Why are Solutions of C₆₀‐Piperazine Purple at pH 11?

Abstract

The C₆₀‐piperazine mono adduct was synthesized by the reaction of C₆₀ and piperazine. The saturated C₆₀‐piperazine aqueous solution was colorless when pH is 8 or below. A purple color was developed when pH is around 9 and the pink color is most intense at pH 11. The color of the C₆₀‐piperazine solution fades out when pH is approaching 12 from 11, and the solution remains colorless when pH is 13 or higher. The UV‐Vis spectra of the C₆₀‐piperazine solution were recorded at pH 4, 11 and 14. The mono‐protonated C₆₀‐piperazine was identified to be responsible for the purple color observed. The computational investigation of the un‐protonated, mono‐protonated and di‐protonated C₆₀‐piperazine was conducted at the PM3 and ZINDO(s) levels of theory. Vibronic coupling of the Jahn‐Teller active vibrational mode to the electronic transition was applied to re‐generate the weak absorption between 550–600 nm in the UV‐Vis spectrum of the mono‐protonated C₆₀‐piperazine.

Keywords:

• C₆₀‐piperazine mono adduct
• vibronic coupling
• Jahn‐Teller active vibrational mode
• PM3, ZINDO(s)

Acknowledgment

We are grateful to Sister Rose Mulligan for her friendly help. H. Xu would like to thank Ari Silver for assistance adapting software. MVK is grateful to RFBR (grant 06‐03‐32446) for the financial support.