[Mitch]

A mystery for the Chemical Illuminati.

Many years ago, I synthesized hexaiodobenzene as a reagent for some Sonogashira reactions I was exploring. After crystallizing the product out of solution, I began filtering the gorgeous orange-red crystals.  As my attention span is small, I also began refilling my manifold traps with liquid nitrogen. Some of the liquid nitrogen splashed onto the filtering hexaiodobenzene crystals and instantly turned yellow-orange. As the vial heats up it will reversibly return to its red-orange color. A video of the process is shown below: Or click me for direct link.


Color Change In Hexaiodobenzene

I've always wondered what caused the color change, as I'm unfamiliar if this is a common phenomenon. The only two-three reasonable explanations I can think of are:  1.) N₂ is complexing into the aromatic ring somehow 2.) Some sort of solid-solid phase transition is occurring 3.) Changing temperatures caused the relative energies of it's Homo-Lumo gap to change by some mechanism.

If you have any thoughts, or thoughtful ideas on what can be done to sort out this mystery lets hear them. Perhaps the Carbon-Based Curiosities will enlighten me.

Mitch

[enahs]

Question:
If you got plenty around, after the color changes back to the red-orange, if you then submerge the vial in the liquid nitrogen to chill is back down without allowing the chemicals to directly contact, does it turn back yellowish?

[Mitch]

Yes it does. Since Nitrogen is a major component of air, it indirectly ends up being the same experiment.

[Borek]

Yes it does. Since Nitrogen is a major component of air, it indirectly ends up being the same experiment.

Not necesarilly - if you use a stopper. Assuming 1:1 50mL of N₂ chilled to 100K can react with 5 grams of hexaiodobenzene.

You can also try to flush nitrogen with argon.

[excimer]

I'm for #2. I've never heard of an aromatic binding to N2 to the point where it's metastable like that. A good experiment to try would be to simply cool the crystals under vacuum and see if they change color like that. If so, it ain't nitrogen. It is possible that cooling it is changing the electronic structure of the bulk, but that can only happen through a change in crystal morphology. #3 suggests intermolecular charge transfer, but I don't know whether temperature affects CT in the solid state. My guess is yes, but, again, through a change in solid-state morphology. Does this color change happen in solution? If so, definitely not a solid-state issue.

Here's the crystal structure, if that helps. http://dx.doi.org/10.1107/S1600536807002279

[enahs]

Yes it does. Since Nitrogen is a major component of air, it indirectly ends up being the same experiment.

Well, I mean, it is easy to pull a vacuum on the container or flush it:).

If you have the time and material, try pulling a vacuum or flushing and then chilling and see if the color change occurs.


I have suspicions it has little to do with the nitrogen and just the temperature.

[Mitch]

It might be easy for you to flush a vial, in this lab it would take several days to set that experiment up. A nuclear chem lab just isn't equipped with any of the right stuff. But, I'll try and set it up this week. Unless someone wants some to play with it too?

[Noel]

I should give it a try.

[AWK]

I think it is a phase transition. A high pressure metallic phase of C6I6 is known.

[Ψ×Ψ]

1: Sounds unlikely if not impossible.
2: Plausible.  I'm not exactly sure how changes in the crystal packing would affect the color.
3: Something like that...the higher vibrational levels of the ground state will be less populated as the temperature decreases.  If it's red-orange, it's probably absorbing around, IDK, 520ish to 550ish nm?  ...Yellowish probably corresponds to absorption somewhere more like 400, 450ish nm.  400ish nm corresponds to higher energy light than 500ish nm--what you'd expect if you knocked out the lower-energy transitions from higher vibrational levels.

Of course, I'm not a laser jock, so this amounts to speculation.  Ask a physical chemist--she/he might actually know what's going on.

[AWK]

2: Plausible.  I'm not exactly sure how changes in the crystal packing would affect the color.

eg; pi-stacking or not

[Ψ×Ψ]

That occurred to me when I read the suggestion in the OP, but I have yet to see it in action.  I don't think I've seen vividly different colors in the solid state from very similar functionalized acenes with dramatically different crystal packing (believe me, we make enough of 'em)--you'd notice a difference in the absorption spectrum, sure...but significant enough to account for a color change like the one in the video?  I don't know.

[AWK]

Many years ago I got yellow and green crystals (different in shape) of a nickel complex during crystallization from water. When I solved their crystal structures I realized the only difference was a mutual orientation and  intermolecular interactions.
Other example comes from organic chemistry - glyoxal (OCH-CHO) is blue as vapor and yellow as a liquid.

[Borek]

Mercury (II) iodide.

[Mitch]

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