Labeling with fluorescent dyes is a potential strategy for quantification but is usually limited by the need for purification steps to remove unbound dye, unless the fluorescence of the bound dye differs from the free dye. In this respect, catechols can act as a donor or acceptor in photoinduced charge transfer (PCT) -depending on its structure -thus affecting the fluorescence intensity of bound dyes. Nitrodopamine-based ligands were shown to act as electron acceptors in donor-excited PCT due to the electron-withdrawing effect of the nitro group. Upon excitation of a conjugated dye, an electron is promoted from the dye's HOMO to its LUMO and then moves into the vacant LUMO of the nitrocatechol group via a non-radiative transition (thus quenching the fluorescence). Unlike FRET, the quenching effect only occurs when the dye is covalently bound to the nitrodopamine-based ligands.