Question

Why does phenophtaleins conjugate base turn pink, while phenophtalein neutral is colorless? is is something with the resonance structuring?

Answer 1

Yes , you are right. Phenolphthalein having a carbon which is attached with two phenol (para position ) and one benzene carboxylate (ortho position) in the case of acidic medium the carbon used to connect these three benzene rings does not allow electrons from one ring to other ring . moreover this form absorbs the light in the UV region.

In the basic form , pheonolic protons are removed, the formed O- electron make a double bond with the benzene ring. Now the elecron is able to move from one ring to other ring. absorption region is shifted from UV to visible region. Thus it shows the pink color