Final answer:
The excited state pKa of 2-naphthol is lower due to the increased stabilization of the conjugate base, the naphthoxy anion, which results from enhanced electron delocalization in the excited state.
Step-by-step explanation:
The ground state pKa of 2-naphthol is 9.5, while the value in the excited state is around 3.5. When a molecule like 2-naphthol is in its excited state, after absorbing energy, its electronic structure changes, often resulting in a rearrangement of electron density within the molecule.
This can have significant effects on acidity. Considering the conjugate base of 2-naphthol, which is the naphthoxy anion, the excited state can lead to increased delocalization of electrons or stabilization of the anion. This increased stabilization of the conjugate base in the excited state makes the proton more easily dissociable, hence the low pKa value in the excited state compared to the ground state.