[asa029]

Hi everyone.

I have recently made an unknown sample that I isolated and purified. I am working on the structural elucidation, but something peculiar happens when I add solvent to the unknown.

The crystals are normally yellow (and an acetone solution is also yellow) but if the same crystals are solved in DCM or chloroform, the solution changes color to green/blue over time (hours). This is however reversible, as the evaporated crystals again become yellow.

I have never seen this before and wondered if anyone knew of a class of compounds that did this in help with the elucidation.

(and its not organometallic:D)

[Honclbrif]

Since its reversible, my first thought is solvatochromism.

http://en.wikipedia.org/wiki/Solvatochromism

Happens fairly frequently, and sometimes to a spectacular degree. I've had compounds go from green to colorless to yellow depending on the solvent. For  fun examples see Nile Blue or Nile Red.

http://en.wikipedia.org/wiki/Nile_blue
http://en.wikipedia.org/wiki/Nile_red

All that said, solvatochromism tends to be pretty rapid. Since the color change is gradual it may also be something else at play such as reversible dimerization or photochromism. Does this happen if you keep it in the dark?

[Yakimikku]

Solvatochromism sounds pretty interesting (I was unaware of this phenomenon--thank you Honclbrif!), maybe you could try different solvents as well and see if there is a correlation between the solvent polarity and color change.

Have you tried to crystallize the material dissolved in DCM or chloroform? Going along with the possibility of dimerization, maybe you could crystallize out the dimer--mount both your yellow crystals and the new ones on a diffractometer and see if they are the same thing or not.

Another idea (probably a better one) would be to do an NMR experiment. Dissolve your compound in CDCl₃, and monitor the NMR spectrum over time. See if there are new peaks arising. If there are, and considering you said it is a slow process, then you could also extract the kinetics of the process.

Hope the suggestions are helpful!

[asa029]

Thank you both for interesting responses. I have never heard of the solvatochronism!

I'll definitely try to see if there is a detectable difference in 1H after time!