[opsomath]

Everyone seems to know that this compound has a green-blue impurity due to air oxidation (maybe); it's even in the Wikipedia article! No one seems to know what that impurity actually is, though, and it's not obvious to me. Any insights or references y'all can provide?

[wildfyr]

The wikipedia article cites this paper (http://scripts.iucr.org/cgi-bin/paper?S0108270194001149). It says that the product can be made pure with sublimation, but upon exposure to (white?) light the green color will reappear.

My shot in the dark is some photo-polymerization which extends the conjugation and gives rise of a colored oligomer. Since its heavier and polymeric, it won't be sublimate.

[opsomath]

Yeah, it was funny to me that the crystallography paper was like "Welp, it's got something in it, but who cares, we have a crystal structure now."

All the polymerization and oxidation products I can think of - the S,S-dioxides, for instance - are the wrong color. Sufficiently long oligothiophenes, I think, are red; the oxides are supposed to be yellow or orange according to some of the synthesis lit. Still got nothing here.

[wildfyr]

You are right. I remember those SI-KCTP polythiophene films were always red.

Do you have a good guess how bithiophene is usually made? Wikipedia is vague "The compound is typically prepared by cross-coupling starting from 2-halothiophenes." Ullman? Kumada? You have more expertise in this than me.

Such crosscoupling does imply some transition metals which commonly are fun colors like green. And who knows what ligands are on there. Its pretty reasonable for a metal ion to be trapped in all those sulfurs, then oxidized from a colorless form to colored one by light.

How would you feel about feeding some bithiophene into an ICP-MS? Wouldn't shock me to find Cu or Ni in there.

[opsomath]

We are working on a paper right now with something similar to your hypothesis "Its pretty reasonable for a metal ion to be trapped in all those sulfurs, then oxidized from a colorless form to colored one by light." So I'm sympathetic...except that the degradation seems to happen even when the starting bithiophene is real, real pure. I suppose that traces of nickel (it's Kumada coupling that's used, btw, or occasionally Suzuki...Grignard the 2-bromothiophene, then drip the organometallic into a Ni(II) catalyst with more 2-bromothiophene) could be retained even after re-recrystallizing it, but it seems just a little unlikely. Yes, easy to check with a Ni method, maybe the spring instrumental class can do a flame AA on my 2,2'-bi.

Another thought: the oxidized stuff, 2,2-dioxides of thiophenes, can act as Diels-Alder dienes. I wonder if that kind of crosslinking can give something green or blue. Also far-fetched.