# Why are Buckminsterfullerene (C60) solutions pink/purple?

A solution of Buckminsterfullerene $$\ce{(C60)}$$ is shown below:

According to the April 8th Wikipedia page on Buckminsterfullerene $$\ce{(C60)}$$:

Solutions of pure C60 have a deep purple color which leaves a brown residue upon evaporation. The reason for this color change is the relatively narrow energy width of the band of molecular levels responsible for green light absorption by individual C60 molecules. Thus individual molecules transmit some blue and red light resulting in a purple color. Upon drying, intermolecular interaction results in the overlap and broadening of the energy bands, thereby eliminating the blue light transmittance and causing the purple to brown color change.[36]

Now, I'm not too bright in the field of quantum chemistry/electrochemistry, so this doesn't click with me very well. Does anyone have a good way to explain in layman's terms why the $$\ce{C60}$$ solutions appear pink/purple? I'm very curious why they experience such a definite color absorption when they are made solely of carbon atoms, while most organic dyes have a host of other heteroatoms and functional groups. Also, why do they exhibit a very specific absorption only when in solution? Any and all contributions are greatly appreciated.

• en.wikipedia.org/wiki/Stacking_(chemistry) Apr 8 '20 at 15:58
• For starters, that wp section is terribly written. "band of molecular levels" is balderdash as much as the term "transmittance".
– Karl
Apr 8 '20 at 15:59
• @Karl Yeah, I knew it couldn't have just been my less-than-perfect electrochem skills that were confusing me. Apr 8 '20 at 16:06
• All pi systems have an absorbance due to the possible electronic excitation from the HOMO to the LUMO. The larger the pi system, the lower the energy, particle in a box and so on. Rings a bell? Benzene in the UV, azulene is blue, beta carotene yellow, etc.
– Karl
Apr 8 '20 at 16:21
• Apr 8 '20 at 16:28