Superphotoacidic properties and pH-switched Stokes shifts in electron-deficient 5-hydroxyisoquinolone derivatives

Abstract

Four electron-deficient 5-hydroxyisoquinolones are among the strongest currently known superphotoacids with excited state pKa* down to −5.8 and ΔpKa up to 9.0, as estimated experimentally via Förster cycle and supported by quantum chemical calculations. A condensed teracyclic derivative is a weaker photoacid with ΔpKa of around three units. The superphotoacidity in turn initiates pH-switched Stokes shifts: irrespective of the form (protonated or deprotonated) absorbing the photon, only emission of the deprotonated form is observed in protic media.