P-chem experiment Absorption Spectra of Conjugated Dyes The polymethine dyes use

Question

P-chem experiment Absorption Spectra of Conjugated Dyes

The polymethine dyes used for this experiment are:

3,3’-diethylthiacyanine iodide Solution 1

3,3’-diethylthiacarbocyanine iodide Solution 2

3,3’-diethylthiadicarbocyanine iodide Solution 3

3,3’-diethylthiatricarbocyanine iodide Solution 4

I did the graph. but i dont know how to find the shoulder. what do you think the shoulder is due to? and what does alpha correct for and how well does alpha correct the wavelength. i attached the graph also. could you plz type the answer? not writing. thx. if you dont know what i am talking about u can see the lab in this link: https://chem.libretexts.org/LibreTexts/University_of_California_Davis/UCD_Chem_105/UCD_Chem_105%3A_Lab_Manual/Lab_7%3A_Conjugate_Dyes

Explanation / Answer

Dyes are colored substances which have extensive uses. Their absorption spectrum shows several absorption peaks generally known as shoulders which represent the absorption of UV radiations due to respective electronic transitions. More the no. of electronic transitions more are the peaks among which one of the peak is maximum which is known as max.

Free electron model proposed by Kuhn allowed to calculate predicted wavelength for conjugated dienes. The parameter was introduced for the dyes containing nitrogen and sulphur heterocycles to predict the wavelength as according to the model. This parameter is added as a correction to observed wavelengths and calculated the theoretical wavelengths.

More the conjugation in the molecule greater is the absorption of lower energy and hence the longer the wavelength recorded almost in visible range and hence the molecule is coloured.

For finding out the shoulder just follow the highest peak in the UV region. You can use scale to find the exact value or roughly the values of can be given as

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