelength (nm)
0.4
0.3
0.3
Table 3. Data comparison of semi-empirical data and experimental results ratio.
Ratios for the data show that there are consistent differences between the results, meaning that
the differences between the computational and experimental data for the three dyes are similar.
Conclusion
Semi-emipirical data differs for each dye the same amount; therefore computational data can be
used to determine what will be observed in the real world or provide relational information of
values. Computational data can provide information about experimental data, but computational
data is only sufficient to the parameters of studied values and computer programs.
References
Spartan 4.0