Photoprotolytic Reactions in Systems Immobilized on Silica Gel Using a Cationic Polyelectrolyte

The patterns of protolytic reactions in the immobilized state are studied using the example of classical systems with photoinduced proton transfer (2-naphthol and its sulfonated derivatives). It is found that the fixation of indicators with a cationic polyelectrolyte leads to a shift of pKa to the acidic region, and the magnitude of the shift depends on the number of sulfo groups in the molecule and reaches 1.2 units for disodium 2-naphthol-3,6,8-trisulfonate. At the same time, the photoprotolytic reaction in the lower singlet-excited state proceeds in the same way as for an unimmobilized substance, and no significant shift of $${\text{p}}K_{a}^{*}$$ is observed. The possibility of creating a flow-through acidity sensor using a change in the ratio of the band intensities of the indicator protonated and deprotonated forms as an analytical signal is demonstrated.