Reversible protonation (deprotonation) of a side-group is a useful and convenient way to affect the reactivity of large organic and biological molecules. We use bifunctional photoacids to demonstrate how the protonation state of a basic side-group (COO-) controls the reactivity of the main acidic group of the photoacid (OH), both in the ground and the electronic excited state of 6-carboxy derivatives of 2-naphthol.