Question

At high concentrations, these dye molecules tend to form dimers in solution.
a. What is the expected relationship between the absorbance at λ_max for the monomer
and concentration?
b. The formation of dimers will produce a shifted absorbance peak. Do you expect this
relationship to hold using the overall concentration of the dye in the methanol solutions? Why or why not? Hint: What relationship do we know about which relates the amount of something to its absorbance? What assumptions do we make when we use this relationship?

DYE #1: 1-1’-Diethyl-2-2’-cyanine iodide
DYE #2: Pinacyanol chloride
DYE #3: 1-1’-Diethyl-2-2’-dicarbocyanine iodide

Answer 1

a. The relationship is called the Lambert-Beer law, and it states that:

Where A is the absorbance, epsilon is the molar absorptivity os the monomer at the studied wavelength, l is the optical path length and c is the concentration of the monomer.

b. It won't hold, because the actual concentration of the monomers will be lower than the total concentration, since part of the monomers will be forming dimers, that do not absorb in the same way as the monomers. Additionally, the dimers also absorb at a shifted wavelenght maximum, but part of their absorbance probably overlaps with the max wavelength of the monomer. This means that we will have an absrobance that is a mixture of absorbance from the monomers and from the dimers, which breaks the condition of the Lambert-Beer law that states that it works for pure substances.