Excited State Proton Transfer (ESPT)

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A large number of studies on the acidity constants pKa* in the excited state of aromatic compounds have been reported showing that the pKa* values are markedly different from the acidity constants in the ground state.^[1] Therefore, proton transfers that are thermodynamically unfavorable in the ground state, may become possible and occur readily in the excited state. In general, it is assumed that these reactions are very fast and therefore, acid-base equilibria can be established during the lifetime of the excited state, though there have been examples where other processes such as phosphorescence can interfere with the establishment of the equilibrium.