I am teaching an undergradute pchem lab on the charge transfer complex of iodine and mesitylene. The solutions are in heptane and approximate concentrations are 0.1 mM iodine and 80-300 mM mesitylene.
In addition to the appearance of the CT band at 328 nm, there is also a blue shift of the iodine absorbance from about 521 to 516 nm as mesitylene concentration increases. Assuming the mesitylene is present in enough concentration to change the dielectric properties of the solvent, I believe one would expect a red shift of the I2 absorbance band. If anyone could please provide an explanation for the blue shift, I would much appreciate it.
There are 4 samples of increasing mesitylene concentration. The more mesitylene, the larger the blue shift of the iodine peak. Without mesitylene, the I2 peak is at 521 nm, and shifts a total of 5 nm to 516 nm for the sample with the largest amount of mesitylene.
Peter - I followed the link you sent me, and they observe a blue shift in both maxima which are separated by about 50 nm.
I posted the spectra overlay above and as you can see the peaks are separated by nearly 200 nm and the absorbance drops to approximately 0 between them so there is no spectral overlap between them.
Likewise, the students analyze the 4 solutions at 4 different temperatures. As formation of the CT complex is exothermic, as they raise the temperature the absorbance and therefore concentration of the 328 nm CT complex peaks drops as temperature increases. Despite this, the blue shift of the I2 maxima remains the same regardless of temperature. If the CT band were overlapping and pushing up the shoulder of the I2 peak then one would expect the I2 blue shift to not shift as much as temperature increases and [CT] is lowered.
Since the I2 maxima blue shift does not change with temperature while [CT] does, it is likely correlated with how much mesitylene was initially added to the solutions. The addition of mesitylene should increase the polarity and dielectric constant of the heptane solvent and one would expect a red shift of the I2 peak, although I do not know if sufficient mesitylene would have been added to observe that effect through that mechanism.
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