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Absorption Spectrum of a Conjugated Dye

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Chem 375-003

Group #5




Using a diode array spectrophotometer, the absorption spectrum of several conjugated dyes in a series were collected. One of these dyes, 3,3 diethylthiatri-carbocyanine iodide, was made to an exact concentration allowing the calculation of the extinction coefficient. The other dyes were studied at unknown concentrations, with the determination of the wavelength of maximum absorption of prime importance.

Introduction & Theory

The color of a substance is determined by the wavelengths of light that it does not absorb, but either reflects or transmits. The ability of a molecule to absorb a photon of a specific wavelength depends on the ability of the molecules electrons to jump to an energy level corresponding to the energy content of the photon. If the photon has sufficient energy to excite an electron, the photon is absorbed. Shortly afterward, the electron drops back to its ground state, emitting a photon as it does so. However, as the drop the ground state takes place at a different rate than the jump to the excited state, the emitted photon is of a different wavelength. In the case of molecules that absorb in the visible region, the fact that the energy absorbed is re-emitted at non-visible wavelengths explains what happens to the absorbed energy. As exact absorbance region of a substance is based upon the (rather unique) energy levels of its electrons, the measurement of absorption spectra serves to both identify compounds as well as to give coincidental evidence as to whether two compounds are similar in structure and composition.


The procedure in Experiments in Physical Chemistry(1) was followed, with modifications as suggested in the class handout. These modifications included specifics on how to use the equipment, as well as specifying 3,3 diethylthiatri-carbocyanine as the only dye for which the extinction coefficient was to be determined. The other dyes were made at unknown concentrations that were found to have maximum absorptions between 0.5 and 1.


In order to determine the extinction coefficient of 3,3 diethylthiatri-carbocyanine, the concentration of the solution measured must be determined. The concentration of the original solution was

As this original solution was diluted by a ratio of 1:20 before a acceptable results were obtained, the measured solution had a concentration of 4.77x10-6M. The path length of the curvette d was 1.0 cm, and the maximum measured absorption A was 0.613174. The extinction coefficient was found to be

The experimental value of max, the wavelength at peak absorbance, is given for each dye on the printouts of their spectra. These values were provided by the software used. The theoretical value of max is given by

Where is an empirical constant for a series of dyes and p is the number of atoms in the polymethine chain. Solving eq. (3) for yields

Table 1 - Calculated vs. Experimental max

Compound (iodide) p Calculated max (nm) Experimental max (nm)
1,1-diethyl-2,2-cyanine 5 0.321 491 490
3,3-diethylthiacarbocyanine 6 0.343 554 556
1,1-diethyl-2,2-carbocyanine 7 0.213 618 604
3,3-diethylthiatri-carbocyanine 9 0.438 745 758

The calculated max is based on an average of 0.329


Comparing the experimental max to the calculated, an average percent error of 1% was obtained.

In all of the dye spectra obtained, fairly large peaks in the range of 200 - 350 nm were observed. This is likely a solvent effect, as this corresponds to red or orange color, which was not observed in the samples. The documentation for the experiment1 listed only methanol as a solvent, and there was discussion during the course of the experiment that ethanol may not have created the same peaks.

The spectrums that were not recorded due to excessive concentration exhibited bands that were much wider than those found when the absorbance was between 0.5 and 1. A plateau of approximately 100 nm was not uncommon and was likely an effect of dimerization and steric effects on the dyes.


1. D. P. Shoemaker, C. W. Garland, J. W. Nibler, Experiments in Physical Chemistry, 6th ed., chapter XIV, The McGraw-Hill Companies (1996).