Question

So in different solvents solvatochromatic dyes produce different colors based on the gap created by said solvent. So I know that adding electron donating groups raises the HOMO and withdrawing group lower the LUMO, I'm having trouble thinking about the effects this would have on the color. on an unsubstituted dye polar solvents produce a red color and non-polar solvents produce a blue color. My dye that had electron donating groups on it, Would the color spectrum become redder even in non polar solvents and why?

Answer 1

Conditions mentioned in the question is : 1. Type of dye : unsubstituted dye 2. Dye contains electron donating group on it + 3. Dye with electron donating group in put in non polar solvent.

Effect of the conditions mentioned will be as follows :

1. Unsubstituted dye : No auxochromes present, if only one pi bond is there then red shift (bathochromic shift) will not be there. Intensity will be lower as compared to substituded dyes.
2 Electron withdrawing group present in second case will act as auxochrome and populate the electrons in LUMO thus lowering it's energy level and energy level of HOMO will rise, thus the gap between LUMO and HOMO will increase, this results in increase in intensity of abosorption will be more towards higher wavelength region so red shift will be there.
3. No in non polar solvent, transition of pi molecular orbital to pi - anti bonding molecular orbital will be smaller than that of the presence of polar solvent , resulting to no significant change due to non polar solvent i.e color spectrum will not become redder any more.