No phenolphthalein mono-anion?

This is a follow-up to the question [here](https://chemistry.stackexchange.com/questions/96259/how-many-protons-does-phenolphthalein-lose-in-basic-solutions). There is no mono-anionic stage shown, does it "disproportionate" to the neutral molecule and the di-anion?




This is an answer to a question of mine, which I posted a few days ago, [Deprotonated form of phenolphthalein](https://chemistry.stackexchange.com/questions/126683/deprotonated-form-of-phenolphthalein/126726#126726). 


I found this paper (it's in Japanese, but the relevant things are legible) <https://www.jstage.jst.go.jp/article/yakushi1947/117/10-11/117_10-11_764/_pdf>, which says that phenolphthalein has 2 pKas, pKa1 = 9.05 and pKa2 = 9.50. It also says that the pink form is the twice deprotonated; the monoanionic form exists, but is colourless.


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